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Coppermills AWTW

rapid gravity filters extension

by Stewart Bell

 (published September 2017)

 

3D model extract showing detail of one of the proposed raw water connections - Courtesy of eight2O

 

Coppermills AWTW is located in North East London and treats reservoir stored water, producing around 500Ml/d of treated water. Construction of the first phase of Coppermills Water Treatment Works were completed in 1969, with a further phase to add primary filtration completed in the 1970s. Modifications to uprate the output of the works to its current design maximum of 680Ml/d was undertaken in the 1980s with links to the London ring main added later that decade. In the 1990s interstage ozonation was incorporated prior to the slow sand filters which had a layer of GAC added to enable the treatment of pesticides. More recently, a transfer pumping station to allow movement of treated water to/from the ring main was added in 2012.

 

Background

The Lea Valley raw water network supplies the works with stored water from the King George V, William Girling, Lockwood, Banbury, High Maynard, East and West Warwick, and Walthamstow No. 4 and No. 5 Reservoirs which are charged variously by abstraction from the River Lea and the New River. The stored water flows to Coppermills AWTW by gravity. There is a small pumped supply from Coppermills Stream which is fed from the reservoirs to maintain a compensation flow in the stream. Raw water is also abstracted from the River Thames at Hampton and delivered to Coppermills by a 100” diameter tunnel to the Lockwood Pumping Station, which feeds Lockwood, Banbury and High Maynard Reservoirs.

 

The works provides several stages of treatment to produce a reliable supply of potable water. Firstly, primary rapid gravity filters (RGFs) remove a large proportion of the suspended solids and algae in the raw water. The filtered water is then ozonated to oxidise dissolved contaminants prior to slow sand filtration (SSF) where the break down products are removed by a range of mechanisms including filtration, adsorption and biological assimilation. Finally, the filtered water is disinfected by chlorination and then chemically conditioned before entering the distribution supply network.

 

Aerial view of part of Coppermills work, showing the existing RGF block (24 (No.) filters top left), new RGF plant is to be constructed in the position of the maintenance buildings and 3 oil storage tanks below (middle left) - Courtesy of eight2O

 

Raw water supply

Historically, the works operators have managed the quality of the raw water supply by careful selection of the feed source.

 

Each of the raw water reservoirs is different in size and layout, which generally results in varying levels of algal activity at any particular time. Over recent years however, environmental changes have led to increased algal activity which means the operators have found it more difficult to manage raw water abstraction effectively.

 

Consequently, the output of the works has been significantly reduced due to the poor quality of water entering the site during algal bloom periods. Typically, during the spring algal bloom multi-celled filamentous algae blind the rapid gravity filters, resulting in the need for more frequent backwashes and loss of filtered water production. This impacts the operation of the rest of the works and reduces the amount of water supplied into the London zone.

 

The water quality experienced in the spring algal bloom in 2008, for example, reduced the output of the works to below 380Ml/d.

 

3D model extract showing the proposed new RGF plant - Courtesy of eight2O

 

Solutions

Thames Water has studied a range of possible solutions to improve/reduce the risk to Security of Supply associated with these algal bloom periods. These considered the nature and quantity of the various algal species experienced, along with their impact upon the current treatment process. Following further exploration of the various solution implementation requirements, these studies concluded that the lowest whole life cost option was to improve the primary filtration at Coppermills, to ensure that up to 550Ml/d of filtered water can be produced, even during periods of poor raw water quality.

 

More detailed investigations and trials of the various options to achieve this improvement in primary filtration considered refurbishment/upgrade of the existing RGF block, a separate new RGF extension and novel pile cloth media filtration. These further studies concluded:

 

Refurbishment/upgrade of the existing filters was theoretically viable using pumice or filtralite in a dual media configuration although a number of key risks remained. These included; increased hydraulic loading rates, a lack of available driving head and concerns regarding potential media loss from the relatively shallow existing filters.

 

The pile cloth media filtration option, either ahead of or in parallel to the existing RGF’s was unproven as a potential solution and the technology is currently unapproved for drinking water treatment. Additionally, further development work would need to be undertaken to evaluate long term performance criteria and to develop new drinking water specific cloth media plus effective cleaning strategies. Although the technology demonstrates a number of potential benefits including small footprint, there remain significant development risks which could not be addressed within the project timescales.

 

The value management process considered both residual risk and solution whole life cost and this clearly indicated that a new parallel block of asset standard RGFs with increased hydraulic gradient, dual media and collapsed pulse washing provided the lowest risk value solution.

 

Residual risks remain around the performance of these new filters with respect to their performance in removing filter penetrating algae (centric diatoms) which can lead to reduced output from the SSFs downstream. It is unclear whether the recent prevalence of this species in recent years is part of a longer term change in algal population or a short term deviation from the norm.

 

3D model extract showing the proposed new RGF plant - Courtesy of eight2O

 

Undertakings

SMBJV (Skanska, MWH & Balfour Beatty JV - part of Thames Water’s eight2O AMP6 capital delivery alliance) has developed the detailed design for a new block of RGFs which will operate in parallel with the existing RGFs and be capable of treating up to 200Ml/d.

 

This new filter block will be delivered along with additional treated water high lift pumps and a new surge protection system. A new workshop will also be provided as the existing one is being demolished to make space for the new filter plant.

