Assessment of anaerobic sewage sludge quality for agricultural application after metal bioleaching

Bioleaching of sewage sludge for copper extraction using Acidithiobacillus thiooxidans: Optimization and ecological risk assessment

In this study, Acidithiobacillus thiooxidans was used for the bioleaching of copper (Cu) from sewage sludge. In order to find optimization conditions, three factors including solid-to-liquid ratio (S/L) (0.01-0.2 %(w/v)), initial element sulfur (S0) (1-10 g/L), and initial pH (1-3) have been investigated. Based on response surface methodology (RSM) determined a significant reduced quadratic model with a p-value of 0.0022 (<0.05 significant level). The maximum Cu recovery was 85.3% in the optimum condition of S/L = 0.16% (w/v), S0 = 8.2 g/L, and pH = 1.4. Furthermore, a kinetic study based on a shrinking core model was performed and the result showed that chemical reaction was rate limiting in the extraction. Toxicity Characteristic Leaching Procedure (TCLP) results after bioleaching showed the bioleaching process detoxified sludge and the bioleached sludge residue was well within the regulatory limits for disposal. The germination seed with adding bioleached and unbioleached sludge to the soil was determined. Various parameters such as Germination Index (GI), Tolerance Index (TI), Vigor Index (VI), and stem length showed that the sewage sludge indices significantly increased than the sample soil with unbioleached sludge.

Effect of Indigenous Microbial Consortium on Bioleaching of Arsenic from Contaminated Soil by Shewanella putrefaciens

The effects of indigenous microbial consortium on removal of As from As-contaminated soil using an Fe(III)-reducing bacterium Shewanella putrefaciens were investigated under circumneutral pH condition. Sequential extraction of As revealed that more than 30% of As was associated with Fe(III)-(oxy)hydroxides in the soil. Bioleaching experiments were conducted anaerobically with a supply of lactate as a carbon source. The highest As removal efficiency (57.5%) was obtained when S. putrefaciens and indigenous bacterial consortium coexisted in the soil. S. putrefaciens and indigenous bacteria solely removed 30.1% and 16.4% of As from the soil, respectively. The combination of S. putrefaciens and indigenous bacteria led to a higher amount of labile As after microbial dissolution of Fe(III)-(oxy)hydroxides. After microbial treatment, soil quality represented by pH and organic content appeared to be preserved. The results indicated that the ecological and physiological understanding of the indigenous microbiome might be important for the efficient application of bioleaching technology to remove As from contaminated soils.

Obtaining edaphic biostimulants/biofertilizers from different sewage sludges. Effects on soil biological properties

We studied the influence of six edaphic biostimulants/biofertilizers (BSs) manufactured by the pH-stat method from different sewage sludge (SS): SS1 (an anaerobic mature sludge, one year old), SS2 (an aerobic young sludge, without maturation) and SS3 (an aerobic mature sludge, four months old), not previously autoclaved (A) and autoclaved (B), by analysing their effects on soil biological properties. Soil enzymatic activities were measured at 1, 3, 5, 7, 15, 30 and 60 days of the incubation period, whereas the 16S rDNA-DGGE profiles were determined at 0, 5 and 60 days. The enzymatic activities were significantly stimulated. The highest stimulation was found in the B2 treatment followed by B3, A2, A3, B1 and A1 treatments. Increasing the number of lower molecular weight proteins in the BS enhances the stimulation of soil enzymatic activities. The application of BS caused at 5 days of the incubation period temporal variations in the soil bacterial community structure.

Bioleaching of heavy metals from sewage sludge: a review

During the treatment of sewage, a huge volume of sludge is generated, which is disposed of on land as soil fertilizer/conditioner due to the presence of nitrogen, phosphorus, potassium and other nutrients. However, the presence of toxic heavy metals and other toxic compounds in the sludge restricts its use as a fertilizer. Over the years, bioleaching has been developed as an environmentally friendly and cost-effective technology for the removal of heavy metals from the sludge. The present paper gives an overview of the various bioleaching studies carried out in different modes of operation. The various important aspects such as pathogen destruction, odor reduction and metal recovery from acidic leachate also have been discussed. Further, a detailed discussion was made on the various technical problems associated with the bioleaching process, which need to be addressed while developing the process on a larger scale.

