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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/234167418 Wastewater Treatment: Biological Chapter · January 2013 DOI: 10.1201/9781003045045-61 CITATIONS...

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/234167418 Wastewater Treatment: Biological Chapter · January 2013 DOI: 10.1201/9781003045045-61 CITATIONS READS 65 47,247 3 authors: Shaikh Zia David W Graham Newcastle University Newcastle University 65 PUBLICATIONS 1,961 CITATIONS 230 PUBLICATIONS 12,825 CITATIONS SEE PROFILE SEE PROFILE Jan Dolfing 286 PUBLICATIONS 12,340 CITATIONS SEE PROFILE All content following this page was uploaded by Jan Dolfing on 17 July 2014. The user has requested enhancement of the downloaded file. Wastewater Treatment: Biological Shaikh Ziauddin Ahammad David W. Graham Jan Dolfing School of Civil Engineering and Geosciences, Newcastle University, Newcastle, U.K. Abstract Rapid urbanization and indiscriminate use of natural resources have placed the environment under increas- ing stress, and different measures are being implemented to prevent further deterioration. For example, treat- ment of our wastes and efficient reuse of our resources are prerequisites to further sustainable existence. As such, various waste treatment technologies have developed with the goal of minimizing negative impacts of wastes on the environment while also potentially recovering value from the wastes. Although many tech- nologies exist, biological processes compare very favorably with non-biological processes because of their sustainability potential, including energy production and resource recovery. Further, carbon, nitrogen, and phosphorus are the main constituents of most wastes, and removal of such elements from waste effluents can reduce environmental stress and minimize ecosystem deterioration. This summary describes typical aerobic and anaerobic biological treatment methods, including activated sludge processes, upflow anaero- bic sludge blanket reactors and other anaerobic systems, and biological nitrogen and phosphorus removal systems, which can be used to treat different types of wastes. An emphasis is placed on methods that also have the capacity to generate potential energy as combustible biogas or nutrients from wastes. Introduction composition of wastewater, including quantity and constit- uents, varies considerably from place to place, depending Human activities and population growth have placed the on suite of sources, social behavior, the type and number environment under increasing stress. Furthermore, in- of industries within a catchment, climatic conditions, water discriminate use of natural resources is accompanied by consumption, and the nature of the wastewater collection Wastewater Waste – increased local and global pollution levels, which are system. Given this variety, wastewater treatment processes reflected in imbalances in our ecosystems. The genera- must be innately versatile, but also sometimes must be tai- tion of large quantities of wastewater with a high organic lored to the specific waste and conditions. The purpose content and toxicants is one obvious product of excessive of this entry is to describe different biological treatment consumption. It has been known for many years that envi- methods and then discuss their relative capacities to treat ronmental discharges of high loads of organic matter can different wastes on the basis of waste characteristics and result in oxygen depletion in receiving waters due to stim- the desire for resource recovery. ulated microbial activity. This oxygen depletion and the presence of trace toxicants found in wastes also negatively influence ecosystems, including reduced biodiversity and Wastewater Treatment Options environmental health. Therefore, negative environmental impacts have driven our need to understand the effect of Special handling and treatment of wastes have been per- pollution on water bodies and develop proper measures to formed for thousands of years in response to their per- reduce discharges, including treatment processes. ceived importance, although approaches have changed as Different technologies are available to treat wastes. perceptions have changed over history. In 4th century b.c. However, biological wastewater treatment methods are in Greece, the Athenian Constitution written by Aristo- most valuable because their economic benefits are high, tle proscribed provisions for the appropriate handling of especially when coupled with waste stabilization and re- sewage. Concern was based on aesthetics, probably odors, source recovery. The optimal treatment processes depend because relationships between domestic wastes and health on the waste type and treatment goals. Wastewater gener- were not yet known. It was not until the mid-1800s that ally originates from two sources: 1) domestic wastewater links between wastes and human health became more ap- from gray water, toilets, and other domestic activities; and parent, which led to a progression of waste management 2) industrial wastewater, generated by industries during approaches and technologies to address health concerns. the normal course of activity, which often rely on the lo- Treatment technologies evolved slowly over time, in- cal sewerage systems for waste processing. Therefore, the cluding physical, chemical, and biological approaches, Encyclopedia of Environmental Management DOI: 10.1081/E-EEM-120046063 Copyright © 2013 by Taylor & Francis. All rights reserved. 