Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting

A batch composting study was performed to evaluate the feasibility of co-composting sewage sludge with lime, aiming at reducing the availability of heavy metals in the sludge compost. Sewage sludge with sawdust as bulking agent was amended with lime at 0, 0.63, 1.0, and 1.63% w/w, and composted for 100 days in laboratory batch reactors. The changes in temperature, pH, electrical conductivity (EC), and extractable heavy metal contents were measured while compost maturity was determined by C/N(organic) and cress seed germination during the composting period. Liming raised pH of compost effectively at the initial stage of composting and caused a decrease in EC through precipitation of soluble ions. Lime amendment also significantly reduced water-soluble and Diethylene triamine pentracetic acid (DTPA)-extractable metal contents. The maximum reductions were 60 and 40% for Cu, 80 and 40% for Mn, 55 and 10% for Zn, and 20 and 25% for Ni at the end of the composting period for the lime-amended sludge as compared to the control. In spite of the inhibitory effect of lime amendment on the decomposition activity of sewage sludge, all treatments reached maturation after 63 days of composting as indicated by the results of C/N(organic) ratio and cress seed germination test results. A lime amendment of < or =1.0% is recommended to co-compost with sewage sludge.     

Residual metal concentrations in soils and leaf accumulations in tobacco a decade following farmland application of municipal sludge

The objectives of this investigation were to examine the long-term residual effects of metal loading through sewage sludge applications on the total vs. diethylene triamine pentacetic acid (DTPA) extractable metal concentrations in soil and leaf accumulations in tobacco. Maryland tobacco (Nicotiana tabacum L.), cv. 'MD 609', was grown in 1983 and 1984 at two sites in Maryland that had been amended in 1972 with dewatered, digested sewage sludge from washington, DC, at rates equal to 0, 56, 112 and 224 mg ha(-1). The metal concentrations in the sludge, in mg kg(-1) dry weight, were: 1300 Zn, 570 Cu, 280 Pb, 45 Ni and 13 Cd. Soil samples collected from the surface horizon and composite leaf samples of cured tobacco were analyzed for total Zn, Cu, Mn, Fe, Pb, Ni and Cd concentrations. The soil samples were also examined for soil pH and DTPA extractable metals. Equations were generated using polynomic and stepwise regression analyses which described the relationships between total vs. DTPA extractable soil metals, and between DTPA soil and soil pH vs. plant metal concentrations, respectively. Significant increases were observed for both total and DTPA extractable metal concentrations for all metals, with all but total Mn and Ni being significant for linear and quadratic effects regarding sludge rates. However, linear relationships were found between DTPA extractable vs. total soil concentrations for all elements except Pb and Ni which were quadratic. Significant increases in plant Zn, Cu, Mn, Ni and Cd and decreases in Fe were observed with increased sludge rates. Plant Pb levels were unaffected by sludge applied Pb. Linear relationships were observed between plant Zn and Cd and DTPA soil metal levels: however, Mn and Cu levels were described by quadratic and cubic relationship, respectively. Relationships between plant Fe and Pb and DTPA extractable concentrations were nonsignificant. Additional safeguards to protect crop contamination from heavy metals such as Cd were discussed.     

调理剂对堆肥产品重金属生物有效性的影响

城市污泥中重金属含量及其生物有效性是限制污泥农用的主要因素,因此,研究污泥堆肥化处理过程中重金属生物有效性,对污泥的农用具有重要意义。实验以城市污泥为原料,以菌菇渣和秸秆为调理剂,设置4个处理:A(污泥∶菌菇渣∶秸秆=1∶0.4∶0.025)、B(污泥∶菌菇渣∶秸秆=1∶0.3∶0.025)、C(污泥∶秸秆=1∶0.12)和D(污泥∶秸秆=1∶0.09),进行好氧堆肥实验,采用BCR顺序提取法测定各种形态的重金属,研究堆肥前后重金属形态的变化规律。结果表明,城市污泥中Cu、Ni、Pb和Cr主要以可氧化态及残渣态存在,生物有效性较低,而Zn和Cd主要以酸溶态和可还原态存在,生物有效性较高;堆肥过程显著降低了Cu、Zn、Ni和Pb的生物有效性,并改变了Cu、Zn、Ni、Pb、Cr和Cd的形态分布,使污泥中的Cu、Zn、Ni、Pb和Cd向着更稳定的可氧化态或残渣态转变;污泥经过堆肥处理后,Cu、Zn和Ni 3种重金属生物有效性关系为:ABCD,与其他处理相比,处理A残渣态的Pb和Cr增加比例较多,综合来看,处理A对重金属生物有效性的降低最为明显,重金属钝化效果最佳。     

