Northern Ireland: Swatragh WwTW

DAWSON-WAM Ltd. was appointed by Northern Ireland Water to design and construct a new wastewater treatment works, on an existing site, to introduce new advances in the wastewater treatment technology to an existing outdated filter-bed facility. The works included the construction of an inlet works, primary, secondary, final treatment plants and sludge storage facilities.

Swatragh is a small rural treatment works for Swatragh village, Co. Londonderry. It serves a current population equivalent of 722 which is expected to grow to 934 for a 2035 design horizon.

Solution development included:

Design of Process, MEICA and Civil works which included the accurate sizing and selection of Primary Settlement, RBC Secondary Treatment Units, Final Settlement Tanks to meet both current and future flows and loads.  During design development it was discovered that a weekly surge in flows to the works was accounted for by discharges from an agricultural/cattle market.  We designed the works to take account of the increased surge flows and loads whilst still being able to maintain treatment and discharge consents under normal low flow conditions.

Procurement, installation and testing of all process and MEICA equipment including:

• 1nr Huber Rotamat inlet screening plant;
• 2nr ST9 Upflow primary settlement package plants with hydrostatic sludge draw-off;
• 2nr DN9 RBC package plants;
• 2nr ST11 Upflow conical final settlement package plants with hydrostatic sludge draw-off;
• Inlet pumping station complete with integral storm storage;
• Sludge handling facilities including sludge pumping station and sludge storage tank.

Civil Engineering construction consisted of:

The excavation and provision of temporary works for the installation of the primary and final settlement tanks.  Each excavation was 4.5 – 5m deep.  Due to very poor ground conditions, we were able to propose the use of secant piled cofferdams which formed a water tight excavation and was also procured through our own specialist in-house piling resources;

Excavation and construction of a reinforced concrete base slab for RBC installation and retaining walls required due to existing site topography. RBCs where backfilled with concrete in strict staged sections due to the delicacy of the GRP housing;

• Inlet works and sludge holding tanks where both constructed in reinforced concrete;
• Inlet pumping station, sludge pumping station and associated valve chambers constructed with traditional concrete ring sections;
• Interconnecting process pipework, asphalt access roads and concrete hard-standing provided site finishes.

Planning was a key element of the project as the site was located on a very small footprint.  To construct this works whilst still maintaining consent standards required the use of a temporary treatment plant.  We proposed the use of a temporary SAFF treatment plant with a single primary and final settlement tank.  We were able to utilise the existing inlet pumping station to transfer sludge to the existing sludge holding tank at the site entrance, thus removing tanker movements within our construction area.  Once operational, the existing works was taken off-line allowing unrestricted uninterrupted construction.

The works was subject to standard NIW level 1, 2 & 3 testing regimes and fully tested/commissioned. After the works was handed over to NIW we continued to provide operational and maintenance assistance to NIW for a minimum period of 6 months.