Electromagnetic Flow Meter

Electromagnetic flow meter operation is based on the fact that the voltage induced by a conductor moving at right angles through a magnetic field will be proportional to the velocity of that conductor as it moves through the field. In the case of the electromagnetic flow meter, the conductor is the stream of water to be measured, and the magnetic field is produced by a set of electromagnetic coils. A typical electromagnetic flow meter is shown in Figure 4.7.

The Point of Compliance with the Water Quality Standards

Mixing Zones

The disposal of wastewater from industrial WWTP in rivers and estuaries is considered in this section. The Water Quality Standards allow the use of mixing zones for discharges that would otherwise exceed the water quality criteria for aquatic life. Mixing zones are areas where the water quality standards may be exceeded, but the exceedances are small enough and short enough so as not to interfere with beneficial uses. Mixing zones are a regulatory recognition that the concentrations and effects of most pollutants diminishes rapidly after discharge due to dilution (Adams 1982). They are established in a manner which limits the duration of exposure for organisms passing through the effluent plume in order to minimize the risk from each discharge. The water quality standards for chronic protection must be met at the boundary of this zone and beyond. A smaller zone in which acute criteria may be exceeded can also be authorized. This zone must be small enough to limit exposure times and therefore not cause acute mortalities or interfere with passage of aquatic organisms in the water body. The mixing zone is not significant for DO analyses because BOD exertion is slow compared to the travel time within the mixing zone. Relative to other constituents, such as toxic chemicals, the characteristics of the mixing zone are important. Clearly, rapid mixing of the effluent with the ambient flow is desirable to decrease toxicity.

The size of mixing zones depends not only on the effluent characteristics but also on the river flow. Typically, lower effluent dilutions and therefore larger mixing zones are achieved for low river flows. Analysis of flow records are therefore needed to establish the magnitude, duration, and frequency of low‐flow period. Regulations in force in the United States often prescribe the duration and frequency of the low flow that must be used for mixing zone analyses, e.g. 7Q10.

For small river depths, vertically mixed conditions are reached within a short distance of the discharge point. Mixing zone analyses can be conducted by simulating the discharge as a plane source extending over the full river depth. For pipe discharges, the width of the source can be assumed to be equal to the river depth. The limited river width constraints the lateral spreading of the plume. The limited river width can be accounted for by introducing image sources with respect to the river banks, as illustrated in Figure 4.8.

Oceans and large lakes, such as Great Lakes, provide extensive assimilation capacity and are used for wastewater disposal by many industries and communities. The wastewater is typically carried to an offshore discharge point by a pipe laid on a buried in the ocean floor, or by a tunnel. The discharge in ocean or deep lakes can be through a single‐port or multiport outfall structure. A configuration of a wastewater plume in the ocean is shown in Figure 4.9. In the first region, called the “initial mixing region” or “discharge nearfield,” the effluent forms a buoyant plume, rapidly rising in the water column. This plume entrains large amounts of ambient water, thereby diluting the effluent (Muellenhoff et al. 1985).

An intermediate goal in point source pollution control which is consistent with the CWA goal of zero discharge is the elimination of the need for a mixing zone. The water quality standards do not prohibit a permit writer from granting a mixing zone for a shore discharge, but shore areas are important biological areas. A permit writer should be sure that granting of the mixing zone on a shoreline will not cause biological effects as specified in the regulation. This may require a demonstration on the part of the permittee in some cases.