Coagulation–flocculation for lignin removal from wastewater – a review

Industrial discharge has tremendously increased inorganic pollutants in water bodies all over the world. Paper and pulp effluent is included in one of the most pollution generating discharges containing complex chemical compounds such as lignin. For clean and healthy water resources, the recovery of lignin from wastewater from the paper and pulp industry is of high importance. Available chemical and biological technologies for removal of lignin have certain drawbacks. Coagulation and flocculation is not only the economic but also the effective method for removal of lignin. The present review highlights available coagulants employed for removal of lignin from paper and pulp wastewater. Each coagulant is pH dependent and shows varied results with change in effluent characteristics. The hydrolysis products of aluminium-based coagulants, iron-based coagulants and copper sulphate have positive charges. These positive charges promote formation of flocs through charged neutralisation or sweep flocculation. In the case of titanium-based coagulants, hydrolysis product is negatively charged and mode is heterocoagulation. Ninety percent recovery of lignin is achieved by using a mixture of oxotitanium sulphate and aluminium sulphate and 80% with aluminium sulphate. Virtually complete recovery of lignin is observed with oxotitanium sulphate.