Evolution of heavy metal speciation during the aerobic composting process of sewage sludge

The contents of heavy metals and their bioavailability to the soil-plant system were the major limitation to the application of sewage sludge compost in soil. This study was conducted to determine the evolution of heavy metal speciation in the course of an aerobic composting, and investigate the influence of changes of composting process parameters including pH, temperature and organic matter (OM) content on distribution of heavy metal speciation in composted sludge. The sequential extraction procedure developed by Tessier et al. was used in sludge compost to determine the heavy metal speciation. Results showed that, during composting, (1) the contents of the residue fraction for Pb, Zn and Cd were decreased but those for Ni and Cr were increased; the Cu residue fraction was almost constant; (2) the contents of the total mobile fractions (including fractions 1-4) for Zn and Pb were significantly increased, but the increase of those for Cu and Ni were not so remarkable; (3) there were significant degrees of correlation between heavy metal fractions and changes of some selected parameters (for example, pH, composting temperature, and OM content). Only the content of the total mobile fractions for Cu could be predictable from its total content. For the prediction of the total mobile fractions of Zn, Ni, Cd and Cr, the R(2) value was significantly increased by the inclusion of other variables such as pH, temperature and OM content.     

Sequential extraction of heavy metals during composting of sewage sludge

The major limitation of soil application of sewage sludge compost is the total heavy metal contents and their bioavailability to the soil-plant system. This study was conducted to determine the heavy metal speciation and the influence of changing the physico-chemical properties of the medium in the course of composting on the concentrations, bioavailability or chemical forms of Cu, Zn, Pb and Ni in sewage sludge. Principal physical and chemical properties and FTIR spectroscopical characterization of sludge compost during treatment show the stability and maturity of end product. The total metal contents in the final compost were much lower than the limit values of composts to be used as good soil fertilizer. Furthermore, it was observed by using a sequential extraction procedure in sludge compost at different steps of treatment, that a large proportion of the heavy metals were associated to the residual fraction (70-80%) and more resistant fractions to extraction X-NaOH, X-EDTA, X-HNO3 (12-29%). Less than 2% of metals bound to bioavailable fractions X-(KNO3+H2O). Heavy metal distribution and bioavailability show some changes during composting depending on the metal itself and the physico-chemical properties of the medium. Bioavailable fractions of all elements tend to decrease except Ni-H2O. Zn and mainly Cu present more affinity to organic and carbonate fractions. In contrast, Pb is usually preferentially bound to sulfide forms X-HNO3. Nickel shows a significant decrease of organic form. Significant degrees of correlation were found between heavy metal fractions and changes of some selected variables (e.g. pH, ash, organic matter, humic substance) during the course of composting. Mobile fractions of metals are poorly predictable from the total content. The R2 value was significantly increased by the inclusion of other variables such as the amount of organic matter (OM) and pH.     

The use of a sequential leaching procedure for assessing the heavy metal leachability in lime waste from the lime kiln at a caustizicing process of a pulp mill

A five-stage sequential leaching procedure was used to fractionate 13 heavy metals (Cd, Cu, Pb, Cr, Zn, Fe, Mn, Al, Ni, Co, As, V, Ba) and sulphur (S) in lime waste from the lime kiln at the causticizing plant of Stora Enso Oyj Veitsiluoto Pulp Mills at Kemi, Northern Finland, into the following fractions: (1) water-soluble fraction (H₂O), (2) exchangeable fraction (CH₃COOH), (3) easily reduced fraction (HONH₃Cl), (4) oxidizable fraction (H₂O₂ + CH₃COONH₄), and (5) residual fraction (HF + HNO₃ + HCl). Although metals were leachable in all fractions, the highest concentrations for most of the metals were observed in the residual fraction (stage 5). It was also notable that the total heavy metal concentrations in lime waste did not exceed the maximal allowable heavy metal concentrations for soil conditioner agents set by the ministry of the Agricultural and Forestry in Finland. The heavy metals concentrations in lime waste were also lower than the maximal allowable heavy metals concentrations of the European Union Directive 86/278/EEC on the protection of environment, and in particular of the soil, when sewage sludge is used in agriculture. The Ca concentration (420 g kg⁻¹; d.w.) was about 262 times higher than the typical value of 1.6 g kg⁻¹ (d.w.) in arable land in Central Finland. However, the concentration Mg (0.2 g kg⁻¹; d.w.) in lime waste was equal to the Mg concentration in arable land in the Central Finland. The lime waste has strongly alkaline pH (12.8) and a neutralizing value (i.e. liming effect) of 47.9% expressed as Ca equivalents (d.w.). This indicates lime waste to be a potential soil conditioner and improvement as well as a pH buffer.     

