Optimization of Phosphorus Removal and Mineral Characterization in Aerated Anaerobic Digestate with Divalent Cation Addition

Abstract

Struvite and other minerals often form at WRRFs when phosphorus-rich activated sludge from EBPR processes undergo anaerobic digestion. Without proper control, these minerals can cause scale deposits that clog pipes and damage dewatering equipment. However, managing when and where these minerals form allows for phosphorus recovery as a sustainable fertilizer or its sequestration into Class A biosolids, creating a nutrient-rich soil product. This approach can reduce phosphorus recycling in solids handling, prevent scaling issues, and enhance the value of biosolids as a marketable, sustainable product. In a digested solids storage tank that is in between anaerobic digestion and final dewatering, precipitation was removed in a controlled pilot setup by manipulating variables such as mixing, aeration to achieve a low solids retention time (SRT) post aerobic digestion (PAD) and by chemical addition. Optimizing this via pilot testing can reduce struvite buildup, enhance phosphorus removal, and provide additional benefits such as nitrogen and COD removal. The pilot setup at Hampton Roads Sanitation District's Atlantic Treatment Plant (ATP) consisted of four 45-gallon tanks operated as daily batch fed continuously stirred tank reactors with a 3-day SRT. Aeration and chemical addition of Mg(OH)2 and Ca(OH)2 at varying Ca2++Mg2+:P ratios were tested under different aeration settings of constant, pH or dissolved oxygen (DO) setpoints. Results showed that DO setpoint-controlled aeration stabilized pH and DO and enhanced OP-P and NH3-N removal, with maximum OP-P removal of 97 percent achieved at a Ca2++Mg2+:P ratio of 1.3:1. NH3-N removal, mainly from struvite precipitation, averaged 10–20 percent, with no nitrification observed likely due to free ammonia inhibition. Alkalinity reduction exceeded predicted levels, suggesting additional coprecipitation reactions occurred. Aeration and mixing consistently achieved about 20 percent total COD removal across the pilot in microaerobic conditions. The chemical equilibrium model Visual MINTEQ was also explored to determine the effects of pH, temperature, chemical choice and dose on minerology. A validated model could then be used to predict scaling potential and characterize minerals formed at WRRFs to better treat these nuisance scaling minerals and determine their potential for harvesting or sequestration as a nutrient-rich product. With the increasing need to not only remove but recover nutrients for beneficial reuse, it is imperative to deepen the understanding of successful phosphorus precipitation in biosolids and how it can be optimized and modified for various solids handling configurations.