Active carbon sanitization?

Question

Does anyone have any suggestions on a good chemical to sanitize an active carbon bed? I have a water plant fitted with an active carbon bed after softeners. The carbon bed is becoming a source of bacteria growth and I would like to sanitize it. any suggestions? Heat is not possible due to the design of the plant.

Answer 1

Today's high technology water purification systems are designed to remove many types of impurities. One of the most difficult to deal with is microbiological contamination. Microbial growth in a water system presents two problems: it not only reduces water quality, but if left unattended, the problem can "grow" to reduce system performance and life. Biofouling can cause a decrease in product water flow rate, a decrease in driving pressure, and can contribute to corrosion of piping and housings. This paper is a primer on the topic of dealing with established and potential microbial problems in crossflow membrane systems.
Applications for reverse osmosis (RO) systems include, as their treatment objective, the reduction of microbes. The term "microbes" includes algae, mold and yeast (fungi), protozoa (Giardia and Cryptosporidium are now well known), and the most popular target, bacteria. Related waterborne particles are virus (DNA particle) and endotoxins, which do not exactly fit the definition, but are often of concern and may need to be removed. Various media and levels of filtration can accomplish removal of these organisms (see Figure 1). The finest filtration, RO, will remove them all.

Not all applications have microbe removal as a primary objective, or even as a concern. However, all membrane systems face the consequences of microbial growth, especially bacteria.

It must first be understood that the presence of bacteria is inevitable, they are found in any and all water systems. Second, while they will always be present, they can be controlled. Third, a microbial contamination problem is much easier to prevent than to correct. This point is especially true for membrane systems- where the polymeric membrane is sensitive to the same forces used to eliminate microbes. Reverse osmosis membranes are the most chemically sensitive of all membrane classes, and the most commonly used for microbial control. Therefore, this paper focuses on RO systems as a model.

Microbial Removal Applications

The focus of this paper is control of microbes in a membrane system. However, it is also useful to understand the performance capabilities of membrane systems for microbial removal.

Currently, the only membrane-based product used for sterilization (the absolute complete removal of viable microbes) is microfiltration membrane (MF), in the form of integrity-tested, pleated-filter cartridges. Flat-sheet and hollow-fiber membranes of the RO, ultrafiltration (UF), and nanofiltration (NF) classes are theoretically capable of sterilization. However, in crossflow modules and systems, the potential exists for seal leaks, membrane flaws, and back-contamination. Crossflow technology is not used alone for sterilization.

Crossflow membrane systems are capable of significant microbial reductions, but their long-term performance is based on both diligent operation and good design. Under these circumstances, membrane systems are capable of consistent three log reductions (1 x l0 reduction, or 99.9% microbe removal) of microbes (Figure 2). for example, two-pass RO systems are commonly used in the pharmaceutical industry to produce United States Pharmacopeia (USP) grade Purified Water, and to pretreat feedwater to stills to help produce USP Water for Injection; a very high standard of purity. Applications in the fields of medicine, food, beverage, and semiconductor chip processing all rely on the microbial removal performance of crossflow membrane systems
MF validated integrity tested cartridge 7+ log reduction
*Theoretical Removal is 100%

Biofilm Formation

If microbial levels are not controlled, they will eventually form biofilms. Established bacteria of most types found in water secrete a polysaccharide-containing slime (glycocalyx) which enhances the bacteria cell's ability to adhere to a surface (Figure 3). Bacteria grow and multiply faster when attached (sessile) than when free-floating (planktonic). Attached cells form a larger colony. The slime layer helps adhere other bacteria cells and nutrients, which float past, and also acts as a protective layer, which resists chemical penetration. This is known as a biofilm

The size, complexity, and resistance to sanitization of the colony grows within this biofilm which is very difficult to penetrate using typical sanitizing agents. They also become a source of recontamination when sanitizing steps do not completely remove the biofilm. A single routine sanitization usually only affects the top layer of the biofilm, so viable bacteria deep in the biofilm will quickly recontaminate the system and high bacteria levels will be seen again within a few days.

Biofilm Removal

To destroy an established biofilm, repetitive sanitizing cycles are usually required. The first step uses a normal biocidal agent. The second step uses a high pH solution, usually sodium hydroxide, to help digest and remove the top layer of bacteria killed by the biocide. Fresh biocide is then reintroduced to the system to kill the next bacterial layer, again followed by caustic.

