Fundamentals of Water Processing

 
Pall Hollow Fiber Filtration System
For more than a century, conventional water treatment plants provided a first line of consumer protection. Now, older technologies such as rapid mix, coagulation and flocculation, sedimentation, or multi-media filtration can no longer ensure that a community's drinking water supply will not be infiltrated by such contaminants as Cryptosporidium, Giardia lamblia and a variety of water-borne viruses.

The specifications for processed water which reaches the domestic, commercial, industrial, or agricultural user are extremely varied in character. The domestic consumer demands a product that is not only safe, but also colorless and free of turbidity, unpleasant taste or odor. For the industrial user, it is generally required that the product be free of chlorides, iron, manganese and dissolved gases. For the agricultural segment, quality may mean the absence of sodium, chlorides, selenium and other elements. Only one water treatment method can effectively and economically address the diverse requirements of every user - membrane filtration.

Membrane technology is rapidly gaining acceptance throughout the world as the most effective and economical water treatment method available. The degree of purification required generally determines what level of filtration is appropriate for a particular application:

  • Microfiltration is ideal for removing suspended particles and bacteria from liquids. Microfiltration membranes are the most porous and normally operate at pressures between 20 to 100 psig. Microfiltration technology is used in the pharmaceutical industry for cell harvesting and in the brewing industry for the cold sterilization of beer.
  • Ultrafiltration removes oils, colloidal solids and other soluble pollutants and allows for recycling of industrial waters. It works well on waste streams with compositional variability reducing, by up to 98%, the amount of waste to be treated or discharged. Produced waters from the oil and gas industries can be effectively treated to remove oil and recycle the water. Ultrafiltration membranes will remove colloids, sugars, and many organic biological solids.
  • Nanofiltration membranes have been developed that have retention capabilities of between 100 and 1000 molecular weight. Since a molecular weight of 200 equates to a large molecule of about 1 nanometer, these membranes are called nanofiltration membranes. Applied pressure requirement is between 75 and 450 psig, (far less than needed to achieve similar flow with reverse osmosis membranes). Ion-selectivity is a significant feature of nanofiltration. Salts with monovalent anions (e.g., Chlorides) are able to pass through the membrane, while salts with polyvalent anions (e.g., Sulfates or Phosphates) are retained. Nanofilters are used for processing surface waters to remove Matter Organic Water (MOW) such as humic and fulvic acids in order to preclude the formation of unwanted disinfection by-products (e.g., Trihalomethane). Nanofiltration technology can achieve a 98% recovery of water from waste. It is considered to be the state of the art in water purification.
  • Reverse Osmosis is the reversal of the osmosis process through applied pressures ranging from 400 to 2,000 psig. Reverse osmosis systems are used to produce drinking water from sea water. Because of the advances made in membrane technology, the use of reverse osmosis is becoming commonplace in the treatment of industrial waters for recovery of by-products. Reuse or discharge of water that will exceed the quality levels of even the most stringent government regulations is now attainable.