2645  2646 Wastewater Treatment: Biological many of which are still used in different sectors. Physical can damage pumps. Sometimes, froth flotation is also used methods are based on the application of physical forces, to remove excessive oils or grease in the wastes. such as screening, mixing, flocculation, sedimentation, flotation, filtration, and gas transfer. Alternately, chemi- Primary Treatment cal processes treat contaminants by adding chemicals or by stimulating specific chemical reactions. Precipitation, Most of the settleable solids are removed from the wastewa- adsorption, and disinfection are common examples of ter by simple sedimentation, a purely physical process. In chemical treatment methods. Physical and chemical meth- this process, the horizontal velocity of the water through ods are often combined, especially in industrial treatment the settle is maintained at a level that provides solids ad- scenarios. In contrast to physiochemical processes, biolog- equate time to settle and floatable material be removed ical processes remove organic contaminants (e.g., biode- from the surface. Therefore, primary treatment steps con- gradable organic material) largely through microbiological sist of settling tanks, clarifiers, or flotation tanks, which activity. Commonly used biological treatment methods in- send separated solids to digestion units and supernatant to clude aerobic treatment in ponds, lagoons, trickling filters, subsequent, typically microbiological, treatment units. and activated sludge plants, and anaerobic treatment[3,4] in similar reactor systems. Processes that combine anaero- Secondary Treatment bic and aerobic unit operations are also common. Secondary treatment uses microbial communities, under The best overall treatment approach depends on the varying growth conditions, to biochemically decompose source and nature of waste, such as production rates, con- organic compounds in the waste that have passed from pri- stituents, and relative concentrations. As such, optimal mary treatment units. An array of reactors are employed process trains and designs should be as simple as possible for biological treatment, which include suspended biomass, in design and operation, while being efficient in removing biofilm, fixed-film reactors, and pond or lagoon systems. key pollutants and minimizing energy consumption and negative by-products. More complex operations are only Secondary Clarification used when absolutely necessary. Within a typical treatment plant, each type of treatment Most biological treatment processes produce excess bio- has a different purpose. For example, the main objective mass through the conversion of waste carbon to new cells. of biological treatment is to treat soluble organic matter in As such, before the final treatment steps, such as disinfec- the wastes, which often requires physical pretreatment to tion or nutrient removal, solids must be separated from the remove solids before biological treatment. For domes- secondary treatment effluents. This is usually by settling, tic wastewater, the main objective is to reduce the organic Wastewater but membranes are also employed. The separated solids Waste – content and, in growing numbers of cases, secondary nu- are either recycled back to the head of the process train or trients (nitrogen, N; phosphorus, P). For industrial waste- sent to digesters for solids reduction and processing, de- waters, the objective is usually to remove or reduce the pending on the type of the digester system. concentration of organic compounds, especially specific toxicants that can be present in some wastes, which is why Tertiary/Advanced Treatment chemical processes are also included in industrial treat- ment systems. However, biological processes are almost Advanced or tertiary treatment consists of processes that always used when possible. are designed to achieve higher effluent quality than at- Biological degradation of organics is accomplished tainable by conventional secondary treatment methods. through the combined activity of microorganisms, in- These include polishing steps such as activated carbon cluding bacteria, fungi, algae, protozoa, and rotifers. To adsorption, ion exchange, reverse osmosis, electrodialysis, maintain the ecological balance in the receiving water, chemical oxidation, and nutrient removal. Although not regulatory authorities have set standards for the maximum technically a tertiary process, final effluent disinfection is amount of the undesirable compounds present in the dis- often performed after secondary or tertiary treatment us- charge water. In a typical wastewater treatment plant, the ing chlorination, ultraviolet methods, ozonation, and other following steps are carried out to achieve the desired qual- methods designed specifically to