Investigating pretreatment methods for struvite precipitation in liquid dairy manure

Phosphorus (P) recovery and re-use is very important today for sustainable nutrient cycling and water quality protection due to the declining global P reserves and increasingly stringent wastewater treatment regulations. P recovery as struvite (MgNH4PO4·6H2O) is a promising technology because it can be used as a slow-release fertilizer. The objective of this study was to investigate different pretreatment methods to enhance struvite precipitation in dairy manure.

Generally there are two challenges that need to be overcome to precipitate struvite in liquid dairy manure. The first is the relatively high calcium (Ca) concentration. The present study investigated the effectiveness of two calcium binding reagents to reduce the calcium inhibitory effects to enhance the struvite precipitation. A chemical equilibrium model (Visual MINTEQ 2.60) was used to determine the pH to acidify manure and to precipitate struvite. Then, bench-scale experiments were conducted to validate the model results using synthetic and untreated manure. First, the manure was acidified (pH 4.5) to liberate the particulate-bounded Mg2+, Ca2+ and inorganic P. Second, EDTA and/or oxalate compounds were added to the manure. Third, pH was increased to 7.5 for struvite precipitation from the liquid filtrate. Results showed that struvite-containing crystals were obtained from the samples treated with calcium removal compounds, with a total suspended solids (TSS) concentration less than 4,000 mg/L.

The second challenge is the low dissolved reactive P (DRP, i.e. PO4-P) to total P (TP) ratio. This study investigated different pretreatment methods for P release to enhance struvite precipitation in liquid dairy manure: enhanced biological phosphorus removal (EBPR), microwave heating (MW) and anaerobic digestion (AD). All of the pretreatment methods resulted in P release but struvite crystals were observed only in precipitates obtained in manure pretreated with MW+H2SO4 acidification. Without oxalic acid to reduce the Ca2+ effects, all the other pretreatment methods were not effective in enhancing struvite precipitation in liquid dairy manure. Hardly any struvite or crystalline-like solids were found in the non-centrifuged samples, regardless of any pretreatment, indicating the effects of suspended solids and organic matter on struvite precipitation.

A partial economic analysis was conducted to assess the chemical costs of P recovery from liquid dairy manure with different pretreatment methods; P recovery amounts as struvite were modeled by Visual Minteq 2.61.Three pretreatment conditions (untreated manure + oxalic acid, MW + H2SO4 acidification, and AD) were selected based on the precipitation results. MW + H2SO4 acidification produced the highest struvite quantities among the pretreatment methods to enhance struvite recovery, also with the lowest chemical addition costs.