Steve Peck, Technical Manager, AXEON Water Technologies
Many customers are familiar with the term TDS, 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 say 2000 ppm for well water. TDS is most commonly associated with sodium chloride, typical table salt; however, it’s actually composed of a variety of salts and minerals.
Hardness and alkalinity are other terms familiar to many folks 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 & magnesium and carbonate & bicarbonate, respectively in the water. Although this information is more descriptive than knowing just TDS alone, it still does not paint the complete picture.
Reverse osmosis (and nanofiltration) purifies water by concentrating the impurities in the feed water (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 of the RO system and is expressed as a percentage. For example, if 5000 gallons per day (GPD) of permeate were produced from 10,000 GPD of feedwater, the recovery rate would be 50%.
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%, then the concentration of salts in the concentrate is twice that in the feed stream. This “concentration factor” increases from 2 to 4 if the recovery rate is increased from 50% to 75%.
The relation between concentration factor (CF) and recovery (R) looks like this:
CF = 1÷ (1-R),
where R = permeate flow ÷ feedwater flow (expressed as a decimal).
(Concentrate polarization factor* is another term used in the industry to predict the probability of scaling).
To best determine the allowable recovery rate, it is important to know which specific ions and minerals are in the feed water because as these impurities become more concentrated, several of them reach a point (i.e. concentration factor) 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 last vessel stage of the RO system.
Several of these “sparingly soluble salts” are listed below in decreasing order of frequency of scaling issues:
Calcium carbonate, CaCO3
Calcium sulfate, CaSO4
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 more often used. SDSI takes into account the effect that 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.
Acid addition can be used to control calcium carbonate; however, acid addition alone will not control other sparingly soluble salts also commonly encountered in an RO system such as calcium sulfate and silica. 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, AXEON S-100 Antiscalant 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. AXEON offers the S-200 Antiscalant for challenging feedwaters containing high levels of metal oxides, silica and scale-forming minerals.
Reverse osmosis (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) such as humic substances and tannins are common to surface water and groundwater. Pretreatment should be considered 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.
By keeping colloidal compounds in suspension, antifoulants, such as the AXEON F-25 Antifoulant 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 becomes time to clean the RO membranes when the production rate decreases 15% compared to the initial production rate (or the differential pressure between the pump pressure and the concentrate pressure has increased by 15%). A routine cleaning regimen that includes AXEON C-10 and AXEON C-20 membrane cleaners will restore the performance of the membranes and the water production rate when applied before the change in operating parameters exceeds 20%.
AXEON C-10 is a low pH cleaner to gently and effectively remove inorganic scalant from the surface of the membrane and AXEON C-20 is a high pH cleaner formulated to remove colloidal and organic foulants.
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. In addition, it is very important to preserve the membranes from microbial growth. AXEON M-100 Membrane Preservative is a safe alternative to the use of chemicals such as formaldehyde or sodium bisulfite.
The above discussion pertained to the prevention or retardation of scaling and fouling. A future topic will focus on the use of membrane cleaners to restore performance after scaling or fouling has occurred. AXEON C-10 low pH cleaner is an excellent cleaning solution to gently and effectively remove inorganic scalants from the surface of the membrane and AXEON C-20 high pH cleaner is formulated to remove colloidal foulants.
For more information please contact us at 800-320-4074, email at firstname.lastname@example.org.
*Concentration polarization factor (CPF) can be defined as the ratio of salt concentration at the membrane surface to the salt concentration in the bulk stream. 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% to 20% greater than in the bulk stream).