Page 117 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 91

2.2.3 Advances in Coagulation Process and Practice
The use of a single coagulant, usually inorganic, is seldom satisfactory to get
the desired coagulation impact in wastewater treatment. However, since the
performance of coagulation process directly affects the overall performance
of the other effluent treatment steps, improving coagulation is a crucial fac-
tor from the point of view of overall effluent treatment strategy. The use of
inorganic coagulants and the practice of coagulation is quite straightforward
in drinking and municipal sewage water treatment. However, this is not the
case with most industrial wastewaters where the number and nature of pol-
lutants is varied and complex; usually, advanced methodologies are required
for treating these high-strength wastewaters. The advances in these mainly
incorporate newer coagulants with specific properties (such as inorganic and
organic polymers, with specific molecular weight or Al/OH ratio), coagu-
lant formulations (intelligent mix of inorganic and organic coagulants to get
benefits of both), coagulant aid, flocculants and flocculation, mixing, and
newer equipment designs and solid-liquid separation. Some other modifica-
tions, such as electro-coagulation, are also being increasingly considered for
wastewater treatment.
Thus, in the context of the conventional coagulants listed above, many
newer coagulant types and hybrid forms are being continuously researched
and enter the market (Tzoupanos and Zouboulis, 2008; Lee et al., 2012).
A review of various hybrid materials comprising inorganic-inorganic,
inorganic-organic, organic-organic, and inorganic-natural polymers has been
presented recently by Lee et al. (2012). This has been very instructive for
researchers working in this area. Some natural coagulants, such as seed
extracts, are also being investigated, although far from being considered for
commercial application at this point of time (Bhuptawat et al., 2007). Polymer
form is increasingly considered to be key factor in improving coagulation
because the large molecular weight and controlled basicity/charged
groups/charge density help to achieve efficient coagulation along with better
flocculation. The most prominent advances among inorganic coagulants
include polymers such as PAC, PAS, and polyaluminumchloro-sulfates, with
substantial variation in their properties due to molecular weight and basicity.
Typically, for PAC, three commercial variations are available: low basicity,
medium basicity, and high basicity based on Al/OH ratio (Gao et al.,
2005). Recently, Yuan et al. (2006) compared performance of PAC with
polyferric chloride and poly-silicate-aluminum-ferric chloride (PSAFC) in
the treatment of dye wastewater. Although generalization on the best coag-
ulant in this regard could not be concluded in the study, it was suggested that