Process plants generally try to minimize the amount of wastewater they generate. However, operations invariably result in production of some wastewater. Proper treatment of this wastewater is crucial for both environmental and economic reasons.
Industrial wastewaters usually contain organic and inorganic matter in varying degrees of concentration. They may include toxic and other harmful materials as well as components that are non-biodegradable or that can reduce the efficiency of many wastewater-treatment operations.
Thus, treatment of industrial wastewaters typically is a very difficult task — far more complicated than municipal wastewater treatment —that requires special methods and sophisticated technologies. These options fall into three categories: physical, chemical and biological. Physical treatment methods include sedimentation, flotation, filtering, stripping, ion exchange, adsorption and other processes that remove dissolved and non-dissolved substances without necessarily changing their chemical structures. Chemical methods include chemical precipitation, chemical oxidation or reduction, formation of an insoluble gas followed by stripping, and other chemical reactions that involve exchanging or sharing electrons between atoms. Biological methods rely upon living organisms using organic or, in some instances, inorganic substances for food.
Biological treatment is more widely used than any other option where reasonably complete treatment is required. It most often serves as the secondary treatment stage to remove major portions of contamination. Other processes handle primary and tertiary treatment to complete the removal of solids and other pollutants.
Some industrial wastewaters are rich in organics and easily biodegradable while others are nutrient deficient, inhibiting or preventing biodegradability. Total dissolved solids and contamination may exceed by many times the levels found in domestic sewage. Industrial wastewaters often also have pHs well beyond the range of 6–9 and may contain high concentrations of dissolved metal salts. To further complicate matters, wastewater flows and characteristics within a plant also can vary with time because of campaign manufacturing or slug discharges on top of the usual discharges. In addition, spillages and dumping that occasionally may occur very adversely can impact the performance of the plant’s wastewater treatment plant. Consequently, it’s always prudent to carefully assess current wastewater and its treatment requirements rather than relying on the past situation. An understanding of the nature of the plant’s operations is vital.
One key parameter for wastewater is its biochemical (or biological) oxygen demand (BOD). This is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given wastewater sample at a certain temperature over a specific time period. Therefore, BOD indicates indirectly the amount of organic compounds in wastewater. The BOD most commonly is expressed in milligrams of oxygen consumed per liter of sample during 5 days of incubation at 20°C.
Another key parameter is chemical oxygen demand (COD), which indirectly specifies the amount of organic compounds in the wastewater. It indicates oxygen consumption and also is given in mg/L.
Both BOD and COD measure the amount of organic compounds in wastewater. However, COD is less specific because it measures everything that can be chemically oxidized rather than just levels of biodegradable organic matter. You can estimate the biodegradability of wastewater by considering its COD and corresponding BOD.
Removing large, suspended and floating solids is the focus of the first stage of wastewater treatment. However, before such treatment takes place, the plant wastewaters usually first go to an equalization tank or system, which acts as a buffer and normalizes varying flow and contamination loads. It’s always best to use a single large concrete tank to which an appropriate coating has been applied. This tank most often is sized based on the difference between expected peak and average flows, with a capacity of 4–8 hours’ worth of difference common.
From the equalization tank, the raw wastewater goes for primary treatment. This usually includes screening to trap solid objects, sedimentation by gravity to remove suspended solids and some adjustments. Primary treatment sometimes is referred to as “mechanical treatment” because it relies on mechanical methods, although chemicals often are used to accelerate the sedimentation process. The design for a primary treatment facility most often includes neutralization (i.e., pH adjustment), coagulation, flocculation and dissolved air flotation (DAF).