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

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.