Effective water management focuses on keeping freshwater withdrawals as low as possible by the implementation of conservation, reuse and recycle policies. Conservation means eliminating waste whereas reuse and recycle strategies seek ways to use water more than once prior to discharge.
Using sound chemical engineering principles, plant wastewater can be regenerated by water treatment methods such as sedimentation, neutralization, precipitation, filtration, and biological systems to produce a treated effluent that is acceptable for reuse and recycle. All to often, however, the term “treated wastewater” implies that it is unacceptable for reuse and only fit for discharge to a municipal wastewater treatment facility or directly into the environment. This notion is quickly fading, however, as plants face a public moral imperative to reuse and recycle treated wastewater.
Economics tends to drive water conservation and reuse projects. Companies fund projects that they believe are in their best financial interest. Because water remains an undervalued commodity, it is frequently in the company’s best financial interest to withdraw fresh water from its source, use it once and then discharge it (often thermally or chemically altered) back into the environment. This practice is in the best self-interest of each water user because it results in the lowest overall operating cost. However, if every company operated in its own best interest, the combined effect would be a depletion of fresh water resources and a negative environmental impact, which would produce a worse outcome for all. This creates a moral dilemma in that if each user acts in their own best self-interest, a worse outcome results for all users. As a result, water conservation projects must be viewed not just on a financial basis, but as an ethical and moral obligation to use natural resources conservatively and efficiently for the benefit of society as a whole. This policy is called “being a good neighbor.”
Industrial plants withdraw water from its source, typically a potable municipal supply, private well or surface supply, for use in various processes. Alternatively, plants in more arid areas of the country are required to use treated municipal wastewater for many industrial applications to reduce the overall demand on potable water supplies. Typical industrial water requirements include steam generation, removing waste heat (cooling water), washing and rinsing, as an ingredient in a food or beverage, and for general purposes such as potable and sanitary use.
The wastewater produced by these processes is sent to a water treatment system where it is treated prior to discharge back into the environment or is reclaimed for reuse in the plant. The key to efficient water management is determining the distinction between when water is suitable for reclamation and when it must be discharged to waste. As more plants are required by environmental regulations to improve the quality of their treated wastewater, the line of demarcation between good quality water and bad quality water becomes blurred.
As a universal solvent, water is never found in its pure state in nature. It contains impurities that affect its overall quality. The average consumer judges water quality by odor, color and taste. If a water supply has no offensive odor, is clear and tastes fine, then it is judged as being of good quality by most consumers. However, industrial processes can contribute harmful impurities that bypass our senses. The concentration of these impurities must be determined by water analysis. The water is considered “polluted” when the impurity concentration is high enough to “adversely and unreasonably” affect the usage of the water in a particular application or process.
Wastewater quality is determined by various parameters that relate to obtaining and maintaining water of a suitable quality for a specific purpose. The assessment of wastewater quality is largely determined by:
BOD (biochemical oxygen demand)
COD (chemical oxygen demand)
Total solids (both dissolved and suspended)
Herbicides, pesticides and other chemicals
The oxygen concentration in fresh water approaches 10 ppm at 60 F. This decreases as the water temperature increases. When wastewater contains high concentrations of substances that increase the oxygen demand, the dissolved oxygen level can be depleted to zero, which makes the water uninhabitable for fish and other aquatic organisms. These impurities include organic compounds, oxidizable nitrogen from nitrite, ammonia and organic nitrogen (proteins, amines and amino acids). The oxygen is also consumed by chemical reducing agents such as ferric iron, sulfite and sulfides.
Various strains of bacteria use organic substances as their food source. Aerobic bacteria use oxygen as part of their metabolic process to break down the organic nutrients into carbon dioxide and water. The BOD test measures the quantity of oxygen used by these organisms in consuming organic matter over a 5-day period at 68 F. This is important to know in order to assess the amount of oxygen that would be consumed by the waste if discharged to a freshwater receiving stream.
In the absence of dissolved oxygen, anaerobic bacteria digest organic food to yield alcohols, organic acids and other organic compounds. Ultimately, anaerobic digestion yields methane and carbon dioxide as the byproducts of bacteria metabolism. Further, anaerobes like sulfate-reducing bacteria (SRBs) convert inorganic sulfate to hydrogen sulfide gas, which imparts an offensive rotten egg smell.
