The effects of environaental fact ors on anaerobic digestion
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Environmental factors which influence biological reactions, such as pH, temperature, nutrients and inhibitors concentrations, are amenable to external control in the anaerobic digestion process.
pH: Acetate and fatty acids produced during digestion (Fig 4.12) tend to lower the pH of digester liquor. However, the ion bicarbonate equilibrium of the carbon dioxide in the digester exerts substantial resistance to pH change. This resistance, known as buffer capacity, is quantified by the amount of strong acid (or alkali) added to the solution in order to bring about a change in pH. Thus the presence of bicarbonate helps prevent adverse effects on microorganisms (methanogens) which would result from low pH caused by excessive production of fatty acids during digestion. Proteins and other organic compounds, as well as bicarbonate, take a part in the buffering capacity and the resistance to changes in pH.
Most microorganisms grow best under neutral pH conditions, since other pH values may adversely affect metabolism by altering the chemical equilibrium of enzymatic reactions, or by actually destroying the enzymes. The methanogenic group of organisms is the most pH sensitive. Low pH can cause the chain of biological reactions in digestion to cease.
Fig. 4.12: Concentrations of volatile fatty acids during digestion when the pH level was kept constant by adding CaO during the entire experiment (Marchaim and Krause 1991).
There are two main operational methods for correcting an unbalanced, low pH condition in a digester. The first approach is to stop the feed and allow the methanogenic population time to reduce the fatty acid concentration and thus raise the pH to an acceptable level of at least 6.8. Stopping the feed also slows the activity of the fermentative bacteria and thus reduces acid production. Once the pH returns to normal, feeding can be recommenced at reduced levels, and then increased gradually, so as to avoid further drops in the pH. A second method involves the addition of chemicals to raise the pH and provide additional buffer capacity. An advantage of chemical addition is that the pH can be stabilized immediately, and the unbalanced populations allowed to correct themselves more quickly. Calcium hydroxide (lime) is often used. Sodium carbonate (soda ash), while more expensive, can prevent calcium carbonate precipitation.
Temperature: The metabolic and growth rates of chemical and biochemical reactions tend to increase with temperature, within the temperature tolerances of the microorganisms. Too high a temperature, however, will cause the metabolic rate to decline, due to degradation (denaturation) of enzymes which are critical to the life of the cell. Microorganisms exhibit optimal growth and metabolic rates within a well defined range of temperatures, which is specific to each species, particularly at the upper limit which, is defined by the thermos/ability of the protein molecules synthesized by each particular type of organism.
Methanogenic bacteria are more sensitive to changes in temperature than other organisms present in digesters. This is due to the faster growth rate of the other groups, such as the acetogens, which can achieve substantial catabolism even at low temperatures (Schmid and Lipper 1969). All bacterial populations in digesters are fairly resistant to short-term temperature upsets, up to about two hours, and return rapidly to normal gas production rates when the temperature is restored. However, numerous or prolonged temperature drops can result in unbalanced populations, and lead to the low pH problems discussed above. Temperature variations can have adverse affects on mesophilic (35°C) digestion, or thermophilic (55°C) digestion. The temperature effect also depends significantly on the solids concentration of the fermentation. When high concentrations of organic loading were used (over 10%), the tolerance for changes of 5 - 10°C is much higher, and bacterial activity returns quickly when the temperature is raised again (Marchaim 1983). Two distinct temperature regions for digestion have been noted: optimal digestion occurs at about 35°C (mesophilic range) and 55°C (thermophilic range), with decreased activity at around 45°C. This response to temperature may be due to effects on methanogenic bacteria, since these appear to exhibit similar optimal regions (Fig. 4.13).
Fig. 4.13: The Effect of Temperature on Methanogens. (After Zehnder and Wuhrman 1977; Huser et al. 1982.)
Well defined mesophilic and thermophilic regions have been noted for activated "fudge and refuse feedstocks (Marina 1961; Pfeffer 1974). For beef cattle manure, raw sewage sludge, and some agricultural residues, the regions are generally the same, although not so well defined (Goluake 1958; Chen et al. 1980; Nelson et al. 1939).
