The levels of methanogens were reduced, while the levels of sulfate reducing
bacteria increased. “Electrical conductivity (EC)” is an estimation of salt content
which is measured by an electrical conductivity meter. EC can be used to know
if there is an accumulation of any salt taking during anaerobic digestion process.
It is important as there are many salts which when accumulated within the
process may inhibit the process and thus may decrease the yield of the biogas.
It is also used to measure the salts present during the loading of the solid or liquid
waste so that its addition does not inhibit the process. To overcome this,
generally dilution of the input wastewater is done to keep the value of the
electrical conductivity at a minimum.
(e) Toxicity.
Compounds of sulfate and sulfur found in the reactor influence both
acetogens and methanogens. This is due to the presence of sulfate-reducing
bacteria (SRB) which can use various substrates for survival and are more
versatile. Sulfate-reducing bacteria present in the wastewaters convert sulfates,
sulfite, and thiosulfate into sulfide. The presence of sulfur compounds reduces
the methane yield. At pH 8.0, sulfide remains in the solution, and below pH 8.0,
hydrogen sulfide is seen. At pH of 7.0, about 80% of the sulfides is present as
hydrogen sulfide. Inhibitory effect of sulfides occurs when the ratio between
COD and sulfides is less than 7.7 (Speece 2008). Decreases up to 50% in biogas
yield are seen when sulfide concentrations are between 50 and 250 mg/L. This
toxicity can be overcome by (Pohland 1992) dilution of the influent, addition of
iron salts for precipitating sulfide from solution, or biological sulfide oxidation.
Ammonium at 100 mg/L was found to be toxic to the anaerobic digestion
process. Salt accumulation can lead to cell death and depends on microbial
acclimatization (Ollivier et al. 1994; Appels et al. 2008; Feijoo et al. 1995).
Chromium, iron, cobalt, zinc, and nickel have also been reported to be toxic at
relevant concentrations (Chen et al. 2008). Phenolic, chlorophenols, halogenated
benzenes, and N-substituted aromatic compounds are inhibitory to microorganisms as it interacts with cell membrane (McDonnell 2007). The addition of
excess chemical when operating the reactor leads to chemical foaming. The
other type of foam is caused due to excess production of biomass in the reactor
called biological foam which is usually brown in color. A baffle is used to
prevent scum production on the medium where the biomass is generated.
Scum is formed due to variation in temperature, mixing, light, and less than
four percent of total solids present in the reactor. Both foam and scum formation
damage the gas pipes and result in reduction of biogas yield. For regular
monitoring of the anaerobic digestion process, fatty acids and total alkalinity
are considered. Volatile fatty acids are produced which may cause a change in
the pH of the reactor and hence lead to lesser biomass production and biogas.
Acetic acid, propionic acid, butyric acid, etc. produced are generally utilized to
produce methane. However, at the same time if there levels are high, they tend to
cause a change in the pH which needs to be adjusted. This is generally done by
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 239
bacteria increased. “Electrical conductivity (EC)” is an estimation of salt content
which is measured by an electrical conductivity meter. EC can be used to know
if there is an accumulation of any salt taking during anaerobic digestion process.
It is important as there are many salts which when accumulated within the
process may inhibit the process and thus may decrease the yield of the biogas.
It is also used to measure the salts present during the loading of the solid or liquid
waste so that its addition does not inhibit the process. To overcome this,
generally dilution of the input wastewater is done to keep the value of the
electrical conductivity at a minimum.
(e) Toxicity.
Compounds of sulfate and sulfur found in the reactor influence both
acetogens and methanogens. This is due to the presence of sulfate-reducing
bacteria (SRB) which can use various substrates for survival and are more
versatile. Sulfate-reducing bacteria present in the wastewaters convert sulfates,
sulfite, and thiosulfate into sulfide. The presence of sulfur compounds reduces
the methane yield. At pH 8.0, sulfide remains in the solution, and below pH 8.0,
hydrogen sulfide is seen. At pH of 7.0, about 80% of the sulfides is present as
hydrogen sulfide. Inhibitory effect of sulfides occurs when the ratio between
COD and sulfides is less than 7.7 (Speece 2008). Decreases up to 50% in biogas
yield are seen when sulfide concentrations are between 50 and 250 mg/L. This
toxicity can be overcome by (Pohland 1992) dilution of the influent, addition of
iron salts for precipitating sulfide from solution, or biological sulfide oxidation.
Ammonium at 100 mg/L was found to be toxic to the anaerobic digestion
process. Salt accumulation can lead to cell death and depends on microbial
acclimatization (Ollivier et al. 1994; Appels et al. 2008; Feijoo et al. 1995).
Chromium, iron, cobalt, zinc, and nickel have also been reported to be toxic at
relevant concentrations (Chen et al. 2008). Phenolic, chlorophenols, halogenated
benzenes, and N-substituted aromatic compounds are inhibitory to microorganisms as it interacts with cell membrane (McDonnell 2007). The addition of
excess chemical when operating the reactor leads to chemical foaming. The
other type of foam is caused due to excess production of biomass in the reactor
called biological foam which is usually brown in color. A baffle is used to
prevent scum production on the medium where the biomass is generated.
Scum is formed due to variation in temperature, mixing, light, and less than
four percent of total solids present in the reactor. Both foam and scum formation
damage the gas pipes and result in reduction of biogas yield. For regular
monitoring of the anaerobic digestion process, fatty acids and total alkalinity
are considered. Volatile fatty acids are produced which may cause a change in
the pH of the reactor and hence lead to lesser biomass production and biogas.
Acetic acid, propionic acid, butyric acid, etc. produced are generally utilized to
produce methane. However, at the same time if there levels are high, they tend to
cause a change in the pH which needs to be adjusted. This is generally done by
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 239