The effect of sodium chloride on the denitrification of saline fishery wastewaters.

The effect of sodium chloride concentration (0.47-100 g l(-1)) on the rates of nitrate and nitrite reduction was studied at 37 degrees C and pH 7.5 in a batch reactor fed with a mixture (v/v) of salmon bloodwater and synthetic substrate and inoculated with biomass adapted to saline conditions (24 g NaCl per litre). NaCl inhibited the nitrate and nitrite reduction rates; the rates were increasingly inhibited at increasing NaCl concentrations in the studied concentration range. An empiric noncompetitive model adequately quantified the effect of NaCl on the rates of nitrate and nitrite reduction. The maximum reduction rate (R(D,max)) in a low salt medium and the inhibition constant (K(I)) were determined for each reductive step. The calculated kinetic parameters were R(D,max) = 25.5 +/- 0.74 mg N (g VSS h)(-1) and K(I)= 34.9 +/- 2.34 x 10(-3) g l(-1) for nitrate reduction and R(D,max) = 18.7 +/- 1.05 mg N (g VSS h)(-1) and K(I) = 62.5 +/- 3.32 x 10(-3) g l(-1) for nitrite reduction. NaCl has a greater effect on nitrite reduction rate than on the nitrate reduction rate.

Organic and nitrogenous matter effects on the denitrification of saline and protein-rich effluents.

The separate effect of protein concentration, nitrate concentration and carbon/nitrogen (C/N) ratio on the rate and efficiency of nitrate reduction was studied in batch reactors fed with a mixture of a synthetic substrate and a saline protein-rich salmon-plant effluent. At a constant nitrate concentration (40 mg L(-1)), the specific rate of nitrate removal decreased by 60% with increasing initial protein concentration (392 to 1900 mg L(-1)) and ammonification prevailed under these conditions; meanwhile at a constant protein concentration (1104 mg L(-1)), the specific rate of nitrate removal increased 58 times with increasing nitrate concentrations (0.5 to 78 mg L(-1)) and denitrification was the main route for nitrate reduction. The C/N ratio had an inverse effect on the specific rate of denitrification; the latter ranged from 227 to 563 [mg NO3(-)-N (g VSS d)(-1)] for a C/N ratio of 163 to 16 [mg TOC (total organic carbon) (mg NO3(-)-N)(-1)], respectively. On the other hand, the ammonia production rate was proportional up to a C/N ratio of 150.

Improvement of nitrate and nitrite reduction rates prediction

Reported models of denitrification rates integrate in an unique parameter the pH-dependent inhibition by HNO2 and the pH effect on the bacterial metabolic activity; furthermore, they do not quantify separately the pH effect on the nitrate and on the nitrite reduction rates. The goal of this work was to quantify both effects on the kinetics of nitrate and nitrite reduction to improve the models’ predictive value. Assays were performed at a pH range of 6.5-9.0 in batch reactors at 37ºC with an activated sludge. At the studied pH range and at below the HNO2 inhibitory concentration (0.004 mg L-1), the maximum nitrate reduction rate diminished 23 percent and 50 percent by decreasing or increasing, respectively, one pH unit from 8.0. The maximum nitrite reduction at pH 8.0 diminished 15 percent at pH 7.0 and 40 percent at pH 9.0. At HNO2 concentrations over the inhibitory concentration, except at pH > 8.0, the maximum nitrate reduction rate diminished 50 percent upon decreasing the pH from 8.0 to 7.0 or increasing it from 8.0 to 9.0. Inclusion of the pH effect in the reported models improved their predictive value; average deviations from the experimental data were reduced from 53 percent to 10.7 percent or 33.8 percent to 10.5 percent for nitrite and nitrate reduction rates, respectively.