Coagulation experiments were performed to
find out the optimum coagulant and alkaline chemical for
the removal of humic acid (HA) from environmental water. This research clearly indicates the
effectiveness of coagulation method using polyaluminum chloride (PAC) with calcium carbonate as
neutralizer and coagulant aid to remove HA. The removal of HA was evaluated from absorbance at 260 nm
and concentration of dissolved organic carbon (DOC). The highest removal ratio was found to be 96.6% for
the absorbance of HA and 91.6% for DOC concentration when 30 mg/L of PAC with calcium carbonate
(CaCO3) was used as an alkaline reagent. In this method, the sludge volume (SV) was reduced about half
compared with the PAC method using NaOH as neutralizer. Ca2+ which is derived from CaCO3 makes HA
colloid size bigger and easier to form
flocks because of the compression in the electrical double layer
(EDL) between HA colloids. The adsorption of HA onto porous CaCO3 particles also plays an important
role. The experiments conducted with sample water collected from Sebangau River, Indonesia consists of
33.4 mg/L of DOC and was found to have a removal efficiency of 86.5%, whereby the
final concentration of
DOC was 4.5 mg/L. These results suggested that CaCO3 requires a lesser amount of coagulants and also
shortens the settling time. This proposed technique provides a more effective and conventional removal
method for HA by coagulation–precipitation processes.