Treatment efficiencies of biogas slurry by earthworm-constructed vertical flow and horizontal subsurface flow constructed wetlands system

Abstract: Anaerobic digestion slurry is wastewater with a characteristics of high solid content, high organic loads and hard for biodegradation. In this paper, Earthworm-constructed Vertical Flow (EVF) and Horizontal Subsurface Flow (HF) Constructed Wetlands System was designed based on the ecological functions of earthworms. The aim of this research was to investigate the potential of integrating earthworms into the vertical flow constructed wetlands to determine if they could mitigate clogging problems as well as to improve the wetland treatment performances. The Constructed Wetlands Systems were filled with sand and ceramsite as the main media. In this paper, hybrid constructed wetland system (a vertical flow wetland and followed by a Horizontal Subsurface Flow Constructed Wetland) was constituted to process anaerobic digestion slurry. Earthworms were added in the VF, a VF without earthworms was set as a contrast at the same time. We also set up two hydraulic loading 10, 20 cm/d, so there were four hybrid constructed wetland systems. After the system was in a stable performance mode, anaerobic digestion slurry was pumped into the systems. Effluents from each sample point were sampled and tested every six days during the two-month operation. Treatment efficiencies and the clogging were determined. Research results showed that when the average influent organic load reached 1.82 kg/(m2∙d), hydraulic loading 10, 20 cm/d, COD removal efficiency reached 90%, TP removal efficiency reached 94% and SS removal efficiency reached 97%, ammonium removal rate reached more than 80%, and the total nitrogen removal efficiency was also maintained above 60%. Adding earthworms in 1st stage (VF), COD, ammonia nitrogen, total phosphorus can meet emission standards under hydraulic loading 10, 20 cm/d. On the other hand, after the treatment of the Horizontal Subsurface Flow (HF) constructed wetland, COD, ammonia nitrogen, total phosphorus, Suspended solid (SS) can meet emission standards. Adding earthworms increased removal of COD, NH4-N, TKN by 2%, 12%, 4%, respectively when the hydraulic loading was 10 cm/d. Improving the hydraulic load reduced greater amount of pollutants but the treatment efficiency decreased. Thus the processing costs were greatly reduced by improving the hydraulic load on the premise of effluent standards. The performance of horizontal subsurface flow wetlands in mitigating TN and TP was better than earthworm-constructed vertical flow wetland. Earthworms had the effect of synchronous treatment of sewage sludge and mitigating clogging. As such sludge handling cost would be greatly reduced, and therefore Earthworm-constructed Vertical Flow and Horizontal Subsurface Flow Constructed Wetlands System have great potential applications in treating anaerobic digestion slurry.