2021 Excellence in Environmental Engineering and Science™ Awards Competition Winner

Grand Prize - Research

BV Innovation Platform - MABR Research Collaborative

Entrant: Black & Veatch
Person in Charge: Sandeep Sathyamoorthy, Ph.D.
Location: Hayward, California
Media Contact: Bruce Moores

Entrant Profile

Black & Veatch (BV) is the owner of the BV Innovation Platform – MABR Research Collaborative. The BV Innovation Platform focuses on realizing resource recovery and sustainable treatment concepts, performing project-oriented research services including full-scale testing, demonstration facility design, treatability testing and technology evaluations.

It assembled the team and served as Principal Investigator. It developed the plan, protocols and tests for the 170-gallon (645 L) hybrid MABR suspended-growth pilot. It further operated the pilot skid, conducted the lab-scale experiments, collected samples, and analyzed the data.

The City of Hayward served as a committee member on the project. It provided space for the lab tests, helped with overnight sampling, shared electrical instrumentation equipment and assisted with the commissioning of the project.

Suez was co-author of the research and provided the pilot skid with design modifications by Black & Veatch. It provided membrane-related support, including relevant data and experience on past projects. It also provided mechanical support for different components of the MABR technology.

Project Description

Research Motivation

Membrane aerated biofilm reactors (MABRs) are an emerging biofilm-based biological nutrient removal (BNR) process intensification technology. Although the MABR process has been studied at lab-scale, a limited number of pilot or full-scale studies have been conducted. There are significant knowledge gaps, particularly related to the value proposition of the technology for process intensification.

A Comprehensive, Integrated Approach to Applied Research

Our MABR research collaborative of Black & Veatch (BV), Suez and the City of Hayward, CA evaluated a pilot-scale hybrid MABR-suspended growth (MABR-SG) process at the Hayward Water Pollution Control Facility (Figure 1). The MABR-SG process enables simultaneous, concurrent nitrification-denitrification within the MABR biofilm and denitrification in the anoxic mixed liquor, resulting in efficient total nitrogen removal in a single tank (Figure 2). An MABR-SG process offers process intensification and reduction in the greenhouse gas emissions. The MABR-SG process impacts water, air, land and system operations by enhancing BNR performance, maximizing the value of existing assets, requiring less energy, reducing GHG emissions and an opportunity produce more energy and return more carbon to the soil.

Our integrated approach consisted of:

Process modeling for in-silico evaluations of the MABR-SG process and to drive the pilot’s research questions and hypotheses.
Pilot-scale operation and monitoring of the MABR-SG process for over 2 years. The MABR-SG pilot system was 645 L (170 gallons) with four distinct zones – a contact zone, the MABR zone and two aerobic zones (Figure 3). A range of SRT and loading conditions were evaluated. The operation involved management of the process, sampling and analyses, and maintenance of the system, sensors and analyzers.
Batch testing to evaluate the nitrifier seeding effect, which is the beneficial transfer of nitrifying bacteria from the MABR biofilm to the mixed liquor. This is cited as a benefit of the MABR-SG process.
DNA-based microbial community evaluations to investigate the microbial community composition and dynamics of the MABR-SG process.

Quality

A Technical Advisory Panel of industry experts from BV, Suez, Hayward and academia provided input and guidance on the research.

A rigorous sampling and monitoring program was employed to characterize the performance of the MABR-SG process. Influent, bioreactor zones and effluent samples were routinely collected throughout the 2-year period and analyzed for chemical oxygen demand (COD), ammonia, total kjeldahl nitrogen (TKN), total phosphorus, alkalinity, nitrate, nitrite and total suspended solids (TSS) and volatile suspended solids (VSS). Analyses were conducted in accordance with standard methods. Additionally, during multiple short-term experiments, samples were collected every 2 hours across each zone of MABR-SG process to quantify diurnal performance. Biofilm and suspended mixed liquor were routinely sampled and the microbial communities were elucidated using high throughput 16S-rRNA sequencing and qPCR.

