Environmentally friendly use of woodchip bioreactors in aquaculture

The project ”Environmentally friendly use of woodchip bioreactors in aquaculture” aimed to 1) reduce the environmental impact of dissolved organic matter leached from woodchips during bioreactors start-up as well as to 2) assess the capacity of woodchip bioreactors to remove commonly used disinfectants, formaldehyde and peracetic acid, from aquaculture effluents.

In work package 1, a laboratory study showed that recirculating water between a woodchip bioreactor and a protein skimmer removed 38% of the dissolved organic matter, measured as dissolved chemical oxygen demand (COD) that leached from woodchips during the first 11 days after bioreactor start-up. Recirculation through protein skimmers can thus be applied as a technically simple method to reduce the environmental impact during bioreactor start-up with removal rates of around 75 g dissolved COD per m3 woodchips achieved in a laboratory study.

In work package 2, laboratory and field studies have shown that woodchip bioreactors effectively remove formaldehyde (FA), the active substance in Formalin. Removal was due to an instantaneous adsorption (52 g FA/m3 woodchips) followed by microbial degradation. Concentration independent (0’ order) removal was 20.5 – 77.1 g FA/m3 woodchips/d at temperatures of 7-23 ºC degrees.

Microbial removal was dependent on temperature, which could be described with a Q10 value of 2.27 and an Arrhenius temperature coefficient of 1.086. Removal rates became limited by FA concentration at FA concentrations lower than 11-15 mg FA/l with first order rate constants of 1.8 – 5.2 (1/d) at temperatures of 7-23 üC degrees.

Removal efficiencies were high, up to 99.8 %, and increased with reduced FA inlet concentration and increased hydraulic retention times in the bioreactor. Removal of formalin lead to an increase in nitrate removal in woodchip bioreactors, indicating that FA was used as a carbon-/energy source for denitrification.

A laboratory study demonstrated that woodchip bioreactors achieved complete removal of peracetic acid (PAA) at inlet concentrations of 50 mg PAA/l, which were much higher than concentration of 1-3 mg PAA/L typically applied in aquaculture. Removal rates of up to 1146 g PAA/m3 woodchips were achieved at inlet concentrations of 175 mg PAA/l and a hydraulic retention time of 3.4 hours.

Woodchip bioreactors continued to remove nitrate even when nitrate inlet concentrations were as high as 350 mg PAA/l. The high treatment efficiencies for FA and PAA observed in this study were based on the large organic surface areas, long retention times and active microbial communities present in woodchip bioreactors, a combination of factors which may have the potential to promote effective removal of other types of disinfectants as well as antibiotics, too, which deserves further investigations.