Effect of solids concentration on bacterial leaching of heavy metals from sewage sludge

Microbial metal leaching from sewage sludge (2-9% w/v) was carried out with the iron-oxidizing bacterium Thiobacillus ferrooxidans. Measurements of pH, oxidation-reduction potential, and concentration of Fe2+ indicated that T. ferrooxidans was effective in removing metals from an incubation bath containing less than 5% sludge solids concentration. Specifically, Cu leaching was completely suppressed at a high solids concentration of 9% (w/v). Results indicated that the deactivation of T. ferrooxidans at a high sludge content was mainly due to the presence of inhibiting materials such as organic matter. A mixed culture of sulfur-oxidizing bacteria was obtained by enrichment from anaerobically digested sewage sludge to enhance the efficiency of the microbial leaching process. These bacteria were much more effective in metal leaching than was iron-oxidizing T. ferrooxidans. At 9% (w/v) solids concentration, the leaching efficiencies of Zn and Cu were 78% (2.66 g/kg dry sludge) and 59% (1.36 g/kg dry sludge), respectively. Therefore, when removing heavy metals from the anaerobically digested sewage sludge, the indigenous sulfur-oxidizing bacteria isolated in the current study were more efficient than T. ferrooxidans, especially at high sludge solids concentrations.     

Bioleaching of heavy metals from livestock sludge by indigenous sulfur-oxidizing bacteria: effects of sludge solids concentration

A technologically and economically feasible process called bioleaching was used for the removal of heavy metals from livestock sludge with indigenous sulfur-oxidizing bacteria in this study. The effects of sludge solids concentration on the bioleaching process were examined in a batch bioreactor. Due to the buffering capacity of sludge solids, the rates of pH reduction, ORP rise and metal solubilization were reduced with the increase of the solids concentration. No apparent influence of solids concentration on sulfate produced by sulfur-oxidizing bacteria was observed when the solids concentration was less than 4% (w/v). A Michaelis-Menten type of equation was able to well describe the relationship between solids concentration and rate of metal solubilization. Besides, high efficiencies of metal solubilization were achieved after 16 d of bioleaching. Therefore, the bioleaching process used in this study could be applied to remove heavy metals effectively from the livestock sludge.     

Bacterial leaching of metals from sewage sludge by indigenous iron-oxidizing bacteria

Bioleaching of metals can be achieved in sewage sludge using Thiobacillus ferrooxidans, which obtains its energy requirements from the oxidation of added ferrous iron. The purpose of this study was to verify the presence of indigenous T. ferroxidans and to evaluate their adaptive capacity and leaching potential. Nineteen sludges (primary, secondary, aerobically and anaerobically digested, oxidation pond) were tested and all of them contained indigenous iron-oxidizing bacteria. The acclimation of these organisms by successive transfers allowed a rise of sludge redox potential over 450 mV and a decrease of sludge pH between 3.8 and 2.2 over a 10-day incubation period. The metal solubilization efficiencies were Cd: 55-98%, Cr: 0-32%, Cu: 39-94%, Mn: 71-98%, Ni: 37-98%, Pb: 0-31% and Zn: 66-98%, were reached with these indigenous strains. The results obtained show that the metal bioleaching may be easily realized by direct acclimation of sludge microflora.     

Bioleaching of heavy metals from anaerobically digested sewage sludge using FeS2 as an energy source

The effect of using FeS2 as an energy source, on the bioleaching of heavy metals (Zn, Cr, Cu, Pb and Ni) and nutrients (nitrogen and phosphorus) from anaerobically digested sludge using isolated indigenous iron-oxidizing bacteria was investigated in this paper. Addition of FeS2 in the range of 0.5-4.0 g l(-1) accelerated the acidification of sludge and raised the oxidation-reduction potential of sludge medium with an inoculation of 15% (v/v) of active bacteria, thus resulting in an overall increase in metal dissolution efficiency. After 16 days of bioleaching at 28 degrees C and an initial pH of 3.0, up to 99% of Zn, 65% of Cr, 74% of Cu, 58% of Pb and 84% of Ni can be removed from the sludge. In contrast, only 94% of Zn, 12% of Cr, 21% of Cu, 32% of Pb and 38% of Ni were leached out in the control without inoculation of iron-oxidizing bacteria and the addition of FeS2. Less than 15% of nitrogen and 6% of phosphorous were lost after 16 days of bioleaching when using FeS2 as the energy source. Comparing to 39% and 45% loss respectively for these two nutrients when using FeSO4.7H2O as the energy source, FeS2 appears to be a more suitable energy source for preserving nutrients in sludge while removing heavy metals from sludge.     

