END-O-SLUDG- Marketable sludge derivatives from sustainable processing of wastewater in a highly integrated treatment plant

Abstract

The safe treatment of wastewater is one of society's most fundamental challenges. Wastewater treatment creates sewage sludge, the processing of which is highly energy-intensive, contributes to greenhouse gas emissions, and is still far from efficient. Scientific investigations and technical developments have focused on five critical areas of sludge management with the ultimate aim of achieving a holistic system solution for sustainable sludge management.

• Sludge reduction investigations: Dissolved Air Flotation was investigated as a method for reducing the volume of sludge produced. Pilot scale trials of the novel process showed that improved TSS removal produced 59% reduction in the organic load to the activated sludge process, a common secondary wastewater treatment step, with a concomitant reduction in the power requirement.
• Sludge treatment investigations: It has been found that using inverted phase fermentation watery sludge could be split into separate solid and liquor fractions which could be converted to biogas in smaller more efficient digesters reducing the capital investment considerably. Furthermore, the digested solid fraction could be treated by micro-milling to break down the sludge matrix further releasing biogas during a secondary digestion resulting in 75% overall organic to energy conversion. Yet, a significant breakthrough was made with the discovery of BIOPOL, a crude biopolymer product derived from digested sludge. It has been found that BIOPOL could provide an excellent alternative to struvite formation as a co-precipitation method for phosphorus recovery from sludge liquor.
• Downstream processing for sludge derivatives: The greatest challenge of the project was to design, build and test a process for making a fertiliser product with sludge cake to exact specification which includes many physical, chemical and biological parameters. The new process makes use of CHP and other sources of waste heat to produce a granular, free flowing, dust free product known as OMF with a guaranteed nutrient content.
• Market development activities: Tests showed that the prototype OMF product could be spread accurately with twin spinning disc fertiliser spreaders up to a bout width of 24 metres, a respectable performance. In farm scale trials the OMF product gave similar yields to conventional fertiliser when applied to cereal and grassland crops. Market data, technical performance results together with the product safety data and environmental findings have been collated to provide a substantial body of evidence in support of the case for End of Waste for OMF as a new sludge derivative.
• Management and sustainability studies: Sustainability assessment methods have been developed and used to compare a number of options for sludge management across Europe. The comparative study showed that under certain scenarios OMF has great potential to be a sustainable option for sludge management with best social, economic and environmental benefits. The methodologies and learning from END-O-SLUDG have been pivotal in the development of a long term sludge management plan for Asturias (Spain).

Overall END-O-SLUDG has made considerable progress in developing innovative holistic solutions for sludge treatment and management including life cycle assessment and on-farm management of biosolids through to End of Waste.