Summary: With conventional water treatment technologies becoming more affordable, access to better water quality will be greater than ever before. In contrast, limited supplies regionally and new water standards will seemingly drive up the cost of raw water. This allows the concept of water reuse to take flight. Some of its advantages are outlined here.
The criteria for water recycling or reuse is to remove whatever contaminants are in the water that make it unusable for a specific application. Obviously, the governing factor here is the quality requirements of this application. As with water treatment in general, specific technologies are used to remove specific contaminants. Table 1 identifies the classes of fouling contaminants found in these water supplies, provides examples, and lists technologies that are appropriate for removal of the contaminants from typical water supplies.
The important thing to realize here is that technologies for water reuse are identical to those required to purify water in general, and there’s no single technology that’s optimum for complete contaminant removal. When considering water reuse, it’s important to realize that the choice of technology will be determined by the quality requirement of the reused water. This means a thorough understanding of treatment technologies is necessary when designing water reuse systems.
In addition to the plethora of existing treatment technologies, new and innovative technologies are constantly being introduced. One example is MBR—membrane bioreactor technology.
Benefits of bioreactors
The most cost effective method of treating waste streams containing biodegradable organic constituents is in a bioreactor—basically a holding tank with or without aeration that utilizes natural, waterborne bacteria to break down the “bioburden” (theoretically, to carbon dioxide and water). Inorganic and non-biodegradable contaminants settle to the bottom of the tank as sludge. The clarified effluent may contain suspended solids consisting of undigested organic material, bacteria and non-biodegradable components, and, without removal of these solids, reuse possibilities are limited. By inserting a microfiltration membrane into the bioreactor tank (or mounting it on the discharge from the tank), the effluent stream is highly filtered, rendering it much more acceptable for reuse. Because suspended solids are so thoroughly removed, polishing technologies such as reverse osmosis (RO) or ion exchange (IX) can be readily applied to further upgrade the quality of this effluent stream.
Motivation to reuse
The incentive to reuse water can usually be attributed to one or more of the following factors:
- Economics—The cost of water or sewering it is expected to rise considerably in the near future. This is a result of the costs of implementing new technologies to meet new and more stringent discharge regulations such as those addressing microbial contaminants, and the new arsenic MCL (maximum contaminant level).
- Availability—As the demand for water increases due to population growth in water-short areas such as the arid southwestern United States, there will be increasing pressures on available water supplies. The total worldwide water volume is fixed, and demands for water will continue to increase.
- Regulatory—As industrial waste discharges become more regulated as a result of identification of new industrial contaminants (or reduction in the acceptable discharge level of known contaminants), costs of additional treatment will make it attractive for some companies to reuse this more expensive treated water rather than simply sewering. Already, the quality of treated discharges in some areas is better than the available water supplies.
So what are the disincentives for water reuse? The most notable are:
- Ignorance—Many companies aren’t aware of the technologies available to improve the quality of this wastewater to allow it be reused.
- Fear—In spite of the fact that technologies exist today to reduce the level of contaminants from virtually any water supply, many companies are afraid that recycled wastewater won’t meet their water quality requirements. See also “Ignorance” above.
- Reluctance—Many companies are afraid to “think outside the box” and don’t bother to examine all the factors, i.e., “Don’t confuse me with facts; my mind’s made up.”
- Economics—In those areas where water is plentiful and low cost, the economics may not favor the cost of the technologies necessary to treat the water so that it can be reused.
More people, more reuse
Any discussion addressing water use, reuse or treatment must acknowledge the fact that these are dynamic fields. Overall water usage will increase to accommodate growing populations. For the reasons cited above, water reuse will become much more common in the future, and technologies will be developed or modified to facilitate reuse activities.
Whereas the feed water in water purification applications—with the exception of seawater—is relatively low in total dissolved solids (TDS) concentration, wastewater streams may be quite high. This is usually the only distinction between water purification and wastewater treatment, but the higher concentrations of contaminants must be taken into consideration when designing water reclamation systems. In some cases, such as those with oily wastes or high concentrations of other organics, relatively “extreme” treatment technologies such as oil-water separators may be required (see Table 2). In many cases, it will be necessary to handle quantities of sludge—insoluble materials that settle out of the solution and must ultimately be hauled away to a landfill—typically after a dewatering step.
The ultimate in recycling is considered to be “zero discharge,” typically defined as complete recovery and reuse of all of the water within a facility. Technically, this is possible, although it may not always be economically justifiable.
As market opportunities for advanced technologies increase, the costs of traditional reclamation technologies—as well as new, more innovative ones—will drop. In concert with this, new promulgated water standards will undoubtedly increase the cost of raw water supplies, contributing to the economic viability of the whole concept of water reuse.
About the author
Peter S. Cartwright, president of Cart-wright Consulting Co., Minneapolis, is a registered professional engineer in several states. He has been in the water treatment industry since 1974 and has published more than 100 papers and articles on related issues. Cartwright has been chairman of several WQA committees and task forces, and has received the organization’s Award of Merit. A member of the WC&P Technical Review Committee since 1996, his expertise includes such high technology separation processes as reverse osmosis, ultrafiltration, microfiltration, electrodialysis, deionization, carbon adsorption, ozonation and distillation. He can be reached at (952) 854-4911, (952) 854-6964 (fax) or email: CartwrightConsul@cs. com and website: www.cartwright-consulting.com.