What are Your Pretreatment System Operating Tips?
Compiled by James McDonald, PE & Mike Henley
(Editor’s note: This article is based on recent discussions in the LinkedIn Ultrapure Water and Industrial Water Treatment groups. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor comments for clarity or length. An important purpose of each group is to provide a forum for practical examination of issues facing endusers of high-purity and industrial water.
Ultrapure Water Group Discussion
Compiled by Mike Henley
(Editor, Ultrapure Water)
Pretreatment System Tips
Mike: When operating a UPW pretreatment system, what tips do you have to share?
Joseph Smith: “In general, pretreatment components are selected to minimize fouling potential for the upstream generation equipment. The typical pretreatment unit processes could include multimedia filtration or cartridge filtration for the removal of suspended particles, Water softeners or antiscalant chemical injection for the removal of incoming hardness, activated carbon filtration or bisulfite chemical injection for removal of the disinfection from the incoming water source. The preparation of an operational procedure table listing the scheduled maintenance, frequency and exceptional condition is a great start. Collecting incoming water data and trending is very helpful in determining an overall maintenance approach to the equipment.”
Mohamed: “The pretreatment process is based on the influent water quality. Types can include:
- A conventional pretreatment system
- Disinfection for microbiological control
- Coagulation-flocculation-sedimentation-filtration for removal the suspended and colloidal material present in incoming water
- Dechlorination for removal the residual chlorine, whether by using activated carbon filtration (ACF) through solid chemical reduction on the surface of activated carbon, or by using sulfite-based products to reduce the residual chlorine that has an impact on RO filtration medium (membranes)
- Cross-flow filtration (RO units) for treating dissolved inorganic solids that present in solution, which is based on their concentration.
Another pretreatment method is using membrane filtration (microfiltration and ultrafiltration). The choice of pretreatment for production of high-purity water is based on the CAPEX, OPEX, and plant scale where the latter will produce high effluent quality that is acceptable to RO membranes.”
Fritz: “I have been operating pharmaceutical water systems for several years now. Choose your system design wisely, which makes the operation much easier. Unfortunately, in most cases the operators cannot choose the design but have to accept the system as it is and optimize it. In the pharmaceutical area, reliability and quality are key factors. Imagine what it costs the company if the system does not produce Purified Water/Water for Injection (PW/WFI) for one day? Another important topic is that if a purified water system breaks down, the pharmaceutical plant must stop all of their processes (compare that to if a tablet press breaks down). That is why a water system is one of the most important pieces of equipment at a pharmaceutical site.
Concerning the pretreatment, the sanitization parameters are quite important. In most cases, softeners are used in pretreatment (the water shall not contain "added substances") that are subject to microbial contamination. A monitoring concept must be set up to keep up the reliability of the system.”
Fab: “Good morning. I prefer PVDF-based UF over sand/multi-media filters. You should change your cartridge filters at least once a month, regardless of the differential pressure, try run the system as much as you can, because not using it means biological growth. Avoid dead-ends and dead-spots in the piping. Rinse the system through if you can.”
Shlomo: “The best way to operate pretreatment is by keeping the process elements to the minimum. Every filter, resin stage, and carbon filtration step retains contamination and can actively act as a growth medium for bacteria, with serious consequences for downstream equipment.
We use technology for reducing the water hardness without media and without salt regeneration. We have been using electrolytic scale reducers (ESR's) for more than 5 years as RO pretreatment with great results. As that they use is electricity.
As for chlorine removal— we install a powerful UV that destroys the chlorine without the use of granular activated carbon or chemicals. The operational costs are substantially lower than regular softener-carbon solutions as water, maintenance, and chemicals are saved.
In this way, our system actively reduces bacteria instead of encouraging bacteria growth. As the system has no moving parts and no media, it is robust as it is simple.”
Elbert: “Basically, we are using electrochemistry to remove scale and to produce free chlorine and UV to destroy microbiological activity and to remove free chlorine. The system is continuously in operation and a part of the pretreated water is recirculated and mixed with the incoming city water. Here the methodology of continuous bioburden reduction starts when free chlorine is introduced in the city water tank. The second step is electrical scale removal. At the cathode, the pH is increase and subsequently, scale is precipitated and automatically removed and flushed to the drain. This lowers the conductivity load on the reverse osmosis system. Free chlorine is produced at the anode, resulting in an absolute killing of the microorganisms. The third step is the hydro optical decomposition (UV)), which also has a double function of removing free chlorine before entering the RO and extinguishing any remaining microorganisms.
Compared to softener and activated carbon filtration technologies as pretreatment for RO and EDI, this approach does not waste water and use chemicals.”
Dattatray: “Basically, to generate high-purity water, an effective pretreatment design plays a major role with treatment steps such as a sand filter, dual softener, ultrafiltration, microfilter, and UV.”
Mauricio: “Cleaning and sanitization. At least one time a year the whole systems should be cleaned, this includes water reservoirs, pipes, vessels, filters, etc. This will reduce the biofilm that builds up over a period of time.
Sanitization should be performed on porous media such as ion-exchange columns, softeners, activated carbon filters, sand filters, etc. There are different sanitization approaches. For example, saturated ion-exchange resins can be sanitized with a sodium chlorine hypertonic solution (high concentration solution) on the saturated media. Activated carbon is especially prone to contamination, a frequent backflush with hot water reduces the risk of contamination, and something similar can be performed on sand filters.
