Addition of nitrate for odor control in sewer networks: laboratory and field experiments

Abstract

This paper presents a biological method for controlling odor problems caused by H2S originating from sewer networks under anaerobic conditions. The proposed method is based on the continuous addition of nitrate which oxidizes dissolved sulphide according to an autotrophic biological procedure and inhibits further sulphide production by sulphate reducing bacteria, until complete denitrification. The proposed method was first tested at laboratory in a 3 l anaerobic batch reactor simulating municipal wastewater of the city of Corfu in respect to sulphate concentration. Addition of nitrate in non-septic (not sulphide containing) wastewater inhibits the production of sulphide until complete denitrification. Heterotrophic denitrification rate was found 4.5 and 3.9 mg NO3- N l-1 h-1 at 250C and 300C respectively. Higher C/N ratio is, probably, responsible for the increased denitrification rate of the lower temperature. Interestingly, addition of nitrate in a septic wastewater led to a preferential autotrophic denitrification with sulphide as electron donor at a rate of 0.8 and 1.5 mg NO3-N l-1 h-1 at 25 and 300C. After complete sulphide oxidation, heterotrophic denitrification takes place inhibiting any further sulfate reducing activity. The effectiveness of the method was validated by field experiments in a 6.7 Km combined sewer network in the city of Corfu, with an average wastewater flow of 500 m3 h-1, an average retention time of 2 h and sulphide concentrations varying from 3 to 27 mg S2- l-1. Continuous addition of 6.9, 15 and 27.7 Kg NH4NO3 h-1 for a period of 4 to 8 hours led to an average sulphide removal efficiency of 84%, 98% and 99%, respectively. Based on these experimental results, a continuous addition of 10 Kg NH4NO3 h-1 is proposed for practical implementation as the optimal dosing, considering sufficient odor control and tolerable increase of the ammonia load. The proposed method is not only effective but also financially interesting taking into account the facility cost and the monthly operational cost, during the summer months of the year.