High purity water treatment systems include several process steps. To prevent oxidation of downstream treatment components such as reverse osmosis membranes and distillation units, residual disinfectants must be removed from the feed water. The use of activated carbon as a disinfection removal technique in high-purity water treatment is more widespread than in other water treatment industries, many of which rely on chemicals to remove chlorine and related compounds. There are many benefits to the use of activated carbon, but also downside risks that must be mitigated to ensure successful design and implementation. Let us consider some design and operational benefits and disadvantages of activated carbon in high-purity water systems.
Activated carbon is a robust and dependable process. When carefully designed, carbon units will remove chlorine and combined chlorine compounds such as chloramines, readily and completely from aqueous streams.
Although activated carbon has a finite capacity for disinfectant removal, small changes in feed water chemistry will not disrupt the removal process.
In addition to chlorine removal, activated carbon offers the added benefit of selectively adsorbing dissolved organic material. This can be an important factor in reducing downstream reverse osmosis (RO) cleanings and help prevent against organic fouling. Also, minimizing organic material reduces the available nutrients in the system which may help to limit microbial activity.
A common alternative technique for chlorine removal is the use of reducing agents such as sodium sulfite. Activated carbon would eliminate chemical handling, under or overdosing of chemicals, and chemical replacement. This is often preferred in high purity environments compared to more industrial water systems and applications.
Compared to chemical injection, the capital expenditure for activated carbon systems can be much higher, especially for units that are designed to be thermally sanitized. Hot water or steam sanitized units would require special materials of construction, like stainless steel, as well as a heat source such as pure steam or plant steam for indirect heating.
Operational costs for activated carbon units include media change outs and energy costs for thermally sanitized systems. Although chemical injection systems may require more frequent maintenance for chemical replenishment, certainly the cost of media replacement for larger activated carbon systems can be higher.
Activated carbon units are designed such that the water is treated by flowing through packed columns filled with granular activated carbon media. The space required for these vessels can become significant or multiple vessels may be required for treating higher flow rates. For organic reduction or chloramine removal, a minimum of 1.0 cubic foot of carbon media or more is required for 1.0 gallon per minute of water flow.
The contact time (or residence time) between the water and carbon media can vary significantly based on the feed water chemistry, the media used, and the percentage of contaminant removal targeted. For significant organic reduction or combined chlorine removal, larger bed volumes are required. For specific contaminant removal, pilot testing may be necessary to demonstrate effectiveness and properly determine the volume of carbon media needed.
Activated carbon units will also release fines into the process stream. The units should be backwashed and rinsed when placed in service and after a bed changeout. The media should be replaced at regular intervals as the carbon particles will embrittle over time. The use of 5-10-micron filtration downstream of carbon units can be employed to prevent these fines from travelling further downstream in the process.
The principal concern regarding the use of activated carbon in high purity water systems in the propensity for microbial proliferation in the activated carbon beds and subsequent downstream microbial contamination. Even for units that are regularly hot water sanitized, effluent water samples from the carbon beds often exceed 500 cfu/ml total viable bacteria, the recognized maximum allowable limit for potable water. The growth of bacteria due to the environmental conditions in the carbon beds is inevitable but can be controlled within reasonable limits with proper design and operation.
Recommendations regarding the use of activated carbon in high-purity water applications include:
The application of activated carbon is more appropriate when organic reduction is a concern. This would include RO based systems operating on surface water supplies where the naturally occurring organic content is higher compared to well water supplies.
Activated carbon for chlorine and chlorine removal is always preferred for distillation-based systems (especially those without reverse osmosis or deionization pretreatment) where any oxidation would be catastrophic
Carbon media should be replaced on a regular basis regardless of bed exhaustion. Six to twelve-month replacement is typical for waters high in organic content or those that contain chloramines. Service exchange activated carbon vessels may be a viable option for lower flow rate systems.
Bacteria should be controlled as much as possible within and downstream of the carbon unit. Control techniques may include thermal sanitization of the bed, avoiding extended periods of stagnation, frequent bed exchanges, and incorporation of UV light downstream of the carbon units.
Proper bed velocities and volumetric flow rates are required to ensure removal of contaminants. While the reaction for chlorine removal via activated carbon is quick, the removal mechanism for organic reduction as well as chloramine removal is slower and will require more contact time or lower volumetric flow rates to be successful.
In high purity applications, activated carbon can be a successful tool in the treatment process, and the good benefits often outweigh the bad and the ugly.
The risks associated with activated carbon in high purity water systems can be mitigated once they have been identified. The recommendations above offer a baseline for proper design and operation. In high purity applications, activated carbon can be a successful tool in the treatment process, and the good benefits often outweigh the bad and the ugly.