Whatever the method used to supply air during composting, the maintenance of aerobic conditions requires large energy inputs and use of costly equipment. On the other hand, suboxic or anaerobic conditions may not grant a sufficient speed of stabilization and sanitization of waste materials and the growing number of rural families that use septic tank toilets in Viet Nam, urgently requires new safe ways to recycle septic tanks sludge, avoiding direct application to farmland. This work compared a conventional anaerobic system with a low oxygen input treatment for composting rice straw together with septic tank biosolids or cow dung. Composting was carried out for 75 days, in open concrete biocells protected from rainfall, after inoculation with selected micro-organisms. The partially aerobic treatment maintained a significantly higher temperature than the anaerobic one, during the whole composting period. Organic C decreased from 32% to 25-27% at the end of process, remaining slightly higher in the anaerobic piles; on the opposite humic C showed a linear increase up to 8 mg g-1, reaching the highest values in the aerobic piles. Both composting systems were able to sanitize the organic wastes since no viable intestinal parasite eggs were found at the end of the process. Compost made from septic tank biosolids represents a valuable source of P, but has lower N and K than compost from cow dung.
Anaerobic composting is a viable alternative to conventional aerobic systems to recycle organic residues and safely treat septic tanks sludge biosolids in rural areas of Viet Nam.
