Dŵr Cymru Welsh Water has invested £23 million at its Five Fords Wastewater Treatment Works (WwTW), near Wrexham, Clwyd in North Wales. The project includes improvements to wastewater treatment, as part of the Habitat quality improvement programme and a new anaerobic digestion plant, which is part of Welsh Water’s AMP5 Sludge Strategy. This paper explains the work undertaken from late 2010 through to March 2012 in upgrading the facility and providing improved sustainable sludge treatment in North Wales.
- SECTOR Wastewater
- LOCATION North Wales
- INVESTMENT £23 million
Sludge treatment plant:
The new sludge treatment plant provides improved sustainable sludge treatment in North Wales and has been sized to treat the additional sludge arising from phosphate removal.
The new plant includes imported sludge reception, sludge thickening, and 2 (No.) 4,000m3 anaerobic digesters, which have been designed to process up to 12,000 tDS/y.
Digester gas is collected and used by 2 (No.) high efficiency 600kWe CHP units, to provide all process heating requirements and sufficient renewable power – to move the whole works toward power self- sufficient service, with savings in operational carbon footprint and operating costs. Treated sludge will be dewatered and recycled to local farmland, as a high-value fertiliser.
The new sludge treatment plant includes 3 (No.) sludge tanker reception bays to safely receive thickened raw sludge imports. Each of these unloading bays is fitted with data logger in order to accurately monitor the source, volume and DS of the sludge received.
The indigenous primary sludge and raw sludge imports are combined in a reception sump and pumped through a series of 4 (No.) strain presses mounted on an elevated platform and the screened sludge is transferred to the screened sludge buffer. Similarly, SAS is pumped direct from the works to a SAS buffer tank.
All sludges are pre-thickened by a series of 4 (No.) gravity belt thickeners. The belt thickeners are provided with dedicated sludge transfer and polymer dosing pumps. In normal operation 2 (No.)
units are used to thicken raw indigenous and imported sludges, 1 (No.) unit is used for thickening SAS, with 1 (No.) unit available as a common standby.
Sludge digestion plant:
All thickened sludge is transferred by dedicated pumps direct to the thickened sludge storage tank and is mixed to maintain consistent characteristics. The sludge digestion plant includes 3 (No.) thickened sludge digester feed pumps (duty, duty, standby) feeding 2 (No.) anaerobic digesters, each with a working volume of approximately 4,000m3, and fitted with jet mixing and pumped recirculation through dedicated heat exchangers.
Sludge is displaced from the digesters via overflow bellmouths and flows by gravity to 2 (No.) existing tanks, providing post digestion buffer storage before conditioning and dewatering using the existing centrifuge dewatering plant. Digested sludge cake is then stored before recycling to local farm land.
Biogas flows from the digesters to a membrane type gas holder through oversized stainless steel pipework with automatic condensate traps installed at the low points to automatically remove condensate as the biogas cools.
Under normal operation, all of the biogas is cleaned to remove siloxane and used in the high efficiency CHP units in order to produce renewable power and hot water for process heating. Also, a dual fuel hot water boiler has been installed in order to produce hot water using either biogas or natural gas during periods when the CHP units are being maintained. Surplus biogas can be transferred to a waste gas burner.
In order to minimise odour nuisance all raw sludge tanks and equipment are covered and potentially odourous air is extracted and transferred to a dedicated odour control plant.