This section discusses variables of industrial wastewaters, how they can be generally classified, the importance of knowing the frequency of generation and discharge.
1- Compatible and noncompatible pollutants
Compatible pollutants can be defined as those pollutants that are normally removed by the publicly owned treatment works (POTW) system. Biochemical oxygen demand (BOD), suspended solids (SS), oil and grease, and ammonia are considered compatible pol¬lutants. The POTW is designed to treat primarily domestic wastewater and the compatible pollutants discharged by industry.
Noncompatible pollutants are defined as those pollutants which are not normally removed by the POTW, may be toxic to a biological IWTS (industrial wastewater treatment system), and may cause pass through or interference with the treatment system. Even some biologi¬cally degradable wastes such as soluble, synthetic cooling oils may cause interference with the heavy metal removal system by inhibiting floc formation. Other examples of noncompatible pollutants include heavy metals such as cop¬per, nickel, lead, and zinc; organics such as methylene chlo¬ride, 1,1,1 trichloroethylene, methyl ethyl ketone, acetone, and gasoline; and sludges containing toxic organics or metals.
From the perspective of the POTW, conventional pollutants sometimes exhibit the characteristics of noncompatible pollu¬tants, and vice versa. Soluble BOD from a food industry may have some harmful effects on a POTW’s secondary treatment system. The accidental discharge of ammonia by a fertilizer manufacturer may disrupt the nitrification/denitrification or stripping tower processes used by the POTW to treat ammo¬nia. On the other hand, some of the heavy metals (usually classified as noncompatible pollutants) are used as micronu¬trients to aid in the production of biological mass and the reduction of BOD. Certain organic chemical wastes such as acetone and isopropanol are biodegradable and, in dilute solutions, are removed by biological action in secondary treatment.
2- Dilute solutions
The discharges from continuous manufacturing processes are normally dilute solutions of compatible and sometimes noncompatible pollutants. They may be discharged to the industry’s pretreatment system or directly to the POTW with¬out any pretreatment. Manufacturing processes such as plat¬ing bath rinses, raw food cleaning, and crude oil dewatering are all examples of dilute solutions of pollutants that may be discharged directly to a POTW sanitary sewer. If a problem occurs in the manufacturing process, a probable result is that the quality of wastewater will change; it may be more laden with pollutants. Some wastestreams from utility services, such as cooling tower and boiler blowdown, are continuous and represent the discharge of dilute solutions.
Another low strength wastewater is storm water runoff from chemical handling and storage areas. Products which may have spilled on the industry’s grounds are washed off during a rainstorm or during the spring thaw. The pollutant concentration is usually too dilute to require pretreatment before discharge to the sewer, but exceeds the discharge standards for discharge to surface waters. While the strength of the storm runoff may be low, the volume that must be treated in addition to normal flow to the pretreatment system or to the POTW can cause hydraulic capacity problems. Excessive flows can be diverted to storage reservoirs or basins and then gradually discharged to the pretreatment system. A great deal of attention is presently focused on cleaning up groundwater sources that have been contaminated by leaking underground storage tanks. Cleanup projects of this nature typically involve large quantities of wastes that may contain high concentrations of solvents, fuels, heavy metals and pesticides. Because of the public attention surrounding groundwater cleanup projects, pretreatment of the contaminated water is almost always required and the result is usually a “high quality” industrial wastewater.
3- Concentrated solutions
Typically, concentrated solutions are batch-generated and the frequency of generation is usually not daily but weekly, monthly, annually, or even longer. These solutions are process chemicals or products that cannot be reconditioned or reused in the same manufacturing process. Concentrated solutions such as spent plating baths, acids, alkalies, static drag out solutions, and reject product may have concentrations of pollutants hundreds or thousands of times higher than the discharge limits of the POTW or higher than can be adequately treated by the pretreatment system if discharged all at once. Time have to be taken for examine and understand each manufacturing process, then identify these concentrated solutions and take the necessary steps to prevent damage to the treatment facilities.
Some wastes may be considered concentrated by the POTW but not by the industry. For example, the ten percent sulfuric acid solution used for pickling parts is considered “Dilute” by comparison to the 98 percent or 50 percent stock solution that the industry uses to make up the pickling solution. When this solution is spent or can no longer be used as a pickling solution, proper treatment and disposal are required. From the industrial manufacturer’s point of view, the solution is spent and no longer concentrated. However, from a wastewater treatment point of view, the solution is concentrated since it contains high concentrations of acid (pH less than 1.0) and heavy metals (1,000 mg/L) compared to the normal pH of 1.0 to 4.0 and heavy metal concentrations of less than 100 mg/L (I.W.T, 1999). Another source of concentrated solutions is the wastewater from equipment cleanup. While the amount of material in the process chemical bath may be considered dilute by industry standards, it forms a concentrated wastestream when discharged during the cleanup of manufacturing equipment. Cleanup wastestreams contain a high concentration of the product during the first washing of the tank, pipe or pump. This discharge of concentrated waste is followed by successive rinses which contain less and less pollutants. If cleanup flow concentrations are not equalized, the cleanup cycle can cause problems in the (IWTS). Spills of process chemicals to the floor, if not contained, can flow directly to the floor drain and the pretreatment or sewer system. The adverse effects on the pretreatment system and POTW are the same as those of any other concentrated solutions. This is why chemical containment areas must not have drains.
