The Sludge Treatment Centre at Dalderse in Falkirk, Scotland, is part of a large waste water treatment works dating from the 1960s and owned by Scottish Water. In 2010, Ramboll investigated using biogas by-products from the works to fuel vehicles or supply the gas grid. Ramboll Energy's current feasibility study determines the measures necessary to implement co-digestion of commercial and industrial waste with sewage sludge, and updates our earlier findings on exploiting the biogas as a commercial energy resource.
At Dalderse, 432 tonnes per day of sewage sludge from several sites is digested in two mesophilic anaerobic digesters, where bacteria break down the sludge to produce methane and other by-products. Anaerobic digestion is standard UK practice for rendering a harmful material safe. The digested sludge is dewatered and recycled to agriculture.
We recommended a phased approach to implementing co-digestion at the plant. First, improve the efficiency of the existing system, by minimising the loss of heat and biogas, improving instrumentation and installing new CHP equipment. In this case, biogas yields could be increased by one-third.
Second, increase the digestion capacity to 564 tonnes per day by constructing additional anaerobic digesters. The new system will treat organic waste feedstock (food and catering waste) as well as the existing sewage sludge load. The two existing digestion tanks could be reconfigured to increase retention time for the sludge thickening process. The quantity of biogas produced would be six times present levels.
Changes in the legislation relating to waste, and the renewable heat incentive, have helped to improve the commercial viability of using biogas. The biogas generated at Dalderse will be converted to heat and power, for transfer to the National Grid, or processed into biomethane by using one of a number of proven techniques for removing contaminants. The upgrade will generate around 160Nm3 per hour of biomethane, suitable for use as a fuel or for feeding back to the gas grid.
Potentially using biomethane to fuel the tankers delivering waste to the plant would reduce Dalderse’s carbon footprint and provide a buffer against increasing fossil fuel prices. We estimate that the upgraded biogas could provide sufficient fuel to run 70 HGVs at 80,000km per year, representing a carbon saving of 5,000 tonnes of CO2-equivalents annually.