B BIOLOGICAL TREATMENT OF WASTEWATER INTRODUCTION SUBSTRATE Biological treatment is the most widely used method for removal, as well as partial or complete stabilization of biologically degradable substances present in waste-waters Most often, the degradable substances are organic in nature and may be present as suspended, colloidal or dissolved matter The fraction of each form depends on the nature of wastewater In the operation of biological treatment facilities, the characteristics of wastewater are measured in terms of its chemical oxygen demand, COD, biochemical oxygen demand, BOD, total organic carbon, TOC, and volatile suspended solids, VSS; concepts of which have been discussed elsewhere.1 Most of the conventional biological wastewater treatment processes are based on naturally occurring biological phenomena, but are carried out at accelerated rates These processes employ bacteria as the primary organisms; however, certain other microorganisms may also play an important role Gates and Ghosh2 have presented the biological component system existing in the BOD process and it is shown in Figure The degradation and stabilization of organic matter is accomplished by their use as food by bacteria and other microorganisms to produce protoplasm for new cells during the growth process When a small number of microorganisms are inoculated into a bacteriological culture medium, growth of bacteria with time follows a definite pattern as depicted in Figure by plotting viable count and mass of bacteria against time.3 The population dynamics of bacteria in biological treatment processes depends upon various environmental factors including pH, temperature, type and concentration of substrate, hydrogen acceptor, availability and concentration of essential nutrients like nitrogen, phosphorous, sulfur, etc., and essential minerals, osmotic pressure, toxicity of media or by-products, and degree of mixing.4 In recent years, cultures have been developed for biological treatment of many hard-to-degrade organic wastes ORGANICS OXYGEN GROWTH FACTORS LYSISED PRODUCTS CO2 H2O ENERGY OTHER PRODUCTS OXYGEN BACTERIA (PRIMARY FEEDERS) DEAD BIOMASS AUTODESTRUCTION OXYGEN CO2 H2O GROWTH FACTORS ENERGY P R O T O Z O A CO2 H2O ENERGY OTHER PRODUCTS OTHER PRODUCTS PROTOZOA (SECONDARY FEEDERS) FIGURE Biological component system existing in BOD process METABOLIC REACTIONS The metabolic reactions occurring within a biological treatment reactor can be divided into three phases: oxidation, synthesis and endogenous respiration Oxidation–reduction may proceed either in the presence of free oxygen, aerobically, or in its absence, anaerobically While the overall reactions 137 © 2006 by Taylor & Francis Group, LLC B A C T E R I A 138 BIOLOGICAL TREATMENT OF WASTEWATER • Organic Matter Oxidation (Respiration) CxHyOz + O2 → CO2 + H2O + Energy • Inorganic Matter Oxidation (Respiration) NHϩ 2O2 → NOϪ ϩ H O + 2Hϩ ϩ Energy • Protoplasm (Cell Material) Synthesis CxHyOz + NH3 + O2 + Energy → C5H7NO2 + H2 Number of Visible Microorganisms Therefore, bacterial respiration in living protoplasm is a biochemical process whereby energy is made available for endothermic life processes Being dissimilative in nature, respiration is an important process in wastewater treatment practices On the other hand, endogenous respiration is the internal process in microorganisms that results in auto-digestion or self-destruction of cellular material.3 Actually, bacteria require a small amount of energy to maintain normal functions such as motion and enzyme activation and this basal-energy requirement of the bacteria has been designated as endogenous respiration Even when nutrients are available, endogenous metabolism proceeds with the breakdown of protoplasm.5 According to Bertalanffy’s hypothesis,6 the microbial growth is the result of competition between two opposing processes: Aufban—assimilation, and LAG LOG Phase Phase Declining Log Growth Stationary Increasing Death Phase Death Phase Death Phase Phase Organic matter metabolized = Protoplasm synthesized ϩ Energy for synthesis and Net protoplasm accumulation = Protoplasm synthesized Ϫ Endogenous respiration “Growth Kinetics” Irvine and Schaezler8 have developed the following expression for non-rate limited growth of microorganisms in logarithmic phase: dN ϭ k0 N dt (1) RE 25 g SP IRA O TIO N 250 g 510 g BOD5 REMOVED O2 ASSIMILATIVE RESPI TIO RA N IO AT PIR ES R S 350 g 275 g O2 ENDOGENOU BIOMASS 120 g ACTIVE BIOMASS FORMED 40 g INACTIVE BIOM AS 10 g BOD UNUSED S D5 BO ASS g BIOMT 10 g UEN 16 FFL E SYSTEM METABOLISM FOR SOLUBLE WASTES Time RE Log Declining Growth Phase Growth Phase Time FIGURE Growth pattern of microorganisms © 2006 by Taylor & Francis Group, LLC SP Endogenous Phase INFLUENT Mass of Microorganisms N AT ION ASSIMILATIVE RESPIRAT S SPIR OU RE SYNTHESIZED ENDOGEN IR AT I N IO INFLUENT ASSIMILATED BOD BOD BIOMASS (SOLUBLE BIOMASS GROWTH AND VSS) UNUSED BOD (SOLUBLE AND VSS) INFLUENT NON-BIODEGRADABLE FSS AND VSS WASTE = SOLUBLES + PARTICULATES FIGURE EFFLUENT Metabolism and process reactions ON EXCESS SLUDGE • Cx H y Oz ϩ Hϩ ϩ NOϪ ϩ Energy → C H NO + CO2 + H2O Protoplasm (Cell Material) Oxidation C5H7NO2 + 5O2 → 5CO2 + 2H2O + NH3 + Energy Abban—endogenous metabolism The rate of assimilation is proportional to the mass of protoplasm in the cell and the surface area of the cell, whereas the endogenous metabolism is dependent primarily on environmental conditions In the presence of enzymes produced by the living microorganisms, about 1/3 of the organic matter removed is oxidized into carbon dioxide and water in order to provide energy for synthesis of the remaining 2/3 of the organic matter into the cell material Metabolism and process reactions occurring in typical biological wastewater treatment processes are explained schematically by Stewart7 as shown in Figure Thus, the basic equations for biological metabolisms are: 510 g BOD5 INFLUENT carried out may be quite different under aerobic and anaerobic conditions, the processes of microbial growth and energy utilization are similar Typical reactions in these three phases are formulated below: BIOLOGICAL TREATMENT OF WASTEWATER or 139 kmax N t ϭNoe k o t * k vs Cn (Cn > Cn ) where: N0 = Number of viable microorganisms per unit volume at time t = Nt = N = Number of viable microorganisms per unit volume at time t 1 * k vs Cn (Cn > Cn ) 2 k (Cn , Cn ) 1 and k = Logarithmic growth rate constant, timeϪ1 In wastewater treatment practices, the growth pattern based on mass of microorganisms has received more attention than the number of viable microorganisms If each microorganism is assumed to have an average constant mass, then N in Eq can be replaced with X, the mass of active microorganisms present per unit volume to obtain: dX ϭ k0 X dt (2) The growth of bacterial population may become limited either due to exhaustion of available nutrients or by the accumulation of toxic substances The growth rate of bacteria starts slowing down, and Eq changes to the form: dN ϭ kt N dt kt = V1 (T, pH, Cs, Cn1, Cn2, … ) Figure shows variation in growth rate kt with change in nutrient concentrations, assuming that T and pH are held constant and substrate concentration, S, is greater than the critical substrate concentration, S*, above which kt, is independent of S Several interesting observations are made from these curves.8 First, the maximum value of kt is essentially constant Second, the shape of the curve and the limiting concentration is different for each nutrient Third, kt is shown to be zero when any of the nutrients is missing Fourth, as the biological reaction proceeds, all nutrients are consumed Thus, even if all nutrients are initially in excess, the growth may eventually become limited Finally, as the concentration drops to zero, a stationary phase is reached, i.e., dN/dt becomes zero In case of a substrate limited system, rate of growth is given by: © 2006 by Taylor & Francis Group, LLC (4) * Cn * Cn Cn + Cn FIGURE k vs nutrient concentration or dX ϭ␮X dt The following simple relationship between specific growth rate of microorganisms, µ, and substrate concentration, S, was developed by Monod9 and has been widely accepted: (3) where growth rate factor kt, varies with time and becomes a function of temperature, T, pH, substrate concentration, S, and concentration of various nutrients, Cn1, Cn2, etc., i.e.: dN ϭ mN dt ␮ϭ dN dX S ϭ ϭ mmax Ndt Xdt K ϩS (5) where K is a constant called half velocity coefficient and µmax is maximum specific growth rate It is postulated that the same amount of substrate is incorporated in each cell formed Therefore, the rate of increase in number or mass of microorganisms in logarithmic growth phase, dN/dt, or dX/dt, is proportional to the rate of substrate consumption, dS/dt, or dL/dt, if the substrate concentration is measured in terms of its BOD, L, and the following relationship can be stated: dX dS ϭY dt dt (6) or ∆X = Y∆S where Y is called the growth yield coefficient, ∆X is the cell mass synthesized in a given time, and ∆S is substrate removed in the same time The substrate utilization rate, q, per unit biomass has been defined as: qϭ dS Xdt (7) 140 BIOLOGICAL TREATMENT OF WASTEWATER where u is the temperature coefficient This equation shows that reaction rates increase with increase in temperature Combining Eqs 4, and yields: qϭ ␮ Y (8) Methods of BOD Removal (9) In wastewater treatment processes, the microorganisms are not present as isolated cells, but are a collection of microorganisms such as bacteria, yeast, molds, protozoa, rotifers, worms