Distributions of zinc, copper, cadmium and lead in a tropical ultisol after long-term disposal of sewage sludge

Heavy metals present in soils constitute serious environmental hazards from the point of view of polluting the soils and adjoining streams and rivers. The distribution of heavy metals in a sandy Ultisol (Arenic Kandiustult) in south eastern Nigeria subjected to 40 years disposal of sewage wastes (sludge and effluents) was studied using two profile pits (S/NSK/1 and S/NSK/2) sited in the sewage disposal area and one profile pit (NS/NSK) sited in the non-sewage disposal area. Soil samples were collected in duplicate from these soil horizons and analyzed for their heavy metal contents. The mean concentrations of Zn, Cu, Cd and Pb in the top- and sub-soil horizons of sewage soil were 79.3, 32, 0.29 and 1.15 mg/kg, respectively. These levels were high enough to constitute health and phytotoxic risks. All the metal levels were much higher in the AB horizon in the sewage than in the non-sewage soil profile, but Pb and Cu contents were also high down to the Bt1 horizon, indicating their apparent relatively high mobility in this soil. There was a significant correlation between organic matter (OM) and Zn (r=0.818**), and between OM and Cd (0.864**) in the sewage soil. The high OM status of the sewage sludge, together with its corresponding low pH, might have favoured metal-OM complexation that could reduce heavy metal mobility and phytotoxicity in this soil.     

贵州省城市污泥重金属组成特征与农用风险评价

分别选取了贵州省7个主要城市的10个典型污水处理厂脱水污泥样品进行检测,统计了2009~2012年污泥中重金属Cu、Zn、Pb、Cd、Ni、Cr、As、Hg的含量。结果表明,城市污泥中重金属含量受到各地区工矿业发展的影响,部分重金属在某一污水处理厂出现了远大于其它地区的极值,具有明显的地域特征。2009~2012年贵州省城市污泥重金属的变化表现为As、Hg呈升高趋势,其它重金属变化趋势不明显,As、Hg升高可能是因为贵州省近年来燃煤消耗量增加,烟气排放污染所致。对污泥重金属的农用风险评价显示,贵阳中部（S2）、黔西地区(S9)污泥农用重金属的生态风险较高,不推荐直接施用,其它各地区污泥农用风险较低,可以进行农用,建议各污水处理厂应根据本地区污泥重金属特征制定合理的污泥处置措施     

江苏省化工园区污水处理厂污泥重金属污染及生态风险评价

污水处理厂的排放污泥是诸多污染物的最终环境归宿之一,其环境影响值得重视。在江苏省全境调查了49家化工园区集中式污水处理厂,对其外排污泥及其浸出液中的5种重金属(砷、镉、铬、汞和铅)含量进行了测定,采用Hakanson潜在生态危害指数法评价了污泥中重金属生态风险。结果表明:49家化工园区污泥浸出液中5种重金属均不超标,但污泥中的砷、铬、汞含量超标,超标率分别为24.5%、6.1%和4.1%,最大超标倍数分别为131、2.12和0.41。污泥中砷含量显著高于我国城市污泥,潜在生态风险程度呈现汞砷铬。从地域分布来看,苏北地区化工园区污泥重金属风险更大。     

Effects of earthworm activity on fertility and heavy metal bioavailability in sewage sludge

The potential for using earthworms (Eisenia fetida) to improve fertility and reduce copper and cadmium availability in sewage sludge was tested by laboratory incubation experiments. Results comparing sewage sludge with and without earthworm treatment showed that earthworm activity decreased the contents of organic matter, total nitrogen, but increased the contents of available nitrogen and phosphorus and had no significant effect on the contents of total phosphorus, total potassium and available potassium. After incubation of the sewage sludge with earthworms for 60 days, the contents of Cu and Cd in the earthworms increased with the increase of additional Cu up to 250 mg kg(-1) and Cd up to 10 mg kg(-1). Bioconcentration factors (BCF) were higher than 1 only for Cd when the addition rate was lower than 5 mg kg(-1), which indicates that the earthworms can only accumulate Cd when the concentration of Cd is low in sewage sludge. Bioavailability of Cd and Cu was evaluated by applying sewage sludge with and without earthworm treatment to soil and then growing cabbage plants. The results showed that earthworm treatment increased the biomass of cabbage and decreased the bioaccumulation of Cd and Cu in the cabbage plants.     

