A process for the elimination of septicity based on controlled addition of nitrate to sewer networks is developed. The process is registered as the NutrioxTM process. The work described in this paper gives the results of a full scale trial in the UK. The trial was set up and managed by Norsk Hydro and independently monitored and evaluated by the Water Research Centre Environmental Management (WRc). The results showed that dosing of nitrate was very effective for suppression of hydrogen sulphide in a rising main. During nitrate dosing the average level of hydrogen sulphide at the works inlet was 0.24 mg/l, with a daily variation within the range of 0-0.4 mg/l. Without nitrate addition the average background level of hydrogen sulphide was 4.2 mg/l, with a daily variation of 1-10 mg/l. The dosing was accomplished without significant breakthrough of nitrate to the treatment works. Dosing of nitrate resulted in an increased removal of soluble BOD across the rising main, but no significant change in total BOD or COD was registered at the treatment plant during the nitrate dosing. The nitrification process in the biological filters at the treatment plant was improved during the dosing period.

Changes in water quality in sewers may have significant effects on the performance of treatment plants. Experiments have been carried out in two pilot scale sewers for studies of biofilm growth and changes of water quality with anoxic (dosage of nitrate) and anaerobic (septic) conditions, respectively. The thickness of the biofilm in the anoxic sewer was 1-2 mm as compared to 0.3-0.6 mm in the septic sewer at a water velocity of 0.5 m/s. An increase in the water velocity from 0.002 m/s to 0.5 m/s gave thicker and denser biofilms. Sulphide formation was prevented in the anoxic sewer, and influent sulphide was efficiently removed. The denitrification rate in the anoxic sewer was 3.5-4.3 g NO3-N/m2d (15°C). This gave a considerable reduction in both organic matter and phosphorus and an increase in pH and alkalinity. The potential to remove organic matter in sewers may be exploited as a pre-treatment to mechanical and chemical treatment plants in order to meet more stringent effluent standards for organic matter discharge. Removal of readily biodegradable organic matter in the sewer may, however, be a disadvantage for treatment plants based on pre-denitrification. A reduced phosphate concentration may give a reduced metal coagulant dose at chemical treatment plants, whereas an increase in pH and alkalinity may give an increased dose. In the septic sewer, there was a small reduction in the organic matter, but no significant net change in total phosphorus, pH and alkalinity on average. The production rate of sulphide was in the range of 0.3-0.7 gS/m2d (15°C). On average there was no change in the concentration of suspended solids in either of the sewers. A sporadic large sloughing of biofilm was observed, particularly in situations with a very diluted wastewater. Sporadic high concentrations of suspended solids may cause separation problems at treatment plants.

Septic wastewater, characterised by the appearance of sulphide, is known to cause problems in sewage systems (corrosion and odour), at treatment plants (e.g. inhibition, sludge bulking) and for human beings (toxicity). Sulphide formation in sewers may be prevented by increasing the redox potential, either by oxygen/air injection (aerobic conditions) or dosage of nitrate (anoxic conditions). The effect on the nitrification capacity in a biofilm process of an anoxic wastewater as compared to a septic wastewater has been studied. The main change in wastewater quality as a result of nitrate dosage is reduced concentrations of organic matter and insignificant sulphide concentrations. The results show that a sulphide concentration of 0.5 mg/l had a considerable negative effect on the nitrification activity. The sulphide and the higher concentrations of organic matter in the septic wastewater caused together a 30-40% reduction of the nitrification capacity as compared to the anoxic wastewater, even with pre-aeration and pre-precipitation with Fe3+. The removal of organic matter in the sewer as a result of the anoxic conditions created by the addition of nitrate, resulted in a maximum nitrification capacity when particulate organic matter was removed by pre-precipitation.

It has generally been accepted that concrete corrosion is caused by bacterial oxidation of hydrogen sulphide in sewage systems. Costs related to sewer replacement and remediation are quite high, but there is limited knowledge and documentation on the relationship between hydrogen sulphide levels and corrosion rates. This is necessary information in order to select the appropriate means of hydrogen sulphide control and to conduct cost-benefit evaluations. This pilot scale study shows that the concrete corrosion rate can be modeled by a Monod type function with Ks=2 ppm H2S in gas and a maximum concrete corrosion rate at 25°C of 16 mm/year. Complete hydrogen sulphide control with 0 ppm H2S in water and gas is required to prevent concrete corrosion. This can be achieved by controlled treatment with nitrate using the Nutriox® Concept where the nitrate dose is based on flow, temperature, sewer design, and sewage concentration. The local conditions will be important for the cost-benefit evaluations, but in general terms, one can say that the longer the hydraulic retention time is, the more cost effective will a controlled treatment with nitrate be for corrosion control.

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