Page 135 - Sustainability in the Process Industry Integration and Optimization

112 Cha p te r F i v e

The paper by Blomquist and Brown (2004) offers a useful review
of wastewater minimization. The authors examined a large number
of preassessment and assessment techniques for respectively
identifying waste minimization focus areas (opportunities) and
options (solutions) during a waste minimization audit. Blomquist
and Brown critically reviewed these techniques and assessed their
relative merits. The preassessment techniques were analyzed in
terms of their ease and speed of implementation; the assessment
techniques were evaluated in terms of their usefulness and
applicability.
Wastewater Treatment
Methodologies for wastewater handling can be subdivided into
different stages of treatment, as follows:

• Pretreatment: Mechanical separation of coarse particles (e.g.,
sticks, plastics).
• Primary treatment: Removal of suspended solids by physical
or physical-chemical treatment. This process may consist of
natural sedimentation or may be assisted via adding
coagulants and/or flocculants or via centrifugation. Primary
treatment also includes neutralization, stripping (e.g.,
elimination of ammonia, NH ), and the removal of oils and
3
grease by flotation.
• Secondary treatment: The removal of colloids and similar
matters from the wastewater. This treatment, which may
include chemical and biological processes, minimizes the
wastewater’s organic load. Processes commonly used include
activated sludge treatment and anaerobic digestion, both of
which lead to the critical removal of phosphate, ammonia,
and oxygen-depleting contaminants.
• Tertiary treatment: This stage comprises physical and chemical
processes that eliminate such pollutants as phosphate,
ammonia, minerals, heavy metals, and organic compounds.
The processes are viewed as a “polishing phase” and are
usually more expensive than conventional techniques. The
necessity of applying this type of treatment is largely dictated
by the following two factors:
(i) Meeting discharge conditions established by environ-
mental quality standards (EQS), which may be stricter
than BAT requirements. Subject contaminants include
ammonia, so-called List I and List II (BAT-CENTRE,
2009) substances, and suspended solids.
(ii) Recycling the wastewater for further use in the factory
as either process water or washing water.