Process Description

Anaerobic processes have been used in wastewater treatment systems for more than a century, initially to stabilize the solids produced.4''"" These bioreactors, called anaerobic digesters, were simple concrete tanks in which the solids were placed as a slurry and allowed to decompose anaerobically. Hydraulic retention times of 60 days or more were common. Gradually, it was discovered that the decomposition could be accelerated by heating the digester to a consistent temperature of about 35°C and mixing it to provide uniform reaction conditions. These discoveries led to the current high rate anaerobic digestion process, which uses HRTs of 15 to 20 days. Anaerobic digestion remains an extremely popular and widely used solids stabilization process, particularly in municipal wastewater treatment.4'4h 72"

Development of high rate anaerobic digestion fostered interest in the use of anaerobic processes to treat high strength industrial wastewaters, leading to the development and use of a wide variety of innovative systems/'Some can be classified as either suspended growth or attached growth systems, but many are hybrid systems, incorporating elements of both. All anaerobic processes, regardless of the type of biomass employed, are described in this chapter because of the similarities of the design approaches employed. Additional details on attached growth systems are provided in Chapters 18 and 21.

The purposeful use of anaerobic digestion to inactivate pathogens in municipal wastewater solids is a relatively new and evolving application.4'70 Just as in aerobic digestion, pathogen inactivation does not occur as a direct consequence of the di-

gestion process per se; rather it is a result of the environmental conditions in the digester. Pathogen inactivation in anaerobic digesters is relatively efficient because of the elevated temperatures that arc typically maintained.

As mentioned above, anaerobic processes are beginning to be used to hydrolyze and ferment a portion of the biodegradable organic matter in wastewater solids, producing VFAs. '' The VFAs are then removed from the solids by elutriation and used to enhance BNR processes, as discussed in Chapter ll. The solids are then concentrated prior to subsequent treatment.

13.1.1 General Description

A general description of the microbiology and biochemistry of anaerobic processes is presented in Chapters 2 and 3, while the kinetics of the transformations are summarized in Section 9.3.2. Although the chemistry, biochemistry, and microbiology of anaerobic decomposition are quite complex, it can be conceptualized as comprising three steps, as summarized in Figure 2.3: (1) hydrolysis of particulate organic matter to soluble substrates; (2) fermentation of those soluble substrates to produce acetic acid, carbon dioxide, and H.; and (3) conversion of the acetic acid, the H:, and a portion of the carbon dioxide to methane.4*1,2 Methane is a sparingly soluble gas, which is evolved from solution and collected for subsequent use. The evolution of methane decreases the chemical oxygen demand (COD) of the waste stream and provides the mechanism for stabilization of the biodegradable organic matter contained in it. Only minimal COD reduction occurs without methane production, and it is associated with the formation and evolution of FF. As discussed in Sections 2.3.2 and 9.3.2, the H:-oxidizing methanogens are fast growing organisms and are present in most anaerobic treatment systems, resulting in conversion of most of the H: produced to methane/* "2 "'' However, since the greatest proportion of the methane produced comes from acetic acid, growth of aceticlastic methanogens is required to achieve significant waste stabilization.

Since COD stabilization in anaerobic processes is directly related to methane evolution, methane production can be calculated from the COD removed in the process, just as the oxygen requirement in an aerobic system can be calculated from a COD balance. As discussed in Section 2.3.2, two moles of oxygen are required to oxidize one mole of methane to carbon dioxide and water. Thus, the COD equivalent of methane is 4 kg COD/kg methane. At standard temperature and pressure (0°C and one atmosphere) this corresponds to 0.35 m* of methane produced per kg of COD converted to methane.4fos For municipal primary solids, the methane equivalent is 0.7 m' of methane produced per kg of volatile solids (VS) destroyed.5* The carbon dioxide content of the gas produced in anaerobic processes ranges between about 30 and 50% and varies depending on the nature of the substrate. For example, the carbon dioxide content is higher when carbohydrates are being treated than when proteins are treated.5"

Lema, et al.4" have summarized those aspects of anaerobic processes that particularly affect their design. They are:

• The very low growth rates that the microorganisms have during methane fermentation.

• The low microbial specific activity, especially at the final step of the process.

