Alternate: Optimizing Energy and Nutrient Control with Two-Stage Anaerobic Digestion

Anaerobic digestion is used in larger wastewater treatment plants in order to generate energy rather than consuming it. However, operating in a reducing environment limits biodiversity and can create undesirable consequences. As such, research has started taking a harder look into the benefits of post-aerobic digestion (PAD) following anaerobic digestion. One of the common issues faced by anaerobic digesters is becoming overloaded with volatile fatty acids (VFAs). This is a result of the acidogens producing VFAs faster than the methanogens can convert the VFAs to methane; limiting the energy produced by the process. Two-stage anaerobic digestion was used to separate out the more rapid stages of hydrolysis and VFA formation from the much slower step of methane production in order to improve the conversion of organic material to usable energy. Optimizing and stabilizing the anaerobic digesters significantly reduces the oxygen demand in a post-aerobic digester, increasing the energy output while reducing the energy requirements. The methanogen reactor consistently achieved 55-60% VSR with the recycle in effect. Dewatering results from the centrifuge manufacturer showed a significant improvement to the dewatering characteristics of the digested biosolids, as the aerobic cultures used are capable of breaking down the extracellular polymeric substances often produced by anaerobic digestion. No coagulant and only 18 active pounds of polymer per dry ton of biosolids were required to achieve 32% TS in the dewatered cake. Gas chromatography results confirmed draeger tube measurements with a reported H2S concentration of 5.6 parts per billion while also confirming the methane concentration at 62% in the biogas. Further research discovered previously published discussions of the ability of nitrates to inhibit the activity of sulfate-reducing bacteria (SRBs) that produce H2S by offering a more thermodynamically favorable electron receptor. This allowed the recycle from the aerobic process step to effectively inhibit the SRBs, reducing the SRB population from the fermentation stage to the methane-producing stage as well as the H2S present in the biogas. This allowed the system to decouple the nitrification and denitrification processes by shifting denitrification to the acid-phase tank This pilot demonstration showed the capabilities of coupling two-stage anaerobic digestion to a conditioning aerobic digester that recycled material through the system. The recycle supplied by the aerobic digester offset variations in the feed material, improving the stability and energy output of the anaerobic digestion process. High volatile solids reduction combines with the reduction of dewatering chemical requirements and higher cake solids to offer substantial cost savings for facilities. The ability of the recycle to reduce the struvite potential in the methanogen reactor reduces maintenance concerns that often accompany anaerobic digestion. This is linked to the elimination of hydrogen sulfide (H2S) in the biogas, thereby removing the necessity for more expensive forms of H2S treatment and continuous maintenance of boilers and CHP units. The removal of H2S allows facilities to more effectively utilize the energy produced through anaerobic digestion on a more consistent basis. The capabilities of this system offer cost savings and solutions for problems that many anaerobic digestion facilities face.