Construction site water run-off successfully treated during new convertor station construction

Global energy transmission leaders Hitachi Energy were constructing two high-voltage direct current (HVDC) converter stations in Norfolk which connect the Hornsea 3 offshore wind farm into the UK’s National Grid.

The convertors invert the 320Kv DC back to AC at 400 kV to be injected directly into the Grid.

The Hornsea 3 offshore wind farm will have a generation capacity of 2.9 gigawatts (GW) when completed and will produce enough low-cost renewable electricity to power more than 3 million UK homes. When complete it will be the single largest offshore wind farm in the world, surpassing the previous record-holder, its sister project Hornsea 2.

Challenge

The works included excavation, ground stabilisation, grading, and drainage, followed by construction of the HVDC convertor stations and associated cable runs across a large area.

A temporary drainage network was installed on site along with two large attenuation storage ponds. As is typical during the construction phase of large projects, the environmental permit governed the pH, concentration of suspended solids and total flow of surface water discharged from site.

In periods of heavy rainfall, the concentration of suspended solids in the surface water flow posed a challenge to the site. The decision was made therefore to install a robust and well-maintained water treatment system to ensure compliance with the environmental permit during the temporary construction phase.

The contractor engaged RVT Group to design and install a comprehensive system that met the site’s requirements.

Solution

RVT Group provided electric submersible pumps to draw water from the underground tank that was collecting the site run-off. This surface water was then fed into a full temporary water treatment system.

The temporary water treatment system consisted of:

• EnviroHub® TU02 Dosing Skid for automatic chemical dosing and pH adjustment
• EnviroHub® Treatment Tank for agitation and continuous flow mixing once chemicals were added
• EnviroHub® Lamella Plate Settlement Tank for expert removal of suspended solid particles from water

This system allowed for effective coagulation, flocculation and settlement for suspended solids removal. A catchment tank and pump then forwarded clean water 150m to a suitable discharge point at a surface watercourse. Plus, an RVT Group engineer provided regular health checks on the system, giving peace of mind on site.

By hiring this thorough water treatment system, Hitachi Energy were able to continue with works, safe in the knowledge that environmental risks were mitigated and under control. Project teams could be confident that works were compliant with environmental regulations because site’s construction water was treated to meet discharge parameters, with site given full visibility of the outcomes.