Production of volatile fatty acids by hydrolysing sludge from Sjölunda WWTP

Sammanfattning: The biological nutrient removal (BNR) processes at Wastewater Treatment Plants (WWTPs) require easily degradable carbon, such as volatile fatty acids (VFAs), to work efficiently. Because of economical, and environmental reasons it is desired to utilize the organic content in the incoming wastewater instead of using an external carbon source. This study investigated the potential of producing VFAs through biological hydrolysis of primary sludge (PS), and activated sludge (AS) from Sjölunda WWTP in Malmö, Sweden. Batchwise hydrolysis at 14.5 °C was conducted in beakers of 4 L to find initial VFA production rates over the first two days of hydrolysis. Three factors that could affect the hydrolysis rate in PS were investigated: volatile solids (VS) content of the sludge, seeding with partially hydrolysed sludge, and addition of reject water coming from the dewatering of anaerobically digested sludge. Hydrolysis of AS gave high release of ammonium (NH4+) and phosphate (PO43-), and is thus not beneficial to use for VFA production at Sjölunda WWTP. PS hydrolysis released lower amounts of NH4+ and PO43- and could thus be beneficial to use. Minimum, average, and maximum initial VFA production rates for the PS mixtures were found to be 0.3, 1.0 and 2.1 mg CODVFA/(g VS × h) respectively. The one experiment with seeding showed a decreased VFA production. Two batches of PS with reject water added were hydrolysed. The first batch showed an increase in VFA production while the second showed a decrease. VFA production per mass VS was found to be independent of VS concentration in the hydrolysed sludge. Sludge mass flow, wastewater flows, and concentrations from Sjölunda WWTP were used to evaluate the potential of removing nutrients through BNR with the produced VFAs as carbon source. With the mean VFA production rate of 1.0 mg CODVFA/(g VS × h) acquired from the experiments, approximately 4 % of the incoming nitrogen and phosphorus could potentially be removed with BNR.