高温堆肥与蚯蚓堆肥对城市污泥重金属形态的影响

采用高温堆肥和蚯蚓堆肥工艺,研究了城市污泥与锯末、粉煤灰或磷矿粉按不同比例混合堆肥前后重金属（Cu、Pb、Zn、Cd和As）交换态、碳酸盐结合态、铁锰氧化物结合态、有机结合态和残留态的变化。研究表明：高温堆肥和蚯蚓堆肥前后各试验污泥的重金属形态和含量呈现出不同的变化,都可以降低污泥中交换态Cu、Pb、Zn、Cd和As的含量;对于Cu和Pb,高温堆肥优于蚯蚓堆肥;对于Zn、Cd和As,蚯蚓堆肥优于高温堆肥。2种堆肥方式中,粉煤灰用量为10%的钝化效果优于20%,磷矿粉的钝化效果同粉煤灰一样。     

Phytoavaelability and extractability of copper and zinc in calcareous soil amended with composted Urban wastes

Abstract

A greenhouse experiment was conducted under simulated field conditions using large‐capacity plastic pots, filled each one with 25 kg of air‐dried calcareous soil. Besides the control, four treatments were prepared by applying separately two rates (20 and 80 Mg ha^‐1) of municipal solid waste (MSW) compost, and co‐composted municipal solid waste and sewage sludge (MSW‐SS). Lettuce was planted and harvested 2.5 months later. The application of composted urban wastes tended to increase Cu concentration in lettuce with respect to the control, but it was only significant when the higher rate of MSW compost was applied. The control showed values of Zn concentration in plant within a deficient range. In general, composted urban wastes treatments had increased Zn concentration values, which were within the sufficiency range. Both treatments with MSW compost increased Cu and Zn uptake in comparison with MSW‐SS co‐compost treatments. At the postharvest, all composted urban wastes treatments increased significantly DTPA‐extractable Cu content in soil with respect to the control; it was also significant the increase in AAAc‐EDTA‐extractable Cu in soil produced by the addition of the higher rate of MSW compost. The application of composted urban wastes increased significantly DTPA‐extractable and AAAc‐EDTA‐extractable Zn contents in soil versus the control, except for the lower rate of MSW‐SS co‐compost. The values of DTPA‐extractable/total ratio for Cu and Zn were under 10%, except for the treatment applying the higher rate of MSW compost which promoted higher values. The values of AAAc‐EDTA‐extractable/total ratio for Cu were above 10% in all treatments including the control. This tendency was also observed in AAAc‐EDTA‐extractable/total ratio for Zn when applying both rates of MSW compost or the higher rate of MSW‐SS co‐compost.     

ESTROM—a Romanian–Swiss research programme

Background, aim, and scope  Composting is an effective treatment process to realize sludge land application. However, nitrogen loss could result in the reduction of nutrient value of the compost products and the stabilization effect of composting on heavy metal concentration and mobility in sludge has been shown to be very limited. Materials and methods  Laboratory-scale experiments were carried out to investigate the effects of bamboo charcoal (BC) on nitrogen conservation and mobility of Cu and Zn during sludge composting. Results  The result indicated that the incorporation of BC into the sludge composting material could significantly reduce nitrogen loss. With 9% BC amendment, total nitrogen loss at the end of composting decreased 64.1% compared with no BC amendment (control treatment). Mobility of Cu and Zn in the sludge may also have been lessened, based on the decline in diethylenetriaminepentaacetic acid-extractable Cu and Zn contents of composted sludge by 44.4% and 19.3%, respectively, compared to metal extractability in the original material. Discussion  Ammonia adsorption capability of BC might be the main reason for the retention of nitrogen in sludge composting materials. Decrease of extractable Cu²⁺ and Zn²⁺ in the composting material mainly resulted from the adsorption of both metals by BC. Conclusions  Incorporation of BC into composting material could significantly lessen the total nitrogen loss during sludge composting. Mobility of heavy metals in the sludge composting material could also be reduced by the addition of BC. Recommendations and perspectives  Bamboo charcoal could be an effective amendment for nitrogen conservation and heavy metal stabilization in sludge composts. Further research into the effect of BC-amended sludge compost material on soil properties, bioavailability of heavy metals, and nutrient turnover in soil needs to be carried out prior to the application of BC-sludge compost in agriculture.     