Metal-contaminated soil remediation by using sludges of the marble industry: toxicological evaluation

The major risks due to metal pollution of sediments consist of leaching to groundwater and potential toxicity to animals and/or plants. The objective of this study was to evaluate by means of an ecotoxicological approach the effects of the addition of cutting marble sludges on the mobile metal fraction of sediments polluted with heavy metals. The study was carried out on two sediments derived from mining activities in Portman Bay (SE, Spain) polluted by heavy metals. These sediments were mixed with sludges left after the cutting of marble. The results obtained by leaching experiments showed that the addition of marble cutting sludge, consisting mainly of carbonates, to a heavy-metal polluted sediment produces a decrease of available metal forms. The carbonate content seems to play a role in chemical stabilisation of metals and in a decrease of toxicity of sediments. The leached solutions have a non-toxic effect. The mild remediation by addition of sludge has moreover effects to long term.     

A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge

The content, behaviour and significance of heavy metals in composted waste materials is important from two potentially conflicting aspects of environmental legislation in terms of: (a) defining end-of-waste criteria and increasing recycling of composted residuals on land and (b) protecting soil quality by preventing contamination. This review examines the effects of heavy metals in compost and amended soil as a basis for achieving a practical and sustainable balance between these different policy objectives, with particular emphasis on agricultural application. All types of municipal solid waste (MSW) compost contain more heavy metals than the background concentrations present in soil and will increase their contents in amended soil. Total concentrations of heavy metals in source-segregated and greenwaste compost are typically below UK PAS100 limits and mechanical segregated material can also comply with the metal limits in UK PAS100, although this is likely to be more challenging. Zinc and Pb are numerically the elements present in the largest amounts in MSW-compost. Lead is the most limiting element to use of mechanically-segregated compost in domestic gardens, but concentrations are typically below risk-based thresholds that protect human health. Composted residuals derived from MSW and greenwaste have a high affinity for binding heavy metals. There is general consensus in the scientific literature that aerobic composting processes increase the complexation of heavy metals in organic waste residuals, and that metals are strongly bound to the compost matrix and organic matter, limiting their solubility and potential bioavailability in soil. Lead is the most strongly bound element and Ni the weakest, with Zn, Cu and Cd showing intermediate sorption characteristics. The strong metal sorption properties of compost produced from MSW or sewage sludge have important benefits for the remediation of metal contaminated industrial and urban soils. Compost and sewage sludge additions to agricultural and other soils, with background concentrations of heavy metals, raise the soil content and the availability of heavy metals for transfer into crop plants. The availability in soil depends on the nature of the chemical association between a metal with the organic residual and soil matrix, the pH value of the soil, the concentration of the element in the compost and the soil, and the ability of the plant to regulate the uptake of a particular element. There is no evidence of increased metal release into available forms as organic matter degrades in soil once compost applications have ceased. However, there is good experimental evidence demonstrating the reduced bioavailability and crop uptake of metals from composted biosolids compared to other types of sewage sludge. It may therefore be inferred that composting processes overall are likely to contribute to lowering the availability of metals in amended soil compared to other waste biostabilisation techniques. The total metal concentration in compost is important in controlling crop uptake of labile elements, like Zn and Cu, which increases with increasing total content of these elements in compost. Therefore, low metal materials, which include source-segregated and greenwaste composts, are likely to have inherently lower metal availabilities overall, at equivalent metal loading rates to soil, compared to composted residuals with larger metal contents. This is explained because the compost matrix modulates metal availability and materials low in metals have stronger sorption capacity compared to high metal composts. Zinc is the element in sewage sludge-treated agricultural soil identified as the main concern in relation to potential impacts on soil microbial activity and is also the most significant metal in compost with regard to soil fertility and microbial processes. However, with the exception of one study, there is no other tangible evidence demonstrating negative impacts of heavy metals applied to soil in compost on soil microbial processes and only positive effects of compost application on the microbial status and fertility of soil are reported. The negative impacts on soil microorganisms apparent in one long-term field experiment could be explained by the exceptionally high concentrations of Cd and other elements in the applied compost, and of Cd in the compost-amended soil, which are unrepresentative of current practice and compost quality. The metal contents of source-segregated MSW or greenwaste compost are smaller compared to mechanically-sorted MSW-compost and sewage sludge, and low metal materials also have the smallest potential metal availabilities. Composting processes also inherently reduce metal availability compared to other organic waste stabilisation methods. Therefore, risks to the environment, human health, crop quality and yield, and soil fertility, from heavy metals in source-segregated MSW or greenwaste-compost are minimal. Furthermore, composts produced from mechanically-segregated MSW generally contain fewer metals than sewage sludge used as an agricultural soil improver under controlled conditions. Consequently, the metal content of mechanically-segregated MSW-compost does not represent a barrier to end-use of the product. The application of appropriate preprocessing and refinement technologies is recommended to minimise the contamination of mechanically-segregated MSW-compost as far as practicable. In conclusion, the scientific evidence indicates that conservative, but pragmatic limits on heavy metals in compost may be set to encourage recycling of composted residuals and contaminant reduction measures, which at the same time, also protect the soil and environment from potentially negative impacts caused by long-term accumulation of heavy metals in soil.     

