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Specialist Chemical Engineering Solutions
Thirtyseven of the sewage treatment sites operated by Anglian Water Services Ltd are being progressively equipped with new phosphate removal systems. Most of these are pre-packaged, factory-built units, which are being brought on-line more quickly and providing other benefits compared with traditionally built systems.
The new units, supplied under a £3 million-framework contract placed with Gee & Company of Birmingham, are designed to dose ferrous chloride for phosphate removal from the final effluent of sewage treatment plant. As stand-alone units, they incorporate storage tanks, bunds, dosing skid and control panels. Although the concept was designed and developed by Gee, the units reflect some additions and technical changes that are specific to the operational needs of Anglian Water.
“At the outset, we sought a single framework partner who would give us a consistency in plant design that would help to simplify through-life maintenance”, explains Anglian Water’s Project Manager, Annabelle Kirkby. “We finally chose Gee partly for reasons of cost and partly because of their experience with similar contracts with other water companies. We were also attracted to their flexible engineering approach, which enabled them to refine certain design features to give us exactly what we wanted”, she says.
Complete packaged system
Designed for the dosing of any chemical that reacts with phosphate, the system is not restricted solely to this application. The same technology has also been applied to the dosing of other chemicals, including sulphuric acid and sodium hypochlorite.
The Anglian Water system uses ferrous chloride, which removes phosphate at the inlet stage and then precipitates it out in the primary tanks. A second dose is added prior to the percolating filters. However, if installed in an activated sludge removal plant, the chemical is only introduced at this second stage.
Using entirely factory-built methods, the packaged units incorporate twin 15 cubic metre capacity Cellmar GRP tanks constructed to BS 4994, Category 1 and up to three sets of duty/stand-by metering pumps mounted above a fabricated bund that also carries the tanks.
The pumps are Signal 200 series units which are controlled by time clocks but - as they have built-in electronics which will accept 4-20 mA control signals - other forms of remote control can easily be retro-fitted. Pump duties can be anything within the working range of three litres/hour right through to 650 litres/hour and are arranged in duty/stand by mode, with auto changeover.
Enclosed in a strongly constructed kiosk, the system is augmented by a second kiosk that contains the electrical control panels. Physical separation in this way means that the controls are isolated from contact with any chemicals or fumes in a dedicated area that has its own heating, lighting and ventilation.
Because of the modular design of the Gee concept, additional slave units incorporating extra tanks and bunds can be added. This gives extra storage capacity where this is required for higher consumption needs. Anglian’s treatment works at March, for example, uses one such slave unit, which doubles the chemical storage capacity there to a total of 60 cubic metres.
Faster and safer
Of the many benefits of modular factory construction, the speed of commissioning is the most notable. Firstly, the units are pre-tested in a controlled environment by the manufacturer. Then, they are delivered to site and craned into position on a prepared concrete slab. This process takes a couple of hours, following which it is the work of no more than three days to connect the dosing lines, cables and telemetry. Typically, the system will be ready to go live within a week.
A requirement for minimal civil preparatory work not only dramatically cuts the need for site works, it also largely takes weather dependency out of the equation. For example, no concrete curing is necessary prior to the lining of bunds.
There are health and safety considerations, too. On the packaged plant, separate, lockable doors are provided to give access to the chemical storage tank area, the dosing pumps and the electrical controls respectively. This enables access to each area to be restricted to authorised personnel only.
The use of twin storage tanks means that if chemical contamination or some other failure should occur in or near one tank, it can be isolated and cleaned whilst the phosphate removal process continues – fed by the other tank. Although the twin tanks are physically connected, they are charged independently. This feature enables the ultrasonic measurement of fluid volume to be taken accurately - and not to be influenced by any foaming during tank filling.
Although used extensively across the Anglian Water region, the Gee solution is not universally deployed by the company for phosphate removal. On nine of the company’s larger sites, where the quantity of chemical storage required exceeds the viable capacity of factory-built systems, traditional civil construction methods remain the preferred option. These sites also use tanks and Signal metering pumps supplied by Gee.
Anglian Water’s Annabelle Kirkby believes that the solution offered by the framework partnership with Gee has worked well. “The manufacture, supply and commissioning of packaged units to cover 29 of our sewage treatment sites and nine traditional sites in a contract period of only 18 months represents a significant achievement. Apart from design consistency – a key part of the Anglian Water specification – Gee has worked enthusiastically with us to give us the design detail and system refinements that we require“, she concludes.
Gee has spearheaded the use of factory-built package plant for chemical dosing.
