Anglian Water Services is progressively equipping 37 of its sewage treatment sites with new phosphate removal systems. Most of these are pre-packaged, factory-built units, which are being brought online more quickly and providing other benefits compared with traditionally-built systems. The new units, supplied under a £3M framework contract placed with Gee & Company of Birmingham, are designed to dose ferrous chloride for phosphate removal from the final effluent of WwTWs. As stand-alone units they incorporate storage tanks, bunds, dosing skids and control panels.
concept was designed and developed by Gee, the units reflect some additions and technical changes specific to the operational needs of Anglian Water.
“At the outset, Anglian sought a single framework partner who would give consistency in plant design that would help to simplify through-life maintenance”, explains the company’s project manager Annabelle Kirkby. “Anglian finally chose Gee, partly for reasons of cost and partly because of the firm’s experience with similar contracts with other water companies. Anglian was also attracted to its flexible engineering approach, which enabled Gee to refine certain design features to give Anglian exactly what it wanted”, she said.
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 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 15m3 capacity Cellmar GRP tanks constructed to BS 4994 Category 1 and up to three sets of metering pumps mounted above a fabricated bund that also carries the tanks.
The pumps are Signal 200 series units controlled by time clocks but, as they have built-in electronics that will accept 4-20mA control signals, other forms of remote control can easily be retro-fitted. Pump duties can be anything within the working range of 3-650 l/h and are arranged in duty/ stand-by mode with automatic change-over. 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 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, giving 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 to a total of 60m3. 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, after which it is the work of no more than three days to connect the dosing lines, cables and telemetry.
FASTER AND SAFER
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 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 region, the Gee solution is not universally deployed by the company for phosphate removal. At 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’s Annabelle Kirkby believes the solution offered by the framework partnership with Gee has worked well. She said: “The manufacture, supply and commissioning of packaged units to cover 29 of Anglian’s 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 specification – Gee has worked enthusiastically with Anglian to give it the design detail and system refinements it requires.”