Sludge digestion and composting

Anaerobic digestion is performed in closed vessels, at ambient temperature but more frequently at 25-35°C, in absence of oxygen, and takes place in two stages: liquefaction and gasification.
During liquefaction, which implies transfer of organics matter into the liquid phase caused by bacteria, mainly volatile acids are produced. The following stage, gasification, is carried out by strictly anaerobic methane gas bacteria. The results of digestion are:
Reduction of volatile organic matter to 50-60%, and therefore reduction of sludge volume;
Destruction of most pathogenic germs;
Production of biogas which may contain up to 65-70% vol. of methane;
Removal of bad odors from sludge.

The digestion can be carried out in the so-called digestion towers which can be also fed in series, with a residence time ranging from 15 to 35 days for heated digesters.
The biogas produced in anaerobic digesters is fed to a generator, while the produced nutrient-rich digestate can be used as fertilizer.
The aerobic bacteria work faster than the anaerobic ones and the reduction of volatile organics is approx. 40%. The process requires more energy and the elimination of pathogenic germs is lower.
The aerobic stabilization is carried out in open aerated tanks by means of air. Retention time ranges between 7 and 12 days, depending on temperature.

Compost is a mixture of organic ingredients that has been decomposed by aerobic bacteria and fungi to be used for fertilizing and soil amendment. Aerobic microrganisms convert the organic substances into heat, carbon dioxide and ammonium. The ammonium in the form of nitrogen (NH4) can be used by plants. The decomposition process is performed by shredding the solid wastes, adding water and ensuring proper aeration by regularly turning the mixture.
Advantages of composting:
Reduced volume of wastes to disposal;
Excellent fertilizer;
Easy process.