Biological filter beds usually consist of a mineral media over which effluent to be treated percolates. The media becomes host to a biomass which is grazed by a series of more complex organisms. If the growth of the biomass is greater than the rate at which the grazing occurs then the biomass film will accumulate in the bed. This will block the interstices of the media and restrict effluent drainage and ventilation of the bed. Under these conditions nitrification is usually the first thing to suffer, however reduction in BOD removal can also be frequently observed.
Greener Waste's remediation products and techniques have been used in such situations as they present an economic alternative to purely physical film removal. The following presents case studies of two such projects:
Case Study One:
Works R consisted of two pairs of circular beds. Each bed was approximately 900 m3 of 50mm granite and two of the beds had inferior performance to the other pair. Previous examination had indicated that media breakdown was not the cause of the loss of performance and that the Bio-Augmentation cleaning program should benefit the system.
The work took place on the worst performing bed over three weeks in August 2002. A bacteria blend is dosed on to the bed and washed in to the media using pumped final effluent. The visual impact was that large quantities of solids were released and this was confirmed by analysis of the filter effluent.
Neutron scatter analysis indicated that the film had reduced from 33% to 11%.
The bed was then re-seeded using a suitable bacteria mix and put back into operation. Average ammonia before the trial from the test bed had been 6.45mg/l and reduced to 1.3mg/l after the cleaning process. Long term analysis for the whole site up to 2006 has now been examined and the results indicate that average BOD has improved from 10 to 7.5mg per litre and average nitrification has improved from 72% to 95%.
Case Study Two:
Works K had two rectangular filters of 2700m3 each one of which had such a history of poor nitrification and it had to be switched off some years previously to maintain the sites compliance. A requirement to re-commission this filter had occurred and performance was still giving cause for concern.
A similar exercise to works R was carried out this time lasting some 10 weeks. Unfortunately the bed could not be isolated from the rest of the site and subsequently any products used also impacted on the other beds. The result was that the de-commissioned bed was successfully re-commissioned with the added benefit of improved performance across the untreated beds due to the effect of bacteria carry over in the returned sludge liquors.
Filter Bed `K` Results
Cost Comparison of Bio-Remediation v Traditional Cleaning Method
The alternative proposal to the Bio-Remediation was to dig out the old media and replace with new. This would have severely jeopardised the ability of the site to meet its compliance requirements as an additional active filter would have had to be taken out of service and temporary treatment processes installed on site during the remedial work. The Bio-Augmentation was carried out without removal of the media and without disruption to site services or operation. The cost comparison (although this case study is old and pricing has now changed) is as follows:
Filter Bed `R` Total Cost for 4 Weeks = £35,000.00
Replacement Cost = £280,000.00
Filter Bed `K` Total Cost for 10 Weeks = £160,000.00
Replacement Cost = £1.2 million
The filter beds were successfully cleaned without the need for the dismantling of distribution equipment, without removing the media, without adverse impact or disturbance on the structure of the bed (retaining walls and drainage tiles) and with minimum interference to site operation.
The Site was able to operate normally and full flow to the filters could be facilitated at night time.
The process enabled significant cost savings to be achieved by the water utility over the traditional cleaning process resulting in a fully operational asset with the benefit of a performance warranty for 36 months.
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