Nitrogen and phosphorus removal from wastewater added to sand by wheat straw addition and wheat plants

Abstract

Wastewater irrigation can add nutrients to soils, but also increase nutrient leaching, particularly in sandy soils. For sustainable use of wastewater, nutrient leaching should be minimized. It is unclear how wheat straw amendment to sand or wheat growth on sandy soil influences removal of N and P from wastewater. This thesis aimed to investigate (1) the ability of wheat straw to remove inorganic N and P from wastewater collected from a sewage treatment plant when mixed into sand at different rates (Experiment 1) and decomposition stages of the straw (Experiment 2), and (2) the effect of wastewater irrigation at different early growth stages of wheat plants on nutrient uptake (Experiment 3). In the first experiment, wheat straw was mixed with sand at 2.5, 5, 7.5, 10, 12.5 g kg-1 in leaching columns before adding wastewater. The control was unamended sand. Leaching was conducted on 4, 8 and 16 days after wastewater addition. With straw amendment, nitrate in the sand-straw mixes was lower than in sand alone while ammonium was higher at 12.5 g straw kg- 1. Over 95 % of inorganic N from added wastewater was removed irrespective of straw rate. Straw amendment had no consistent effect on P leaching. In the second leaching column experiment, sand was mixed with wheat straw at 12.5 g straw kg-1 and incubated moist for 7 or 14 days or added just before adding wastewater (fresh straw). The control was unamended sand. Leaching was conducted 4, 8 or 16 days after wastewater addition. With straw amendment, available N in the sand-straw mixes was highest in fresh straw on day 16. Leachate inorganic N was much lower than in sand alone irrespective of straw decomposition stage. In both leaching column experiments, very little N2O was released, suggesting that denitrification was not an important process. Likely mechanisms for nutrient removal by straw are dissimilatory nitrate reduction to ammonium and nutrient binding to straw. It was concluded that mixing wheat straw into sandy soil prior to wastewater application can substantially reduce inorganic N leaching. In a pot experiment, sandy soil was left unplanted (control) or planted with wheat, which was grown for 7, 14 or 21 days before wastewater addition. All pots received reverse osmosis (RO) water for 20 days. Half of the planted pots and unplanted pots were irrigated with wastewater from day 21 to 35, the other pots still received RO until day 35. Wastewater irrigation increased N uptake compared to RO irrigation only in plants that were 21 days old before wastewater addition but had little effect on plant growth and on inorganic N and P in soil. However, presence of wheat reduced available N and P in soil compared to unplanted soils which would reduce potential of nutrient leaching after wastewater irrigation. It can be concluded that inclusion of organic amendments and/ or suitable crops are the potential options for wastewater reuse on sandy soils. Field experiments should be carried out to confirm the applicability of these effects.