This biocide/caustic cycle may need to be repeated several times until the entire biofilm is removed. For a well-established biofilm, 5 or l0 cycles are commonly required.

Continuous or Periodic Sanitization?

There are two basic approaches for controlling bacterial growth in a water system. One is to maintain a residual level of biocidal agent within the system (continuous dosing). This is similar to the common technique where municipal water treatment facilities inject enough chlorine, or chloramines, into their treated water to provide a residual throughout the distribution system. Chlorine is the most common biostat used in the United States (at least presently), and a typical minimal target residual is 0.2 mg/L. In Europe, ozonating municipal water systems is popular, but maintaining a residual is difficult due to the short half-life of ozone in water. In the U.S., the most commonly used chemical in point-of-use water treatment systems is chlorine, in the form of sodium hypochlorite (household bleach).

The second approach is to periodically sanitize the system. Whether a periodic or continuous approach is used will depend on the quality of the product water required. For instance, those systems producing "ultrapure water" where no chemical residual can be tolerated in their product water must employ periodic cleaning and sanitizing instead of continuous dosing of a biostatic chemical. Most systems using continuous dosing will also need a regular, although less frequent, cleaning and sanitizing regimen. Even when ultraviolet (UV) lights post-treatment with heat or biocide addition in the storage and distribution system is done, the whole system will require periodic sanitization.

System Designs---Pretreatment Equipment

A typical RO system consists of depth media prefiltration and chemical pretreatment, followed by reverse osmosis filtration, optional deionization skid, then a storage tank and distribution system (Figure 4). Ultrapure water systems are more complex (Figure 5). Pretreatment components chosen depend on feedwater quality, and are usually intended to protect the reverse osmosis (RO) membrane from fouling. The various pretreatment components themselves have different tendencies for microbial growth, and thus should be included in the engineering review for both design and operational strategies for microbial control.

Figure 4



A backwashable dual-media filter is used to remove large particles (10 to 20 microns and larger) and also precipitated iron (the oxidized ferric state). The large surface areas of the media in these filters provide a favorable environment for bacteria to grow and multiply, especially if the feedwater does not have residual chlorine. With a residual chlorine level of from 0.5-1.0 mg/L, dual-media filters are not typically the source of biological problems.

Where dissolved iron (ferrous) is present, a useful technique is to add chlorine to oxidize and precipitate it, to allow its filtration. An added benefit is the control of microbial growth. If iron is not being removed, and a 0.2-0.5 mg/L free chlorine residual is not consistently present otherwise, a good strategy is to inject chlorine to control growth in the pretreatment equipment (even if it must be removed later in the system, this step usually pays off).

A water softener is often used instead of pH reduction to prevent salt precipitation of the RO membrane, but this unit process is generally not a source of microbial problems–since the high ionic concentration of the regenerant brine solution is an unfavorable environment for bacteria.

Activated carbon (AC) is commonly used when organics or chlorine need to be removed. Granular activated carbon filters have very large surface areas available for biogrowth. Because the porous media fix organics which can be used as nutrients on its surfaces, and since the carbon upstream in the bed acts to catalyze the reduction of chlorine and remove it from the water before traveling through the whole media bed, AC filters commonly become contaminated and are well known as sources of microbial contamination. Designing AC filters with a constant recirculation loop to assure flow, and adding a UV light to this loop, is a technique to retard microbial growth. Activated carbon beds are chemically sanitized on a periodic basis, using a sodium hydroxide solution of pH 11 to 12 in a static soak of at least 4, up to 24 hours. Rinse-out of the caustic will take a long time. In critical applications, live steam is often used for sanitization (provided the tank and all other components are capable of withstanding the pressures and temperatures required, typically 121°C at atmospheric 15 psig for 2 hours). Steaming degrades the carbon somewhat, and fines are created which are typically backwashed out (with pure water). Hot water at 180°F for 2 hours can also be used.

Chemical injection for pretreatment is common in RO systems. The feed tanks of such chemicals as antiscalent, coagulant or chlorine reducing agents (e.g., sodium metabisulfite) may themselves become sources of contamination. Check with the system supplier or chemical manufacturer to determine the appropriate feed tank conditions to prevent this. A regular, complete change of the feed stock and thorough cleaning of the tank is often the best approach.

