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.

Severn Trent Water has completed a £15M upgrade of Cheltenham’s Hayden STW.

The plant, which serves a pe of around 105,000, has been dealing with sewage

since the turn of the last century.

“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


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


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


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.

Coastal Project

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


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


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

consent standards.

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