kill residual organisms in ity of the effluent before it can be safely discharged into the wastewater after all previous treatment steps. the receiving water. Pretreatment/Preliminary Treatment Biological Treatment Options Pretreatment is primarily used to protect pumping equip- Biological processes are classified according to the pri- ment and promote the success of subsequent treatment mary metabolic pathways present in the dominant different steps. Pretreatment devices such as screen and/or grit re- microorganisms active in the treatment system. As per the moval systems are designed and implemented to remove availability and utilization of oxygen, the biological pro- the larger suspended or floating solids, or heavy matter that cesses are classified as aerobic, anoxic, and anaerobic. Wastewater Treatment: Biological 2647 Aerobic Processes ment processes also can be classified based on the growth conditions in the reactor (see Fig. 1). In this case, the two Treatment processes that occur in the presence of molecu- main categories are suspended growth and attached growth lar oxygen (O2) and use aerobic respiration to generate cel- processes. lular energy are called aerobic processes. They are most metabolically active, but also generate more residual solids Suspended Growth Processes as cell mass. In these processes, the microorganisms, which are respon- Anoxic Processes sible for the conversion of waste organic matter to simpler compounds and biomass, are maintained in suspension These are processes that occur in the absence of free within the liquid phase. However, there are different types molecular oxygen (O2) and generate energy through an- of aerobic and anaerobic suspended growth processes. aerobic respiration. Microorganisms use combined oxy- Aerobic processes include activated sludge, aerated la- gen from inorganic material in the waste (e.g., nitrate) goons, and sequencing batch reactors, whereas anaero- as their terminal electron acceptor. Anoxic processes are bic processes include bag digesters, plug-flow digesters, common biological nitrogen removal systems through stirred-tank reactors, and baffled reactors with organisms denitrification. primarily in the liquid phase. Anaerobic Processes Attached Growth Process These are the processes that occur in the absence of free In these processes, the microorganisms responsible for de- or combined oxygen, and result in sulfate reduction and grading the waste are attached to surfaces (e.g., stones, in- methanogenesis. They usually produce biogas (i.e., meth- ert packing materials), or are self-immobilized on flocs or ane) as a useful by-product and tend to generate lower granules in the system. Attached growth processes can be amounts of biosolids through treatment. aerobic or anaerobic. Aerobic attached growth processes Apart from a classification based on microbial metabo- include trickling tilters, roughing filters, rotating biological lism and/or oxygen utilization, biological wastewater treat- contactors, and packed-bed reactors. Anaerobic systems Wastewater Waste – Fig. 1 Different biological treatment processes. S, substrate concentration available to microorganisms; Sbulk, substrate concentration in the bulk of the liquid; ASP, activated sludge process; SBR, sequencing batch reactor; TF, trickling filter; RBC, rotating biological contactor; ACP, anaerobic contact process; AF, anaerobic filter; UASB, upflow anaerobic sludge blanket; AFBR, anaerobic fluidized bed reactor.  2648 Wastewater Treatment: Biological include upflow packed-bed reactors, down-flow packed- surface or surface aerators designed to supply adequate bed reactors, anaerobic rotating biological contactors, dissolved oxygen to the water for the microorganisms to anaerobic fluidized bed reactors, upflow anaerobic sludge thrive. The wastewater flows through the tank and resident blanket (UASB) reactors, and various hybrid anaerobic re- microorganisms consume organic matter in the wastewa- actors (HAR). UASBs are widely used reactors for the an- ter. The aeration tank effluent flows to the clarifier where aerobic treatment of industrial and domestic wastewater. the microorganisms are removed. The clarifier supernatant is then transferred to disinfection or treatment units, and then ultimately discharged to the receiving water. Biosolids Aerobic Biological Waste Treatment from the settler are recycled back to the head of the treat- Processes ment system or sent to digesters for further processing. Typical aerobic waste treatment systems provide a loca- Aeration Tanks tion where microbes are exposed to molecular oxygen (O2) to oxidize complex organics present in the waste, produc- Aeration tanks are usually designed uncovered, open to ing carbon dioxide, simple organics, and new cell biomass. the atmosphere. Air is supplied to the microorganisms by The activated sludge process (ASP) is very well known two primary methods: mechanical aerators or diffusers. and the most widely used biological treatment process in Mechanical aerators, such as surface aerators and brush developed countries. aerators, aerate the surface of the water mechanically and promote diffusion of