2021 Virtual Undergraduate Research Symposium

2021 Virtual Undergraduate Research Symposium

Aerobic Polishing Treatment for Anaerobic Bioreactor Effluent

Aerobic Polishing Treatment for Anaerobic Bioreactor Effluent


PROJECT NUMBER: 30 | AUTHOR: Emily Phaneuf​, Civil and Environmental Engineering

MENTOR: Junko Munakata-Marr, Civil and Environmental Engineering

GRADUATE STUDENT MENTOR: Carolyn Coffey, Civil and Environmental Engineering


Conventional wastewater treatment processes are energy intensive, creating interest in engineering low-energy wastewater treatment systems. Anaerobic technologies do not require aeration, which can account for up to half of a facility’s energy consumption. A pilot-scale bioreactor was tested to evaluate the effectiveness of an anaerobic baffled reactor (ABR) paired with an anaerobic fixed film reactor (AFFR) for raw domestic wastewater treatment. This system has a low energy consumption rate compared to conventional wastewater treatment because the anaerobic biology eliminates the need for added oxygen. Additionally, biogas may be collected from this system for energy reuse. Weekly sampling demonstrated that this system is effective for solids removal: the EPA secondary standard for total suspended solids (TSS) of 30 mg/L is consistently met by the ABR-AFFR. However, biochemical oxygen demand (BOD5) requirement (30 mg/L) is not met by this system. Therefore, the goal of this research is to evaluate aerobic post-treatment of residual BOD in ABR-AFFR effluent. A 10-liter batch reactor was implemented to aerobically treat effluent from the ABR-AFFR. This system was operated as both an unmixed and a continuously mixed batch reactor with constant aeration. For each replicate experiment, TSS and BOD5 were measured initially and after 18- and 24-hour hydraulic residence times. Percent change was calculated for each of these water quality parameters to evaluate effectiveness relative to initial water quality. Percent reduction in BOD was satisfactory (69% and 70% removal for 18- and 24- hour HRTs, respectively, in an unmixed reactor; 51% and 55% for 18- and 24-hour HRTs, respectively, in a mixed reactor). However, results for solids measurements were more variable, showcasing the complexities of approaching solids production and removal. Overall, although BOD5 was not always reduced to the EPA secondary standard, this experiment serves as a proof of-concept for aerobically treating residual BOD from the ABR-AFFR.


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Emily is a junior studying environmental engineering at Mines. She first began research in 2018 within the civil/environmental engineering department as a part of the FIRST fellowship and has continued in research since as a part of the MURF. As an undergraduate, she has conducted research as a part of ReNUWIt, a multi-institution engineering research center focused on urban water management; in particular, she has been involved with projects focused upon domestic wastewater treatment utilizing anaerobic bioreactors. In the future, she would like to continue exploring methods to treat residual biochemical oxygen demand after anaerobic treatment. After graduation, she hopes to pursue a career in consulting with an emphasis on drinking water or wastewater treatment.


  1. Awesome talk and poster! Apologies if this is a remedial question, but I’m not familiar with wastewater treatment. What is the method through which the BOD falls in the ABR? And will you continue your experiments focusing on mixed or unmixed ABRs?

    • Thank you! BOD declines in the ABR because microbial communities present metabolize carbon-containing compounds as a food source. In the future, I hope to attempt to run trials operating a continuous flow mixed reactor (this experiment used batch reactors – water sample is introduced and remains for the entire experiment instead of having flow feeding constantly in and out).

  2. Great poster and talk Emily! At the end you mention that your future work will focus on improving the method for BOD levels. Will you continue to analyze how the changes you make in the ABR affect TSS levels as well?

    • Absolutely. Although my primary goal in pursuing this project is to decrease effluent BOD to meet the EPA secondary standard, it will be important for me to continue to monitor total suspended solids as well to ensure that the EPA secondary standard for TSS is met, especially because BOD consumption is related to increased biomass. Ideally, I would like to implement some type of physical removal method in future systems to aid in the removal of accumulated biomass.

  3. Great job! Very easy to follow and very interesting! How would you implement a continuous flow stirred reactor, and what do you expect to see from this?

    • Thanks so much! The existing reactor has ports that would allow for functioning as a continuous flow reactor. In this experiment, I simply filled the reactor using a line connected to the ABR and left both openings closed to prevent flow in/out of the reactor. However, if operating as a continuous flow reactor, I would connect a line to the ABR to receive inflow while also using the other opening to allow for flow out of the system. From an operational standpoint, it would be valuable to use a continuous flow reactor because if a batch reactor was permanently implemented into the system, a basin would be needed to store flow from the ABR. Use of a batch reactor in this experiment made for easier setup and operation since this experiment serves merely as a proof of concept for utilizing aeration to remove residual BOD, but would not be ideal for permanent implementation.

  4. Thanks, Emily — very cool! Can you comment on how the rate of aeration chosen? And, somewhat tangentially: how dependent is this process (or aerobic wastewater treatment in general) on the bubble size?

    • Thanks! With regards to your first question, the diffuser chosen was designed for a 10 gallon aquarium (the reactor used was only 10 liters). Therefore, diffuser capacity was considered sufficient to ensure oxygen saturation and to encourage aerobic metabolisms.
      As for the bubble size, smaller bubbles would increase oxygen transfer efficiency and would likely also reduce operating costs in a full-scale system. It is difficult to say that increased oxygen transfer efficiency would translate to improved operating efficiency, however, since it is possible that the mechanism of BOD removal could be partially aerobic and partially anoxic. (Further investigation of chemical and biological mechanisms would be necessary to say for sure.) However, it must also be considered that smaller openings used to create these smaller bubbles may also be more likely to become clogged, potentially leading to maintenance costs.
      Thanks again for such great questions!

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