July 7th 2010

    Visit to the engineering facilities of Anglian Water at their
                           Colney and Little Melton Plants

We met at Colney Water Treatment Works, where Darren Livock, Anglian Water Supply Manager, started by giving us an enthusiastic talk regarding the differing challenges presented by water abstraction from various aquifers and river.

Each source requires tailored treatment to deal with different forms of pollutant. While mentioning the work done at Heigham water treatment works, where the majority of the water is taken from the river, he concentrated on the abstraction of water from boreholes at Colney, Barford and Little Melton.

He also described how the water supplying the water towers of Hethersett, Easton and East Carlton is processed. Darren also answered all the numerous questions that we asked him, including details of the process of drilling boreholes and their subsequent servicing.

We then visited the water treatment plant here, that treats the water abstracted from the two boreholes on site at the rate of 75 l/s. The main pollutants are pesticides and nitrates. The pesticides are removed by passing the water through activated charcoal filters. The charcoal is recharged by burning - where the trapped pesticides are burnt off and little of the carbon burns. The nitrates in the water, 40 mg/l at the time of our visit, were below the legal maximum of 50 mg/l and less than Anglian Water's own limit of 45 mg/l. The water is blended with water from "old" water sources that are low in nitrates. Chlorination is also performed at this plant.

Before leaving the site, we inspected the head of one of the two boreholes and the 400-Watt standby generator, powered a 6-cylinder Volvo diesel engine, kept warm and ready to provide power to the entire site within 10 seconds of mains failure.warm and ready to provide power to the entire site within 10 seconds of mains failure.

We then drove to the newer Little Melton works, where "old" water from the two boreholes at Little Melton, known as "Marlingford" and from the two boreholes by Easton College - "Barford", comes to be treated. Each of these two sites supplies 35 l/s. This water has no nitrates but has high levels of iron dissolved.

In order to remove this, venturi pumps introduce air into the water and it goes into a contact tank where the dissolved iron reacts with the oxygen and comes out of solution. A little chlorine is also added at this point to further assist the reaction. The water then passes through sand beds, which filter out the iron oxide.

The sand beds are cleaned by back flushing and the effluent then goes to the sewer. There are four sand beds, three in use at any one time, one is back flushed every 24 hours and then allowed to so settle before it can be used again. This process is totally automatic.

The treated water from this plant is combined here with that from Colney, resulting in water, at the time of our visit having 20 mg/l of nitrates. The majority is then pumped to the three water towers at Hethersett, Easton and East Carlton, Hethersett being the controlling tower. The remainder is added to the Norwich main with the addition of ammonia, that keeps the chlorine in solution for a longer period.

After the fascinating tour and having even more questions answered to our satisfaction, we let Darren return to their duties. The time was now approaching two o'clock!

Report and photos by Ferrers July 2010