oxygen to water from the atmosphere. Activated Sludge Process The concentration of dissolved oxygen in the liquid can be controlled by adjusting the speed of the rotors. Both Classic ASPs are aerobic suspended cell systems. Mineral- mechanical aerators and diffusers are the largest energy ization of waste organic compounds is accompanied by the consumers in aerobic biological wastewater treatment pro- formation of new microbial biomass and sometimes the cesses. Diffusers bubble air directly into the tank at depth removal of inorganic compounds, such as ammonia and and are usually preferred because of higher oxygen trans- phosphorus, depending on the particular process design. fer efficiencies. Activated sludge processes were first conceived in the early As previously indicated, aeration provides O2 to the mi- 1900s with the word “activated” referring to solids that croorganisms and also serves to mix the liquor in the tank. catalyze the degradation of the waste. It was subsequently Although complete mixing is desired, there are usually discovered that the “activation” part of the sludge was a “dead zones” in the tank where anaerobic/anoxic condi- complex mixture of microorganisms. The liquid in acti- tions develop in poorly mixed areas. It is desirable to keep Wastewater Waste – vated sludge systems is called the “mixed liquor,” which these zones to a minimum to minimize undesired odors includes both wastewater and the resident organisms. and also problems with sludge bulking, which can reduce There have been several incarnations of the ASP. The settling efficiency in secondary clarifiers. most common designs use conventional, step aeration, and continuous-flow stirred-tank reactors. A conventional Secondary Clarifiers ASP consists of standard pretreatment steps, an aeration tank, and a secondary clarifier, an example of which is Clarifiers are used to separate the biomass and other solids shown in Fig. 2. The aeration tank can be aerated by sub- coming out of the aeration tank by means of gravity set- Fig. 2 Activated sludge process. Wastewater Treatment: Biological 2649 tling. The flow rate of the liquid is maintained in such a SRT = VX/(QXe + QwXw) way that the upflow velocity of the liquid is less than the settling velocity of the biosolids present in the liquid. As where SRT is the mean cell residence time (day); V is the noted, some of the settled biosolids are returned back to volume of aeration basin (e.g., L); X is the mixed liquor the aeration tank to increase the solids’ contact time with suspended solids concentration (mg/L); Q is the volumet- the wastes and also maintain the desired biomass levels ric flow rate (e.g., L/day); Xe is the effluent suspended sol- in the aeration tank. ids concentration (mg/L); Qw is the waste sludge flow rate (e.g., L/day); and Xw is the waste sludge suspended solids Important Operating Parameters in Activated concentration (mg/L). Sludge Systems Sludge Volume Index Key operating parameters and typical values for activated sludge systems are provided in Table 1. All parameters The sludge volume index (SVI) is another key parameter ultimately are used to guide and pseudo-control biosolids and used to describe the settling characteristics of the levels, and they profoundly affect process performance. sludge. The SVI is expressed as the volume occupied by The total suspended solids in the aeration tank are known 1 g of sludge (mL/g) after 30 min of settling time. Well- as mixed-liquor suspended solids (MLSS). This term re- settled sludge normally yields a clear separation between fers to the amount of solids in a certain volume of the water the water and the sludge. However, if the sludge has any (usually milligram of solids per liter). The actual biomass problems, such as bulking, pinpoint floc formation of tiny, fraction of the solids is estimated as the solids that can poorly settling floc, or ashing, the interface between the be volatilized at 550°C. The volatile fraction is known as sludge and the water may not be seen clearly. Such condi- mixed-liquor volatile suspended solids (MLVSS). There- tions usually result from problems in the aeration tank and fore, MLVSS is frequently used as a proxy for the active cause reduced effluent quality because of poor settling in biomass treating the waste. MLVSS ranges from about the clarifier. 70% to 90% of the MLSS concentration in most activated sludge systems. Dissolved Oxygen Concentration Solid Retention Time Microorganisms in an activated sludge system require ad- equate oxygen to oxidize organics in the waste. The basic The most important design parameter in activated sludge oxidation reaction for organics degradation can be approx- Wastewater Waste – systems is the mean cell residence time of cells in the reac- imated as (stoichiometry not provided) tor, also known as the sludge age or solid retention time (SRT). The SRT can be controlled by manipulating the CHON + O2 + microorganisms ® rate at which excess sludge is wasted and is influenced by CO2 + H2O + NH3 + more microorganisms hydraulic flow conditions through the reactor. It is the ratio of the total solids in the system and the total solids