Obtaining biostimulant products for land application from the sewage sludge of small populations

Abstract

Large amounts of sewage sludge are generated in wastewater treatment plants during the water purification process. In small populations – defined as urban settlements with less than 2000 population equivalent – sewage sludge is normally produced in anaerobic systems (e.g. septic tanks, Imhoff tanks or anaerobic ponds) or in extended aeration systems. According to the European Directives 86/278/EEC and 2006/12/EC, sewage sludge must be treated before its final fate. Although there are various alternatives, land application of sewage sludge after its composting has been widely promoted because of its high nutrient content. However, the employment of composted sludge as a soil amendment has some limitations, including social rejection. In this paper, a hydrolytic process for obtaining an agricultural biofertiliser from sewage sludge has been assessed. The enzymatic hydrolysis of sludge produced two by-products: an insoluble paste and a nutrient-enriched liquid that constituted the fertilising product. This biofertiliser had higher contents of organic matter, proteins, potassium (K) and sulphur (S) compared to the fresh sludge. Protein content was characterised by a low molecular size, thus increasing the bioavailability of nitrogen (N). Further, high levels of enzymatic activity were detected in soil after the addition of the obtained biofertiliser.