Tighter emissions controls and limited markets for sludge-derived fuel are behind Northumbrian Water’s decision to convert its biggest sewage treatment plant to enhanced anaerobic digestion.
The Bran Sands plant treats effluent from local chemical plants along with domestic sewage from Middlesbrough. It deals with a sewage load equivalent to waste from 1.9 million people.
In June, the company announced it would invest £32 million in the plant to shift from drying sludge, for use as a fuel, to sludge digestion - a significant change in its disposal strategy.
Bran Sands has always been an innovative site. It was developed in the mid-1990s in response to the 1998 ban on sludge disposal at sea and the rise in sludge generation due to the 1991 urban wastewater treatment Directive.
It is the only major UK site to dry sludge for fuel and includes a nine-megawatt gas-fired power station. This originally provided heat for the drying process and generated energy for use in the sewage treatment process or export to the grid.
But high gas prices forced Northumbrian to stand the plant down a few years ago and today it is used only intermittently to support the national grid (ENDS Report 385, pp 34-38 ). The company finds it cheaper to dry sludge using bought-in electricity.
Because of the waste incineration Directive’s stringent emission controls, other outlets for sludge-derived fuel are limited and this is likely to have pushed up gate fees. According to site manager David Charlton, some 90% of the plant’s production is sent to cement kilns.
Given these limitations, a change of tack is perhaps not surprising. Northumbrian now plans to build a ‘thermal hydrolysis advanced digestion’ facility, which will produce larger amounts of methane than ordinary anaerobic digestion and will generate 4.7MW of renewable electricity - over half the site’s requirement.
The plant will be built by Aker Kvaerner Engineering and involves dewatering sludge and cooking it in pressure vessels at 165°C before digestion. The process will destroy 60% of volatile solids in the sludge - 10% more than ordinary anaerobic digestion, said Mr Charlton.
The company is wary of giving detailed predictions of the plant’s performance, but says it expects it will be only 5% more expensive than an equivalent conventional anaerobic treatment system, while delivering 35% more energy.
Mr Charlton said the process would reduce the company’s carbon footprint by 1,000 tonnes of CO2 per year. He was unable to put this in the context of the company’s total emissions, but a rough calculation based on its reported 2005/06 electricity use of 548GWh suggests it is less than 0.5%.
Sludge production at the works should be cut by 35% in terms of dry solids, the company says. But because the product will end up as sludge cake with a 70% moisture content, rather than as granules which are only 8% water, the total weight of sludge requiring disposal will actually rise.
The sludge cake will be sent for land spreading, according to a company spokesman, following its experience of sending lime-treated sludge from smaller sites to farmers.
The plant is due to be finished in 2009. The company’s second largest site, at Howden on Tyneside, may opt for a similar development.