Chemical and Biological Leaching Methods to Remove Heavy Metals from Sewage Sludge: A Review

Allinor J.I,Anyadiegwu C.I.C,Ejele A.E,Ibeneme S.I,Lugard N Ukiwe

doi:10.24297/jac.v4i3.948

Allinor J.I, Anyadiegwu C.I.C + Show 3 more

Open Access

https://doi.org/10.24297/jac.v4i3.948

Copy DOI

Abstract

The removal of heavy metals (HMs) in sewage sludge (SS) is important since sludge is often disposed or applied on farmland to enhance soil fertility. The present study reviewed two conceptual approaches (chemical and biological leaching) of removing HMs present in SS. In the chemical leaching method, traditional acid treatment together with novel methods such as aeration, complexation and sequential extraction procedure have been reviewed extensively. Certain factors influence the removal of HMs in SS. These factors include; pH, leaching agent, redox potential, and contact time. Nitric acid (HNO3), hydrochloric acid (HCl), sulphuric acid (H2SO4), phosphoric acid (H3PO4), ethylenediamine tetraacetic acid (EDTA), as well as Thiobacillus thiooxidans and Thiobacillus ferrooxidans are the most widely studied leaching agents and substrates involved in the chemical and bioleaching processes. However, the bioleaching process has been proposed as a safe, efficient, economical, environmental friendly method to remove HMs in SS due to its simplicity, high yield of metal extraction, low acid consumption, and low sludge solids concentration. Nevertheless, the present review has noted that most researchers are of the opinion that more studies are needed in the bioleaching method in order to enhance its commercial attraction.

Highlights

Heavy metals (HMs) are members of elements that exhibit metallic properties with high atomic number and atomic weight as well as a specific gravity greater than 5.0
In an attempt to investigate the extraction of certain heavy metals (HMs) from sewage sludge (SS) using different acid types, Gaber et al (33) observed that at pH 2.43, citric acid appear to be highly effective in extracting Cu (86%) and Zn (88%) in 24 h and 90 and 96 % of Cr and Ni, respectively in 5 days leaching time
A review of chemical and biological leaching processes to remove HMs from SS has been undertaken in the present study

Summary

Introduction

Heavy metals (HMs) are members of elements that exhibit metallic properties with high atomic number and atomic weight as well as a specific gravity greater than 5.0. HMs includes the transition metals, some metalloids, lanthanides, and actinides They are toxic and pose a threat to both humans and the ecosystem when their occurrence in the environment exceeds acceptable limits (1), (2). There are two main types of SS, namely; a) Coagulated SS: This is SS with a gelatinous appearance containing high concentrations of aluminum or iron salts with a mixture of organic and inorganic materials and hydroxide precipitates. Sludge conditioning is a process whereby sludge is treated with chemicals or various other means to prepare it for dewatering process (6). Sludge conditioning make use of chemicals whose main function is either to lower or raise the pH value of the sludge such that smaller particles coagulate to larger ones to releasing water (7). The dewatered sludge and the coagulate are digested separately using a leaching process.

Chemical leaching

Biological leaching

Findings

Conclusion