Removal of heavy metals from anaerobically digested sewage sludge by isolated indigenous iron-oxidizing bacteria

The removal of heavy metals (Cr, Cu, Zn, Ni and Pb) from anaerobically digested sludge from the Yuen Long wastewater treatment plant, Hong Kong, has been studied in a batch system using isolated indigenous iron-oxidizing bacteria. The inoculation of indigenous iron-oxidizing bacteria and the addition of FeSO4 accelerated the solubilization of Cr, Cu, Zn, Ni and Pb from the sludge. pH of the sludge decreased with an increase in Fe2+ concentrations and reached a low pH of 2-2.5 for treatments receiving both bacterial inoculation and FeSO4. After 16 days of bioleaching, the following heavy metal removal efficiencies were obtained: Cr 55.3%, Cu 91.5%, Zn 83.3%, Ni 54.4%, and Pb 16.2%. In contrast, only 2.6% of Cr, 42.9% of Cu, 72.1% of Zn, 22.8% of Ni and 0.56% of Pb were extracted from the control without the bacterial inoculation and addition of FeSO4. The residual heavy metal content in the leached sludge was acceptable for unrestricted use for agriculture. The experimental results confirmed the effectiveness of using the isolated iron-oxidizing bacteria for the removal of heavy metals from sewage sludge.     

Removal of DEHP in composting and aeration of sewage sludge

The potential of composting and aeration to remove bis(2-ethylhexyl) phthalate (DEHP) from municipal sewage sludge was studied with two dewatered sludges: raw sludge and anaerobically digested sludge. Composting removed 58% of the DEHP content of the raw sludge and 34% of that of the anaerobically digested sludge during 85 days stabilisation in compost bins. A similar removal for the anaerobically digested sludge was achieved in a rotary drum in 28 days. Less than 1% of DEHP was removed with the compost leachate. Although DEHP removal was greater from raw sludge compost than anaerobically digested sludge compost, the total and volatile solids removals were on the same level in the two composts. In the aeration of raw sludge at 20 degrees C the DEHP removals were 33-41% and 50-62% in 7 and 28 days, respectively. Both composting and aeration are concluded to have the potential to reduce the DEHP contents typically found in sewage sludges to levels acceptable for agricultural use.     

氧化亚铁硫杆菌对电动力处理城市污泥中重金属影响研究

分析了氧化亚铁硫杆菌对污泥中重金属形态及对电动力处理污泥中重金属的去除率影响.氧化亚铁硫杆菌处理后的污泥,pH值降低,部分重金属从稳定态转化为可溶态.经氧化亚铁硫杆菌预处理后的泥样和未经预处理的泥样,在同样条件下用电动力处理,处理结果为:预处理后Cd的去除率从61.85%上升到72.54%,Cu从34.26%上升到63.51%,Zn从65.06%上升到7L 22%.预处理对Cu的影响比Cd、Zn大得多.用电动力处理Cu超标污泥中重金属时,将污泥先用氧化亚铁硫杆菌预处理,可有效地提高其去除效果,处理后的污泥有利于农用.     

Degradation of inhibitory substances by heterotrophic microorganisms during bioleaching of heavy metals from anaerobically digested sewage sludge

The presence of organic acids was found to be inhibitory to the bioleaching of sewage sludge and the objective of the present study was to elucidate the roles of heterotrophic microorganisms in removing organic acids during the bioleaching of heavy metals from anaerobically digested sewage sludge. Microbiological analysis showed that acetic and propionic acids posed a severe inhibitory effect on iron-oxidizing bacteria as reflected by a sharp decrease in their viable counts in the first 4d and it only started to increase 2d after the depletion of both acids. Biodegradation of these inhibitory organic acids was revealed by sharp increases in total fungi and acidophiles between day 3 and day 5 which coincided with degradation of organic acids. This was further confirmed by the increases in total counts of both acetate and propionate degraders in the same period. Two yeast strains Y4 and Y5 with strong ability to degrade acetate and/or propionate were isolated and identified as Pichia sp. and Blastoschizomycetes capitatus, respectively. B. capitatus Y5 was an more important player in removing the inhibitory organic acids during the bioleaching process since it could utilize both acetate and propionate as sole carbon source while Pichia sp. Y4 was an strict acetate degrader. Results from the present study not only provided the evidence for biodegradation of organic acids by heterotrophs, but also disclosed a biological mechanism for the initiation of bioleaching of organic acid-laden sewage sludge.     

