Properties of biosolids from sludge treatment wetlands for land application

Abstract

Sludge treatment wetlands consist of constructed wetlands which have been upgraded for sludge treatment over the last decades. Sludge dewatering and stabilisation are the main features of this technology, leading to a final product which may be recycled as an organic fertiliser or soil conditioner. In this study, biosolids from full-scale treatment wetlands were characterised in order to evaluate the quality of the final product for land application, even without further post-treatment such as composting. Samples of influent and treated sludge were analysed for pH, Electrical Conductivity, Total Solids (TS), Volatile Solids (VS), Chemical Oxygen Demand (COD), Dynamic Respiration Index (DRI), nutrients (Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP) and Potasium (K)), heavy metals and faecal bacteria indicators (E. coli and Salmonella spp.). According to the results, sludge water content and therefore sludge volume are reduced by 25 %. Organic matter biodegradation leads to VS around 43-44 %TS and COD around 500 g•kgTS-1. The values of DRI24h (1000-1500 mgO2∙kgTS-1∙h-1) indicate that treated sludge is almost stabilised final product. Besides, the concentration of nutrients is quite low (TKN~4 %TS, TP~0.3 %TS and K~0.2-0.6 %TS). Both heavy metals and faecal bacteria indicators meet current legal limits for land application of the sludge. Our results suggest that biosolids from the studied treatment wetlands could be valorised in agriculture, especially as soil conditioners.