Enhanced phosphorus recovery from anaerobic dairy effluent through acidification or ozonation pretreatments combined with biochar and iron chloride

Abstract

Anaerobically digested (AD) dairy effluent is rich in phosphorus (P) and nitrogen (N) and requires treatment before discharge. Effluent from the AD is often centrifuged to reduce the suspended solids, however, the supernatant liquid AD fraction (LAD) still contains high concentrations of P, N, and small diameter suspended solids. A promising approach for recovering nutrients from LAD is to use biochar as a filter aid to remove N, P, and suspended solids. The recovered biochar can be used to recycle nutrients by amending it to soil as a biochar-based fertilizer. To aid in P and suspended solid removal from the LAD, iron (Fe) can be dosed with biochar, and can be preceded by acidification or ozonation pretreatment to increase solids separation. In this study, five biochar doses (1, 2, 4, 6, and 8 g L⁻¹) and four Fe doses (2, 3, 6, and 15 mmol L⁻¹) were evaluated, with pretreatments consisting of acidification of LAD to pH 5 or 6.5, or ozone dosing applied prior to biochar and iron addition. Increasing Fe dose concentration from 2 to 15 mmol L⁻¹ in the 6 g L⁻¹ biochar treatment increased total-P removal from 24 to 44%. Acid pretreatments at pH 6.5 and 5 in the 6 g L⁻¹ biochar treatment with Fe dose of 15 mmol L⁻¹ increased TSS removal from 50 to 86%, and increased total-P removal from 64 to 85%. Ozonation also decreased TSS and increased the P concentration in the recovered solids. These results offer insights into the conditions needed to optimize production of a biochar-based fertilizer using dairy anaerobic digester effluent, which is a key step toward realizing the sustainable bioeconomy potential of dairy operations.