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; 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, 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.

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 rejected 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.