Making Phosphorus Fertilizer from Dairy Wastewater with Aluminum Water Treatment Residuals

Core Ideas Al‐WTR sorbed P from dairy wastewater and subsequently applied as fertilizer to lettuce. The Al‐WTR acted as a P source, increasing lettuce yield and plant P concentrations. Al‐based WTR loaded with P can compete successfully with common fertilizers. Phosphorus recovery and recycling can improve the sustainability of nutrient use in agriculture. In this study, P was recovered by reacting aluminum‐based water treatment residuals (Al‐WTR) with organic matter‐rich, high P concentration (60–100 mg L –1 ) dairy wastewater. The resulting P‐rich (500–700 mg kg –1 Olsen P, ∼10 g kg –1 total P) Al/O‐WTR was applied as fertilizer in screen houses using romaine lettuce ( Lactuca sativa L. var. longifolial ) planted in Typic Haploxerolls with limited bioavailable P (<10 mg kg –1 ). Two experiments were conducted in 10‐L pots: (i) fertilizer comparison, with liquid fertilizer (N, P, K, and micronutrients), slow release fertilizer (N, P, K, and micronutrients), liquid fertilizer with no P (control), and liquid fertilizer with ∼5 g Al/O‐WTR per kg soil (∼15 t Al/O‐WTR ha –1 ); (ii) Al/O‐WTR rate comparison (∼0, 2, 3, 5, and 7 g kg soil –1 , equivalent to 0, 6, 10, 15, and 22 t Al/O‐WTR ha –1 ). In the fertilizer comparison, lettuce biomass was significantly greater in all treatments (>140 g pot –1 ) compared to the control (57 g pot –1 ). In the rate comparison experiment, lettuce biomass was also significantly greater in the Al/O‐WTR treatments (412 to 512 g pot –1 ) compared to the control (309 g pot –1 ). The highest additions of Al/O‐WTR produced the highest lettuce yield. Lettuce yield and P concentration were similar across treatments, indicating that Al/O‐WTR can provide sufficient P to perform similarly to common fertilizers. Recovery and reuse of P using Al‐WTR may therefore improve the sustainability of nutrient cycling in agriculture.