Use of lignite fly ash as an additive in alkaline stabilisation and pasteurisation of wastewater sludge

In this study, the possibility of using lignite fly ash in low doses for reducing the pathogen levels in wastewater sludge was investigated. The results showed that using fly ash alone in doses of 40%, 80% and 120% (on a dry weight basis), did not produce an alkaline environment for an efficient removal of pathogens. However, using fly ash in conjunction with the minimum amount of quicklime may act as an effective way of fecal coliform removal in both alkaline stabilisation and pasteurisation processes. It was shown that using fly ash in doses of 80% and 120% in alkaline stabilisation and pasteurisation processes prevented the pH decays and regrowth of pathogens during 60 days of storage period. The results of the study confirmed that alkaline pasteurisation process produces a product which is more resistant to pH decays and regrowth of fecal coliforms compared to that of alkaline stabilisation. Consequently, the overall results of this study indicated that the minimum lime and fly ash dosages required to generate a Class B biosolid were 10-15% and 80%, respectively. On the other hand, heating sludge to 50°C prior to the addition of 10-15% quicklime and 80% fly ash followed by further heating to 70°C and then sustaining at this temperature for 30 minutes were sufficient to generate a Class A biosolid.