An advantage of thermophilic digestion is that the rate of methane production is approximately twice that of mesophilic digestion, so reactors can be half the volume of mesophilic digesters, and still deal with the same volume of material, while maintaining the same gas production. Strong, warm, soluble industrial wastes give specific gas yields (volume of gas per volume of digester per day) of up to 8 volumes of gas per volume of digester per day with immobilized cell designs. With warm (>55°C) wastes this has obvious advantages. However, with wastes which are at ambient temperatures, such an animal manures, considerable energy is needed to raise the temperature of the waste to 55°C. A number of detailed studies of gas yields and energy consumption have been carried out (Shelef et al. 1980; Converse et al. 1977; Schellenbach 1980; Hashimoto et al. 1981).
Shelef et al. (1980) found that thermophilic digesters could accept higher organic loads than mesophilic systems at the same HRT. This advantage became more pronounced as the retention time decreased. With cattle manure at 12% total solids and HRT of 6 days, they obtained specific yields of 5.5, versus 3.0 in mesophilic digesters, and found that only 20% of the energy produced was used for heating and mixing.
However, Converse et al. (1977), using dairy manure of 15.8% total solids, found that thermophilic operation (HRT = 6.29, T = 60°C) gave lower net energy yields than mesophilic operation (HRT = 10.4, T = 35°C). Schellenbach (1980) concluded that mesophilic cultures gave a higher methane yield per pound of volatile solids than thermophilic, and that thermophilic cultures were more unstable and sensitive to mechanical or operational disruptions. This point has been raised by a number of researchers, although there is disagreement as to how unstable thermophilic digestion is. Full scale mechanically stirred thermophilic systems of 2 - 3% solids content require temperature controls of ±0.5°C, while mesophilic systems tolerate variations of ±2°C (Gerber 1954, 1975, 1977). On the other hand, when over 10% solids are fermented, a much wider range of temperature controls are needed in thermophilic digestion (Marchaim 1983). Hashimoto et al. (1981) concluded that thermophilic digestion gave a higher net energy production per unit of capital cost than mesophilic digestion. Excellent results were obtained with an influent concentration of 8 - 10% volatile solids and detention times of 4 - 5 days. Marchaim (1983) and Shelef et al. (1980, 1983), in Israel, showed that up to 16% solids concentration can be loaded to commercial systems (of 200 m3) with small effects on temperature changes.
In a number of papers, published recently, the "psychrophilic digestion" - digestion in temperatures of 10° 25°C, is reported (Wellinger 1989; Paris et al. 1988). Using the UASB- reactor, it could be demonstrated that, at temperatures as low as 10°C, digestion was successful (Grin et al 1985; Lettinga 1983; Verstraete 1986). The start-up of low-temperature digestion is one of the major constraints for the application of this technique. When longer retention time are used, with special attention to keeping volatile acids concentration low (Zeeman et al. 1988; Wellinger 1989), and using a mesophilic inoculum at temperatures higher than 20°C, the psychrophilic process can be successfully operated. This success in anaerobic digestion at low temperatures may alter the attitude of many farmers and factories to working with anaerobic digestion in cold countries. De Man et al. (1988) showed in a recent publication that anaerobic digestion systems (EGSB and UASB) can operate at temperatures as low as 8°C, when low strength soluble wastewaters were treated.
Nutrient Effects: In addition to an organic carbon energy source, anaerobic bacteria appear to have relatively simple nutrient requirements, which include nitrogen, phosphorus, magnesium, sodium, manganese, calcium, and cobalt (Speece and McCarty 1964). Nutrient levels should be at least in excess of the optimal concentrations needed by the methanogenic bacteria, since these are the most severely inhibited by slight nutrient deficiencies. Nutrient additions are often required in order to permit growth in digestion of industrial wastes and crop residues. However, nutrient deficiency should not be a problem with most manures and complex feedstocks, since these substrates usually provide more than sufficient quantities.
