Water Service is standardising its approach to small-scale, rural wastewater treatment. The ongoing work at Loughguile is a typical example of its strategyWork currently being undertaken at Loughguile, in County Antrim, exemplifies nicely the approach being adopted by Water Service to the rationalisation and standardisation of its small rural works (see p13).
The Loughguile scheme falls under the utility's northern framework for small-to-medium (£250,000-£5M) WwTW and sewerage projects. Operating the northern framework is Consent Solutions a joint venture between Black & Veatch and Northern Irish civils contractor Dawson WAM. The other frameworks are:
- East - Biwater/Graham
- South - Bernadette Seamus Gillan
- West - Shearwater (Purac, Charles Brand, Atkins, Gida)
Water Service undertook its own study of the Loughguile catchment's current and enhanced PEs and gave Consent Solutions the raw data upon which to assess process options. The final treatment solution was agreed at a joint workshop with the client and its regional project manager, RPS Consulting Engineers. The project is valued at £3M with a 60/40 civils/M&E split. Black & Veatch's role includes all of the civil, mechanical, electrical and process design - including that for the off-site pumping stations, sewerage system and system hydraulics.
Under the scheme, a WwTW serving 30 dwellings at Ballyknock will be decommissioned and replaced by a pumping station directing flows through a new 1,700m pipeline to Loughguile. The works at Ballyknock occupies a large site, and its replacement with a pumping station may allow Water Service to sell off a significant portion of the land. The works at Corkey will also be decommissioned and converted to a pumping station. Flows from a pumping station at Ben Vista, which currently feeds Corkey, will be reversed through a new 1,700m pipeline to carry combined flows from Corkey and Ben Vista to the new works at Loughguile. The new pipeline from Ben Vista to Loughguile is 1,950m.
The existing works at Loughguile occupies a constrained site and comprises inlet works with overflow system and automatic screen. This is followed by primary settlement, sludge draw-off tank, media filter and final settlement. A number of years ago, a reed bed was constructed as a temporary measure to polish effluent and maintain compliance. Discharge is to a small river. Niall Rogers, Black & Veatch's project manager, described the existing setup as "a typical rural works, exactly what you'll find across the province".
The new works is being constructed within the boundaries of the existing site, and the current WwTW is to remain online and within compliance throughout the upgrade. Another challenge was the site hydraulics. Although the inlet sits slightly above the works, Rogers describes the rest of the site as "like a billiard table", which did not give designers an awful lot to play with to achieve a gravity-fed treatment train.
The process driver at Loughguile is a stringent ammonia consent of 1mg/l, which in itself eliminated a lot of process options. Optioneering narrowed the field down to a choice of MBR or SBR, the joint Water Service/RPS/Consent Solutions workshop finally settled on the former. The MBR's small footprint being a significant factor, it would have been a struggle to fit a more conventional form of treatment on such a confined site. As part of its standardisation drive, Water Service has invested heavily in MBRs in recent years. The utility has "really embraced the technology", observed Rogers.
A Copa flat-sheet MBR system is being used, but Black & Veatch opted for a four-cell, as opposed to two-cell, configuration. Maintaining the correct MLSS level is essential to sustain effective MBR treatment, but this parameter is dependent upon influent loading. Having four cells gives the works' operators more control over MLSS levels because cells can be taken on or off line in response to influent composition. The number of cells being brought online will be determined at commissioning stage and will depend upon influent composition.
In a further example of the flexibility four cells provides, Rogers noted that if a problem develops in a cell on a two-cell configuration, only half the flow can be passed. With the four-cell system unique to Loughguile and Pomery, however, the membrane packs have been sized to pass full flow on three cells.
Although the site is fully automatic, the choice of an MBR may increase the level of maintenance Water Service will have to undertake, when compared with the existing process. This, according to Rogers, is a trade-off the utility has had to make in order to meet the ammonia consent: "It will require some thorough maintenance because you have some pretty high-tech equipment, with lots of instrumentation which - in comparison to what was here originally - is a great leap forward."
Using an MBR also required a specially designed inlet works. "Effective up-front screening and grit removal is of paramount importance for the well being of the MBR process," says Rogers. There have been some instances of unwanted solids and grit bypassing the screens and entering the MBR cells resulting in damaged or clogged membranes. To prevent this, a custom-made inlet package was designed in conjunction with Haigh Engineering. The package, with standby on every element, has a stone trap and storm overflow screen upfront, then provides 6mm then 3mm screening. The inlet works, described as "a pretty beefy piece of M&E equipment" is built of standard Haigh components, configured in a slightly unorthodox manner. The package cost £200,000.
The original upgrade design called for a circular sludge tank sited above ground in a corner of the site and the storm tank to be constructed on the site of the existing reedbed. Following a value engineering exercise, however, it was decided to incorporate these structures in the single concrete structure, which also houses the MBR cells and control gallery.
As part of Water Services' standardisation drive, the MBR and inlet designs used at Loughguile have been adopted at other sites in the Consent Solutions region. One such example is Pomery in Tyrone. Pomery is slightly larger than Loughguile with 2006 PE of 1,267 and 2030 enhanced PE of 2,000.
Pomeroy, is an even more constrained site, but does not present the same hydraulic challenges. "The site," says Rogers, "is like a cliff face." The topography has made work difficult and a substantial bite has been taken out of the hillside to accommodate the MBR, sludge and storm tank structure.
Rogers is keen to point out the extent to which local suppliers have been involved in both schemes. All the controls and panels have been produced locally, as have the generators. The M&E installation has been undertaken by a contractor from the province, as was the new sewerage work and site pipework.
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