Treating domestic sewage produces sludge as a waste stream, and getting rid of it is probably the biggest problem currently facing by the U.K. water industry. There was a time when the U.K. dumped five million tons of it annually into the North Sea, but that practice, quite properly, stopped in 1998.
One treatment for sewage sludge is to anaerobically digest it to generate biogas that can be burned as a fuel to provide heat or electricity, but the process involves expensive machinery, rendering it only economic on large scale. Central sludge processing plants have been built to serve large conurbations in the U.K., but the problem remains in small rural works. There the only options are landfills, which taxes have made expensive — the industry’s customers object to paying more — or land spreading. Where there is forestry, land spreading is a real alternative but tightening environmental standards have ruled out disposal of sewage sludge on agricultural land unless the sludge is treated, again making this route uneconomic for small works. So what’s the answer?
We believe that co-digestion of sewage sludge with domestic waste is the way forward. It’s a solution that has been used in Denmark, but it has yet to catch on in the U.K.
Anaerobic digestion is a process that uses bacteria that live in the absence of oxygen to convert organic matter into biogas (methane). It can be applied to any organic matter including domestic waste, industrial waste, agricultural waste, sewage sludge or even special-grown biofuel crops like maize silage.
Although the anaerobic digestion process is the same whether it is treating sewage sludge or domestic waste, it is unusual for both types of waste to be treated together in the U.K. This is mainly due to a lack of commonality between the organisations involved — sewage sludge is disposed of by the water utility that generates it, while domestic waste disposal is the responsibility of local government.
In Denmark there is a more “joined up” approach with a major program for biogas production using anaerobic digestion technology. Kruger, part of Veolia Water Solutions & Technology (VWS), was responsible for developing much of the anaerobic digestion technology currently being used in Denmark and they designed a benchmark plant at Grindsted Kommune, a cooperative in a mainly agricultural area with local food processing industries. It co-digests organic household waste, primary and secondary sewage sludge, food wastes from supermarkets and restaurants and food industry wastewater to produce almost 7,000m3/day of biogas. This generates electricity and district heating. The odourless digested sludge is dewatered and then spread on agricultural land.
The greatest challenge to the engineers responsible for the program was not the technology, but arranging for household waste to be sorted prior to collection. The success of the project is in no small part due to the cooperation of the local public, and the policy of good communications that has provided their motivation to sort their waste at source — supported by a fines system for households that fail to sort.
The Danish model clearly works and has demonstrated the ability of current anaerobic digester technology to co-digest household waste, industrial waste and sewage sludge. For rural sewage treatment works in the U.K. there is a valuable lesson to be learned. There is already a culture of sorting and recycling waste, so the Danish approach could be easily adopted, given the political will.
Aside from the benefits of electricity generation and district heating, the reduction in the quantity of waste going to landfill is significant and there are clear environmental benefits to be gained. Anaerobic digestion technology is continuing to develop and, along with the latest high efficiency CHP systems, it is possible to maximize the economic benefits of energy from waste, and we think it’s high time it happened here in the U.K.