Environmental factors

Temperature

Temperature is a key variable in biological treatment. Anaerobic digesters are generally operated in one of two temperature ranges: mesophilic (3040 °C) or thermophilic (50-65 °C). Most anaerobic reactors are operated in the mesophilic ranges. Methane has been produced at temperatures of 10 °C or lower, but for reasonable rates of methane production, temperatures should be maintained above 20 °C. The rate of methane production approximately doubles for each 10 °C temperature change in the mesophilic range (Stronach et al., 1986). In municipal wastewater plants, anaerobic treatment is usually carried out near the mesophilic range (25 °C to 40 °C) with an optimum at approximately 35 °C (Parkin and Owen, 1986).

There are some advantages of using thermophilic digestion including higher rates of degradation and therefore smaller digester size with less capital cost, faster solid-liquid separation and better control of bacterial and viral pathogens (Mackie and Bryant, 1995). In thermophilic temperature ranges, reaction rates proceed faster than under mesophilic conditions, so that the loading potentials of anaerobic bioreactors are significantly higher. However, the fact that thermophilic wastewater treatment is hardly ever applied can probably be attributed to the conflicting and sometimes disappointing results. In comparison to mesophilic operational systems, thermophilic reactors seem to be less stable. Another disadvantage is the energy required to heat the influent to reactor temperature (Parkin and Owen, 1986; van Lier et al, 1996).

Psychrophilic digestion operates at temperature of 15-20 °C. There have been studies of anaerobic processes at psychrophilic temperatures aimed at decreasing or eliminating expenses required for anaerobic digester heating systems. Safley and Westermann (1990) suggest that reasonable methane yields are possible for anaerobic digestion at low temperatures if organic loading rates are appropriately reduced. In addition, Dague et al. (1998) report that an anaerobic sequencing batch reactor (ASBR) performs well while treating low-strength wastewater at 5-20 °C.

In practice, a stable and uniform temperature is imperative for consistent and efficient reactor operation that results in the best treatment. Temperature fluctuation has a net adverse effect on digester performance and contributes to instability of anaerobic treatment (Droste, 1997).

pH and alkalinity

The pH is perhaps the most important anaerobic process control parameter. A favorable pH range for methanogenic bacteria is between 6 and 8 with an optimum pH for the group as a whole near 7.0. Many studies report that the pH required in anaerobic systems for good performance and stability is in the range of 6.5-7.5, although stable operation has been observed outside this range. Clark and Speece (1971) reported effects of adverse pH levels of 3.8-9.4. They found that steady methane production occurs at pH levels as low as 4. However, the rates were lower than for the same reactor operating at more optimal pH. No inhibition of methane production was observed between pH 6 and 8, and temporary pH shock did not have long-lasting effects.

The anaerobic process may eventually fail if the pH gets as low as around 6.0 and remains there for some time. Acidogenic bacteria produce organic acid, which tends to lower the pH of the anaerobic reactor. Under normal conditions, this pH reduction is buffered by the bicarbonate produced by methanogens. Under adverse environmental conditions, the buffering capacity of the system can be upset, eventually stopping the production of methane. An increase in volatile acids thus serves as an early indicator of system upset. Therefore, excess alkalinity or the ability to control pH must be present to guard against the accumulation of excess volatile acids. Anaerobic processes can operate over a wide range of volatile acid concentrations if proper control is maintained. A constant pH lends stability to the process. Automatic pH control is considered more economical than adding pH chemicals in a random manner because fewer chemicals are consumed. Common materials to increase the alkalinity are lime, soda ash, ammonia, ammonium bicarbonate, sodium hydroxide or sodium bicarbonate. Generally, lime, sodium hydroxide and ammonia are the least expensive of these chemicals (Parkin and Owen, 1986; Anderson and Yang, 1992).

Nutrient requirements

The low growth yields of anaerobes from a given amount of substrate result in lower nutrient requirements compared with aerobes. The nutrients required in highest concentration are nitrogen (N) and phosphorous (P). A common empirical formula of bacterial composition is C5H7O2N. Using that formula, nitrogen comprises approximately 12% of bacterial cells. Nitrogen is used in the synthesis of protein, enzymes, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Phosphorous is required to synthesize energy-storage compounds, RNA and DNA (Parkin and Owen, 1986). For a typical activated sludge process, the COD : N : P requirement is approximately 100 : 5 : 1 on a mass basis. The theoretical minimum COD : N : P ratio of an anaerobic system is 350 : 7 : 1 for a highly loaded system, whereas for a lightly loaded system it is 1000 : 7 : 1 due to the reduced net synthesis of biomass. In addition to the nitrogen and phosphorous required for anaerobic microbial systems, some sulfide precursor may be needed. Anaerobic systems have significantly higher sulfur content in the biomass than aerobic cells. Therefore, the empirical cell formulation of anaerobic cells can be considered as C5H7O2NP0.06S0.1 (Speece, 1996).

According to Zehnder et al. (1980), a substrate sulfur content of approximately 0.001-1.0 mg/L is required for optimal growth and methane production.

There are a number of trace elements required for successful anaerobic digestion: nickel and cobalt have been shown to promote methanogenesis. For typical wastes, these elements will normally be present in excess amounts (Rittmann and McCarty, 2001).

Toxicity

Toxicants, components in the wastewater causing adverse effects on bacterial metabolism, are responsible for the occasional failure of anaerobic digesters. Inhibition of methanogenesis is generally indicated by reduced methane production and increased concentration of volatile acids. Most toxic materials are stimulatory at low concentrations, but become inhibitory as the concentrations increase. From a control standpoint, toxic materials need to somehow be reduced in concentration to below a toxic threshold (McCarty, 1964). The following are some toxicants that are known to cause problems in anaerobic digesters.

