Production of renewable bioproducts and reduction of phosphate pollution through the lime pretreatment and acidogenic digestion of dairy manure

Acidogenic digestion with lime pretreatment was proposed as a means of reducing soluble reactive phosphorous concentrations in dairy cow manure while producing calcium carboxylate salts as a chemical platform for production of fuels and chemicals via the MixAlco Process. When concentrated in small geographic areas, large dairy operations can have environmentally harmful impacts on their watersheds, due to excess nutrients causing eutrophication of local water bodies. In Central Texas, as in many other areas, phosphorous contamination from confined animal feeding operations is a leading cause of human induced eutrophication. It was found that lime pretreatment followed by acidogenic or methanogenic anaerobic digestion led to substantial reductions in soluble reactive phosphate (SRP) levels while also producing potentially valuable products. Dairy manure filtrate was analyzed for SRP before and after the process steps of lime pretreatment and anaerobic digestion. An average reduction of SRP by 89% was observed through the two step process for lime pretreatment coupled with acidogenic digestion. Acidogenic digestion was demonstrated to produce mixed organic acids at mesophillic temperatures and primarily acetic acid at thermophillic temperatures. Using the MixAlco Process, the carboxylate salts of these organic acids could potentially be converted into marketable fuels and chemicals.