Is pilot testing required for all membrane plants? The short answer is, “No.” The politically correct answer is, “It depends on what you are trying to achieve with the pilot testing and also your budget limitations.”
|There are a number of typical goals when conducting a membrane pilot study.|
Typical goals for conducting a membrane plant pilot study are:
- To obtain the process design parameters for the full scale plant;
- To obtain adequate operational data to estimate operation and maintenance cost of a full-scale plant;
- To optimize chemicals, flux and recovery rates and cleaning regimes;
- To familiarize the operation staff with membrane technologies;
- To show compliance with regulatory requirements; and
- To conduct research for new technology applications or optimizing current technologies.
Let us discuss each goal separately and use them as guides for making the decision regarding the necessity of pilot testing.
Obtaining Design Parameters
For most clean groundwater sources such as deep confined aquifers, where dissolved solids such as salts and contaminants (arsenic, radionuclides, nitrate, etc.) are to be removed, the design parameters for reverse osmosis (RO) and nanofiltration membrane systems are well known with significant data from decades of operating these plants. Utilizing these resources, coupled with computer model projections, typically results in very accurate design parameter estimates. This is especially true if data from other plants using the same aquifer is available. Pilot testing in these situations would not be necessary unless required by local regulatory agencies.
For low-pressure membrane plants, microfiltration (MF) and ultrafiltration (UF) utilizing groundwaters or groundwaters under the direct influence of surface waters, the same argument applies. These membranes are designed for high particulates, turbidity and microorganisms. These systems can be (and have been) designed with success utilizing conservative, but reasonable process design parameters.
For surface waters, including flashy rivers, high organic content reservoirs and lakes as well as tidal waters and seawater, it is a different case. These sources tend to vary in temperature, chemical composition and organic/metal/solids loading seasonally and during storm events. In most of these situations, but not all, pilot testing will result in a focused and tailored design, minimizing surprises and resulting in a more reliable and efficient facility. Pretreatment of membrane systems should be one of the primary targets for such pilot studies.
Estimating Life-Cycle Costs
The same logic as item No. 1 applies here. A side benefit of piloting could be to obtain guaranteed life-cycle costs (power, chemicals, cleaning regime and membrane replacement) from manufacturers. The bid documents can be prepared based on a life-cycle cost and not just the capital cost. This approach typically requires longer-term pilot studies (four to six months) to capture seasonal changes.
Optimizing Operational Parameters
If piloting is imperative, the only way to achieve optimum operating parameters is to schedule the pilot testing to capture the peak events and seasonal impacts.
Spring may result in higher turbidity due to snow melts and late spring rainfalls in some areas and can cause reservoir turnover. Depending on the membrane type and conservatism factor used in design, as well as budget limitations, in most cases spring is not the best time to pilot.
Summer typically results in a better understanding of taste/odor and algae control for MF/UF and pretreatment requirements and fouling for seawater RO.
|Membrane pilot studies can optimize operational parameters.|
Autumn presents challenges in areas with hardwood cover in the watershed. These areas experience more organics by the decay of leaves. Autumn’s cooler air temperatures and wind on the reservoirs will typically produce more organics as well as iron/manganese issues and therefore would be a better piloting period for these locales. Cleaning regimes, coagulation optimization and fouling impacts as well as taste and odor controls become a major part of the pilot study.
Winter yields the coldest water temperatures for surface water sources and therefore reduction in membrane flux due to the viscosity of the water. All membrane manufacturers have accurate membrane-specific temperature correction factors which can be used. As long as the extreme cold temperatures are utilized during design and plant sizing, winter is not typically a mandatory season to pilot.
Familiarizing Operational Staff
Although this is an important benefit of pilot testing (if conducted), it not a necessity, since there are many other means of providing training for the operators. Training programs such as the American Membrane Technology Assn. (AMTA) and affiliates the Southeast Desalting Assn., the South Central Membrane Assn. and the Southwest Membrane Operators Assn. offer hands-on training. Various manufacturers’ training classes can provide more comprehensive training.
There are no national standards for membrane piloting. Regulations are very specific to each state and local jurisdiction. Contact your local regulatory agency early in project planning to get an understanding of their requirements. Remember all leading manufacturers of MF/UF have gone through comprehensive national testing protocols such as challenge tests (by the U.S. Environmental Protection Agency, NSF Intl., etc.) multiple times and there is no need to repeat these tests if they are using the same membranes. Also, inexpensive bench tests are sufficient in establishing rejection properties of membranes for specific contaminants. Share this information and results of previous studies with your regulatory agency.
Typically this is done through manufacturers, national industry organizations and state/federal funding programs and not by municipalities, unless they have a direct interest in the technology implication.
If piloting is required, it should be a meaningful program tailored for the site conditions. Such comprehensive programs could cost $50,000 to $200,000 for smaller systems with limited pilot program scope to more than a million dollars for large mini-plant scale facilities with an extended long-term study. Remember, there is extensive involvement from the consultant and facility operators, and significant laboratory costs, coordination and installation of temporary housing, water, sewer, power, Internet and phone for pilot units. If multiple manufacturers are piloting to bid a project, these requirements are multiplied. Just remember, pilot units are not like vacuum cleaners that plug in and start collecting data.
Ben Movahed, P.E., BCEE, is president of WATEK Engineering Corp. and chair of publications for AMTA. He has over 25 years of experience in water treatment facility design with emphasis on membrane and desalination plants. For more information, please contact the author at 301.933.9690 or [email protected]