When conducting sanitization of any component, it is important to ensure that any sample valves, drain valves by-pass lines that could harbor bacteria are also sanitized. It is also important to sanitize the components of the system in the order the water flows. This will minimize the potential of contaminating the freshly sanitized component by the component immediately upstream.

Figure 5



RO Machines and Elements Design

An RO machine is fairly complex, and may have many rough surfaces, crevices, and dead flow areas in its piping and instrumentation. The membrane elements have an especially large amount of surface area available to support microbial growth. Proper design of the RO machine will minimize bacterial growth potential, and there are many design considerations that can affect microbial control.

1. Sanitary Plumbing Design. A proper sanitary design requires there be few, if any, dead legs (where the length of dead-end pipe is more than 3-4 pipe diameters) and as many smooth internal surfaces as practical. It should also allow for routine cleaning and sanitization, and be completely drainable as well. Where heat or aggressive chemical sanitization is anticipated, or polished surfaces required, stainless steel is preferred over plastic piping.

2. Materials of Construction. Smooth surface materials such as polished stainless steel are more resistant to biofouling than the rougher surfaces found in polyvinyl chloride (PVC) and other pipe, and threaded joint fittings. These microenvironments of low flow are areas where bacteria can lodge, adhere, and grow.

3. Sanitary Valves and Instrumentation. Diaphragm valves are more sanitary than ball valves, because the active portion remains in the flowing stream, while the closed ball valve will be stagnant. Many common gauges and instruments are now available in sanitary design, at added cost.

4. Sanitary Design Membrane Elements. Because membrane elements have such large surface areas, they are particularly prone to biofouling. A membrane element design that eliminates the so-called "brine" or "concentrate seal" and the dead-flow areas behind these seals is preferred (e.g., Full-Fit™or Durasan® designs). This design innovation from the 1980's allows a continuous flow past the membrane element at all points within the housing.

5. Post-Treatment Protection. Some means to protect the water from microbial contamination after it has been purified by the RO machine may be required. Regardless of the method used, a recirculation loop within the storage and distribution system is preferred to maintain a minimum velocity of 6 feet per minute. Several methods are available. One is to heat the water to 80°C, and continuously recirculate it. A second is to continuously inject ozone, an extremely powerful biocide, at 0.2-0.5 ppm concentrations (post-membrane with no recirculation). Ozone does not contaminate product water, since its breakdown products are oxygen and water. A third is to circulate the product water through 254 nm wavelength, ultraviolet (UV) lights, located on recirculation loops and at points of use, although they will not destroy an established biofilm. All of these methods are effective at protecting the product water from microbial recontamination, but the system design, operation, and use of the water must all be considered in selecting their use.

While cost-effect designs should always be used in an RO machine, only applications where bacterial reduction is critical for the end use will usually support the extra cost of sanitary-design components. Examples are found in food, medical, pharmaceutical, and semiconductor chip manufacturing applications. Here it is common to pay extra for stainless, polished piping, and sanitary style valves, gauges, and instruments. Where steam or aggressive chemical sanitization is anticipated, use of stainless steel predominates over plastic piping. However, most RO applications can bear the small incremental cost of the more sanitary membrane element design.

System Operation

Proper operation is at least as important as proper sanitary design for controlling microbes in a membrane-based treatment system. There is no practical design for an RO system, which does not require diligent and knowledgeable operation to assure low microbe counts.

Although not always practical, continuous operation can prevent areas of stagnant water, which leads to biofilm formation. If the system cannot be run continuously, a programmed automatic flush cycle for 15 minutes every 4 hours will help dislodge bacteria from the system before they firmly adhere to the surface, inhibiting biofilm formation and flushing them from the system. This will reduce the frequency of the required cleaning and sanitizing schedule.

When to Sanitize

Proper operation also includes preventative maintenance, such as routine cleaning and sanitization. The key to determining the cleaning and sanitization frequency is to review operating and performance parameters over a period of time. Important operating criteria include pre and post filter pressures, flow rates, temperatures, and microbial levels.

Two general rules of thumb for cleaning and sanitizing an RO system are when the product water flow rate decreases by 10% to 15%, and when there is an increasing trend in the bacteria level found in the parameters such as flow rates, and pressures. However, in applications where critical bacterial limits exist, it is necessary. Bacteria samples can either be sent to a local lab, or commercially available bacterial samplers can be used on site. The samplers are designed to culture bacteria cells, which can then be enumerated by visual inspection after 48 to 72 hours incubation. Microbial counts should be recorded over time, to establish a regular cleaning and sanitation schedule. With a regular schedule, the system is maintained to prevent a critical level of fouling and biogrowth in an efficient manner.

