By C.Rockey and D.Metcalfe, South West Water, U.K.
Summary
South West Water (SWW) is nearing the completion of an exciting project to provide consumers with a state-of-the-art drinking water production facility. The pioneering Mayflower facility will replace an ageing water treatment works (WTW) in Plymouth and provide up to 90 million litres of top-quality drinking water to consumers in Plymouth and the South Hams, Devon. Following long-term pilot-scale research SWW and their delivery alliance H5O agreed to build an innovative pre-treatment facility comprising suspended ion exchange (SIX®), inline coagulation (ILCA®) and ceramic microfiltration (CeraMac®) (PWNT, Netherlands) at a new treatment works, Mayflower WTW. These novel technologies were selected in part due to their potential to increase the robustness of the treatment process and improve treated water quality. Clear water quality goals were established, including providing an absolute barrier to particles (including cryptosporidium); reducing dissolved organic carbon (DOC), disinfection by products (DBPs) and chlorine demand; and increasing biological stability in distribution, to ensure the investment would meet current and future challenges. Construction and commissioning is due for completion during late 2018.
Introduction
SWW has had a long-term strategic goal to re-locate the existing Crownhill WTW in Plymouth to a new site on high ground above the city on the edge of Dartmoor. This location is important as it provides a strategic link between all three of our supply areas and our largest impounding reservoir by gravity.
Over time the costs and risks of operating and maintaining the existing facility have been increasing. Meeting long-term water quality goals is becoming more challenging due to the impacts of climate change and increasing levels of DOC in the source waters. These factors alongside our desire to continue to exceed our consumer and regulatory obligations (at all times) drove the search for a solution to best meet our goals for this supply area by 2020.
As a starting point for this strategic investment SWW reviewed its goals for any potential treatment process which included:
- Robust treatment barriers ideally providing an absolute barrier to cryptosporidium
- Enhanced DOC removal to reduce the formation of all DBPs in light of the progression from prescribed regulatory standards and towards “minimising the formation of DBPs”
- Highly automatable, compact, robust and efficient process
- Forward-looking innovative approach
- Keep consumers’ bills down in the long term
The SIX CeraMac process which was in development by PWNT (Netherlands) was identified at the time as potentially meeting these goals. Following initial tests SWW invested in a large-scale pilot plant in order to gain insight into the technical, economical and operational feasibility of these processes through a long-term evaluation. The key goals of the pilot were to gain confidence and operational experience of these innovative processes whilst collecting a large data set encompassing all raw water conditions. This information could be used to inform decisions and reduce risk in any subsequent design and build process, and ultimately the long term operation of these processes.
Pilot scale research to inform design and operation
The pilot research was invaluable in:
- Fully understanding the challenges present in the SWW raw waters
- Optimising the processes to enable the most effective operation possible and confirming efficacy
- Understanding the limitations and opportunities for design
- Gaining direct operational experience and training staff ahead of any full-scale build
- Demonstrating the benefits of the innovative approach and dispelling urban/operational myths (providing reassurance/building confidence).
After two years of detailed and collaborative pilot research we determined a slightly amended process was feasible and could provide significant improvements in water quality, robustness and operability of treatment. From these conclusions we progressed to full-scale design.
SWW is a partner in the Interreg DOC2Cs project which is supported by the European Regional Development fund which supports technological innovation. The other partners within this cross-border collaboration are De Watergroup (BE), Lille University (FR), Delft University of Technology (NL) and PWNT (NL). The project partners are collaborating to accelerate the development of innovative technologies to improve DOC removal.
Innovative Processes
SIX – Suspended Ion Exchange was initially proposed as the sole membrane pretreatment for the removal of DOC and pilot results showed that SIX was very effective in removing DOC during the initial raw water conditions. Specialist organic characterisation techniques demonstrated that SIX preferentially removed the lower molecular weight (LMW) organic compounds but the removal of high MW (HMW) organic compounds was limited. When the concentration of these HMW organics was high the membranes fouled much more rapidly which lead to the need to increase cleaning of the membranes. Due to this finding a further pretreatment process was required to enable efficient membrane operation.