The influence of zeolite type A on heavy metal transfer in the activated sludge process

In areas which are highly urbanised, where water re-use is frequently necessary, the importance of “heavy metal” removal during sewage treatment cannot be overestimated. One critical factor controlling the forms of the metals in the sewage is the chemical matrix of the sewage; as a consequence of their chelating capacity detergent builders are important in modifying the sewage matrix. Laboratory simulations of the activated sludge process have been studied to estimate the impact of the detergent builder zeolite type A on the removal of heavy metals from sewage in this treatment process.Experiments have also been undertaken in which the influent concentrations of the builder were doubled, in an endevour to simulate the effect of a “wash day”. It is apparent from the data presented that under conditions of constant loading, zeolite type A did not significantly affect the concentrations of any of the metals studied with the exception of zinc. However, removal of some of the metals was less than in the presence of a condensed phosphate solution. Under conditions of shock loading the improvement in metal removal which has been observed when condensed phosphates were added under conditions o of variable loading was not observed when zeolite type A was added under similar conditions.     

Effect of sewage on biodegradability of hydrocarbons in refinery wastewater

The effect of the microbial flora of municipal sewage, with special reference to faecal coliform and faecal streptococci groups, on the hydrocarbons biodegradation of the refinery wastewater from the Moustorod Oil Refinery has been studied. The results indicate that sewage with its microbial flora seems to have a significant role in the biodegradation of hydrocarbons in refinery wastewater. In addition, the early degradation in the wastewater seeded by sewage reveals that faecal streptococci, in addition to other microbial flora, have a significant role in the biodegradation of hydrocarbons.     

