EFFECTS OF RAINFALL

The sudden dilution of a river by heavy rainfall can be a disruptive factor in a water treatment plant. The location of a river water intake should be carefully chosen with this problem in mind. In the operation of a treatment plant, it is common practice to adjust the chemical dosages according to effluent water quality. However, there are many water supplies so variable that it is necessary to base changes in chemical treatment on raw water characteristics, rather than on finished water quality. This imposes a hardship on the treatment plant operators requiring their constant attention to analysis and control.

Tides create another important influence on surface water quality in that they slow, or actually reverse, normal river flow. This is particularly pronounced during periods of low rainfall. The change in water quality between high and low tide sometimes justifies the installation of raw water supply reservoirs to receive water at low tide when the river flows unimpeded and quality is at its best. Plants so equipped stop pumping at high tide when saline bay waters move upstream into the upper channel.

Another characteristic of surface waters is seasonal temperature changes. This complicates treatment, particularly affecting the coagulation process in the winter. Low temperatures also create problems with air-binding of filters due to the increased solubility of gases and higher water viscosity. This binding causes pressure drop through the filter beds to increase, releasing gas and disrupting flow.

Another effect of temperature change occurs in water-cooled systems of industrial plants where heat exchange equipment is usually designed for the least favorable condition—the higher summer temperatures of surface waters. In winter, when the temperature is low, the flow must often be restricted to prevent over cooling. Lower water velocities may allow silting in heat transfer equipment, which can lead to corrosion and to pressure loss when higher cooling rates are needed.

Because rivers are warmer in the summer, designers take this into account in most water-dependent systems. But a complication arises in that many waste-waters contain heat from plant processes, and this added heat compounds the natural rise of the summer, sometimes producing an effluent warm enough to create an unhealthy condition for aquatic life. Pollution discharges not only add to the heat load of the river, but they usually also add to the oxygen demand and may have a pronounced influence on the oxygen content of the river water.

source: The NALCO Water Handbook