Ammonia-nitrogen. Ammonia-nitrogen-containing waste or its precursors are of concern because of the potential inhibitory effects of ammonia on the anaerobic digestion microbial consortia (Angelidaki et al., 1993; Poggi-Varaldo et al., 1997). Ammonia is usually formed in anaerobic processes as a result of mineralization of organic nitrogen in wastes rich in protein or urea. The excess ammonia-nitrogen in the fermentation medium could cause an inhibitory effect in three different ways. First, free ammonia, which is more toxic for anaerobic microflora than the ammonium ion, is formed during the fermentation process. Second, amination of a-ketoglu-taric acid by ammonia-nitrogen coupled with rapid disappearance of a-ketoglutaric acid from the metabolic pool of the tricarboxylic acid cycle could cause difficulties in the metabolism of organic compounds. Finally, buildup of ammonia-nitrogen may result in undetected accumulation of volatile fatty acids (VFAs) because ammonia will keep the pH above 7 (Krylova et al., 1997; Sterling et al., 2001).

Ammonia-nitrogen is generally inhibitory to methanogens at levels of 1500-3000 mg/L. However, ammonia inhibition can be tolerated in concentrations as high as 7000 mg/L with no significant decrease in methane production if a long acclimation time is allowed. Toxicity caused by continuous addition of ammonia decreases as solids retention time (SRT) is increased (Parkin and Owen, 1986).

Sulfide. Sulfide toxicity is a common problem with wastewaters containing high concentrations of sulfate. Sulfate is used primarily as an electron acceptor in anaerobic wastewater treatment and is converted to sulfide.

Sulfide in complex with heavy metals - such as iron, zinc or copper - is not toxic. It is the soluble form - primarily un-ionized hydrogen sulfide - that is most inhibitory. Concentrations of soluble sulfide from 50 to 100 mg/L are tolerated with little or no acclimation. Concentrations up to 200 mg/L are tolerated after some acclimation. Concentrations above 200 mg/L are quite toxic (McCarty, 1964). Theoretically, 600 mg/L of sulfate will produce 200 mg/L of sulfide. Hydrogen sulfide (H2S), one of the sulfide species formed, is a relatively insoluble gas and is partially stripped from solution through normal gas production. At a normal pH during anaerobic treatment, almost all soluble sulfide is H2S or HS- (Rittmann and McCarty, 2001). H2S is formed by bacterial sulfate reduction and the decomposition of sulfur-containing organic substrates. Acid-forming bacteria are less sensitive to H2S than methanogens. Within the latter group, hydrogen oxidizing bacteria are considered to be more sensitive than acetoclastic methanogens.

H2S is also a toxic and odorous gas even at low concentrations. It poses a health hazard and esthetic problems for workers and those who live around anaerobic systems. The H2S in anaerobically produced gases not only causes an odor nuisance, but is also corrosive and quite detrimental to the operation of combustion engines used in energy recovery. Furthermore, H2S is oxidized to sulfur dioxide during combustion, creating air pollution (Arogo et al, 2000; Rittmann and McCarty, 2001).

Sulfide production in anaerobic systems has some benefits. Sulfide serves as an essential nutrient for biological growth, and a certain amount (50100 mg/L) is actually desirable because it helps remove heavy metals from solution in the digester. It also helps maintain a low oxidation-reduction potential, which is required for successful treatment operation (Speece, 1996).

Cation toxicity. Some organic wastes have relatively higher concentrations of normal alkali and alkaline earth salts and this can inhibit the anaerobic process. If one attempts to control very high volatile acid concentrations through the addition of sodium hydroxide or other sodium-containing bases, high salt concentration could readily affect the activity of microorganisms and interfere with their metabolism (Rittmann and McCarty, 2001). McCarty (1964) reports that sodium concentrations in the range of 100-200 mg/L are beneficial for the growth of mesophilic anaerobic microorganisms. For anaerobic granular biomass at mesophilic temperatures, sodium concentrations of 5, 10 and 14 g/L caused 10, 50 and 100% inhibition of methanogens, respectively, at neutral pH (Rinzema et al, 1988).

Acclimation is a factor that could affect the characteristics of sodium inhibition. Acclimation appears in anaerobic digesters treating wastewaters from mussel or sea-food processing units that contain high concentrations of sodium. Adaptation of methanogens to high concentrations of sodium

Table 23.2 Cation concentrations reported to be inhibitory to anaerobic microorganisms

Cation

Moderately inhibitory (mg/L)

Strongly inhibitory (mg/L)

Sodium

3500-5500

8 000

Potassium

2500-4500

12 000

Calcium

2500-4500

8 000

Magnesium

1000-1500

3 000

over prolonged times could increase the sodium tolerance of these microbes (Soto et al., 1993; Feijoo et al., 1995).

Another phenomenon associated with sodium toxicity is the antagonistic effect. Here, if a cation such as sodium is present in an inhibitory concentration, this inhibition might be relieved if another cation such as potassium is added. With the stimulatory concentrations of the various cations present, they help reduce the extent of inhibition caused by any of the other cations present at a moderately inhibitory concentration (Feijoo et al., 1995; Speece, 1996). Table 23.2 shows a summary of concentrations of various common cations that may cause inhibition (Parkin and Owen, 1986).

Feedback inhibition. Anaerobic treatment systems may also be inhibited by several intermediates produced during the process. High concentrations of these intermediates such as VFAs are toxic by virtue of feedback inhibition. In order to avoid some of these problems, Ghosh and Klas (1978) suggested that two-phase anaerobic digestion be used to spatially separate acidogenic bacteria from methanogenic bacteria. They report that some advantages of phase separation are enhanced stability and increased resistance to toxicants.

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