Concentration vs. 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 liter 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 discharge 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 discharge rate standards. The discharge 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 discharge 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 POTWs 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.

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.

Hours of Operation vs. 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.