Our renewable energy programme makes a significant contribution to controlling
net emissions, and we again expanded this programme in 2006/2007.
Severn Trent Water converts the methane from the digestion of sewage sludge into electricity, using combined heat and power (CHP) units. In 2006/07 its in-house renewable energy production was approximately 154,000 MWh. This represents 17% of total electricity used by Severn Trent Water.
In addition to Severn Trent Water's 34 CHP sites, there are hydro power installations at three sites - Clywedog, Vyrnwy and Ladybower - with combined electricity generating capacity of approximately 900KW.
In forthcoming years, implementation of regulations such as the Urban Wastewater Directive and the Water Framework Directive will increase Severn Trent Water's energy requirements. In order to offset that, we plan to expand our renewable energy production even further. There is an ongoing programme to increase the total number of sewage treatment works with CHP units, and we are also looking at new opportunities. These include biomass energy crops, micro-generation, wind turbines and extending hydro power.
A significant proportion of the sludge treated by Severn Trent Water in anaerobic digestion is secondary sludge, which is difficult to digest effectively. New technology being trialled at Minworth Sewage Treatment Works helps to increase its digestibility. It will enable the digesters to convert more solids to biogas, which has a twofold benefit: the additional gas can be used in the CHP engines to generate electricity; and there is less sludge for recycling.
The technology is called Sonix, and it uses high-powered ultrasound to break up flocs and cells. This increases their digestibility by up to a factor of two. The technology was developed by a consortium of water companies in the late 1990s, and Severn Trent Water supported the development through its R&D programme. Minworth is the first installation of Sonix in the UK.
The trial has been running since February 2007, and the initial performance test is due to be completed by September 2007.
Severn Trent Water is also commissioning a new process called acid phase digestion at its Derby sewage works. Acid phase digestion is a modification of anaerobic digestion, and it aims to enhance digestor performance by splitting up the microbiological processes involved. Similar processes trialled in other water companies have delivered increases of 20% in gas production and solids destruction. This again will produce additional gas for use in the CHP engines, and will reduce the volume of sludge for recycling.
Severn Trent Water's first acid phase digestion reactor is being designed for installation at Derby, and is scheduled to go into operation by October 2008.
