The MOB Comes to Reno: Putting Mobile Organic Biofilm Technology to the Test

Introduction
Recent years have seen a surge in process intensification processes for water reclamation facilities. This paper details one of these processes, the Mobile Organic Biofilm (MOB TM) process, which is currently being tested in a comprehensive, 18-month long, 7 MGD ADF demonstration study at the Truckee Meadows Water Reclamation facility (TMWRF).

Facility Background
TMWRF has an enhanced biological phosphorus removal in a short-SRT anaerobic/oxic (AO) process, tertiary nitrification in nitrifying trickling filters (NTFs) and tertiary denitrification with methanol in fluidized bed reactors. The facility is required to meet extremely low effluent nutrient limits of 500 lbs/day TN and 134 lbs/day TP from the TMDL for the Truckee River. These load limits correspond to 2.0 and 0.54 mg/l at present average flows of 30 MGD, and 1.36 and 0.37 mg/l at the permitted future flow rate of 44 MGD.

In recent years, TMWRF has been challenged by cold weather stability of the NTFs, making permit compliance difficult. The NTFs also contain high capital and operating costs.

TMWRF is testing MOB as an intensification process that will allow the NTFs to be removed from service, increasing the reliability of nitrification and subsequent permit compliance. TMWRF is also seeking reduced operating costs from removing a pump station to the NTFs and reducing the methanol required to denitrify.

MOB Process Overview
MOB can be helpful for wastewater treatment facilities that have treatment capacity limitations. Ground-up kenaf stems are utilized as organic biofilm carriers in the bioreactor (Figure 1.) The kenaf fibers form kernels for biofilm growth to promote granulation in the reactors. The carrier is allowed to flow between reactor zones and into clarifiers. The WAS is then screened to capture the kenaf, which is then recycled back to the bioreactors. Kenaf-seeded floc also exhibits enhanced settling. Figures 2 and 3 show the initial kenaf addition to the reactor and the study's media retention screen.

Demonstration Study Goals and Testing Plan
Study Goals
Four primary criteria were established to define success of the study:
1. Achieve effluent ammonia concentrations below 1 mg/l in secondary effluent
2. Achieve NOx effluent less than 6 mg/l in secondary effluent after the anoxic mixers are installed
3. Achieve aerobic effluent OP of 0.5 mg/l or less
4. Maintain SVI values below 120 ml/g

Study Results
The MOB Demonstration study has achieved consistent low ammonia over the course of the study after nitrification had been established to the present. The system has not been able to operate at low DO concentrations due to EBPR concerns, leading to high effluent nitrate (Figure 3). EBPR performance has been strong during periods of the study, but high RAS nitrate values have often inhibited phosphorous removal. Figure 4 shows effluent phosphate over the course of the study. SVI values have been steady at roughly 120 ml/g after early foaming and sludge floatation were addressed and are lower than the comparison trains, as shown in Figure 5.

Troubleshooting
The study has had difficulty with MOB deposition in the secondary clarifier slab and under the fine bubble diffusers in the aeration basins. Clarifier deposition was prevented by running a once a day sweep at 100% flow on the RAS pumps to scour the clarifier floor. Aeration tank deposition was primarily caused by power outages at the facility that allowed the MOB granules to settle below the fine bubble diffusers. The aeration intensity at the downstream end of the reactor was insufficient to re-suspend the granules. Plant staff were able to re-suspend kenaf with a floating mixer.

TMWRF has also experienced higher than expected loss of kenaf solids through the media retention screen. This was initially addressed by replacing a pump in the system causing high shear and reducing the particle size of the kenaf carriers over time. After replacement and replenishing the kenaf in the reactor basin, screen loss was still observed. Consideration should be given to more frequent media replenishment than is currently expected and potentially to a different screenings pore size.

Testing
TMWRF has retained the University of Nevada-Reno to perform kinetic testing throughout the study to calculate the relative nitrification, denitrification, P-release and P-uptake rates of both suspended and attached growth. Results have been interesting, with some compelling evidence that both nitrifiers and PAOs are being retained in the granules. Raw plots of one set of comparison kinetic tests are shown in Figure 6.

Conclusions
The 15 months of the TMWRF MOB Demonstration study has shown a process capable of meeting all of the goals set out in the study after modifications are implemented. The study has demonstrated consistent nitrification performance. Current unstable EBPR performance and insufficient denitrification are being addressed by a post-anoxic zone placed into service in January 2025, Figure 7.

The study has led to several lessons to be communicated to other utilities interested in the technology. These are:
- Minimum aeration intensities for granule resuspension,
- Clarifier mechanism requirements to prevent deposition,
- Screening details,
- Pump type compatibility Process objectives in similar short-SRT EBPR facilities.
This paper will enable these lessons learned to be communicated to others considering the same process.