Optimizing Energy and Nutrient Control with Two-Stage Anaerobic Digestion

INTRODUCTION Anaerobic digestion is often used in larger wastewater treatment plants (WWTPs) in order to generate energy rather than consuming it. However, operating only 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. Having a long history of researching and improving aerobic digestion methods, Thermal Process Systems (TPS) sought to apply its existing technologies and knowledge of aerobic digestion to a two-stage anaerobic digestion system. One of the common issues faced by anaerobic digesters is becoming volatile fatty acid (VFA) overloaded. This occurs as a result of the acidogens making 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 primary goal of coupling aerobic digestion to anaerobic digestion with a recycle stream was to improve the stability of the anaerobic digester and resulting product by increasing the volatile solids reduction (VSR), improving biogas production and quality, reducing the overall hydraulic retention time (HRT) for the digestion process, and improving the dewatering characteristics. This was primarily achieved by establishing process control parameters around the pH of the anaerobic digester by varying the recycle rates. METHODOLOGY Operation of the pilot unit commenced without utilizing a recycle from the aerobic digester to establish a baseline of operations. During the initial phase of the pilot demonstration, the operational liquid levels of each tank was set so the fermentation tank, methanogen reactor, and aerobic digester operated at 2-, 15-, and 8-day hydraulic retention times (HRTs), respectively. Each phase of the pilot demonstration lasted for three sludge ages of steady-state operation. After the first phase of operating in a plug-flow fashion, the recycle was initiated at 60% of the daily feed amount. The recycle rate was later increased to 80%, 100%, and 150% of the daily feed amount to observe the ability of the system to reduce the struvite precipitation potential in the anaerobic digester by decreasing the ammonia and pH. Continuous monitoring of the system was carried out with the use of Endress & Hauser temperature/ORP/pH probes, liquid level pressure transducers, and vacuum/pressure gauges. The routine liquid analysis of each digesters' contents was carried out with the use of Hach colorimetric test kits. The routine biogas analysis was conducted with the use of Sensidyne gas detection tubes. Theoretical calculations of the potential for struvite formation were performed with the 'Struvite Tool' Microsoft Excel plug-in produced by the Office of Water Programs at CSU Sacramento. Dewatering results were obtained by sending samples to Centrysis Centrifuge Systems in Kenosha, Wisconsin. RESULTS The plug-slow operation of the pilot unit resulted in the methanogen reactor becoming VFA overloaded. This was accompanied by a decrease in the VSR, biogas production, and methane concentration in the biogas. Initiating the recycle rapidly lowered the VFA to alkalinity ratio (VFA/ALK) in the reactor; bringing it to literature values that designate a healthy anaerobic digester. The VFA/ALK was maintained at proper values without spikes in the VFA concentration or foaming events in the methanogen reactor for the remainder of the pilot demonstration. 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. Not having a requirement for standard metal salt coagulants reduced the amount of phosphorus in the biosolids cake; bringing the nitrogen to phosphorus ratio in the cake to the range that agronomic studies suggest makes up a healthy soil. Increasing the recycle through the system effectively allowed the aerobic digester to remove a larger mass of ammonia from the methanogen reactor daily. The decreasing ammonia was accompanied by a controlled pH adjustment in the reactor. The methanogen reactor maintained less than 500 mg/L of ammonia and a pH of 6.8 throughout the steady-state portion of this phase of the pilot; levels that theoretically eliminate the production of struvite. The actual data obtained during the pilot demonstration showed the decreasing ammonia and pH subsequently led to an increase in the soluble ortho-phosphate present in the digester. These effects were not accompanied by a decrease in the VSR or biogas production. Another observed result that was observed was the H2S in the biogas became non-detectable with the 1 part per million gas detection tubes. The GC results confirmed this observation 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 (SRB) that produce H2S by offering a more thermodynamically favorable electron receptor. The nitrates supplied by the recycle from the aerobic digester inhibited the SRBs while promoting a sulfur-oxidizing, nitrate-reducing bacteria (Lomas, 2005). This allowed the system to decouple the nitrification and denitrification processes by moving denitrification to the fermentation tank. DISCUSSION AND CONCLUSIONS 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. Maintaining high VSR increases the total amount of methane produced through anaerobic digestion. High VSR 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 H2S in the biogas which removes the need for more expensive forms of H2S treatment and continuous maintenance of boilers and CHP units. The removal of H2S allows facilities to 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.