Characterisation of bacterial diversity in fresh and aged sewage sludge biosolids using next generation sequencing

Sewage sludge, often referred to as biosolids, is generated in large quantities by wastewater treatment plants. It contains macro-and micronutrients which are essential for plant growth and so represents a valuable agricultural resource. Prior to land application, pathogens are carefully monitored to reduce the risk of crop and soil contamination, however, to date there has been limited investigation of agriculturally beneficial bacteria indigenous to the biosolids. This study investigated shifts in the composition of the bacterial community alongside the physicochemical properties of biosolids of increasing age, from freshly dewatered to those stockpiled for approximately four years. With stockpiling, there was a significant increase in ammonium content, ranging from 801 mg/kg in the fresh biosolids to 8,178 mg/kg in the stockpiled biosolids and a corresponding increase in pH ranging from 6.93 to 8.21. We detected a ten-fold increase in Firmicutes, from 4% relative abundance in the fresh biosolids compared to 40% in the older, stockpiled biosolids. Plant growth promoting bacteria (PGPB) of the Proteobacteria family, particularly of the Devosia and Bradyrhizobium genera were identified in the freshly dewatered and the older, stockpiled biosolids. Land application of the biosolids studied here could reduce fertiliser costs, provide a means of pH correction to acidic soils and a potential source of bacteria beneficial for crop growth.