4- Concentration versus mass of the pollution
An understanding of the concentration and the mass of a pollutant in an industrial waste is needed to determine the effects on the industry’s pretreatment system, the POTW collection, treatment, and disposal systems, and the sampling of the industry’s discharge. The concentration of a substance in wastewater is normally expressed as milligrams per litre (mg/L) and is a measurement of the mass per unit of volume. The mass of a substance is normally expressed in pounds or kilograms and is a weight measurement. A mass emission rate is a measurement of weight per unit time and is usually expressed as pounds or kilograms per day. Many of the electroplating and all of the metal finishing categorical standards are written in concentrations, whereas most of the other categorical standards are written as mass emission rate standards. The mass emission rate standards recognize that with more production and water, the mass of pollutant will also increase. This approach prevents dilution of the pollutant to meet concentration limitations. The mass emission rate of a substance can be calculated by knowing the concentration of the pollutant in the wastewater and the volume of wastewater.
The effects of pollutant concentration and mass on the POTW collection, treatment, and disposal systems are generally the same as their effects on the IWTS. However, hydraulic problems in any portion of the POTW system could cause pollutants to pass through the POTW untreated, even though the mass of the pollutant did not change. If the daily mass loading is the same, but the instantaneous mass emission rate is highly variable, the POTW’s collection system may not equalize the slug loading of a highly concentrated solution. The result may be interference with the treatment system, causing violations of either or both effluent and sludge disposal limitations.
5- Frequency of generation and discharge
Important to both the operation of the industry’s pretreatment system and the POTW’s collection, treatment, and disposal systems is the frequency of industrial waste generation and discharge. Wastewater sampling to investigate process problems and to determine compliance with the discharge limits are also affected by the hours of discharge.
5-1 Hours of operation versus discharge
Normally, the hours of operation are also the hours of discharge to the IWTS. Thus the operator can generally expect to receive flow for treatment during the hours of operation. If the production is constant, the discharge volume and chemical constituents will also be constant. Several common situations where an industrial waste must be treated after the normal production hours are described below:
1. The “wet” processes run for one shift, but the “dry” processes run for two. The dry processes may require utilities such as compressed air or a boiler, each having a wastewater discharge.
2. In industries with long collection systems, production and wastewater flow to the system may stop, but the IWTS may continue to operate and discharge until the wastewater in the collection system has been processed.
3. Spills, accidental discharges or storm water flow that goes to the IWTS may cause the IWTS to operate outside of the normal production hours.
4. A food processing plant operates for one or two shifts, generating some wastewater, but most of the equipment cleaning operations occur on an off shift. The cleaning generates most of the wastewater volume.
5. The IWTS has an equalization tank either at the beginning of the IWTS or at the end of the manufacturing system. Discharge from the equalization tank to the rest of the IWTS may continue after production stops because it is programmed to pump to the next unit process until it reaches its low level.
Equalization of the wastewater is an important factor affecting the actual hours of wastewater discharge to the IWTS and sewer. In order to deliver a relatively constant flow and concentration of pollutants to the IWTS, large wastewater collection sumps, equalization tanks or storage tanks may be used. As noted above, these equalization devices may also lengthen the time of discharge beyond the actual hours of operation of the manufacturing facility. Equalization of industrial wastewater flows can also be beneficial to the POTW. By lengthening the hours of discharge from the industry, there is an effective increase in the available hydraulic capacity of the POTW collection system because of the decreased industrial flow rates. Due to the normal diurnal variation in domestic wastewater flows (peak flows usually occur between 8:00 a.m. and 6:00 p.m.), the hydraulic capacity of a sewer may be exceeded if a large industrial flow is allowed to be discharged to the sewer during a short period. Therefore, it may be necessary for the industry to discharge only at night. Sampling of this discharge would then be shifted to the night-time hours.
