Maple Lodge STW Solves Its Ferric Sulfate Challenge

Low-maintenance peristaltic metering pumps were the answer to this plant’s chemical dosing problems

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Maple Lodge STW Solves Its Ferric Sulfate Challenge

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Maple Lodge Sewage Treatment Works is a large sewage works in Maple Cross, Hertfordshire, northwest of London. Part of Thames Water, the U.K.’s largest water and sewerage company, it serves nearly 500,000 residents and businesses, receiving up to 300,000 cubic meters of wastewater per day. When the plant is at maximum capacity it can generate enough renewable energy to power the equivalent of 9,000 homes. 

Maple Lodge STW was experiencing challenges with diaphragm pumps in their ferric sulphate application. One issue was clogging in the pump suction line and valves, especially when the levels in the three 60 cubic meter tanks were low. When the periodic clogging occurred, the pumps would need to be taken apart and cleaned, which was time-consuming, expensive and messy. 

Another issue was nuisance alarms that would shut the pumps down. These alarms where triggered by sensitive sensors that were built into the system to monitor the diaphragm pumps. When variations in the suction line pressures occurred due to clogging in the suction port, changes in the tank level or changes in the concentration of the chemical when the tanks were at low levels, the system would go into an alarm condition and service was required on the diaphragm pumps to get them up and running again.

The diaphragm pumps were installed inside a secure room located next to the storage tanks. The pump suction inlet elevation was about 2 meters above the bottom of the 60-cubic-meters ferric sulphate tanks. All was well with the pumps when the tank level was higher than 2 meters and the pumps were operating under a flooded suction. However, when the tank level fell below 2 meters, the pumps were required to lift the more concentrated ferric causing the alarms and clogging.

Hearing that peristaltic pumps were more effective at pumping fluids with particles that can clog diaphragm pumps and that they can self-prime against discharge pressure, Pete Thomas, operator with Thames Water, decided to install a Blue-White M-4 series peristaltic metering pump. The M-4 pump can accept 14 different tubes of various sizes and materials allowing the pump to meter most water treatment chemicals including sodium hypochlorite, 98 percent sulfuric acid, oil-based emulsion polymers, lime slurry and ferric sulphate at discharge pressures from a full vacuum to as high as 125 psi. The specific model M-426-MNP pump now in use at Maple Lodge STW uses the “NP” tube, which has a maximum output of 600 liters per hour and a maximum pressure capability of 30 psi. The pump was installed in June of 2014 and has pumped at 150 liters per hour against 20 psi without fail.

The only maintenance required on the M-4 is the periodic replacement of the pump tube assembly, a five-minute procedure. Although the tubes are estimated to last over three months in this application, Thames Water decided to replace them monthly due to the low replacement cost (less than $100) and the ease of maintenance compared to the overall cost of repairing the diaphragm pumps.

Learn more about the M-4 series metering pumps at


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