Wood chips and wheat straw as alternative biofilter media for denitrification reactors treating aquaculture and other wastewaters with high nitrate concentrations

This study evaluated wood chips and wheat straw as inexpensive and readily available alternatives to more expensive plastic media for denitrification processes in treating aquaculture wastewaters or other high nitrate waters. Nine 3.8-L laboratory scale reactors (40 cm packed height × 10 cm diameter) were used to compare the performance of wood chips, wheat straw, and Kaldnes plastic media in the removal of nitrate from synthetic aquaculture wastewater. These upflow bioreactors were loaded at a constant flow rate and three influent NO₃–N concentrations of 50, 120, and 200 mg/L each for at least 4 weeks, in sequence. These experiments showed that both wood chips and wheat straw produced comparable denitrification rates to the Kaldnes plastic media. As much as 99% of nitrate was removed from the wastewater of 200 mg NO₃–N/L influent concentration. Pseudo-steady state denitrification rates for 200 mg NO₃–N/L influent concentrations averaged (1360 ± 40) g N/(m³ d) for wood chips, (1360 ± 80) g N/(m³ d) for wheat straw, and (1330 ± 70) g N/(m³ d) for Kaldnes media. These values were not the maximum potential of the reactors as nitrate profiles up through the reactors indicated that nitrate reductions in the lower half of the reactors were more than double the averages for the whole reactor. COD consumption per unit of NO₃–N removed was highest with the Kaldnes media (3.41–3.95) compared to wood chips (3.34–3.64) and wheat straw (3.26–3.46). Effluent ammonia concentrations were near zero while nitrites were around 2.0 mg NO₂–N/L for all reactor types and loading rates. During the denitrification process, alkalinity and pH increased while the oxidation–reduction potential decreased with nitrate removal.

Wood chips and wheat straw lost 16.2% and 37.7% of their masses, respectively, during the 140-day experiment. There were signs of physical degradation that included discoloration and structural transformation. The carbon to nitrogen ratio of the media also decreased. Both wood chips and wheat straw can be used as filter media for biological denitrification, but time limitations for the life of both materials must be considered.