8.2.1 Outline of large scale biomethanation

Anaerobic digestion has been in practical use for a long time. Its industrial installations had started as early as in around 1900. Since then, the anaerobic digestion systems have been continuously improved and enlarged to treat a wide range of biomass wastes such as food industry wastewater/waste, garbage, livestock waste, night soil and sewage sludge among others.

8.2.2 Large-scale anaerobic digestion systems

A typical anaerobic digestion system in large-scale is depicted in Fig. 8.2.1. The function of each unit process is described below.

(a) Pre-treatment process

It is often required for an effective anaerobic digestion that the received biomass waste is conditioned in a pre-treatment process such as removal of foreign matters not suitable for anaerobic digestion, pulverization, dilution by water, thickening, and/or acid or alkali treatment. Some biomass wastes such as garbage, which is a mixture of various organic and inorganic matters, and, thus is not always consistent in its composition and properties, are subjected to mechanical and/or magnetic separation in order to get rid of indigestible materials such as metals and plastics. The separated biomass waste is pulverized and added with dilution water to prepare waste slurry for the next unit process.

(b) Slurry storage tank

Prepared slurry is temporarily stored in a slurry storage tank for leveling dairy fluctuations in both quality and quantity. If the ambient temperature is suitable, microbial activities of acidogens in the storage tank may increase. If this occurs, accumulation of organic acids in the slurry can cause a decrease in pH to as low as around 4. The slurry storage tank must be designed to resist corrosion due to such low pH conditions.

(c) Methane fermenter (anaerobic digester)

Three major functional microbial groups are active in the methane fermenter. These three functions that take place sequentially are; hydrolysis, acidogenesis and methanogenesis. The final products of the reactions are methane and carbon dioxide. The hydrolysis reaction is often the rate-limiting pathway of an anaerobic digestion process on not-readily biodegradable or recalcitrant biomass such as sewage sludge and ligneous biomass, whereas the methanogenesis is likely to be the rate-limiting pathway on readily biodegradable biomass such as garbage and starchy wastewater. In order to establish an efficient anaerobic digestion system, it is important to consider the rate-limiting pathway and to select the most suitable reactor design for the properties of anticipated biomass waste. For example, the overall rate of anaerobic digestion on readily biodegradable biomass waste heavily depends on the density of active methanogens in the reactor, thus the reactor should be designed to maximize the density or mass of the methanogens within the system.

(d) Fermentation wastewater treatment

Fermentation wastewater discharged from the anaerobic digester usually contains high concentrations of organic matters, nitrogen compounds and phosphorus compounds. The fermentation wastewater should be treated to reduce the concentrations of these pollutants to meet the standards for final discharge to a receiving body of water or sewer system. The most typical fermentation wastewater treatment system is the activated sludge process with tertiary treatment.

(e) Biogas utilization

Since most of the biomass wastes contain proteins (a source of nitrogen and sulfur) and sulfate salts, the biogas contains certain concentrations of hydrogen sulfide and ammonia. The biogas produced from sewage sludge, which sometimes contains a considerable amount of silicones, may contain siloxanes as well. Since these impurities can possibly cause damage to biogas utilization facilities such as a gas engine, gas boiler, gas turbine and fuel cell, and/or cause secondary air pollution, a biogas utilization unit process is often equipped with a desulfurization device and/or siloxane remover prior to the gas holding tank.