Efficient dewatering and heavy-metal removal in municipal sewage using oxidants

Municipal sewage sludge has become an environmental problem globally. With the rapidly growing urban population and continued construction of large-scale sewerage facilities, the development of effective sludge treatment, disposal, and management strategies are highly desirable. Municipal sewage sludge, for instance, has been widely used in agriculture to increase soil fertility. However, the environmental issues associated with sludge disposal are complicated, considering that sludge contains heavy metals, toxic organics substances, and pathogens that can significantly affect the environment, plants, and human health at large. Moreover, lack of effective sludge treatment policies and high operating costs have remained the main factors hindering the treatment of municipal sewage sludge. Despite the extensive research on several oxidation processes for efficient dewatering sludge, i.e., the use of Fenton’s reagent, peracetic acid, little has been reported for removal of heavy metals from sludge about the use of oxidants including CaO₂.

To this note, a team of researchers at Hunan Agricultural University: Jing Yuan (Ph.D. Student), Weining Zhang (M.S. Student), Professor Zhihua Xiao, Ms. Xihong Zhou, and Professor Qingru Zeng investigated the feasibility of three oxidants, namely, Fenton’s reagent, CaO₂ and potassium sulfate in treating sludge. Their main objective was to evaluate the performances of the three oxidants in sludge dewatering and heavy metal removal. Their research work is currently published in the Chemical Engineering Journal.

In their approach, the research team started their experiment work by treating the municipal sewage sludge with the three oxidants. Next, heavy metals in the supernatant were separated via the addition of CaO. CaO was particularly preferred because it is a simple and cost-effective process for recycling the treated sludge. Lastly, the authors presented an in-depth insight on the contents of the heavy metals and the changes in the supernatant water quality before and after CaO treatment.

The authors observed that all of the three oxidants could effectively remove the moisture content of the sludge besides significantly reducing the SRF value used in the characterization of the sludge filtration performance. Treatment with Fenton’s reagent and potassium sulfate proved more effective for removing Pb, Zn, Cu, and Cd heavy metals, especially Cd. On the other hand, the metal removal performance of CaO₂ was not satisfactory. Nonetheless, the addition of CaO to sludge treatment allowed the effective separation of metals in the supernatant. Consequently, the oxidants exhibited excellent dewatering performance for municipal sewage sludge. The water content was reduced from 99.08% to 66.5% for CaO₂,74.7% for potassium sulfate, and 76.1% for Fenton’s reagent. Furthermore, the authors reported that the treated sludge was easy to transport and safe for agricultural use as per the set standards.

In summary, the study presented the use of oxidants for efficient dewatering and removal of heavy metals in municipal sewage sludge. Results showed that Fenton’s regent and potassium sulfate oxidants are more suitable for dewatering and removal of Pb, Zn, Cu, and Cd. Besides, the addition of CaO to the supernatant allowed easy separation of metals in the supernatant. Most importantly, the treated sludge was safe for agricultural use. In a statement to Advances in Engineering, the team leader, Professor Qingru Zeng, stated that the newly presented method is viable for efficient and cost-effective municipal sewage sludge treatment for agricultural application.