Page 111 - Membranes for Industrial Wastewater Recovery and Re-Use

90 Membranes for Industrial Wastewater Recovery arid Re-use

Scale composition depends on the relative mineral content of the recirculating
stream. A summary of solubility constants associated with common mineral
precipitates has been presented in Table 2.14. While the chemical content of
each system has been a function of the source water and the Rc, it is possible to
apply chemical equilibrium models, such as Argo Analyser (Pig. 4.7) to evaluate
the potential for scale formation. As a first approximation, the solubility
relationships can be used to identify the chemical constituents that are likely to
form deposits. Typically, when reclaimed water is used as a source water, the first
calcium salt to precipitate is calcium phosphate unless the water has been pre-
treated for phosphorus removal.
Scale control can be accomplished through chemical precipitation followed by
solids removal (sedimentation, filtration, etc.) to reduce the concentration of
minerals in the recirculating stream. Chemicals used to promote upstream
precipitation include lime, caustic soda, alum, and various formulations of
organic or inorganic polymers. Acidification or addition of scale inhibitors can
control scaling by increasing the solubility of minerals in the recirculating
stream. Phosphate specifically can be removed biologically, and multivalent ions
may be removed by ion exchange.
The solubility of mineral precipitates that form from hydroxides, phosphates,
or carbonates typically increases with decreasing pH. To prevent scale
formation, the pH of the water is reduced to about 7 using sulphuric acid. The
additional sulphate and lower pH convert calcium and magnesium carbonates
into more soluble sulphate compounds. It is important to control the amount of
acid added to maintain some residual alkalinity in the system, since excess acid
can cause accelerated corrosion. Acids used to control pH of the recirculating
stream include sulphuric, hydrochloric, and citric acids. Alternatively gases can
be used to acidify the water such as carbon or sulphur dioxide. Chemical
chelators such as ethylenediamine tetraacetic acide (EDTA) and polymeric
inorganic phosphates can also be added, often in-line (Fig. 3.13), to increase the
solubility of scale forming constituents.

Biological growth
The warm, moist environment in cooling towers coupled with the availability of
nitrogen, phosphorus, and organics provides an ideal environment for microbial
growth. Typically, microbial growth results in biofilm formation and fouling, in
which microbial products encourage the attachment and growth of
heterogeneous deposits containing both microorganisms and inert materials, on
heat exchanger surfaces. These biofilms then interfere with heat transfer and
water flow. During extended operating periods, portions of the biofilm slough off
of the surface. This microbial biomass contains particles and other debris that can
settle, further inhibiting effective heat transfer. Some types of microorganisms
release corrosive by-products during their growth such as organic acids (e.g.
acetic) or inorganic acids (e.g. hydrogen sulphide) leading to microbially induced
corrosion (MIC), a phenomenon exacerbated by standing water conditions.
Bacteria that may be present in cooling water include Pseudomonas, Klebsiella,
Eneterobacter, Acinetobacter, Bacillus, Aeromonas, and Legionella (Adams et al.,