Nitrite and nitrate as electron acceptors for biological sulphide oxidation 2

Authotrophic denitrification with sulphide using nitrate (R1) and nitrite (R2) as electron acceptor were investigated at bench scale. Different SRT (5 and 20 d) have been tested in R1 while R2 was operated at SRT = 13 d. The results indicated that the process allows to achieve complete sulphide removal in all testes conditions. Tested sulphide loads were estimated form the H2S produced in a pilot scale anaerobic digester treating vegetable tannery primary sludge; nitrogen loads originated from the nitrification of the supernatant. Average nitrogen removal efficiencies higher than 80% were observed in all the tested conditions once steady state was reached. A maximum specific nitrate removal rate equal to 0.35 g N-NO3 g SSV d have been reached in R1. Due to sulphide limitation, an incomplete denitrification have been observed and nitrite and thiosulphate tend to accumulate especially in the presence of variable environmental conditions both in R1 and R2. Lower SRT caused higher NO2 /NO3 reduced ratios (0.22 and 0.24, with SRT of 5d and 20 d, respectively) using nitrate as electron acceptor in steady state condition. Temperature decrease caused sudden NO2 /NO3 reduced ratio increasing in R1 and NO2 removal decreasing in R2.