Correlation between inorganic (heavy metals) and organic (PCBs and PAHs) micropollutant concentrations during sewage sludge composting processes

The nature and congener composition of PCBs and PAHs present in sewage sludge composting processes was investigated. These studies included analysis of the most significant process parameters (such as pH, temperature, weight percentage variation) and in addition heavy metals whose typical composting speciation and behaviour were also considered in order to better understand organic compound time profiles. The significant correlation found between Pb, Cd, Cu and PCBs and between PAHs and Hg implies that quite a strong adsorption of PCBs onto organic matter takes place and also provides evidence for the volatilisation of PAHs. Chemical characteristics of inorganic species and organic compounds are summarised to account for the observed correlation and time trend profiles. Moreover, single congener concentrations demonstrate that the number of Cl substituents for PCBs and condensed benzene rings for PAHs determine to what extent they can be broken down for biodegradation and removed through volatilisation respectively.     

中国部分城市污泥中矿质元素形态与生物可利用性研究

近年来随着城市污水处理厂数目迅速增加,低成本、高效率的污泥农用受到越来越多的关注。研究从北到南的五家污水处理厂消化污泥中矿质元素的含量和形态分布,为污泥资源化的合理利用提供科学参考。结果表明：消化污泥总体呈高有机质、高氮磷、低K、高矿质元素营养的相似性,但其理化性质如阳离子交换容量(CEC)、pH等差异较大,对矿质元素的形态分布与生物可利用性有明显的影响。同种元素在不同污泥中含量的差异较大,相差数倍,甚至十倍以上。污泥中不同元素含量明显不同,Fe的浓度最高,次之是Mg、Zn、B的浓度最低。连续提取研究表明,污泥pH对污泥中矿质元素的形态分布有明显影响,酸性污泥(厦门污水处理厂)中各矿质元素酸溶／交换态百分率明显高于其他污泥。Cu主要分布在氧化态;Zn的三态分布百分率均在10％以上;Mg、Mn的酸溶／交换态含量较高;Fe主要分布在残渣态;B氧化态和还原态的百分率较高。建议在全国范围内进行一次污水处理厂污泥品质调查,为污泥合理农用提供科学依据。     

焦化废水处理过程外排污泥中重金属形态特征及其潜在环境风险

焦化废水处理过程所排放污泥中重金属的含量及化学形态是否构成环境风险将直接影响污泥处置方法的选择,为此,实验采用BCR顺序提取法分析了焦化废水处理站外排污泥中重金属(Cd、Hg、Pb、Cr、As、Ni、Zn、Cu和Mn)的形态特征,并采用地累积指数(Igeo)和潜在生态危害指数(RI)评价了重金属对土壤的潜在环境风险。研究结果表明:除Ni主要以可氧化态存在外,焦化废水外排污泥中其他几种重金属元素主要存在于残渣态,重金属元素的含量低于《城镇污水处理厂污染物排放标准(GB18918—2002)》中的控制限值;与城市污泥相比,焦化废水外排污泥具有低Pb、Cr、Zn、Cu含量,而高Cd、Hg、Mn含量的特点;基于Igeo和RI的评价结果,Cd和Hg是外排污泥中具有一定环境风险的元素,需要考虑其下游去向。焦化废水处理外排污泥中主要存在残渣态重金属成分,不表现为很高的环境风险,其处置应重点考虑其中有机污染物特别是POPs。     

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.     