Comparison of bio-dissolution of spent Ni-Cd batteries by sewage sludge using ferrous ions and elemental sulfur as substrate

Bioleaching of spent Ni-Cd batteries using acidified sewage sludge was carried out in a continuous flow two-step leaching system including an acidifying reactor and a leaching reactor. Two systems operated about 30d to achieve almost complete dissolution of heavy metals Ni, Cd and Co in four Ni-Cd batteries. Ferrous sulphate and elemental sulfur were used as two different substrates to culture indigenous thiobacilli in sewage sludge. pH and ORP of the acidifying reactor was stabilized around 2.3 and 334mV for the iron-oxidizing system and 1.2 and 390mV for the sulfur-oxidizing system. It was opposite to the acidifying reactor, the pH/ORP in the leaching reactor of the iron-oxidizing system was relatively lower/higher than that of the sulphur-oxidizing system in the first 17d. The metal dissolution, in the first 12-16d, was faster in the iron-oxidizing system than in the sulphur-oxidizing system due to the lower pH. In the iron-oxidizing system, the maximum solubilization of cadmium (2500mg l(-1)) and cobalt (260mg l(-1)) can be reached at day 6-8 and the most of metal nickel was leached in the first 16d. But in the sulphur-oxidizing system there was a lag period of 4-8d to reach the maximum solubilization of cadmium and cobalt. The maximum dissolution of nickel hydroxide (1400mg l(-1)) and metallic nickel (2300mg l(-1)) occurred at about day 12 and day 20, respectively.     

Heavy metals removal from anaerobically digested sludge by chemical and microbiological methods

The metal removal from anaerobically digested sludge was studied by chemical treatment and microbial leaching processes in laboratory reactors. The removal of metals increased with decreased sludge solid concentration and pH. In the acid treatment process, a pH of 1.5 was required to reduce the copper concentration in the sludge to an acceptable level. In the microbial processes mixed culture gave 10% better solubilisation of metals than in single culture. The acid requirement was lower for the microbial process. The cost of sludge treatment in terms of chemicals was found to be decreased by 80% in microbial leaching.     

Enhancing aerobic digestion potential of municipal waste-activated sludge through removal of extracellular polymeric substance

A protease-secreting bacteria was used to pretreat municipal sewage sludge to enhance aerobic digestion. To enhance the accessibility of the sludge to the enzyme, extracellular polymeric substances were removed using citric acid thereby removing the flocs in the sludge. The conditions for the bacterial pretreatment were optimized using response surface methodology. The results of the bacterial pretreatment indicated that the suspended solids reduction was 18 % in sludge treated with citric acid and 10 % in sludge not treated with citric acid whereas in raw sludge, suspended solids reduction was 5.3 %. Solubilization was 10.9 % in the sludge with extracellular polymeric substances removed in contrast to that of the sludge with extracellular polymeric substances, which was 7.2 %, and that of the raw sludge, which was just 4.8 %. The suspended solids reduction in the aerobic reactor containing pretreated sludge was 52.4 % whereas that in the control reactor was 15.3 %. Thus, pretreatment with the protease-secreting bacteria after the removal of extracellular polymeric substances is a cost-effective and environmentally friendly method.     

Middle-thermophilic sulfur-oxidizing bacteria Thiomonas sp. RAN5 strain for hydrogen sulfide removal

Hydrogen sulfide (H2S) is one of the most toxic and offensively odorous gases and is generated in anaerobic bioreactors. A middle-thermophilic sulfur-oxidizing bacterium (SOB), Thiomonas sp. strain RAN5, was isolated and applied for H2S removal from both artificial and anaerobically digested gas. When a bioreactor containing medium inoculated with RAN5 was aerated continuously with artificial gas (containing 100 ppm H2S) at 45 degrees C for 156 hr, the H2S concentration in the vented gas was reduced by 99%. This was not affected by the presence of other microbes in the bioreactor The H2S removal efficiency of the RAN5 bioreactor for anaerobically digested gas was greater than 99% at influent H2S concentrations ranging from 2 to 1800 ppm; the efficiency decreased to 90% at influent H2S concentrations greater than 2000 ppm. Thiomonas sp. strain RAN5 cannot survive at room temperature, and thus its leakage from a wastewater treatment plant would not damage sewage systems. These data suggest that Thiomonas sp. strain RAN5 may be a useful microorganism for H2S removal.     

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.     

