Ask most engineers—at least those actively engaged in the design of wastewater treatment plants (WWTPs)—when membrane bioreactor (MBR) technology makes sense and you will probably get the following response: “MBR technology should be considered when the site footprint is small and when you are trying to produce high-quality effluent.” Both are great reasons to think MBR, but the sentiment is lagging behind the facts and a growing trend.
MBR technology is routinely making more sense than sequencing batch reactors (SBR), ditches and even moving bed bioreactors (MBBR); and not just for postage stamp-size real estate conundrums. So, where does this lingering misperception about MBR as a niche technology come from? Old ideas and dated facts on cost and energy.
In order to make the best available technology (BAT) truly available, we need to change outdated MBR paradigms.
By reviewing technical literature or surfing the Internet, one can easily find the following comments associated with MBR costs:
- • “MBR operating costs are the same order of magnitude as alternative treatment.”
- • “[MBR] membrane equipment costs have come down from $3 per gal to $0.75 per gal …”
- • “[MBR] membrane prices have dropped eightfold in the last decade.”
This growing body of information reflects a new reality that MBR technology is not only the BAT, it also is competitive categorically in most WWTP applications. In fact, in a survey of five 2011-2012 engineering evaluations, MBR systems fared well against conventional technologies in terms of capital cost and net present worth. This sampling includes results from five completely separate analyses conducted by five different engineers for different clients on unrelated projects.
The common theme is that MBR is now competitive with alternative technologies being considered for mechanical plants. Despite higher equipment prices as compared to SBRs and other technologies, MBRs make up ground in the following categories:
- • Solids handling (more than 40% cost savings);
- • Concrete/buildings (50% to 80% cost savings);
- • Equalization (more than 50% cost savings compared to SBR); and
- • Constructability/phasing (more than 50% cost savings due to less site work).
Moreover, remote monitoring capabilities, pending nutrient limits and projected growth are examples of non-cost factors that routinely favor the selection of MBRs.
Across the country, in different applications, at a variety of flows, the trend toward MBR systems is clear. But there is still one question hindering the unfettered use of MBRs anywhere and everywhere: Is MBR technology too energy-intensive? The answer is that it should not be.
The Energy Question
The idea that MBRs take more energy is not unfounded, but it is somewhat misguided in two key areas: First, the notion that less membrane air scouring translates into lower energy bills is not consistent with energy data. Second, reducing air scouring energy is not always the best or only way to make plants more efficient.
More to the point, industry data suggest that the best way to reduce energy bills is to improve turndown, simplify systems, increase flux and hold system suppliers accountable for plant performance, not just air scouring.
An easy way to differentiate membrane technologies is to compare so-called air scour rates: Some take more and some take less. While this may be a good way to compare equipment, it has not proven to be a good metric for predicting energy bills at city hall.
In a recent survey of 13 full-scale WWTPs, actual energy bills were normalized and averaged over time (Figure 1). All of the surveyed plants have been in service for at least one year and many of them for much longer (the oldest plant was commissioned in 2002). With the exception of Plant 5, each bar represents the average of at least 12 months of real data found in actual energy bills and daily monitoring reports filed with EPA.
Taken as a whole, two surprising trends emerged from the study:
- 1. Plants using theoretically more efficient membrane technology consume substantially more energy (based on utility bills).
- 2. Plants where the supplier (system provider) is responsible for total energy consumption perform markedly better than others.
Extracting specific conclusions from such data can prove challenging, if not academic, given all of the variables involved, but the trends appear to hold up under a microscope. For example, Plants 1, 6 and 7 are located in the same region, were built around the same time and are similar in most aspects. The same can be said for Plants 2, 8 and 9. In all cases, the plants using reportedly more efficient membrane technology (taking X air scour) are much less efficient overall than those using membranes requiring double the air scour rate (2X).
Moving the Needle
A review of technical literature on how to improve MBR energy efficiency yielded some surprising results. While air scouring reduction is a common theme, several other themes were just as important:
- • Improving turndown (the ability to incrementally match energy input to demand);
- • Simplifying systems (taking out parasitic loads such as pumps, mixers and blowers);
- • Operating at higher flux rates; and
- • Making the supplier responsible for plant performance.
On paper, most MBR plants should be expected to consume roughly 4,500 kWh per million gallons. This would be the total plant demand, including aerobic digestion and disinfection. The liquid stream should account for roughly 70% of that demand, or 3,150 kWh per million gallons. By comparison, the National Association of Clean Water Agencies found that 49 out of 54 conventional activated sludge (CAS) plants surveyed consumed less than 3,000 kWh per million gallons (for liquid stream)—about the same as MBR. With the exception of Plants 3, 5 and 11, most of the U.S. installations do not readily meet this bar as compared to the two Japanese plants that do: Plant 12 (Sanpou) and Plant 13 (Moriyama).
Moriyama and Sanpou were supplied by, and are now operated by, the Kubota Corp. Moriyama is a 1.3-million-gal-per-day (mgd), full-scale demonstration facility that has been running for more than a year. Starting with a conventional MBR system, Kubota removed mixers and pumps, installed more efficient membrane technology, employed proportional air scouring, implemented siphon filtration and increased flux in an effort to improve efficiency. The result was a decrease in energy consumption from 3,440 kWh per million gallons to 1,625 kWh per million gallons (without a significant increase in maintenance).
Although it is a full-scale plant, Moriyama is a demonstration facility as part of a joint effort with the Japanese government and a scalping plant with regulated throughput. The ability to run at the best efficiency point explains, in part, the plant’s efficiency; and also is an argument for turndown. More impressive than Moriyama is the 16-mgd Sanpou plant, which uses an even older submerged membrane unit type, but still operates efficiently at an average consumption of 1,852 kWh per million gallons (within 10% of a nearby CAS plant).
The energy data for the Kubota plants does not account for weaker wastewater strength, solids management and disinfection. Cited efficiencies, however, are comparable to the referenced CAS data and validate the ability of MBR systems to run efficiently with the right design, operation and accountability for overall plant performance. For comparison purposes, the total adjusted demand for each plant was reported here as 2,494 kWh per million gallons.
Changing the Paradigm
MBR technology is a viable, affordable wastewater treatment technology that should be considered whenever mechanical plants are on the table. While important innovations are being made at the commodity level, operating at a plant’s best efficiency point, simplifying systems by removing unnecessary equipment and improving operating procedures may have a greater impact on energy than air scouring rates. In fact, industry data support that making suppliers responsible for plant performance—not just theoretical air scouring targets—may move the needle the farthest toward lower energy bills.
Using the latest data to expand MBR technology’s reach