Treatment of agricultural wastewater with constructed wetlands.

摘要

Animal manure contains essential plant nutrients such as nitrogen, phosphorus and potassium along with high number of bacteria, viruses and parasites. Pollutants contained in manure enter water bodies both as diffuse or non-point source and as point source from concentrated livestock production systems. Pathogens may flow to the water bodies when manure is applied to fields prior to rainfall. Nutrients that reach water bodies cause eutrophication and pathogens pose health risk.;Two separate studies were conducted to evaluate the applicability of constructed wetlands in treating pollutants originating from animal manure. One constructed wetland system was spiked with high number of E. coli and bacteriophage P22 for a short period of time to simulated tile-drain flow and the number of E. coli and bacteriophage P22 in the effluent were monitored in winter and summer seasons. The other constructed wetland system was continuously supplied with diluted dairy wastewater and removal of pollutants and recovery of nutrients were measured.;On average, 0.54 and 0.69 log reduction of E. coli were obtained in summer and winter months, respectively from the surface flow (SF) wetlands subjected to pulse loading. With similar loading, 3.16 and 1.23 log reduction of E. coli were obtained from subsurface flow (SSF) wetlands in summer and winter months, respectively. E. coli removal in subsurface flow wetland was higher than in surface flow wetland in both seasons. Two models one based on the convection dispersion equation (CDE) and the other based on colloid filtration theory did not adequately describe E. coli removal in constructed wetlands. Higher removal of bacteriophage P22 was observed in both SF and SSF wetlands in both winter and summer months in the wetlands subjected to pulse loading. P22 removal rates in SSF wetlands were 41 times the removal rate in SF wetlands in winter and 19 times in summer. The CDE model could accurately describe bacteriophage P22 removal in constructed wetlands.;In the wetland systems that were subjected to continuous manure loading, chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and E. coli in influent and effluent were measured. Duckweed was harvested every week to explore the nutrient recovery potential. Average COD, TN and TP removal obtained in surface flow wetlands from dairy wastewater were 28%, 28% and 16% respectively. Average annual mass removal of COD, TN and TP in the wetlands were 2137 g COD/m2/year, 149.5 g N/m2/year and 10.3 g P/m2/year, respectively. First order removal model that includes background concentration was found more suitable than first order model or DUBWAT model for predicting effluent COD, TN and TP removal in constructed wetlands. Average N and P recovered by harvesting duckweed across all the wetlands were 22.4 g N/m2/year and 5.6 P/m 2/year, respectively.