The use of aeration in constructed wetlands and the potential for earthworm and crop production

Abstract

Constructed wetlands (CWs) are used in a variety of industries to treat effluent for safe reuse or discharge. They can however take up large areas of land. Adding oxygen can increase treatment efficiency, as it is vital to many nutrient removal pathways, such as nitrification and aerobic respiration. Increasing efficiency can decrease the cost of treatment and reduce land area needed to operate a CW. The study aimed to find the best method of aerating wetlands treating brewery effluent, the potential production of worms and plants in the system and their effects on water treatment. The first experiment tested a hybrid constructed wetland, using small (1.0 m3) ebb and flow filtration additions to a horizontal subsurface flow constructed wetland against nanobubble aeration in a similar horizontal flow wetland cell. It was not possible to add replicates to the study, but findings prompted further investigation into the hybrid setup. Experiment two focussed on the ebb and flow filters within the hybrid system comparing the presence of plants and worms. The ebb and flow filters increased dissolved oxygen (DO) from an average inflow of 3.89 mg/L to an outflow of 5.70 mg/L and decreased the ammonia content of the effluent from 14.8 mg/L to 11.2 mg/L. Swiss chard (Beta vulgaris) was successfully grown in the system and was found not to affect DO. The addition of both Swiss chard and earthworms (Eisenia fetida) was found to decrease PO43- by 17.69 ± 1.36 %. Experiment 3 compared the previous ebb and flow filters to trickle filters with the addition of celery (Apium graveolens) to both systems to determine which filter treated the effluent more effectively. The ebb and flow filters performed better than the trickle filters, increasing the DO by 37.54 ± 0.06 % compared to 19.22 ± 0.06 %. Celery increased this change in both systems, to 44.16 ± 0.06 % in the ebb and flow filters and 30.96 ± 0.06 % in the trickle filters. The ebb and flow filters were also able to decrease the NH3-N concentration by 36.1 ± 0.2 %. This effluent could sustain plant growth; Swiss chard grew at a rate of 0.04 ± 0.02 g/g/d and celery at 0.2 ± 0.1 g/g/d. Ebb and flow filters are better at aerating brewery effluent and decreasing nutrient load than nanobubble pumps and trickle filters. The addition of ebb and flow filters to the CW treating brewery effluent has the potential to improve nutrient removal and provide an edible crop.