Icy solution to a sticky problem
When Anglian Water needed a solution to tackle a water discolouration problem it decided to try an alternative cleaning method which promised to keep disruption to customers to the absolute minimum.Customers in the North Norfolk town of Wells-next-the-Sea had suffered some discolouration of their water periodically over time. The cause was manganese sediment that had evaded treatment and entered the distribution network.
During 2007 extreme flow velocities in the pipelines caused by a burst on the network combined with high flows during peak summer demand caused discolouration of customer supplies. Conventional flushing and intervention had been used three times since then to remedy the problems but indications from water quality samples showed that the problem still existed. The company was aware that manganese deposits and other sediment were still causing a problem and it wanted them removed from the network.
Robin Peck, Anglian Water's innovation project manager, explains: "We knew we were faced with a historical discolouration problem. The size of the large diameter 10" main in the Wells network had made it difficult to effectively clean using flushing and the problem was never quite resolved. We knew that water in the area contained manganese but any cleaning programme needed to be as quick, efficient and effective as possible to prevent any disruption to customers.
"We scheduled the work for the off-peak season as Wells attracts a lot of holiday makers in the summer. This way we could manage the demand on the system to compensate for some parts of the network coming off supply, we wanted a solution which would enable us to return the cleaned main to service as quickly as possible."
In total, 2.2km of main were to be cleaned, the majority of which was plastic pipe although a sizeable portion was a large diameter cast iron main. As well as returning the main to service immediately, Anglian Water specified a number of other key requirements that would indicate whether the clean had been successful or not. These included:
· Removal of discolouration and turbidity
· Removal of iron and manganese deposits
· No post-clean elevated iron levels in water samples
Anglian Water chose to use ice pigging, a technique adapted from the food technology industry, that enables the thorough cleaning of a range of diameter pipes using ice slurry. Ice pigging, delivered in the UK through a partnership between Morrison Utility Services (Morrison US) and Agbar Environment (the global licence holder), needs no enabling works as the ice is simply inserted into one end of an isolated section of main through an a hydrant or air valve. It takes about 20 minutes to insert the ice, forming a flowable pig between 10 and 100m long, depending on pipe diameter.
The upstream valve is opened to drive the ice pig along the pipe under normal operating mains pressure. Its progress is monitored and controlled at the discharge hydrant. By modifying the ice fraction the ice slurry can be specifically designed according to the pipe material, lining material, type of sediment to be removed and the condition of the main.
The ice pig is formed from chlorinated water and substances removed from the inside of the pipe are captured within the ice and deposited on exit. The effect of the ice on the inside of the pipe also makes it especially effective at removing adherent manganese deposits one of Anglian Water's specific requirements.
Ice pigging uses less water than traditional cleaning techniques that in 'water poor' areas like East Anglia and the South-east makes sense for the sustainability of supplies overall. The ice pigging technique offered Anglian Water the flexibility to clean the large diameter cast iron main just as effectively as the smaller 8" PVCu main saving time and money. A 75% ice fraction was used at a flow rate of 12l/s. The mains were cleaned in two lengths with the ice making one pass per length.
Following the deployment of the ice pigging technique at Well-next-the-Sea the indications are good that it has solved the discolouration problem permanently. In total 27.4kg of sediment was removed from both the large diameter cast iron main and smaller plastic main, 21.9kg of this sediment was removed from cast iron main alone.
The mains were isolated for less than three hours in total ensuring that customers' supplies were not disrupted. No elevated iron levels were detected post clean indicating that the cast iron mains cleaning had been effective.
Peck adds: "Thinking about our customers, ice pigging is done quickly without any excavation and the associated noise and disruption. Supplies might only be interrupted briefly, if at all and there are no risks to the quality of the water which runs through the main following the clean as no harsh chemicals are used.
"The results of the trial at Wells were first class; given the amount of dirty ice removed we were surprised that the mains could be returned to service immediately. We will continue to test water from this part of the network following the clean and so far I am pleased to say that we have not detected a return of the problems we were experiencing. I am satisfied that we have found a solution that appears to be working."