Volatility of heavy metals during incineration of tannery sludge in the presence of chlorides and phosphoric acid.

Knowledge of the behaviour of heavy metals in the combustion process is a most important factor in selecting disposal alternatives for waste materials. Accordingly, in this work, the vaporization behaviour of highly concentrated heavy metals (Pb, Zn, Cu and Cr) in tannery sludge were investigated experimentally. The sludge was spiked with various chlorine compounds (i.e. PVC, FeCl3, CaCl2 and NaCl) and pre-treated with phosphoric acid in order to evaluate the capacity of enhancement and weakening of the volatility of the heavy metals contained in tannery sludge. The experimental results show that the vaporization percentages increased with increasing ratio of Cl/sludge and temperature, and the accelerating and increasing effect of the addition of chlorides on the vaporization percentage of heavy metals was dependent on the release capacity of chlorine radicals. The vaporization percentages of lead and zinc increased by 15-20%, whereas those of copper and chromium increased by only about 3 and 10% at 800 degrees C. However, heavy metals were not expected to be completely released in the combustion process in spite of the high ratio of Cl/sludge. Alternately, heavy metals contained in tannery sludge can be immobilized effectively by pre-treatment with phosphoric acid. When the 85% phosphoric acid accounted for 10% of dry basis of tannery sludge, the phosphate-treated sludge showed the lowest vaporization percentage of about 3-15% with formation of Ca18Cu3(PO4)14, Ca9Cr(PO4)7, Ca19Zn2(PO4)14 and PbMgP2O7 in the bottom ash.     

A study of heavy metals and their fate in the composting of tannery sludge

Tannery waste is categorized as toxic and hazardous in Malaysia due to its high content of Cr (in excess of 500 mg/kg) and other heavy metals. Heavy metals, when in high enough concentrations, have the potential to be both phytotoxic and zootoxic. Heavy metals are found as contaminants in tannery sludge. This investigation aimed to identify the fate of chromium, cadmium, copper, lead, and zinc concentrations in tannery sludge throughout a 50-day composting cycle. The results of this study showed a general increase in the removal of Cr, Cd, Pb, and to a much smaller extent Zn and Cu, manifested by a decrease in their overall concentrations within the solid fraction of the final product (the decreases were likely the result of leaching). Furthermore, in using a sequential extraction method for sludge composting at different phases of treatment, a large proportion of the heavy metals were found to be associated to the residual fraction (70-80%) and fractions more resistant to extraction, X-NaOH, X-EDTA, X-HNO3 (12-29%). Less than 2% of the metals were bound to bioavailable fractions X-(KNO3+H2O).     

Treatment of copper industry waste and production of sintered glass-ceramic.

Copper waste is iron-rich hazardous waste containing heavy metals such as Cu, Zn, Co, Pb. The results of leaching tests show that the concentration of these elements exceeds the Turkish and EPA regulatory limits. Consequently, this waste cannot be disposed of in its present form and therefore requires treatment to stabilize it or make it inert prior to disposal. Vitrification was selected as the technology for the treatment of the toxic waste under investigation. During the vitrification process significant amounts of the toxic organic and inorganic chemical compounds could be destroyed, and at the same time, the metal species are immobilized as they become an integral part of the glass matrix. The copper flotation waste samples used in this research were obtained from the Black Sea Copper Works of Samsun, Turkey. The samples were vitrified after being mixed with other inorganic waste and materials. The copper flotation waste and their glass-ceramic products were characterized by X-ray analysis (XRD), scanning electron microscopy and by the toxicity characteristic leaching procedure test. The products showed very good chemical durability. The glass-ceramics fabricated at 850 degrees C/2 h have a large application potential especially as construction and building materials.     

