Southern Water is celebrating finding a solution to the scaling up of its plate heat exchanger and condensers at its newly opened state-of-the-art wastewater treatment works at Ford, West Sussex. The company has worked closely with Environmental Treatment Concepts (ETC), a specialist in electronic descaling, which has resolved the problem by installing the Scalewatcher ENiGMA Descaling System onto the drying system. This has resulted in a dramatic reduction in time spent cleaning the plates from 12 hours a day to just 6 hours a month.

Opened in 1999, the Ford wastewater treatment works is part of Southern Water's Bognor Regis and Littlehampton wastewater scheme, which has been introduced to eliminate the daily release of waste into the sea. Part of a multi-billion pound environmental programme by Southern Water, the Ford site is one of six new treatment works built along the South Coast to meet EEC regulations.

The £53 million scheme enables wastewater from Littlehampton, Bognor Regis and the surrounding areas to be pumped through 16km of underground pipelines and to undergo a series of treatment processes before the cleaned wastewater is released back into the environment through a 3.3km outfall. The Ford plant treats up to 62 million litres of wastewater from more than 130,000 people every day. The wastewater treatment process involves a number of procedures resulting in the production of a valuable, organic fertiliser.

When the wastewater arrives at the site it flows through 6mm screens to remove debris and then through a grit trap, known as a detritor, to remove grit and sand. Following sand and grit removal the wastewater is piped into one of three settlements tanks, each holding 2,000 tonnes, where between 60 and 70 percent of the remaining solid waste is settled out.

The settled wastewater then passes to aeration tanks where it is mixed with a biologically active sludge that starts a microbiological action to reduce organic pollutants remaining in the wastewater. Once this process is completed, the solid material is removed from the bottom of the tank and transferred to the sludge-recycling centre.

The first stage of the recycling process involves blending the different sludges to produce a homogenous mixture of water and solid material. Polymers are added to thicken the mixture and this is fed into one of three large digester tanks where it remains for up to 14 days. During this time a temperature of 35°C is maintained in the digester, allowing bacteria within the waste to break down organic matter to water, carbon dioxide and methane gas. The methane gas is stored on site in a gasholder. A proportion of the gas is used to provide the necessary heat to maintain the temperature of the digestion process with the remainder being used to heat the dryer.

The second stage of digestion cools the material and feeds it into a centrifuge dewatering plant, which separates the solid material from the water and creates a "cake". The "cake" is fed into a huge drying machine, which heats the cake to temperatures of 90°C. This evaporates the remaining water to produce virtually odour-free, dry granules, which are then sold as fertiliser to customers for £10 per tonne. This is a major financial saving for Southern Water, which used to have to transport solids to landfill at a cost of £1,000 a tonne.

Prior to the Scalewatcher ENiGMA Electronic Descaling System being installed, the plate heat exchangers were producing high differential pressures every 24 hours that necessitated the plates having to be individually descaled with jet washing and wire brushing. This procedure took 12 hours each and every day. The high levels of scale were due to the wastewater treatment centre being situated in a hard water area and also that the condensed water is a by-water product of the sewage inflow.

Following a site survey by an ETC engineer it was agreed that Scalewatcher ENiGMA Electronic Descaling units would be fitted onto the drying system to treat the make-up water and on the feed to the pipe taking the cooling water from the condensers to the heat exchanger.

"We were initially sceptical", says Mark Day, Process Scientist. "However, the Scalewatcher ENiGMA Electronic Descaling system has now been installed for two months and instead of having to shutdown everyday for up to 12 hours we now just wash down the heat exchanger plates every couple of weeks which takes between 3 and 4 hours. In addition, we have seen a remarkable change in the condensers, which, during inspections, show that the scale is now soft and being removed from the sides."

"The thermal drying is a critical section in the recycling system and one that we cannot afford to shutdown", concludes Mike Walker. "The Scalewatcher ENiGMA Electronic Descaling System is working way beyond our expectations and will in the long term save us costs on preventative maintenance, labour and downtime".

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