COD measures the total organic content that can be oxidized by potassium dichromate in a sulfuric acid solution. The organics are decomposed to carbon dioxide and water by chemical reaction. The COD test is limited, however, in that it does not distinguish between organic matter that is biodegradable and non-biodegradable.
pH measures the acidity or basicity of the water. Most aquatic organisms tolerate a pH range of 5.5 to 9.5 and thrive in a pH range of 6.5 to 8.5. pH values less than 7 are acidic. Values greater than 7 are basic or alkaline.
Impurities are either dissolved in water or suspended. Suspended solids are readily removed by settling or filtration whereas soluble substances can not be filtered. These substances may impart color to the water and adversely affect the turbidity and clarity. The total solids test measures the amount of suspended and dissolved solids in the wastewater effluent.
WASTEWATER TREATMENT SYSTEMS
Industrial wastewater treatment systems are designed to remove harmful pollutants prior to discharge of the treated effluent from the plant site. Various mechanical and chemical methods are used to accomplish this goal including sedimentation, coagulation, clarification, filtration, and pH adjustment.
Clarifiers: Clarifiers are commonly used to enhance the efficiency of settling out suspended solids. Engineers have come up with various clarifier designs that efficiently collect and concentrate settled sludge from the waste stream. This process is augmented by the feeding of coagulants such as aluminum sulfate (alum), sodium aluminate, ferric chloride and ferrous sulfate (copperas). In addition, long chain polyelectrolytes are used alone or in conjunction with chemical coagulants to increase the particle size and thereby increase the settling rate.
Flotation: In some cases, dissolve air and gravity separation are used to float impurities to the surface where they are skimmed off for disposal or further treatment. Oil-water separators, for example, remove free oil from oily water waste streams. If the oil is emulsified, chemical additives such as alum, calcium chloride or sulfuric acid are used to break the emulsion and allow the oil to float to the surface.
Biological systems: Depending on the characteristics of the waste stream, various biological treatment systems are used to duplicate natural processes of decomposing organic waste in man-made equipment. Aerobic and anaerobic organisms are cultured to breakdown organic wastes to yield a final decomposition product of carbon dioxide and water. In the case of anaerobic digestion, methane and hydrogen sulfide are produced.
Trickling filters: Biological filters (aka trickling filters) are simple systems that employ a bed of rocks on which a bio-layer of beneficial aerobic and anaerobic organisms are cultured. The bacteria digest the organic nutrients as the water passes through the filter bed. This process has been used with success in many waste water treatment designs such as those in pulp, paper and textile manufacturing.
Activated sludge: The activated sludge concept is widely used for treating organic waste streams. Here clarified waste is aerated to form active masses of organisms (activated sludge) which consume the organic matter to yield carbon dioxide and water. The activate sludge is settled out and the clarified water decanted for disposal or reuse. BOD reduction of 85 to 95% is achievable in 6 hours of aeration. With extended aeration, 99% reduction is possible. The effluent from this process is clear and very low in organics.
Other methods: Other treatment methods are available to regenerate process wastes for recycle and reuse. Granulated activated carbon (GAC) is effective in adsorbing organics, removing odors and improving taste. Pressure filters, ultrafiltration and reverse osmosis processes are commonly used to treat brackish (high dissolved solids) waters and removing suspended and colloidal material. Ion exchange is used to recycle more steam condensate by virtue of removing corrosion by-products and hardness. In addition, oxidation-reduction systems are effective in treating insoluble metallic hydroxide waste streams. In essence, these and many other methods are available to the environmental engineer to produce a final waste effluent that is suitable for recycle or reuse.
Regeneration of industrial wastewater for reuse or recycle is an integral part of minimizing fresh water withdrawals, reducing water consumption and minimizing waste. The enhanced water quality produced by these mechanical and chemical processes makes the decision of whether to discharge or reuse treated waste somewhat arbitrary. In many cases, treated wastewater is equal to or better than the quality of the source water; and yet it is often discharged to waste instead of being reused simply because we label it treated “wastewater.”
Because of the relative low cost of water and wastewater, it is difficult to justify water conservation projects based on economics and return on investment analysis. Going forward, environmental regulations and policies will be promulgated that require industry to make more efficient use of fresh water resources. This may not be in the best financial interest of the industrial consumer, but it is viewed as in the best interest of the environment and the public at large. As we’ve seen, good engineering and scientific methods are available to achieve these goals.