Page 114 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 93

systems. To assess fouling propensity of possible feedwater sources the National
Engineering Laboratory based at Grangemouth have developed device for
assessing biofilm formation through thermal resistance measurements. Results so
far have revealed fouling rates of recovered municipal effluent to be generally
higher than other sources, such as canal water, and highly dependent on flow
velocity. On the other hand, examples exist of employing membrane treatment for
the production ofboiler feedwater from secondary municipal effluent (Section 5.1).

Metallic corrosion
In cooling systems, corrosion can occur when an electrical potential between
dissimilar metal surfaces is created. The corrosion cell consists of an anode,
where oxidation of one metal occurs, and a cathode, where reduction of another
metal takes place. The presence of dissolved minerals can accelerate the corrosion
reaction. Dissolved oxygen and certain metals (manganese, iron, and
aluminium), which can arise in cooling intakes depending on the water source
(Salvito et a]., 2001), promote corrosion because of their relatively high
oxidation potentials.
Mild steel and copper alloys are used extensively in heat exchanger systems
due to their excellent heat transfer capabilities. However, the build-up of
minerals in cooling water tends to be aggressive towards these materials.
Analysis of corrosion is complex due to the interplay of water quality variables
that can either induce or prevent corrosion. A summary of water quality
determinants significant for corrosion control is given in Table 3.4. The
corrosion potential of cooling water can be controlled by the addition of chemical
corrosion inhibitors. The concentration of chemicals in the recirculating stream,
increases with increasing Rc and may require removal from blowdown water
prior to discharge.

3.1.8 Governing legislation and guidelines
Regulatory requirements and industrial guidelines relating to the use of water
for cooling and boiler applications are fairly localised. In the USA regulatory
requirements address water intake structures, air emissions, and discharge of
blowdown water. Currently, under Section 3 16(b) of the Clean Water Act, it has
been proposed to implement more stringent protection measures at water intake
structures to protect fish, shellfish, and other aquatic life (USEPA, 2002a). This
proposed regulation could lead to the need for retrofitting of once-through cooling
water intakes and ultimately lead to increased use of recirculating systems.
Airborne emissions from coal-fired power plants and waste-to-energy facilities
are a subject of increasing scrutiny in urban environments. Increased emissions
control requirements coupled with uncertainties about the long-term
availability of fossil fuels may result in changes in the distribution of energy
sources. As coal-fired plants are displaced by alternative fuels, changes will also
occur in the quantities of cooling water required. It has been estimated that
conversion from coal-fired power plants to alternative fuels could reduce
evaporative consumption by 25% (Powicki, 2002).