Accurate effluent treatment at tannery

The delivery of unfailingly accurate flow rates, unaffected by fluctuations in head or viscosity allied to the ability to handle effluent containing wool residues without clogging, are critical in the success of a Watson-Marlow Bredel SPX-50 hose pump in the treatment plant at Stirling Leather's woolskin tannery.

Effluent from the tannery, normally with a pH of 2-5 to 3, contains sodium chloride, chrome liquor, organic fats and detergents in varying concentrations as well as wool residues. The latter, although not present in large volumes and barely noticeable on visual inspection, caused serious problems with clogging when a submersible pump was used on the duty.

The pump operates for eighteen hours a day, five days a week pumping untreated effluent from an underground holding tank directly to run-down screens, a height of approximately 4.5 metres. The holding tank is continuously agitated and aerated by blowers to prevent solids from settling out. When effluent in the tank falls to its lowest level suction lift can be as high as 6 metres.

With large solids and wool fibres screened out, the effluent is flocculated and passes to a flocculation tank. Terry Koerner, Production Manager at Stirling Leather, emphasises the importance of accurate flow rates at this stage in the treatment process. "The SPX-50's ability to achieve and maintain the specified flow rate, normally 0.5 l/s, is crucial to efficient flocculation. This and the fact that it has no difficulty handling the wool residues makes it the ideal pump for the duty."

As well as the downtime involved in clearing clogged impellers, with the submersible pump previously fitted, flow varied with fluctuations in head and the viscosity of the effluent, with the result that from time to time too much (or too little) flocculant was being added to the flocculation tank.