and insect larvae in a gelatinous mass.13 These microorganisms tend to collect in a biological floc, called biomass, which is expected to possess good settling characteristics The biological oxidation or stabilization of organic matter by the microorganisms present in the floc is assumed to proceed in the following sequence:13,14 and q ϭ qmax S K ϩS Under conditions of rate limited growth, i.e., nutrient exhaustion or auto-oxidation, Eq becomes: dX dS ϭY Ϫ bX dt dt (10) where b is the auto-oxidation rate or the microbial decay rate In absence of substrate, this equation is reduced to: dX ϭϪ bX dt (11) Several kinetic equations have been suggested for analysis and design of biological wastewater treatment systems and the following have been applied frequently:10–13 q SX dS ϭ max dt ( K ϩ S ) (12) dS ϭ qSX dt (13) dS S2 ϭ qX dt S0 (14) where S0 is the initial substrate concentration Combining Eqs 10 and 12 gives the net specific growth rate: ␮ϭ q YS dX ϭ max Ϫ b Xdt K ϩ S (15) A similar kinetic relationship can be obtained by combining Eq 10 with Eqs 13 and 14 Effect of Temperature One of the significant parameters influencing biological reaction rates is the temperature In most of the biological treatment processes, temperature affects more than one reaction rate and the overall influence of temperature on the process becomes important The applicable equation for the effect of temperature on rate construct is given by: kT = k20u T–20 © 2006 by Taylor & Francis Group, LLC (16) (a) An initial high rate of BOD removal from wastewater on coming in contact with active biomass by adsorption and absorption The extent of this removal depends upon the loading rate, the type of waste, and the ecological condition of the biomass (b) Utilization of decomposable organic matter in direct proportion to biological cell growth Substances concentrating on the surface of biomass are decomposed by the enzymes of living cells, new cells are synthesized and end products of decomposition are washed into the water or escape to the atmosphere (c) Oxidation of biological cell material through endogenous respiration whenever the food supply becomes limited (d) Conversion of the biomass into settleable or otherwise removable solids The rates of reactions in the above mechanisms depend upon the transport rates of substrate, nutrients, and oxygen in case of aerobic treatment, first into the liquid and then into the biological cells, as shown in Figure 5.15 Any one or more of these rates of transport can become the controlling factors in obtaining the maximum efficiency for the process However, most often the interfacial transfer or adsorption is the rate determining step.14 In wastewater treatment, the biochemical oxygen demand is exerted in two phases: carbonaceous oxygen demand to oxidize organic matter and nitrogenous oxygen demand to oxidize ammonia and nitrites into nitrates The nitrogenous oxygen demand starts when most of the carbonaceous oxygen demand has been satisfied.15 The typical progression of carbonaceous BOD removal by biomass with time, during biological purification in a batch operation, was first shown by Ruchhoft16 as reproduced in Figure The corresponding metabolic reactions in terms of microorganisms to food ratio, M/F, are shown in Figure This figure shows that the food to microorganisms ratio maintained in a biological reactor is of considerable importance in the operation of the process At a low M/F ratio, microorganisms are in the log-growth phase, characterized by excess food and maximum rate of metabolism However, under these conditions, the settling characteristic of biomass is poor because of their dispersed O2 O2 DISPOSITION OF ASSIMILATED BOD BIOLOGICAL TREATMENT OF WASTEWATER O2 O2 SUBSTRATE L BI HEMICA OC CELL REACTION TRACE ELEMENTS WASTE PRODUCTS FLOC PARTICLE DISSOLVED OXYGEN 141 1.0 UNUSED BOD 0.5 ASSIMILATIVE RESPIRATION IS S NTHE L SY INITIA ENDOGENOUS RESPIRATION NET BIOMASS INCREASE 0.2 0.5 SHORT-TERM AERATION CONVENTIONAL 10 20 EXTENDED AERATION RELATIVE ORGANISM WEIGHT (M/F) R2 R2 R2 Y OX FIGURE Metabolic reactions for the complete spectrum LIQUID FILM G EN REACTOR C C ∆r DISSOLVED R1 SUBSTRATE R2 R2 BIOCHEM REACTION RD BYPRODUCT R2 E AT TR BS SU PRODUCTS ∆r SUBSTRATE CELL O2 CO2 CELL MEMBRANE LIQUID FILM Mass transfer in biofloc FIGURE Reduction of total carbonaceous oxygen demand, (%) 100 continued aertion under these conditions results in autooxidation of biomass Although the rate of metabolism is relatively low at high M/F ratio, settling characteristics of biomass are good and BOD removal efficiency is high Goodman and Englande17 have suggested that the total mass concentration of solids, XT , in a biological reactor is composed of an inert fraction, Xi, and a volatile fraction, Xv , which can be further broken down into an active fraction, X, and non-biodegradable residue fraction, Xn, resulting from endogenous respiration, i.e.: D Total BO 90 XT = Xi + Xv = Xi + X + Xn 80 The total mass concentration of solids in wastewater treatment is called suspended solids, whereas its volatile fraction is called volatile suspended solids, X In a biological reactor, volatile suspended solids, X, is assumed to represent the mass of active microorganisms present per unit volume 70 Net ad sorbed 60 (17) and sy nthesiz ed 50 ized 40 Oxid TOXICITY 30 20 10 0 12 16 Aeration time, hr 20 24 FIGURE Removal of organic inbalance by biomass in a batch operation growth; also, the BOD removal efficiency is poor as the excess unused organic matter in solution escapes with the effluent On the other hand, high M/F ratio means the operation is in the endogenous phase Competition for a small amount of food available to a large mass of microorganisms results in starvation conditions within a short duration and © 2006 by Taylor & Francis Group, LLC Toxicity has been defined as the property of reaction of a substance, or a combination of substances reacting with each other, to deter or inhibit the metabolic process of cells without completely altering or destroying a particular species, under a given set of physical and biological environmental conditions for a specified concentration and time of exposure.18 Thus, the toxicity is a function of the nature of the substance, its concentration, time of exposure and environmental conditions Many substances exert a toxic effect on biological oxidation processes and partial or complete inhibition may occur depending on their nature and concentration Inhibition may result from interference with the osmotic balance or with the enzyme system In some cases, the microorganisms become more tolerant and are considered to have acclimatized or adapted to an inhibitory concentration level of a toxic substance This adaptive response or acclimation may result from a neutralization of the toxic material produced by the biological activity of the microorganisms or a selective 142 BIOLOGICAL TREATMENT OF WASTEWATER growth of the culture unaffected by the toxic substance In some cases, such as cyanide and phenol, the toxic substances may be used as substrate Rates of acclimation to lethal factors vary greatly Thus, the toxicity to microorganisms may result due to excess concentrations of substrate itself, the presence of inhibiting substances or factors in the environment and/or the production of toxic by-products.19–23 The influence of a toxicant on microorganisms depends not only on its concentration in water, but also on its rate of absorption, its distribution, binding or localization in the cell, inactivation through biotransformation and ultimate excretion The biotransformations may be synthetic or nonsynthetic The nonsynthetic transformations involve oxidation, reduction or hydrolysis The synthetic transformation involve the coupling of a toxicant or its metabolite with a carbohydrate, an amino acid, or a derivative of one of these According to Warren19, the additive interaction of two toxic substances of equal toxicity, mixed in different proportions, may show combined toxicity as shown in Figure The combined effects may be supra-additive, infra-additive, no interaction or antagonism The relative toxicity of the mixture is measured as the reciprocal of median tolerance limit Many wastewater constituents are toxic to microorganisms A fundamental axiom of toxicity states that all compounds are toxic if given to a text organism at a sufficiently high dose By definition, the compounds that exert a deleterious influence on the living microorganisms in a biological treatment unit are said to be toxic to those microorganisms At high concentrations, these substances kill the microbes whereas at sublethal concentrations, the activity of microbes is reduced The toxic substances may be present in the influent stream or may be produced due to antagonistic interactions Biological treatment is fast becoming a preferred option for treating toxic organic and inorganic wastes in any form; RELATIVE TOXICITY, 1/TLm SUPRA-ADDITIVE INTERACTION STRICTLY ADDITIVE INTERACTION INFRA-ADDITIVE INTERACTION NO INTERACTION ANTAGONISM SOL A 100 75 50 75 SOL B 25 50 25 100 SOLUTION COMBINATIONS FIGURE Possible kinds of interactions between two hypothetical toxicants, A and B © 2006 by Taylor & Francis Group, LLC BIOLOGICAL TREATMENT OF WASTEWATER solid, liquid or gaseous The application of biological processes in degradation of toxic organic substances is becoming popular because (i) these have an economical advantage over other treatment methods; (ii) toxic substances have started