Immobilization of copper flotation waste using red mud and clinoptilolite

The flash smelting process has been used in the copper industry for a number of years and has replaced most of the reverberatory applications, known as conventional copper smelting processes. Copper smelters produce large amounts of copper slag or copper flotation waste and the dumping of these quantities of copper slag causes economic, environmental and space problems. The aim of this study was to perform a laboratory investigation to assess the feasibility of immobilizing the heavy metals contained in copper flotation waste. For this purpose, samples of copper flotation waste were immobilized with relatively small proportions of red mud and large proportions of clinoptilolite. The results of laboratory leaching demonstrate that addition of red mud and clinoptilolite to the copper flotation waste drastically reduced the heavy metal content in the effluent and the red mud performed better than clinoptilolite. This study also compared the leaching behaviour of metals in copper flotation waste by short-time extraction tests such as the toxicity characteristic leaching procedure (TCLP), deionized water (DI) and field leach test (FLT). The results of leach tests showed that the results of the FLT and DI methods were close and generally lower than those of the TCLP methods.     

Management of incinerator residues in Flanders (Belgium) and in neighbouring countries. A comparison

This paper covers the Flemish legislative tools concerning the management of bottom ash, fly ash and APC residue from municipal waste incinerators, with respect to their contamination with heavy metals. The situation in Flanders is compared to the one in the Walloon region, The Netherlands, Germany and France. Waste management in the countries considered differs on the level of available management options, of leaching tests and of limit values. To make an indicative comparison of leaching tests and limit values in the different countries, leaching tests were carried out on bottom ash and fly ash, and the results are compared to the relevant limit values for recycling and landfilling of the different countries. The comparison of legislations as well as the leaching results show that discrepancies in waste management between the different regions and countries exist. Recently, European limit values for landfilling became available. European legislation on recycling, however, has not been developed and urgently needs to be considered and drafted as the market for recycling can be expanding rapidly.     

Clay-sewage sludge co-pyrolysis. A TG-MS and Py-GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Wastewater sewage sludge was co-pyrolyzed with a well characterized clay sample, in order to evaluate possible advantages in the thermal disposal process of solid waste. Characterization of the co-pyrolysis process was carried out both by thermogravimetric-mass spectrometric (TG-MS) analysis, and by reactor tests, using a lab-scale batch reactor equipped with a gas chromatograph for analysis of the evolved gas phase (Py-GC).Due to the presence of clay, two main effects were observed in the instrumental characterization of the process. Firstly, the clay surface catalyzed the pyrolysis reaction of the sludge, and secondly, the release of water from the clay, at temperatures of approx. 450-500 °C, enhanced gasification of part of carbon residue of the organic component of sludge following pyrolysis.Moreover, the solid residue remaining after pyrolysis process, composed of the inorganic component of sludge blended with clay, is characterized by good features for possible disposal by vitrification, yielding a vitreous matrix that immobilizes the hazardous heavy metals present in the sludge.     

Study of the treatment effectiveness of a solidification/stabilization process for waste bearing heavy metals

The sludge from a steel processing unit bearing zinc, lead, iron, and manganese was solidified with ordinary Portland cement. The waste was stabilized in the specimens with a waste/binder ratio range of 0.16–4.0. On the basis of the available leaching and unconfined compressive strength, the performance of the solidified/stabilized waste was compared for different numbers of curing days. It was found that curing up to 28 days resulted in a performance improvement, as shown by less leaching of heavy metals and the increased unconfined compressive strength of the specimen. The treatment effectiveness of the solidification/stabilization process was assessed for the metals Pb, Zn, Fe, and Mn, and was found to be 89%, 95%, 74%, and 90%, respectively, for an optimum ratio of 4.0 after 28 days of curing.     

