Besides increasing the energy efficiency of the treatment plants public bodies can also recover energy through the anaerobic digestion of sludge, thus improving the whole energy balance related to the operation of a treatment plant.
There are three main types of sludge: primary sludge (deriving from primary treatment), excess sludge (from biological treatment), and sludge or suspended solids (generated in a tertiary treatment, if applied).
Carbon balances of the aerobic and anaerobic biodegradation show that under aerobic conditions, about half of the organic compounds are transferred to biomass, whereas under anaerobic conditions, it is only 1–5 per cent as most of the organic carbon is metabolised to biogas.
The sludge is anaerobically digested in airtight tanks and the biogas formed is collected and stored to be used for on-site generation of electricity and heat in biogas motors. Even if the electrical efficiency of biogas motors is low compared to other techniques (between 30 and 40 per cent), the heat of the gas motors is also used to preheat the raw sludge as the temperature in the anaerobic digestors has to be maintained at around 37°C.
When the anaerobic digestor(s) has spare capacity, waste or waste water with a high organic load (e.g. from food processing industries or distilleries) can be co-fermented resulting in higher gas production, as most of the organic compounds are converted to biogas (methane and carbon dioxide). This also results in lower sludge formation and electricity consumption for activated sludge aeration, as the concentrates do not have to be aerobically treated. Thus, the co-fermentation of biodegradable concentrates, instead of their aerobic treatment, is a win-win situation.