Determination of the major factor responsible for soluble organic matter release during nitrite/free nitrous acid pre-treatment of waste activated sludge.

Soluble chemical oxygen demand (SCOD) release by free nitrous acid (FNA)/NO system is usually called "FNA disintegration", despite lack of evidence that FNA is the main agent responsible for organic matter breakdown. The aim of this study was to investigate whether FNA or NO is the primary disintegration factor of thickened secondary sludge in a wide spectrum of process parameters (T = 48 h, 0-2280 mg NO-N/L, pH 3.2-6.4 and FNA between 0 and 47.4 mg HNO-N/L). Statistical analysis based on multiple regression and the Akaike Information Criterion showed that NO, not FNA, is a main disintegrating factor leading to SCOD release (p = 0.005206 and 0.00009 respectively) and that the FNA concentration is without statistical significance (p = 0.800234 and 0.328099 respectively). These findings are important as understanding key factors is essential for productive future research and technology development. Moreover, these findings give doubts about the role of FNA in its other applications such as inhibition of nitrification.