Energy generation from biosolids could solve disposal problems

The conference, which took place at the annual IWEX trade show for the water and effluent treatment industry at the National Exhibition Centre in Birmingham on 30 October, revealed a variety of ways in which biosolids can be disposed of in a more environmentally sustainable way than incineration. Biosolids are traditionally disposed of by being spread on farmland, but recent and upcoming changes in legislation and decreases in farming have made it necessary for water companies to find alternative methods of disposal. In the future, long periods of bad weather, or another foot-and-mouth style farming crisis, could bring about real problems with biosolid disposal, said Stephen Riches of the Anglian Water (AWG) Technology Group.

According to Steve Clay, Process Engineering and Development Manager for Severn Trent Water, the processes that have greatest potential for biosolid disposal are: co-generation, use in cement kilns, power generation, gasification, pyrolysis and hydrolysis. However, if biosolids are to be used, in particular for power generation, the material needs to consist ideally of 30-35% dry solids, resulting in the question of where the heat comes from to dry the biosolids without reducing the total energy from the material available for export, he said.

One example of energy generation from sewage derived fuel is the proposed Biosolids Reduction and Energy Generation plant (BREG), run by AWG and energy company TXU Europe, designed to recycle biosolids in a sustainable manner not reliant on the agricultural market, as well as to generate electricity from a renewable source, and produce an ash by-product suitable for use in the aggregate or construction industries. The plans for the BREG plant, which is to be situated next to Anglian Water’s Corby sewage treatment works, are currently awaiting review by the planning authorities, and the plant is anticipated to be operational by the summer of 2003.

Biosolids can also be used in co-generation plants, such as Longannet Power Station outside Glasgow, a 2,400 MW coal fired plant, once the largest operational power station in Britain. Sludge derived fuel from the Greater Glasgow area is only a small proportion of the mix, but does equate to a wastewater population equivalent of several million, says Philip Turner, Water Operations Manager at Scottish Power, the company that runs the station. One significant problem that Turner has observed both at Longannet and at other power plants using biosolids is that power workers are often not keen on the idea of having sewage delivered to their work place. However, compared to coal, which can have big swings in contaminant content, sewage derived fuel is quite stable, says Turner.

Gasification of biosolids to produce a gas fuel is another alternative, but one which brings further challenges. The process releases impurities and organic fragments, including sulphur and nitrogen compounds, tars and inorganic salts, some of which cause corrosion and fouling problems in equipment, says Erik Rensfelt from the Swedish research and development company TPS Termiska Processer AB. This means that the gas product needs to be thoroughly cleaned, firstly by removing particulate matter with filters, and then by chemical manipulation, such as the conversion of nitrogen into product ammonia gas.

Alternatively, the energy stored in biosolids could be put to use through fertilisation of short rotation coppice, says Rensfelt. “The sludge is not used directly in the thermal process,” he said. Currently, his company is engaged in ARBRE (Arable Biomass Renewable Energy), a joint venture with the Kelda Group (formerly Yorkshire Water), which runs a new 8 MW bioenergy plant near Leeds, which predominantly uses short rotation willow coppice, with a drive time to the plant of not more than 45 minutes. The plant also uses forestry waste.