上海城市污水厂污泥处理与利用系统分析

上海市城市污水处理规划的实施，将在2020年前使该市的城市污水厂污泥产生量从现状的85tDS/d增长至1300－1500tDS/d，将形成可观的污泥消纳压力。预期了该市的污水污泥产生状况，以现有污水污泥处置与利用技术为基础，测算了可行的污泥处置与利用方向及相应的容量，以及满足容量利用的前处理要求。以此为基础，推荐了适宜的容量利用方案，并建议以生物稳定化干化处理为污泥处置与利用前的改性处理步骤，且可在适当的条件下增加厌氧消化环节，提高处理体系的水平与容量宽余度。此污泥处理与利用体系可达到污泥消纳运行的柔性化和可靠性，兼具环境安全性与经济合理性。     

The effects of FeCl3 on the distribution of the heavy metals Cd, Cu, Cr, and Zn in a simulated multimetal incineration system

Recognizing that waste-derived chlorine can enhance heavy metal emissions by forming volatile metallic chlorides during municipal solid waste (MSW) combustion, and that in Taiwan, FeCl3-containing sewage sludge may either be landfilled or coincinerated with other MSW, this study thus investigated the effects of FeCl3 on the speciation and partitioning of heavy metals in a multimetal incineration system by using a tubular furnace and FeCl3-spiked simulated wastes. The molar ratio of chlorine content to heavy metal content (referred to as the Cl/ M ratio), ranging from 3 to 200, was used as a parameter to evaluate the effects of chlorine on the movement of heavy metals between the incinerator discharges. Results indicate that speciation and partitioning were related to the affinity between the chlorine and the heavy metals and between chlorine and hydrogen in the combustion system. The effectiveness of increasing the Cl/M ratio to the formation potential of metallic chlorides and on the shift of heavy metals from the bottom ash to the fly ash and/or the flue gases was found to have in increasing order as follows: Zn>Cu>Cr, a phenomenon basically reflecting the volatility of the heavy metals and their chlorides formed during combustion.     

Toxicity evaluation of sewage sludges in Hong Kong.

Anaerobically digested sewage sludges collected from four wastewater treatment plants located in Sha Tin, Tai Po, Yuen Long, and Shek Wu Hui in Hong Kong were subjected to chemical characterization and toxicity testing to provide preliminary assessment of their suitability for land application. All sewage sludges were slightly alkaline with pH range of 8.3-8.7. Electrical conductivity (EC; 0.69 dS m(-1)) and soluble NH4-N content (996 mg kg(-1)) of sewage sludge from Yuen Long were lower than that of other plants. Concentrations of heavy metals were determined as total, extractable, and water-soluble fraction using mixed acid digestion, DTPA (pH 7.3), and distilled water, respectively. Yuen Long sludge was most polluted with Zn and Cr higher than the pollutant concentration limits listed in Part 503 of USEPA, owing to the effluent coming from the tannery industry. High concentration of Ni was found in sludge from Sha Tin that originated mainly from the electroplating industry. DTPA-extractable Zn contents were high in sludges from Yuen Long (1247 mg kg(-1)) and Shek Wu Hui (892 mg kg(-1)), while 3.7 mg kg(-1) of DTPA-extractable Cr was found in Yuen Long sludge. Metal speciation of sludges showed that Pb was major in residual phase while Cu, Cr, and Ni in organic and residual phases, and Zn did not show any dominant chemical phase. The sludge extracts did not exert significant adverse effect on seed germination of Brassica chinensis (Chinese cabbage), but Yuen Long sludge caused a reduction in root growth. In view of its lower EC and soluble ammonia contents, the high concentration of Zn and Cr in Yuen Long sludge would likely be responsible for this adverse effect on root growth. Therefore, Yuen Long sludge would likely have a more serious impact on soil quality and plant growth as compared to other sludges. This would require further verification from greenhouse and field experiments.     

Mechanical performance and capillary water absorption of sewage sludge ash concrete (SSAC)