Impact of sewage sludge spreading on heavy metal speciation in tropical soils (Réunion, Indian Ocean)

This study was designed to evaluate the impact of sewage sludge spreading on tropical soilborne heavy metal speciation. Sludgeborne heavy metal speciation was also assessed, and the potential mobility of the elements was classified as follows: Zn>Ni>Cu approximately Cr. Two sequential extraction procedures were applied to study Cr, Cu, Ni and Zn speciation in control soils and sludge-amended soils. We demonstrated that sewage sludge spreading over a 2-year period did not have an impact on soilborne heavy metal concentrations but affected speciation of the most mobile fractions of Ni and Zn. Both protocols were consistent for the organic matter fractions, with an increase in Cu, Zn and Cr concentrations in the amended soil as compared to the control soil. In addition, we highlighted that the two protocols characterized different pools of organic matter and that organic compounds remained in the solid matrix after extraction. With respect to the reducible fraction, completely opposite results were obtained with the two protocols and the solid residue study revealed that the two schemes were ineffective in characterizing iron and manganese fractions.     

污水污泥中重金属污染物的溶出过程研究

污水污泥中污染物尤其是重金属的水体浸出作为重要的二次污染途径备受关注。为了探讨污水污泥中重金属的溶出规律,对上海7个污水处理厂的污水污泥进行了重金属总量分析,并采用水平振荡法对上述污水污泥中重金属进行了溶出动力学研究。结果表明,污水污泥中Cu、Cr和Zn的含量均较高,之后依次是Pb、As、Cd和Hg;污水污泥中重金属溶出动力学过程分为快速反应和慢速反应两个过程,可以用Elovich方程进行模拟;污水污泥中各重金属的溶出负荷差异较大,Cu、Cr和Zn的溶出负荷较大,其他较小,但各重金属相对于污水污泥中重金属总量的溶出百分比差异不大,基本处于10%以下。     

The use of a sequential leaching procedure for heavy metal fractionation in green liquor dregs from a causticizing process at a pulp mill

A five-stage sequential leaching procedure was used to fractionate heavy metals (Cd, Cu, Pb, Cr, Zn, Fe, Mn, Ni, Co, As, V, Ba and Ti) in green liquor dregs into the following fractions: (1) water-soluble fraction (H2O), (2) exchangeable fraction (CH3COOH), (3) easily reduced fraction (HONH3Cl), (4) oxidizable fraction (H2O2 + CH3COONH4), and (5) residual fraction (HF + HNO3 + HCl). The green liquor dregs were derived from a causticizing process at a pulp mill at Kemi, Northern Finland. According to the leaching studies, the leachability of heavy metals in the water-soluble fraction varied between 0.5 and 2 mg kg(-1) expressed on a dry weight (d.w.) basis, indicating relatively low bioavailability of the metals. However, the concentration of Mn (2065 mg kg(-1); d.w.) showed a strong and of Zn (17.6 mg kg(-1); d.w.), Ni (39.7 mg kg(-1); d.w.) and Ba (32.0 mg kg(-1); d.w.) slightly tendency to be extracted in the exchangeable fraction. In addition, Zn, Mn, Ni, Co, V and Ba showed clear leachability in the easily reduced fraction, as well as Cd, Cu, Cr, Zn, Mn, As and Ba in the oxidizable fraction. For Cd, Cu, Cr, Zn, Mn, Ni, Co, Ba and Ti, the sum of leachable heavy metal concentrations in fractions 1-5 agreed relatively well with the "total" heavy metal concentrations. Recoveries of the sum of fractions 1-5 were 84-56% of those obtained by the US EPA method 3052 (i.e. concentrations obtained after microwave oven digestion with a mixture of HF and HNO3).     

Heavy metal accumulation in wheat plant grown in soil amended with industrial sludge

