Witches Oak Water Treatment Works of Severn Trent

To attain the goal of water treatment with low carbon emissions in line with the UK’s Green Recovery Scheme, Severn Trent Water is designing a new 89 Ml/d water treatment works at their Witches Oak site utilising PWNT’s innovative technologies.

Once completed, the Witches Oak Water Treatment Works (WTW) will be the 7th utility in the world to implement full-scale process technologies of PWNT.

The Witches Oak site will include in-line coagulation and adsorption (ILCA®) and ceramic microfiltration by CeraMac®. Suspended ion exchange (SIX®) is also being evaluated on a pilot-scale for a future asset implementation cycle.

Witches Oak Water Treatment Works of Severn Trent
PWNT are working closely with Nijhuis Saur Industries group partners to design and build the Church Wilne WTW.

Source Water Challenges

The River Trent poses many water quality challenges for conventional treatment, with difficult-to-remove organics,  nitrate and ammonia. The ILCA® and CeraMac® serve as core treatment units and each contributes in a different way to the finished water quality requirements for the treatment works. The SIX®, which is also under consideration for future implementation, could also contribute to achieve finished water quality goals, should further dissolved organic carbon (DOC) be required.

Overall Treatment Train

The planned treatment process is shown in Figure 1. Constructed wetlands will serve to treat for ammonia and nitrate, ILCA® is for flocculation, CeraMac® serves as the filtration barrier (reducing turbidity and removing pathogens such as cryptosporidium), Granualar Activated Carbon (GAC) is for polishing, and ultraviolet light irradiation (UV) is for primary disinfection. Final disinfection will be with chlorine. SIX® is a possible future addition to remove DOC.

Figure 1 - Proposed treatment train

About the Technologies

The SIX® process (Figure 2) is based on an advanced plug flow resin contacting reactor followed by a specially designed lamella settler. The settled resin is then completely regenerated and re-used. The continuous adsorption process operates under stable kinetics.

SIX® would be implemented as pre-treatment to coagulation, and is expected to remove up to 60 percent dissolved organic carbon (DOC) based on tests with this river water. While DOC is not regulated, it does contribute to the formation of disinfection by-products (DBPs) after the treated water is chlorinated which can be harmful to human. SIX® will lower the DOC concentration and thus also reduce the formation potential for DBPs. SIX® might also remove bromide, which would lessen the formation of brominated DBPs. SIX® with its anion exchange resin will remove nitrate.

One of the key advantages of the SIX® process is that it allows for the use of different types of resin from different manufacturers. An anion exchange resin is used for DOC removal, and pilot-testing will confirm the resin dosage that is needed to achieve the desired water quality goals.

As a process, the SIX® power use has been minimised with air mixing of the resin in the contactors and low pressure loss through the process. The single pass system which regenerates the resin on a timed basis means that a lower dose can be used because the resin adsorption sites are available all of the time (i.e., there is no bleed stream for regeneration leaving used resin in the system).  Also, by keeping the resin clean, optimum adsorption is maintained.

Figure 2 – The SIX® Process

CeraMac® is a microfiltration filter system with a nominal pore size of 0.1 um.  The membrane is supplied by Metawater (NGK) of Japan. This process will serve as the filtration barrier to meet finished water turbidity limits. The CeraMac® is also credited with 4.0 log Cryptosporidium, 4.0 log Giardia, and 1.0 log virus removal.

CeraMac® operates with multiple membrane elements in a single vessel. Flocculated water from the ILCA® will enter the CeraMac® system, were particulates and turbidity will be removed.  The CeraMac® system will be backwashed based on a loading to the system and a chemically enhanced backwash (EBW) regime with chlorine washes and acidic peroxide washes will be used to help control fouling during operation. Clean-in-place (CIPs) will be performed with traditional CIP chemicals: chlorine and acidic peroxide.

The CeraMac® system is proven to be a sustainable filtration system for surface water plants. This is due to several features which are optimized to minimize the emissions over the life of the plant. First, the CeraMac® system is designed to match the expected life of the ceramic membrane. In practice, this is >20 years, based on the oldest membranes in operation by Metawater being nearly 25 years old and still in operation. The CeraMac® vessel and internals are also durable, and expected to be in operation over this time span. Second, the energy use of the CeraMac® is low compared to other filtration systems. With optimised pretreatment, the fouling can be minimised, thus also minimising the energy use for pumping water through the membranes. The CeraMac® uses a high pressure backwash (at up to 5 bar), and this provides a rapid and extremely effective backwash of the membranes. This translates into less downtime for backwashing and better performance recovery after backwashing (availability generally >95%). The air-spring backwash of the CeraMac® is also more energy efficient than a pumped backwash, because the air pressure is not lost during operation, and only a low energy use is needed to maintain the system air-spring. Finally, the components of the CeraMac® system are nearly 100% recyclable.  There are no plastic components or unique potting materials that cannot be recycled.

ILCA® is the flocculation mixing stage after coagulant and pH control chemicals are added. Coagulation will remove a fraction of DOC as well as colour, which will assist with meeting DBP regulatory limits and will help achieve optimized operation of the UV system (reduces plant power usage).

ILCA® was developed by PWNT as a pre-treatment flocculation for CeraMac® filtration. Coagulation is provided upstream via a static mixer or an overflow weir for chemical mixing, and the flocculation occurs in the ILCA®. It is not necessary to form large, settleable flocs for filtration by CeraMac®, so smaller pin flocs are acceptable. These are formed in ILCA® which has a shorter residence time compared to conventional flocculation systems. ILCA® has no stirrers or mechanical moving parts, but instead uses specially designed removable plates to create the G-values that are needed. This means there is no energy use for the ILCA® system. The ILCA® can be pressurised or in tanks/towers open to the atmosphere. The design and options for the ILCA® offer a lower capital and operating cost, a smaller footprint, and lower maintenance tasks when compared to traditional flocculation options.

Minimal Operator Attention

The Witches Oak WTW is being designed to operate with minimal operator attention.

  • The SIX® contactors and regeneration system are fully automated. During normal operation, adding salt to the brine system is the only non-automated action.
  • The ILCA® is fully automated and requires no operator intervention during normal operation; only the filling of the chemical tanks will require operator attention.
  • The CeraMac® system is also fully automated. Backwashes, EBWs and integrity tests (i.e., pressure decay tests) are automatically scheduled and executed. The operator will be able to manually stop/start and request wash operations, which are automatically planned by the CeraMac® logic.
Minimal Operator Attention
The design has been developed with operability and maintainability at the forefront of all decisions.

Low Emissions Advanced Treatment

As utilities look to new sources to improve resiliency within their systems, it is quite often the case that these sources are more challenging to treat.  Advanced treatment, such as with SIX® and CeraMac®, aid in the treatment of these sources, to help meet the finished water quality goals as well as provide robust and reliable treatment.  The proposed processes for the Witches Oak WTW are also fully automated and will require minimal operator attention.  With ceramic membranes as the filtration barrier, there is also considerably less risk associated with the breakthrough of Cryptosporidium and Giardia compared to other filtration alternatives.

Utilities are also aiming to implement low carbon treatment where possible. These SIX®, ILCA®, and CeraMac® processes were designed for ease of operation, longevity and reliability. Carbon footprint evaluations have shown that these technologies use less energy and have a long operational life – two of the main contributors to carbon emissions. The CeraMac® is also nearly 100% recyclable, an important factor when looking at the whole-life-carbon-footprint of a technology.

With these benefits, the Witches Oak WTW serves as an example of the synergy of advanced treatment and low emissions operation to treat a challenging source. Customers can expect excellent water quality, that has been produced with a lower environmental impact.

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