Srinivasa Varadhan, Ph.D., PE, PEng (Ontario) will present “Design and Implementation of a Moving Bed Bioreactor for Sulfolane Treatment” at Innovations in Process Engineering Conference (IPE) 2022 at the Hyatt Regency in Miami, Florida at 9:00 a.m. EST on Thursday, June 23, 2022.

Srinivasa Varadhan’s co-authors are Richard Hodges, Geosyntec; Janet Goodfellow; and Scott Forbess, Geosyntec.

Srini Varadhan is a Principal Environmental Engineer based in Ontario, Canada with more than 12 years of engineering, research, and consulting experience focused on large-scale and complex soil and groundwater remediation programs, process engineering design and implementation for groundwater and wastewater treatment, mine water management, remediation system operation and maintenance, feasibility study pilot testing, site strategy development, and project delivery.

As one of the newest conference offerings from the Water Environment Federation (WEF), the IPE will focus on innovative treatment technologies and serve as a technical platform for discussions around the continued development of technological innovations and new practices for water treatment.

The Water Environment Federation is a nonprofit association that provides technical education and training for thousands of water quality professionals who clean water and return it safely to the environment. WEF members have proudly protected public health, served their local communities, and supported clean water worldwide since 1928.

Abstract

Conventional moving bed bioreactor (MBBR) technology was adapted for treating sulfolane, a solvent currently used in several petrochemical and manufacturing industries, and a contaminant of emerging concern in groundwater at several industrial facilities globally. There are limited existing technology case studies for full-scale treatment of sulfolane, which include sorption, activated sludge, and aerobic degradation in a fixed-bed bioreactor. Depending on the groundwater chemistry and microbial dynamics, fixed-bed bioreactors often present intensive operation and maintenance (O&M) requirements resulting from periodic backwashing due to biofilm growth and challenges with spent biologically activated carbon disposal. Therefore, as part of this study, an alternative sulfolane treatment system was evaluated. The project goal was to meet the sulfolane discharge water quality objective (WQO) of 20 μg/L and reduce O&M requirements for a target peak groundwater flowrate of 600 gpm. Consequently, the feasibility of adapting a conventional moving bed bioreactor (MBBR) technology for sulfolane treatment was assessed. This paper presents the procedures and results of testing, scale-up, and performance of a full-scale MBBR system for aerobic sulfolane treatment as an alternative to a fixed-bed bioreactor.

Preliminary microcosm and bench-scale studies were performed to confirm aerobic sulfolane degradation using free-floating or planktonic inoculum. The studies successfully demonstrated sulfolane degradation in systems containing only native planktonic bacteria and with planktonic inoculum obtained from the existing fixed-bed bioreactor. The results suggested more reliable and rapid sulfolane degradation using the inoculum from the existing bioreactor when compared to native planktonic bacteria (Figure 1). To expand upon the preliminary findings, additional bench-scale studies were performed to evaluate performance under a range of conditions, including variable sulfolane concentrations and water temperatures that a full-scale system might experience during operation using site groundwater. The bench-scale studies were operated for a short duration, typically 24 hours each to inform parameters that needed further evaluation. The additional bench-scale studies illustrated consistent degradation across varying sulfolane concentrations but greater sensitivity to temperature variations, with slower sulfolane degradation kinetics below 66°F, the temperature of site groundwater (Figure2).

Based on the observations noted from the bench-scale studies, a mini pilot test was set up to evaluate sulfolane degradation performance over a longer term. Nutrient loading requirements and the effect of suspended fixed-film growth on sulfolane degradation were also evaluated. This mini pilot test was operated for 10 days. Data from the mini pilot demonstrated long-term sulfolane degradation and indicated very low nutrient requirements, consistent with the generally accepted carbon:nitrogen:phosphorus (C:N:P) ratio of 100:5:1formula for sustaining aerobic bacterial growth (Droste, 1997). Notably, data indicated that if allowed time to develop and acclimatize, the native planktonic bacteria could achieve sulfolane degradation comparable to the inoculum obtained from the existing bioreactor. Therefore, the native planktonic bacteria developed and acclimated in the mini pilot groundwater was used as an inoculum for future testing.

