Evaluation of passive reduction of nitrate from greenhouse effluent by planted bioreactors

A number of pilot-scale gravel and wood-chip hybrid bioreactors planted with select species, together with unplanted units, were evaluated for their nutrient removal capabilities from the typical greenhouse effluent with high levels of nitrate and salts. Two levels of nutrient solution (high and low loading: HL/LL) were prepared to simulate the typical characteristics of the greenhouse effluent. The wood-chip bioreactor with Typha angustifolia exhibited the highest consistent nutrient treatment with an average nitrate reduction in the LL phase of 88.4% (28.2 g N m −3 media day −1 ) and phosphate reduction of 34.4%. The nitrate reduction in this bioreactor was the highest among the values reported in the literature. The near-complete denitrification developed provided a nitrate-limiting environment as evidenced by an average 21.5% sulfate reduction. The distinct increase in the outflow organic carbon (as BOD 5 ) from the wood chips in the bioreactor planted with T. angustifolia appeared to be the key explanation for the efficient denitrification, while the other vegetated bioreactors resulted in 19.0%–36.5% nitrate reduction and low outflow BOD 5 near the end of the experiment, indicating carbon limitation in these bioreactors.