Leachate quality and denitrifying gene abundance during establishment of turfgrass irrigated with tailored membrane bioreactor effluent

Abstract

Decentralized membrane bioreactors provide flexibility of management and wastewater treatment quality for sustainable wastewater reclamation and reuse in light of urban population growth and local water shortages. Wastewater reuse by subsurface turfgrass irrigation using sequencing batch membrane bioreactor (SBMBR) effluent seasonally tailored for nitrate content provides the added benefits of optimized nutrient recovery, local aquifer recharge, reduced fertilizer costs, surface and groundwater protection and improvement of the public's perception of reuse. As part of a long term investigation, the feasibility of this reuse strategy was investigated by collecting data during the turfgrass establishment phase addressing 1) nutrient leaching from warm- and cool-season turfgrasses 2) changes in soil salinity and 3) turfgrass health. Denitrification gene nirK abundance in soil with respect to depth, nitrogen content and grass type was also quantified using quantitative polymerase chain reaction assays. Eighteen turfgrass test plots were established with either a cool-season Kentucky bluegrass [Poa pratensis (L.)] Colorado Blue[TM] blend, or a warm-season buffalograss [Buchloe dactyloides (Nutt.) Eng.] cv. 'Legacy' in randomized complete block split-plot design with treatment factors in triplicate to assess the feasibility of turfgrass irrigation with a) effluent with seasonally tailored nitrate content or b) tap water and additional granular fertilizer. Ceramic cup soil suction lysimeters were installed at 10, 20 and 60 cm depths to collect soil leachate. Frequency domain soil sensors were installed at 10 and 60 cm depths to monitor soil moisture, temperature and salinity, and soil was sampled at 10, 20 and 60 cm depths. Soil and leachate samples, collected prior to and immediately following 5 weeks of treatment application, were analyzed for plant nutrients and components of salinity, sodium absorption ratio and cation exchange complexes. Genomic microbial DNA was extracted from soil samples on both sampling dates and nirK abundance quantified. Turfgrass irrigation with seasonally tailored SBMBR effluent appears to be a viable strategy for wastewater reclamation and reuse, based on observations made during turfgrass establishment. Turfgrass health and soil salinity were not affected by any of the experimental factors. Mean 60 cm nitrate concentration after treatment did not violate the federally mandated limit of 10 NO3- -N mg L-1 although concentrations from 5 of 18 lysimeters did. Average nitrate concentrations were significantly higher at 10 cm compared to lower depths, and in all plots following treatment application. nirK abundance was significantly lower in deeper soil samples and weakly correlated with treatment type, grass type and sampling date. A high level of variability in data was observed for the first sampling date but results from subsequent samplings suggest that variability should reduce over time. The continuation of the investigation into subsequent growing seasons is strongly recommended in order to improve the quality of statistical analyses by collecting more data, reducing variability within data sets and to assess long term changes in experimental factors.