Langsett Water Treatment Works Magnetic ION exchange (Miex®) Plant
As part of the Yorkshire Water AMP6 Large Treatment Stream Framework, Morgan Sindall Sweco Joint Venture (MS2JV) were appointed by Yorkshire Water to design and deliver works at Langsett Water Treatment Works (WTW). The works, as agreed with the Drinking Water Inspectorate (DWI), included the provision of a Magnetic Ion Exchange (MIEX®) Plant to remove colour from the upland raw waters.
Langsett Water Treatment Works (WTW) is located 17km north west of Sheffield in South Yorkshire, on the edge of the Peak District National Park. The WTW treats raw water which is extracted by gravity from two upland impounding reservoirs at Langsett and Midhope. In low water conditions, extraction is supplemented by low-lift booster pumps. The works supplies 60MLD to the Sheffield and Barnsley areas.
In common with many upland sources, both Langsett and Midhope have suffered from increasing levels of raw water colour originating from the extensive peat moorlands within the reservoir catchments.
The existing treatment process at Langsett WTW comprised:
- Inlet works
- Chemical dosing/coagulation
- Clarification using superpulsators
- Rapid gravity filtration
- Manganese removal
To offset the impact of deteriorating raw water colour and to continue to supply water of an acceptable quality to the distribution network, increasing amounts of coagulants had been used to aid the flotation and filtration of organic compounds. This resulted in solids overloading the existing WTW which in turn had an adverse impact on the treatment capacity. Based on trend data, YWS forecast that the raw water colour would continue to deteriorate over time, leading to a further reduction in the works capacity.
An additional risk related to coloured upland raw water is the formation of Trihalomethanes (THM), a Group B carcinogen, that can result from the chlorination of colour not removed by existing treatment process.
YWS had a Drinking Water Inspectorate (DWI) undertaking to address the raw water colour issues at Langsett WTW by 31 December 2019.
YWS’s notional solution to the raw water problem was the installation of a Magnetic Ion Exchange (MIEX®) plant as a pre-treatment process upstream of the WTW. MIEX® is an innovative ion exchange process technology that has been developed and supplied by US-based IXOM Watercare Inc. and is used to remove Dissolved Organic Carbon (DOC) and to reduce Disinfection By-Products (DBP’s) such as THM at water treatment plants. Langsett was YWS’s fourth MIEX® plant and is the first of the ‘next generation’ MIEX® installations, incorporating enhancements based on 10-years of operational learning from plants at Albert WTW (Halifax), Ewden WTW (Sheffield) and Graincliffe WTW (Bradford), as well as IXOM’s improvements in resin technology and performance.
The MIEX® process uses a specially developed magnetised ionic resin to attract the negatively charged dissolved organic carbon (DOC) compounds that can cause colouration in untreated water. The raw water enters a high rate contactor vessel where it is mixed with MIEX® resin and the ion exchange process takes place, as negatively charged target ions within the DOC exchange for chloride ions on the resin’s active side in a process known as adsorption. Flow then enters lamella separators where the MIEX® treated raw water is separated from the resin, the magnetic resin beads forming large agglomerates that settle to the base of the unit. Resin is recycled to the contactor with a small proportion being sent for regeneration. This is an integral part of the process which involves the resin being separated from the water and mixed with brine to exchange with chloride target anions off the resin. The brine is generated on site as part of the process using salt saturators.
The MIEX® process requires a compact physical footprint and results in a small waste stream. The special gold-based resin is a costly component at approximately £12,000/tonne, but it is continuously regenerated within the process, with only a nominal requirement to replace small quantities lost as consequence of normal process operation.
At Langsett WTW, the MIEX® process acts as a raw water pre-treatment phase, upstream of the existing treatment processes. This involved the construction of complex connections to the live raw water mains to feed the new process. These critical connections were designed to limit the duration of any shut-down periods for the existing WTW. The enhancements provided by the project also included:
- Inlet works at the head of the MIEX® process
- Site service water and washwater pumping station and associated site-wide distribution pipework
- Power upgrade system linked into the new and existing works (comprising HV Transformer, LVDB, Standby Generator and fuel oil reception/storage area)
- Site access roads
The scheme was delivered by MS2JV (a design and build joint venture between Sweco and Morgan Sindall Infrastructure) under an NEC Option C target cost delivery contract as part of YWS’s AMP6 Large Treatment Framework. YWS entered into a parallel design contract with IXOM to supply the MIEX® process design and equipment; MS2JV then ordered the MIEX® plant and media direct from IXOM.