Disposal of sewage sludge from waste water treatment plants is a serious environmental problem of increasing magnitude. Waste water treatment generates as much as 70 g of dry solids per capita per day. Although one of the disposal solutions for this waste is through incineration, still almost 30% of sludge solids remain as ash. This paper presents results related to reuse of sewage sludge ash in concrete. The sludge was characterised for chemical composition (X-ray flourescence analysis), crystalline phases (X-ray diffraction analysis) and pozzolanic activity. The effects of incineration on crystal phases of the dry sludge were investigated. Two water/cement (W/C) ratios (0.55 and 0.45) and three sludge ash percentages (5%, 10% and 20%) per cement mass were used as filler. The mechanical performance of sewage sludge ash concrete (SSAC) at different curing ages (3, 7, 28 and 90 days) was assessed by means of mechanical tests and capillary water absorption. Results show that sewage sludge ash leads to a reduction in density and mechanical strength and to an increase in capillary water absorption. Results also show that SSAC with 20% of sewage sludge ash and W/C = 0.45 has a 28 day compressive strength of almost 30 MPa. SSAC with a sludge ash contents of 5% and 10% has the same capillary water absorption coefficient as the control concrete; as for the concrete mixtures with 20% sludge ash content, the capillary water absorption is higher but in line with C20/25 strength class concretes performance.     

Lime stabilization and land disposal of cold region wastewater lagoon sludge

Effects of lime [Ca(OH)₂] stabilization upon the pathogenic population in accumulated solids associated with the operation of two aerated wastewater lagoons in Alaska and two facultative wastewater lagoons in northern Utah were evaluated. The subsequent drying, at a temperature of 12 °C, of the lime stabilized sludges on sand and soil beds was also investigated. The lime stabilization of the lagoon sludges was evaluated by dosing the sludges with lime and applying sludges to bench scale drying beds. Lime addition produced high fecal coliform reduction, and the limed sludges readily dewatered on both sand and soil beds.     

A spatial multicriteria decision making tool to define the best agricultural areas for sewage sludge amendment

Sewage sludge amendment on agricultural soils has recently become a practice of heightened interest, as a consequence of sewage sludge production increase. This practice has benefits to soil and crops, however it may also lead to environmental contamination, depending on the characteristics of the fields. In order to define the suitability of the different agricultural fields to receive sewage sludge, a spatial tool is proposed. This tool, elaborated in GIS platform, aggregates different criteria regarding human exposure and environmental contamination.The spatial tool was applied to a case study in the region of Catalonia (NE of Spain). Within the case study, each step of the tool development is detailed. The results show that the studied region has different suitability degrees, being the appropriate areas sufficient for receiving the total amount of sewage sludge produced. The sensitivity analysis showed that “groundwater contamination”, “distance to urban areas”, “metals concentration in soil” and “crop type” are the most important criteria of the evaluation.The developed tool successfully tackled the problem, providing a comprehensive procedure to evaluate agricultural land suitability to receive sewage sludge as an organic fertilizer. Also, the tool implementation gives insights to decision makers, guiding them to more confident decisions, based on an extensive group of criteria.     

Identification of metal toxicity in sewage sludge leachate

Sewage sludge is a source of organic matter and nutrients with the potential for being used as a fertilizer. However, metals in sewage sludge might accumulate in soil after repeated sludge applications, and metal concentrations might reach concentrations that are toxic to microorganisms, soil organisms and/or plants. This toxicity might change with time due to kinetic factors or abiotic factors such as freezing, drying or rainfall. The objective of this study was to determine toxicity of sewage sludge leachate from a lysimeter with 50 cm of sludge applied. Attempts were also made to identify the cause of toxicity of the sludge leachate by toxicity identification and evaluation (TIE) techniques. Sludge leachate was collected monthly during 1 experimental year (August 2001 to August 2002). Metal concentrations were analysed, and the toxicity was determined with Daphnia magna (48-h immobility). The effect of EDTA or sodium thiosulphate addition, filtration through a CM-resin or a Millex-resin on toxicity was also tested. The results showed that toxicity of the sludge leachate apparently varied during the year, and that filtration through the CM-resin reduced most of the toxicity followed by the addition of EDTA. None of the other treatments reduced the toxicity of the sludge leachate. This indicated that one or more metals were responsible for the observed toxicity. Further calculations of toxic units (TU) suggested that Zn contributed most to the toxicity. Results also indicated that Ca concentrations in the sludge leachate reduced the toxicity of Zn.     

