Effect of synthetic and natural coagulant on lignin removal from pulp and paper wastewater

The effects of synthetic polymer (HE, PEI, and PAM) and natural polymer (chitosan) and chemical (alum) coagulant on the removal of lignin (black liquor color, and TOC) from pulp and paper industrial wastewater were investigated. It was observed that the formation of flocs was rapidly taking place which was an indication of fast coagulation flocculation process. PAM which is a nonionic polymer had poor effect as compared to HE and PEI which are cationic polymers. Both HE and PEI resulted in good brown colored precipitate at pH of about 6. These coagulant resulted in removal of about 80% of color and 30% of TOC from alkaline (black liquor) wastewater by gravity settling in 30 minutes. alum resulted in good precipitate with less than 30 minutes settling time by gravity. The percent color removed by alum was the same as the one removed by HE or PEI, but the TOC removed was higher by this chemical coagulant (about 40%). The natural coagulant, chitosan, resulted in the highest removal in both color and TOC as compared to both synthetic polymers and chemical coagulant. Up to 90% of color and 70% of TOC were observed to be removed by chitosan.