Characteristic of heavy metals in biochar derived from sewage sludge

This study investigates the characteristic of heavy metals (Pb, Zn, Cu, Cd, Cr, Ni and As) in biochar derived from sewage sludge at different pyrolysis temperatures (300, 400, 500, 600 and 700 °C). The heavy metal concentrations, chemical speciation distribution, leaching toxicity, and bio-available contents were investigated using ICP-OES after microwave digestion, a sequential extraction procedure recommended by the Community Bureau of Reference (BCR), an improved nitric acid–sulphuric acid method, and diethylenetriamine pentaacetic acid (DTPA) extraction method, respectively. The results showed that a great percentage of the heavy metals remained in biochar, the concentrations of heavy metals in biochar (except Cd in B7) were higher than that in sludge, and the enrichment of the heavy metals in biochar enhanced with the pyrolysis temperature. Although the effect of pyrolysis temperature on the chemical speciation distribution, the leaching toxicity and the bio-available contents of heavy metals in biochar was inconsistent, the potential risk of biochar on soil and groundwater contamination was lower than sewage sludge.     

Environmental and economic life cycle assessment for sewage sludge treatment processes in Japan

Life cycle assessment for sewage sludge treatment was carried out by estimating the environmental and economic impacts of the six alternative scenarios most often used in Japan: dewatering, composting, drying, incineration, incinerated ash melting and dewatered sludge melting, each with or without digestion. Three end-of-life treatments were also studied: landfilling, agricultural application and building material application. The results demonstrate that sewage sludge digestion can reduce the environmental load and cost through reduced dry matter volume. The global warming potential (GWP) generated from incineration and melting processes can be significantly reduced through the reuse of waste heat for electricity and/or heat generation. Equipment production in scenarios except dewatering has an important effect on GWP, whereas the contribution of construction is negligible. In addition, the results show that the dewatering scenario has the highest impact on land use and cost, the drying scenario has the highest impact on GWP and acidification, and the incinerated ash melting scenario has the highest impact on human toxicity due to re-emissions of heavy metals from incinerated ash in the melting unit process. On the contrary, the dewatering, composting and incineration scenarios generate the lowest impact on human toxicity, land use and acidification, respectively, and the incinerated ash melting scenario has the lowest impact on GWP and cost. Heavy metals released from atmospheric effluents generated the highest human toxicity impact, with the effect of dioxin emissions being significantly lower. This study proved that the dewatered sludge melting scenario is an environmentally optimal and economically affordable method.     

Phosphate fertilizer from sewage sludge ash (SSA)

Ashes from sewage sludge incineration are rich in phosphorus content, ranging between 4% and 9%. Due to the current methods of disposal used for these ashes, phosphorus, which is a valuable plant nutrient, is removed from biological cycling. This article proposes the possible three-stage processing of SSA, whereby more than 90% of phosphorus can be extracted to make an adequate phosphate fertilizer. SSA from two Swiss sewage sludge incinerators was used for laboratory investigations. In an initial step, SSA was leached with sulfuric acid using a liquid-to-solid ratio of 2. The leaching time and pH required for high phosphorus dissolution were determined. Inevitably, dissolution of heavy metals takes place that would contaminate the fertilizer. Thus in a second step, leach solution has to be purified by having the heavy metals removed. Both ion exchange using chelating resins and sulfide precipitation turned out to be suitable for removing critical Cu, Ni and Cd. Thirdly, phosphates were precipitated as calcium phosphates with lime water. The resulting phosphate sludge was dewatered, dried and ground to get a powdery fertilizer whose efficacy was demonstrated by plant tests in a greenhouse. By measuring the weight of plants after 6 weeks of growth, fertilized in part with conventional phosphate fertilizer, fertilizer made from SSA was proven to be equal in its plant uptake efficiency.     

Waste paper and clinoptilolite as a bulking material with dewatered anaerobically stabilized primary sewage sludge (DASPSS) for compost production

Environmental problems associated with sewage sludge disposal have prompted strict legislative actions over the past few years. At the same time, the upgrading and expansion of wastewater treatment plants have greatly increased the volume of sludge generated. The major limitation of land application of sewage sludge compost is the potential for high heavy metal content in relation to the metal content of the original sludge. Composting of sewage sludge with natural zeolite (clinoptilolite) can enhance its quality and suitability for agricultural use. However, the dewatered anaerobically stabilized primary sewage sludge (DASPSS) contained a low concentration of humic substances (almost 2%), and the addition of the waste paper was necessary in order to produce a good soil conditioner with high concentrations of humics. The final results showed that the compost produced from DASPSS and 40-50% w/w of waste paper was a good soil fertilizer. Finally, in order to estimate the metal leachability of the final compost product, the generalized acid neutralization Capacity (GANC) procedure was used, and it was found that by increasing the leachate pH, the heavy metal concentration decreased. The application of the sequential chemical extraction indicated that metals were bound to the residual fraction characterized as a stabilize fractions.     

