Hybrid MABR Technology for Nitrogen Removal in a Low Suspended Growth SRT

This paper focuses on the hybrid MABR technology for nitrogen removal at a WRRF operating in a low suspended growth SRT (i.e., cBOD mode). To date, full-scale MABR installations have been limited to WRRFs already performing nitrogen removal, whereby the MABR is working in a complementary fashion to the existing suspended growth activated sludge.

The novel application of using MABR in a low suspended growth SRT was piloted at Central Contra Costa Sanitary District (Central San) in the San Francisco Bay Area. Under this application, the hybrid MABR biofilm exclusively performed the ammonia/total nitrogen removal (i.e., non-complementary with the low suspended growth sludge). The focus of this presentation can be broken into three areas:
1) Background, objectives, and results
2) Sumo® modeling results
3) Carbon, energy, and greenhouse gas (GHG) emissions potential benefits for this hybrid MABR in a low suspended growth SRT mode

Background, Objectives, and Results
Central San is one of the five largest wastewater treatment plans in the San Francisco Bay Area with a permitted capacity of 53.8 mgd average dry weather flow. The Bay Area recently adopted total inorganic nitrogen (TIN) load limits (40 percent baywide aggregate TIN load reduction). Transitioning from secondary treatment to nitrogen removal at Central San is challenging due to plant capacity, site contaminated soil, and a return stream from incinerator air pollution control equipment containing cyanide (a known inhibitor of nitrifiers). MABR was selected for piloting as it has the potential to remove TIN within existing tankage while addressing the other listed challenges.

At the pilot onset, Central San had not yet received permit limits. The overarching piloting goal was to evaluate whether existing tankage could be retrofit with this intensification technology in a relatively fast construction project (~1 year) with minimal modifications to the secondary process.

Central San's pilot included four core objectives:
1. Confirm ability to form an ammonia removing biofilm on the membranes (addresses cyanide concerns)
2. Evaluate ammonia and TIN removal while maintaining low suspended growth SRT
3. Determine specific ammonia removal rates to confirm ability to remove ammonia/nitrogen in existing tankage
4. Determine the optimal MABR cassette's location at Central San

MABR Pilot Setup and Results
The pilot process flow diagram is presented in Figure 1. The pilot can be fed from different locations across the plant (see Figure 2). Details for each phase are provided in Table 1.

A summary for each phase (approximately 2-months each) and the corresponding results:
- Phase I (secondary clarifier influent feed): to demonstrate the hybrid MABR ammonia removal efficacy while maintaining a relatively low suspended growth SRT. The ammonia removal performance in Phase I is presented in Figure 3. Some key observations:
- Ammonia Removal occurred rapidly within initial 2 weeks with an effluent steady state of 5-10 mg N/L.
- Ammonia removal rate (approximately 2.1 g/m2/d) was similar to other full-scale installations with a longer SRT (typically 2-2.5 g/m2/d).
- TIN removal was minimal as expected due to limited available carbon at this feed location.
- Phase II (anaerobic selector feed): to verify that the hybrid MABR biofilm can effectively remove nitrogen in the presence of ambient cyanide. The ammonia and TIN removal performance are presented in Figure 4 and Figure 5, respectively. Some key observations:
- Ammonia Removal was similar to Phase I, showing robust and reliable removal.
- Up to 40% TIN removal was observed. TIN removal improved from Phase I, but it was still limited due to carbon availability.
- Phase III (blend of anaerobic selector and primary effluent): to stress the system with the highest possible cyanide, ammonia, and carbon levels. The ammonia loading and TIN removal performance were initially similar to Phase II as presented in Figure 6. Some key observations:
- Ammonia Removal was similar to Phases I and II, showing robust and reliable removal.
- Up to 50% TIN removal was observed.
- Upon increasing the loading rates, the nitrifying biomass appears to be the limiting factor as evidenced by an increase in effluent ammonia levels.

Full-Scale Hybrid MABR Modeling Results for this Low Suspended Growth SRT Mode
Sumo® was used as the modeling platform as its features lends itself to such attached growth systems. The datasets used were as follows: daily pilot grab samples, influent and effluent ammonia and nitrate online probes, daily composites from the raw influent and primary effluent samplers during the phases II and III, and diurnal sampling from July/August 2019.

A comparison of the data against the calibrated model is presented in Figure 7. Overall, the model was deemed reasonably calibrated to compare various MABR layouts and relative TIN removal performance.

The energy efficiency benefits of MABR are well documented with an emphasis on oxygen transfer efficiency (e.g., Castrillo et al., 2019) and simultaneous nitrogen removal. While attractive, how does a hybrid MABR operating in a low versus high suspended growth SRT compare in terms of cost, carbon, energy, and GHG emissions? For comparative purposes, a Modified Ludzack-Ettinger (MLE) configuration was used as a baseline while comparing the low versus high suspended growth SRT as shown in Table 2.