• The very low values of the half-saturation coefficients, which means an extraordinary affinity of the microorganisms for Iheir substrates.

• The importance of internal and external resistances to mass transfer.

• The inhibition produced by chemicals present in the wastewater or produced in the process.

• The necessity of keeping the physico-chemical parameters within relatively limited ranges to maximize the activity of the microorganisms.

• The need to design and operate a system that can handle fluctuations in wastewater flow and composition.

These challenges are addressed in the design of anaerobic bioreactors by providing a uniform reactor environment and an SRT that is sufficiently long to ensure the growth of all the necessary microorganisms. The mechanisms by which these objectives are achieved are discussed below.

Figure 13.1 provides a schematic of an anaerobic bioreactor that illustrates its four major components: (1) a closed vessel, (2) a mixing system, (3) a heating system, and (4) a gas-liquid-solids separation system. Table 13.1 relates those components to the aspects identified by Lema et al.411 Anaerobic bioreactors are typically constructed of either concrete or steel, although earthen basins are used for some low-rate processes. An enclosed vessel is used to exclude dissolved oxygen and ensure the development of anaerobic conditions. The bioreactor is often insulated to minimize heat loss. Mixing is provided to increase the homogeneity of the reaction environment and to reduce the resistance to mass transfer. Uniform bioreactor conditions minimize the impacts of the inhibitory materials produced as metabolic intermediates, keep bioreactor physico-chemical parameters within limited ranges, and minimize the impacts of influent flow and composition fluctuations. Due to the high affinity of the reactions for their substrates, performance is not severely impacted by the uniform bioreactor environment. Several methods are used to mix the bioreactor, including devices such as gas recirculation or mechanical mixers, recirculation of bioreactor effluent to the influent, or bioreactor configurations that use the influent and recirculation flows to mix the contents. Gas evolution during treatment results in a degree of mixing that can be significant in certain bioreactor configurations. The

Figure 13.1 Anaerobic bioreactor.

Table 13.1 Relationships Between the Components of Anaerobic Bioreactors and the Aspects of Anaerobic Reactions that Affect Process Design

Anaerobic reactor component

Anaerobic reaction Gas-liquid-solids aspect of Lema et al.40 Closed vessel Mixing system Heating system separation system

1. The very low specific growth rates.

2. The low specific activities.

3. The very low values of the half-saturation coefficients.

4. The importance of internal and external resistances to mass transfer.

Provides optimum reaction conditions by excluding dissolved oxygen.

Provides optimum reaction conditions by excluding dissolved oxygen.

Provides optimum reaction conditions by excluding dissolved oxygen

Provides intimate contact between microorganisms and their substrates to maximize achievable specific growth rates.

Provides intimate contact between microorganisms and their substrates to maximize acheivable specific growth rate.

High reaction conversion efficiency possible even though well mixed conditions are typically utilized.

Mixing helps to overcome the adverse impacts of internal and external mass transfer resistance.

Allows maximum microorganism specific growth rate by maintaining temperature for optimal growth.

Allows maximum microorganism activity by optimizing temperature.

Allows optimal microorganism activity even though rate is reduced by mass transfer resistance.

Accumulation of active microorganisms allows operation at increased SRT.

Accumulation of active microorganisms allows high total activity, in spite of low specific activity.

Accumulation of active microorganisms and increased SRT can increase total reaction rate and compensate for effects of mass transfer resistances.

5. The inhibition produced by chemicals.

Excludes one reaction inhibitor, oxygen.

Minimizes buildup of reaction intermediates by providing uniform reactor environment.

6. The necessity of keeping the physico-chemical parameters within relatively limited ranges.

7. The need to design and operate a system that can handle fluctuations in wastewater flow and composition.

Excludes one reaction inhibitor, oxygen.

Excluding oxygen allows optimal reaction rates in spite of fluctuating loading conditions.

Minimizes variations in reactor environment.

Reduces the variation in reactor environmental conditions in spite of variations caused by fluctuating loading conditions.

Minimizes production of reaction intermediates by maintaining temperature for maximum biological activity.

Reduces variation in one environmental factor, temperature.

Allows optimal reactor temperature to be maintained in spite of fluctuating loading conditions.

