variety of models is needed-one mathematical model cannot provide answers to all questions. These models are analogous to the emissions-to-air-quality models used in developing air pollution control programs.

To be successful, there must be good predictive capability for the dominant factors that determine the engineering choices for satisfying the standards. These factors are determined by sensitivity analyses as well as the experience of the modeler. Thus for engineering purposes it is not necessary to understand every process if more knowledge would have no effect on the choice of control strategy. For example, it is not necessary to understand the behavior of a certain pollutant at a location where the input is far below any possible threshold value of concern. Or another example, if the range of uncertainty of a biological effect is a factor of 10, the effect may be of no importance if the upper end of the range is fully acceptable.

The preceding discussion relates to the wastewater disposal system for a municipality including source control programs, a treatment plant, and outfall. The three kinds of information required apply equally well to all other types of pollutant sources and the approach to devising an engineering system. In the case of CSOs, for example, it is necessary to have a set of water quality objectives; some knowledge of the environment; and information on the amount, quality, frequency, and distribution of existing CSOs.

TABLE 4.9 Typical Length and Time Scales of Effects Associated with Typical Pollutant Problems for Coastal Wastewater Discharges

Pollutant

General Length Scale

General Time Scale

Ammonia toxicity (if any)

One or two kilometers

Few hours

Other acute toxicity (if any)

Few kilometers

Less than 1 day

Coliforms

Up to 10 kilometers

Up to a week

Bacterial pathogens

Up to 10 kilometers

Up to a week

Viral pathogens

Up to 10 kilometers

Up to 4 months1

Protozoan pathogens

Up to 10 kilometers

Up to 3 months2

Deposition of organic matter

10 kilometers

Few days

Oil and grease (wastewater origin)

Few kilometers

Few days

BOD-caused DO decrease

Few kilometers

Few days

Nutrients (nitrogen)

Up to 100 kilometers

Months to a few years

Regional hypoxia

Up to 100 kilometers

Months to a few years

Heavy metals (sediments)

Few kilometers

Years to decades

Synthetic organics (sediments)

10s of kilometers

Decades

1 Melnick and Gerba 1980, Goyal et al. 1984.

2 G. Vessey, MacQuarie University, Sydney, Australia, personal communication, 1992.



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