Pretreatment and Its Purpose

It is common to value-engineer the pretreatment stage of a high-purity water treatment system, often with detrimental long-term consequences for operation and maintenance. While pretreatment is sometimes viewed merely as the removal of suspended solids or particulates, it encompasses much more, particularly in high-purity water treatment systems.

Pretreatment is essential to condition water before it undergoes downstream primary treatment operations. The requirements for pretreatment can vary due to different feed water qualities and primary deionization techniques used. The three most common primary treatment technologies—reverse osmosis (RO), ion-exchange (often referred to as DI, even though RO and distillation are also deionization processes), and distillation—each require specific feed water qualities for efficient operation. In some cases, RO and DI can be considered pretreatment technologies if positioned upstream of distillation. RO and distillation processes concentrate feed water during operation, which can lead to increased fouling or scaling especially in waste streams.

The general purpose of pretreatment is to reduce the potential for oxidation, scale, and fouling in primary treatment processes. This is critical because these processes typically have the highest capital and operational expenditures in the system. We can categorize these concerns, and the commercially successful technologies to address them, as follows:

Oxidation: Oxidation is perhaps the greatest concern for primary treatment technologies because it is irreversible. It is commonly initiated by residual disinfectants such as chlorine and chloramines, which can oxidize thin-film composite RO membranes and cause pitting and corrosion in stainless steel components of distillation units. Common techniques for the reduction of oxidizing compounds include activated carbon, chemical injection, and UV light.

Fouling: Fouling can occur on RO membranes and DI resin beds due to particulates, colloids, organic material, or a combination of these impurities. The result is poor operational performance, increased energy or chemical consumption, and more frequent cleaning. Effective filtration and organic removal are the best remedies to minimize the fouling potential of feed water.

Biofouling: A subset of fouling, biofouling is caused by microbiological impurities. Control methods include the addition of residual disinfectants (e.g., chlorine), temperature management, and UV light. Physical filtration, periodic sanitization, and several other design features can also help remove and control bacteria in pretreatment systems.

Scaling: Scaling involves the precipitation of sparingly soluble compounds, typically magnesium and calcium salts, on the surfaces of RO membranes or distillation unit water contact surfaces. This affects product water quality and water flux through the membranes, while in distillation units, scale can limit heat transfer and overall performance. The most common method to prevent scaling is to remove scaling compounds from feed water through water softening. Chemical treatments may also be employed ahead of RO membranes. Additional scaling compounds, such as iron, silica, and other sparingly soluble compounds, should also be addressed if present in elevated concentrations in the feed water.

Other feed water contaminants, such as carbon dioxide, alkalinity, pH, ammonia, and ammonium, may influence the operation of downstream equipment, though they typically do not have as detrimental an effect.

Design & Maintenance

To minimize the risk of oxidation, scaling, or fouling, a robust approach to design, operation, and maintenance is critical. System design should account for the worst-case feed water conditions and any predictable seasonal fluctuations. Information regarding the successful operation of high-purity water treatment systems with existing feed water can be invaluable for design. For municipal feed water, be aware of any changes in treatment that could affect the incoming water chemistry. Note any advisories from the municipality regarding deviations from potable water standards, as these may impact the suitability of the water for pharmaceutical applications or necessitate additional testing to ensure compliance with potable water standards.

The pretreatment design should be based on three key factors:

1. Feed Water: Each feed water has unique properties, leading to significant variations in pretreatment design. Surface water supplies often contain more organic material and exhibit higher microbial activity, while groundwater sources typically have higher mineral content and may contain elevated concentrations of various silica compounds.
2. Downstream Processes: The pretreatment requirements will vary depending on the primary treatment option chosen. Impurities such as dissolved solids may be less consequential for a vapor compression still but are critical to prevent fouling in RO systems. Consideration should be given not only to the choice of primary treatment technology but also to desired recoveries, throughput, cleaning frequency, and specific manufacturer requirements.
3. Final Water Quality: While the main purpose of the pretreatment system is to protect primary treatment processes, final product water quality should not be overlooked. Primary operations may have a finite ability to remove certain impurities. If any of these are critical quality attributes for the final product water, additional treatment may be required either upstream or downstream of the primary techniques. This could include dissolved gas management, endotoxin control, or reduction of individual ions or organic impurities, such as nuclease compounds or nitrosamines.

Common pretreatment operations can effectively remove various classes of impurities. For instance, granular activated carbon is effective at removing residual disinfectants, such as chlorine, while also significantly reducing organic content.

Particular attention should be paid to controlling bacteria throughout the generation system. In the initial stages, residual disinfectants help in microbial control. However, once these disinfectants are removed, maintaining control becomes more challenging. At this point, additional treatments such as UV light should be considered, along with sound sanitization methods for all water contact surfaces.

The pretreatment system is often the simplest area to reduce capital investment for pharmaceutical water systems. Since this part of the system does not require hygienic design, inexpensive components can be utilized. However, because the criticality of the water is not as high at this stage, it may be tempting to use substandard components, design processes aggressively, or neglect adequate operational monitoring. As discussed, the functionality of pretreatment is vital, and any initial capital savings may lead to significantly higher operation and maintenance costs in the future.

In fact, the pretreatment section is where the best value can be found in conservatively designing each operation. While materials used in construction typically do not require sanitary design, slightly larger filter housings, vessels, and UV units may not add much to overall costs. Increasing the surface area of filtration, right-sizing carbon and softener beds, and using more or specialized media can justify the incremental investment. Additionally, providing process redundancy can be an inexpensive way to enhance system uptime. While undersized components may save initial costs, they will result in shorter run cycles, increased downtime, and higher maintenance needs.

Monitoring Pretreatment

Each pretreatment process serves a specific purpose and should be regularly monitored to verify performance. For feed water from a surface source, this is particularly important because the characteristics of the water can change seasonally and without warning, even if it remains classified as potable. Ideally, performance parameters for each pretreatment operation should be identified and monitored in real time. For instance, at-line monitoring of hardness downstream of a water softening system would continuously ensure proper operation, regardless of any changes in feed water hardness concentration. If performance parameters cannot be monitored in real time, they should be tested at a reasonable frequency.

In unmonitored pretreatment systems, the first indication of a problem may be a failure in the downstream primary treatment operations, which can be significantly more expensive to rectify. Proper pretreatment minimizes the need for cleanings and sanitizations of downstream components. Regardless of maintenance costs, system downtime and potential interruptions in water supply can be far more costly to facility operations.

As mentioned above, the importance of controlling and monitoring microbial activity throughout the pretreatment system cannot be overstated. In addition to being a requirement under cGMP for pharmaceutical water systems, maintaining a state of microbial control through pretreatment will reduce challenges for the remainder of the generation system and, ultimately, the storage and distribution network.

Recommendations

Pharmaceutical water systems generally consist of various unit operations, all of which are interdependent. Inadequate design or maintenance of one upstream process can have a cascading effect on others, ultimately impacting product quality. Although the significance of pretreatment processes may not be comparable to that of downstream processes, failures or inadequacies in pretreatment can lead to operational issues in the system. While the capital expenditures for pretreatment are typically lower than those for primary treatment operations such as RO, DI, and distillation, the design and operation of pretreatment are equally important.