Accumulation of active microorganisms allows increased SRT to be maintained, thereby limiting accumulation of reaction intermediates.

Accumulation of active microorganisms increases reaction rates in spite of adverse environmental conditions.

Accumulation of active microorganisms provides increased reactor biomass needed to treat peak process loadings.

configuration of the feed distribution system can also encourage mixing. Heating is typically provided to maintain temperatures that are constant and near the optimum values for the biomass. Methane gas produced by the system is generally used to fire boilers that provide the necessary heat.

Relatively long SRTs are required in anaerobic processes because of the low maximum specific growth rates of methanogens. Long SRTs also minimize the buildup of inhibitory reaction intermediates and allow the process to respond better to fluctuations in wastewater flow and composition. In some instances, the necessary SRT is achieved by providing a sufficiently long HRT.4,os In other cases, the necessary SRT is provided by separating solids from the treated effluent and retaining them in the bioreactor, thereby achieving an SRT that is significantly longer than the HRT."';,"S The gas-liquid-solids separation device is critical to the performance of such systems because the efficiency of liquid-solids separation determines the extent to which active biomass can be accumulated. Gas separation from the solids is necessary to facilitate liquid-solids separation. Several approaches are used to retain active biomass in anaerobic treatment systems; they are described in Section 13.1.4.

A wide range of bioreactor configurations exists, depending on the type of waste, the type of gas-liquid-solids separation provided, and the treatment objectives. Four different process types are considered: (1) anaerobic digesters, (2) low-rate anaerobic processes, (3) high-rate anaerobic processes, and (4) solids fermentation processes. The first three are used to stabilize organic matter by converting it to methane and carbon dioxide. Solids fermentation processes are used to produce VFAs to enhance the performance of BNR systems.

13.1.2 Anaerobic Digestion

Anaerobic digestion (AD) is used for the stabilization of particulate organic matter and Figure 13.2 provides a schematic of the process. An anaerobic digester is well mixed with no liquid-solids separation.467" Consequently, the bioreactor can be treated as a continuous stirred tank reactor (CSTR) in which the HRT and SRT are identical. An SRT of 15 to 20 days is typically used, although SRTs as low as 10 days have been used successfully and longer SRTs are employed when greater waste

Figure 13.2 Anaerobic digestion.

stabilization is required."'72 5 Many anaerobic digesters are cylindrical concrete tanks with a cone-shaped bottom and steel or concrete covers, although other materials and configurations can be used. Diameters range from 10 to 40 m, and sidewall depths from 5 to 10 m. Mixing is required and is provided by internal mechanical mixers, external mechanical mixers that recirculate the tank contents, gas recirculation systems of various types, or pumped recirculation of the tank contents. Historically, relatively low volumetric power inputs have been used to mix anaerobic digesters. More recent experience suggests, however, that such practices may cause a significant portion of the bioreactor volume to be inactive, as well as in significant short-circuiting of feed to the effluent." In contrast, tracer testing has demonstrated that newer approaches can produce essentially completely mixed conditions, thereby minimizing inactive volume and short-circuiting.

Methane produced by the process is combusted and used to heat the feed stream and digester contents. Bioreactor temperatures in the mesophilic range (~35°C) are typically maintained,4"'* 2 7" although numerous investigations of the use of thermophilic operating temperatures (~55°C) have been conducted.Gas storage is typically provided to accommodate variations in gas production rates, thereby facilitating the operation of boilers and other equipment using the gas as a fuel source. External pressurized storage is sometimes used, but more frequently gas is stored in the digester under a cover that floats on the digester contents, as illustrated in Figure 13.3.J"S272'7'

Historically, anaerobic digesters treating municipal wastewater solids have experienced operating problems associated with the accumulation of grit in the bottom and floating scum on the surface.4"72 75 Consequently, bioreactor configurations have been developed that have improved mixing characteristics and reduced potential for grit and scum accumulation. One is the egg-shaped digester, illustrated in Figure 13.4.4"75 Developed in Germany, it is receiving increasing interest in the United States, where several full-scale installations currently exist. The large height-to-diameter ratio and the steeply sloped lower and upper sections of the vessel result in improved mixing, reduced grit and scum accumulation, and easier removal of any

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