HNO₂ treatment of sludge: An alternative way of sludge usage as fertilizer

The research was conducted to evaluate the sludge quality as fertilizer after heavy metal removal by using free nitrous acid (FNA, HNO₂) solution. To meet up the Korean criteria of fertilizer, FNA treatment with the ultrasonic process was studied here. The sample was taken from a local wastewater and sewage treatment plants and FNA with FNA plus ultrasonic used to treat them. As and Cd concentrations were met the Korean criteria after FNA treatment. In contrast, Al of sludge from sewage treatment plant did not decrease down to the criteria, possibly suggesting that it may be related to the coagulants of the sludge during thickening and dewatering. FNA treatment showed satisfactory results in comparison with the other acidic treatment of citric acid, oxalic acid, HNO₃ or HCl. Removal efficiency of the Fenton-like reaction was more effective than the FNA method. Despite this result, the FNA treatment may be better for sludge recycling as fertilizer because the Fenton-like process removed the necessary organic matter, nitrogen as well as metals. Moreover, the addition of ultrasound process has in conserving time and expenditure in metal removal and FNA with ultrasound became more effective in shorter reaction time. Therefore, FNA treated sludge could be a better option for sludge recycling as fertilizer. In conclusion, a proper plan of sludge management should be required for future work to use sludge as fertilizer.