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Quality assessment of composts in the Greek market: The need for standards and quality assurance
Journal of Environmental Management 80 (2006) 58–65 www.elsevier.com/locate/jenvman
Quality assessment of composts in the Greek market: The need for standards and quality assurance Katia Lasaridi a,*, Ioanna Protopapa a, Maria Kotsou a, George Pilidis b, Thakis Manios c, Adamantini Kyriacou a a
Harokopio University, 70 El. Venizelou, 17671 Kallithea, Athens, Greece b University of Ioannina, Dourouti, 451 10 Ioannina, Greece c Technological Education Institute of Crete, School of Agricultural Technology, Heraklion, Greece Received 28 February 2005; received in revised form 18 July 2005; accepted 18 August 2005 Available online 5 December 2005
Abstract An extensive data survey and study of the Greek market for composts or products marketed as such was carried out in order to acquire a comprehensive image of the local situation, in view of the proposed operation of large municipal solid waste (MSW) composting facilities and EU legislation changes. Physical and chemical parameters (moisture, organic matter, electrical conductivity, pH and heavy metals), stability indicators (self-heating potential, germination index) and biological indicators (microbial population, pathogen indicators and selected pathogens) were analyzed for the assessment of product quality. Results revealed wide variations even within the same group of products, which is particularly significant for parameters directly related to environmental protection and public health. The heavy metal content ranged from levels exceeding the fairly lenient Greek standards to below the stringent limits for AC class compost in Austria. About 25% of the composts examined met the heavy metal limits for the EU eco-label award. Salmonella spp. was not detected in any of the composts but Staphylococcus aureus and Clostridium perfringens were found in 17 and 96% of the composts respectively. Pathogen indicator microorganisms were present at levels above suggested limits in all the composts. The high variability of such important parameters in composts available on the Greek market suggests an urgent need for establishing quality assurance procedures and mechanisms in the country. Moreover, the wide range of limit values within EU member states suggests the need for developing EU compost quality standards, in order to harmonize the compost markets. q 2005 Elsevier Ltd. All rights reserved. Keywords: Compost quality; Pathogens; Stability; Heavy metals; Compost microbial community
1. Introduction Compost quality is a difficult to define and often elusive term, meaning different things to different people according to their professional background and national legislations. Nevertheless, compost quality lies at the core of the issue of composting and biological treatment in general, as it defines the marketing potential and the outlets of the product and in most cases, the viability of the treatment plant, but also the long-term acceptability of biological treatment as a valuable option in the waste hierarchy (Lasaridi, 1998).
* Corresponding author. Tel.: C30 210 9549164/9549150; fax: C30 210 9514759. E-mail address: [email protected] (K. Lasaridi).
0301-4797/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2005.08.011
Compost quality refers to the overall state of the compost in regard to physical, chemical and biological characteristics, which indicate the ultimate impact of the compost on the environment. The quality of compost is determined by the sum of its different features and properties; specifications are usually determined by minimum admissible levels of required substances or maximum tolerable limits for unwanted ones (de Bertoldi, 1993; Hogg et al., 2002). The criteria that are relevant to the evaluation of quality depend on what purpose the compost is used for, the relevant environmental protection policies and the market requirements (Gillett, 1992; Kehres, 1992). For example, composts intended as growing media should meet more stringent quality criteria compared to composts that will be used as landfill cover. A number of characteristics determine compost quality, such as particle size distribution, moisture, organic matter and carbon content, concentration and composition of humus-like substances, nitrogen content and forms of N, phosphorus and
