Using vegetable tannin and polyaluminium chloride as coagulants for dairy wastewater treatment: A comparative study

The aim of this work was to compare the performance of tannin and polyaluminium chloride (PAC) in the coagulation process for dairy industry wastewater. The conditions of coagulation/flocculation with regards to the dosage, pH, velocity gradient and slow-mixing time were optimized and some parameters such as floc aggregation (K-A) and breakage (K-B) coefficients were computed. Additionally, it was carried out a thermogravimetric analysis of the obtained sludge for both coagulants. The results showed no statistical differences between the performance of the two coagulants based on chemical oxygen demand, colour, turbidity and total solids removal. Particularly, the PAC showed higher alkalinity consumption and increased the electrical conductivity of the clarified wastewater, while tannin showed good performance within a wider pH, ranging from 5.0 to 10.0. Moreover, although the greatest K-A values were observed for PAC usage, the tannin use presented higher resistance to floc breakage during the slow-mixing time. The obtained sludge from the experiments using tannin accounted for higher volatile solids and fixed carbon, and lower ash content when compared to PAC. In conclusion, tannin is a promising alternative for dairy wastewater treatment regarding its process performance and wastewater treatment residuals management.