Microbial characterization during composting of municipal solid waste

This study investigates the prevailing physico-chemical conditions and micr obial community; mesophilic bacteria, yeasts and filamentous fungi, bacteri al spores, Salmonella and Shigella as well as faecal indicator bacteria: to tal coliforms, faecal coliforms and faecal Streptococci, present in a compo st of municipal solid waste. Investigations were conducted in a semi-indust rial pilot plant using a moderate aeration during the composting process. O ur results showed that: (i) auto-sterilization induced by relatively high t emperatures (60-55 degreesC) caused a significant change in bacterial commu nities. For instance, Escherichia coli and faecal Streptococci populations decreased, respectively, from 2 x 10(7) to 3.1 x 10(3) and 10(7) to 1.5 x 1 0(3) cells/g waste dry weight (WDW); yeasts and filamentous fungi decreased from 4.5 x 10(6) to 2.6 x 10(3) cells/g WDW and mesophilic bacteria were r educed from 5.8 x 10(9) to 1.8 x 10(7) bacteria/g WDW. On the other hand, t he number of bacterial spores increased at the beginning of the composting process, but after the third week their number decreased notably; (ii) Salm onella disappeared completely from compost by the 25th day as soon as the t emperature reached 60 degreesC; and (iii) the bacterial population increase d gradually during the cooling phase. While Staphylococci seemed to be the dominant bacteria during the mesophilic phase and at the beginning of the t hermophilic phase, bacilli predominated during the remainder of the compost ing cycle. The appearance of gram-negative rods (opportunistic pathogens) d uring the cooling phase may represent a serious risk for the sanitary quali ty of the finished prod-act intended for agronomic reuse. Compost sonicatio n for about 3 min induced the inactivation of delicate bacteria, in particu lar gram-negatives. By contrast, gram-positive bacteria, especially microco ccus, spores of bacilli, and fungal propagules survived, and reached high c oncentrations in the compost. (C) 2001 Elsevier Science Ltd. All rights res erved.