The technologies used for municipalities and industries are very similar with the exception of domestic and industrial waste collection requiring a pre-selection of the biowaste that will be mixed in the digestors.
Industrial and Municipal Digestion Plants
Large-scale industrial digestion plants are used for biowaste and kitchen waste digestion. Biowaste is collected in special bins in every single household in Canada. Depending on the season it consists more or less out of kitchen and/or garden waste. Kitchen waste is collected in restaurants, hotels, universities and so on. Years ago most of the kitchen waste was fed to pigs. But nowadays, as there are more and more restrictions because of hygiene it is becoming increasingly more common to put kitchen waste into biogas plants. Before digestion, biowaste and kitchen waste have to be treated. This pretreatment includes foreign objects removal, grinding, sieving and homogenisation. Plastics, stones, forks, knives - any of which may hamper the digestion process itself or damage any pump, mixer or whatever have to be taken out of the process.
Grinding means to reduce particle size so that all parts can be pumped and mixed readily. Paper shredder can be used to mix the paper with the other biowastes. As kitchen waste has to be hygienic - normally at a temperature of 70° C with a retention time of one hour - particle size must be no greater then 1 cm. A sieve is used to remove all large particles just before entry into digestion. Of course, everything has to be slurred with liquid to make it pumpable. All the pretreatment is performed within a hall. To digest biowaste is much more complicated than manure - and kitchen waste digestion is even more difficult. Depending on the amount of kitchen waste included in the biowaste, you may have to use a two-stage digestion process technology. In such a case a so called hydrolysis tank is necessary. In it the first biological processes take place. The hydrolysis gas consists mainly of CO2 and can either be added to the biogas from the digester or to a biofilter for odour treatment before being emitted to the environment. After hydrolisation is finished the substrate is pumped to the digester. In it all the biogas production takes place. The retention time depends on the substrate, the temperature and the intention of the operator. As there are tendencies for the substrate to develop swimming and sedimentation layers, one of the most important tasks of the process technique is to avoid exactly this. Complete mixing is absolutely necessary.
Tanks are mainly coated steel tanks; the standard is an industrial one. The produced biogas is treated in a manner very similar to farm-scale and cofermentation biogas plants. Gas engines and diesel gas engines are in operation. But in most cases gas engines are used. Power can be several MW. Large-scale cofermentation biogas plants have large gas holders and emergency flares in case the engine(s) is (are) not in operation and biogas has to be burnt. The gas system may include a blower, condensate trap, desulphurisation unit and so on. Everything is controlled by a gas system control unit. The digested substrate is dewatered. The compost is either spread onto the fields for agricultural use or composted (cured). After this standard composting people can buy high quality compost for their own use or the compost is used in nurseries, gardening, viniculture and landscape-gardening.
After dewatering the liquid phase is pumped to the beginning of the process and recycled for slurrying of the dry input substrate. This is done to avoid spoiling fresh water all the time. Mechanically it is fairly simple but there are several microbiological problems. As there are lots of dissolved salts within the liquid phase high concentrations may develop there after several cycles. This may hamper the growth of microorganisms - depending on the input substrate. All the large-scale digestion plants are controlled by an overall process control system. There are lots of devices for measurement and safety reasons. At night everything is run automatically; during day there are operators on site - especially for pretreatment, repair and maintenance reasons and for accepting the biowaste and kitchen waste.
Large-scale digestion plants are constructed for one reason only: to make a profit. Therefore the plant must operate day and night. The investment costs may be up to several million dollars. Depending on the input substrate, the pretreatment aggregates have to be engineered and constructed. The hydraulic retention time may vary as a function of the input substrate and the pretreatment. This has a direct impact on the digester volume. Therefore a great deal of know how is needed to construct a large-scale digestion plant.
Biowaste digestion is not simple. After many years of engineering such kinds of plants, BIO-METHA with their partners are able to offer competent know-how.
