The recycling or disposal of sludge in a sustainable manner is a major challenge for the water industry, with the UK generating around 15,000 tonnes of sludge each year.
Sludge is a dried solid by-product of wastewater treatment, accounting for just 0.1% of wastewater as a whole. Most sludge (80%) is treated and recycled to land (for agriculture, reclamation, composting and other uses), disposed of through thermal destruction (with excess heat from this process being used for Combined Heat and Power) or, the least favoured option, sent to landfill.
Recycling to land
In general, recycling of treated sludge (called biosolids) to land is the most sustainable option, as it can reduce the need for artificial fertiliser and improve soil structure. In 2010-11, for example, 80.3% of sludge was recycled to land, 18% was disposed of through thermal destruction and 0.7% went to landfill.
Controls and choices
The industry has a limited control on the volume of sewage received and the amount of sludge produced. Several factors combine to increase sludge volumes: population increases, new property connections added to the network, and continued implementation of legislation such as the European Urban Waste Water Treatment Directive, which specifies standards in relation to discharges to the environment.
The choices for recycling must also depend upon local circumstances and requirements for transport, energy use and other resources. Many wastewater companies choose a number of different outlets to ensure a flexible approach, adapted to regional requirements.