Acute Digester Heating Issue Uncovers Chronic Plant Ailment

PROBLEM STATEMENT/PURPOSE: In the Spring of 2020, the Little Patuxent WRF digesters had been running for a little over year when an anomaly occurred; the digesters began dropping in temperature. The purpose of this case study is to walk through the complex troubleshooting process that led to discovering an unexpected root cause. The troubleshooting and eventual solution to the temperature issue shed light on the fact that it was just a symptom of a much more systemic ailment. The clue that led to the discovery of the systemic issue was a mysterious sudden significant change in sludge density that appeared in historian data on Christmas Day 2019. The case study will then detail the investigation into the chronic issue and the eventual testing and recommendation of mitigation strategies. The forensic team included the Little Patuxent operations group, HDR experts, and industry experts notably the late Dr. David 'FlocDoc' Jenkins. The case study will wrap up with key take aways from both a process mechanical design and an operations perspective. This case study will focus on the trouble shooting methodology. BACKGROUND: Between 2017 and 2021 the Little Patuxent WRF in Savage, MD underwent a 2-phase biosolids upgrade with the first phase incorporating gravity and belt thickening, anaerobic digestion, phosphorous removal, sludge holding, and centrate deammonification. The 2nd phase added a dryer facility that included sludge conveyance, Cake storage, belt dryers, dried biosolid conveyance and storage. The case study that is the topic of this abstract occurs during a period when phase 1 was fully constructed and online and phase 2 was still under construction. METHODOLOGY: The initial temperature drop trouble shooting involved heat and mass balance evaluations centered around the digester heating recirculation and heat exchange system (heating loop). These evaluations showed that flow through the heating loop was slowing causing a compounding issue of heat exchanger fouling due to inadequate scouring velocity and a laminar flow condition both of which degrade heat exchange capacity. These issues occurred suddenly after the digesters had already been in service for over a year. The cause was found to be gas entrainment in the pump suction piping as well as gas binding at the recirculation pump impeller. Piping and pump modifications were made to address the acute issue of gas release in the heating loop. The case study will detail the modifications made, but in general they involve: - Modify piping to convey versus trap gas release - Trial and eventual purchase of a different type of recirculation pump. Parallel to the modifications made to heating loop the investigation continued into finding the root cause of the out of the ordinary gas release in the sludge. Online sludge density data revealed that the sludge density rapidly decreased from an SG of 1 to an SG below 0.95 at the end of 2019. Figure 1 - Historian data showing a divergence in digester pressure level vs. radar level beginning on Dec. 25 2019 Grab samples taken at various digester depths confirmed the accuracy of the online density monitoring. 3 potential causes around the date of the density change were identified an investigated. Those potential causes were: - A polymer spill - An annual dairy shutdown - A filament bloom in the bioreactors The case study will detail how the cause was narrowed down to the filamentous bacteria Microthrix that thrives during the cold season. Microthrix mitigation strategies were developed, tested, and implemented. These included but are not limited to: - PAC dosing trial - Chlorination at various locations - Selection control o Swing zones o Internal recycle location o Scum removal improvements Figure 2 - This chart summarizes the key parameters during the initial troubleshooting period. The digester radar-pressure level is the sludge density indicator, and it shows that by late August 2020 Microthrix impact on the digesters had subsided. The most recent data will be included in the case study. RESULTS: With the mitigation strategies implemented on the liquid treatment side of the process, Microthrix continues to cause difficulties during the cold season. Microthrix thrives on grease and the grease flow to the plant has only increased since 2020. A change that improved primary scum removal ended up being the most effective mitigation. The County is also working on rehabilitating a grease digester that has been offline for 20 years to further limit the impact grease has on the process. The pump and piping modifications originally implemented on one primary digester have continued to perform well and are now being implemented on the 2nd primary digester. DISCSSION/CONCLUSIONS: Filamentous bacteria such as Microthrix and Nocardia can be a perennial ailment on the liquid treatment side at wastewater plants that can also have downstream effects on the solids treatment side of the process. This case study will conclude with the author's lessons learned from a process mechanical perspective, and the co-authors lessons learned from an operational perspective. Generally these include but are not limited to: - Microthrix: o Results from PAC trial o Results from chlorination trial o Results from selection changes - most effective o Generally Microthrix is hard to combat in greasy environments - Digester heating loops should be designed to covey gas to the maximum extent possible - Digester recirculation and mixing systems should react to changing sludge density.