Originality and Innovation

Key findings include:

An MABR-SG process can support significant process intensification.

Operation of the MABR-SG process at a suspended SRT of only 1.5 days, comparable to BOD removal process, resulted in removal of approximately 40% inorganic nitrogen (Figure 4).
When the MABR-SG process was operated at a suspended SRT of 3-4 days, an effluent ammonia concentration of less than 1 mgN/L was achieved (Figure 5).
qPCR results indicate the nitrifier fraction in the MABR-SG process is comparable to a 8-10 days SRT BNR process (Figure 6).

The observed nitrification rate of the MABRSG process is variable and depends on multiple factors.

The nitrification rate performance of the MABR biofilm is related to multiple factors, including in-situ ammonia, BOD, nitrite and nitrate concentrations and the operating suspended SRT (Figure 7). Our research was the first to elucidate the dependence of the ammonia oxidation rate on multiple parameters.
Our results suggest that the use of oxygen within the biofilm results from balance and competition between nitrifiers and heterotrophs (Figure 8).
Heterotrophs are strongly influenced by the diffusion of BOD into the biofilm. A high MABR zone BOD concentration results in competition for available oxygen within the biofilm between autotrophic ammonia oxidizing bacteria and heterotrophic BOD oxidizing bacteria (Figure 9).

Biofilm structure and performance are strongly influenced by the BOD/N ratio of the influent.

The biofilm structure and competition between heterotrophs and nitrifiers is influenced by the BOD/nitrogen ratio of the influent being treated.
At higher BOD/N ratios, a regular high performing biofilm is produced. At very low BOD/N ratios, a patchy and thinner biofilm forms (Figures 10, 11).
With excessive BOD loading, the biofilm grew thick and slimy due to the accumulation of extracellular polymeric substances resulting in reduced biofilm activity and performance (Figure 12).
Biofilm management through the scour is a critical operational tool to enhance MABR performance (Figure 13).

Complexity of the Problem

The challenges of testing MABRs at pilot scale include variability in the influent, which is an inevitable reality of “real-world” systems, lack of a controlled environment and equipment malfunction/failure.

To overcome complexities associated with pilot-scale research, a multi-discipline team of professionals collaborated on the sampling, operation of the facility and analyses. This research dove deeper than typical “demonstration” work done at pilot scale. Experiments were carefully designed for specific and untested aspects. For example, the research systematically evaluated the seeding hypothesis – which has not been accomplished at this scale and is an essential basis for the value-proposition of MABR technology.

Contribution to Social and Economic Advancement

The research filled key knowledge gaps in the applicability of the MABR-SG process to achieve intensified nutrient removal. Existing biological reactors could be retrofitted to achieve a 50% to 100% increase in treatment process capacity without construction of new concrete tanks. In the San Francisco Bay Area alone, 39 utilities are working toward a collective goal to reduce their nutrient loading. This research suggests that the MABR-SG process could significantly reduce capital and operational costs of BNR upgrades, resulting in net environmental benefits (Figure 14). As utilities focus on the need for energy-efficient, low-GHG nutrient removal as part of a broader resource recovery and optimization portfolio, the MABR-SG hybrid process evaluated by this research provides an effective tool. It shows MABR technology is ready to be used in more applications.

Click images to enlarge in separate window.


Figure 1: Aerial Photograph of the Hayward WPCF which was the loation of the MABR-SG pilot. The pilot system is shown in the inset.	Figure 2: The unique counter-diffusional biofilm integral to the MABR-SG process with the substrate (ammonia-nitrogen and COD) diffusing in from the bulk liquid and the electronc acceptor (oxygen) supplied from the lumen results in concurrent nitrification and denitrification within the biofilm coupled with denitrification using excess COD in the bulk liquid. Together, this provices a total nitrogen removal solution in a single bioreactor.

