Separation and settlement

Vexamus Water looks at how using Lamella separators in settlement tanks can improve potable water treatment process and help WTWs keep within more stringent parameters

The requirement for safe and reliable water treatment process is still as important as ever. However, pressure on available space, tougher planning permission that stipulates the need for minimal environmental impact and the need to manage costs are proving to be demanding parameters for ordinary settlement tanks. With their track record in clarifying a wide range of water and wastewaters, the Johnson Lamella, a plate-type separator - with more than 6,000 installed in municipal and industrial applications - may be worth considering.

The patented flow control system ensures an even flow across the plates, allowing entrained solids to slide from the plates to the sludge layer on the floor of the tank. By ensuring the flow enters the slots in the bottom edge of the spaces fitted to each plate, disturbance of the sludge layer is avoided. This is a very important difference between Johnson Lamellas and other types.

Removal of sludge from concrete tanks containing Lamella plate-packs is achieved by hydraulic or electrically actuated Zickert floor scrapers (available from Vexamus Water). The scraper comprises a series of triangular profile stainless steel plates, fitted to a rigid frame that slides on high-density polythene sections fixed to the floor of the tank. The reciprocating action of the scraper aids the thickening of the sludge as it is transported along the length of the tank towards the sludge hoppers located at the inlet end of the concrete tank. Maintenance is minimal because of the lack of moving wetted parts.

Since the *Bouchier Report, published in 1998 by the UK Drinking Water Inspectorate (DWI) stated the need for an additional clarification stage in washwater recirculation to reduce Cryptosporidium, the ability of Lamella to meet the required specifications and provide a smaller footprint has gained recognition. Vexamus Water has supplied Lamellas on washwater applications at some of the largest WTWs in the UK.

One of the latest installations, which has been completed in the north of England, involves the use of the Johnson Lamella plate separator in newly constructed tanks for clarification of chemically dosed raw water. At this WTW, the Lamella is designed to treat a flow of almost 1,300m3/h and serves a population of slightly more than 100,000.

The process has three parallel streams, each comprising a series of flocculation stages prior to the Lamellas. Sludge removal is achieved by Zickert electrically actuated floor scrapers. Clarified effluent flows from the Lamellas to a series of rapid gravity sand filters and the backwash water from the filters is transferred to a free-standing Lamella to remove the entrained solids and recycle the clarified water to the inlet of the works. This installation is the first of its type in the UK, although the Johnson Lamella has been used in similar applications for many years in the US.


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