Recycled glass succeeds in beating sand into effluent

The savings are attributed to the greater filtration effectiveness of glass and its ability to be more easily cleaned. Better filtration is put down to glass's more angular shape and the ability to combine particle sizes to optimise filtration. Glass is also easier to backwash than sand, which means it doesn't have to be changed as frequently, say the researchers.

J E Hartley, has just completed a full-scale trial of recycled glass filtration media (RGFM) in two-stage filters at its Thorganby plant, near York. The company's effluent discharge consents allow up to 350m3/day with a maximum suspended solids concentration of 30mg/l. Hartley's effluent exhibits varying and sometimes challenging characteristics, associated with the carrots, turnips, peas, bans, potatoes and celery that it handles.

The trials showed that RGFM outperformed sand as a filtration medium. They looked at operating cycles for different crops, which were optimised for backwash frequency. Two-stage pilot scale filters were tested on-site, so the advantages of using different-sized glass media over a single stage sand filter could be gauged. The results showed the effectiveness of a combination of coarse and medium grade glass in 'polishing' the final effluent.

The final results showed that for certain effluents, two-stage glass filters can remove up to 90% of suspended solids compared to 32% for sand. Hartley site manager John Pick says: "The results to date are very encouraging and, provided we maintain the improved performance level through different crop changes we'll consider upgrading our current filtration process to include recycled glass."

The trial was funded by the Waste and Resources Action Programme (WRAP) and undertaken by consultant Aqua Enviro. WRAP aims to raise glass recycling in the UK from 42% to 60% by 2008.

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