Long-term field application of sewage sludge increases the abundance of antibiotic resistance genes in soil

Sewage sludge and manure are common soil amendments in crop production; however, their impact on the abundance and diversity of the antibiotic resistome in soil remains elusive. In this study, by using high-throughput sequencing and high-throughput quantitative PCR, the patterns of bacterial community and antibiotic resistance genes (ARGs) in a long-term field experiment were investigated to gain insights into these impacts. A total of 130 unique ARGs and 5 mobile genetic elements (MGEs) were detected and the long-term application of sewage sludge and chicken manure significantly increased the abundance and diversity of ARGs in the soil. Genes conferring resistance to beta-lactams, tetracyclines, and multiple drugs were dominant in the samples. Sewage sludge or chicken manure applications caused significant enrichment of 108 unique ARGs and MGEs with a maximum enrichment of up to 3845 folds for mexF. The enrichment of MGEs suggested that the application of sewage sludge or manure may accelerate the dissemination of ARGs in soil through horizontal gene transfer (HGT). Based on the co-occurrence pattern of ARGs subtypes revealed by network analysis, aacC, oprD and mphA-02, were proposed to be potential indicators for quantitative estimation of the co-occurring ARGs subtypes abundance by power functions. The application of sewage sludge and manure resulted in significant increase of bacterial diversity in soil, Proteobacteria, Acidobacteria, Actinobacteria and Chloroflexi were the dominant phyla (> 10% in each sample). Five bacterial phyla (Chloroflexi, Planctomycetes, Firmicutes, Gemmatimonadetes and Bacteroidetes) were found to be significantly correlated with the ARGs in soil. Mantel test and variation partitioning analysis (VPA) suggested that bacterial community shifts, rather than MGEs, is the major driver shaping the antibiotic resistome. Additionally, the co-occurrence pattern between ARGs and microbial taxa revealed by network analysis indicated that four bacterial families might be potential hosts of ARGs. These results may shed light on the mechanism underlining the effects of amendments of sewage sludge or manure on the occurrence and dissemination of ARGs in soil.     

Toxicity of copper in sewage sludge

Sewage sludge is a source of organic matter and nutrients, but a major obstacle for its recycling is that the municipal wastewater sludge has low but significant levels of contaminants. This investigation, on the acute toxicity of copper in sewage sludge, was conducted with three organisms, Daphnia magna, Lemna minor and Raphanus sativus (seeds). The toxicity of the leakage water from sewage sludge spiked with CuSO(4) was studied for 64 days. The toxicity increased during the first 8-16 days and then started to decrease. The first increase in toxicity was due to ammonia, but after 32 days, a dose-related effect of copper was found. After 64 days, L. minor had an EC50 of 3800 mg Cu/kg dw for 7 days growth inhibition, a LOEC of 3200 mg Cu/kg dw and a NOEC of 1600 mg Cu/kg dw. D. magna had an EC50 of 18100 mg Cu/kg dw (24-h immobility) and a NOEC of 12800 mg Cu/kg dw. Root elongation of R. sativus was reduced at 25600 mg Cu/kg dw. Both for Daphnia and Lemna, the pH of the leakage water had an effect of the toxicity. This means that chemical speciation and bioavailability is very important for the hazard assessment of copper in sludge and soil.     

Effect of sewage sludge amendment on soil microbial activity and nutrient mineralization