leaving Organics are consumed by microorganisms, and new mi- the system. crobial cells are synthesized with ratio of organisms pro- duced relative to the organics consumed being the sludge yield. As noted, oxygen is supplied by mechanical aerators or diffusers in the aeration tank. Required oxygen levels Table 1 Typical design parameters for ASP. in the system depend on the process, but the design goal Process components is to minimize oxygen addition due to energy costs. The or variables Typical values Reference dissolved oxygen concentration can be controlled by ei- Aeration tank ther adjusting the speed of the air pump or throttling the Depth (m) 5–8 air pipes. Air pumps are more widely used to aerate the Width (m) 7–12 wastewater because of their lower operational and main- SRT (day) 5–15 tenance costs. MLSS (kg/m3) 1500–4000 SVI (kg/m ) 3 40–150 Food-to-Microorganism Ratio F/M 0.2–0.4 The food-to-microorganism ratio (F/M) is a good in- Organic loading rate 20–60 dicator for designing and regulating the operation of (kg COD/m3day) the aeration tank. The F/M ratio is expressed as the Oxygen requirement 1.4–1.6 amount of organic biodegradable material [milligrams (kg/kg COD removed) of 5-day biological oxygen demand (BOD5)] available  2650 Wastewater Treatment: Biological for the amount of microorganisms present (mg MLVSS) Common Microorganisms in Activated Sludge per day. Systems F/M = (QSo)/X Activated sludge is a complex mixture of broadly differing microorganisms. Major categories are as follows: bacte- ria, fungi, algae, protozoa (e.g., flagellates, ciliates, and roti- where F/M is the food-to-microorganism ratio (day-1); So is fers), and viruses. Viruses and pathogenic bacteria are often the influent BOD5 concentration (mg/L); X is the MLVSS present in wastewater, which is the primary reason for hav- concentration (mg/L); and Q is the volumetric flow rate ing post-biological disinfection steps in treatment plants. (L/day). The targeted F/M ratio for any treatment system var- ies depending on the design of the system, and values can Attached Growth Processes range widely. However, since influent BOD cannot be controlled, MLVSS is typically modulated by varying the Attached growth processes, such as trickling filters (Fig. 3), return activated sludge rate from the secondary clarifier, can achieve similar treatment objectives as activated the goal being to maintain an optimum F/M ratio for spe- sludge systems. Conversion processes in these systems cific activated sludge design. are typically mass transport limited: microorganisms in the outer layers of the biofilm contribute most to the Organic Loading Rate overall substrate removal. The support material in trickling filters is chosen to provide sufficiently large pore spaces to The amount of organic matter in wastewater is commonly allow air through the trickling filter regardless of biofilm measured by BOD5, chemical oxygen demand (COD), or growth and water trickling down the filter. Wastewater is the total organic carbon content.[8,9] If there are excess or- distributed using rotary arms at the top and then trickles ganics in the influent or inadequate organisms in the aera- down the filter. Trickling filters are mainly used for the tion tank, incomplete treatment will result. oxidation of carbon and ammonia, but can also achieve Wastewater Waste – Fig. 3 Aerobic trickling filter. Wastewater Treatment: Biological 2651 denitrification when convection of air through the system tially accomplished in four major reaction stages involving is optimized. different microorganisms in each stage.[15,16] Stage 1: Hydrolysis—The organic waste material mainly consists of carbohydrates, proteins, and lipids. Anaerobic Wastewater Treatment Complex and large substances are broken down into sim- Processes pler compounds by the activity of the microbes and the extracellular enzymes released by these microbes. The Anaerobic treatment technologies are widely practiced in hydrolysis or solubilization is mainly done by hydrolytic different industries on the basis of their requirement and microbes such as Bacteroides, Bifidobacterium, Clos- suitability. The processes have some advantages and dis- tridium, and Lactobacillus. These organisms hydrolyze advantages in treating different wastes, and few of them complex organic molecules (cellulose, lignin, proteins, are summarized in Table 2. Under anaerobic conditions, lipids) into soluble monomers such as amino acids, glu- organic matter is degraded through the sequential and syn- cose, fatty acids, and glycerol. These hydrolysis products trophic metabolic interactions of various trophic groups of are used by the fermentative acidogenic bacteria in the prokaryotes, including fermenters, acetogens, methano- next stage.[14,17] gens, and sulfate-reducing bacteria (SRB).