An essential nutrient can become toxic to organisms if its concentration in the substrate becomes too great. In the case of nitrogen, it is particularly important to maintain an optimal level to achieve good digester performance without toxic effects. The imbalance between the high nitrogen content and the carbon source cause toxicity by generating ammonia.
Influence of carbon/nitrogen ratio on digestion
Nitrogen present in the feedstock has two benefits: (a) it provides an essential element for synthesis of amino acids, proteins and nucleic acids; and (b) it is converted to ammonia which, as a strong base, neutralizes the volatile acids produced by fermentative bacteria, and thus helps maintain neutral pH conditions essential for cell growth. An overabundance of nitrogen in the substrate can lead to excessive ammonia formation, resulting in toxic effects. Thus, it is important that the proper amount of nitrogen be in the feedstock, to avoid either nutrient limitation (too little nitrogen) or ammonia toxicity (too much nitrogen). The composition of the organic matter added to a digestion system has an important role on the growth rate of the anaerobic bacteria and the production of biogas.
The components of the feedstock are utilized selectively by different bacteria present in the digester. This is especially true with different ratios of organic matter to nitrogen. Bacteria need a suitable ratio of carbon to nitrogen for their metabolic processes. C:N (carbon to total nitrogen) ratios higher than 23:1 were found to be unsuitable for optimal digestion, and ratios lower than 10:1 were found to be inhibitory, in studies on thermophilic anaerobic digestion of poultry manure, cow manure and mixtures of manures with paper or cellulosic materials (Kimchie, 1984). The experiments were performed with urea as a nitrogen source. Several reports examined the inhibitory effect: Hashimoto (1986) examined the effect in mesophilic and thermophilic conditions for over ten volume turnovers, and examined acclimation conditions and correlation to total volatile acids (VFA) concentrations. Velsen (1979) reported that nitrogen concentrations as high as 5000 mg/l can be tolerated by sewage sludge methanogens; and De Baere et al. (1984) reported initial signs of inhibition at about 8000 mg/l. Various studies have shown that free ammonia is far more toxic than the ammonium ion. Wiegant and Zeeman (1986) recently proposed a scheme for the inhibition of thermophilic methane digestion by high ammonia concentration. Ammonia acts as a strong inhibitor of the formation of methane from H2 and CO2. It has only a minor effect on the formation of methane from acetate. The inhibition of the hydrogen consumption leads to an inhibition of propionate breakdown, which acts as an inhibitor of the acetate consuming methanogens. Schwartz (1986) examined the ammonia stress on bacteria in an anaerobic sludge blanket reactor, and concluded that the high concentration of ammonia caused inhibition of anaerobic activity, but did not result in irreversible damage to the biomass in the reactor.
All the above experiments were performed with diluted cow manure (around 2.5 - 4% VS) to which NH4Cl was added. The carbon/nitrogen (C/N) ratio of the feedstock has been found to be a useful parameter in evaluating these effects, and in providing optimal nitrogen levels. A C/N ratio of 30 is often cited as optimal (Fry 1975; NAS 1977; BORDA 1980; UNEP 1981; Kimchie 1984; Marchaim 1989). Since not all of the carbon and nitrogen in the feedstock are available to be used for digestion, the actual available C/N ratio is a function of feedstock characteristics and digestion operational parameters, and overall C/N values can actually vary considerably from less than 10 to over 90, and still result in efficient digestion.
In these studies, a compound that instantly liberates ammonium (NH4+) or ammonia (NH3) was used. From their results, it was concluded that the effect of adding NH4+ on the inhibition of biogas production was instantaneous and the systems succeeded in recovering from the inhibition only in a few cases. In these experiments, inorganic nitrogen was used and in low organic loadings.
