Articles & Case Studies

Esher Sewage Treatment Works

Posted: Tuesday 23rd May 2006


Esher Sewage Treatment Works (STW) situated about 1.5 km north of the centre of Esher adjacent to the river Mole, serves a domestic and trade population of 123,106 in the Esher catchment, which is mixed urban and rural. The works was built in 1960 and then expanded in 1982. Flows to the works arrive via deep gravity trunk sewers and pumping mains from four terminal pumping stations.

Esher STW had a new Freshwater Fisheries Directive undertaking that imposed a new Ammonia consent standard. The effluent consents changed from 25/12/4 (SS:BOD:NH3) to 25/12/2 (SS:BOD:NH3). Thames Water’s brief was to ensure compliance with the new Ammonia consent at Esher STW, by March 2005.

The sewage treatment works comprised of two streams of treatment. The first stream treated 40% of incoming flows and consisted of six primary settlement tanks, 12 slag media filters and eight humus tanks. The second stream treated 60% of incoming flows and consisted of two crude sewage oxidation ditches and four final settlement tanks. In June 2003, the Trident South Alliance, which includes Black & Veatch and Costain, were tasked to investigate options of achieving the new ammonia consent at Esther STW. In order to achieve the new consent, the following three options were investigated:

· Option 1 - Passing 100% of flows through the oxidation ditches with the addition of a 2-stage anoxic tank upstream of the oxidation ditches

· Option 2 - 40% flows through single filtration and 60% through oxidation ditches

· Option 3 - 40% flows through double filtration and 60% through oxidation ditches

· Other options of new package plants were also investigated

Option 1 was the preferred option as it achieved least cost, whilst meeting the new consent.


The original total project cost was an estimated £7M. This was considered too high by Thames Water, so the Trident South Alliance was challenged with reducing the total project cost to below £5M.

The project scope was challenged by the team, and many items removed from the scope of work. For example, replacing the primary settlement tank scraper bridges item was removed, and reducing the number of chemical storage tanks from four to two. The team also looked at re-using the existing assets on site as much as possible. The scope of work for separating the raw and Surplus Activated Sludge (SAS) process streams was defined, and most of the scope used existing pipework and structures in some way. For example, the existing SAS pipeline and picket fence thickeners used for thickening SAS were re-used for transporting and thickening the raw sludge, and an existing sludge pumping station wet well was divided into two, to use as a separate raw sludge transfer well, and the other as a liquor/drainage well.

The scope of work required for modifying the process stream with regard to the hydraulics, was optimised and the minimum work required was put forward with the hydraulics still being acceptable. This included modifying existing weir levels, increasing pipe diameters – all of which avoided major construction work in the inlet works.

One of the main innovations at this stage of the project, which resulted in a large cost saving on design and construction, was the solution of a combined pumping station and 2-stage anoxic zone within a single glass coated steel tank.

As a result of the challenges and innovations, the total project cost was reduced to £4.8M.


The scope of work of the project was:

Inlet Works Area
  • Chemical dosing in inlet works for chemically assisted settlement in the Primary Settlement Tanks (PSTs)
  • Refurbishment of Pista Grit Trap
  • Two new storm weir plates
  • Connection of two chambers via new pipework
  • New FFT flow measurement

Primary Settlement Tank Area
  • Two new weir plates in main distribution chamber
  • New pipe from main distribution chamber to PSTs 5-8 feed chamber
  • New settled sewage collection chamber
  • PST auto de-sludge system

Oxidation Ditch Area

  • 2-stage Anoxic Tank with integrated pumping station
  • Two new horizontal aerators in oxidation ditches
  • Upgrade Return Activated Sludge (RAS) pumps
  • Make RAS well hydraulically safe

Final Settlement Tank Area

  • Re-arrangement of final effluent pipework

· New washwater pumping station and sampling point

· New Final Settlement Tank (FST) weir plates on two tanks


  • Decommission Filter stream
  • New bypass to storm land treatment area
  • New site washwater pipe system

Sludge Stream

· New raw sludge picket fence feed pumps

· Re-use picket fence thickeners for thickening raw primary sludge

· Upgrade liquor pumps

· SAS thickening unit and polymer system

· Replace corroded panels in sludge holding tanks

· New air mixing in three sludge holding tanks


· New transformer

· New Generator

· New Motor Control Centre

A comprehensive project programme was compiled with input from the design, construction and commissioning teams, which gained the approval of Thames Operations. The programme was very tight, through the design, construction and commissioning phases, for the compliance date of March 2005 to be met.


Throughout the design phase, there were many further challenges and innovations made which resulted in a reduction in costs, as well as minimising construction work. Examples of some of the savings made through the design phase of the project are shown in the table below:

Scope Change

Effect on project

Re-locate new washwater booster pump set in existing RAS pumping station dry well, and re-use 20 year old redundant final effluent wet well.

