Affiliations: Department of Environmental Science and Engineering,
Tsinghua University, Beijing 100084, China | Chinese Research Academy of Environmental Sciences,
Beijing 100012, China | Institute of Geographical Science and Natural Resource
Research, Chinese Academy of Sciences, Beijing 100101, China
Abstract: Livestock wastewater has been a major contributor to Chinese
cultural eutrophication of surface waters. Constructed wetlands are under study
as a best management practice to treat wastewater from dairy and swine
operations, but the removal efficiency of pollutants is relatively low.
Enhancing the treatment efficiency of livestock wastewater by effluent
recirculation was investigated in a pilot-scale vertical-flow constructed
wetland. The wetland system was composed of downflow and upflow stages, on
which narrow-leaf Phragmites communis and common reed Phragmites Typhia are
planted, respectively; each stage has a dimension of 4 m^2
(2 m × 2 m). Wastewater from facultative pond was fed into the system
intermittently at a flow rate of 0.4 m^3/d. Recirculation
rates of 0, 25%, 50%, 100% and 150% were adopted to evaluate the effect of the
recirculation rate on pollutants removal. It shows that with effluent
recirculation the average removal efficiencies of NH_4-N,
biological oxygen demand (BOD_5) and suspended solids (SS)
obviously increase to 61.7%, 81.3%, and 77.1%, respectively, in comparison with
the values of 35.6%, 50.2%, and 49.3% without effluent recirculation. But the
improvement of TP removal is slight, only from 42.3% to 48.9%. The variations
of NH_4-N, dissolved oxygen (DO) and oxidation-reduction
potential (ORP) of inflow and outflow reveal that the adoption of effluent
recirculation is beneficial to the formation of oxide environment in wetland.
The exponential relationships with excellent correlation coefficients
(R^2>0.93) are found between the removal rates of
NH_4-N and BOD_5 and the recirculation
rates. With recirculation the pH value of the outflow decreases as the
alkalinity is consumed by gradually enhanced nitrification process. When
recirculation rate is kept constant 100%, the ambient temperature appears to
affect NH_4-N removal, but does not have significant
influence on BOD_5 removal.
