Severn Trent spends £18M on flagship STWs
Severn Trent has upgraded Cheltenham's Hayden STW, dramatically improving environmental performance at a cost of £15M. Secondary treatment has also been added to Lydney STW, one of Severn's few coastal works. Mary Monro reports.
"The overall aim of the work is to enable the River Chelt, into which effluent is discharged, to achieve its Class 2 river quality status," explained Chris Moore, Severn Trent¹s assistant sewage treatment manager for North Gloucestershire.
"Effluent makes up around 90% of the total flow of the river during dry weather, so the effluent itself has to achieve this standard." Consent levels were; BOD 25mg/l, SS 35mg/l, and ammonia 10mg/l (summer) or 15mg/l (winter).
However, when new levels of 10mg/l BOD, 20mg/l SS and above all much reduced ammonia levels of 3mg/l (summer) and 5mg/l (winter) were imposed by the EA, it became clear the treatment process had to be improved and the old bacteria beds had to go.
Treatment is now by the activated sludge process (ASP) a process which Severn Trent is installing at most of its STWs.
To make way for the new treatment stages, the original filter beds and sludge drying beds had to be demolished.
The ASP consists of a diffused air plant with four aeration lanes, with a combined average treatment rate of 35Ml/d and a maximum of 75Ml/d. Average retention time in the lanes is around 11hrs.
Work has also been carried out on the storm overflow tanks, to minimise pollution risk.
The first of the five tanks has been converted to a blind tank to trap sludge and the walls raised to give a capacity of 10,000m3. Allied to this change, the sludge handling process has been upgraded. Two new digesters constructed, replacing the original single digester installed in the 1970s. There was also a requirement to retain as many of the existing assets as possible. Structures such as the detritors have been kept and fitted with new M&E equipment.
Much of the rubble from the demolition work has been reused on site, minimising costs and traffic on local roads.
The outline design work was carried out in-house, with Amey Construction as principal contractor, Wabag as sub-contractors for the M&E work, Haswell Consulting Engineers as detailed designers and Severn Trent staff as site managers.
Mr Moore said the project was quite a challenge, as the works had to be kept in operation and it was not the only project under way in the area. He said: "Hayden is the largest works in the area, but we also had similar work going on at Tewkesbury and Brockhampton STWs. The consultant for one was the same, but there were different contractors and different suppliers, which made things a bit complicated."
There was another element which further complicated the situation at Hayden. The plant is effectively used a showcase, as one of Severn's visitor centres.
Parties of children from schools in the area and other groups are welcomed and it was felt that as far as possible these visits should continue. Some of the redundant tanks have been converted to a pond and others to demonstrate filtration processes, as part of educational scheme in conjunction with a local school. Safe routes and procedures were worked out to allow the visits to continue during construction work.
Severn Trent is often thought of as the only land-locked water company in the UK. However, at Lydney STW on the Severn estuary, the company has been faced with the same problems as companies with long coastlines. Here, the estuary is wide enough to be classified as a coastal area. The STW handles a mainly domestic flow (15% trade waste) of 30Ml/d. For years, treatment was limited to screening followed by primary settlement, with settled sewage then being discharged directly into the estuary.
Now, however, following the requirements of the urban wastewater treatment directive, consents of 25mg/l BOD, 150mg/l SS and 125mg/l COD have been set by the EA, requiring some form of secondary treatment.
Following an initial appraisal, and taking into account the required consent levels, an ASP design was again chosen as the most suitable option for treating the flow to the plant of around 12Ml/d. All of the design works were carried out in-house by Severn Trent's process engineering division at Warwick, who came up with a proposal for a non-nitrifying, four-chamber ASP, using diffused air. Each cell is 19m2 and has a retention time at average flow of just 6 hrs.
The contractor, Birse, had to take into account the possibility of flooding as Lydney is in an area of very low-lying land where the River Lyd flows into the Severn estuary. When the tide comes in, the water levels can rise extremely fast.
This hazard also had to be taken into account in the design. All new installation work has been designed to withstand a potential 100 year flood level of 9.5m AOD.
Fortunately space was not a problem in this case and the new ASP was constructed on a greenfield site alongside the existing primary settlement tanks.
The sludge-handling facilities have also been upgraded. Any surplus activated sludge is now pumped to a new belt press upstairs in the former presshouse and dosed with liquid polyelectrolyte.
The thickened sludge is discharged to a new holding tank which has been added to the existing three conditioning tanks, and which has been fitted with a cover and odour filter.
Installation of the activated sludge plant has increased power requirements and an additional supply line has been installed. The work has cost nearly £3M, but has been completed well ahead of deadline.
The first series of samples, taken after the ASP had time to become
biologically stable, showed the effluent to meet all of the EA's discharge