Nitrogen processing gradients in subsurface-flow treatment wetlands—influence of wastewater characteristics

Abstract

Nitrification, an aerobic microbial process, is generally considered to be the rate-limiting step for N removal in subsurface-flow (SSF) constructed wetlands treating organic wastewaters. SSF wetland nitrogen (N) processing gradients were investigated using cascade mesocosms comprised of five planted ( Schoenoplectus tabernaemontani), gravel-filled tanks operated in series, in order to determine the effects of organic substrate availability (measured as chemical oxygen demand, COD) and partial pre-nitrification on rates of N removal. Duplicate sets of cascades supplied with 23 mm d −1 of four different organic wastewaters provided COD:N ratios of ∼2–15 (<1–30 in individual tanks) and a range of N species balances (34–88% ammoniacal-N, NH 4-N; <1–40% oxidised N, NO x -N). Mass balances for organic N (Org-N), NH 4-N and NO x -N, and COD removal were calculated for each tank of the cascades. Concurrent measurements were made of plant growth and N uptake, sediment N accumulation and selected biogeochemical indicators (redox potential, pH, and CO 2 and CH 4 emissions). Using a simplified model of sequential N transformations and sinks, average net rates of N mineralisation ranged from 0.22 to 0.53 g m −2 d −1, nitrification from 0.56 to 2.15 g m −2 d −1, denitrification from 0.47 to 1.99 g m −2 d −1 (60–84% of measured N removal) and plant assimilation from 0.28 to 0.47 g m −2 d −1 in the cascade tanks. Nitrification and denitrification occurred concurrently with COD removal, even in the upstream stages of cascades receiving the higher-strength wastewaters. Surprisingly, neither net areal nitrification rates, nor first order nitrification rate constants ( k A) were correlated with COD removal rates (as a measure of heterotrophic oxygen demand). Nitrification rates were correlated with average NH 4-N concentrations in the cascade tanks, and were closely paralleled by net denitrification rates. Although k A for N mineralisation, nitrification and total N removal were highest for the partially pre-nitrified wastewater tested, considerably higher areal mass removals were achieved in the cascades receiving more concentrated ammonium-rich wastewaters. The oxygen demand required to support full nitrification of ammonia and mineralised Org-N in the cascades (without accounting for competitive heterotrophic oxygen demand) was in the upper range of that expected to be able to be supplied through surficial and plant-mediated oxygen transfer. Implications for understanding the nature of coupled nitrification–denitrification and COD removal in SSF treatment wetlands are discussed.