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Wastewater Technology Fact Sheet: Anaerobic Lagoons

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Wastewater Technology Fact Sheet: Anaerobic Lagoons United States Environmental Protection Agency Wastewater Technology Fact Sheet Anaerobic Lagoons DESCRIPTION are not applicable to many situations because of large land requirements, sensitivity to environmental An anaerobic lagoon is a deep impoundment, conditions, and objectionable odors. Furthermore, the essentially free of dissolved oxygen, that promotes anaerobic process may require long retention times, anaerobic conditions. The process typically takes especially in cold climates, as anaerobic bacteria are place in deep earthen basins, and such ponds are used uneffective below 15° C. As a result, anaerobic as anaerobic pretreatment systems. lagoons are not widely used for municipal wastewater treatment in northern parts of the United States. Anaerobic lagoons are not aerated, heated, or mixed. The typical depth of an aerated lagoon is greater than Process eight feet, with greater depths preferred. Such depths minimize the effects of oxygen diffusion from the An anaerobic lagoon is a deep earthen basin with surface, allowing anaerobic conditions to prevail. In sufficient volume to permit sedimentation of settlable this respect, anaerobic lagoons are different from solids, to digest retained sludge, and to anaerobically shallower aerobic or facultative lagoons, making the reduce some of the soluble organic substrate. Raw process analogous to that experienced with a single- wastewater enters near the bottom of the pond and stage unheated anaerobic digester, except that mixes with the active microbial mass in the sludge anaerobic lagoons are in an an open earthen basin. blanket. Anaerobic conditions prevail except for a Moreover, conventional digesters are typically used shallow surface layer in which excess undigested for sludge stabilization in a treatment process, whereas grease and scum are concentrated. Sometimes lagoons typically are used to pretreat raw wastewater. aeration is provided at the surface to control odors. Pretreatment includes separation of settlable solids, An impervious crust that retains heat and odors will digestion of solids, and treatment of the liquid portion. develop if surface aeration is not provided. The discharge is located near the side opposite of the Anaerobic lagoons are typically used for two major influent. The effluent is not suitable for discharge to purposes: receiving waters. Anaerobic lagoons are followed by aerobic or facultative lagoons to provide required 1) Pretreatment of high strength industrial wastewaters. treatment. 2) Pretreatment of municipal wastewater to allow The anaerobic lagoon is usually preceded by a bar preliminary sedimentation of suspended solids as a screen and can have a Parshall flume with a flow pretreatment process. recorder to determine the inflow to the lagoon. A cover can be provided to trap and collect the methane Anaerobic lagoons have been especially effective for gas produced in the process for use elsewhere, but this pretreatment of high strength organic wastewaters. is not a common practice. Applications include industrial wastewaters and rural communities that have a significant organic load from Microbiology industrial sources. Biochemical oxygen demand (BOD) removals up to 60 percent are possible. The Anaerobic microorganisms in the absence of dissolved effluent cannot be discharged due to the high level of oxygen convert organic materials into stable products anaerobic byproducts remaining. Anaerobic lagoons such as carbon dioxide and methane. The degradation process involves two separate but interrelated phases: bacteria can be adversely affected by small fluctuations acid formation and methane production. in pH substrate concentrations, and temperature, but the performance of acid-forming bacteria is more During the acid phase, bacteria convert complex tolerant over a wide range of conditions. When the organic compounds (carbohydrates, fats, and proteins) process is stressed by shock loads or temperature to simple organic compounds, mainly short-chain fluctuations, methane bacteria activity occurs more volatile organic acids (acetic, propionic, and lactic slowly than the acid formers and an imbalance occurs. acids). The anaerobic bacteria involved in this phase Intermediate volatile organic acids accumulate and the are called “acid formers,” and are classified as pH drops. As a result, the methanogens are further nonmethanogenic microorganisms. During this phase, inhibited and the process eventually fails without little chemical oxygen demand (COD) or biological corrective action. For this reason, the methane- oxygen demand (BOD) reduction occurs, because the formation phase is the rate-limiting step and must not short-chain fatty acids, alcohols, etc., can