Chemical composition of organic acid digest from a municipal wastewater treatment plant and chemical modeling of nuisance struvite formation and phosphorus recovery as brushite

Abstract

Modern municipal wastewater treatment often includes an anaerobic digestion step with a hydraulic retention time of ~1.5 days before the main anaerobic digestors. This step, often termed the organic acid digester or “acid digester” for short, produces a sludge characterized by lower pH, higher volatile fatty acid (VFA) concentrations, and high soluble phosphorus (P) concentrations, particularly if fed sludge from an enhanced biological phosphorus removal process. Here, the analysis of major ions, organic and inorganic, in the organic acid digest from a wastewater treatment plant (WWTP), Nine Springs, Madison, Wisconsin (USA), is reported for a period of 13 weeks. The bioengineered origin of this organic acid digest makes its composition unlike natural waters of any kind. From the chemical composition, the course of subsequent processes can be predicted by chemical modelling. Methanogenesis using VFAs as substrates is predicted to raise digest pH to the point where struvite is expected to form, and nuisance struvite is routinely observed at Nine Springs. Addition of calcium hydroxide to organic acid digest is expected to precipitate ~90% of soluble P from solution as the calcium phosphate mineral brushite at near neutral pH, confirmed by a pilot plant at Nine Springs. This study demonstrates the possible application of chemical analysis and chemical modelling in wastewater systems to predict and possibly adaptively manage waste streams to curb nuisance struvite formation and predict P recovery, and could be refined through broader testing and application at other wastewater treatment facilities.

BioWin Modeling of CalPrex Phosphorus Recovery from Wastewater Predicts Substantial Nuisance Struvite Reduction

“…Dosing the CalPrex reactor with lime caused a predicted pH response curve characteristic of a Lime dose-response curves for CalPrex performance were produced by feeding the CalPrex reactor a series of lime doses and calculating a steady-state performance. Dosing the CalPrex reactor with lime caused a predicted pH response curve characteristic of a buffered system with an inflection point at pH 7.6 (Figure 3), here calculated by BioWin but similar to the results calculated using Visual Minteq [28,33]. The titratable acidity of VFAs and soluble phosphates characteristic of acid digestate neutralizes added hydroxides and buffers pH, but once these are neutralized, the pH becomes much more sensitive to lime addition.…”

“…These functions were replicated within the BioWin configuration using a series of unit processes parameterized to match their pilot equivalents in function. The target pH for lime dosing of the CalPrex reactor in our BioWin configurations is 7.0, at which point almost all solubilized P is expected to precipitate out as brushite [28]. Dewatering and clarification parameters were informed by the performance of the pilotscale equivalents.…”

“…CalPrex acts on the effluent of an organic acid digester, typically from a two-phase anaerobic digestion system, that is fed waste-activated sludge (WAS) from an EBPR process. The result is that organic acid digestate has a high concentration of soluble orthophosphate and a chemical composition conducive to phosphate precipitation [28]. In CalPrex operation, sludge from the organic acid digester is first dewatered, and the mildly acidic, phosphate-rich liquid is reacted with calcium hydroxide in a clarifier to a pH of ~6.5-7 to precipitate brushite from the solution.…”

Improving BioWin Modeling of Phosphorus Solubilization in Acid-Phase Digesters

“…Hydrolysis, fermentation, and acidogenesis sequentially break down complex organic molecules into volatile fatty acids (VFAs) and soluble inorganic nutrients [9,10]; the primary nutrients released include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Subsequent methanogenesis from metabolizing the VFA substrate raises the pH and can precipitate N, P, and Mg out of solution, most commonly as the ammonium magnesium phosphate mineral struvite (NH 4 MgPO 4 •6H 2 O) [8,12]. Struvite is considered a nuisance precipitate by the wastewater industry because it clogs pipes and reduces the reactor volume, necessitating costly maintenance and repairs; its prevention and removal is an ongoing area of research [8,[12][13][14][15][16][17][18][19][20][21][22][23][24].…”

“…Subsequent methanogenesis from metabolizing the VFA substrate raises the pH and can precipitate N, P, and Mg out of solution, most commonly as the ammonium magnesium phosphate mineral struvite (NH 4 MgPO 4 •6H 2 O) [8,12]. Struvite is considered a nuisance precipitate by the wastewater industry because it clogs pipes and reduces the reactor volume, necessitating costly maintenance and repairs; its prevention and removal is an ongoing area of research [8,[12][13][14][15][16][17][18][19][20][21][22][23][24]. In addition, the sludge generated from processes such as enhanced biological phosphorus removal (EBPR) contains elevated P loads from the digestion of polyphosphate-accumulating organisms, which can exacerbate struvite precipitation issues [8,12,15,25].…”