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potassium, heavy metals, salinity and the nature of ions responsible for it, cation exchange capacity, water holding capacity, porosity and bulk density, inert contaminants, pathogens, and state of maturity or stability (Lasaridi, 1998). However, the most important, from the point of view of standards for the protection of public health, the soil and the environment in general, are those relating to pathogens, inorganic and organic potentially toxic compounds (heavy metals, PCBs, PAHs, phthalates etc.) and stability, the latter determining compost nuisance potential, nitrogen immobilization and leaching and phytotoxicity (Brinton, 2000; Deportes et al., 1995; Hogg et al., 2002). Currently, compulsory and voluntary compost standards in different countries are characterized by a great degree of heterogeneity, stemming from the effort to combine two often contradicting targets: maximum environmental and public health protection on the one hand and maximum organic matter recycling on the other. Moreover, the precautionary approach adopted in the EU and the risk assessment approach prevailing in the USA, may lead to broad differences in the accepted limit values for a number of critical parameters, such as heavy metals (Hogg et al., 2002). Even within the EU there is a wide variation among the limit values adopted by the member countries, with the north being usually more stringent than the south, reflecting mainly the varying level of progress on source separation of the biodegradable fraction of MSW, but also the different needs in soil organic matter. The adoption of the long debated ‘biowaste directive’ would alter this situation, leading to a more homogeneous European market for composted products. However, the delivery of a stand-alone ‘biowaste directive’ is currently uncertain and the discussion for including relevant standards and targets in a framework directive, such as a ‘soil framework directive’ or the revision of the waste framework directive is still at a preliminary level. All national compost standards include compost sanitization criteria for human pathogens and occasionally for plant pathogens. These criteria may refer to the product (absence of Salmonella, absence or low levels of fecal coliforms and fecal streptococci), the process (setting a minimum period for which the compost should maintain a temperature higher than a designated level) or both. Maximum permissible values are set for heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) although the limits vary widely (Deportes et al., 1995; Hogg et al., 2002). Similar values are set for foreign matter (glass, plastics and stones) in most national specifications, usually defined as maximum allowed content on a dry weight basis and in reference to their particle size (Hogg et al., 2002; Brinton, 2000). The degree of compost stability and its nitrogen content are particularly important for its agronomic use and are increasingly more often defined in compost specifications. Compost stability is no longer defined as the C/N ratio, as was often the case in older standards, but usually on the basis of compost microbial activity measured through the respiration activity (e.g. AT4, Dynamic Respiration Index, SOUR) or the self-heating potential (Rottegrade) (FCQAO, 1994; Hogg et al., 2002; Lasaridi and Stentiford, 1998).
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In Greece the existing compost quality specifications refer to composts from MSW and sewage sludge and require that Salmonella and enterobacteriaceae be absent with no further reference to plant pathogens or process specifications. As enterobacteriaceae are commonly encountered in various environmental media, such as soil, the requirement for their absence in composts is particularly stringent and difficult to meet. On the other hand, rather lax limits for heavy metal content are permitted, following the 86/278/EEC Directive on the use of sewage sludge in agriculture. Maximum allowable limits for compost foreign matter are also set. Parameters for the determination of compost stability, nitrogen content and/or phytotoxicity are not defined (Anon, 1997). The EU eco-label criteria for soil improvers and growing media (Decision 2001/688/EC) are also applicable in Greece, but they are rather demanding in terms of quality requirements and application rates and thus relevant only for the top products on the market. For the remaining compost products available on the Greek market, whether produced locally or imported, it is presumed that they should at least conform to the requirements for the MSW composts (Anon, 1997). However, currently there are neither mechanisms nor organizations responsible for the analysis, control and certification of the composts available on the market. This results in low quality products making their way onto the market, creating consumer distrust of compost products in general. It also distorts competition between high and low quality compost and may allow the use of products that are harmful to public health, plants and the environment. The aim of this study was to identify, register and analyze all compost products available on the Greek market, in order to derive their quality profile and examine their compliance with Greek standards. Moreover, as more stringent EU-wide standards are forthcoming, compost compliance with more stringent limit values established in different EU countries was also examined.
