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Applying MBRs without breaking the bank
Membrane bioreactors (MBRs) may have a reputation for being costly, but they do not have to be. A survey of 137 MBRs on the U.S. mainland shows that an MBR using flat-plate (FP) technology typically is half to three-quarters the capital expenditure (CapEx) of hollow-fiber (HF) systems.
There is some data that suggests MBR systems are competitive with several other technologies, including oxidation ditches, sequencing batch reactors, moving-bed bioreactors and biological nutrient removal plants. There is insufficient data to say definitively that an FP system costs less, but the data suggests that it can be cost-effective versus conventional systems treating to the same quality effluent.
CapEx certainly is important, but what about operating expenditure (OpEx)? While there is a tremendous amount of data publicly available regarding energy and chemical consumption of MBR systems, there is little to no data published on labor requirements of these systems. Labor can account for more than half of a plant’s operating budget, so why is it being ignored? The survey of 137 MBR plants was conducted by a third-party analyst and shows that an FP MBR requires 50% less labor to run than HF systems.
While the idea of installed cost comparisons is nothing new, the execution of those comparisons can be challenging, as no two wastewater plants are exactly alike. This is especially true for newer technologies like MBR, where the number of installations (and thus data) is somewhat limited compared to conventional systems. The worldwide installed base for MBRs is about 20 years, but that of conventional systems is more than 100 years.
Another challenge specific to the cost comparison of different MBR technologies shows that there are several drivers, including tighter effluent limits, space limitations and labor rates. They also are used in a myriad of applications, including municipal, industrial, tribal, resort, greenfield, retrofit and package plant—all with different cost impacts. Procurement methods and MBR system supplier scope of supply have a large impact on the final project costs.
To combat these challenges, focus has been placed on MBR installations treating municipal-type (non-industrial) wastewater with flows less than 10 million gal per day (mgd). Beginning in 2008, MBR installed cost data was gathered from multiple sources, including city documents, press releases, news articles, contractors and operators/owners.
Upon initial review of the data, two things became clear: Installation costs show significant variations below 0.1 mgd, and the amount of data beyond 5 to 6 mgd is limited due to the lower installed base in the U.S. A majority of the municipal wastewater plants in the U.S. are less than 6 mgd. As such, the original data set of less than 10 mgd was culled to MBR plants between 0.5 to 6 mgd, and what remained were 84 data points of two types (43 FP MBR facilities and 41 HF MBR facilities).
The data in Figure 1 (see page 10) shows significant cost differences between FP and HF systems. Below is a list of the top 10 reasons FP systems cost less than HF systems:
1. Reduced equalization. FP membranes used in MBR systems generally have higher flux rate capabilities at longer durations than HF systems. Equalization, therefore, can be reduced or eliminated from the equation. For example, during independent testing with the King County (Wash.) Department of Natural Resources, HF peak fluxes were on the order of 22 to 25 gal per square foot per day (gfd) during trials lasting four hours. Similar peak-flow tests were performed for an FP system, but peak fluxes were 25 to 30 gfd and trials lasted 24 to 72 hours. The same FP membrane was approved for use up to 42 gfd by the California Department of Public Health.
2. Less screening. FP membranes have a large, flat surface and are held rigidly in place with defined and consistent spacing between each membrane. HF systems have bundles of fibers tightly packed together where hair and fibrous material can bind around them and reduce surface area or, worst case scenario, pull the fibers from their potting. Fine screens measuring 3 mm typically are used for FP systems, and 1 to 2 mm for HF systems. Most screening manufacturers require grit removal and coarse screening in front of 2-mm or smaller-opening fine screens. All of these factors add up to a lower headworks capital cost for FP systems.
3. Process intensification. FP systems typically have a larger nominal pore size than HF systems, reducing the amount of pressure required to drive flow through the membranes. Running at a lower pressure reduces the fouling rate on the membranes, which allows for a higher operating mixed liquor
concentration. Operating at higher mixed liquor suspended solids (MLSS) concentrations reduces the process volume required for biological treatment.
4. Optimized clean-in-place (CIP). Because FP membranes are held rigidly in place with no membrane flexing or movement (i.e., air scour turbulence causes HF membranes to move vigorously), there is better air scour contact across the entire surface of the membrane sheet. Uniform distribution of air scour across the membrane surface increases air scour efficiency, reducing or eliminating the need for other types of membrane cleaning. There are four industry-accepted methods for routine membrane cleaning: recovery cleaning, maintenance cleaning, backwashing and air scour. Some FP systems require only air scour and maintenance cleaning.
5. No tank liners. If recovery cleaning is not required, tank liners are not required to prevent chemical corrosion of the concrete tanks. Recovery cleaning is where the mixed liquor is removed from the basin, the tank is filled with chemical and the membranes are allowed to soak for four to 24 hours.
6. No bulk chemical storage tanks. Spent chemical from the recovery cleaning process usually has to be neutralized. Neutralization and recovery cleaning chemicals require onsite storage tanks.
7. Reduced electrical. Fewer/smaller ancillary components mean fewer motor starters and installation labor hours, and smaller programmable logic controllers and motor control centers.
8. Simplified controls. See No. 7.
9. Fewer recycle streams. The packing density of HF membranes typically is higher than that of FP systems, so the membrane basins generally are smaller. The larger FP membrane volume, however, allows for more oxygen to be transferred into the MLSS and keeps the residual dissolved oxygen (DO) at or below 2 ppm. With a DO less than 2 ppm, the recycle can be brought directly from the MBR basin to the anoxic basin without the need for deoxygenation. One less recycle stream is required for FP systems, eliminating some pumps, motor starters, controls and associated maintenance.
10. Smaller footprint/buildings. The culmination of all of the above results in less equipment, less space and smaller buildings.
When determining the lowest cost of ownership, the capital cost is only one component of the analysis. Each of the 10 reasons listed gives a clear picture why FP systems not only cost less to build but also to operate.
Reducing system complexity directly correlates to reducing the number of full-time employees needed to maintain a system. When the third-party analyst gathered total installed cost data, it also polled the plants for the number of full-time employees at each site. While not all of the 137 surveyed plants replied with their labor requirements, quite a few responded. The data shows that an FP MBR averages about half of the manpower requirements of HF systems. To normalize the data, it is reported in terms of the number of full-time employees versus plant capacity (in mgd).
The minimum number of operators is approximately one employee per mgd for all types of MBR systems. This is not fuzzy math: The numbers are based on 40-hour work weeks. Some of the plants are contract operated, where an operator is only on site a few hours a day or week, accounting for less than one employee.
While there is no significant difference on the low end, there is on the average and maximum number of full-time employees per mgd. An average FP MBR has 1.8 operators per million gallons of treated wastewater per day, and HF systems average 4.1 operators—more than a 2:1 difference. This difference can equate to hundreds of thousands of dollars saved per year in operations and maintenance costs.