造纸白泥和粉煤灰的添加对污泥消化液中氮磷回收的影响

污泥厌氧消化液中含有丰富的氮磷，若直接排放到环境中，将会对附近水体造成严重污染。由于消化液中Mg^2＋和Ca^2＋的含量很低，严重影响了氮磷的回收效果。把造纸白泥和粉煤灰引入到污泥厌氧消化液氮磷的回收当中，可以明显地提升消化液pH和提高PO4^3-P和NH3-N回收率。实验结果表明：当造纸白泥添加量为4g／（L·h）时，曝气12h后，pH可达10．19，此时PO4^3-P和NH3-N回收率分别达到64％和45％；而当粉煤灰添加量为4g／（L·h）时，曝气12h后，pH达到9．63，PO4^3-P和NH3-N回收率分别为46％和41％。但仅用曝气方式处理，12h后，pH值仅为8．52，PO4^3-P和NH3-N回收率分别只有20％和18％。实验结果还表明，水力停留时间（HRT）越大，pH上升速度越快，幅度越大，氮磷的回收效果就越好。     

Stabilization of heavy metals in municipal sewage sludge by freeze–thaw treatment with a blend of diatomite,FeSO4, and Ca(OH)2

In this work, the effects of diatomite with 15% FeSO₄?7H₂O and 7.5% Ca(OH)₂ on sludge stabilization were investigated using batch leaching tests. The influence of cell rupture caused by freezing and thawing on stabilization was also evaluated. The results indicated that the optimal diatomite percentage was 2%. Cell rupture by freezing and thawing reduced heavy metal leachability, followed by cell death and decrease of organic groups. The concentration of heavy metals in sludge leachate increased after cell rupture, indicating that the heavy metal leachability was reduced after freezing and thawings. Moreover, the stabilization effects were generally improved after freezing and thawing. As compared with the stabilization of the original sludge, the unstable fractions decreased and the residual fractions of the heavy metals increased in the stabilized sludge after cell rupture.

Implications: This study developed a method to stabilize heavy metals in municipal sewage sludge. Diatomite combined with FeSO₄·7H₂O and Ca(OH)₂ improved the treatment of sewage sludge contaminated by heavy metals. Cell lysis by freeze–thaw treatment reduced the risk of leaching heavy metals caused by cell death and decreased major organic groups in the sludge.     

Emission characteristics for co-combustion of leather wastes,sewage sludge,and coal in a laboratory-scale entrained flow tube furnace

Four different mixed fuels consisted of leather waste, coal, and sewage sludge were combusted in a lab-scale entrained flow fluidized bed furnace. The influence of blending ratio on emission characteristics of SO₂, NO_x, HCl, particulate matter (PM), heavy metals, and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) was studied. Results showed that the mixing of coal with sewage sludge had a complex effect on the emission characteristics. On the one hand, with more sewage sludge blending in the mixed fuel, the acid gas pollutant (SO₂, NO_x) decreased a lot, and the recovery of volatile heavy metals (Cd, Pb) increased at the same time. Furthermore, the leaching toxicity of Cr in the fly ash and bottom ash went down below the national standard with the adding of sewage sludge. On the other hand, the mixing of sewage sludge which consisted of more ash content resulted in the increase of the PM emission. Moreover, the high content of Cu and chlorine in the sewage sludge can promote the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) when the fuel 3 and 4 were combusted. Most importantly, the concentration of toxic PCDD/Fs in the flue gas produced from fuel 3 and fuel 4 was successfully controlled down below 0.20 ng I-TEQ/Nm³ by the active carbon.

     

污泥酸化速率影响因子的探讨

采用城市污水厂污泥作为嗜酸微生物菌株来源,通过添加一定量的单质硫,使其中的嗜酸硫杆菌群大量增殖,并使污泥pH大幅降低.取得的培养物可用于废旧干电池中重金属沥滤等的处理.由于培养物对重金属沥滤效率和污泥的酸化速率密切相关,为此进行了不同的污泥种类、加硫量、污泥浓度和曝气强度对污泥酸化速率影响的实验.实验表明,初沉泥、二沉泥和混合浓缩污泥都能迅速利用硫产酸;加硫量(以100 mL污泥计)在0.5、1.0、2.0、4.0 g时,快速酸化的趋势相同.0.5 g的加硫量显示出略微慢的酸化速率,最后达到的最低pH在1.2左右,其他三个都降至1.0以下.污泥质量分数在0.5%、1.0%、2.0%、3.0%、4.0%时,也具有相同的酸化趋势,0.5%质量分数的污泥下降速率最快,这与较低浓度下污泥对pH的缓冲能力较小有关;曝气强度在0.45 L/min和0.3 L/min差别较小,5 d内能迅速降低pH,0.2 L/min的酸化速率较慢,足够长的时间(12 d)也能将pH降至2.5左右,0.1 L/min的曝气强度的酸化速率最慢,不能达到预期的酸化效果.     