In order to examine whether the high organic loading in a thermophilic anaerobic digestion system has a vital influence on gas production in the presence of ammonia and organic nitrogen, the addition of NH4Cl to high organic loading systems, and the addition of blood in thermophilic conditions were examined. Thermophilic anaerobic digestion of rumen content from cattle, with and without the addition of nitrogen, as ammonium chloride or blood, to a C:N ratio of 14.3:1, did not show inhibition or improvement of biogas production (Marchaim 1989). When ammonium chloride was added, in addition to blood, to a C:N ratio of 10.6:1, biogas production began to decline sharply. When only blood was added (without ammonium chloride) to a C:N ratio of 11.1:1, no inhibition of biogas production occurred. After an electrical breakdown of several hours, which stopped mixing and heating, even the mixtures that contained blood without ammonium chloride showed a decline of gas production, from which the systems could recover to normal gas production. In the systems which contained NH4Cl the biogas production stopped completely.
High organic loading values, which caused higher buffer capacity and organic nitrogen levels, and avoiding a sudden high ammonia concentration, were the main explanation for not experiencing inhibition under regular conditions. The main reason for changes in gas production with time depended largely on the origin and composition of the rumen content.
In contrast to assumptions based on the literature, inhibition of the thermophilic digestion process, due to the addition of NH4+ was not found at these high concentrations. One may think that the higher organic loading used for digestion (around 10% in added material, in comparison with 2 - 6% in others' work (Kimchie 1984, Hashimoto, 1986; Wiegant and Zeeman 1986) is the main factor in balancing the ammonia effect. Since inhibition was found to be caused by NH3 and not by NH4+ (Wiegant and Zeeman 1986) the influence of buffer capacity, caused by high organic loadings, is of major importance.
Toxicity Effects: Anaerobic fermentation has a reputation of being sensitive to toxicants, and, moreover, methanogenesis is reported to be the most sensitive step. A common misconception about anaerobic digestion is, however, that the process cannot tolerate toxic substances, and that the biota die when exposed to toxicants. Due to the prolonged generation times, the recovery periods can be considerably extended if the toxicant is indeed bactericidal. However, studies on toxicity recovery of methanogenic strains indicate that some toxicants, found in agricultural and industrial wastes, exhibit a bacteriostatic or reversible effect on the methanogens, at the low concentrations normally encountered. Methanogenic bacteria were able to acclimatize to levels of many times those causing inhibition to unacclimatized methanogens (Speece and Parkin 1983; Speece 1985).
Environmental conditions such as pH, hydraulic retention time (HRT), total solids (TS) and organic loading rates (OLR) influence the sensitivity of bacteria, the response to toxicity and acclimatization characteristics (Hashimoto et al. 1980). For example, long HRT increases the potential for acclimatization and, in general, minimizes the severity of response to toxicity. Another important environmental factor involves toxicities of excessive quantities of many common, relatively non-toxic, organic or inorganic substances, which become inhibitory at high OLR values. The threshold toxic levels of inorganic substances vary, depending on whether these substances act singly or in combination. Certain combinations have a synergistic effect, whereas others display an antagonistic effect (Kungelman and McCarty 1965; Kungelman and Chin 1971).
Inorganic cations, such as Ca++ , Mg++ , Na+ , K+ , Fe++ or NH4+, which have a stimulatory effect at low or normal concentration, exhibit inhibitory effect at higher concentrations. Inorganic ions such as SO4=, NO3-, are potential inhibitors of methanogenesis in their ability to be alternative electron acceptors (Winfrey and Zeikus 1977). Sulfide (S) which is essential for most methanogens, is toxic above 200 mg/l, and made insoluble when heavy metals are present (Stafford et al. 1981; Zeikus 1977).
Toxic compounds affect digestion by slowing down the rate of metabolism at low concentrations, or by poisoning or killing the organisms at high concentrations. The methanogenic bacteria are generally the more sensitive, although all groups involved in digestion can be affected. Due to their slow growth, inhibition of the methanogens can lead to process failure in completely mixed systems, due to "washout" of the bacterial mass (i.e. the draining of bacteria through the outlet at a faster rate than their generation in the digester).