Reduction in construction time, as this avoided constructing new pumping station and avoided shutting down the whole works flow to install new pumping station on existing final effluent pipeline. Net saving over £100,000.

Install Nivus-type flowmeters in existing open channels in inlet works, instead of 900mm dia magflometer on crude sewage pipe.

Avoided need for large excavation, removing concrete surround, and installing 900mm dia magflowmeter on existing 900mm dia concrete pipe. Also avoided need for works shut down. Large saving in construction costs.

Avoided the use of the existing ‘radio network and PLC5 platform’ 15 year old control system.

Reduced costs, whilst time was also not wasted working on a system which in itself needed to be upgraded.

Adopted the re-use of existing power supply from the nearby Microstrainer Building, and hence reduced length of feeder cable to the new Raw Sludge & Works Drainage MCC.

Reduction in cost and construction programme.


The contractor was able to make further savings due to increased knowledge of the site conditions and plant workings. Examples of some of the savings made through the construction phase of the project are shown in the table below,

Scope Change

Effect on project

Settled Sewage Collection Chamber moved closer to Anoxic tank

Saved 80m of 1m dia pipework, 2.5m below ground level, adjacent to shallow pumping main pipework. Reduced cost, programme and mitigated health and safety risk of working in deep trench.

Re-used existing redundant pipework for liquor/drainage pipework to North of oxidation ditch.

Saved 20m of 200mm dia pipework above ground. Reduced cost, programme. Environmental saving through re-using existing pipework.

Raised large diameter pipework between connection chambers.

Reduced excavating time, breaking out concrete in ground, reduced risk of striking underground services. Reduced need for costly 1.2m dia bends.

The construction team was also highly innovative in their methods of construction including:

  • Using hydro-demolition instead of using traditional hand-held heavy breakers, which can lead to hand arm vibration syndrome (The construction team worked over 50000 hours with zero reportable accidents).
  • Recycling the concrete from the redundant sludge drying beds to use as sub-base and capping material.
  • Night-working during the wire cutting and removal of the 37” filter feed pipes located within the settled sewage collection chamber to minimize effects on process. Close liaison with Thames Water Operations made this work possible.


The complex commissioning phase of the project required detailed planning, which was undertaken by the entire project team at commissioning workshops. The objective was to convert Esher STW from two separate process streams (filter and crude ditch) into a single settled sewage ditch stream, and define the routes and processes for the sludge flows. At the workshops, the interdependency of each item to be commissioned was analysed to determine the correct commissioning sequence. Contingency plans were also discussed in the event of any process area failing.

Esher STW was also one of the trial sites in the OM (Operating Manual) to SOM (Site Operating Manual) project initiated by Thames Water Asset Management and Waste Water Quality Management groups. The commissioning team worked together with Thames Water Operations in developing the company wide new SOM and SAMM (Site Asset Maintenance Manual) formats.


Esher STW originally had an ITC (Initial Target Cost) agreed for £4,534,093, however the projected Final Actual Cost (FAC) during the project was maintained at or below the ITC. As well as the challenges and innovations mentioned above, Subcontracts were negotiated and orders placed under the ITC value. For example, savings through ordering the SAS drum thickener from CPL Environmental Ltd. A saving of £30,500 was made as two SAS drum thickeners, one for the Horley STW project and one for Esher STW project were ordered. This successful liaison and innovative thinking in the Alliance, carried on through for the duration of the project. This can be clearly identified when comparing the Final Actual Cost (FAC) against the ITC.

During the second half of the construction phase, an additional item of scope was added into the project, a Screenings Handling Plant. Due to the short remaining time to procure, order and manufacture, good supply chain management was essential not to cause delay to the Take Over Certificate (TOC) date. The plant was procured through Spirac Ltd for £80,000, and a revised ITC was submitted and agreed. The revised ITC value was agreed for £149,452, bringing the total to £4,683,545.

The Esher STW project was completed in March 2005, with an FAC of £4,370,119. Against the ITC the total saving was £313,426.


The flow to treatment was switched to the new process stream, and the project was successfully commissioned in February 2005. The plant has been working successfully since project completion. Ann Weston, the Thames Water Project Manager had this to say about the project on a site visit in June 2005,

“I went to Esher today for a walk round with Grahame to admire the beauty of the finished project. Everything was looking very good and all the plant appeared to be working well.

She added: “You all need to be congratulated on the completion of a very successful project. Operations are very pleased with the result and we now have a works that is within the quality consent and working well. We also completed the project below Control Cost which is another bonus. I hope that I can work with you all again sometime in the future. You were a great team to work with.”

Contact Information

Julie O'Fee
Black & Veatch

Tel - 0173 785 6553
Email -

This case study was supplied by;

Christopher French
Tel - 0121 745 2666
E-mail -

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