5-2 Discharge variations
Industries that have daily, weekly, or seasonal manufacturing cycles will show variations in wastewater generation. Business cycles for each of the various segments of the industrial community will have an effect on production, and therefore on the generation of wastewater. The food processing industry provides a good example of daily, weekly, and seasonal variations in discharge quantity and quality. For example, an industry that processes citrus peel to make pectin is dependent on when the peel arrives at the industry’s plant. This may mean anywhere from three to six days per week. As the season progresses, the type of peel changes from orange to lemon, and the sugar content changes yielding a slightly different type of wastewater. After the citrus season, the plant is completely shut down. In certain industries, variations in the quantity of wastewater reflect the nature of the business or the business cycle of the particular business segment. In a small shop producing printed circuit boards, it is typical to have a 30-day turnaround with sales, ordering, and development taking place during the first part of the month. Production is slow while making test boards, but once the board is developed, production proceeds at a rapid pace to produce the boards for shipment in the last week of the month. The printed circuit board industry is subject to both downturns and upturns in the market. The major pollutant from the industry is copper and, consequently, the quantity of copper discharged to the industrial sewer fluctuates according to market and production cycles.
Variations in the quality of industrial waste can also occur due to market forces or environmental concerns requiring a different type of product. In the metal finishing industry, for example, companies are moving from cadmium-plated metal, an environmentally more hazardous substance with more stringent discharge limitations, to zinc-plated parts. Knowledge of the industry, the manufacturing processes, and market forces are valuable tools needed by the industrial waste treatment plant operator to anticipate variations in industrial discharges.
5-3 Continuous and intermittent discharges
Discharges from manufacturing facilities usually reflect the type of manufacturing process used at the facility. Processes which are continuous tend to produce wastewater on a continuous basis, with relatively constant volume and quality. Batch processes, or activities that occur once per shift, per day, or per week, tend to produce an intermittent discharge. Also, as a general rule-of-thumb, the larger the manufacturing process, the more likelihood there is of a continuous discharge. Examples of manufacturing processes that have continuous discharges include rinsing or cleaning of parts or food, processing of crude oil, either at the well head or refinery, air or fume scrubbing, papermaking, and leather tanning. Intermittent discharges of wastewater are characterized by discharges of a volume of wastewater separated by a time period between discharges.
These typically occur at the beginning or ending of a manufacturing process or during equipment cleanup, a spill, replacement of spent solution, or disposal of a reject product. Intermittent discharges also tend to be more concentrated and of smaller volume than the wastewater normally discharged. For an industrial pretreatment facility, the intermittent discharges and the variations in waste generation determine the design capacity of the system.
5-4 Industrial effluents
Whereas the nature domestic wastewater is relatively constant, the extreme diversity of industrial effluents calls for an individual investigation for each type of industry and often entails the use of specific treatment processes. Therefore, a thorough understanding of the production processes and the system organization is fundamental.
There are four types of industrial effluents to be considered:
1- General manufacturing effluents: Most processes give rise to polluting effluents resulting from the contact of water with gases, liquids or solids. The effluents are either continuous or intermittent. They even might only be produced several months a year (campaigns in the agrifood-industry, two months for beet sugar production, for example). Usually if production is regular, pollution flows are known. However, for industries working in specific campaigns (synthetic chemistry, pharmaceutical and parachemical industries), it is more difficult to analyse the effluents as they are always changing.
2- Specific effluents: Some effluents are likely to be separated either for specific treatment after which they are recovered, or to be kept in a storage tank ready to be reinjected at a weighted flow rate into the treatment line. Such as, pickling and electroplating baths; spent caustic soda.
3- General service effluents: These effluents may include wastewater (canteens, etc.), water used for heating (boiler blowdown; spent resin regenerants), etc.
4- Intermittent effluents: These must not be forgotten; they may occur from accidental leaks of Products during handling or storage, from floor wash water and from polluted water, of which storm water may also give rise to a hydraulic overload.
For the correct design of an industrial effluent treatment plant, the following parameters must be carefully established (I.W.T, 1999):
– types of production, capacities and cycles, raw materials used,
– composition of the make-up water used by the industrial plant,
– possibility of separating effluents and/or recycling them,
– daily volume of effluents per type,
– average and maximum hourly flows (duration and frequency by, type),
– average and maximum pollution flow (frequency and duration) per type of waste and for the specific type of pollution coming from the industry under consideration.
Since it can seriously, disturb the working of certain parts of the treatment facilities (glues, tars, fibers, oils, sands, etc.).
When a new factory is being designed, these parameters will be ascertained after analysis of the manufacturing processes and compared with data from existing factories. The amount and degree of pollution depend on the methods of manufacturing. For an example, in piggeries industry the method of cleaning will affect both the degree of pollution and water processing consumed as shown in Table 1.
Table. 1 Effects of producing methods on pollution degree (Brault, 1991)
The effluents are rather acidic which is due to lactic fermentation or to sulphitation (pH 4 to 5). When a wet technique is used to extract starch, the pollution comes from the evaporation of water and its made up of volatile organic acids. A notably soluble protein-rich pollution may come from the glucose shop. The general wastes of potatoes processing is presented in Table 2.
Table 2 Potato processing wastes (Brault, 1991)
Ref.
Brault. “Water Treatment Handbook”. 1991
I.W.T, “Industrial Waste Treatment, V1&V2”. California State University, USA 1999