3.2 What types of aerobic digestion methods exist?

Several methods exist in the aerobic digestion process, each with its unique approach to promoting the growth of beneficial microorganisms responsible for decomposing pollutants. Let’s explore some of them.

Conventional aerobic digestion stabilises sludge in unheated open digesters through diffused air or surface mechanical aeration. Sludge can be anaerobically pre-treated and further thickened to reduce the required digester volume. It’s important to note that solids concentrations in the aerobic digesters should not exceed 3%.

Aerobic digestion using pure oxygen is a variation of conventional aerobic digestion, where pure oxygen is supplied to the medium instead of air. The concentration of solids in the digester can be as high as 4% without reducing the oxygen transfer rate to the biomass. This process is suitable for large treatment plants with limited space. The reaction is highly exothermic, increasing the process efficiency and making it suitable for cold-climate regions.

Autothermal thermophilic aerobic digestion (ATAD) is a form of aerobic digestion that uses air or oxygen to aerate the sludge. The heat needed to raise the temperature is supplied by the aerobic digestion of organic and cellular material. The higher temperature can be achieved by insulating the digester and increasing the oxygen supply, compared to conventional aerobic digestion.

The high temperatures destroy most pathogens and avoid the need for disinfection. Further, in the ATAD process, a lower retention time is required, and a higher reduction of solids content is achieved. The design temperature is 45 – 70°C, and the retention time is 2 to 10 days.

The upcoming topics will focus on conventional aerobic digestion because of the available experience in treating faecal sludge in emergency settings.