Water is essential to sustain life, and an adequate, safe and accessible supply must be guaranteed to everybody. Improving access to safe water results in tangible benefits to health and economic development, and as such, every effort should be made to achieve a water quality as safe as possible.
In several regions around the world, water reuse is becoming increasingly important. Water reuse has several drivers. First, water reuse is related to freshwater availability, which depends on the natural environment and climate, plus the industrial and urban pollution related to the use of water. Affordability of water and the price of water is a driver for reuse in areas where the price is close to the affordability. Unfortunately, in many regions the actual water price does not reflect the affordable price or the cost to produce.
Second, population growth and urbanization, which go hand in hand with industrialization and an improvement of the living standard, create a local imbalance between water supply and demand. This is currently occurring in Asia and influencing policymakers to stimulate water reuse.
Water reuse has some practical challenges. First, the lack of infrastructure drives toward modular technology offerings. Second, the need to achieve very stringent quality specifications of reused water drives technology development.
The health concerns associated with chemical constituents of drinking water differ from those associated with microbial contamination and those that arise primarily from the ability of the chemical constituent to cause adverse health effects after prolonged periods of exposure. There are few chemical constituents in water that can lead to health problems resulting from a single exposure, except through massive accidental contamination of a drinking water supply. Moreover, experience shows that in many but not all such incidents, the water becomes undrinkable owing to unacceptable taste, odor and appearance. There are many chemicals that may occur in drinking water; however, only a few are of immediate health concern in any given circumstance.
In recent years, there has been increasing interest in a group of chemicals generally known as emerging organic pollutants, or emerging constituents. These are chemicals used in everyday life, and their presence in environmental waters is explained either by a point-of-source contamination or incomplete removal in wastewater treatment. Examples of compounds of particular interest include endocrine disruptors, human and animal antibiotics, disinfection byproducts, pesticides, insecticides, herbicides and various pharmaceutical drugs. All of the aforementioned chemicals have an unwanted effect on natural ecosystems and in humans. The challenge is to reduce the pollution and prohibit its presence in drinking water.
Reverse osmosis (RO) and nanofiltration (NF) membrane technologies have been touted as suitable for cost-effective desalination and the removal of a wide range of low-molecular-weight trace organic constituents. Membrane systems can be installed as treatment methods for producing potable water from a surface source, or to integrate membrane technologies as quaternary treatment options to an existing municipal or industrial wastewater treatment plant. These systems are widely known as water reuse systems.
The end-use of wastewater from a municipal source can be utilized for the following: agriculture and landscape irrigation, groundwater recharge, nonpotable reuse (e.g., a car wash), alternative industrial water source (e.g., cooling water) and indirect potable reuse. The latter case has been given special attention since the improved treatment would minimize the exposure of these compounds to nature and humans.
The widespread acceptability of RO/NF for the above uses, however, will require careful assessment of the expected membrane rejection of undesirable organics and their sorption by the membrane (cause of irreversible fouling). DOW Water Solutions is participating actively in these efforts by running several research projects under real-life conditions. Studies are carried out in collaboration with global research institutes and universities such as Spain’s Universitat Rovira i Virgili in Tarragona. The main focus of the latter collaboration is to determine what rate of improvement can be achieved when RO and ultrafiltration (UF) units are connected to an existing wastewater treatment plant as a stand-alone unit or as an integrated membrane system.
A state-of-the-art membrane treatment consists of an integrated membrane system—for instance, DOW UF membranes as a pretreatment for Filmtec RO/NF elements. There are several advantages in combining these two treatment methods. The key to a successful RO/NF operation is to have a proper, suitable pretreatment system in place; this is especially important with wastewater, which has a high fouling potential. When a UF membrane system is installed, RO/NF membranes can be designed to operate at a high flux rate, minimizing not only the footprint of the plant but also the initial capital investment and operating costs.
From an operational point of view, a good pretreatment section minimizes the membrane fouling. This leads to several additional benefits, as there is a lower required system feed pressure and lower cleaning frequency. A lower cleaning frequency leads to less system downtime and minimizes the use of cleaning chemicals. As a result, the operation becomes more environ- mentally friendly and cheaper to operate. In the long run, fouling prevention and fewer cleanings will lead to longer membrane lifetime, and thus lower costs of membrane replacements.