You can`t always rely on those old rules of thumb for wastwater treatment!

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Source: Strathkelvin Instruments Ltd.

Cumbria Waste Management (CWM) wastewater treatment studies have produced some surprising results proving that you can’t always rely on those old rules of thumb for treating wastewaters.

CWM is a local authority waste disposal company with 3 operational landfill sites at Distington near Workington, Hespin Wood near Carlisle and Flusco near Penrith. Prior to disposal to sewer, all of the leachate from these landfills is treated on-site in biological treatment lagoons that are run as sequential batch reactors (SBR). These routinely achieve removal rates of >99% for both ammoniacal nitrogen and BOD.

As a consequence of the introduction of the Landfill (England & Wales) Regulations 2002, the disposal of all liquid wastes to landfill will be banned from October 2007, therefore a new disposal route needs to be found for these wastes. As the Distington leachate treatment plant has been identified as having spare treatment capacity, and should in theory be able to successfully treat most of the predominantly aqueous non-hazardous wastes being landfilled at present, it has been proposed that a Liquid Waste Acceptance Facility be built adjacent to the existing plant. This would provide a continuing disposal service to current customers and hopefully an increasing source of revenue in future through new business.

Of paramount importance in the introduction of off-site wastes, which could be potentially detrimental to the treatment process, is safeguarding the bacterial biomass. Partial or complete loss of the treatment capability of the biomass could result in exceedance of the site discharge consent with possible financial penalties and/or prosecution. In addition, the tankering of leachate to an off-site WWTP would increase disposal costs substantially, and re-seeding and re-establishment of an SBR would take many weeks. Therefore being able to rapidly deal with incoming tankered wastes, and continuously monitoring the plants performance would be key activities in protecting the plants operation.

After witnessing an on-site demonstration of the Strathtox respirometer by Strathkelvin personnel using the Distington plant’s own bacteria where the ease of use and especially the speed in obtaining useful results was shown, CWM decided to purchase the instrument. Strathtox will be used to test incoming liquid wastes, the purpose of which is to act as one of the main lines of defence in ensuring that the health of the biomass is not compromised.

Despite the fact that the treatment of tankered liquid waste through the leachate treatment plant has not yet commenced, CWM have still been able to utilise the Strathtox to investigate in advance the possible inhibitory effects of many of the liquid wastes which have been envisaged as eventual candidates for treatment, such as interceptor wastes, soluble oil and water-miscible solvent solutions. Most of these tests have produced the anticipated results with regards to depression/elevation of respiration rates, however, some unusual findings have also come to light.

For example, it was found that solutions of simple substrates that had previously been assumed to be readily metabolised by the biomass (namely glucose, sucrose and methanol) did not exhibit anything like the expected increase in oxygen uptake and therefore biodegradation capacity. Plans to incorporate one of these substrates into CWM’s standard feed solution to be used in the Strathtox tests were therefore abandoned, instead various feed solutions of sodium acetate (one of the major degradation products found in leachate) were trialled and were successful in eliciting the desired response. Similarly, CWM found that biomass from the Distington plant could comfortably treat leachate from any of the three CWM landfills whereas the respiration rate of the Hespin Wood biomass was significantly decreased when exposed to leachate from the Distington site (previously a hazardous waste site which accepted heavy metal compounds, phenolics, herbicides, etc.).

From these observations it was concluded that the bacteria in the biomass were more effective in metabolising those substrates which they were ‘used to’, i.e. those that they had been regularly exposed to in the leachate. However the biomass can, given time, evolve metabolic pathways to deal with simpler substrates (as proved with CWM’s own in-house laboratory trials with methanol), or become adapted to deal with moderately high concentrations of toxic compounds (as is the case at Distington).

The outcome of their tests with the Strathtox to date have shown that it would be unwise to take for granted the ability of the biomass to instantaneously treat what would previously have been considered innocuous liquid wastes.

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