[12,13] Metabolic Stage 2: Acidogenesis—Fermentative acidogenic bac- interactions between these microbial groups lead to the teria convert simple organic materials such as sugars, transformation of complex organic compounds to simple amino acids, and long-chain fatty acids into short-chain compounds such as methane, carbon dioxide, hydrogen organic acids such as formic, acetic, propionic, butyric, sulfide, and ammonia. The digestion process is essen- valeric, isobutyric, isovaleric, lactic, and succinic acids; alcohols and ketones (ethanol, methanol, glycerol, and acetone); carbon dioxide; and hydrogen. Generally, ac- idogenic bacteria have high growth rates and are the most abundant bacteria in any anaerobic digester. The high Table 2 Advantages and disadvantages of anaerobic activity of these organisms implies that acidogenesis is wastewater treatment. never the rate-limiting step in the anaerobic digestion Advantages process. The volatile acids produced in this stage are High efficiency: Good removal efficiency can be achieved in the further processed by microorganisms characteristic for system, even at high loading rates and low temperature. the acetogenesis stage. Simplicity: The construction and operation of these reactors are Stage 3: Acetogenesis—In this stage, acetogenic bacte- ria, also known as obligate hydrogen-producing acetogens, Wastewater relatively simple. Waste – Flexibility: Anaerobic treatment can easily be applied on either convert organic acids and alcohols into acetate, hydrogen, a very large or a very small scale. and carbon dioxide, which are subsequently used by meth- anogens and SRB. There is a strong symbiotic relationship Low energy consumption: As far as no heating of the influent is needed to reach the working temperature and all plant between acetogenic bacteria and methanogens. Metha- operations can be done by gravity, the energy consumption of nogens and SRB use hydrogen, which helps achieve the the reactor is almost negligible. low hydrogen pressure conditions required for acetogenic Energy recovery: Energy is produced during the process in the ­conversions. form of methane. Stage 4: Methanogenesis—It is the final stage of an- aerobic digestion where methanogenic archaea convert the Low sludge production: Sludge production is low, well stabilized, and has good dewatering property. acetate, methanol, methylamines, formate, and hydrogen produced in the earlier stages into methane. The growth Low nutrient and chemical requirement: Especially in the case rate of methanogens is very low, and therefore, in most of sewage, an adequate and stable pH can be maintained without addition of chemicals. cases, this step is considered as the rate-limiting step of the anaerobic process, although there are also examples where Disadvantages hydrolysis is rate limiting. Low pathogen and nutrient removal: Pathogens and nutrients are partially removed and hence post-treatment is needed. UASB Reactors Long start-up: Due to low growth rate of methanogenic organisms, the start-up takes longer time. The most common and widely used anaerobic reactor is Possible bad odor: Hydrogen sulfide is produced. Proper the UASB reactor. It is an attached, self-immobilized handling of biogas is required to avoid bad smell. cell system, which consists of a bottom layer of packed Necessity of post-treatment: Post-treatment of the anaerobic sludge bed (sludge blanket) and an upper liquid layer, as effluent is generally required to reach the discharge standards shown in Fig. 4. for organic matter and pathogen. Wastewater flows upward through a sludge bed con- Source: Data from Seghezzo et al. sisting of bacterial aggregates floating blanket, and the  2652 Wastewater Treatment: Biological Important Operating Parameters in Anaerobic Reactors Different operating parameters such as pH, temperature, HRT, and nutrients, among others, and their disturbances can manifest in case of industrial wastewaters treatment in anaerobic reactors, even under normal operational condi- tions.[26,27] Some of these factors are discussed below. pH The optimum degradation is achieved when the pH value of wastewater in the digester is maintained between 6.5 and 7.5. In the initial period of fermentation, as large quantities of organic acids are produced by acidogens and acetogens, a drop in pH occurs inside the digester. This low pH condi- tion inhibits methanogens and subsequently reduces meth- ane production. As the digestion proceed, the pH increases owing to the conversion of organic nitrogen to NH4. When the methane production level is stabilized, the pH range remains buffered between 7.2 and 7.8.[28,29] Waste Composition To attain optimum degradation, wastewaters have to be nutritionally balanced in terms of carbon (C), nitrogen (N), phosphorous (P), and sulfur (S). The C/N/P ratio of 700:5:1 is recommended for efficient anaerobic diges- Fig. 4 Upflow anaerobic sludge blanket reactor. tion. A fairly high concentration of acetate is required to prevent SRB outcompeting methanogens for acetate and Wastewater Waste – hydrogen. microbes present in the sludge bed convert the complex Temperature organic materials to methane, carbon dioxide, and hydro- gen. The granular sludge (1–5 mm in diameter) has high Methanogens are inactive at extremely high and low tem- biomass content (MLVSS) and specific activity, and good peratures. Few psychrophilic methanogens have been settling properties. The upward flow of the liquid inside discovered, which can grow at a temperature range of 4– the reactor is obtained by means of effluent recirculation. 