An incubation experiment was performed to study the effect of sewage sludge on microbial respiration and nutrient mineralization in a sandy soil as an indication of its effects on soil biological properties and nutrient transformation. Sewage sludge was amended with a sandy soil at 0, 25, 50, 150 and 350 g kg⁻¹ fresh weight. An increase in the sludge amendment rate caused an increase in both pH and electrical conductivity (EC). However, pH decreased while EC increased and then decreased along the incubation time. Nevertheless salinity and heavy metal contents of the soil sludge mixture were all within the safety guidelines. Soluble NH₄⁺, NO₃²⁻ and PO₃²⁻ increased after amending the soil with sewage sludge, but increasing the application rate to 350 g kg⁻¹ of sludge decreased the N and P mineralization efficiency and created an adverse effect on nitrification. The daily CO₂ evolution pattern was the same in all treatments that CO₂ evolution increased initially and then decreased till the end of the incubation period. All the treatments had peak CO₂ evolution at day 7, except for the soil amended with 350 g kg⁻¹ of sludge which had peak CO₂ evolution at day 2. Similarly, the percentage of C-mineralization decreased with an increase in sludge amendment rate. The present experiment indicated that an application rate of 50–150 g kg⁻¹ sludge for sandy soil would have the optimal beneficial effect on the soil in terms of microbial activity and nutrient transformation.     

Assessment of polychlorinated dibenzo-p-dioxins and dibenzofurans in sludges according to the European environmental policy

The amount of sewage sludge generated in Europe is expected to surpass the 10 million tons/year in 2006 as a result of the waste water treatment process according to the Water Policy in European Union. Sewage sludge is what is left behind after water is cleaned in waste treatment plants and is characterized for this high content in nitrogen and phosphorous that could be of great importance in agriculture as fertilizer or soil conditioner. On the other hand, pollutants like metals and organic contaminants are usually removed from water and are accumulated in the sewage sludge, reaching the food chain if their concentrations are not below the safe limits established by the European legislation. The latter issue is of great concern nowadays and in this sense, different works alert against the use of the sewage sludge in agriculture arguing that serious illnesses, even resulting in death as well as adverse environmental impacts are associated to the application of sewage sludge. This work is a continuation of a former comprehensive survey on of priority organic pollutant in sludges for agricultural purposes carried out by our group in Catalonia and this time is focused on the polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), one of the most toxic group of organic compounds listed in the Work Document on Sludge which is the reference tool in this field in Europe and is also included in the Stockholm Convention. Eighty eight samples were collected from the end of 2003 to April 2006 and the concentrations detected were lower than the 100 ng/kg I-TEQ limit recommended by the European legislation (EC, 2000). Thus, sewage sludges generated in Catalonia do not represent a threat to human health if they are used as fertilizers in agriculture.     

Impacts of sewage irrigation on heavy metal distribution and contamination in Beijing, China

A potential hazard to Beijing was revealed due to the accumulation trend of heavy metals in agricultural soils with sewage irrigation, which results in metal contamination and human exposure risk. Samples including soils and plants were collected to assess the impacts of sewage irrigation on the irrigated farming area of Beijing. Concentrations of the five elements Cd, Cr, Cu, Zn, and Pb were determined in samples to calculate the accumulation factor and to establish a basis for environmental protection and the suitability of sewage irrigation for particular land use in the urban-rural interaction area of Beijing. Using reference values provided by the Beijing Background Research Cooperative Group in the 1970s, the pollution load index (PLI), enrichment factor (EF), and contamination factor (CF) of these metals were calculated. The pollution load indices (sewage irrigation land 3.49) of soils indicated that metal contamination occurred in these sites. The metal enrichment (EF of Cd 1.8, Cr 1.7, Cu 2.3, Zn 2.0, Pb 1.9) and the metal contamination (CF of Cd 2.6, Cr 1.5, Cu 2.0, Zn 1.7, Pb 1.6) showed that the accumulation trend of the five toxic metals increased during the sewage irrigation as compared with the lower reference values than other region in China and world average, and that pollution with Cd, Cu, Zn, and Pb was exacerbated in soils. The distributions of these metals were homogeneous in the irrigation area, but small-scale heterogeneous spatial distribution was observed. Irrigation sources were found to affect heavy metal distributions in soils. It was suggested that heavy metal transfer from soils to plants was a key pathway to human health exposure to metal contamination. However, with the expansion of urban areas in Beijing, soil inhalation and ingestion may become important pathways of human exposure to metal contamination.