The existing sewage infrastructure for the community of Kirkwall, the principal town of the Orkney Islands, currently discharges untreated sewage into Kirkwall Harbour. The system has developed historically, without using any treatment facilities and with minimum monitoring. In the past, operations for the disposal of waste water relied heavily on the natural processes of the adjacent waters.
Legislation now demands that significant improvements be made to the quality of waste water discharge and these improvements will be incorporated into the Kirkwall Waste Water Treatment Project. North of Scotland Water Authority (NoSWA) proposes to construct a new waste water treatment plant and associated long sea outfall at Head of Work, an isolated location on Orkney Mainland, connecting to an existing rising main sewer. North of Scotland Water Authority has invited Dredging & Construction to design, construct, install and test the Kirkwall Long Sea Outfall and associated pipework on land from the boundary of
the WWTP to the discharge point offshore. This new outfall will discharge treated effluent approximately 300 metres offshore into The String and has a designated lifetime of sixty years.
The waste water effluent, which will be discharged through the new
outfall, will improve coastal water quality. This will contribute to the
quality of the local fish produce and could encourage new fishing
ventures and tourism through the provision of cleaner waters
The line of the outfall pipe was to be drilled through rock (Raasay
Flags) and required specialised plant and equipment. It was
therefore important that, prior to drilling, detailed geotechnical,
bathymetric and seismic surveys be carried out to determine the
precise location and depth of the directional drilling operations. This
information was critical, influencing both the accuracy and speed of
After setting out and the establishment of an entry point, a pilot
hole was drilled through the rock using a 250mm diameter drill
head. This pilot drilling exited 328 metres offshore and at a depth of
20 metres below sea level. Due to the strong current allowing
between twenty and thirty minutes of diving time, it was only
possible to connect a winch wire to the drill string at slack water.
With a 40 Tonnes winch set up on the cliff top reaming commenced
to establish an enlarged hole of 660mm diameter.
With reaming completed a previously fabricated 560mm diameter
PE80 pipe was pulled through the drilled hole into its final position.
Following cleaning and testing of the outfall pipe a local diving team
working from the multicat “Forth Constructor” attached the
prefabricated GRP four outlet diffuser to complete the outfall.
Land based pipeline works consisted of open cut trenching across
the agricultural fields of Head of Work through rock to install a
900mm diameter PE pipe. Finally a reinforced concrete manhole
was constructed to enable the ultimate connection between outfall
and land pipelines to be made.
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