6°C. Most of the methanogens can grow well from 25°C Because of the high density of biomass present in the self- to 65oC temperatures. The optimum temperature for the immobilized granular sludge, the reactor is able to support growth of the mesophilic methanogens is 35–37°C. a high SRT, which is diverse from the hydraulic retention When the ambient temperature goes down to 10°C, gas time (HRT) and require no support material. The major production virtually stops. Satisfactory gas production drawback of the UASB is the requirement of high HRT takes place in the mesophilic range, from 30°C to 40°C. to achieve desired biodegradation. Maintenance of high HRT demands huge reactor volume. These problems are Loading Rate overcome by using HAR where the advantages of AFBR are coupled with UASB operation by maintaining a high High organic loading rate may lead to acid accumulation upflow velocity (4–8 m/hr) inside the reactor. With and reduction of methane production. Similarly, if the plant higher upflow velocity, better mass transfer is obtained in is underfed, the gas production will also be low. the reactor, which reflects on the higher degradation with less HRT operation. The main purpose of these reactors Retention Time is to achieve better degradation of waste and increase the production of biogas (methane) in a substantially reduced- The retention time depends on the growth rate of the mi- size anaerobic reactor. crobial population and reactor configuration (attached Wastewater Treatment: Biological 2653 cell or suspended cell system), waste strength, and waste Nitrosovibrio, and Nitrosolobus. In the nitrite oxidation ­composition. stage, Nitrobacter, Nitrospira, Nitrospina, Nitrococcus, and Nitrocystis are known to be involved in the produc- Toxicity tion of nitrate.[10,43] Ammonia uptake rate varies accord- ing to reactor configuration, substrate type, and influent The presence of toxicants in the wastewater, such as oxy- ammonium concentration. Denitrification is the second gen (lethal to obligate anaerobes), ammonia, chlorinated stage of the nitrogen removal process. It is a heterotrophic hydrocarbons, aromatic hydrocarbons, heavy metals, and bioconversion process carried out by the heterotrophic de- long-chain fatty acids, among several others, may also re- nitrifiers under anoxic conditions. The oxidized nitrogen sult in occasional failures of anaerobic digesters. The compounds (NO2– and NO3–) are reduced to nitrogen gas presence of trace amount of metals (e.g., nickel, cobalt, by the denitrifiers that use nitrite and/or nitrate as termi- molybdenum) also stimulates the growth of microbes. Ex- nal electron acceptors and organic matter as carbon and cess volatile fatty acid (VFA) concentrations are reported energy source. Pseudomonas, Alcaligenes, Paracoccus, to inhibit the growth of several microbial species. The Thiobacillus, and Halobacterium are commonly found in undissociated forms of VFA can diffuse across the cell dentrification systems. membrane and dissociate intracellularly, which results in Few advanced processes, including partial nitrification, reduction in growth rate.[35,36] The 50% inhibition of ace- anaerobic ammonium oxidation (Anammox) and autotro- toclastic methanogenesis in granular sludge was observed phic nitrogen removal (Canon) are also being practiced in at a concentration of 13,000, 3,500, and 15,000 mg/L of different treatment plants according to the characteristics acetate, propionate, and butyrate, respectively. Small of the wastewater. A combined system of partial nitrifi- amounts of sulfide, a vital sulfur source, are beneficial for cation and Anammox is advantageous as no extra carbon methanogens. Acetoclastic methanogens are the most addition is needed, a negligible amount of sludge is pro- sensitive in terms of sulfide inhibition. Fifty percent in- duced, and less energy and oxygen are required compared hibition was observed at total sulfide concentrations of with the conventional two-stage process. 220–980 mg/L over the pH range 6.5–8.0. Sharon Process Granule Deterioration The Sharon (single-reactor high-activity ammonium re- Lipids present in the wastewater creates problem by form- moval over nitrite) process is used for removal of ammonia ing long-chain fatty acids during hydrolysis in the anaero- through nitrite formation.