Chemical fractionation and risk assessment of trace elements in sewage sludge generated from various states of Pakistan

In the developing world, rapid urbanization and industrialization produces an enormous volume of wastes daily. This study was aimed to explore the potential and risks associated with sewage sludge through the characterization and fractionation technique. Sewage sludge samples were collected from various wastewater treatment in five different cities of Pakistan. Considerable amounts of macro-elements were detected in all types of sewage sludge samples. The pHw of all sewage sludge were neutral to slightly alkaline in reaction. Total organic carbon (TOC) was maximum (18.73%) with Coca-Cola sewage sludge (CSS) while the minimum (14.69%) was with Water and Sanitation Agency (WASA) sewage sludge (WSS). Percent relative distribution of cadmium (Cd) was higher in residual fraction (F4) up to 52% in the Nestle wastewater treatment plant, Sheikhupura (NSS). The chromium (Cr) concentration in Kasur sewage sludge (KSS) was extremely in mobile fraction (exchangeable) as compared with all other sludge samples, therefore showing a higher level of risk assessment code. While in the case of Iron (Fe), mobility was less and its maximum portion was noted in residual fraction (F4) of all sewage sludge samples. Percent distribution of manganese (Mn) showed variable trends for different sewage sludge samples. Zinc (Zn) concentration showed high mobility (exchangeable fraction) in case of NUST wastewater treatment plant, Islamabad (NTS) (31.16%) and WSS (37.83%) as compared with other sewage sludges. The risk assessment code indicated that Zn and Ni had a medium level of risk with I-9 Sector wastewater treatment plant, Islamabad (ISS), CSS, KSS, and NSS whereas these pose a high risk with NTS and WSS. Based on physicochemical properties, nutrients, trace elements, mobility, and risk assessment code, it was concluded that KSS should not be recommended at any application rate while NTS and WSS may be used at low application rates whereas ISS, CSS, and NSS may be used for agricultural crop production.