What is it about?
Organic acid digest is the product of the first 1-1/2 days of anaerobic digestion at a municipal wastewater treatment plant. It is different from any natural water because of its bioengineered origin, a mixture of primary sludge and waste activated sludge. It has unusually high concentrations of organic anions like acetate and proprionate, and also high concentrations of ammonium and phosphate. Its pH is ~5 and has an ionic strength of ~0..1 M.
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Why is it important?
Organic acid digest is normally processed by further anaerobic digestion, which converts the organic anions into biogas (methand and carbon dioxide) and raises the pH to the point that the dissolve phosphate reacts with the dissolved ammonium and magnesium to form an ammonium magnesium phosphate mineral called struvite. Nuisance struvite is the bane of the existence of treatment plant operators, clogging pipes and heat exchangers and necessitating either expensive treatments or equipment stoppages. Alternatively, dissolved phosphates in organic acid digest can be reacted with added calcium hydroxide to form the calcium phosphate mineral called brushite, which can remove ~90% of dissolved pH at pH 7.
Perspectives
I first encountered organic acid digest in 2008 when I was conducting research at the Nine Springs Wastewater Treatment Plant in Madison, Wisconsin. I noticed that two-thirds of the soluble phosphate disappeared into the sludge between the organic acid digester and the anaerobic digester. It occurred to me that phenomenon could be utilized to more completely remove and recover phosphate at treatment plants by adding calcium hydroxide to both raise pH to 7 and to supply calcium to allow the process to go to completion.
Phillip Barak
University of Wisconsin Madison
Read the Original
This page is a summary of: Chemical composition of organic acid digest from a municipal wastewater treatment plant and chemical modeling of nuisance struvite formation and phosphorus recovery as brushite, PLOS Water, June 2023, PLOS,
DOI: 10.1371/journal.pwat.0000120.
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