The effect of treating farm dairy effluent with varying rates of poly-ferric sulphate on reducing phosphate leaching through agricultural subsurface drains : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

The application of farm dairy effluent (FDE) to land is a significant source of phosphate entering waterways. Studies have shown that treating FDE with 250 mg Fe/L of poly-ferric sulphate (PFS) solution can significantly reduce phosphate leaching, however it remains unknown if this can be achieved at lower rates of PFS treatment. A lower treatment rate would reduce cost, which could increase farmer use, and thus potentially help to reduce overall phosphate loss to water. The objectives of this study were therefore to: 1) determine the effect of different poly-ferric sulphate treatment rates on the reduction of phosphorus leaching from subsurface tile drainage systems; and 2) determine the effect of land application of treated effluent on GHG emissions, by quantifying nitrous oxide (N 2 O), carbon dioxide (CO 2 ), and methane (CH 4 ) emissions from the soil lysimeters receiving effluent treated with a range of rates of poly-ferric sulphate. A field trial was conducted to evaluate the impact of different PFS treatment rates on the concentrations of dissolved reactive phosphate (DRP), total dissolved phosphate (TDP), and total phosphate (TP) in drainage water. This water was collected from soil drainage units that underwent two effluent applications over a one-year period. The study investigated the efficacy of three PFS rates (250 mg Fe/L, 167 mg Fe/L, and 83 mg Fe/L) and compared the results with those of untreated FDE, relative to background (control) leaching losses. Treatment rates of 250 mg Fe/L and 167 mg Fe/L proved equally effective, reducing the DRP leaching loss factor by 111.13% and 107.77%, respectively. The P250 mg Fe/L and 167 mg Fe/L treatment rates reduced the TDP leaching loss factor by 115.47% and 112.85%, respectively, and the TP leaching loss factor was reduced by 85.02% and 90.72%, respectively. Reductions greater than 100% indicated that the treatment reduced leaching losses to levels below those of the control (no effluent applied). The 83 mg Fe/L rate reduced the DRP and TDP leaching loss factors by 80.56% and 80.17%, respectively, and did not cause a significant reduction in the TP leaching loss factor. This suggests that P leaching can be significantly reduced using a lower rate of PFS than previously reported. The results were attributed to the reactions which occur between the phosphate in effluent and the ferric hydroxides formed in the PFS-treated effluent. These reactions reduce the concentration of DRP in the treated effluent and reduce the mobility of phosphate within the soil matrix when PFS- treated effluent is applied to land. Greenhouse gas emissions of N 2 O, CO 2 , and CH 4 showed no significant difference between the untreated FDE treatment, PFS treatments, and the control.