appearing even in municipal wastewater treatment plants normally designed for treating nontoxic substrates; and (iii) biological treatment systems have shown a resiliency and diversity which makes them capable of degrading many of the toxic organic compounds produced by the industries.24 Grady believes that most biological treatment systems are remarkably robust and have a large capacity for degrading toxic and hazardous materials.25 The bacteria and fungi have been used primarily in treating petroleum-derived wastes, solvents, wood preserving chemicals and coal tar wastes The capability of any biological treatment system is strongly influenced by its physical configuration As mentioned previously, the Michelis–Menten or Monond equation, Eq 5, has been used successfully to model the substrate degradation and microbial growth in biological wastewater treatment process However, in the presence of a toxic substance, which may act as an inhibitor to the normal biological activity, this equation has to be modified The Haldane equation is generally accepted to be quite valid to describe inhibitory substrate reactions during the nitrification processes, anaerobic digestion, and treatment of phenolic wastewaters.24,26,27 Haldane Equation ␮ ϭ ␮ max S S ϩ K ϩ S ր Ki (18) SPECIFIC GROWTH RATE, m, h–1 where Ki is the inhibition constant In the above equation, a smaller value for Ki indicates a greater inhibition The difference between the two kinetic equations, Monod and Haldane, is shown in Figure 9, in which the specific growth rate, ␮, is plotted for various substrate concentrations, S The values for ␮max, Ks and Ki are assumed to be 0.5 h–1, 50 mg/L and 100 mg/L, respectively Behavior of Biological Processes The behavior of a biological treatment process, when subjected to a toxic substance, can be evaluated in three parts: Is the pollutant concentration inhibitory or toxic to the process? How does it affect the biodegradation rate of other pollutants? Is the pollutant concentration in process effluent reduced to acceptable level? Is there a production of toxic by-products? Is there an accumulation of toxic substances in the sludge? The above information should be collected on biological systems that have been acclimated to the concerned toxic substances Pitter28 and Adam et al.29 have described the acclimation procedures Generally, biological processes are most cost-effective methods to treat wastes containing organic contaminants However, if toxic substances are present in influents, certain pretreatment may be used to lower the levels of these contaminants to threshold concentrations tolerated by acclimated microorganisms present in these processes Equalization of toxic load is an important way to maintain a uniform influent and reduce the shock load to the process Also, various physical/chemical methods are available to dilute, neutralize and detoxicate these chemicals 0.5 MONOD EQUATION 0.4 0.3 0.2 HALDANE EQUATION 0.1 300 400 200 100 SUBSTRATE CONCENTRATION, S,mg/L FIGURE Change of specific growth rate with substrate concentration (inhibited and uninhibited) © 2006 by Taylor & Francis Group, LLC 143 144 BIOLOGICAL TREATMENT OF WASTEWATER Genetically Engineered Microorganisms TYPES OF REACTORS Three types of reactors have been idealized for use in biological wastewater treatment processes: (a) Batch Reactors in which all reactants are added at one time and composition changes with time; (b) Plug Flow or Non-Mix Flow Reactors in which no element of flowing fluid overtakes another element; and (c) Completely Mixed or Back-Mix Reactors in which the contents are well stirred and are uniform in composition throughout Most of the flow reactors in the biological treatment are not ideal, but with negligible error, some of these can be considered ideal plug flow or back-mix flow Others have considerable deviations due to channeling of fluid through the vessel, by the recycling of fluid through the vessel or by the existence of stagnant regions of pockets of fluid.31 The nonideal flow conditions can be studied by tagging and following each and every molecule as it passes through the vessel, but it is almost impossible Instead, it is possible to measure the distribution of ages of molecules in the exit stream The mean retention time, t- for a reactor of volume V and having a volumetric feed rate of Q is given by t ϭVրQ In non-ideal reactors, every molecule entering the tank has a different retention time scattered around t- Since all biological reactions are time dependent, knowledge on age distribution of all the molecules becomes important The distribution of ages of molecules in the exit streams of both ideal and non-ideal reactors in which a tracer is added instantaneously in the inlet stream is shown in Figure 10 The spread of concentration curve around the plug flow conditions depends upon the vessel or reactor dispersion number, Deul, where D is longitudinal or axial dispersion coefficient, u is the mean displacement velocity along the tank length and l is the length dimension.32 In the case of plug flow, the dispersion number is zero, whereas it becomes infinity for completely mixed tanks Treatment Models Lawrence and McCarty11 have proposed and analyzed the following three models for existing continuous flow © 2006 by Taylor & Francis Group, LLC INFLOW Q OUTFLOW Q PLUG FLOW Q OUTFLOW BACK-MIX FLOW Plug Flow Condition (Dispersion Number = 0) Conc of tracer C/C One of the promising approaches in biodegradation of toxic organics is the development of genetically engineered microorganisms Knowledge of the physiology and biochemistry of microorganisms and development of appropriate process engineering are required for a successful system to become a reality The areas of future research that can benefit from this system include stabilization of plasmids, enhanced activities, increased spectrum of activities and development of environmentally safe microbial systems.30 INFLOW Q Non-ideal Flow Condition (Large Dispersion Number) Uniformly Mixed Condition (Dispersion Number = 0) Time of Flow to Exit / Mean Retention Time FIGURE 10 Hydraulic characteristics of basins aerobic or anaerobic biological wastewater treatment configurations: (a) a completely mixed reactor without biological solids recycle, (b) a completely mixed reactor with biological solids recycle, and (c) a plug flow reactor with biological solids recycle These configurations are shown schematically in Figure 11 In all these treatment models, the following equations can be applied in order to evaluate kinetic constants,33 where ∆ indicates the mass or quantity of material: • Solid Balance Equation C ⎡⌬Cells ⎤ ⎡⌬Cells ⎤ ⎡⌬Cells⎤ ⎡⌬Cells ⎤ ⎢ Reactor ⎥ ϭ ⎢Growth ⎥ Ϫ ⎢ Decay ⎥ Ϫ ⎢ Effluent Loss⎥ (19) ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ • Substrate Balance Equation ⎡⌬Substrate ⎤ ⎡⌬Substrate ⎤ ⎡⌬Substrate ⎤ ⎢ Reactor ⎥ ϭ ⎢ Influent ⎥ Ϫ ⎢Growth ⎥ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ ⎡⌬Substrate ⎤ Ϫ⎢ ⎥ ⎣ Effluent Loss⎦ (20) Parameters for Design and Operation Various parameters have been developed and used in the design and operation of biological wastewater treatment processes and the most significant parameters are: ux– Biological Solids Retention Time, or Sludge Age, or Mean Cell Retention Time, is defined BIOLOGICAL TREATMENT OF WASTEWATER Reactor Q,So 145 substrate removed, (S0 – Se), and influent substrate concentration, S0 A desired treatment efficiency can be obtained by control of one or more of these parameters separately or in combination Q,X,Se X, Se I- Completely Mixed-No biological solids recycle BIOLOGICAL TREATMENT SYSTEMS Q,So Reactor (Q+Qr) X, Se X, Se Settling Tank (Q–W), Se Xe Sludge w,Xr Qr, Xr, Se II- Completely Mixed-Biological solids recycle Reactor (Q+Qr) Q,So X, Se Settling Tank (Q–W), Se Sludge w,Xr Qr, Xr, Se III- Plug Flow-Biological solids recycle FIGURE 11 Treatment models as the ratio between total active microbial mass in treatment system, XT , and total quantity of active microbial mass withdrawn daily, including solids wasted purposely as well as those lost in the effluent, ∆XT /∆t Regardless of the fraction of active mass, in a well-mixed system the proportion of active mass wasted is equal to the proportion of total sludge wasted, making sludge age equal for both total mass and active mass U– Process Loading Factor, or Substrate Removal Velocity, or Food to Microorganisms Ratio, or Specific Utilization, is defined as the ratio between the mass of substrate utilized over a period of one day, ∆S/∆t, and the mass of active microorganisms in the reactor, XT ¯ t – Hydraulic Retention Time or Detention Time, or Mean Holding Time, is defined as the ratio between the volume of Reactor, V, and the volumetric feed rate, Q BV – Volumetric Loading Rate or Hydraulic Loading Rate is defined as the ratio between the mass of substrate applied over a period of one day, ST /∆t and the volume of the reactor, V E – Process Treatment Efficiency or Process Performance is defined as percentage ratio between the © 2006 by Taylor & Francis Group, LLC The existing biological treatment systems can be divided into the following three groups: (a) Aerobic Stationary-Contact or Fixed-Film Systems: Irrigation beds, irrigation sand filters, rotating biological contactors, fluidized bed reactors, and trickling filters fall in this group In these treatment processes, the biomass remains stationary in contact with the solid supportingmedia like sand, rocks or plastic and the wastewater flows around it (b) Aerobic