Optimising the preparation of activated carbon from digested sewage sludge and coconut husk.

Preparation of activated carbon from sewage sludge is a promising way to dispose of sewage sludge as well as to produce a low-cost adsorbent for pollutant removal. This research work aimed to optimise the condition for activated carbon preparation from anaerobically digested sewage sludge with the additive coconut husk. The sewage sludge sample was mixed with the additive coconut husk. The preparation condition variables investigated involved the concentration of the ZnCl2 solutions, heating temperature, dwell time and heating rate in pyrolysis and the mixing ratio of coconut husk to sewage sludge. Surface area, pore size distribution, aqueous phenol adsorption capacity and the production yield of the final products were determined and compared. Experimental results revealed that low concentrations of ZnCl2 solution tended to improve the microporosity of the final product. Heating temperature had a considerable impact on the surface area, pore size distribution and phenol adsorption capacity of the final products, whereas dwell time and heating rate performed comparatively insignificantly. The effect of increasing the mixing ratio of coconut husk to sewage sludge was principally to increase the microporosity of the final products. The activated carbon with the highest BET surface area was produced with the activation of 5 M ZnCl2 solution and, thereafter, pyrolysis at a heating temperature of 500 degrees C for 2 h with a heating rate of 10 degrees C/min. The mixing ratio of 1:4 in terms of coconut husk to sewage sludge based on their dried weights was found to be most cost effective.     

Leaching of heavy metals and alkylphenolic compounds from fresh and dried sewage sludge

Reusing sewage sludge as a soil fertiliser has become a common alternative to disposal. Although this practice has a few benefits, it may contribute to the medium- and long-term contamination of the trophic chain because sewage sludge may contain heavy metals and organic contaminants. As the leaching of contaminants may depend on the sludge pre-treatment, the leaching of heavy metals (Cu, Ni, Pb, Zn and Cr) and alkylphenolic compounds (APCs) (octylphenol (OP), nonylphenol (NP), nonylphenol-mono-ethoxylate (NP₁EO)) was investigated in five fresh and 40 °C dried sewage sludge samples from north-eastern Spain. FT-IR analyses and full-scan GC-MS chromatograms showed that sludge drying changed the nature of organic compounds leading to changes in their solubility. Moreover, sludge drying led to a higher relative contribution of dissolved organic carbon than the particulate organic carbon in the leachates. Leaching of Pb, Zn and Cr was below 5 % in both fresh and dried sludge samples, whereas Cu and Ni leached at rates up to 12 and 43 %, respectively, in some of the dried sludge samples. The leaching yields of OP, NP and NP₁EO ranged from 1.3 to 35 % for fresh samples, but they decreased from 0.8 to 3.4 % in dried samples. The decrease in the leachability of APCs observed in dried sludge samples might be attributed to the fact that these compounds are associated with particulate organic matter, with significantly lower concentration or even absent in dried sludge than in fresh sludge samples. Therefore, it is recommended to dry the sludge before its disposal.     

The influence of organic amendment and nickel pollution on tomato fruit yield and quality

The effects of organic fertilization (sludge application) and/or different levels of Ni pollution on tomato fruit yield, quality, nutrition, and Ni accumulation were investigated. The mass loading of sewage sludge solids used in this study for the amendment of a calcareous soil with low organic matter content was 2% (w/w). A control with no sewage sludge amendment was also included (S). Nickel was added to the sludge amended soil at 0, 60, 120 and 240 mg kg-1 concentrations. Sewage sludge addition to the calcareous soil significantly increased fruit yield but did not adversely affect the quality and nutritional status of the tomato fruit. The results demonstrated that sewage sludge could be successfully used as a horticultural fertilizer. Only the highest addition rate of Ni (240 mg kg-1) to an organic amended calcareous soil had negative effects on fruit yield and quality, and caused a Ni accumulation in fruit that could be considered as a hazard for human health. Thus, no toxic problems will be encountered in tomato fruit due to Ni pollution provided the total Ni (soil Ni plus Ni incorporated with sludge amendment) concentration is kept below the maximum concentration of Ni allowed for agricultural alkaline soils in Spain (112 mg Ni kg-1).