YWS’s notional feasibility design was taken by MS2JV and refined to improve constructability and process efficiency, as well as to maximise the use of gravity-fed raw water from the reservoirs. A fully detailed design was developed including use of a 3D REVIT design model to incorporate all construction works and M&E installation packages. The model was stored in a Common Date Environment accessible by all sub-contractors and the construction team. This enabled a greater co-ordination between the various designers, and proved to be of great benefit in clash detection for a project with considerable and complex M&E components. The model also assisting in virtual realisation and assessment of human factors such as access and lifting. The project utilised a BIM compliant document numbering system to BS 1192.
Compliance with YWS’s process safety requirements was paramount throughout development of the detailed design. Every area of the proposed works was subject to the IChemE 7-stage hazard study process including different stages of detailed hazard study and access, lifting and maintenance reviews. Throughout the design process, MS2JV worked closely and collaboratively with IXOM and the wider supply chain to maximise the potential for programme and safety/accessibility benefits resulting from constructability improvements.
Construction work began on site in November 2017 and was substantially completed in July 2019. The MIEX® Plant comprises a complex reinforced concrete substructure set substantially below ground level within the existing quarry floor. This maximised available reservoir capacity via gravity feeds from the Langsett and Midhope reservoirs, as well minimising visual amenity impact with respect to the site’s location immediately adjacent to the Peak District National Park. In total, approximately 6,000m3 of fractured sandstone was excavated, crushed, screened and re-used in construction on site, thereby avoiding hundreds of potential off-site vehicle movements in a highly environmentally sensitive location.
The proximity of the existing eight-metre high quarry face immediately adjacent to the works area necessitated consideration of measures to protect personnel and assets from potential rockfalls both during construction and on completion of the works; to address this, a system of temporary and permanent rockfall netting was installed around the working area.
The MIEX® Plant substructure comprises a multi-level/multi-compartment water-retaining reinforced concrete box structure up to 8m in depth, incorporating various chambers, channels, openings, and cast-in pipework connections. Although the structure was significantly rationalised and simplified from the outline design concept, the layout remained complex and consequently a considerable number of concrete pours were required to form the finished structure. Approximately 1430m3 of structural concrete and 200t of reinforcement were used in the substructure construction, that involved 35 separate concrete pours.
The substructure was substantially completed and hydraulically tested in September 2018, this to allow erection of the superstructure. This comprises a steel portal frame building 33m long x 22.5m wide x 11.5m high with external profiled steel sheet cladding. The portal frame incorporates longitudinal crane rails at high level to accommodate a 5t SWL travelling crane beam which can service the whole process area footprint within the building.
The completed MIEX® structure was handed over for M&E installation in November 2018. Mechanical equipment specified and supplied by IXOM was installed by MS2JV’s subcontractors in accordance with the requirements of IXOM’s design. The M&E installation proved the benefits of using a 3D model as a collaborative working tool as it facilitated a short, intensive but safe installation period within a confined working area. The standard use by MS2JV of CLIP (Construction Lean Improvement Programme) techniques with direct subcontractor participation contributed significantly to the overall success and timely completion of this critical phase of the project.
Commissioning of the MIEX® Plant and associated works commenced in June 2019, with process trials beginning in August 2019. Beneficial Completion was achieved in mid-November ahead of the DWI Regulatory Compliance Date of 31st December 2019.
The added value that Sweco delivered as part of the project included:
- Compliance with YWS’s Process Safety requirement including the facilitation of the IChemE 7-stage hazard study process for each process area
- Optimisation of hydraulic design to maximise the reservoir capacity available via gravity feeds
- Use of a 3D REVIT design model for both clash detection and virtual realisation/assessment of human factors such as access and lifting
- On site re-use of 6,000m3 of fractured sandstone
- Collaborative delivery approach that included multiple suppliers and sub-contractors
The project outturn was £17.3m, this against the £18.4m target cost.