Suspended-Contact Systems: Activated sludge process and its various modifications, aerobic lagoons and aerobic digestion of sludges are included in this group In these treatment processes, both the biomass and the substrate are in suspension or in motion (c) Anaerobic Stationary-Contact and Suspended Contact Systems: Anaerobic digestion of sludges and anaerobic decomposition of wastewater in anaerobic lagoons fall in this category A typical layout of a wastewater treatment plant incorporating biological treatment is shown in Figure 12 Primary sedimentation separates settleable solids and the aerobic biological treatment is designed to remove the soluble BOD The solids collected in primary sedimentation tanks and the excess sludge produced in secondary treatment are mixed together and may be digested anaerobically in digesters Trickling filter and activated sludge processes are most common secondary treatment processes for aerobic treatment and are discussed in detail Discussion of sludge digestion by anaerobic process and use of biological nutrient removal as a tertiary treatment have also been included In addition to conventional pollutants present in municipal and industrial wastewaters, significant concentrations of toxic substances such as synthetic organics, metals, acids, bases, etc., may be present due to direct discharges into the sewers, accidental spills, infiltration and formation during chlorination of wastewaters It is import to have a knowledge of both the scope of applying biological treatment and the relevant engineering systems required to achieve this capability Thus, the kinetic description of the process and the deriving reactor engineering equations and strategies for treatment of conventional and toxic pollutants are essential for proper design and operation of biological waste treatment systems.24 146 BIOLOGICAL TREATMENT OF WASTEWATER Pretreatment Raw Wastewater Primary Treatment Screening and Grit Removal Flotation Sedimentation Oil Separation Disposal FIGURE 12 Secondary Treatment (Biological) Final Tertiary Treatment Sedimentation Effluent Activated Sludge Trickling Filters Anaerobic Lagoons Aerated Lagoons Stabilization Ponds RBC Sludge Digestion Typical wastewater treatment sequence MICROBIAL FILM IC B RO NUTR IC B RO © 2006 by Taylor & Francis Group, LLC IENTS AE AE FILTER MEDIUM OXYGEN END PRO DUCTS AIR Trickling Filters Wastewater is applied intermittently or continuously to a fixed bed of stones or other natural synthetic media resulting in a growth of microbial slime or biomass on the surface of this media Wastewater is sprayed or otherwise distributed so that it slowly trickles through while in contact with the air For maximum efficiency, food should be supplied continuously by recirculating, if necessary, the treated wastewater or settled sludge or both Oxygen is provided by the dissolved oxygen in influent wastewater, recirculated water from the air circulating through the interstices between the media to maintain aerobic conditions Active microbial film, biomass, consisting primarily of bacteria, protozoa, and fungi, coats the surface of filter media The activity in biological film is aerobic, with movement of oxygen, food and end-products in and out of it as shown in Figure 13 However, as the thickness of the film WASTE WATER AN The available information strongly indicates that immobilized biological systems are less sensitive to toxicity and have a higher efficiency in degrading toxic and hazardous materials.34 Fixed-film wastewater treatment processes are regarded to be more stable than suspended growth processes because of the higher biomass concentration and greater mass transfer resistance from bulk solution into the biofilm in fixed-films.35 The mass transfer limitation effectively shields the microorganisms from higher concentrations of toxins or inhibitors during short-term shock loads because the concentrations in biofilms change more slowly than in the bulk solution Also, since the microorganisms are physically retained in the reactor, washout is prevented if the growth rate of microorganisms is reduced.34,35 The biofilm systems are especially well suited for the treatment of slowly biodegradable compounds due to their high biomass concentration and their ability to immobilize compounds by adsorption for subsequent biodegradation and detoxification.34 FIGURE 13 Process of BOD removal in trickling filters increases, the zone next to the filter medium becomes anaerobic Increased anaerobic activity near the surface may liquify the film in contact with the medium, resulting in sloughing or falling down of the old film and growth of a new film The sloughed solids are separated in a secondary settling tank and a part of these may be recirculated in the system Two types of trickling filters are recognized, primarily on the basis of their loading rates and method of operation, as shown in Table In low-rate trickling filter, the wastewater passes through only once and the effluent is then settled prior to disposal In high-rate trickling filter, wastewater applied BIOLOGICAL TREATMENT OF WASTEWATER 147 TABLE Comparison of low-rate and high-rate filters Parameters Low-Rate Filters High-Rate Filters US gallons per day per square foot 25 to 100 200 to 1000 Million US gallons per day per acre 1.1 to 4.4 8.7 to 44 Cubic metre per day per square metre 1.0 to 4.1 8.1 to 40.7 Hydraulic Loading Organic Loading (BOD) Pounds of BOD per day per 1000 cubic feet to 25 g of BOD per day per cubic metre 1100 to 13000 80 to 400 400 to 4800 Generally absent Always provided R = 0.5 to High nitrified, lower BOD Not fully nitrified, higher BOD Recirculation Effluent Quality 25 to 300 220 to 1100 Pounds of BOD per day per acre-foot to filters is diluted with recirculated flow of treated effluent, settled effluent, settled sludge, or their mixture, so that it is passed through the filter more than once Several recirculation patterns used in high-rate filter systems are shown in ASCE Manual.36 Sometimes two filter beds are placed in series and these are called Two-Stage Filters The advantages and disadvantages of recirculation are listed below: (c) Amount of sludge solids to digesters may be increased The ACE Manual36 lists the following factors affecting the design and operation of filters: (a) composition and characteristics of the wastewater after pretreatment, (b) hydraulic loading applied to the filter, (c) organic loading applied to the filter, (d) recirculation, system, ratio and arrangement, (e) filter beds, their volume, depth and air ventilation, (f) size and characteristics of media, and (g) temperature of wastewater Advantages of Recirculation (a) Part of organic matter in influent wastewater is brought into contact with growth on filter media more than once (b) Recirculated liquid contains active microorganisms not found in sufficient quantity in raw wastewater, thus providing seed continually This continuous seeding with active microorganisms and enzymes stimulates the hydrolysis and oxidation and increases the rate of biochemical stabilization (c) Diurnal organic load is distributed more uniformly Thus, when plant flow is low, operation is not shut off Also, stale wastewater is freshened (d) Increased flow improves uniformity of distribution, increases sloughing and reduces clogging tendencies (e) Higher velocities and continual scouring make conditions less favourable for growth of filter flies (f) Provides for more flexibility of operation Disadvantages (a) There is increased operating cost because of pumping Larger settling tanks in some designs may increase capital cost (b) Temperature is reduced as a result of number of passes of liquid In cold weather, this results in decreased biochemical activity © 2006 by Taylor & Francis Group, LLC Assuming that the flow through the packed column could be approximated as plug flow, and if BOD removal rate occurs by first order reaction, Eq 13, then the formula to use in trickling filters will become: dS ϭ qSX = k f S dt or Se Ϫk t ϭe f S0 (21) Another equation suggested for application in trickling filters13 is: Se 1 ϭ ϭ t S0 ϩ qXt ϩ k f (22) where trickling filter rate coefficient, kf , is a function of active film mass per unit volume and remains constant for a given specific area and uniform slime layer Contact time, t, 148 BIOLOGICAL TREATMENT OF WASTEWATER is related to filter depth, H, volumetric rate of flow per unit area, Qa , and specific surface area of filter media, Av Sinkoff, Porges, and McDermott37 have proposed the following relationship based on their experiments: ⎡A ⎤ t ϭ c1 H ⎢ v ⎥ ⎣ Qa ⎦ n (23) c1 is assumed to be a constant and exponent n ranges between 0.53 and 0.83 depending upon the type of filter medium and the hydraulic characteristics of the system Substitution of this value of t in Eq 21 gives: Se ϭ expϪ k f S0 n n ⎡ Av ⎤ Ϫk Ј f H րQa ⎢ ⎥ Hc1 ϭ e ⎣ Qa ⎦ (24) Eckenfelder13 suggests that the amount of active surface film covering the filter medium decreases with depth H; therefore, combining Eqs 22 and 23 and substituting c1 ϰ 1/Hm, gives: Se 1 ϭ ϭ (25) Љ n n n S0 ϩ k f Av H (1 Ϫ m ) ր Qa ϩ k f H (1 Ϫ m ) ր Qa For treatment of domestic wastewater on rock filters, Eckenfelder has obtained the values of n = 0.5, m = 0.33 and k Љf = 2.5 with H in ft and q in MGD/acre Several empirical relationships for process efficiency in trickling filters have been proposed and successfully applied Most significant of these are the National Research Council Formula and Rankin’s Formula which have been described in detail in