Denitrification of Aquaculture Wastewater using Agricultural by-products as Biofilter Media

Abstract

This research evaluated wood chips and wheat straw as inexpensive and readily available biofilter media for denitrification as alternatives to commonly used plastic media. Nine 3.6-L laboratory scale reactors (40 cm packed height x 10 cm dia) were built to evaluate performance of wood chips, wheat straw, and Kaldnes plastic media to reduce nitrate from synthetic and real aquaculture wastewater. The upflow bioreactors were loaded at a constant flow rate and three influent concentrations of 50, 120, and 200 mg NO₃-N/L each for at least four weeks, in sequence.

Experiments showed that both wood chips and wheat straw are comparable to Kaldnes plastic media in terms of reducing nitrate. As high as 99% of nitrate (as high as 200 mg NO₃-N/L) were reduced. Pseudo-steady state denitrification rates at 200 mg NO₃-N/L influent averaged 1365 ± 39 g NO3-N/m³/day for wood chips, 1361 ± 80 g NO₃-N/m³/day for wheat straw, and 1326 ± 74 g NO₃-N /m³/day for Kaldnes. These values were not even the maximum potential since nitrate profile along reactors showed substantial reduction in the lower half of the reactors with rates more than double than what was estimated for the whole reactor. Treatment of real wastewater also showed comparable performance with that at the 50 mg NO₃-N /L synthetic experiments.

COD consumption per NO&#8323;-N removed was highest in Kaldnes media (3.41 to 3.95) compared to wood chips (3.34 to 3.64) and wheat straw (3.26 to 3.46). COD/NO&#8323;-N ratios for the real wastewater were low (<2.0). Effluent ammonia concentrations were close to zero while nitrites were around 2.0 mg NO&#8322;-N/L for all reactor types. There was alkalinity production and pH increased corresponding to NO&#8323;-N removed. On the other hand, the oxidation-reduction potential also decreased with nitrate removal.

Wood chips and wheat straw lost 16.2% and 37.7% of their masses, respectively over 140 days. There were physical signs of degradation like discoloration and structural transformation. Carbon/Nitrogen (C/N) ratio and lignin mass of the media also decreased. Considerable amount of microbial biomass also developed inside the reactors decreasing porosity.

By Polymerase Chain Reaction &#8212;Denaturing Gradient Gel Electrophoresis (PCR-DGGE) analysis, it was shown that the bacterial populations differed among reactor types and with column height.