Unveiling the role of sediments in phosphorus removal in pilot-scale constructed wetlands for swine wastewater treatment

The accumulation rate, fractions, and sorption capacity of phosphorus in sediments determine the removal efficiency and service life of constructed wetlands (CWs). Nine pilot-scale three-stage surface flow CWs were constructed to treat three loading rates of lagoon-pretreated swine wastewater, and surface sediment samples at initial and one-year treatment were collected to analyze the phosphorus fractions and sorption capacity. After one-year treatment, concentration of total phosphorus (TP) in sediments increased for high loading rates of wastewater, but remained stable for low loading rates. The annual accumulation rate of TP in sediments (Ma) was -43-445 mg kg-1 yr-1 at surface loading rate (SLR) of 36-355 g P m-2 yr-1. Their association could be described well using a sigmoid model, i.e., Ma = -23 + 538/(1 + exp.(-(SLR-262)/48)) (R2adj = 0.897, RMSE = 40.8, p < 0.01), indicating that the phosphorus accumulation rates in sediments were loading rate-dependent. The sum of inorganic phosphorus fractions contributed to 80-100% of the TP concentration, and accumulation of aluminum-bound phosphorus (AlP) and iron-bound phosphorus (FeP) was responsible for variability of TP concentration in sediments. Phosphorus sorption capacity of CW1 sediments increased by 1.3-1.8 times, attributed to increased pH, and concentrations of ammonium oxalate-extractable aluminum and iron in sediments due to the wastewater input. Selecting iron and aluminum-rich materials preferentially as substrates and regulating the ratio of metal ions to phosphorus in wastewater should be alternative enhancement strategies of CWs for phosphorus removal.