ASCE Manual.36 Eckenfelder and O’Connor13 have reported a value of 1.035 for overall temperature coefficient, u, in Eq 16 An adjustment in process efficiency due to variation in temperature should be provided Activated Sludge Process It is a biological treatment process in which biologically active mass, called activated sludge, is continuously mixed with the biodegradable matter in an aeration basin in the presence of oxygen The combination of wastewater and activated sludge is called the mixed liquor The oxygen is supplied to the mixed liquor either by diffusing compressed air or pure oxygen into the liquid or by mechanical aeration The activated sludge is subsequently separated from the mixed liquor by sedimentation in a clarifier and a part of this sludge is recirculated to the aeration basin The rest of this sludge, indicating net excess production of biological cell material, is disposed of Activated sludge treatment plants vary in performance due to variation in unit arrangements, methods of introducing air and wastewater into the aeration basin, aeration time, concentration of active biomass, aerator volume, degree of mixing, etc Some important types of activated sludge processes are discussed below and their operating parameters are summarized in Table TABLE Activated sludge process parameters Conventional Organic Loading Rate—Bv 1b BOD5 per day per 1000 cubic feet g BOD5 per day per cubic metre Step Aeration Short Term Biosorption Pure Oxygen Complete Mixing Extended Aeration Aerated Lagoons 30–40 50–150 100–400 30–70 150–250 125–180 10–20 480–640 Parameters 800–2400 1600–6400 480–1120 2400–3200 2000–2880 160–320 80 0.2–0.5 0.2–0.5 2–5 0.2–0.5 0.4–1.0 0.6–1.0 0.05–0.2 0.2 Process Loading Factor, U 1b BOD5 per day per 1b 1b MLVSS or kg BOD5 per day per kg MLVSS Sludge Age, days, θx 3–4 3–4 0.2–0.5 3–4 0.8–2.3 14–ϱ 3–5 ¯ Aeration Time, hours, t 6–7.5 6–7.5 2–4 0.5–1.5 (aeration) 1–3 3–5 20–30 70–120 BOD5 removal, %, E 90–95 90–95 60–85 85–90 88–95 85–90 85–90 85–90 Normal Return Sludge Average Resign Flow ϫ 100 Primary Settling Required * 30 (15–75)* Yes 50 (20–75)* 20 (10–50)* 100 (50–150)* 25 (20–50)* 100 (50–150)* 100 (50–200)* Yes No Provision in design should be made for these maximum and minimum values © 2006 by Taylor & Francis Group, LLC Optional Yes Optional No No BIOLOGICAL TREATMENT OF WASTEWATER Kinetic Rate: Depending upon the design and operating conditions, one or more of the kinetic rate Eqs 10, 12, 13 and 14 for BOD removal can be applied to different types of the activated sludge processes Oxygen Requirement: Oxygen is used to provide energy for synthesis of biological cells and for endogenous respiration of the biological mass The total oxygen requirement, ∆O2, can be expressed with the following equation; ∆O2 = aЈ∆S + bЈXT (26) where aЈ is the fraction of BOD removed that is oxidized for energy and bЈ is the oxygen used for endogenous respiration of the biological mass, per day In conventional aeration basins, an hourly oxygen demand of 50 to 80 mg/L per 1000 mg/L of VSS is exerted near the beginning of the tank and is reduced to 20 mg/L per 1000 mg/L of VSS in the course of to hours.14 Excess Sludge Yield: By applying material balance for volatile suspended solids in activated sludge system, and using the concept shown in Figure 3: Excess solids in activated sludge system = Nonbiodegradable suspended solids in influent + Biomass Synthesized during BOD removal – Biomass broken down by endogenous respiration or BOD OF SETTLED MIXED LIQUOR ⌬X ϭ fX ϩ a⌬S Ϫ bXT SLUDGE DISPOSAL (27) where: ∆X = Net accumulation of volatile suspended solids, g/day f = Fraction of volatile suspended solids present in the influent which are non-degradable X0 = Influent volatile suspended solids, g/day Temperature Effect: According to Eckenfelder and O’Connor,13 the value of temperature coefficient in Eq 12 varies between 1.0 for low loading rates to 1.04 for high loading rates Friedman and Schroeder38 have studied in detail the effect of temperature on growth and the maximum cell yield occurred at 20°C Elements of a conventional activated sludge system are shown in Figure 14 In this system, the settled waste is mixed with the return sludge at the inlet end of the aeration tank The microorganisms receive the full impact of any shock load and respond accordingly with sudden increase in oxygen demand during growth By the time microorganisms leave the aeration tank, the organic matter has been stabilized and the microorganism population starts dying off Thus, the microbial population undergoes a continual shifting and never reaches a relatively constant equilibrium.7 A mass of activated sludge of three to four times the mass of the daily BOD load must be kept in the system in order to consume all the new food and also acquire good settling properties These types of plants have been used for treating domestic wastewaters of low biochemical oxygen demands In conventional activated sludge plants BOD OXIDIZED BOD ADSORBED AND SYNTHESIZED BOD OF SETTLED EFFLUENT TIME AIR DIFFUSERS INFLOW PRIMARY SETTLING TANK AERATION BASIN RETURN SLUDGE EXCESS SLUDGE FIGURE 14 Conventional activated sludge © 2006 by Taylor & Francis Group, LLC 149 SLUDGE SECONDARY EFFLUENT SETTLING TANK 150 BIOLOGICAL TREATMENT OF WASTEWATER that have plug flow design, high BOD in influent causes higher oxygen demand at that point in the mixed liquor and this oxygen demand diminishes as the flow passes down the aeration tank Most of the plants designed these days are provided with tapered aeration, with highest air supply near the inlet end and lowest near the outlet end of the aeration tank Modifications of the Conventional Activated Sludge Process B Short Term Aeration or High Rate or Modified Activated Sludge These systems have very high loading rates, both in terms of organic and volumetric loading, and low mixed liquor volatile suspended solids, thus requiring small aeration tank capacities and reduced air requirements Because of shorter aeration time and lower mass of organisms, this process provides an intermediate degree of treatment Organic matter is removed largely by synthesis, thus exerting a high rate of oxygen demand and producing a relatively large volume of sludge per unit mass of BOD removed Since the sludge still contains certain unstabilized organic matter, the settled sludge in secondary settling tanks should be removed rapidly in order to avoid its anaerobic decomposition and floatation The flow diagram is similar to the conventional system as shown in Figure 14 C Contact Stabilization or Biosorption The elements of this type of plant are shown in Figure 16 This system is ideally suited to the treatment of wastewaters in which a large portion of BOD is BOD OF SETTLED MIXED LIQUOR A Step Aeration Activated Sludge Step aeration process, developed by Gould39 at New York City, offers more flexibility than the conventional activated sludge process In this process, wastewater is introduced at four or more points along the aeration tank in order to maintain a uniformly distributed loading In addition to evening out the oxygen demand, this also keeps sludge reaerated in the presence of substrate This process remains biologically more active instead of reaching the endogenous phase near the end of the conventional aeration tank Step aeration system layout and fluctuations in BOD in aeration tank are shown in Figure 15 This method has been successfully employed in the treatment of domestic wastewaters and industrial wastewaters of similar nature SLUDGE DISPOSAL TIME DISTRIBUTED LOADING INFLOW PRIMARY SETTLING TANK EXCESS SLUDGE STEP AERATION BASIN RETURN SLUDGE SLUDGE FIGURE 15 Step aeration activated sludge © 2006 by Taylor & Francis Group, LLC SECONDARY SETTLING TANK BIOLOGICAL TREATMENT OF WASTEWATER present in suspended or colloidal form The suspended BOD is rapidly absorbed in a short period, ½ to 1½ hours, by the well-activated organisms and a part of soluble BOD is metabolized In the activation tank, the sludge is reaerated for bio-oxidation and stabilization of adsorbed food; and when returned to the aeration tank, it is activated for higher BOD removal as compared to the conventional plant where sludge has become lean and hungry in the absence of a food supply The additional advantage of this process is the reduced overall tank volume required as compared to the conventional system However, the operation of such plants is more complex and less flexible than conventional ones throughout the aeration tank In effect, the organic load on the aeration tank is uniform from one end to the other end and consequently a uniform oxygen demand and a uniform biological growth are produced It is assumed to reduce the effect of variations in organic loads that produce shock loads on conventional units, retain a more biological population and hence, produce a more uniform effluent, and be able to treat organic wastes of any concentration and produce an effluent of any desired concentration.5 Using Treatment Model II, Figure 11, as an example of a completely mixed system, Lawrence and McCarty11 have shown analytically that although the complete-mixing will reduce the shock loads due to variations in organic loads, plug flow type conventional units, Treatment Model III, are more efficient Assuming that Eq 13 is applicable for BOD removal rate, and since the BOD in a completely mixed aerator, S, is equal to the effluent BOD, Se, therefore under steady state conditions: D Completely Mixed Activated Sludge “Complex mix” approach is