Geochemical aspects of phosphate recovery from livestock manure
Creators: name: Rabinovich, Alon
Publisher: name: No Publisher Supplied
Year: 2021

description: Accumulation of excess phosphorous from livestock manure application to crop soil is a major driver for harmful algal bloom formation in surface waters. While this practice as part of a crop nutrition management plan is agronomically sound, it is driving P deposition into vulnerable watersheds and thus in incurring stricter limitation on livestock manure soil application. The process of struvite (MgNH4PO4·6H2O) production out of livestock manure liquids at alkaline condition (pH 8-10) is an expanding practice for P recovery and removal, but high concentration of dissolved organic matter and calcium hinders struvite precipitation and limits the recovery treatment effectivity. In addition, calcium in livestock manure forms insoluble phosphate minerals such as hydroxyapatite (Ca10(PO4)6(OH)2) during the recovery treatment. Due to long residence time of livestock manure effluent in storage lagoons, ≤ 90 % of its phosphate content is complexed to colloids and unavailable for recovery. Common practices to improve phosphate recovery as struvite are acidification and anaerobic digestion pretreatments and magnesium dosing during treatment to suppress hydroxyapatite formation, all of these significantly increase operation costs. Another disadvantage of alkaline struvite production is high pH of effluent discharge that may harm crop or soil fertility and high overall residual nutrients content. This study evaluated applying aerated fluidized bed reactor approach for struvite recovery from LWE, together with aerobic pretreatment to free up complex phosphate and a combined aeration plus constructed wetland treatment post-treatment. It was hypothesized that this synergic approach will: Effectively recover phosphate without the need for digestion or acidification pretreatment of livestock manure effluent; apply post treatment to address the high pH of discharge effluent and supplement other nutrients removal. In order to further elucidate the geochemistry of interaction between struvite, dissolved organic matter and calcium, a constant composition model simulated struvite precipitation with various dissolved low molecular weight organics and calcium in simulated livestock wastewater effluents. Results from bench and pilot scale aerated fluidized bed reactor trials found an improved phosphate recovery rate when lower calcium and dissolved organic matter occurred, and that high calcium and dissolved organic matter also increased the time needed for phosphate precipitation as struvite in aerated fluidized bed reactor. This was further corroborated by constant composition model showing that the anionic low molecular weight phenol and benzoate increased struvite growth rate by an order of magnitude and suppressed the hinderance effect of calcium on struvite growth. Optimization of magnesium dosing for phosphate recovery from livestock manure found that minimal dosing was sufficient to maximize struvite content in recovered solids and that the thermal stability of ammonium (NH4) in recovered struvite improved by ≤ 34%. Finally, an evaluation of supplement treatments found that application of aerobic microbial pretreatment for cattle manure increased the content of recoverable phosphate by up to 92%, and aeration of alkaline effluent from aerated fluidized bed reactor neutralized its pH values.