2. Materials and methods 2.1. Market research The different compost materials available on the Greek market at the time of this study (January–June 2003) were identified through both desk and field research. Sources for the desk study included the Internet, agricultural magazines, environmental and agricultural exhibition guides, associations for the certification of biological agriculture products and personal contacts with compost producers. This was followed by a three months on-site market research in the wider Athens area, in super-markets, garden centers and agricultural provisions stores. A total of 28 products were identified and acquired in their original packaging. The products were classified into six groups according to their main substrates, as specified in the product description on the packaging and a code system was used instead of brand names to ensure confidentiality (Table 1). After closer examination it was
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Table 1 Description of the different groups of composts and soil improvers identified on the Greek market in the period from January to June 2003 Group code
Nunber of products
Description
K1–K7 F1–F5 P1–P4
7 5 4
V1–V5
5
M1–M4
4
W1–W3
3
Composts containing manures Composts of exclusively plant origin (withdrawn fruits, olive mill press-cake, seaweed, etc) Various peat products, marketed as composts. Biological indicators were not determined for these products as they did not contain waste materials Various products, the composition of which could not be clearly identified from the label information, marketed as composts and believed to be such, with the exception of V4, which was leonardite and was not analyzed for biological indicators Fresh and spent mushroom compost-not available in the market. The spent mushroom compost is utilized as a component in potting mixtures sold in bulk by various local producers Compost from mechanically separated MSW-not sold on the market but used as landfill cover and for coalmine restoration
proven that five of them (P1–P4 and V4) were not composts despite being marketed as such. 2.2. Analytical methods Moisture and organic matter (OM) content were determined through the weight loss at 105 and 550 8C, respectively. Electrical conductivity (EC), the pH value and the self-heating potential (Rottegrade) were determined according to FCQAO (1994). Phytotoxicity, a parameter partly related to stability, was calculated as the Germination Index (GI) of Lepidium sativum seeds (Agrocementi Ltd, Athens), using a modified version of the method proposed by Zucconi et al. (1985). Twenty five seeds were placed on five layers of filter paper pads wetted with 5 ml of 1:10 compost aqueous extract in petri dishes incubated in the dark at 25 8C for 48 h. Tap water was used as control. All aforementioned analyses were performed in triplicate. Heavy metals (Pb, Ni, Zn and Cu) were determined after digestion of 1 g (dw) of pulverized sample with pure nitric acid, using AAS (Shimazu 6800 Flame Atomic Absorption Spectrophotometer) (Sposito et al., 1983). The total mesophilic bacteria, the spore forming bacteria, the staphylococci, the yeasts and the filamentous fungi were determined by the dilution plate method as described by Hassen et al. (2001). For the fecal coliforms the eosin-methylen blue-lactose sucrose agar was used as described by Ichida et al. (2001). Colonies were randomly isolated and microscopically examined (Gram-bacilli, oxidase negative). For the fecal streptococci the Slanetz and Bartley medium was used as described by Vuorinen and Saharinen (1999). Colonies were randomly isolated and microscopically examined (GramC cocci, catalase negative). The population of Clostridium perfringens was determined by the Most Probable Number method, as described by Bezirtzoglou and Romond (1990). The incidence of Salmonella spp. was determined in 25 g compost samples (25 g compost sample C225 ml peptone water for pre-enrichment, 37 8C, 16–20 h). Enrichment was operated in both Rappaport– Vassiliadis and tetrathionate broth (42 8C, 24 h). Streaking onto SS agar, XLD agar and BS agar petri dishes, resulted in several suspected colonies, which were identified with the API 20E system (Biomerieux) (BAM, 2001).
3. Results and discussion Most products were available in 20 l bags and their price ranged from 20 to 35 V. Some products, mainly derived from manures, were available in bulk at prices ranging from 25 to 40 V/m3. The MSW composts were not marketed but were used for landfill and coal-mine restoration, with the transport costs covered by the municipality operating the plant. The spent mushroom composts were offered at prices ranging from 0 to 10 V/m3 to form the basis for various soil improvers, empirically formulated and available locally in bulk. Moisture and organic matter content varied widely, even among composts of the same group. This may be confusing or misleading for consumers, especially when materials are marketed on a weight basis. Moisture content varied from 6% for an MSW compost (W1) to 70% for a peat-based product (P3). Several national standards set an upper limit value for moisture content to prevent ‘selling water’ and the development of anaerobic conditions during storage. Eighteen samples (64%) exceeded the Greek limit of 40% moisture, while 13 (46%) exceeded the most commonly met limit value of 45% (Italy, Luxembourg and Germany). However, all samples satisfied the minimum requirement of 25% dry matter of the eco-label standard for soil improvers and growing media (EC, 2001). All composts contained more than 40% organic matter, aside from two samples from the group of manures (K1, K2) and two from the group of products of exclusively plant origin (F2, F3). To receive the EU eco-label, products should contain no less than 20% OM, which was met by all samples examined. However, the parameter does not provide any insight on the quality, type and stability of the organic matter, which is why it is not specified in any of the statutory national standards. It is believed that a statutory minimum value for OM content could be useful, to prevent blends with e.g. soil from being offered as composts, but this should be defined in parallel with product stability (Hogg et al., 2002). The high OM content of the Greek composts indicates that such blending is probably not practiced. All samples tested from almost neutral to slightly alkaline, with pH values ranging from 6.3 to 8.9, apart from one product containing peat, which was more acidic (Fig. 1). Greek
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Fig. 1. pH values and electrical conductivity of the products analyzed.