with respect to combining the return sludge and wastewater in order to maintain the entire contents of the aeration chamber in essentially a homogenous state Wastewater introduced into the aeration basin is dispersed rapidly throughout the mass and is subjected to immediate attack by fully developed organisms throughout the aeration basin Biological stability and efficiency of the aeration basin is enhanced by this design Layout of a completely-mixed activated sludge plant and variation in BOD are shown in Figure 17 In this mathematical analysis, McKinney5 considered the complete mixing activated sludge process as the one in which the untreated wastes are instantaneously mixed dS S0 − Se = = qXSe dt t or BOD OF SETTLED MIXED LIQUOR Se ϭ S0 ϩ qXt BOD OF SETTLED MIXED LIQUOR SLUDGE DISPOSAL TIME IN II TIME IN I EFFLUENT INFLOW PRIMARY SETTLING TANK EXCESS SLUDGE FIGURE 16 SECONDARY SETTLING TANK AERATION (SORPTION) BASIN-I RETURN SLUDGE Biosorption (contact stabilization) activated sludge © 2006 by Taylor & Francis Group, LLC 151 ACTIVATION TANK-II (28) 152 BIOLOGICAL TREATMENT OF WASTEWATER BOD INFLUENT SLUDGE DISPOSAL INFLOW PRIMARY SETTLING TANK EFFLUENT TIME AERATION BASIN RETURN SECONDARY EFFLUENT SETTLING TANK SLUDGE EXCESS SLUDGE FIGURE 17 E F Complete mixing activated sludge In recent years, several wastewater treatment plants have been designed to operate with pure oxygen instead of conventional use of air in activated sludge treatment process The obvious advantage of pure oxygen aeration is the higher oxygen concentration gradient maintained within the liquid phase, and this condition permits higher concentration of biomass in the aeration tank This process has been shown to be more economical due to less energy requirements and in some cases has produced a better quality effluent Significant increase in volumetric loading rate, reduction in sludge production, elimination of foaming problems and decrease in treatment costs are claimed to be advantages.40 A pure oxygen activated sludge system developed by Union Carbide Corporation is shown in Figure 18 This process is operated at MLSS values between 3000– 10000 mg/L and the settling rate of sludge is considerably improved Extended Aeration Extended aeration plant is the one where the net growth rate is made to approach zero, i.e., rate of growth becomes approximately equal to rate of decay This is achieved by increasing the aeration time in order to keep the sludge in the endogenous growth phase for a © 2006 by Taylor & Francis Group, LLC considerable time In practice, it is impossible to operate an extended-aeration system without sludge accumulation, because certain volatile solids, mainly polysaccharides in nature and inert organisms in activated sludge process, accumulate in the plant Excess sludge is not generally wasted continuously from an extended aeration, but instead, the mixed liquor is allowed to increase in suspended solids concentration and a large volume of the aeration tank content or return sludge is periodically pumped to disposal Oxidation ditch plants are designed and operated on this principle Layout of a typical extended-aeration plant and variation in BOD in aeration tank are shown in Figure 19 G Aerated Lagoons These are similar to the activated sludge system but without recirculation of sludge Mechanical or diffused aeration devices are used for supplying oxygen and also providing sufficient mixing All suspended solids may or may not be kept in suspension, depending upon the degree of mixing Deposited solids may undergo anaerobic decomposition Mathematically, the BOD removal rate in aerated lagoons is given by Eq 13 and assuming the aerated lagoon to be a completely mixed system, without recycle and maintaining sufficient turbulence, BIOLOGICAL TREATMENT OF WASTEWATER AERATION TANK COVER GAS RECIRCULATION COMPRESSORS CONTROL VALVE AGITATOR OXYGEN FEED GAS EXHAUST GAS WASTE LIQUOR FEED STAGE BAFFLE MIXED LIQUOR EFFLUENT TO CLARIFIER RECYCLE SLUDGE FIGURE 18 153 Schematic diagram of “unox” system with rotating sparger this equation becomes similar to Eq 28 In practice, this equation has proven to represent a generalized response function for design of most aerated lagoons.33 The exact solid level in an aerated lagoon can be approximated by applying a material balance around the lagoon, under equilibrium conditions: Rotating Biological Contactors Solids In + Net Synthesis In Basin = Solids Out or X0 + (Y∆S – b Xet) = Xe or Xe ϭ X ϩ Y ⌬S ϩ bt (29) Because of a very low solid concentration, the detention time in aeration basins is very high and a large volume of aeration basins is required Therefore, the temperature variation exerts a profound effect on the rate of BOD removal Eckenfelder and Ford10 have given a relationship for estimating the lagoon temperature at both extreme conditions Once this temperature is established, a corrected kT value should be obtained from Eq 16, using u equal to 1.035 and then adopted in the kinetic Eq 28 Several other modifications in the activated sludge process have been discussed elswhere;41 but most of these modifications are similar in concepts to one or more of the types discussed above For example, in Hatfield and © 2006 by Taylor & Francis Group, LLC Kraus systems the supernatant from digestion tanks or even digested sludge are added to the reaeration tank to provide nutrients Similarly, an Activated Aeration Plant is a combination of a conventional activated sludge process and the short-term aeration process As mentioned earlier, the traditional aerobic biological wastewater treatment processes have been divided into two groups: fixed film or stationary contact systems like trickling filters and suspended contact systems like activated sludge process Rotating biological contactors, RBC, are more like trickling filters in operation, but adopt certain characteristics of suspended growth systems In this process, large lightweight plastic disks of 2–4 m diameter are half submerged in the wastewater flowing continuously through cylindrical bottomed tanks The disks are rotated slowly at a speed of 1–2 rpm The biomass grows on the plastic disks and the substrate is absorbed by this biomass while it is submerged in the wastewater The oxygen absorption occurs when the biomass is in direct contact with air, generally at a rate higher than that obtained in trickling filters These units have been operated successfully at extreme temperature conditions both for municipal and industrial wastewaters having very high BOD values Antoine and Hynek42 have concluded that RBC are stable, versatile and competitive with the activated sludge process In Canada, an important parameter regulating the pulp and paper wastewater treatment is toxicity reduction, measured by rainbow trout standard bioassay tests The results of bioassay tests conducted by Antoine43 showed RBC was effective in treating the toxic paper mill wastewater, when 154 BIOLOGICAL TREATMENT OF WASTEWATER BOD OF SETTLED MIXED LIQUOR BOD SLUDGE BOD TIME INFLOW AERATION BASIN SETTLING TANK EFFLUENT RETURN SLUDGE - 100% SLUDGE WASTED PERIODICALLY FIGURE 19 Extended aeration activated sludge it was operated at disk speeds of 13 and 17 rpm and flow rates of 1.9 to 2.5 LPM (0.5 to 0.65 USGPM) Similarly, Antoine observed that the RBCs were able to produce acceptable effluents for boardmill, kraft and sulfite wastewaters For sulfite wastewater, the loading rate had to be reduced to increase the detention time On the other hand, the suspended growth treatment of pulp and paper wastes has not consistently produced effluents of an acceptable level B.C Research had conducted tests on the use of the rotating biological contactor process for refinery waste containing phenols and observed it to be an effective method with proper control on operation.43 Anaerobic Treatment In this process, anaerobic bacteria stabilize the organic matter in absence of free oxygen Anaerobic treatment has been used widely for stabilization of sludges collected from primary and secondary settling tanks and recently is being adopted for treatment of soluble wastes in anaerobic lagoons, anaerobic filters, etc One of the important advantages of anaerobic processes © 2006 by Taylor & Francis Group, LLC over aerobic processes is a high percentage conversion of organic matter to gases and liquid and a low percentage conversion to biological cells McCarty44 has mentioned that efficient anaerobic treatment of soluble wastes with BOD concentration as low as 500 mg/L is now feasible Wastes with lower BOD can also be treated anaerobically, although the waste treatment efficiency will not be of the same magnitude as expected from aerobic treatment Anaerobic treatment of wastewaters takes place in two stages as shown in Figure 20 In the first stage, complex organic materials like protein, fats, carbohydrates, are converted into simple organic acids by acid forming bacteria, but with little change in BOD or COD value In the second stage, these fatty acids are converted to carbon dioxide and methane, thereby stabilizing the BOD or COD In a conventional anaerobic treatment process, the substrate is fed into the digester continuously or intermittently In most of the existing digesters, the contents are mixed, mechanically or with compressed gas collected from digesters There is no recirculation of digested sludge and the system is a typical flow through system The hydraulic detention time, t in BIOLOGICAL TREATMENT OF WASTEWATER Complex Organic Material (Proteins, Fats, Carbohydrates) FIGURE 