In order to meet the more stringent phosphorous removal requirements that come into force between 2020 and 2025, wastewater utilities will need to choose the right chemical dosing technology. Kevin Wheeler, managing director, WES explores the options

elivering chemicals accurately is a fundamental part of many controlled industrial processes and almost all municipal water or wastewater treatment regimes. With the water industry gearing- up for the 1000 plus, new final effluent phosphorus consents, which will come into force before 2025 as part of the Water Industry National Environment Programme (WINEP), and the new Asset Management Plan for 2020–2025 (AMP7), determining the right dosing system has never been more important. With several alternative technologies, dozens of equipment manufacturers and hundreds of product variants available on the market, that choice can seem challenging. A dosing system comprises a chemical storage tank, a metering pump, control system, and associated valves, pipework and accessories. Selection is based on a number of criteria. The first is the chemical being handled and the concentration used, which determines the materials used for these parts. Material choices must be made carefully. Some materials perform poorly when exposed to highly concentrated chemicals, but, are safe to use at lower concentrations. Even small parts made from inappropriate materials, such as internal valve components or the diaphragms used in pressure gauges, may introduce a potential failure point to a system. Some chemicals may produce heat or vapours during handling, and these characteristics must be accommodated in the system design. High specific gravity, viscous or lumpy materials will require appropriate pumps and fittings, and special precautions are needed at both low and high temperatures.


The range of output volumes and system pressures required will determine the size, specification and capabilities of the components used in the chemical dosing system. A further key choice will be the metering

or dosing pumps, which are designed to deliver precisely measured quantities of chemical, provide suction if required to extract the chemicals from the storage tank, and pressure to transport it through the distribution pipework to its point or points of use. The three main dosing pumps used are: diaphragm pumps;


peristaltic pumps; and progressive cavity pumps. Diaphragm pumps are available in

numerous configurations, each with their own set of drawbacks and advantages, and use a reciprocating action to move a flexible diaphragm, adjusting the volume

Progressive cavity pumps use a helical

of an internal tube or cavity. A known volume of chemical is alternately drawn into the cavity through an inlet valve, then pushed out into the chemical delivery pipework through an outlet valve. Chemical flow is not continuous, but rather a series of pulses. Peristaltic pumps, meanwhile, use a rotary motor drive to move a series of rollers over a flexible tube inside a rigid housing. The rollers squeeze the tube, forcing chemical out into the distribution pipework. As these pumps do not reciprocate, the chemical flow they generate is continuous, and the absence of internal valves improves their ability to cope with liquids containing solids and gas liberating chemicals such as sodium hypochlorite. Disadvantages include limited discharge pressure capability and the need for the periodic replacement of the pump tube, which can wear over time.

A dosing system comprises a chemical storage tank, a metering pump, control system, and associated valves, pipework and

accessories, and selection of these components is based on a number of criteria

rotor turning inside a specially shaped stator. The motion of the rotor against the stator results in the creation of a series of fixed shape cavities that transport chemical through the pump to the distribution pipework. Progressive cavity pumps work well with slurries, viscous materials and chemical products that are sensitive to shear, but have a limited range of capacity adjustment and the complex shapes and tight manufacturing tolerances required for their operation leads to high capital and maintenance costs. Real world installations often require

compromises in the location and working conditions of system components. Dosing pumps may have to be installed above storage tanks, for example, requiring a pump design capable of generating sufficient suction pressure. Parts may be exposed to wind, rain, extreme temperatures or intense sunlight, which can degrade some materials. The availability of suitably skilled operators and maintenance personnel may be limited. Wherever possible these factors should be considered and accommodated at the system design stage. Further, many chemical addition

Choosing the right chemical dosing system might seem a daunting prospect, but WES is here to help

processes, particularly those related to potable water supply, require continuous uninterrupted operation. This may require parallel chemical metering streams to be installed to provide a duty-standby functionality, with pre-assembled packages preferred so installation times and process interruption is minimised. Choosing the right chemical dosing system might seem a daunting prospect, but WES is here to help. Because the company isn’t linked to any single equipment manufacturer, it can provide customers with unbiased advice about the technologies, approaches and solutions that best meet their needs.



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