Evaluating the potential of phosphorous recovery from wet milling and dry grind corn facilities using chemical precipitation

Phosphorous (P) flows plays a vital role in defining the sustainability of Food-Energy-Water nexus related to crop production, animal production, water pollution, resource recycling, and energy flows. Majority of domestic ethanol production in United States uses corn as its raw material. The co-products produced during the corn-ethanol production such as distillers grain (dry milling) and corn gluten feed (wet milling) are utilized as animal feed. About 50 - 80 % of the phosphorous in corn is in the form of phytate (inositol hexakisphosphate) which cannot be digested by non-ruminant animal and is released in the environment with animal waste. \n\nSide streams formed during the corn-ethanol production, such as light steep water (wet milling) and thin stillage (dry milling), contain phosphorus concentrations that are several orders of magnitude greater than wastewater, potentially enabling cost effective recovery, yet precipitation of phytate in these side streams is poorly understood. Lab-scale experimental analysis was conducted over a wide range of pH values and calcium to phosphorous molar ratios and results have shown substantial potential for chemical precipitation to recover phosphorous from these side streams.\n\nOur analysis for light steep and thin stillage observed total phosphorus removal of greater than 90% could be achieved and showed sensitivity to both pH and calcium to phosphorus molar ratio. Results from batch experiments for diluted light steep filtrate samples (100x) have shown that final total phosphorous could be reduced to as low as 25-30 (mg/l as PO43-) . Results for batch experiments for diluted thin stillage samples (33x) showed a slightly higher final total phosphorous concentration of around 40-45 (mg/l as PO43-). \n\nSolids produced from full-strength experiments for specified conditions were characterized using various characterization techniques such as elemental analysis, SEM, XRD, particle-size analysis, total proteins analysis and 31P NMR. The solids analysis indicates co-precipitation of proteins and presence of other metal ions such as magnesium and potassium in the precipitated samples.\n\nOur study indicates that excess phosphorous in light steep and thin stillage could be removed/recovered by using appropriate pH conditions and calcium to phosphorous ratios. The precipitation is not kinetically constrained and is expected to be thermodynamically controlled and thus the minimum concentrations that can be achieved under similar pH and Ca:P molar ratio should remain constant with change in dilution. Phosphorous recovery from these side streams has the potential to reduce excess phosphorous in the wastewater and the co-products generated by these plants. This could reduce direct and indirect phosphorous pollution caused by the plants.