Water Purification Methods for Pharmaceutical Uses

Water purification is critical at every phase of production in a pharmaceutical or medical device manufacturing setting. From pre-treatment to injection, distribution, and storage, water is one of the most versatile solvents and needs rigorous purification to remove any dissolved substances that may affect its quality. In this article, we discuss the top three water purification methods and look at their advantages and disadvantages.

Distillation

This method involves heating water to its boiling point, then cooling the vapor using cold water as it passes through the condenser. Once the vapor condenses, it becomes liquid and is collected. There are various designs, such as single effect, multiple effects, and vapor compression. Multiple effects and vapor compression are preferred for larger purification systems as they have a greater generating capacity and are more efficient.

Advantages

• This method is reusable and thus is more cost-efficient.
• It can be used as a pre-purification step as it removes a wide range of impurities.

Disadvantages

• It cannot guarantee the absolute elimination of endotoxins and inorganic ions.
• Some volatile organic impurities, especially those with boiling points of less than 100˚C, can escape into the condenser and appear in the distillate.
• The high temperatures involved could produce organochlorides when the chlorine in tap water reacts with organic impurities in the water. 
• It is an expensive method as it requires large amounts of water and energy.

Deionization

As the name suggests, this purification method involves removing inorganic ions, cations such as sodium, calcium, magnesium, aluminum, and anions such as chlorides, sulfates, and nitrates. These purification systems have charge-bearing resins used to exchange the ions and are periodically regenerated using either an acid or a base.

The cation-exchange resins are regenerated using either sulfuric acid or hydrochloric acid, and they exchange a hydrogen ion with the positive ions (cations). On the other hand, anion-exchange resins are regenerated using bases, such as potassium or sodium hydroxide, and they exchange a hydroxide ion with negative ions (anions). The anion-exchange resin also removes some endotoxins as the free endotoxin has a negative charge. The exchanged hydroxyl and hydrogen ions then combine to form pure water.

The anionic and cationic resins can either be designed to be in separate beds or as a mixed bed exchanger that holds both. Mixed bed exchangers are preferred as they are more efficient. They, however, have a complicated regeneration process.

Advantages

• It effectively eliminates dissolved inorganic ions.
• Resins can be regenerated.

Disadvantages

• It has a limited capacity as ions cannot be retained when all ion-binding sites are taken up.
• It does not eliminate bacteria, pyrogens, and particulate or organic matter. 
• Chemical regeneration of resins can result in the generation of organic impurities and particulate matter.

Reverse Osmosis

Reverse osmosis removes about 95-99% of all impurities. Osmosis occurs when water moves across a semi-permeable membrane from a high concentration region to a low concentration region under osmotic pressure. For reverse osmosis, the inverse occurs.

This purification method involves using semi-permeable membranes that are big enough to allow water molecules to permeate but are too small for chemical ions. These membranes reject all bacteria, organic matter, particles, and pyrogens with a molecular weight greater than 200 Daltons. Hydraulic pressure is applied to the concentrated solution to counter the osmotic pressure so that pure water moves from the concentrated solution.

Advantages

• It successfully removes microorganisms, colloids, particles, and ions.
• It requires minimal maintenance.

Disadvantages

• Yield slow rates of flow as the membranes are restrictive.
• A pre-treatment step is required to safeguard the membrane from damage by scaling, organic deposits, or particulates.