Bioleaching of heavy metals from anaerobically digested sewage sludge

The effects of sulfur concentration, initial pH of the sludge and sludge solid content on metal bioleaching were examined using anaerobically digested sewage sludge procured from a typical sewage treatment plant in Delhi, the capital city of India. Experiments on effect of sulfur concentration were carried out using 0-4 g L(-1) of elemental sulfur to optimize the concentration of elemental sulfur for efficient bioleaching. For the type of sludge (20 g L(-1) solid content) used in the present study, 2 g L(-1) of elemental sulfur was found sufficient in metal bioleaching in the following order: Zn 86%, Cu 71.5%, Mn 70%, Ni 58.3% and Cr 43.8%. Changes in pH, sulfate concentration and oxidation-reduction potential (ORP) as a function of time were experimentally monitored. A rapid change in the above parameters took place in 4-6 days followed by a slow change until the 10th day. The change in pH with time was observed to vary at different initial pH values (pH 7 to pH 3) of the sludge; however, there was not much difference in the final pH achieved and final metal solubilization which ranged from: Cu 83.6-94.2%, Ni 27.7-29.8%, Zn 89-94.8%, Mn 67.5-79% and Cr 34.1-44.1% The results of the present studies strongly indicate that using 2 g L(-1) elemental sulfur, indigenous sulfur oxidizing microorganisms can bring down pH to a value needed for significant metal solubilization. Also, bioleaching can be carried out successfully over a wide range of initial pH values of the sludge. Further, at higher sludge solid concentration than 20 g L(-1), lower metal solubilization was achieved due to the buffering capacity of the sludge.

Chemical treatment of sludge: in-depth study on toxic metal removal efficiency, dewatering ability and fertilizing property preservation

The presence of toxic metals in municipal sewage sludge restricts the application of this biomass in agricultural area. A chemical leaching process using a combination of inorganic acid and two oxidants has been developed for sludge decontamination. The present study investigated the effects of the concentrations of sulphuric acid (H2SO4), hydrogen peroxide and ferric chloride on metal solubilization from sewage sludge, as well as preservation of fertilizing properties (nutrient content) and dewatering ability of the treated sludge. The analysis of the results from batch leaching tests has allowed to define the optimal conditions for the reagent concentrations, which are 56 kg Fe3+ tonne(-1) of dry sludge solids (tds), 8 kg H2O2 tds(-1), and enough H2SO4 to reach a pH between 2.0 and 2.5 but less than 142 kg H2SO4 tds(-1). Finally, under these conditions, oxidoreduction potential values are found to be between 450 and 475 mV.

Pilot-plant study of wastewater sludge decontamination using a ferrous sulfate bioleaching process

The objective of this research was to investigate the performance of the ferrous sulfate bioleaching (FSBL) process in a pilot plant for decontamination and stabilization of wastewater sludge. Batch and continuous experiments, conducted with two 4-m3 bioreactors using indigenous iron-oxidizing bacteria (20% v/v of inoculum) with addition of 4.0 g ferrous sulfate heptahydrate per liter of sludge initially acidified to pH 4.0, were sufficient for effective heavy metal (cadmium, copper, manganese, zinc, and lead) removal yields. The average metal removal yields during the FSBL process were as follows: cadmium (69 to 75%), copper (68 to 70%), manganese (72 to 73%), zinc (65 to 66%), and lead (16%). The FSBL process was also found to be effective in removing both fecal and total coliforms (abatement > 5 to 6 log units). The nutrients content (nitrogen, phosphorus, and magnesium) were also preserved in decontaminated sludge.

Effect of anaerobic digestion and initial pH on metal bioleaching from sewage sludge

The effects of anaerobic digestion and initial pH on the bioleaching of metals from sewage sludge were investigated in shake flask experiments. A strain of Acidithiobacillus thiooxidans was employed in the assays using secondary and anaerobic sludges, which resulted in similar solubilization yields of the metals chromium, copper, lead, nickel, and zinc for both the sludges investigated. The effect of initial pH (7.0 and 4.0) on metal bioleaching was assayed by using the anaerobic sludge inoculated with indigenous sulfur-oxidizing thiobacilli. Although the time required to reach the end of the experiment (final pH close to 1.0) was shortened at initial pH of 4.0, final metal solubilization was not significantly different for both initial pH values, resulting in higher solubilization yields for copper, nickel, and zinc (higher than 80%). Chromium and lead presented solubilization yields close to 50%. The results obtained in this work showed that the metal bioleaching process can be applied to sewage sludge regardless of the type of sludge and without the requirement of pH adjustment.

Effects of several applications of digested sewage sludge on soil and plants

Sewage sludge production has significantly increased during the last years in European Union (EU) countries, being primarily used for agricultural purposes. In this study, digested sewage sludge was applied to greenhouse soil over a three-year period (2001--2003), with three sludge treatments in the first two years (2, 4, and 6 kg m(-2)) and three more applications using a greater quantity in the last year (6, 8, and 10 kg m(-2)). The effects of sewage sludge application on soil and on a leafy crop (Lactuca sativa L.) were studied. Mineral elements, organic matter, pH, and heavy metals were measured in soil and plant tissues. Pathogen and indicator microorganism dynamics in soil were also determined after each sludge application. Results showed that sewage sludge applications increased organic matter, P, and N Ca content in soil. Furthermore, Zn, Pb, Ni, and Cu content increased in soils, primarily after high doses of sludge. The highest yield value was obtained in the second-year harvest, since the last sludge application did not increase yield. Fecal coliform numbers decreased significantly one month after sludge application. However, total coliforms, Clostridium sulphite-reducers and Salmonella, were present in soils three months after sludge application.