It can be understood that the cattle milk CIP wastewater
contained high concentrations of solutes and
colloids (Table 1). The pH of cattle milk CIP wastewater
(I) was neutral, and it has already formed big
flocs (high turbidity and low CODf), so there is no
need to study the pH effect on its coagulation.
Therefore the cattle milk (II) and sheep milk CIP
wastewater with two different contaminant concentrations
were used to study the coagulation efficiencies
under different pH.
It was found that the removal efficiencies of turbidity
and COD were correlated with pH in the condition
of 25 mg/l chitosan (Fig. 4), and increasing pH
decreased both the removal efficiencies of turbidity
and COD. Either the wastewater from the cattle milk
CIP or the sheep milk CIP wastewater, a portion of
COD was contributed from the solid particle of protein
and fat. The removal efficiencies of turbidity (99% and
95% for cattle milk and sheep milk CIP wastewaters,
respectively) shown that chitosan could remove the
solids in both the CIP wastewaters (Fig. 4). For the
removal efficiency of COD, it was 70% for the cattle
milk CIP wastewater. The ratio of CODf/CODt was 0.3
before coagulation, meaning that 70% of the COD was
particulates and 30% of the COD was dissolved
(Table 1), thus chitosan remove most of the solids but
not the dissolved solute. However, the COD removal
efficiency of sheep milk CIP wastewater was 54%, and
the value of CODf/CODt was 0.96 before coagulation,
indicating that solid form COD was only 4%, so
chitosan could remove dissolved COD in sheep milk
CIP wastewater. Almost all the turbidity was removed
after coagulation, the ratio of CODf/CODt is close to
one, i.e., most of the CODt is composed of CODf. It
was also shown that the zeta potential decreased as the
pH of wastewater increased, and in pH above 7, the
surface zeta potentials were all negative (Fig. 4). In the
condition of 25 mg/l chitosan and pH 7, the surface
charge were neutralized and resulted in high COD and
turbidity removal efficiencies, but they decreased as
pH declined.