[45,46] In this process, both auto- bic reactor. Long-chain fatty acid imparts toxic effect to trophic nitrification and heterotrophic denitrification take Wastewater Waste – acetogenic and methanogenic microbes. It also becomes place in a single reactor with intermittent aeration. The adsorbed onto the sludge, inducing sludge flotation and re- denitrification in the Sharon process is achieved by add- sulting in washout. Some long-chain fatty acids also act ing methanol as a carbon source. Although the process is as surfactant at neutral pH and obstruct the floc formation not suitable for all wastewaters due to a high temperature by lowering the surface tension between water and the hy- dependency, the Sharon process is suitable for removing drophobic bacteria and promote their washout. Addition nitrogen from waste streams having high ammonia con- of polyelectrolytes (calcium salts) may prevent inhibition centrations (>0.5 g/L). to some extent, but it does not prevent flotation. Anaerobic Ammonium Oxidation Biological Removal of Nitrogen Anaerobic ammonium oxidation (Anammox) is a highly exergonic, lithoautotrophic biological conversion process The conventional biological nitrogen removal is a two- where ammonia becomes converted to nitrogen by the step process, nitrification followed by denitrification. The activity of a group of planctomycete bacteria. These process is slow due to low microbial activity and yield. microorganisms use CO2 as the sole carbon source and Nitrification involves a chemolithoautotrophic oxidation have a capability to oxidize ammonia to gaseous nitro- of ammonia to nitrate under strict aerobic conditions. This gen by using nitrite as the electron acceptor in an anoxic oxidation is a result of two sequential oxidative stages: am- condition. monia to nitrite (ammonia oxidation) and nitrite to nitrate (nitrite oxidation). Different microorganisms involved in Combined Nitrogen Removal these stages use molecular oxygen as an electron accep- tor and carbon dioxide as carbon source. The oxidation of Ammonia-rich wastewater can be treated by Anammox, ammonia to nitrite is performed by nitrifier microorgan- which requires nitrite as precursor. Thus, before feeding isms such as Nitrosomonas, Nitrosococcus, Nitrosopira, into the Anammox process, ammonia has to be preoxidized  2654 Wastewater Treatment: Biological to nitrite. Thus, a partial Sharon process can be used before tivated sludges. It can accumulate phosphate of an amount the Anammox process to improve the nitrogen removal ef- of 0.9%–1.9% of dry cell weight. ficiency. Partial nitritation (conversion of 55%–60% of ammonium to nitrite) is achieved in the Sharon process without heterotrophic denitrification. Nitrite-rich waste is Conclusion then treated in an Anammox reactor. In the partial Sha- ron–Anammox digester, overall 83% ammoniacal nitrogen Biological treatment processes have a proven track record removal can be obtained from the waste stream has a total of dealing adequately with various kinds of wastes gener- nitrogen load of 0.8 kg N/m3/day. ated by human activities. They mimic natural processes occurring in streams and rivers. Waste treatment processes are increasingly engineered in such a way that they per- Canon Process form this task efficiently with a minimal input of energy. Traditionally, treatment has relied on technological ap- The Canon (completely autotrophic nitrogen removal over proaches designed to mimic aerobic processes occurring nitrite) process is also the combination of partial nitritation in the water column of streams and rivers. To become and Anammox processes. In this process, two groups of truly sustainable, however, we must move away from aerobic and anaerobic microorganisms (e.g., Nitrosomo- energy-consuming aerobic processes and switch to an- nas and planctomycetes) perform two sequential reactions aerobic treatment processes, again mimicking natural pro- in a single and aerated reactor. The nitrifiers consume cesses, but now those occurring in the anaerobic sediments oxygen and oxidize ammonia to nitrite. Consumption of of the aforementioned streams and rivers. For example, oxygen creates an anoxic condition the Anammox process there is a new focus in the water industry to integrate these needed. The performance of the Canon process is very two processes into systems where the waste is initially di- much dependent on operational parameters such as dis- gested in an anaerobic step followed by an aerobic pol- solved oxygen, biofilm thickness, nitrogen-surface load, ishing step. Only by integrating these two processes, and and temperature. variants thereof such as partial nitrification and Anammox wastewater treatment, will waste treatment become truly energy efficient and sustainable. Finally, it should be noted Biological Phosphorus Removal that anaerobic digestion to methane is not the only sustain- able option. Great strides are now being made in microbial The removal of phosphorus from the wastewater by the fuel cell technology within waste treatment with chemi- biological means is known as biological removal of phos- cal energy from wastes being captured as electricity. All Wastewater Waste – phorus. The groups of microorganisms that are largely told, we are finally beginning to see again that wastes are responsible for phosphorus removal are known as the not problems to be solved but are valuable resources, and polyphosphate-accumulating organisms (PAOs). These new technologies continue to be developed to capture this organisms are able to store phosphate as intracellular poly- capacity. phosphate, leading to phosphorus removal from the bulk liquid phase through PAO cell removal in the waste ac- References tivated sludge. Enhanced biological phosphorus removal can be achieved through the ASP by recirculating sludge 1. Van de Kraats, J. Editorial. Eur. Water Pollut. 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