By David H. Paul
Summary: In this second installment of a three-part series, the author discusses three more important considerations when embarking on the choppy seas of the membrane water treatment industry. This article expounds on tips 4-6 of 10 for water treatment dealers to keep in mind when selecting the appropriate membrane system.
Many public and private utilities and water treatment companies are evaluating use of membrane technologies to meet current and anticipated drinking water needs. Whether evaluating, designing or building and installing membrane water treatment equipment, there are several important factors to consider.
We have chosen 10 tips for improving the design, operation and maintenance of a membrane water treatment system. To reiterate, the 10 tips are:
- Identify your feed water for fouling and scaling threats,
- Select a proven membrane product,
- Be conservative in the design,
- Conduct a well-designed 2,000-hour (minimum) pilot test,
- Critically evaluate the pilot test data,
- Select an experienced designer/contractor,
- Train personnel very well,
- Don’t wet the membrane until you’re ready to run continuously,
- Take complete start-up data, and
- Implement and monitor performance trends.
Last month’s Part 1 covered tips 1-3. We’ll discuss tips 4-6 here. Next month’s Part 3 will cover the remainder.
A well-designed pilot test
Scaling problems within reverse osmosis (RO) and nanofiltration (NF) membrane units are defined as those that occur due to dissolved feed water substances that precipitate into solid form within a unit due to feed water concentration, which results from permeate production. Fouling problems within RO and NF units are defined as those primarily caused by living, dead, and/or non-living suspended solids plugging the surface of the membrane and/or the flow channels within the membrane elements.
As discussed in Part 1, the maximum concentration points for various sparingly soluble compounds in RO and NF membrane units are relatively understood. If scaling threats were the only concerns within a particular pre-treated feed water, we could use one of many software programs to somewhat accurately predict recovery rates at which a unit could be operated to keep it from scaling. Small-scale pilot testing, in many cases, wouldn’t be required.
While we can predict scaling fairly accurately, we unfortunately can’t predict fouling very accurately. This is especially true for surface water and groundwater under the influence of surface water. Living, dead, and/or non-living particle loading can change drastically due to seasonal changes and short-term variations in weather conditions. The results of greater-than-expected fouling during normal operation include higher energy costs, higher chemical cleaning costs, and higher membrane element replacement costs.
Higher energy costs
Electricity to operate an RO or NF unit’s high-pressure feed pump is typically the single biggest operating cost. The electricity needed is proportional to the amount of pressure required to produce a given volume of permeate. When fouling occurs on the surface of the membrane, more feed pressure must be applied to produce the same amount of permeate because the fouling layer produces a pressure drop across it.
Fouling on the surface of the membrane and/or in the feed water flow channels within the membrane elements also produces a greater feed-concentrate pressure drop. This is due to increased friction and a greater pressure drop across the plugged areas. This, again, requires the feed water pressure to be increased to produce the same flow rate of permeate.
For example, the percentage costs—determined by a pilot study conducted for a 700 gallon per minute (gpm), permeate, groundwater RO facility—were the following:
- Power 40 percent
- Pre-filter replacement 4 percent
- Antiscalant 29 percent
- Acid 6 percent
- Membrane element replacement 20 percent
- Other 1 percent
This was based on a fee of four cents per kilowatt-hour for power. As power fees and/or power requirements (due to fouling, scaling or other reasons) increase, the overall increase in costs to produce a given amount of permeate can be dramatic.
Higher chemical cleaning costs
Fouling is removed (with varying degrees of success) by chemical cleaning. A faster fouling rate results in the requirement for more frequent chemical cleanings. For very large units, using proprietary chemicals, the cost can run up to several thousand dollars per cleaning.
Higher element replacement costs
Many facilities budget to replace membrane elements every 3-5 years. With increased fouling rates and chemical cleaning frequency, however, it’s common to replace membrane elements in a shorter time period.
For a plant that has two 250-gpm (permeate) RO units with, for instance, 175 membrane elements that cost $700 each, the replacement cost for the equipment only (not including shipping or installation) would be over $120,000. For a 15-25 million gallon per day (mgd) facility, which is common in Florida and predictably in other states, membrane element replacement costs are in the millions of dollars.
For larger systems (see Figure 1), therefore, it’s imperative to understand that fouling rates will occur when operating on a given feed water. A small-scale pilot test is an economical way to document fouling rates as well as other parameters.
A pilot-scale unit has the same basic operating design as the anticipated future full-scale unit, only smaller. The smaller size allows for less capital investment for the pilot study; yet, the pilot unit’s performance should mimic the full-scale unit.
In many areas of the world, surface water changes with the seasons. Almost all surface waters will change with variation in weather, e.g., rainfall. To accurately document operating costs (power consumption, cleaning chemicals, chemical injection dosages, pre-filter replacement schedule, membrane element replacement schedule, etc.), it would be ideal to operate a relatively small pilot-scale unit for several years. This would increase the chance of operating the unit on all common variations of the feed water.
While there have been, and will be, long-term pilot studies that last for years, this isn’t common. It’s more common to operate a pilot unit for a shorter period of time. Two thousand hours (83 days) is the minimum time period that a pilot unit should be operated. A 2,000-hour pilot study should occur during the highest fouling time period. For most surface waters, this is during the summer when biofouling is typically highest.
Conducting a 2,000-hour pilot study greatly improves the ability to predict costs while testing the design of a membrane unit. Modifications to the operating design may be implemented during the pilot study to determine the most cost-effective way to operate the system.
Evaluating pilot test data
It takes a trained professional to accurately interpret 83 days of pilot study data and extrapolate to predict a unit’s performance over three-to-five years of operation. So, it’s easy for the untrained eye to get “snowed.” The author recommends that a neutral third party conducts the pilot study and/or evaluates the pilot test data. As a minimum, to track the performance of an RO/NF pilot unit, the following data must be obtained:
- Flow rates—permeate and concentrate for each stage,
- Conductivity—feed, permeate, and concentrate for each stage,
- Pressure—feed, interstage, permeate, concentrate,
- Temperature—feed, pre-treated feed, concentrate,
- pH—feed, pre-treated feed, permeate, concentrate, and
- Silt Density Index—feed, pre-treated feed.
From these data, normalized salt rejections, permeate flows and pressure drops can be calculated. The data are inputted into performance software programs so trends can be documented. The data are then typically extrapolated over a three-to-five year time frame.
Selecting an experienced pro
The common purchasing philosophy is to award a contract to the lowest bidder. There are many companies that are delving into designing, constructing and installing membrane water treatment systems. Some have little experience.
If you’re a company that wants to get into this business, be sure to have in-house expertise, or hire the expertise so that your design, installation and operation are excellent. Otherwise, you may not last long in the business.
If you’re a company that plans to purchase an RO/NF unit, ensure that you select a company with a proven track record and check out references.
If you’re considering membrane water treatment for the first time, there are things to watch out for and consider. This series of three articles covers 10 important tips. In this article, the most important tip discussed was to conduct and evaluate the data from a well-designed pilot study.
About the author
David Paul is president of David H. Paul Inc, an advanced water treatment training and technical services firm in Farmington, N.M. He has over 25 years of experience in advanced water treatment. Paul has published over 100 technical articles and papers. He has created and administers a 4,000-page, college-accredited correspondence training program plus three on-campus college degree programs in advanced water treatment. He holds a bachelor’s degree in biology and a master’s degree in microbiology from New Mexico State University. Paul can be reached at (877) 711-4347, (505) 327-2934 (fax), email: email@example.com or website: www.dhptraining.com