Septic sewer treatment reduces load on STWs

Sewers treated with Nutriox are much less likely to suffer from odour problems and corrosion by sulphuric acid. They also produce effluent which is easier for bacteria to digest at the treatment works. Peter Minting reports on a study by Anette Asoy of HydroCare.

Nutriox helps to promote the digestion of effluent before it reaches the sewage treatment works (STW), according to Anette Asoy, concepts development manager at HydroCare.

The findings will have important implications in the UK, where Nutriox is already used by water companies at around 500 sites, to stop sewers producing hydrogen sulphide (H2S), after turning septic in anaerobic conditions.

This well-known problem often occurs in sewers with low flow rates and a major build-up of organic matter.

The resulting H2S leaves pipes badly corroded after reacting with water to form sulphuric acid, and when concentrations in air exceed 2-3ppm, it becomes a serious odour nuisance. Higher concentrations can present a serious health hazard to maintenance staff.

To combat this problem, Nutriox, which is basically a calcium nitrate solution, works by increasing the redox potential of nitrate relative to sulphate. This enables denitrifying bacteria to predominate at the expense of sulphate reducing bacteria, reducing the risk of septicity.

In the German study of the Bitterfeld sewer network, the effects of adding the solution were closely monitored over a period of two weeks.

Samples were taken at 17 points on the network, including 24hr samples at pumping stations and three samples per day at manholes.

In comparison with a control period without Nutriox dosing, the report concluded: ŒThere was no net change in suspended solids concentration, because the degradation of suspended organic matter balanced the production of biomass,' but Œthe change in waste water quality may result in an increased removal of organic matter in the pre-settling step of 34 per cent, which then results in a corresponding reduced load on the biological step.'

Alkalinity was also observed to increase.
Critical factors affecting the degree of effluent quality improvement were the proportion of wastewater in the network actually treated with Nutriox, along with the retention time and flow rate.

Potential problems identified included an unacceptably high total nitrogen level of 13.6 mg/l at the final sampling point, probably due to inadequate mixing, and suspected precipitation of phosphate compounds on to the sewer walls which could lead to later peaks in concentration when disturbed.

HydroCare has now developed computerised controllers which monitor key effluent parameters throughout sewer networks, such as flow, temperature and sulphide and optimise dosing levels.

The company is hoping to demonstrate that Nutriox is a cost-effective option in a market where Œnitrate salts have previously been considered too expensive.


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