Surface-water discharge falls under the National Pollutant Discharge Elimination System (NPDES), authorized by the Clean Water Act, while federal and state laws require permits for surface-water discharge. The NPDES limits flow, conventional pollutants (e.g., biological oxygen demand, pH, total suspended solids, fecal coliform, oil and grease), toxicants (e.g., metals, volatile organics), and non-conventional pollutants (e.g., ammonia, nutrients). Some states have the authority to establish their own water-quality standards, which can be more stringent than federal standards established by the U.S. Environmental Protection Agency.

NON-CHEMICAL TECHNOLOGIES

The essential function of water treatment is to produce a change in water chemistry. More than 30 suppliers are known to provide non-chemical means of changing water chemistry, which can be grouped into five basic categories: magnetic, alternating-current (AC) induction, electrostatic, ultrasonic, and mechanical energy (cavitation).

Magnetic

Devices of this type (Figure 1) generally have one or more permanent magnets mounted to the outside or inside of a pipe. No other equipment or power is required. The orientation of the poles of the magnet(s) and the velocity of the water to be treated are critical to performance. Magnetic devices act through the Lorenz force (charged particles moving through a magnetic field), influencing ions in water to prevent scale. The process causes scale particles to precipitate within a water stream and be carried along with the water. Magnetic devices generally are not as effective in controlling bacteria and corrosion.

AC induction

These devices (Figure 2) consist of induction coils wrapped around a pipe or induction coils placed in cooling-water flow. AC is sent through the coils, imparting an induced electric field, along with variable magnetic fields, to water. Depending on the supplier, various frequencies, wave shapes, and strengths can be used. The generated current can be constant or pulsed. These devices are intended for scale, corrosion, and biological control.

Electrostatic

These devices (Figure 3) generate a powerful electrical field that affects the electrically charged dissolved ions of scale-causing minerals. Crystals grow (controlled precipitation) and remain suspended in the solution. Scale control is the main benefit of the technology; chemicals usually are required for corrosion and biological control.

Ultrasonic

In these devices (Figure 4), ultrasonic waves are generated using high-power, high-intensity ultrasonic transducers. As the sound waves pass though, water alternately is compressed and decompressed, with the potential for cavitation to result. Mostly, the devices are sold for microbiological control.