Groundbreaking trials show that recycled glass is not only a highly effective filtration medium but also better for the environmentFinal results of groundbreaking trials show that using recycled glass as a
filtration medium offers numerous advantages over the traditional mediums, including dramatically increased suspended solids removal from effluent and much lower costs, as well as being kinder to the environment.
Wakefield-based consultants Aqua Enviro has just published its concluding report on independent industrial-scale trials of recycled glass filtration media (RGFM) at two sites in Yorkshire and the Humber, which took place between April and November 2005.
The report highlights that the tests have identified that a two-filter system using RGFM, which is manufactured to BSI PAS 102 specifications, has been successful in removing up to 90% of suspended solids in trials on chemical plant and food processing effluent. RGFM can therefore be instrumental in treating effluent and helping companies to meet Environment Agency consent levels for discharge.
Reduction of suspended solids is important, as they have a negative effect on the environment by de-oxygenating water, with a consequent detrimental impact on fish habitat. Excessive growth of blanket weed brought about by suspended solids can also affect river flow, as silt and other particles build up when they are caught in the weed.
The report adds that the trials demonstrated the glass-based system was capable of treating suspended solid shock loadings of more than 1,200mg/l and still recovering within 36 hours.
Aqua Enviro consultant, Mark Lowe, says: "The thinking behind the two-filter system was that one would remove larger solid particles and the second would polish the effluent by banishing smaller ones. This was what happened in practice and the method proved highly economic for removing suspended solids and reducing soluble COD before water reuse, as it doesn't demand an initial chemical treatment phase, which is necessary when single-stage sand filters are used.
"The tests have also shown RGFM delivers more efficient backwash performance than other filtration mediums. This process - which involves clean water being pumped backwards through the filters to flush them clean of sludge - is required less often and is more energy-efficient with glass because it has a lower specific density.
"Unlike sand, RGFM has also shown no marked tendency to clog up, or
blind, when faced with high loadings of suspended material." Lowe added the trials had shown the greater flexibility of glass particle sizing was a crucial reason for its performance being better than that of sand. He said: "As glass is a processed product, it can be sized to treat specific solid types. As the size of the solids decreases, so does that of the glass required to remove them."
The Aqua Enviro report covers trials which took place at the sites of vegetable processor JE Hartley, at Thorganby, near York, and Croda Chemicals Europe, at Rawcliffe Bridge, near Goole.
Results from the two locations echo those obtained previously from similar tests at kitchen roll and toilet tissue producer Georgia Pacific GB, at Bury, near Manchester, and Yorkshire Water's Malton WwTW in the north of that county.
As a result of the latest tests, Hartley is now considering upgrading its processes to include a two-filter, glass-based system. Croda has ordered a full-scale filter system, to be commissioned in the spring, which will be monitored with a view to developing a stronger business case for the use of glass.
JE Hartley has been supplying frozen vegetables to the wholesale manufacturing and retail trades for more than 20 years. Its plant handles vegetables such as carrots, turnips, peas, beans, potatoes and celery.
Its effluent exhibits changeable and challenging characteristics, with its treatment process previously discharging up to 350m3/d with a maximum suspended solids concentration of 30mg/l.
Two large-scale rigs with a volume of 10m3 were operated for six weeks during
the full trial. This used coarse, followed by medium grade, glass - the combination which had performed best during an earlier pilot test.
The rigs ran on a four-hour operational cycle, with backflush following four hours of forward flow at an average rate of 4.4m3/h. Samples of the influent were collected after the first and second filters.
During the first fortnight, the influent had average suspended solid levels of 9mg/l, with the filters achieving an average removal efficiency of 37%. Later, however, the average concentration increased to 28mg/l - it rose as high as 70mg/l at one stage - and mean removal efficiency soared to well above 40%.
John Pick, site manager at JE Hartley, says: "The tests revealed the most effective size of RGFM varied according to the type of crop being processed. Overall, however, it's obvious the two-stage glass filter system produced a far higher quality effluent than that which our own onsite sand-based system can manage.
"Although we have a long and proud history of complying with environmental
standards, it's obvious our existing system is removing very few solids and is clearly not helping us to meet our Environment Agency consent.
"It's also encouraging that the glass-based system improved backwash efficiency and meant it was necessary less often, as this obviously opens the door to cost savings.
"The overall results of the trial are very encouraging and provided we can maintain the improved performance level through different crop changes - the products and washings we produce vary seasonally - there's every chance we'll upgrade our current process to include recycled glass.
"We're always trying to improve our impact on the environment and see RGFM as
a plus, because it will help us contribute further to sustainability."
Croda manufactures chemicals used in products made by high-street names,
with the Rawcliffe Bridge site specialising in lanolin and derivatives.
The company's products include surfactants for cosmetic creams and lotions, especially lipids for dietary supplements and fatty acid amides, the uses of which include adding "slip" to plastic bags, enabling them to be opened more easily.
Tim Uppard, process engineer at the Rawcliffe Bridge site, says: "Our introduction of the Integrated Pollution Prevention and Control standard, due to take effect in early 2006, requires the maximum level of suspended solids in our effluent to be reduced to 50mg/l, compared with our average before the trial began of 250mg/l.
"We were therefore keen to find the most suitable tertiary treatment to achieve compliance. We also wanted to find a way of reusing a proportion of our process water, as part of our continuing environmental improvement programme."
The trial, which involved a large-scale rig operating for up to 24 hours a day, saw medium-grade glass utilised in the first filter and fine grade material in the second, the combination shown to be best by the results of an earlier pilot test.
Although the full trial showed the levels of suspended solids and COD in Croda's effluent to be highly variable, it indicated the glass-based treatment, combined with a membrane system, would reduce concentrations.
Suspended solid removal
Suspended solid removal during the trial reached 90%, and was consistently at 45-55%, with the COD figure reaching a high of more than 70%. Uppard said: "We were already considering installing a membrane system to reuse a proportion of our wastewater.
"The trial has also shown that using the two-stage glass system enables us to cope with very high levels of solids without any costly pre-filtration process being necessary or the filters becoming fouled.
"In contrast, a sand filter would only be able to cope with 100mg/l at most. So, if we bought one, we'd also have to spend between £100,000 and £150,000 on a chemical pre-treatment process to reduce concentrations before filtration, and then pay the ongoing costs of operating it.
"Following the installation of a full-scale plant, we'll now do some further work to assess the most suitable system for reducing solid levels further, so we can meet any
further discharge consents and have the option of reusing water.
Uppard said Croda was extremely proud to be trialling a recycled material as a means of cost reduction and meeting discharge consents. It was now planning to publicise its participation in the tests as a way of enhancing its environmental image.
The JE Hartley and Croda trials were funded by the Waste and Resources Action Programme (WRAP), a Government-funded, not-for-profit company, established to improve the UK's recycling performance and promote resource efficiency.
Copies of Aqua Enviro's final report on the trials are available at www.wrap.org.uk or 01295 819652.
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