Microbial Trend Analysis

When using microbial levels to set a cleaning/sanitizing schedule, it is useful to do a trend analysis of the data. Even though an RO system will remove microbes, a high level in the feedwater will eventually contaminate the downstream water. For this reason, bacteria levels should be monitored in the feed as well as the product water. Seasonal variations are common, depending on the location and water source, so some baseline testing may be needed throughout the year. Because bacterial growth is dependent on circumstances, which can be complex, it is not always predictable or repeatable. Therefore, it is important to determine whether the bacteria count in the product water is trending toward a higher level or there is simply a transient spike (Figures 6-8). If bacteria counts increase slowly over time, the nature of the contamination is different than for a biofilm, which is characterized by a rapid exponential increase. A spike may indicate a low-flow area agitated under unusual circumstances, or the result of a long shutdown without proper biocidal controls. A gradual increase in levels may be natural and not practically preventable.

aning and Sanitizing Agents

There are many types of cleaners and sanitizers available . These can be placed in two major categories. Oxidizers, which include ozone, chlorine, hydrogen peroxide and peracetic acid are very effective against most kinds of microorganisms, but degrade polymeric membranes and are completely incompatible with some. Ozone, for instance, will immediately degrade any RO, NF or UF membranes.
Common Sanitizing Agents Agents Effective Concentration Time
Cleaning and Sanitizing Techniques

A typical cleaning/sanitizing procedure includes the recirculation of the chemical agent for 20 to 30 minutes followed by a 20 to 30 minute soak. This is followed by an additional recirculation for 15 to 20 minutes. The agent is then flushed from the machine using purified water. It is always important to check with the membrane manufacturer for chemical and membrane compatibility, and the recommended contact times before using any biocide cleaning/sanitizing regime.

Sanitizing PA/TLC Membranes

The polyamide polymer family membranes have grown to be the most popular RO membranes in use, eclipsing cellulose acetate blends (CA) in the 1980's. Virtually always made in a composite morphology, these membranes are more commonly referred to as "thin-layer composite" (TLC), "thin-film composite" (TFC), etc. They are all based on chemically sensitive polyamide polymers.

The aggressive chemical nature that makes some biocides effective also degrades PA membranes-- some drastically. Many oxidizing agents cannot be used. Ozone is out of the question for any RO membrane. Free chlorine in any dosage will degrade PA, so chlorine in most forms is discouraged. Hydrogen peroxide is tolerable, but is slow acting and requires high concentrations for effective action. The use of peracetic acid for PA membranes was popularized in the kidney dialysis market eight to ten years ago, and remains common today.

It has been learned, painfully, that to use peracetic acid or other oxidizing agents, the membrane must first be cleaned of iron and other metals (acid cleaning cycle). If not removed, these dramatically catalyze the degradation of PA membrane by oxidizing agents possibly resulting in severe membrane damage. An acid cleaning cycle is usually effective in removing these metals.

This chemical weakness of the PA/TLC membranes has led to the use of the CA RO membranes in many installations where low level chlorine dosing, or regular sanitizing of the entire RO machine with membrane in place is desired. While not as effective in rejecting salts and less tolerant of high pH fluctuations than PA membranes, CA RO membranes are tolerant of low levels of chlorine. In fact, some of the most severe cases of biofouling occurred in large-scale RO systems installed in electronic applications, where CA membrane was replaced with PA and chlorine residuals eliminated.

Summary

Microbial contamination, and particularly biofilms, can cause significant quality and operational performance problems for RO water treatment systems. Components with large surface areas, such as AC filter beds and the membrane elements, are the most prone to problems. While microbes cannot be eliminated, they can be controlled. The first step is proper system and component design; the goal is to eliminate areas conducive to stagnant flow and bacterial attachment. Operational techniques are also required, including continuous or at least regular periodic operation, and continuous biocide dosing. These are not always practical, but reduce the frequency of the cleaning/sanitizing cycle. While highly variable in required frequency, a periodic cleaning/sanitizing cycle is nearly inevitable.

Answer 2

Try Chlorine.