Unfortunately, there is not a single answer. Each system is unique, and so are the recommendations for cleaning and or sanitization process conditions. A good operation manual must be always included in your cleaning procedures for every single component.
Lastly, despite systematic and proper work, be ready for the unexpected. Bacteria are living beings— they mutate and become resistant to chemicals and temperature. They are designed to survive. When they build biofilms, they protect each other. Single bacteria can reproduce to generate millions of its class. In those cases, you will need a different and more severe disinfection method.”
Industrial Water Treatment Discussion
Compiled by James McDonald, PE
James: How often should a cooling tower be cleaned?
Vinod: “Twice a year, depending on cooling tower size, type, and use. For heavy industrial power plants, it can be cut down to once a year, or cleaned if water gets polluted and changes color.”
Romain: “In France, once a year is mandatory.”
Malik: “Normally once per year, but it mainly depends upon turbidity levels and chemical dosing.”
Jonathan: “Down under, it's purely a regulatory dictated requirement, generally depending on the classification of the tower, industry, and regulatory body (Health Department, etc...) that has jurisdiction. Typically, it's quarterly.”
Jan: “With a good treatment, once a year.”
William: “I would say once a year. Here in Québec, cooling towers operate for six months only most of the time. But, the cleaning needs to be done perfectly. In Québec, we have high standards since the Legionella outbreak a few ago that killed 13 people and caused more than 250 people to get very sick. The new regulations have severe requirements.”
Keith: “I agree that a minimum of annually is a good practice. This helps minimize solids accumulation where Legionnaires bacteria disease can "hide out" from the biocide program. More frequent cleaning may be required in some circumstances.”
Vashisth: “Cleaning of cooling towers mostly depends on various factors like surrounding environment, type of make-up water being used, and type of blowdown strainer being used. According to cooling water conditioning, the rate of microbiological organism growth and suspension capacity of chemicals being dosed also matters. For example, in the iron and steel industry, the cooling tower basin is full of mud of powdered iron particles, which are mostly contained in surrounding environment.”
Craig: “As and when required. London’s air is so dirty and polluted that some of the towers I've maintained are cleaned three to four times a year”
Jake: “At least six monthly cleanings, regardless of its condition.”
Joe: “You truly have to know the laws, industry, and facility to answer this. For example, a tower next to a quarry may be air washing minerals, a hospital may have JCAHO regulations, or a country/locality may have guidelines. At a minimum, cleaning should be once per year.”
Andi: “There are key performance indicators for cooling towers, such as the approach temperature difference, among others. You can refer to some or all of the indicators to determine requirement of cooling tower cleaning.”
Mitchell: “A minimum of twice per year, according to NYC/NYS Regulations. Towers should be inspected frequently and cleaned as needed beyond the requirements.”
Tomas: “There are many and varied factors that influence the need for tower water cleaning: environmental conditions, the type of solids in the air, the number of concentrations, inlet and outlet temperatures, inlet water quality, filtration system for the circulating water, type of ground around the tower, elevation from the ground to the air inlet, contaminants that may be provided by the circulating water, and much more. Because of all this, it is impossible to determine the time between cleaning cycles.”
Vinod: “The OSHA Technical Manual – Section III: Chapter 7—states that cooling towers should be cleaned and disinfected at least twice a year. Normally, this maintenance will be performed before initial start-up at the beginning of the cooling season and after shutdown in the fall.”
Tim: “Normally, this maintenance will be performed before initial start-up at the beginning of the cooling season and after shutdown in the fall. That is it!”
James: Do you express your nitrite test results as NO2 or NaNO2, and what difference does it make?
Dominick: “Unbeknownst to many samplers, many (most) nitrite titration test kits express their results as NaNO2. Still, I've seen logs indicating an NO2 control range, yet nobody did the required math/calculations for conversion. For example, a very popular titration kit tells you to multiply number of drops X40; however, that only derives NaNO2. The correction (in their particular case) would be to multiply number of drops X27 to derive NO2. Most individuals, even experienced water treaters miss this in the field.”
Jan: “Most test kits/test strips give the result in NO2/NO3. To go from NO2 to NaNO2, multiply with 1.5.”
Marc: “If your test result is in sodium nitrite, multiply by 0.67 for the actual nitrite. The standard tests end point includes the sodium ion, unless the reagent states otherwise in the conversion.”
Clive: “Nitrite and chromate are anodic inhibitors, a.k.a. dangerous inhibitors (i.e., they can intensify corrosion [pitting] when underfed). The conversion factors can put the concentrations into dangerous territories.”
Adam: “If you are aware of the difference and which test you are performing, you should have an appropriate control range either way. I tend to report results as NaNO2 and have a range calculated accordingly.”
Mark: “Interesting that a chemical expression is worth a debate. How about molybdate versus molybdenum...what do you think?”
Jan: “Molybdenum is indeed a similar situation, you can express it as Mo; MoO3, or Na2MO4.H2O.....only the last two are stable and commercially available.”
David: “I've always seen it expressed as elemental N.”
MSP: “In both ways you could express, you only need to mention in the report whether it is expressed as NaNO2 or NO2. The end user could make out using the conversion factor.”
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