The thermally dried sludge rolling off the production line at Bran Sands recently landed Entec and client Northumbrian Water the Engineering Council's annual award for the Built Environment.
Sludge specialists from all over the world have beaten a laudatory path to the £70M Regional Sludge Treatment Centre at Bran Sands since it began work last August. Northumbrian Water is now able to end 60% of the North East's sludge being dumped at sea.
Entec's sustainable solution heats and pasteurises raw sludge at 400degreesC in a closed loop system that recycles thermal heat around the plant, allowing for a slight bleed of gaseous emissions. By maximising energy efficiency, minimising emissions to the atmosphere and producing sludge pellets of high calorific value, Bran Sands boasts the first sludge plant to achieve authorisation under Integrated Pollution Control (IPC) legislation.
Entec project manager for the RSTC Tony Hill said: 'We have been awarded a licence by the Environment Agency for operation of a plant that has acceptable emissions to land, air and water.'
Entec's recommendation to NWL, to thermally dry up to 90,000t of sewage sludge a year from the North East for the next 20 years, was made with the Government's waste hierarchy - prevention, reduction, reuse, recovery and disposal - in mind.
Since sewage sludge cannot be prevented or significantly reduced, NWL has opted for maximum re-use and has an end product that could be gasified into a synthetic fuel, providing energy for the Bran Sands site into the new millennium.
'Most sludge drying plants in Europe treat digested sludge. Digestion reduces the carbon content in the sludge and produces methane gas. Drying raw sludge is even more sustainable because it retains a higher calorific value and therefore produces a more valuable end product,' says Hill.
Sludge entering the RSTC at 3-6% dry solids is pumped through the dewatering presses, producing a 30% dry solids 'cake'. Residual sludge liquors are returned to the neighbouring Effluent Treatment Works. The cake is then dried in three 4.8m diameter drums rotating like gigantic tumble driers. The dried sludge, now at 95% dry solids is transferred to a pelletisation plant. Finished pellets are cooled by air and transported first to storage silos and then into lorries.
One tonne of pellets emerges from the drying plant for every 20t of sludge going in. The pellets can be used as fertiliser in griculture, horticulture and forestry or as a source of carbon for steel, bricks and cement manufacture.
Pellet versatility is vital if NWL is to avoid the landfill option, which is becoming prohibitively expensive as a result of taxes. The tiny pellets could ultimately replace the natural gas that powers the combined heat and power plant at Bran Sands.
By heating the pellets at 850C in a reduced oxygen zone, a synthetic gas of hydrogen, carbon monoxide and methane is produced which could provide fuel for the adjoining CHP plant.
Gas produced would be cleaned, compressed and used to power a third gas turbine that would deliver 5.5MW of electricity and 7.5MW of usable heat. The gasification plant, using up to 42,000t of dried product a year, could be extended and the existing two turbines on the CHP modified for 'pellet power'. Each tonne of pellets would produce one MW/h of electricity, leaving 200kg of ash that could be used in building materials.
Entec is designing the gasification project which has been partly funded by a 1.5M ECU EU THERMIE grant. Construction is planned to start this year and to be on line by October 2000.
Successful completion of the RSTC project relies on the integration of the drying plant with the CHP and gasification plant and this responsibility lies firmly with Entec. Hill is confident this can be achieved. 'We were responsible for the integration of the Andritz plant with the whole of the sludge treatment centre and the rest of the Bran Sands site,' he said.
'Working closely with the various contractors was shown to have paid off when the project was delivered on time in August 1998 after 15 months of construction.'
Dredging International (UK)
N G Bailey