Water Quality Modeling

Water quality modeling involves the prediction of water pollution using mathematical simulation techniques. A typical water quality model consists of a collection of formulations representing physical mechanisms that determine position and momentum of pollutants in a water body. Models are available for individual components of the hydrological system such as surface runoff; there also exist basin‐wide models addressing hydrologic transport and for ocean and estuarine applications. Often finite difference methods are used to analyze these phenomena, and, almost always, large complex computer models are required (USEPA 1985d; CORMIX 1996; Section D.2).

Formulations and Associated Constants

Water quality is modeled by one or more of the following formulations:

• Advective fate and transport formulation
• Dispersive fate and transport formulation
• Streeter–Phelps formulation
• Surface heat budget formulation
• DO saturation formulation
• Reaeration formulation
• Carbonaceous deoxygenation formulation
• Nitrogenous biochemical oxygen demand formulation
• Sediment oxygen demand formulation
• Photosynthesis and respiration formulation
• pH and alkalinity formulation
• Nutrients formulation (fertilizers)
• Algae formulation
• Zooplankton formulation
• Coliform bacteria formulation (e.g. Escherichia coli)

WWTP BOD, SS, and Fecal Coliform Removal Efficiencies: Meet Water Quality Standards

The major goals of the combined primary, secondary, and sometime tertiary treatment are to remove the conventional pollutants including organics and pathogens (e.g. BOD, COD, SS, fecal coliform), to meet the water quality standards and NPDES permit requirements (CWA 1972; Davis and Masten 2014; USEPA 1995b, 2010a). Primary treatment will typically remove 60% of the SS in raw wastewater and 35% of the BOD₅. The major goal of secondary treatment is to remove the soluble BOD₅ that escape the primary process and to provide added removal of SS. Secondary treatment is achieved by using biological processes shown in Figure 4.11, which is a schematic diagram of a pulp and paper mill’s process WWTP (Das 1991, 1993). The WWTP includes effluent from both sides of the bleach plant that receives primary clarification. A surge basin, which is connected to the primary clarifier, is not commonly used. However, in the event of a spill (black liquor for example), overflows can be retained in the surge basin for about six hours, based on the mill’s total wastewater flow.

Following primary clarification, nutrients (ammonia and phosphoric acid) are added to the primary effluent. A high‐purity oxygen‐activated sludge secondary treatment, under the trade name UNOX®, is followed by secondary clarification. The overflows from secondary five clarifiers are collected in a wet well, and the effluent is discharged to the Columbia River through a 310 ft long diffuser section on the submerged outfall pipe at a rate of 65 MGD (Figure 4.11).

This WWTP secondary treatment provides the same biological reactions that would occur in the receiving water if it had adequate capacity to assimilate the wastewater. The secondary treatment processes are designed to speed up these natural processes so that the breakdown of the degradable organic pollutants can be achieved in a relatively short time. Although secondary treatment may remove more than 85% of the BOD₅ and SS, it does not remove significant amounts of nitrogen, phosphorus, or heavy metals, nor does it completely remove pathogenic bacteria and viruses. The NPDES permit often requires effluent to meet at least 85% removal of BOD₅ and SS, fecal coliform counts 200 MPN in 100 ml effluent, and pass standard bio‐assessment tests for selected freshwater (e.g. trout or salmon, fathead minnow) and marine saltwater species (e.g. mysid shrimp, Champia parvula, sand dollar, Echinarachnius parma).

In case where secondary levels of treatment are inadequate, additional treatment processes are applied to the secondary effluent. These tertiary treatment processes may involve chemical treatment and filtration of the wastewater. Some of these tertiary treatment processes can remove as much as 99% BOD₅, SS, total‐phosphorus, bacteria, and 95% of the nitrogen. These processes can produce a sparkling clean, colorless, odorless effluent which could be applied on land and for other beneficial reuse.

Most of the impurities removed from the wastewater do not simply vanish. Some organic compounds are broken down into carbon dioxide and water. Most of the impurities are removed from wastewater as solid, that is sludge. Because most of the impurities removed from water are present in sludge, therefore, sludge handling and disposal must be carried out carefully to achieve the pollution control (also discussed this in Section 4.9.2).