Many customers in the carwash industry are familiar with the term TDS, or total dissolved solids. TDS is a very important parameter in the design of reverse osmosis (RO) systems. TDS is often reported in parts per million (ppm), such as 300 ppm for tap water or 2,000 ppm for well water, and is most commonly associated with sodium chloride, or table salt. However, it’s actually composed of a variety of salts and minerals.
Hardness and alkalinity are other terms familiar to many people, especially in discussions of traditional water softening. The reporting of both the hardness and alkalinity of water provides information relative to the levels of calcium and magnesium as well as carbonate and bicarbonate, respectively, in the water. Although this information is more descriptive than knowing just TDS alone, it still does not paint the complete picture.
A closer look at RO
RO (and nanofiltration) purifies water by concentrating the impurities in the feedwater (i.e., salts) into a waste stream. The industry terms “permeate” and “concentrate” refer to the purified water and waste stream, respectively. The ratio of the permeate separated from the feedwater entering the RO system is defined as the “recovery” rate and is expressed as a percentage. For example, if 5,000 gallons per day (GPD) of permeate were produced from 10,000 GPD of feedwater, the recovery rate would be 50 percent.
As the recovery of permeate is increased (i.e., more pure water produced), the quantity of salts is increased in the concentrate (waste) stream. For example, if the recovery rate is 50 percent, then the concentration of salts in the concentrate is twice that in the feed stream. This “concentration factor (CF)” increases from two to four if the recovery rate is increased from 50 percent to 75 percent. The relation between the CF and the recovery (R) looks like this: CF = 1 ÷ (1-R), where R equals permeate flow divided by feedwater flow (expressed as a decimal).
The concentrate polarization factor (CPF) is another term used in the industry to predict the probability of scaling and can be defined as the ratio of salt concentration at the membrane surface to the salt concentration in the bulk stream. The CPF is a function of the permeate recovery rate and the membrane element geometry. In a typical RO system, the CPF ranges from 1.13 to 1.2, meaning that the concentration of salts at the membrane surface is 13 percent to 20 percent greater than in the bulk stream.
Determining the allowable recovery rate
To best determine the allowable recovery rate, it is important to know which specific ions and minerals are in the feedwater. As these impurities become more concentrated, several reach a point (i.e., CF) where they are no longer dissolved in water but precipitate and deposit onto the membrane surface and cause scaling. Scaling occurs on the concentrate end of the final (tail) elements of the RO system’s last vessel stage.
Several of these “sparingly soluble salts” are listed below in decreasing order of frequency of scaling issues:
- Calcium carbonate, CaCO3
- Calcium sulfate, CaSO4
- Silica, SiO2
- Barium sulfate, BaSO4
- Strontium sulfate, SrSO4
- Calcium phosphate, Ca3(PO4)2
- Iron hydroxide, Fe(OH)2.
Since groundwater and surface water commonly contain calcium carbonate at concentrations close to saturation, calcium carbonate is the most frequently encountered scalant in operating RO systems. The solubility of calcium carbonate is dependent on the pH of the concentrate stream. At lower pH (acidic conditions), calcium carbonate tends to stay dissolved in the concentrate.
One indicator of the potential for calcium carbonate scaling is the Langelier Saturation Index (LSI), which compares the pH of the concentrate stream with the saturation pH. The LSI is used for brackish waters (TDS < 10,000 ppm). An LSI > 0 indicates that the calcium carbonate in the concentrate is beyond the saturation point.
For very high brackish waters (TDS > 10,000 ppm), such as seawater, the Stiff & Davis Saturation Index (SDSI) is used more often. SDSI takes into account the effect increasing ionic strength of high TDS water has on the solubility of sparingly soluble salts. The density of ions in water with high TDS interferes with the precipitation of the sparingly soluble salt.
An alternative to conventional water softening
As a less-expensive alternative to conventional water softening, antiscalants are often used to effectively control a broad variety of inorganic scalants over a large concentration range. Antiscalants interfere with precipitation through either threshold inhibition or crystal modification. For example, one antiscalant solution is injected into the feedwater to retard the precipitation of scalants, thereby allowing higher recovery than could be achieved otherwise.
Silica scale is especially difficult to remove from the membrane surface. In the presence of aluminum or iron, silica forms insoluble aluminum and iron silicates; therefore, if a silica scaling potential exists, aluminum and iron must be removed from the feedwater. An antiscalant is helpful in silica scale control by slowing agglomeration of scale particulate. An antiscalant is available in the market that challenges feedwaters containing high levels of metal oxides, silica and scale-forming minerals.
Within the carwash industry, the use of RO for a spot-free rinse reduces the amount of detergent used in the wash process leading to lower costs for treatment when reclaiming wash water. The antiscalants used in the RO process are safe to use and do not interfere with the water reclamation system.
Removing particulates with RO
RO (and nanofiltration) systems are intentionally designed for the removal of the dissolved ions but not the suspended solids (particulates). Equipment such as media filtration, cartridge filtration and ultrafiltration are specifically designed to remove the particulates. Smaller, fine particulates that find their way through this pretreatment equipment, such as silt, clay, suspended solids, biological slime, silica and iron flocs, end up on the surface of the membrane.
In addition, dissolved organics known as natural organic matter (NOM), including humic substances and tannins, are common in surface water and groundwater. Consider pretreatment when the concentration of total organic carbon (TOC) exceeds 3 ppm. These colloidal particles and organic compounds are referred to as “foulants.” Fouling affects the lead elements of the first stage.
Treating feedwater with antifoulants
By keeping colloidal compounds in suspension, antifoulants are used to treat feedwaters with high potential for fouling by silt organics, colloids, tannins and fine particulates.
During RO system operation, it is normal for the production of RO permeate to slowly decrease over time (when running the system at the same pressure and temperature) due to the eventual buildup of scalants and foulants. It is time to clean the RO membranes when the production rate decreases 15 percent compared to the initial production rate (or the differential pressure between the pump pressure and the concentrate pressure has increased by 15 percent). A routine cleaning regimen will restore the performance of the membranes and the water production rate when applied before the change in operating parameters exceeds 20 percent.
When an RO system is not operating for an extended period or placed into storage, dissolved nutrients from the water previously concentrated at the surface of the membrane can create an ideal environment for the growth of microorganisms. A permeate flush prior to shutdown is beneficial in purging the membrane. Furthermore, it is important to preserve the membranes from microbial growth.