20 Acid Producing Organic Acid (Acetic Acid, Propionic Acid, ) CH4 + CO2 + Bacterial Cells Methane Producing + Bacteria 155 + H2S + N2 Bacteria Bacterial Cells + CO2 + H2O + H2O + Humus Matter Sequential mechanism of anaerobic waste treatment the conventional process becomes equal to the solid retention time, ux Recently, several modifications have been made in the conventional anaerobic treatment process McCarty44 has grouped the basic anaerobic process designs into Conventional Process, Anaerobic Activated Sludge Process, and Anaerobic Filter Process Operating conditions of these process designs are shown in Figure 21 It is suggested that the conventional process be used for concentrated wastes like sludges where economical treatment can be obtained by keeping hydraulic detention time, t equal to the desired solid retention time, ux The economic treatment of diluted wastes, however, requires hydraulic detention time, t, much below the desired solid retention time, ux , and thus, anaerobic contact processes become more applicable.44 Anaerobic treatment processes are more sensitive to operating parameters and their environments as compared to aerobic processes The best parameter for controlling the operation of anaerobic treatment is the biological retention time or solid retention time, SRT A minimum SRT exists below which the critical methane producing bacteria are removed from the system faster than they can reproduce themselves In practice, SRT values of two to ten times this minimum value are used Thus, the hydraulic detention time and solid retention time maintained in anaerobic treatment processes are very high and the net growth of biological solids becomes very low due to significant decay as given by Eq 12 Mixing of the digester content is becoming a common practice The advantages of mixing are better contact between food and microorganisms, uniform temperature, reduction in scum formation, accelerated digestion and distribution of metabolic inhibitors Certain cations, such as sodium, potassium, calcium, or magnesium show a toxic or inhibitory effect on anaerobic treatment when present in high concentrations, as shown in Table 3.45 Soluble sulfides exhibit toxicity because only they are available to the cells If the concentration of soluble sulfides exceeds 200 mg/L, then the metabolic activity of methanogenic population will be strongly inhibited leading to the process failure.21 Concentrations up to 100 mg/L can be tolerated without acclimation and sulfide concentrations between 100 and 200 mg/L can be tolerated after acclimation © 2006 by Taylor & Francis Group, LLC MIXING CH4+ CO2 EFFLUENT (Q1Le, ∆S/∆T) INFLUENT (Q1Le) ∀, L, S CONVENTIONAL PROCESS MIXING CH4+ CO2 INFLUENT (Q1Le) EFFLUENT (Q1Le) MIXED LIQUOR ∀, L, S RETURN WASTE ORGANISMS ∆S/∆T ANAEROBIC ACTIVATED SLUDGE PROCESS CH4+ CO2 EFFLUENT (Q1Le, ∆S/∆T) 1L CONTACT MEDIA INFLUENT (Q1Le) ANAEROBIC FILTER PROCESS FIGURE 21 Basic anaerobic process designs TABLE Stimulatory and inhibitory concentrations of light metal cations to anaerobic processes Cation Stimulatory Con., mg/L Strong Inhibitory Con., mg/L Sodium 100–200 8000 Potassium 200–40 12000 Calcium 100–200 8000 75–150 3000 Magnesium 156 BIOLOGICAL TREATMENT OF WASTEWATER Depending on pH, ammonia can be toxic to anaerobic bacteria and free ammonia is more toxic If concentration of free ammonia exceeds 150 mg/L, severe toxicity will result, whereas the concentration of ammonium ions must be greater than 3000 mg/L to have the same effect At a concentration of 1600 mg/L as N, ammonia can upset the process.20 The volatile acids cause little inhibition in anaerobic reactors at neutral pH.21 Operating parameters of conventional anaerobic digesters are shown in Table system as shown in Figure 22 is considered necessary for nutrient removal.46 In the first stage, carbonaceous BOD is reduced to a level below 50 mg/L In the second stage, the ammonia, present in effluent from the first stage, is oxidized to nitrites and nitrates by nitrosomonas and nitrobacters, respectively, as shown below: NHϩ 3O2 ⎯Nitrosomonas → NOϪ ϩ 2H O ϩ 4Hϩ ⎯⎯⎯⎯ 2 NOϪ ϩ O2 ⎯Nitrobacter → NOϪ ⎯⎯⎯ NUTRIENT REMOVAL Biological nitrification and denitrification is one of the common methods for nitrogen removal from wastewaters In warmer climates, nitrification may occur to a considerable degree in conventional aerobic biological treatment processes, followed by serious adverse effects of denitrification in settling tanks and/or the receiving bodies of water In northern cold climates, below 18°C, a three-stage biological The third stage accomplished denitrification–conversion of nitrites and nitrates to atmospheric nitrogen under anaerobic conditions: 3NOϪ ϩ CH OH → 3NOϪ ϩ CO ϩ 2H O 2NOϪ ϩ 2CH OH → N ϩ CO2 ϩ H O ϩ 2OHϪ TABLE Operating parameters of conventional anaerobic digesters Parameters Unmixed – Loading Rate, Bv 0.02–0.05 – Volatile Solids Reduction percent 1.6–3.2 10–15 50–70 – Detention time, days E 0.1–0.3 0.32–0.80 30–90 1b VSS/day/cubic ft kg VSS/day/cubic metre ¯ t Mixed 50 Mixing Absent Present pH 6.8–7.4 6.8–7.4 Temperature, °C 30–35 30–35 PHOSPHORUS AND BOD REMOVAL NITRIFICATION DENITRIFICATION Coagulating Chemical Application (Optional Points) Air Raw Wastewater Settling Air Aeration Tank Settling Return Sludge Waste Sludge FIGURE 22 Waste Sludge Methanol Aeration Tank Settling Return Sludge Waste Sludge Typical three-stage treatment process for nutrient removal © 2006 by Taylor & Francis Group, LLC Reaction Tank Settling Return Sludge Waste Sludge Effluent BIOLOGICAL TREATMENT OF WASTEWATER A supplemental source of carbonaceous BOD must be added in this stage to reduce the nitrates to nitrogen gas in a reasonable period of time This has been accomplished either by adding a cheap organic substrate like methanol or by bypassing a part of the wastewater containing carbonaceous BOD in the first stage In some cases, the carbonaceous and nitrogeneous oxidation steps are combined in a one-stage aerobic biological system Another system uses fixed-film reactors, such as gravel beds, separately for nitrification and denitrification stages Effluent nitrogen concentrations of mg/L have been proposed as the upper limit in a biological process Many full scale biological nitrogen removal facilities are now in operation Nitrifying bacteria are subject to inhibition by various organic compounds, as well as by inorganic compounds such as ammonia Free ammonia concentrations of 0.1 to 1.0 mg/L and free nitrous acid concentrations of 0.22 to 2.8 mg/L, start inhibiting Nitrobacters in the process.20 The majority of phosphorus compounds in wastewaters are soluble and only a very small fraction is removed by plain sedimentation The conventional biological treatment methods typically remove 20 to 40 percent of phosphorus by using it during cell synthesis A considerably higher phosphorus removal has been achieved by modifying the processes to create “luxury phosphorus uptake.” Factors required for this increased phoshorus removal are plug-flow reactor, slightly alkaline pH, presence of adequate dissolved oxygen, low carbon dioxide concentration and no active nitrification.46 However, the most effective method of phosphate removal is the addition of alum or ferric salts to conventional activated sludge processes Nomenclature Av Bv = = D = E H K = = = Ki L = = N0 = Nt = ∆O2 = Q = Qa = Specific surface area of filter media, Length–1 Volumetric loading rate; mass per unit volume per unit time Longitudinal dispersion coefficient, (Length)2 per unit time Process treatment efficiency, ratio Filter depth, length Half velocity coefficient = substrate concentration at which rate of its utilization is half the maximum rate, mass per unit volume Inhibition constant, mass per unit volume Substrate concentration around microorganisms in reactor, measured in terms of BOD, mass per unit volume Number of microorganisms per unit volume at time t = N = Number of microorganisms per unit volume at time t Amount of oxygen requirement, mass per unit time Volumetric rate of flow, volume per unit time Volumetric rate of flow per unit area, Length per unit time © 2006 by Taylor & Francis Group, LLC Qr = R S ∆S Se = = = = S0 = T U V X = = = = ∆X = Xe = X0 = Xr = XT Y aЈ = = = b bЈ = = c1 f = kf ,kЈ ,kЉf f k0 kt kЈ l m n q qmax t tu w u 157 Volumetric rate of return flow, volume per unit time Recycle ratio Substrate concentration, mass per unit volume Substrate removed, mass per unit time Effluent BOD or final substrate concentration, mass per unit volume Influent BOD or in the initial substrate concentration, mass per unit volume Temperature, °C Process loading factor, time–1 Volume of the reactor, volume Mass of active microorganisms present per unit volume Cell mass synthesized, mass per unit time Effluent volatile suspended solids, mass per unit volume Influent volatile suspended solids, mass per unit volume Volatile suspended solids in return sludge, mass per unit volume Total mass of microorganisms in the reactor, mass Growth yield coefficient, dimensionless Fraction of BOD removed that is oxidized for energy Microorganisms decay coefficient, time–1 Oxygen used for endogenous respiration of biological mass, time–1 Constant = Fraction of volatile suspended solids present in the influent which are non-degradable = Rate coefficient in filters, time–1 = Logarithmic growth rate constant, time–1 = Growth rate factor, time–1 = Growth rate factor, (time)–1 (mass per unit volume)–1 = Length dimension in reactor, Length = Constant = Trickling filter exponent = dS/Xdt = Substrate utilization rate per unit biomass = Maximum substrate utilization rate per unit biomass = Contact time in filter or any other reactor, time = V/Q = Mean retention time, time = Mean displacement velocity in reactor along length, length per unit time = Volumetric rate of flow of waste sludge, volume per unit time = Temperature coefficient for microbial activity 158 BIOLOGICAL TREATMENT OF WASTEWATER ux m = Mean cell retention time, time = dx/Xdt = Specific growth rate of microorganisms, time–1 mmax = Maximum specific growth rate of microorganisms, time–1 D/ul = Reactor dispersion number, dimensionless M/F = Microorganisms to food ratio in a reactor dL/dt = Rate of waste utilization measured in terms of BOD, mass per unit volume per unit time dN/dt = Rate of growth in number of microorganisms, Number per unit volume per unit time dS/dt = Rate of substrate consumption, mass per unit volume per unit time ∆S/∆t = Mass of substrate utilized over one day, mass per unit time ST /∆t = Total mass of substrate applied over a period of one day, mass per unit time dX/dt = Rate of growth of mass of active microorganisms, mass per unit volume per unit time ∆XT /∆t = Total quantity of active biomass withdrawn daily, mass per unit time REFERENCES MacInnis, C., Municipal Wastewater, Encyclopedia of Environmental Science and Engineering, Vol 1, edited by J R Pfafflin and E.N Ziegler, Gordon and Breach, New York Gates, W.E and S Ghosh, Biokinetic Evaluation of BOD Concepts of Data, Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, Vol 97, no SA3, June 1971, pp 287–309 McKinney, R.E., Microbiology for Sanitary Engineers, McGraw-Hill Book Company, Inc., New York, 1962 Stanier, R.Y., M Doudoroff and E.A Adelberg, The Microbial World, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1957 McKinney, R E., Mathematics of Complete Mixing Activated Sludge, Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, 88, SA3, May 1962, pp 87–113 Tsuchiya, H.M., A.G Frederickson and R Avis, Dynamics of Microbial Cell Populations, Advances in Chemical Engineering, Vol 6, edited by T.B Drew, J.W Hoopes, Jr and T Vermeulen, Academic Press, New York, 1966 Stewart, M.J., Activated Sludge System Variations, Specific Applications, Proceedings of the Fifteenth Ontario Industrial Waste Conference, June 1968, pp 93–115 Irvine, R.L and D.J Schaezler, Kinetic Analysis of Date from Biological Systems, Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, Vol 97, No SA4, August 1971, pp 409–424 Monod, J., The Growth of Bacterial Cultures, Annual Review of Microbiology, 3, 371, 1949 10 Eckenfelder, W.W and D.L Ford, Water Pollution Control, Jenkins Publishing Company, Austin, Texas, 1970 11 Lawrence, A.W and P.L McCarty, Unified Basis for Biological Treatment Design and Operation, Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, 96, SA3, June 1970, pp 757–778 12 Pearson, E A., Kinetics of Biological Treatment, Advances in Water Quality Improvement, edited by E.F Gloyne and W.W Eckenfelder, Jr., University of Texas Press, Austin, 1970 13 Eckenfelder, W.W and D.J O’Connor, Biological Waste Treatment, Pergamon Press, New York, 1961 © 2006 by Taylor & Francis Group, LLC 14 Fair, G.M., J.C Geyer and D.A Okun, Water and Wastewater Engineering, Vol 2, John Wiley and Sons, Inc., 1968 15 Bewtra, J.K., Droste, R.L and Ali, H.I., The Significance of Power Input in the Testing and Biological Treatment of Industrial Wastewater, Treatment and Disposal of Liquid and Solid Industrial Wastes, edited by K Curi, Pergamon Press, New York, 1980, pp 23–47 16 Ruchhoft, C.C., Studies of Sewage Purification-IX, Public Health Reports, 54, 468, 1939 17 Goodman, B.L and A.J Englande, Jr., A Unified Model of the Activated Sludge Process, Journal of the Water Pollution Control Federation, 46, February 1974, p 312 18 Parker, H.W., Wastewater Systems Engineering, Prentice-Hall Inc., Englewood Cliffs, 1975 19 Warren, C.E., Biology and Water Pollution Control, W.B Saunders Company, Toronto, 1971 20 Eckenfelder, W.W., Jr., Principles of Water Quality Management, CBI Publishing Company, Inc., Boston, 1980 21 Grady, C.P., Jr and H.C Lim, Biological Wastewater Treatment— Theory and Applications, Marcel Dekker, Inc., New York, 1980 22 Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of Environmental Science and Technology, Vol I, edited by E Ziegler and J Pfafflin, Gordon and Breach Science Publishers Inc., New York, 1982, pp 81–102 23 Bewtra, J.K., Toxocity Effects on Biological Processes in Waste Treatment, New Directions and Research in Waste Treatment and Residual Management, Vol 2, Proceedings of International Conference held at the University of British Columbia, Vancouver, B.C., June 1985, pp 807–827 24 Gaudy, A.F., Jr., W Lowe, A Rozich and R Colvin, Practical Methodology for Predicting Critical Operating Range of Biological Systems Treating Inhibitory Substrates, Water Pollution Control Federation Journal, Vol 60, No 1, 1988, pp 77–85 25 Grady, C.P.L., Jr., Biodegradation of Hazardous Wastes by Conventional Biological Treatment, Hazardous Wastes and Hazardous Materials, 3, 1986, pp 333–365 26 Gaudy, A.F., Jr., A.F Rozick and E.T Gaudy, Activated Sludge Process Models for Treatment of Toxic and Nontoxic Wastes, Water Science and Technology, Vol 18, 1986, pp 123–137 27 Godrej, A.N and J.H Sherrard, Kinetics and Stoichiometry of Activated Sludge Treatment of a Toxic Organic Wastewater, Water Pollution Control Federation Journal, Vol 60, No 2, 1988, pp 221–226 28 Pitter, P., Determination of Biological Degradability of Organic Substances, Water Research, 10, 1976, pp 231 29 Adam, C.E., D.L Ford and W.W Eckenfelder, Jr., Development of Design and Operational Criteria for Wastewater Treatment, Enviro Press, Inc., Nashville, 1981 30 Pierce, G.E., Potential Role of Genetically Engineered Microorganisms to Degrade Toxic Chlorinated Hydrocarbons, Detoxication of Hazardous Wastes, edited by J.H Exner, Ann Arbor Science Publishers, Ann Arbor, 1982, pp 315–322 31 Levenspiel, O., Chemical Reaction Engineering, John Wiley and Sons, Inc., New York, 1967 32 Murphy, K.L and B.I Boyko, Longitudinal Mixing in Spiral Flow Aeration Tanks, Journal of the Sanitary Engineering Division, Proceedings of the American Society of Civil Engineers, 96, SA2, April 1970, pp 211–221 33 Parker, C.E., Anaerobic–Aerobic Lagoon Treatment for Vegetable Tanning Wastes, Report prepared for the Federal Water Quality Administration Environmental Protection Agency, U.S Government Printing Office, Washington, D.C., December 1970 34 Stevens, D.K., Interaction of Mass Transfer and Inhibition in Biofilms, Journal of Environmental Engineering, Vol 114, No 6, 1988, pp 1352–1358 35 Toda, K and H Ohtake, Comparative Study on Performance of Biofilm Reactors for Waste Treatment, Journal of General Applied Microbiology, Vol 31, No 2, 1985, pp 177–186 36 Wastewater Treatment Plant Design, American Chemical Society of Civil Engineers Manual of Engineering Practice, No 36, New York, NY, 1977 37 Sinkoff, M.D., R Porges and J.H McDermott, Mean Residence Time of a Liquid in a Trickling Filter, Journal of the Sanitary Engineering BIOLOGICAL TREATMENT OF WASTEWATER 38 39 40 41 42 Division, Proceedings of the American Society of Civil Engineers, 85, SA6, 1959 Friedman, A.A and E.D Schroeder, Temperature Effects on Growth and Yield for Activated Sludge, presented at 26th Purdue Industrial Waste Conference, Lafayette, Indiana, May 4–6, 1971 Gould, R.H., Tallmans Island Works Opens for World’s Fair, Municipal Sanitation, Vol 10, No 4, April 1939, p 185 McWhirter, J.R., Oxygen and the Activated Sludge Process, Chapter in The Use of High Purity Oxygen in the Activated Sludge Process, Vol edited by J.R McWhirter, CRC Press Inc., West Palm Beach, 1978 Srinda, R.T and R.F Ward, Activated Sludge Processes: Conventional Processes and Modifications-Applications, presented at Short Course in Water Quality Control, Department of Civil Engineering, University of Massachusetts, Amherst, Mass., March 1970 Antoine, R.L and R.J Hynek, Operating Experience with Bio Surf Process Treatment of Food Processing Wastes, Proceedings of 28th Industrial Wastes Conference, Purdue University, Lafayette, Indiana, May 1973 © 2006 by Taylor & Francis Group, LLC 159 43 Antoine, R.L Fixed Biological Surfaces—Wastewater Treatment, CRC Press, Cleveland, Ohio, 1976, pp 93–122 44 McCarty, P.L., Anaerobic Treatment of Soluble Wastes, Advances in Water Quality Improvement, edited by E F Gloyne and W W Eckenfelder, Jr., University of Texas Press, Austin, 1970, pp 336–352 45 McCarty, P.L., Anaerobic Waste Treatment Fundamentals, Public Works, Vol 95, No 9–12, 1964, pp 95–126 46 Bouck, D.W., Nutrient Removal in Three-Stage Processing, Chapter in Advances in Water and Wastewater Treatment—Biological Nutrient Removal, edited by M.P Wanielista and W.W Eckenfelder, Jr., Ann Arbor Science, Ann Arbor, MI, 1978, pp 65–78 J.K BEWTRA N BISWAS University of Windsor ... disposal In high-rate trickling filter, wastewater applied BIOLOGICAL TREATMENT OF WASTEWATER 147 TABLE Comparison of low-rate and high-rate filters Parameters Low-Rate Filters High-Rate Filters... 1980 22 Bewtra, J.K., Biological Treatment of Wastewater, Encyclopedia of Environmental Science and Technology, Vol I, edited by E Ziegler and J Pfafflin, Gordon and Breach Science Publishers Inc.,... Bewtra, J.K., Droste, R.L and Ali, H.I., The Significance of Power Input in the Testing and Biological Treatment of Industrial Wastewater, Treatment and Disposal of Liquid and Solid Industrial Wastes,