WWEM - Abstract

Abstract Information :

In a recent project with Northumbrian Water Ltd, a Hach Solitax solids measurement probe was used to monitor and control the sludge recirculation flow rate to maintain a solids setpoint via SCADA on two Lamella clarifiers. The site suffered from flow restrictions during peak demand periods due to clarifier performance even with large increases in coagulant use.rnrnRecently, Wear Valley WTW has seen the raw water quality deteriorate due to extreme weather events. Raw water in Burnhope reservoir is challenging to treat with a combination of very low conductivity, low alkalinity, and long periods of very high colour/organic content. The change in raw water quality has put extra pressure on the current coagulant and lime systems. Financially, the impacts come from the increased chemical consumption as well as reduced flow requiring other more expensive site to help meet customer demand. Wear Valley is a gravity fed works, and higher production reduces overall company carbon footprint. rnrnPrevious work with Hach at a wastewater site showed that controlling the solids flow rate of a varying sludge thickness improves process efficiency. It was hypothesised that this work could be reapplied to a Lamella clarifier drinking water plant to improve quantity and quality of water. Wear Valley uses two Lamella clarifiers in parallel. The trial allowed one Lamella to be used as a reference throughout the process for comparison. The Solitax data was used to discover a varying sludge thickness which was known to cause treatment issues. Whilst this was not monitored, it was difficult to draw trends and understand the process completely. rnrnAfter creating a calculator using the Solitax data and flow data, sludge was recirculated to maintain a constant solids dose by manually altering the pump speed. Settled water quality was compared to the uncontrolled Lamella 2. Due to the significant improvement in quality from Lamella 1, the process was adopted across the whole plant and implemented into SCADA for automatic control. Following the PLC controlling system installation, ferric dose has been reduced by approximately 15% during high colour raw water conditions. During poor raw water periods, site is now maintaining almost maximum flows, 10-15% higher than previously. rnrnDaily production improvements during winter of up to 4ML/d equates to an expected £40,000/year total in cheaper production savings. During high colour periods a 15% reduction in ferric is around £9,000/year, added to an expected 5% reduction during the rest of the year would total £15-20,000/year ferric cost reduction. rnrnAs well as the improved clarifier performance, sludge draw-off from clarifiers is more constant. Improvements have been seen on press and centrifuge performance as site does not have a clarifier sludge thickener. The reduction in sludge volume has reduced the likelihood of emergency tankering and associated costs.rn rnAlthough there have been huge increases in the quantity of the water produced by Wear Valley, it was the internal water quality specifications that limited throughput. This graph shows clarified iron concentrations at various flows, the introduction of the Solitax has kept a lower settled iron than previously seen. rnIn the future, Northumbrian Water Ltd. are working on optimising the sludge recirculation set points. Data analysis during this winter will allow deeper understanding of the process efficiencies needed as the water quality changes with the seasons. Further improvements are expected by altering the solids loading set point depending on real time conditions. Flows through the works will be compared to previous years. rnrn