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.     

Hybrid cement-assisted dewatering,solidification and stabilization of sewage sludge with high organic content

Sewage sludge with high organic content is particularly difficult to dewater before disposal in landfill. In this study, different hybrid cement binders were investigated to evaluate their ability to dewater the sewage sludge with high organic content. After 7 days of stabilization, the CASC (Mayenite/Sulfoaluminate cement) hybrid binder showed an excellent efficiency on both water content reduction and strength development; the water content and unconfined compressive strength value of solidified sludge reached 52.43 % and 109.55 kPa, respectively, at 8 % binder/sludge mass rate. The horizontal vibration leaching test (HJ 557-2009) indicated that leachability of heavy metals of the CASC-solidified sludge was far lower than that of non-solidified sludge and CAPC-solidified sludge. Furthermore, SEM and XRD analyses suggested that certain hydrates formed in the solidification process might have accelerated the depletion of interstitial water and strength development in the CASC-solidified sludge.     

Impacts of ultrasound on selective leaching recovery of heavy metals from metal-containing waste sludge

Lab synthesized metal-bearing sludge (LSMS) was used in series of designed lab tests to evaluate impacts of ultrasound on selective separation of heavy metals through acid leaching. The tests eliminated the potential of induced bias generated by utilizing field sludge that were produced from different location sources. The results showed that metal pairs of Cu and Fe, Cu and Cr, and Cr and Fe inside LSMS could be practically separated with one metal being contained in a liquid phase and another in a solid phase through acid leaching processes enhanced by ultrasound. With assistance of ultrasound, the acid leaching demonstrated a more efficient segregation between metals within LSMS than a conventional leaching that doesn’t have ultrasonic enhancement, and the tests provided in a generic means that ultrasonically enhanced acid leaching could cost-efficiently recover heavy metals from metal-containing waste sludge.     

Agronomic characterisation of different types of sewage sludge: Policy implications

Spain is one of the main municipal sewage sludge producers of Europe. This paper aims to agronomically characterise different types of sewage sludge stabilised by different methods (anaerobically digested, composted, and pelletised) and deliver policy recommendations from the results of this characterisation. Anaerobic sewage sludge quality is found to be better in plants with a lower volume of water processing. Composted sludge shows the best quality from a heavy metal point of view, but its low available nitrogen content increases the input of heavy metals when spread, as compared to digested or pelletised sludge. Pelletised sludge has higher heavy metal content than anaerobically digested sludge. Despite the good quality of the sludges, future regulations, especially with regard to Cd levels, will limit the use of this waste in agriculture.     

Neutralization of an extremely acidic sludge and stabilization of heavy metals in flyash aggregates

An extremely acidic, heavy metal-rich sludge (pH=-1.2) was scrubbed with a Class-F fly ash in order to simultaneously neutralize the acidity and stabilize the heavy metals contained in both wastes. This paper outlines the leaching behavior of the aggregate material generated by scrubbing. For proper fly ash/sludge ratios, the fly ash acted as an outstanding neutralizer for the acidic waste. Leaching of heavy metals from the aggregate samples was below the environmental limits within a pH range between 3 and 9. Subsequent washing of the leached aggregate with acidic CALWET solutions did not result in an additional release of heavy metals. It is proposed that coordinative bonding of the metal cations onto neutral surface sites and electrostatic adsorption led to stabilization of the heavy metals within the aggregate structure below hydrolysis pHs.     

