Lignite fly ash was used as an additive in three different alkaline stabilisation processes and the effects on some chemical properties of wastewater sludge were investigated. The results of the study indicated that sludges added with fly ash only in a dose of 40% (on a dry weight basis) generally caused no significant differences in the sludge properties. However, ammonium-nitrate nitrogen, available phosphorus and soluble boron concentrations were slightly reduced in sludges which were added with 80% and 120% fly ash. On the other hand, alkaline stabilisation (10-15% quicklime + 40-120% fly ash) and alkaline pasteurisation (10-15% quicklime + 40-120% fly ash + heating at 70 C for 30 min) processes caused a significant decrease in available forms of nitrogen and phosphorus and a significant increase in pH. Although a strong relationship between fly ash dosage and available nitrogen concentration was obtained in alkaline stabilisation process, no significant correlation was found between the two parameters in alkaline pasteurisation process. The results also suggested that using fly ash with doses of 40%, 80% and 120% in alkaline pasteurisation process caused no significant alteration in sludge properties. Heating and quicklime addition were expected to be the influential factors. (c) 2007 Elsevier Ltd. All rights reserved.