Research indicated that coagulation, powdered activated carbon or heated aluminium oxide particles had been previously applied as ceramic membrane pre-treatments. Despite our desire to avoid the traditional application of coagulants research indicated this would be the most effective pretreatment for mitigating fouling from HMW organic compounds. Further pilot testing was undertaken to evaluate in-line coagulation/absorption (ILCA) as a second pre-treatment for DOC removal between the ion exchange and membrane processes.
ILCA – Pilot testing confirmed that coagulation flocculated the HMW organic compounds (the opposite MW fraction removed to that that is removed by SIX), preventing them from strongly binding to the membrane and allowing them to be removed with a chemical-free backwash. Due to the removal of a large portion of the DOC present in the raw water by SIX (the first stage of the process), the coagulant dose required was significantly (>50%) reduced. This fact in combination with the very high solids loading capacity of Metawater ceramic membranes made inline coagulation possible – a process where the coagulant and pH correction chemicals are injected into the forward flow / with no clarification process prior to filtration. The design of the in-line flocculation zone was optimised from pilot testing at both SWW and PWNT research facilities to ensure the coagulant/DOC flocs presented to the membrane had the correct characteristics.
CeraMac – Ceramic membranes are significantly more robust than the polymeric membranes typically used in water treatment and can run at higher fluxes/productivity, be cleaned more aggressively to fully restore permeability, have a longer life etc. Ceramic monolith membranes offer benefits over polymeric membranes, due to the removing the risk of fibre breakage and subsequent operational repairs. Whilst they are more expensive per unit filtration area than polymeric membranes, they have longer lifetimes prior to replacement (e.g. 20 years vs. seven for polymeric membranes) and can achieve higher flux/productivity which on a whole-life costs basis the economic case for investing in ceramic membranes may be made. Optimising the treatment to achieve a high membrane flux was a major goal of the pilot work, to further improve the economic feasibility and reduce the footprint of the process.
During pilot work with the membrane we also focussed on the use of various chemical cleaning regimes including ozone and chlorine. This research showed that ozone was more effective than chlorine in restoring the membrane condition and the waste streams were simpler to remediate and less environmentally damaging than chlorine solutions. This insight from the pilot research was one of many applied to the full-scale design for Mayflower WTW.
Water Quality benefits
As SIX and CeraMac removed opposite fractions of DOC, they are very complementary – when combined (SIX followed ILCA) very high removal of DOC can be achieved. Pilot-scale testing showed residual DOC levels could be halved in comparison with a conventional coagulation sand filtration process, despite a much smaller coagulant dose. Treatment to reduce levels of DOC is important and in our situation the DOC reduction is expected to deliver:
- Lower DBP formation potential – pilot testing showed that DBPs were reduced by c 60%
- Less chlorine required and lower more stable chlorine residuals to consumers – chlorine demand and decay were shown to be significantly reduced, improved acceptability
- Improved operation and efficiency of downstream processes:
- Membrane operation – higher fluxes, lower operating pressure, reduce chemical cleaning, simpler operation achieved, lower cost
- Operation of UV disinfection – more stable and higher treated water UV transmittance and the provision of two independent DOC barriers (SIX and ILCA) reduces risk and operational cost
- Useable life of granular activated carbon (GAC) is increased by reducing DOC and in particular the lower molecular weight fractions that are targeted by ion exchange. Less frequent regeneration leads to lower costs, operational risk and disruption
- More stable treated water quality reducing biological re-growth in the distribution network and domestic plumbing systems, reducing the risk of the negative impacts on appearance, taste or smell
Conclusion
Pilot research has enabled South West Water to work with its H5O Delivery Alliance and PWNT to engineer an innovative drinking water treatment facility on the edge of Dartmoor. The Mayflower will soon provide South West Water the benefits of enhanced removal of DOC and the robustness of a ceramic membrane filtration. Meeting our goals and most importantly ensuring consumers can trust their tap water is of the highest quality and always good to drink.
