Persistency of bacterial indicators in biosolids stabilization with coal fly ash and lime

Alkaline coal fly ash and lime were tested for their effectiveness in patho gen removal from biosolids at different time intervals and temperatures. Co al fly ash at 10 and 35% w/w was mixed with dewatered biosolids and then th e ash-biosolids mixture was mixed separately with 0, 1.1, 2.2, 4.4, 8.5, 11 , and 18% calcium oxide (w/w on a dry weight basis) with and without heatin g to 55 degreesC. Total bacteria, salmonella, and total coliforms were moni tored at various time intervals. Both ash-biosolids mixtures with or withou t lime amendment had a significantly lower total bacterial population than the biosolids control, but the residual indigenous bacterial flora in the a sh and lime stabilized biosolids still maintained a population of greater t han 10(4) g(-1) dry biosolids. Alkaline-stabilized biosolids with a lime am endment rate greater than 8.5% could maintain pH greater than or equal to 1 2 for more than 2 hours, which effectively removed total coliforms and salm onella in the mixture. Heat treatment to 55 degreesC and a storage time of 14 days provided an added advantage resulting in a further reduction in pat hogens for all treatments. It is recommended that 10% ash-biosolids mixture should be amended with a minimum of 8.5% lime on a dry weight basis for at least 2 hours to achieve acceptable levels of salmonella and total colifor ms to ensure no pathogenic risk following land application.