Figure 3: Details of the MABR-SG pilot employed for the research. (L) A view of the inside of the pilot bioreactor illustrating the four zones. (R) A Simplified process flow diagram of the pilot system.	Figure 4: Total inorganic nitrogen removal (TIN) across the MABR-SG process when operated at suspended SRTs ranging from 0.3 to 3 days. (middle panel) Operation at 1.5 days results in a 40% TIN removal of TIN. (lower panel) At a suspended SRT of 3 days, and effluent TIN of less than 15 mg-N/L can be achieved.

Figure 5: Diurnal profiles of primary effluent (PE) ammonia-nitrogen (PENHx-N) and MABR-SG process effluent ammonia (Eff.NHx-N) and nitrate (Eff.NO3-N) at a suspended SRT of 1.5 days (top panel) and 3 days (bottom panel). Operation of the MABR-SG process at a suspended SRT of 3 days results in a total nitrogen of approximately 10 g-N/L and an ammonia-N concentration of 1 mg-N/L over 18-h of the day.	Figure 6: qPCR was used to investigate the microbial population of the mixed liquor (left of each bar chart) and MABR biofilm (right of each bar chart) at each SRT. Results suggest that the percentage of nitrifying bacteria (ammonia and nitrite oxidizing bacteria) in the biofilm is comparable to that in the seed sludge which is operated at a much longer aerobic SRT. This emphasizes the intensification value, even at the microbial community level, of the MABR-SG process.

Figure 7: Ammonia removal rate in the nitrification zone as a function of the in-situ ammonia concentration. The theoretical ammonia removal rate basd on Monod kinetics is shown for reference. Our research was the first pilot-scale research to elucidate that the nitrifiation rate does not vary as a monotonic function of the ammonia concentration.	Figure 8: Relationship between ammonia removal rate and oxygen transfer rate in the MABR biofilm. The competition between ammonia oxidizing bacteria and heterotrophs for oxygen supplied by the MABR to the biofilm plays a strong role in mediating the nitrogen removal performance in the MABR-SG process.

Figure 9: Hetrotrophic activity and the related oxygen consumption are influenced by the diffusion of BOD into the biofilm and the return of nitrite and nitrate (NOx) which is denitrified. At high suspended SRTs, the MABR experiences a high NOx-loading rate and BOD is therefore used for denitrification. At shorter suspended SRTs, the heterotrophic activity in the biofilm is greater resulting in more BOD removal, rather than nitrogen removal within the film.	Figure 10: Images of the MABR biofilm at different soluble COD to ammonia rates. (R) a moderate sCOD/N ratio of 7 in the primary effluent results in a uniform film across the entire MABR surface. (L) In contrast, a low C/N ratio of 2.5 results in patch "bio-islands" growing on the MABR.

Figure 11: A rendition of the hypothesized distribution and relative thickness of the MABR biofilm when treating influent with an sCOD/N ratio of 7 (L) and 2.5 (R). We hypothesize that the most significant impact is on the thickness and contribution of meterotrophs within the biofilm.	Figure 12: Excessive loading of the MABR at a high BOD for extended periods resulted in significant EPS production within the MABR and reduced performance.

Figure 13: Effective MABR biofilm management through a scour regime is critical to maintain treatment performance in the MABR-SG process. (L) Gradients of oxygen and COD (biodegradable organic matter) is critical to ensure distinct activity zones within the MABR biofilm. (R) Air scour rate and frequency are both crticial operational parameters which mediate biofilm health and performance.	Figure 14: Results from our research were used to develop comprative designs of conventional and MABR-SG process for upgrade of a POTW in the SF Bay area. The MABR-SG process results in a smaller bioreactor volume, reduced aeration costs. Importantly, no supplemental carbon (e.g., in the form of methanol) would be required with the MABR-SG system. Overall, this results in a very significant environmental benefit.