Polyelectrolytes are commercial coagulant aids.
Synthetic polyelectrolytes are currently the most widely
used chemicals in the treatment of industrial wastewaters.
Generally, a little amount of polyelectrolyte dosage is
enough to reach high efficiency. Because of they have
some advantages including the possibility of structuration
in response to specific requirements, greater purity, higher
quality, stability and greater efficiency [19]. With
polyelectrolytes as coagulant aids, the metal coagulant
dosage can be reduced without cutting down the
performance [20]. Yu et al, [21] reported that the charge
density and molecular weight of polyelectrolyte play
important role in the coagulation.
Effluent COD and COD removal efficiencies versus
different polyelectrolyte dosages are given in Figure 3. At
the lowest concentration of polyelectrolyte, COD removal
efficiency was 74% and effluent COD concentration was
6.4 g/L . With increase in polyelectrolyte concentration,
removal efficiency increased up to 87%. At a higher
concentration than 50 mg/L, effluent COD decreased thus
50 mg/L polyelectrolyte dosage was accepted as optimum
dosage. Optimum concentration of polyelectrolyte forms
a bridge between particles and cause good flocculation.
However high concentration of polyelectrolyte forms an
envelope on the suspending particles and causes them to
remain in suspension thus removal efficiency decreases
[18]. Similar result was obtained from this study and
when the polyelectrolyte concentration was increased,
process performance was decreased.