Based on the findings of the preceding tests, a flow-through pilot study was set up to collect data relevant for the design of a full-scale bioreactor. Initially, the objectives of the pilot study were to evaluate: 1) the hydraulic retention time (HRT) required to reduce the anticipated influent sulfolane concentrations to the WQO; 2)sensitivity to influent sulfolane variability; 3) operational requirements (nutrients and dissolved oxygen [DO]);and 4) temperature sensitivity. Initial data indicated sulfolane degradation was inconsistent when using planktonic bacteria over a longer duration; therefore, the use of inert, suspended MBBR media was added to the pilot study reactors to evaluate whether degradation would be enhanced if the existing planktonic cultures were allowed to develop as a fixed-biofilm culture.

Data collected after the addition of the MBBR media demonstrated more effective sulfolane removal achieved by fixed-biofilm microorganisms as opposed to planktonic microorganisms. The pilot system performance was then optimized by balancing the quantity of inert media with hydraulic retention time (HRT) to optimize the size and cost of the full-scale MBBR system (Tables 1 and 2). At the conclusion of the optimization activities, the pilot system was able to consistently and reliably degrade sulfolane concentrations of more than 2,000micrograms per liter (μg/L) to less than the 3.3 μg/L laboratory detection limit using 20% inert media by volume and an HRT ranging from 6 to 8 hours (Figure 3). The pilot study also demonstrated that series operation of the bioreactors to achieve a cumulative 6-to-8-hour HRT was more effective than one bioreactor with a 6 to 8 hour HRT, likely due to mixing kinetics.

Sensitivity analysis of the pilot system was subsequently performed to assess potential operational limitations during scale-up. The pilot system demonstrated low sensitivity to nutrient requirements due to relatively low carbon loading. However, decreased sulfolane degradation kinetics resulting from low microbial activity were observed at temperatures below 60°F, with rapid recovery at 70°F or higher. DO concentrations ranging from 6to 10 mg/L were determined to be optimal for sustained performance.

Based on the results of the pilot test, application of the MBBR technology for sulfolane treatment was accepted as an alternative to a fixed-bed bioreactor and was scaled-up to a full-scale MBBR system in 2021. The MBBR system utilizes two 125,000-gallon MBBR reactors in series, each containing 30% MBBR media by volume and an overall HRT of approximately 8 hours at a design peak flow rate of 600 gpm. To date, the MBBR system has reliably degraded influent sulfolane concentrations ranging from 400 to 800 μg/L to an effluent concentration below laboratory detection limits during routine operation at up to 400 gpm. Consistent with the low organic carbon loading, the nutrient requirements are minimal, and microbial activity continues to be sustained by the nutrient loading already present in the extracted groundwater. The MBBR system is equipped with a coarse bubble diffuser network to maintain the MBBR media in a constantly mixed and suspended state. To date, the microbial slough-off from the MBBR system has been minimal, consistent with the relatively lower biofilm generation potential as a result of low organic carbon and nutrients loads.

The testing data demonstrated that the HRT, the characteristics of the inoculum, the media fill ratio, nutrient and DO concentration, and the operating temperature range were all critical parameters necessary to achieve effective sulfolane treatment in an MBBR system. This work demonstrates that depending on project objectives and site-specific conditions, the conventional MBBR technology can be adapted for effective groundwater remediation of sulfolane and potentially extended for the treatment of other contaminants in groundwater.

More Information

About the event: Innovations in Process Engineering
About Water Environment Foundation: WEF Home
For consultation regarding Moving Bed Bioreactors and Sulfolane Treatment, contact Srini Varadhan at SVaradhan@Geosyntec.com.
Learn more about Srini Varadhan: Srinivasa Varadhan (geosyntec.com)