Electrochemical degradation of pulp and paper mill wastewater. Part 2. Characterization and analysis of sludge

In this paper, we report the characteristics and the management of the sludge generated by the batch electrochemical (EC) treatment of the black liquor (BL) of a small paper mill based on agricultural waste as raw material. The study shows that the sludges obtained from the EC treatment of the BL at its natural pH without any additive and with 625 mg dm(-3) sodium chloride (NaCl) or 10 mg dm(-3) polyacrylamide (PAA) during the EC process had very good settling characteristics. PAA addition hastens the EC process, and the sludge settling rate improves considerably. The settling velocity of the sludge obtained from the EC treatment without any additive could be satisfactorily correlated with the model of Richardson and Zaki. The results of the gravity filtration of the treated BL showed that the addition of NaCl (625 mg dm(-3)) improved the filtration characteristics and reduced the values of the specific cake resistance (alpha). The values of the specific cake resistance (alpha) and the resistance of the filter medium (R-m) were in the range of 3.25-4.67 x 10(11) m kg(-1) and 2.37-2.98 x 10(9) m(-1), respectively. Prolonged use of iron anodes produces a large number of dents on their surface. SEM images of the electrodes show that the dents formed because of the anode dissolution during the EC process. The sludge has lower ash content and fixed carbon and higher volatile matter than that of Indian coal. Thermal analysis showed good combustion characteristics and complete oxidation of the EC process sludge at about 400 degrees C, with a heating value of 11.33 MJ kg(-1). The sludge can be dewatered, dried, and used in the furnace/incinerators for its heat recovery, and the ash may either be blended with organic manure for use in agriculture/horticulture or may be blended with clay/coal fly ash to make bricks/ceramic tiles for the building industry.