The concentrations of different forms of Zn, Cu, Mn, Ni, Cd, Cr, Pb and Fe metals were determined for the roadside sludge collected from pickling-rolling and electroplating industrial area. In sludge the relative abundance of total heavy metals were Fe>Mn>Cr>Ni>Cu>Pb>Zn>Cd and DTPA-extractable metals were in the order--Fe>Ni>Mn>Cr>Cu>Zn>Pb>Cd. Pot-culture experiment was conducted in soils amended with sludge (0%, 10%, 20%, 30%), pretreated with lime (0%, 0.5% and 1%). The soils were alkaline in nature (pH>8.3) with organic carbon contents were 0.34% and 0.72%. The most abundant total and bio-available metal was Fe. Two wheat seedlings were grown in each pot containing 3kg sludge-amended or control soil and the experiment was conducted till harvesting. Application of sludge increased both total and bio-available forms of metals in the soils, while lime application decreased the bioavailability of heavy metals in sludge-amended soils. The content of organic carbon showed positive correlation with all metals except Zn, Cr and Pb. CEC also showed a strong positive correlation (R2>0.7) with Fe, Mn, Cu, Ni and Cd. Though wheat plants are not accumulators, the translocation efficiency was appreciably high. The translocation factor from shoot to grain was found smaller than that of root to shoot of wheat plants. This makes an implication that the heavy metal accumulation was proportionally lesser in grain than in shoot. In, 10% sludge with 0.5% lime-amended soils; each of these toxic heavy metals was found to be within permissible range (USEPA). Hence, on the basis of present study, the best possible treatment may be recommended.     

Implications of rhizospheric heavy metals and nutrients for the growth of alfalfa in sludge amended soil

Rhizospheric distribution of nutrients and heavy metals in sludge amended soil was investigated using the rhizobag technique to give an indication of the release of metals from wastewater sludge. DTPA-extractable Zn, Cd, Ni and Mn, and available P, K and NH4+-N in the rhizosphere were markedly depleted when soil was amended with sludge. There was no conspicuous depletion or accumulation of DTPA-extractable Cu in the rhizosphere when the soil was amended with sewage sludge but DTPA-extractable Fe accumulated in the rhizosphere when the soil was amended with increasing amounts of sludge. The pH value in the rhizosphere increased with distance from the roots when soil was amended with larger amounts of sludge. The exchangeable fraction of Cu in the rhizosphere was depleted whether or not the soil was treated with sludge. Carbonate, oxide, organic and residual fractions of Cu and Zn were depleted in the rhizosphere at a distance of 0-2 mm from the roots when soil was amended with 50% sludge. Application of sewage sludge had a positive effect on alfalfa growth. With an increase in sludge amounts, the concentrations of Fe, Cu and Zn in alfalfa shoots did not change. Soil amendments with less than 25% sludge did not increase the availability or mobility of heavy metals. The depletion in rhizospheric DTPA-extractable Zn, Cd and Ni indicates that with the sole exception of Cu, release of metals from sludge amended soil was very limited.     

Remediation of metal contaminated soil with mineral-amended composts

This study examined the use of two composts derived from green waste and sewage sludge, amended with minerals (clinoptilolite or bentonite), for the remediation of metal-contaminated brownfield sites to transform them into greenspace. Soils contaminated with high or low levels of metals were mixed with the mineral-enhanced composts at different ratios and assessed by leaching tests, biomass production and metal accumulation of ryegrass (Lolium perenne L.). The results showed that the green waste compost reduced the leaching of Cd and Zn up to 48% whereas the composted sewage sludge doubled the leachate concentration of Zn. However, the same soil amended with composted sewage sludge showed an efficient reduction in plant concentrations of Cd, Cu, Pb or Zn by up to 80%. The results suggest that metal immobilisation and bioavailability are governed by the formation of complexes between the metals and organic matter. The amendment with minerals had only limited effects.     

Change of PCBs and forms of heavy metals in sewage sludge during thermophilic anaerobic digestion

Determination of seven congeners of PCBs was carried out for sewage sludge before, during and after thermophilic digestion. The overall content of heavy metals (Zn, Cu, Ni, Cd, Pb, Cr) in sludge before and after digestion was determined. Moreover the concentration of heavy metals in particular chemical fractions of the sludge was analyzed. After the thermophilic digestion total concentration of seven PCBs was reduced by 47%, which suggests that thermophilic digestion affects PCB reduction positively. On the 10th d of the process, concentration of lower chlorinated PCBs increased, whereas those of higher chlorinated PCBs decreased. The thermophilic digestion process showed no accumulation of the studied heavy metals in the mobile fractions (exchangeable and carbonate) of the stabilized sewage sludge, except for nickel. The highest increase in zinc, copper, cadmium, and chromium concentration was observed in the organic-sulfide fraction, whereas the highest increase in lead was found in the residual fraction of the sludge. In case of nickel both fractions of organic-sulfide and exchangeable-carbonate fractions were enriched.