80
3 Advanced Technologies (Biological and Thermochemical) …
for solubilizing microbial cells but they are expensive due to high energy consumption. The most used mechanical pre-treatment technique is sonication, which also
can be categorized as an advanced oxidation method [77, 79].
3.2.1.5 Effects of Operating Conditions on AD
Various parameters such as temperature, pH, alkalinity and solids content can affect
the rates of the different stages of the digestion process. The value and stability of
pH are very important during the AD as the methanogenic bacteria are extremely
sensitive to pH with the optimum range of 6.6–7.6 [69, 88]. Temperature is another
important factor that can affect the growth rate and metabolism of microorganisms,
hence the population dynamics in the anaerobic reactor. Increasing temperature leads
to enhanced solubility of organic compounds as well as improved biological and
chemical reaction rates and death rate of pathogens. However, very-high-temperature
ranges such as thermophilic conditions would result in the increased fractionation of
free ammonia, which is an inhibitor substance for microorganisms [69]. Alkalinity
levels can also reveal the performance of an anaerobic process. In a steady system, the
alkalinity levels are best between 1000 and 5000 mg CaCO 3 L
−1 , lower levels could
be a warning about the impending reactor failure [88]. Some of the best operating
conditions for anaerobic digestion include the following:
– No dissolved oxygen,
– No inhibitory substances (such as heavy metals, sulfides, and HMF),
– Sufficient nitrogen and phosphorous,
– Carbon to nitrogen ratio of 25–30:1 [70],
– VFA concentration in the substrate between 2000 and 3000 mg/L [70].
3.2.1.6 Industrial Applications of AD
AD is probably one of the most commonly used methods to stabilize agricultural
biomass, solid wastes, and wastewater sludge. Currently, there are over 1270 sewage
treatment facilities producing biogas in the USA alone. Approximately, 247 anaerobic digesters are operating at commercial livestock farms and 98 facilities are using
food-based materials [71]. For example, the Fair Oaks Farm in Indiana, USA, uses
the dairy cow manure as the feedstock to produce methane through AD using mixed
plug-flow anaerobic digesters, generating 865 MMBTU of compressed biogas per
day. The biogas is used as the fuel for the entire farm operation and milk-hauling truck
fleet, saving millions of dollars in the fuel costs, lowering emissions and reducing
the farm’s carbon footprint [89].
In another example in Beijing China, a large-scale Centralized Biogas Supply
Project uses chicken and swine manure in upflow solid reactors, producing
207,000 m
3 biogas per year. The biogas is used as a cooking fuel for 540 households
in Shangdongkuo village, the largest central biogas supply in Changping district. The
project reduces odors and pathogens in the village and uses the generated digestate
3 Advanced Technologies (Biological and Thermochemical) …
for solubilizing microbial cells but they are expensive due to high energy consumption. The most used mechanical pre-treatment technique is sonication, which also
can be categorized as an advanced oxidation method [77, 79].
3.2.1.5 Effects of Operating Conditions on AD
Various parameters such as temperature, pH, alkalinity and solids content can affect
the rates of the different stages of the digestion process. The value and stability of
pH are very important during the AD as the methanogenic bacteria are extremely
sensitive to pH with the optimum range of 6.6–7.6 [69, 88]. Temperature is another
important factor that can affect the growth rate and metabolism of microorganisms,
hence the population dynamics in the anaerobic reactor. Increasing temperature leads
to enhanced solubility of organic compounds as well as improved biological and
chemical reaction rates and death rate of pathogens. However, very-high-temperature
ranges such as thermophilic conditions would result in the increased fractionation of
free ammonia, which is an inhibitor substance for microorganisms [69]. Alkalinity
levels can also reveal the performance of an anaerobic process. In a steady system, the
alkalinity levels are best between 1000 and 5000 mg CaCO 3 L
−1 , lower levels could
be a warning about the impending reactor failure [88]. Some of the best operating
conditions for anaerobic digestion include the following:
– No dissolved oxygen,
– No inhibitory substances (such as heavy metals, sulfides, and HMF),
– Sufficient nitrogen and phosphorous,
– Carbon to nitrogen ratio of 25–30:1 [70],
– VFA concentration in the substrate between 2000 and 3000 mg/L [70].
3.2.1.6 Industrial Applications of AD
AD is probably one of the most commonly used methods to stabilize agricultural
biomass, solid wastes, and wastewater sludge. Currently, there are over 1270 sewage
treatment facilities producing biogas in the USA alone. Approximately, 247 anaerobic digesters are operating at commercial livestock farms and 98 facilities are using
food-based materials [71]. For example, the Fair Oaks Farm in Indiana, USA, uses
the dairy cow manure as the feedstock to produce methane through AD using mixed
plug-flow anaerobic digesters, generating 865 MMBTU of compressed biogas per
day. The biogas is used as the fuel for the entire farm operation and milk-hauling truck
fleet, saving millions of dollars in the fuel costs, lowering emissions and reducing
the farm’s carbon footprint [89].
In another example in Beijing China, a large-scale Centralized Biogas Supply
Project uses chicken and swine manure in upflow solid reactors, producing
207,000 m
3 biogas per year. The biogas is used as a cooking fuel for 540 households
in Shangdongkuo village, the largest central biogas supply in Changping district. The
project reduces odors and pathogens in the village and uses the generated digestate