Anaerobic Digestion with Nutrient Control - What if your Digesters were the Sidestream?

Traditional Anaerobic Digestion processes can create undesirable challenges at WWRFs. Nitrogen recycle from dewatering can have a significant impact on the overall loading to the facility and Phosphorus released under anaerobic conditions can negatively impact dewatering performance. These problems increase chemical and energy costs and create many operational challenges. These proven processes are well-established and employed (independently) at several facilities but, when they are used in concert with the addition of a recycle loop, the solids process effectively becomes sidestream nutrient removal. Managing these nutrients in solids processes that are optimized for the desired biology has other side-effects including decreased H2S production, reduced odor and more robust digester performance. Mesophilic Anaerobic Digestion (MAD) is commonly used in larger wastewater treatment plants in order to stabilize solids and generate biogas for heat and energy. While various staged digestion approaches have been understood for years and implemented at some facilities, the added complexity (often with only marginal benefits) has caused many WRRFs to stick with conventional MAD. The benefits of post-aerobic digestion (PAD) following anaerobic digestion and and Acid reactor ahead of MAD have been generally understood, if not widely used. However, those two 'add-ons' had traditionally been viewed as independent. Data from a PAD-only facility that has been in operation for 10 years will be discuss in conjunction with recent pilot testing at several facilities that consistently confirms significant results when both an Acid reactor and PAD are added to conventional MAD digesters with a recycle loop to capitalize on the biology and chemistry resulting from the conditions in those reactors. H2S concentration in the biogas was significantly reduced, without the addition of ferric chloride or other chemicals, which would significantly extend the life of H2S removal systems and reduce odor and corrosion challenges and lower chemical costs. Because the process better controls the pH and Ammonium concentration (through nitrification-denitrification) through the anaerobic digester system, ammonia toxicity to methanogens is reduced and the stoichiometry is not conducive to struvite formation. In addition, this nutrient control within the solids process has the ability to eliminate the need for additional sidestream tankage and processes. Dewatering significantly improved with added VS destruction and because aerobic biology more effectively breaks down the Extracellular Polymeric Substances (EPS) the cells form during anaerobic digestion. In one pilot study, coagulant was eliminated and polymer dose dropped to achieve 32% TS in the dewatered cake. Through reduced chemical use, optimized digester chemistry, increased solids destruction and other consequential benefits, the capabilities of this system offer savings and solutions for several common MAD challenges that outweigh the costs and drawbacks.