Zero Discharge vs. Indirect Reuse
Water scarcity and water pollution continue to be crucial issues in the world. Therefore, wastewater reuse is becoming one of the crucial areas of new technology development. Advanced wastewater reuse often is possible only with application of membrane separation technologies.
Reuse of effluent from municipal sewage treatment plants has been implemented on a large scale worldwide. Usually such plants take secondary treatment effluent and continue treatment with ultrafiltration (UF), reverse osmosis (RO) and disinfection (UV, chlorination or ozone treatment). Current membranes and fouling prevention strategies are more than adequate for efficient and economically feasible municipal plant wastewater reuse.
Industrial wastewater reuse presents more challenges. Often, such streams contain fats, oils and grease (FOGs), proteins and other macromolecules (polysaccharides) that can cause severe membrane fouling. Efficient pretreatment strategies, antifouling processes and cost-effective membrane cleaning must be used if such processes are to have any chance to succeed.
Industrial wastewater can be reused for floor washing, irrigation, cooling systems, toilet washing, vehicle washing and other indirect reuse applications. Another option is direct reuse in production/process and zero discharge with reuse of both solids removed and treated water. For instance, mineral oil-rich streams (e.g., those from vehicle and train washing, laundry, petroleum processing and metal casting) can be treated with UF to remove oils and surfactants. Water can then be further processed with RO to remove salts and small molecules. Cleaned water can be reused in the process, while oil-rich concentrate can be processed and used in pavement manufacturing, for instance. Such cases are indeed a win-win situation.
Power plants and the automotive and paper and pulp industries are other examples in which zero-discharge systems have been installed and successfully operated. In general, streams with a high volume (flows of more than 500,000 gal per day) and low amount of contaminants (FOGs below 100 mg/L and total suspended solids [TSS] below 100 mg/L) that can foul membranes account for most of the successful installations.
Food Manufacturing Streams
On the other hand, reuse of food-manufacturing wastewater is still a challenge. Such streams usually have high amounts of solids and FOGs. Contaminant concentrations vary significantly hourly, daily and seasonally. Often, well-trained plant operators that are available at petroleum refineries or power plants, for instance, are not around, even at Fortune 500 company-owned large plants.
If water is to be directly reused in food manufacturing, even RO is often not enough to remove the small amounts of impurities that can influence odor and taste. Activated carbon adsorption and ozonization are needed to remove these contaminants. Another problem is that UF does not remove all the organic impurities present in such wastewater, meaning that RO membranes encounter streams with a combination of salts and organic components.
Even when new technologies such as membrane bioreactors (MBRs) are used to remove almost all organic contaminants, precipitation of insoluble salts can still significantly reduce the lifetime of membranes. Membrane washing can reduce the lifetime of expensive RO membranes to less than six months in some cases. Wastewater softening prior to RO can often help. What to do with RO concentrate is another active issue; evaporation and drying are energy-inefficient and expensive processes.
One of the reasons membrane systems are so popular is that they can be fully automated. If one uses flocculation as pretreatment to remove FOGs and solids, jar testing is almost always needed for the best results. Currently, many membrane manufacturers are trying to develop fouling-free systems. Vibration at the membrane/liquid interface, ultrasound and gas sparging are among the methods used to reduce membrane fouling. Any such process requires a significant amount of energy.
Also, if really dirty water is to be treated, systems require a large surface area of membranes. One must consider that frequent membrane cleaning reduces lifetime and how much energy is needed to run RO membranes. If brine water is treated—in the pickling industry, for instance—pressures higher than those used to run sea desalination are needed.
If municipal plants are not available nearby or potable water is scarce, there are only two choices left for food industry: relocate or install water reuse systems. Proper pretreatment is a key to the success or failure of such plants. To minimize jar testing and pretreatment system adjustments and avoid overdosing of treatment chemicals, one has to install large equalization tanks (EQs) with good mixing.
Ideally, EQs should collect and mix wastewater produced over a 24-hour period. The pH adjustment and coagulants/precipitant additions can also be achieved in EQs. Turbidity sensors can then be installed with alarm systems to prevent wastewater from entering membranes, unless clean and without treatment chemical overdose. If economically feasible, MBRs are a better choice than UF because MBRs can reduce organic loads close to zero. RO membranes can then be used to remove inorganic contaminants. Softening process water also can help reduce precipitative fouling of RO membranes.
While water reuse in large-volume applications such as petroleum refining, paper and pulp manufacturing and in power plants and the automotive industry is becoming routine, reusing water at food manufacturing plants is still a complex and expensive choice. Direct reuse and the zero-discharge approach are usually too expensive for most food manufacturing plants.
Removing TSS, FOGs and part of organic contaminants loads and using municipal plants to remove dissolved organics is still the most economically feasible and practical solution. Another option is using partially treated water for floor washing or irrigation, for instance.