 

The filters

The filters have been designed in accordance with Thames Water’s asset standard for rapid gravity filtration upstream of slow sand filtration. 12 (No.) 90m2 twin cell, dual media (sand/anthracite) units will be provided; sized to operate at 8m/h when one filter is off-line for backwashing. If one filter is out of service for maintenance, the filtration rate can increase to 8.8m/h when one of the remaining 11 (No.) filters goes off line for backwashing.

 

The clean backwash system is equipped to provide up to three washes per filter per day. This elevated wash frequency may be required under extreme algal loading conditions and if this were the case, the new filter block can still produce 160Ml/d by reducing the filtration rate to 6.9m/h.

 

Collapsed pulse (simultaneous air and water) washing will ensure the filters are effectively cleaned after each cycle and chlorinated water will be used for part the backwash sequence to control biological fouling of the underdrains. Further detailed consideration has been given to minimise the impact of weed growth and zebra mussel accumulation in the new filters by minimising surfaces where this can occur and by providing safe access for future cleaning operations.

 

3D model extract showing the proposed new RGF plant and raw water connections

Courtesy of eight2O

 

Off-site manufacture/on-site assembly

A modular, off-site manufacturing and on-site assembly approach has been adopted for the project to minimise time on site during the construction phase. The RGF shells and clean backwash tank will be constructed using the Dutchland system which is a post tensioned, precast concrete solution supplied by Shay Murtagh.

 

Modular Leopold XA block floors from Xylem will provide the filter underdrains system; with launders, access walkways and frontal pipework pre-fabricated in steel and assembled on site by sub-contractor MEPS.

 

The benefits of BIM have been fully embraced for the project with detailed models developed for each phase of delivery; BIM360 has been used to coordinate and communicate the design effectively to all parties. Furthermore, 4D Syncro planning has enabled project assembly to be rehearsed prior to construction thus giving a high level of confidence in the programme.

 

Integrating the new filters

Much of the focus in design has been on how the new filters will be integrated within the existing treatment process and hydraulic profile. The new filters are designed to work in parallel with the existing filters and supply filtered raw water to the west ozone re-lift pumping station which in turn feeds the ozone contact tanks and SSFs.

 

There are 24 (No.) existing RGFs arranged in four banks of six units. Each filter has a filtration area of 155m2 and contains 500mm of 14/25 sand as filter media with a 350mm support layer of gravel.

 

The filters have PCI A-type filter floors with earthenware laterals. Air scour is provided by a partial separate air lateral system, with some air also introduced via the water laterals. The existing wash rates are not sufficient to provide collapse pulse conditions during the combined phase or adequate fluidisation during the rinse phase.

 

There are four distinct raw water inlet shafts which feed the rapid gravity filters:

 

►  The Spine Tunnel serves the majority of the rapid gravity filters (shaft W).

►  RGFs 1 and 2 are typically supplied by Walthamstow No. 5 Reservoir (shaft X).

►  RGFs 7, 8, 13 and 14 by the Coppermill Stream (shaft Y).

►  RGFs 19 and 20 by the East Warwick Reservoir (shaft Z).

 

The two northern banks of filters (RGFs 1 to 12) and the two southern banks of filters (RGFs 13 to 24) are normally washed independently; therefore it is possible for two RGFs to be undergoing a wash simultaneously. It is possible for two RGFs from each bank to be off-line for washing; one RGF undergoing a wash while the other RGF is either in drain down or the re-start stage of the wash sequence.

 

3D model extract showing detail of one of the proposed raw water connections

Courtesy of eight2O

 

Filter standards

Thames Water’s standard for the new filters is based on a requirement for dual media with air scour and backwash rates that provide collapse pulse conditions during the combined air scour backwash stage followed by a fluidising rinse/re-grade stage.

 

This has been demonstrated to provide much greater resilience to algal challenges enabling plant output to be maintained and gives an opportunity for OPEX to be reduced during periods of good quality raw water. However, the deeper dual media configuration and increased clogging head allowance means that gravity feed is not possible so pumped connections from the existing raw water feeds are required.

 

The elevated levels do allow dirty washwater to drain by gravity to the existing double D culverts that conveys the washwater from the existing RGF block and east and west effluent pumping station to Reservoir No 3 for settlement.

 

Due to a number of constraints including site layout, existing pipework configuration and sheer scale, the decision has been taken to connect to shafts X, Y and Z but not W. Historically, shaft W supplies the better quality raw water during algal bloom periods and this will continue to be directed to the existing filters whilst the new filters will treat generally poorer quality water from the other sources. Under challenging raw water conditions, the new block will treat up to 200Ml/d of raw water thereby reducing the hydraulic load on the existing filters. When raw water quality conditions improve this can be reduced to 75Ml/d to minimise OPEX with the existing filters treating an increased proportion.

 

3D model extract showing detail of one of the proposed raw water connections

Courtesy of eight2O

 

To minimise the risks and impact of the connections to the raw water shafts a solution has been devised whereby canister pumps will pump directly from new saddle connections to the existing raw water feed mains. This avoids the need to construct separate wet wells in a heavily congested area, significantly reduces the depth of excavations and avoids future problems with cleaning and access.

 

Programme

Construction of the new filter plant is scheduled to commence with the construction of the new clean backwash tank in July 2017. Commissioning will start late 2018, with handover of the filter plant in June 2019.

 

The editor and publishers would like to thank Stewart Bell, Head of Water Process with Skanska Utilities, for providing the above article for publication.

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