Waste water works meets stricter consent levels

Black and Veatch Contracting has recently undertaken substantial improvements to Llandrindod Wells Waste Water Treatment Works, on behalf of Dwr Cymru Welsh Water.

Black and Veatch Contracting has recently undertaken substantial improvements to Llandrindod Wells Waste Water Treatment Works, on behalf of Dwr Cymru Welsh Water.

The works, which mainly receives gravity fed domestic sewage from the main catchment sewer network at an average flow rate of 56 litre/s, has to meet stricter consent levels set down by the Environment Agency Wales.

Black and Veatch Contracting has been working closely with Cost Consultants ChandlerKBS and EC Harris as members of the Welsh Water Capital Alliance to deliver the GBP 1.28 million project.

The Capital Alliance is a strategic partnering team made up of mainly specialist engineering and construction companies, formed to deliver around 60% of Dwr Cymru Welsh Water's GBP 1.2 billion capital investment programme during 2000-05.

The improvements include the construction of a new inlet works concrete channel structure to incorporate an automated 6mm aperture screening system.

This comprises a screenings compactor, flow measurement, grit settlement, and storm overflow /return pipe work connecting to a new 475m3 storm retention tank, which has an automated cleaning system installed.

In addition, a new 13.3m diameter concrete formed final settlement tank complete with automated half bridge tank scraper system is also under construction.

Black and Veatch Contracting has designed the works improvements to readily accept further plant expansion in order to increase processing capacity and receive future flows from pumping stations sited in other catchment areas.

The majority of site improvement schemes undertaken by Black and Veatch Contracting necessitates that the existing works operation is maintained with minimal disruption.

The limited construction area at Llandrindod WwTW required installation of a temporary bypass arrangement around the existing inlet channels so that inlet flows can be maintained to the works during the improvements.

The existing channels were then demolished and the new concrete channel structure to house screening equipment constructed.

In both normal flow and storm flow conditions, sewage entering the works via the new inlet arrangement passes down the right hand channel and flows through a 6mm perforated screen, which is used to remove solids.

Controlled effluent flow is then directed to the primary settlement tanks via a grit settlement chamber.

The flows are monitored by a Magflow flowmeter.

Prior to commencement of construction, Black and Veatch Contracting (working in conjunction with sister company Black and Veatch Consulting) undertook ground surveys and trial bore holes in the proposed construction area, adjacent to the River Ithon.

The land was subsequently stabilised by introducing pile-driven columns down to the underlying bedrock.

Storm flows that were originally screened by a 12mm bar screen and then directly discharged into the River Ithon will now be intercepted by the new inlet screening arrangement.

New pipe work will then pass screened storm flows to the storm retention tank, which will provide 2 hours storm storage capacity.

The stored effluent is then pumped back to the head of the works for processing when the inlet returns to low flow condition.

During construction, the existing storm excess outfall and the treated effluent outfall from the Llandrindod Wells Waste Water Treatment Works site were identified as being unsatisfactory by representatives from both the Environment Agency Wales and Countryside Commission for Wales.

However, after extensive negotiations, and working closely in conjunction with the two governing bodies, Black and Veatch Contracting designed and constructed a new combined outfall (complete with headwall) to replace them.

In less than a week, work was completed on the outfall and associated new headwall.

Inspection of the newly completed structures was duly undertaken and permission to discharge via the new outfall was granted by the Environment Agency Wales.

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