Page 110 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 89

exchangers and condensers and (4) metallic corrosion. These water quality
problems can result from any water source (fresh, reclaimed, or salt) unless
appropriate preventive measures are incorporated into the cooling water system.
Definitions of each cooling water quality issue are given below. Chemicals such
as chlorine and chelating agents are added to prevent biofouling and inhibit
mineral build-up. As the water volume is reduced through evaporation and drift,
the concentration of these chemicals and their by-products increases. Cooling
towers can also contain chemicals from the ambient air.

Scaling
Scaling refers to the formation of mineral deposits, usually on hot surfaces,
which can compromise heat exchanger efficiency. As with dense membrane
processes, calcium deposits (calcium carbonate, calcium sulphate and calcium
phosphate) are the predominant form of scale. Such deposits are somewhat
heterogeneous in nature (Fig. 3.12) and their accumulation on the surfaces of
heat exchangers can reduce the heat transfer efficiency, causing overheating
of metal and, ultimately, boiler tube failure. In addition, the presence of deposits
can provide habitats for the growth of microorganisms within the cooling
system. Magnesium scales (magnesium carbonate and phosphate) can cause a
similar problem. Silica scales are particularly problematic, since silica is largely
insoluble and forms very tenacious deposits. Silica can volatilise at the high
temperatures of boiler systems and become entrained in the steam (Dyson, 2001;
Troscinski and Watson, 1970; Vanderpool, 2001). As the steam cools within the
turbine, the silica can crystallise on the turbine nozzles or blades leading to
increased frictional resistance and reduced steam velocities. Because the deposits
are not uniformly distributed, the turbine rotors become imbalanced and
produce excessive vibrations.

Figure 3.12 Electron micrograph of calcium deposit from cooling tower operations. The length of the white
bar represents IO wm