Saving time and money by ion exchange

SELCHP was commissioned in 1994 to incinerate up to 420,000 tonnes annually of household waste from Lewisham, Greenwich, Westminster and Bromley and generates 35MW of electricity for export to the National Grid – sufficient for 48,000 homes. SELCHP is a successful commercial partnership between the public and private sectors in a consortium of local authorities and specialist companies, the operating partner of which is Veolia. 

Electricity is generated by steam turbines using a traditional steam-water circuit. Heat from the incinerator produces 395°C superheated steam in 46bar water tube boilers. This steam drives a turbine generator with exhaust steam being condensed in air cooled condensers and returned to the boiler via a deaerator where demineralised make-up water is added to replace steam and water losses.

The on-site make-up demineralisation plant had been installed in 1993. It was limited in output to 10m3/h and operated for about 8h between regenerations. However, the regeneration time was 4h and although this was not an issue under normal operation, it created a bottleneck when it came to refilling the boiler. Operations manager, Graham Duff, explains: "After repairing a tube failure or similar boiler shut down we have to refill the boiler with 120m3 of demineralised water. With 80m3 of deionised water production between regenerations and the long regeneration time this could take up to a day.” A replacement plant was required to deliver high purity water meeting a 0.1µS/cm conductivity and 20µg/l silica specification. 

Veolia Water Technologies, Process Water provided the solution with a two-stream Rapide Strata+ 18m3/h packaged deioniser system together with conductivity, pH and silica monitoring, PLC controller and HMI. The two streams operate as duty and standby but, when demand is high, can run together to deliver 36m3/h. When demand for make-up water is low and the treated water tank is full, the duty unit automatically switches to recycle mode. In this mode, treated water is recirculated from the treated water tank, through the Rapide Strata+ and back to the tank. The recycle mode operates for 20min every 2h and keeps the resin beds in good condition, as well as polishing the water in the treated water tank to maintain quality. 

The Rapide Strata+ is a three stage, reverse flow regenerated ion exchange demineralisation system, incorporating two of Veolia’s special technologies: HiPol and SCION. HiPol is a long established process, which uses a high rate cation exchanger (the two small cylinders in the photograph) downstream of the conventional cation and anion exchangers to remove sodium leakage and produce treated water of 0.06μS/cm. SCION is a short cycle ion exchange process using uniform sized resin beads to achieve high flow rate, high chemical efficiency and a regenerHiPol polisher

The short regeneration time of the Rapide Strata+ means that the boiler can be back in service 6-8h earlier and, with boiler down time costing about £4,000 per hour in lost revenue, that is a saving of about £30,000. Graham Duff says: "In the two years that we’ve had the plant, the reduced down time alone has repaid the investment.”

Aside from the reduced downtime, there is another benefit from the new plant. By comparison with the old demineralisation plant, the treated water quality from the Rapide Strata+ is lower in conductivity and daily consumption of regenerant chemicals is about 40% lower. Regeneration wastewater is also lower by about 20%, helping to reduce the facility’s overall water footprint.