Immobilisation of heavy metal in cement-based solidification/stabilisation: a review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.     

Land application of biosolids. Soil response to different stabilization degree of the treated organic matter

The effect of land application of biosolids on an agricultural soil was studied in a 2-month incubation experiment. The soil microbial biomass and the availability of heavy metals in the soil was monitored after the application of four different composting mixtures of sewage sludge and cotton waste, at different stages of composting. Land application caused an increase of both size and activity of soil microbial biomass that was related to the stabilization degree of the composting mixture. Sewage sludge stabilization through composting reduced the perturbance of the soil microbial biomass. At the end of the experiment, the size and the activity of the soil microbial biomass following the addition of untreated sewage sludge were twice those developed with mature compost. For the mature compost, the soil microbial biomass recovered its original equilibrium status (defined as the specific respiration activity, qCO2) after 18 days of incubation, whereas the soil amended with less stabilized materials did not recover equilibrium even after the two-month incubation period. The stabilization degree of the added materials did not affect the availability of Zn, Ni, Pb, Cu, Cr and Cd in the soil in the low heavy metal content of the sewage sludge studied. Stabilization of organic wastes before soil application is advisable for the lower perturbation of soil equilibria status and the more efficient C mineralization.     

Characterization of a thermal power plant air heater washing waste: a case study from Iran.

In Iran most of the electricity is generated by thermal power plants. As a result of fuel oil burning in winter time, the air heaters of the boilers have to be washed and cleaned frequently. The wastewater originating from air heater washing is then treated in an effluent treatment plant by chemical precipitation followed by dewatering of the sludge produced. The resulting waste is classified as specific industrial waste that should be characterized in detail under the Waste Management Act of Iran. The quantity of this waste produced in the studied power plant is about 20 tonnes year(-1). In the present investigation, the first to be carried out in Iran, seven composite samples of dewatered sludge from air heater washing wastewater treatment were subjected to investigation of the physical properties, chemical composition and leaching properties. The most likely pollutants that were of concern in this study were heavy and other hazardous metals (Cd, Co, Cr, Mn, Ni, Pb, Zn and V). The results revealed that mean pH, wet and dry density and moisture content of the waste were 6.31, 1532 kg m(-30, 1879 kg m(-3) and 15.35%, respectively. Magnetite, SiO2, P2O5, CaO, Al2O3 and MgO were the main constituents of the waste with a weight percentage order of 68.88, 5.91, 3.39, 2.64, 2.59 and 1.76%, respectively. The toxicity characteristic leaching procedure test results for some heavy and other hazardous metals showed that mean elemental concentrations of Cd, Co, Cr, Mn, Ni, Pb, V and Zn in leachate were 0.06, 1.55, 5.49, 36.32, 209.10, 0.58, 314.06 and 24.84 mg L(-1), respectively. According to the Waste Management Act of Iran this waste should be classified as hazardous and should be disposed of in accordance with hazardous waste disposal regulations.     

Flow analysis of metals in a municipal solid waste management system

This study aimed to identify the metal flow in a municipal solid waste (MSW) management system. Outputs of a resource recovery facility, refuse derived fuel (RDF) production facility, carbonization facility, plastics liquefaction facility, composting facility, and bio-gasification facility were analyzed for metal content and leaching concentration. In terms of metal content, bulky and incombustible waste had the highest values. Char from a carbonization facility, which treats household waste, had a higher metal content than MSW incinerator bottom ash. A leaching test revealed that Cd and Pb in char and Pb in RDF production residue exceeded the Japanese regulatory criteria for landfilling, so special attention should be paid to final disposal of these substances. By multiplying metal content and the generation rate of outputs, the metal content of input waste to each facility was estimated. For most metals except Cr, the total contribution ratio of paper/textile/plastics, bulky waste, and incombustible waste was over 80%. Approximately 30% of Cr originated from plastic packaging. Finally, several MSW management scenarios showed that most metals are transferred to landfills and the leaching potential of metals to the environment is quite small.