Water & Wastewater Treatment Magazine
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E ffective coagulation control is a vital step in ensuring the safety of potable water. Most natural water sources will contain dissolved and suspended par- ticles, which get into water through land erosion, minerals dissolving, the decay of vegetation and from discharged organic and artificial waste. Failure to remove these contaminants can result in the spread of bacteria, algae and viruses. Cryptosporidium is a particularly nasty parasite that can cause severe diarrhoea illness, with potentially fatal results in extreme cases. Resistant to chlorine and many other filtering tech- niques, it can be found in any surface water source, making efficient and effec- tive treatment processes at a microscopic level vital to ensure potability. Removal of waterborne particles is achieved by coagulation and floccula- tion. Coagulation is essential for good clarification and filtration performance in removing organic compounds such as manganese, dissolved phosphorous, colour, iron and suspended particles. The process typically uses either aluminium or iron-based salts (coagulant) to bind to- gether small particles (colloids) in water including clays, proteins, metal oxides and dissolved organic substances that will not settle or float. Shepherding the flocs Most particles suspended in water have a negative charge, making them difficult to remove. Chemical reaction between coagulant and colloids causes the destabilised particles to overcome their negative charge and bind together to form larger particles. Through flocculation, these large particles bind together to form even larger particles known as "flocs" which can be more easily filtered from the raw water. When adding coagulant, operators will o…en overestimate the amount need- ed, to ensure that no dangerous Natural Organic Matter (NOM) enters the water supply. While this may make the process look better initially, there is the risk that long term filter efficiency can be impaired if turbidity levels are too low coming off the clarification process. Over time, fine (sub-micron) particulates can penetrate deep into the filter bed reducing the ef- ficiency of the filter, potentially passing through to the next stage of the process. Significant over-dosing can also lead to high residuals of coagulant in treated water, elevated colour (when dosing a fer- ric based coagulant) and post-treatment precipitation of particles, which can cause turbidity, deposition and coating of pipes in the distribution system. Coagu- lant is also expensive. Increasing disposal costs and landfill restrictions are putting pressure on water utilities to either mini- mise sludge, or otherwise incur high costs for disposing of it responsibly. Maximise coagulation process efficiency An optimised coagulation process not only results in maximum removal of particles and NOM, but also reduces turbidity and minimises cost. Technolo- gies are entering the market to help water treatment facilities improve coagulation control at all stages of the treatment pro- cess, including pre and post-treatment. Newer measurement tools and techniques can provide more accurate measurements than previously possible and put the data in context. For instance, ABB's RVG200 paperless recorder feeds measurements through a fully configur- able algorithm. To help determine the dosing calcula- tion, data is collected from sensors measuring specific parameters, including UV dissolved organics, colour, turbidity, pH/ORP, aluminium and iron. Some tips for optimising coagulant dosing efficiency Measurements should be made at key points throughout the water treatment process in order to review and advise on treatment efficiency. Suggested locations include: l Raw Water – Feed Forward l UV Dissolved Organics / Colour, turbidity, pH l Dosed Water – Feed Back l pH l Clarified Water – Feed Back l UV Dissolved Organics / Colour, turbidity, residual Fe/Al, pH l Filtered/Final Water – Confirm l UV Dissolved Organics / Colour, turbidity and residual Fe/Al, pH Where practical, measurement equip- ment should be installed as close to sam- pling points as possible, while ensuring easy access for operation and mainte- nance. Sample lines should have local flushing points available, with minimal U-bends. Where instruments are being employed to monitor and/or control the coagulation process then the following tasks should be undertaken: l Physically check that instruments are working correctly on a regular basis l Carry out necessary calibration/main- tenance tasks as deemed necessary for the application l Flush sample lines to ensure all solid depositions have been cleared l Ensure that calculated coagulant dose is being dosed accurately l Always suspect a straight line when viewing trended results. The Knowledge: water treatment How technology can help to eliminate coagulant overdosing Julian Edwards of ABB Measurement & Analytics explains the importance of taking a holistic approach to coagulation control in order to accurately calculate dosing levels. 28 | FEBRAURY 2020 | WWT | www.wwtonline.co.uk