Water & Wastewater Treatment

26 | MAY 2020 | WWT | www.wwtonline.co.uk The Knowledge: phosphorus removal Recognising that delivery of the Water Industry Environmental Programme requires industry and supply chain col- laboration, Ofwat is consulting on a com- petitive funding mechanism to encourage collaborative innovation initiatives. Going further, cooperation and joint planning could help businesses to coordinate their activities, avoid conflicting peak demands for resources and comply more cost-ef- fectively with regulatory requirements. In view of today's competitive environment, this kind of collaboration would require a fundamental shi in culture. Design aims of a standardised dosing system A key design aim for standardisation of dosing systems is versatility, enabling a relatively small number of products to cover a wide variety of application needs. Compact systems are immediately more versatile than their larger predecessors as they expand location choices. Today there is a particular need for systems which will fit easily into smaller sites that now require dosing for the first time. If the system has inbuilt weather protection, its locations can be both indoor and outdoor. As well as being weatherproofed, every part of the system's design and construction must be robust enough for durable, reliable and safe performance. Ferric sulphate and ferric chloride are amongst the most common dosing chemicals, but a versatile system should be able to accommodate alternatives. This will allow switching to a different chemical if increased industry demand reduces the availability and increases the price of the operation's first choice. Materials used in the pumps, valves, pipes and other components should be specified to suit that range of chemicals. The system's chemical storage tank capacity needs careful consideration to optimise the convenience and cost- effectiveness of deliveries, and so avoids runouts. Reliable dosing level control is essen- tial, and the system must allow for rates to be set and varied in ways appropriate to different site sizes and circumstances. For instance, some very small sites with less stringent limits on chemical levels may be dosed at a suitable fixed rate. On a small-to-medium site whose phosphorus input load varies through the day, dosing can be programmed to follow a predicted diurnal profile. A period of continuous sampling and analysis is used to deter- mine that pattern, and this is repeated at intervals to check for any change. Larger sites are o en served by auto- matic dosing adjustment which follows the flow rate of the input water. To avoid overdosing when storms deliver very high volumes of water with relatively low levels of phosphorus, the maximum dosing rate can be capped. If a larger site needs very precise matching of dosing rates to input water content, data from sensors monitor- ing phosphorus levels can be fed back to the control system. As an additional safeguard, enabling immediate reporting of any failure, dos- ing systems should be equipped with telemetry. Finally, all of the features and functions summarised here should be de- livered as a complete, pre-assembled and pre-tested package designed for quick and easy installation and commissioning. The DS1500 unit from WES gives a good illustration of what a standard chemical dosing system would look like. Its design and capabilities could easily slot into the industry's current think- ing process around simplification and standardisation. Compact and tough, and ideal for indoor and outdoor applications requiring small to medium capacity, it is fully enclosed within a weatherproof, polyethylene, rotationally moulded structure. Its safety features include an integral bund, automatic leak detection cut-off, splash screens, and separate lockable doors for the fill/control compartments. Power and signal cables, along with dosing lines, are concealed and protected by a vertical tunnel through the bund. Construction material options allow for use of different chemicals. A 1,700-litre polyethylene chemical storage tank, with a 1,500-litre working volume, is part of the integrated design. Ideal for taking 1,000-litre deliveries from commercial chemical suppliers, it can also be filled by pumping from an on-site intermediate bulk container (IBC). Customers can choose from a variety of digital dosing pumps, including single and duty/standby set-ups, according to their preferences and applications. Dosing rates can be entered directly via the screen of the pumps' user-friendly interface. The system offers plug-and-play simplicity, with very wide turn-down ratios and no need for complex stroke rate calculations. Dosing rate is simple to control manually, over a range from 0.025 to 150 litres per hour, or automati- cally via input/output connections to the pumps. Additional options include duty/ standby pump changeover and telemetry functions. "Producing much larger numbers of dosing sys- tems and reducing their price, without compro- mising on performance and integrity, will require collaboration and radical change." Plug and play: the DS1500 chemical dosing system from WES

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