A two-stage process for simultaneous thermophilic sludge digestion, pathogen control and metal leaching

Cleanup of sewage sludge spiked with Cd, Cu, and Zn: Sludge quality and distribution of metals in the "soil-plant-water" system

Corn was grown under greenhouse conditions in the presence of uncleaned versus cleaned municipal sewage sludge to assess the effectiveness of a chemical leaching process that uses an inorganic acid and strong oxidants to clean biosolids (i.e., to remove metals without reducing their agronomic potential). Specifically, our study analyzed physicochemical sludge from the Montreal (MSL) wastewater treatment plant (WTP) and biological sludge from the Becancour (BSL) WTP. Both biosolids were spiked with individual metals (dry weight basis): Cd (100 mg kg-1), Cu (3000 mg kg^-1), and Zn (5000 mg kg^-1), or their mixture. MSL biosolid enrichment led to the solubilization of added metals and removed 84-88% of Cd, 78-79% of Cu, and 79-81% of Zn. Similarly, BSL biosolid enrichment resulted in the removal of 86-88% of Cd, 80-81% of Cu, and 88-89% of Zn. The fractional distribution of metals varied between biosolids depending on their production process, stabilization, and initial metal concentration. In the MSL biosolids, only Cu partitioning was influenced by spiking, cleanup, and washing. The three metals (Cd, Cu, and Zn) occurred either in their crystalline phase or were linked to Fe oxide, organic matter, or carbonate. In the BSL biosolids, the metals that were not in their crystalline phase were only associated with Fe oxide. This study demonstrated that 99% of Cd and Cu and 97% of Zn contents remained in the soil; however, biosolid cleanup generally decreased metal concentrations in plants, leachate, and/or soil.

Bioleaching of heavy metals from wastewater sludge with the aim of land application

Presence of heavy metals in the wastewater sludge has greatly hindered sludge land application. Bioleaching has been developed for heavy metal removal from sludge. The pH of the sludge is declined by microorganisms with S or FeS as energy source. Sludge considered to be used in land is mainly due to its fertilizer values as it contains nitrogen, phosphorus, and potassium. Therefore, it is important to understand how the bioleaching would impact on sludge characterization. In addition, pathogens are great threat to human health. The ability of pathogen elimination of bioleaching is highly concerned. In this review, the major heavy metals in the sludge are summarized. The change of nitrogen, phosphorus, and potassium after bioleaching is stated. The pathogen elimination due to bioleaching has been discussed. The work has provided an insight of research need in sludge bioleaching with the aim of residual sludge land application.

Effects of Solid Content and Substrate Concentration on Bioleaching of Heavy Metals from Sewage Sludge Using Aspergillus niger

The presence of heavy metals in sewage sludge not only affects the performance of sludge anaerobic digestion process but also restricts the land application of treated sewage sludge. Therefore, a fungi-mediated bioleaching process for simultaneous metal leaching and sludge digestion by Aspergillus niger was developed to treat the sewage sludge containing heavy metals in this study. The effects of two important parameters, sludge solid content and substrate (sucrose) concentration, on the performance of fungal bioleaching were investigated in this study. The results showed that the rate of pH reduction increased with increasing sludge solid contents and sucrose concentrations. In this study, the efficiency of metal removal decreases in the order of Mn > Zn > Ni > Pb. The efficiencies of metal leaching and solid degradation (SS and VSS) were found to be decreased with an increase of sludge solid content and a decrease of sucrose concentration. At 2 days of reaction time, the maximum efficiency of metal solubilization was 95, 56, 21 and 13% for Mn, Zn, Ni and Pb, respectively.

Heavy metal bioleaching and sludge stabilization in a single-stage reactor using indigenous acidophilic heterotrophs

Simultaneous sludge digestion and metal leaching (SSDML) have been reported at mesophilic temperature. It is generally perceived that while sludge stabilization is effected by heterotrophs at neutral pH, metal bioleaching is done by acidophilic autotrophs. However, little information is available on the microbial communities involved in the process. This study carried out SSDML in a single-stage reactor using sludge indigenous microorganisms and looked at the bacterial communities responsible for the process. Volatile suspended solids were reduced by more than 40%. The concentration of zinc, copper, chromium, cadmium and nickel decreased by more than 45% in the dry sludge. Acidophilic species of Alicyclobacillus genus were the dominant heterotrophs. A few heterotrophic bacteria were detected which can oxidize iron (Alicyclobacillus ferrooxydans, Alicyclobacillus ferripilum and Ferrimicrobium acidiphilum). Acidithiobacillus ferrooxidans (autotroph) was responsible for the oxidation of both iron and sulfur which lead to a change in the pH from neutral to acidic. The presence of acidophilic heterotrophs, which can oxidize either iron or sulfur, enhanced the efficiency of SSDML process with respect to sludge stabilization and metal leaching. This study shows that it is possible to carry out the SSDML in a single-stage reactor with indigenous microorganisms.

Decontamination of heavy metal laden sewage sludge with simultaneous solids reduction using thermophilic sulfur and ferrous oxidizing species

A possibility of using simultaneous sewage sludge digestion and metal leaching (SSDML) process at the thermophilic temperature to remove heavy metals and suspended solids from sewage sludge is explored in this study. Though thermophilic sludge digestion efficiently produces a stable sludge, its inability to remove heavy metals requires it to be used in tandem with another process like bioleaching for metal reduction. Previously, different temperature optima were known for the heterotrophs (thermophilic) responsible for the sludge digestion and the autotrophs involved in bioleaching (mesophilic), because of which the metal concentration was brought down separately in a different reactor. In our study, SSDML process was carried out at 50 °C (thermophilic) by using ferrous sulfate (batch-1) and sulfur (batch-2) as the energy source in two reactors. The concentration of volatile suspended solids reduced by >40% in both batches, while that of heavy metals zinc, copper, chromium, cadmium and nickel decreased by >50% in both batch-1 and batch-2. Lead got leached out only in batch-1. Using 16S rRNA gene-based PCR-denaturing gradient gel electrophoresis analysis, Alicyclobacillus tolerans was found to be the microorganism responsible for lowering the pH in both the reactors at thermophilic temperature. The indicator organism count was also below the maximum permissible limit making sludge suitable for agricultural use. Our results indicate that SSDML at thermophilic temperature can be effectively used for reduction of heavy metals and suspended solids from sewage sludge.

Copper emission during thermal treatment of simulated copper sludge

This study evaluates Cu emissions in air-particulate and gas phases during thermal treatment of simulated copper sludge by a rotary kiln. Influences of operating parameters, including treatment temperature (400-700 degrees C), rotary speed (0.89-2.00 rpm) and copper content in sludge (1% to 5% by weight) on copper emissions were investigated. The toxicity characteristic leaching procedure (TCLP) and scanning electron microscopy (SEM) were also conducted to evaluate copper leaching and the surface structure of thermally treated sludge, respectively. The results indicated that (1) low Cu emissions in air-particulate and gas phases were associated with the two operating conditions of 400-500 degrees C at 0.89-1.39 rpm and 600-700 degrees C at 2.00 rpm; (2) temperatures and rotary speeds did not affect gaseous copper emission, except for the operating condition of 400 degrees C at 2.00 rpm; (3) rising copper content of sludge at 600 degrees C and 2.00 rpm increased the particulate copper emission, but not the gaseous copper emission; (5) the TCLP copper leaching concentrations of sludge treated at 400 degrees C were obviously higher than those treated at 500-700 degrees C; however, all of the thermally treated products agreed with the Taiwan EPA TCLP regulations.