Evaluation of tephra for removing phosphorus from dairy farm drainage waters

Abstract

Research was conducted in the Manawatu region, New Zealand, to investigate the ability of Papakai tephra to remove phosphorus (P) from dairy farm mole and pipe drainage waters. The capacity of this tephra to adsorb P was quantified in the laboratory using a series of column experiments and was further evaluated in a field study. In a column experiment, the P adsorption capabilities of 2 particle size factions (0.25–1, 1–2 mm) of Papakai tephra were compared with that of an Allophanic Soil (Patua soil) known to have high P adsorption properties. The experiment used a synthetic P influent solution (12 mg P/L) and a solution residence time in the columns of c. 35 min. By the end of the experiment, the 0.25–1 mm tephra removed an estimated 2.6 mg P/g tephra at an average P removal efficiency of 86%. The 1–2 mm tephra removed 1.6 mg P/g tephra at an average removal efficiency of 58%. In comparison, the Patua soil removed 3.1 mg P/g soil at a P removal efficiency of 86%. Although, the Patua soil was sieved to 1–2 mm, this size range consisted of aggregates of finer particles, which is likely to have contributed to this material having a higher P adsorbing capacity. A field study was established on a Pallic Soil, under grazed dairy pastures, to compare drainage water P concentrations from standard mole and pipe drainage systems (control) and drainage systems incorporating Papakai tephra. The 2 tephra treatments involved filling mole channels with 1–4 mm tephra (Mole-fill treatment) or filling the trench above intercepting drainage pipes with ‘as received’ tephra (Back-fill treatment). Over an entire winter drainage season, the quantity of total P (TP) lost from the control treatment drainage system was 0.30 kg P/ha. The average TP losses for the Mole-fill and the Back-fill treatments were 45% and 47% lower than the control treatment, respectively.