be used by be inhibited. For the design of an anaerobic lagoon to many microorganisms, and thereby exert an oxygen work, it must be based on the limiting characteristics of demand. these microorganisms. The methane-production phase involves an Establishing and maintaining equilibrium intermediate step. First, bacteria convert the short- chain organic acids to acetate, hydrogen gas, and The system must operate at conditions favorable for the carbon dioxide. This intermediate process is referred performance of methanogenic bacteria. Ideally, to as acetogenesis. Subsequently, several species of temperatures should be maintained within the range of strictly anaerobic bacteria (methanogenic 25 to 40 C. Anaerobic activity decreases rapidly at microorganisms) called “methane formers” convert the temperatures below 15 C , when water temperature acetate, hydrogen, and carbon dioxide into methane drops below freezing, and biological activity virtually gas (CH4) through one of two major pathways. This ceases. The pH value should range from 6.6 to 7.6, process is referred to as methanogenesis. During this but should not drop below 6.2 because methane phase, waste stabilization occurs, represented by the bacteria cannot function below this level. Sudden formation of methane gas. The two major pathways of fluctuations of pH will inhibit lagoon performance. methane formation are: Alkalinity should range from 1,000 to 5,000 mg/L. 1) The breakdown of acetic acid to form methane and Volatile acid concentration is an indicator of process carbon dioxide: performance because the acids are converted to methane at the same rate that they are formed if CH3COOH CH4 + CO2 equilibrium is maintained. Volatile acid concentrations will be low if the lagoon system is working properly. 2) The reduction of carbon dioxide by hydrogen gas As a general rule, volatile acid concentrations should to form methane: be less than 250 mg/L. Inhibition occurs at volatile acid concentrations in excess of 2,000 mg/L. Table 1 CO2 + 4H2 CH4 + 2 H20 presents optimum and extreme operating ranges for methane formation. The rate of methane formation Equilibrium drops dramatically outside these extreme ranges. In addition to adhering to the above guidelines, sufficient When the system is working properly, the two phases nutrients such as nitrogen and phosphorus must be of degradation occur simultaneously in dynamic available. Concentrations of inhibitory substances, equilibrium. That is, the volatile organic acids are including ammonia and hydrogen when a high converted to methane at the same rate that they are concentration of sulfate ions are present, and formed from the more complex organic molecules. The growth rate and metabolism of the methanogenic TABLE 1 IDEAL OPERATING RANGES Typically, anaerobic lagoons are used in series with FOR METHANE FERMENTATION aerobic or facultative lagoons, enhancing the operation Parameter Optimum Extreme of both types of lagoons as aerobic or facultative Temp. ( C) 30-35 25-40 lagoons providing further treatment of the effluent. Initial treatment in an anaerobic lagoon often renders pH 6.6-7.6 6.2-8.0 the waste more amenable to further treatment and Alkalinity (mg/L as 2,000-3,000 1,000-5,000 reduces the oxygen demand. CaCO3) Volatile acids (mg/L 50-500 2,000 as acetic acid) Anaerobic lagoons often are used in small or rural Source: Andrews and Graef, 1970. communities where space is plentiful but costs are a concern. Low construction and operating costs make concentrations such as calcium, should be kept to a anaerobic lagoons a financially attractive alternative to minimum. Excessive concentrations of these inhibitors other treatment systems, although sludge must produce toxic effects. Depending on its form, ammonia occasionally be removed. can be toxic to the bacteria as well as affect its concentration. Concentration of free ammonia in ADVANTAGES AND DISADVANTAGES excess of 1,540 mg/L will result in severe toxicity, but concentrations of ammonium ion must be greater than Some advantages and disadvantages of anaerobic 3,000 mg/L to produce the same effect. Maintaining a lagoons are listed below: pH of 7.2 or below will ensure that most ammonia will be in the form of ammonium ion, so higher Advantages concentrations can be tolerated with little effect. Table 2 provides guidelines for acceptable ranges of other More effective for rapid stabilization of strong inhibitory substances. organic wastes, making higher influent organic loading possible. TABLE 2 CONCENTRATIONS OF INHIBITORY SUBSTANCES Produce methane, which can be used to heat Substance Moderately Strongly buildings, run engines, or generate electricity, Inhibitory (mg/L) Inhibitory mg/L) but methane collection increases operational Sodium 3,500-5,500 8,000 problems. Potassium 2,500-4,500 12,000 Calcium 2,500-4,500 8,000 Produce less biomass
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