In order to control and adjust operation, to minimize toxic effects, it is important to identify inhibition in its early stages. The two main indicators of inhibition are:
a. Reduction in methane yield, indicated by two or more consecutive decreases of more than 10% in daily yield at a constant loading rate;
b. Increase in volatile acids concentration, generally occurring when the total volatile acids (expressed as acetic acid) exceed the normal range of about 250 to 500 ppm (mg/l).
The major toxicants usually encountered with natural feedstocks are ammonia, volatile acids, and heavy metals.
Ammonia: Ammonia toxicity is often a problem in feedstocks with a high protein content. Ammonia is rapidly formed in a digester, by deanimation of protein constituents. Free ammonia has been found to be much more toxic than ammonium ion, and thus ammonia toxicity thresholds are very sensitive to pH below 7.0. In general, free ammonia levels should be kept below 80 ppm, to prevent inhibition (Anderson et al. 1982). A much higher concentration, about 1,500 - 3,000 ppm ammonium ion can be tolerated (McCarty 1964a; Fischer et al. 1979; Hart 1963; Schmid and Lipper 1969). Concentrations of free ammonia and ammonium ion are related by equilibrium reactions and pH.
Volatile Acids: High concentrations of volatile acids such as acetate, propionate or butyrate, are associated with toxicity effects. It is not clear whether these acids are themselves toxic, or whether acid buildup (pH <6.8) is merely a manifestation of toxicity. Among these acids, inhibitory effects have been demonstrated only for propionate, and only at relatively high concentrations of greater than 1,000 ppm (Hobson and Shaw 1976).
Heavy Metals: Certain heavy metals are toxic to anaerobic organisms, even at low concentrations. Heavy metal ions inhibit metabolism and kill organisms by inactivating the sulfhydryl groups of their enzymes in forming mercaptides (Mosey et al. 1971). Since these reactions involve metal ions, it is the soluble fraction that is the toxic form, and toxic effects are thus affected by the solubilities of heavy metals under various digester conditions (Theis and Hayes 1979). Since many heavy metals form insoluble sulfides or hydroxides under pH conditions in the range of those found in digesters, one way to avoid heavy metal toxicity is to add chemicals such as sulphates, which will form non-toxic complexes or insoluble precipitates. Toxic substances can also be removed from the feedstock or diluted to below the toxic threshold level.
Biodegradability of digester feedstock
In general, most natural organic wastes can be digested; lignin is the major exception. In Developing Countries, the primary substrate is cattle dung, due to large cattle populations. This is a good substrate, since it is moderately degradable, and is well balanced nutritionally (C/N = 25:1).
Swine and poultry manures produce even more biogas per unit weight, and at higher rates, with lower C:N ratio and higher risk of failure of digestion operations. Human wastes (nightsoil), as well, are high in nitrogen (C/N = 6), and can also be digested. Carbohydrate wastes could be added to raise the C/N ratio and provide more gas.
Agricultural residues (e.g., wheat, rice straw) are usually readily available, but have a high C/N ratios (over 40). They can only be digested in a mixture with manures and nightsoil. These wastes are usually partially biodegradable, and can be made more so by physical size reduction, and by pre-composting. However, problems can arise with these materials because they float in the digester and form hard scum on the surface. The high lignin content of this material, which is not degradable, gives the fibrous feature to the digested slurry, used after fermentation as a soil conditioner.
Plants, such as water hyacinth, duckweed, etc., can also be degraded easily, and give quite high gas yields. In these cases, digestion of these weeds can solve the problem caused by excess weed growth in canals, while providing energy as well. Since their primary productivity is very high, the opportunity exists to create an "energy farm", by cultivating these weeds, perhaps in wastewater, which would also solve the problem of wastewater treatment. They absorb toxicants from the sewage and therefore the digested slurry obtained is limited in its uses.
Wastes generated in urban areas (garbage, organic domestic and industrial wastes) are in principle also amenable to anaerobic digestion. However, these feedstocks have not been thoroughly explored in Developing Countries.
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