Southern Water spends £25m to meet NEP conditions


Southern Water spends £25m to meet NEP conditions

Southern water has invested £25m on upgrading the Millbrook wastewater treatment works due to the need to cut nitrogen levels in effluent water being discharged into the Test Estuary.


The main driver behind this project is due to the National Environment Programme (NEP) specifying a new discharge permit condition of 10 mg/l of total nitrogen for wastewater treatment works. The end result will be a much greener treatment of wastewater prior to its release into the Solent.


Originally constructed in the 1930s, the wastewater treatment works, located in Southampton’s Western Docks, serves a population of approximately 135,000 people. It has undergone two upgrades in its lifetime – one in the 1960s, the other in the 1990s – and incorporates a carbonaceous activated sludge plant (ASP). This upgrade, however, is considered to be its most significant yet.


The £25m project was delivered by Costain, MWH (4Delivery) and Veolia Water as part of Southern Water’s AMP5 plan and ran from November 2011, finishing this month.


Southern water’s Senior Project Manager, Stewart Garrett, said: “A key driver for this upgrade was the section of the National Environmental Programme (NEP) which specifies a new discharge permit condition of 10 mg/l of total nitrogen (TN). This was a new requirement that wasn’t being measured previously. The main challenge for Southern Water was how this could be achieved using the existing assets at the WwTW while maintaining treatment capacity on a site that had space issues. It was clear that Millbrook’s existing ASP was not capable of producing effluent that met the new standards.”


Serving such a large region, it was clear that upgrade works had to be carried out whilst keeping the WwTW running in order to maintain the current condition of the plant. Close collaboration between all delivery partners resulted in the following construction:


  1. A treatment tank the length of ten buses.
  2. 68 beams holding the tank.
  3. 2 final settlement tanks.


Over 10,000 cubic metres of concrete, 11,000 tonnes of reinforced steel, 1,500 metres of new pipeline and 3,000 metres of new ducting was used during the construction phase of the upgrade.


90% of waste material created during the upgrade works so far has been recycled or reused on site. All concrete that had to go off-site was sent to recycling centres to be used by those in the building trade.


Overall, the upgrade project has been successful. Stewart Garrett said: “Although the capcity at the WwTW has not increase, influent is now treated to a much higher standard. As a result, effluent discharges into the Test Estuary contain less nutrients and the spread of environmentally damaging algae growth has been cut.”