statutory standards for MSW and sludge derived composts (Anon, 1997) require that their pH lies within the range of 6.0–8.0, which was not met by 12 samples (43%) mainly from the K and V groups. In most national standards that define pH limits (e.g. Italy, Belgium), composts should have a pH value within the range of 6.0–8.5 to ensure compatibility with most plants (Hogg et al., 2002), a condition not met by seven samples (25%). The EC decision on awarding the eco-label requires that information is provided on the product pH but does not set limit values. The Greek standards’ upper limit of 4.0 dS/m for electrical conductivity, a level considered tolerable by plants of medium sensitivity, was surpassed by eight samples (29%). Five of these (K5, K6, F5, V2 and V4) showed values from 6 to over 12 dS/m (Fig. 1), indicating toxicity due to salts for most plants, if used undiluted in potting mixtures. Such usage restrictions were usually not indicated on the label. The high phytotoxicity (Fig. 2) exhibited by these samples may be, at least in part, attributed to their high EC values. Iannotti et al. (1994) found a strong inhibition of cress seed germination by MSW compost water extracts, at all maturity levels, which was mainly attributed to the high salinity of the compost. The EC decision on the eco-label award sets an upper conductivity limit of 1.5 dS/m for growing media only, i.e. materials other than soil in situ, in which plants are grown. This was met by six products (21%), half of which were peat-based. Stability of the composts was estimated by the self-heating potential and GI (Fig. 2). According to the former, as used in the Rottegrad definition in Germany, the Netherlands,
Luxembourg and elsewhere, all samples would be classified as mature composts (class V for all samples except for W3, which was class IV). Results were more diversified for GI, which varied from 25% for K6, to 151% for K7. Five samples (18%), mostly from the class of manure composts, exhibited a GI below 80%, indicating phytotoxicity. There was a significant correlation between GI and electrical conductivity (p!0.001), indicating that for composts with a wide range of conductivity values, GI is mainly governed by this parameter rather than compost stability. The EC decision on the eco-label award demands that products shall not adversely affect plant emergence or subsequent growth, but does not specify testing methods and limit values. Its only other reference to stability is indirect, requiring that products do not give off offensive odors. Stability is increasingly recognized as an important compost quality characteristic, as poorly stabilized composts may be unsuitable, even harmful, for some applications. Plant damage due to the presence of phytotoxic compounds, reduced nitrogen availability or soil oxygen depletion, as well as odor nuisances and potential regrowth of pathogens are among the main problems related to unstable composts (Lasaridi, 1998; Saviozzi et al., 2004). There is still no clear agreement on methods for determining compost stability and many believe that it may not be possible to accurately assess stability on the basis of a single parameter. Nevertheless, several countries (Austria, Belgium-Flanders, Denmark, Germany, Luxembourg, Netherlands, Sweden, UK, Canada, Australia, New Zealand, states in the USA) have implemented some form of measurement for stability in their statutory or voluntary
Fig. 2. Range of the germination index values and the self-heating potential, measured as maximum temperature above ambient, for the samples analyzed.
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Table 2 Heavy metals concentration in the composts examined and selected limit values (in mg/kg dm) Compost code
Ni
Pb
Zn
Cu
K1 K2 K3 K5 K7 F1 F2 F3 F4 F5 V3 M4 W1 W2 W3 Eco-label limits Greek limits Austrian limits (AC)
18.4 46.2 48.5 266.5 65.8 123.3 49.4 72.2 337.7 2.3 59.6 70.6 28.4 29.4 37.9 50.0 200.0 25.0
9.2 28.5 8.1 24.1 7.2 8.6 14.1 9.0 44.9 6.2 3.7 11.5 479.5 642.7 770.1 100.0 500.0 45.0
234 569 73 3984 269 118 190 309 721 99 260 109 732 497 565 300 2000 200
27.5 68.8 11.2 45.3 25.0 25.9 24.8 26.0 28.6 2.0 16.4 14.3 346.4 271.9 286.8 100.0 500.0 70.0
standards, most often based on respiration activity (including the self-heating test) or phytotoxicity assessment (Hogg et al., 2002). With the exception of nickel, the concentration of the heavy metals determined was higher for the MSW composts (Table 2), although some manure and plant residue composts also exhibited particularly high levels of nickel and zinc. Few composts did not comply with the rather lenient limits defined in Greek legislation (two for Ni, two for Pb and one for Zn). Other southern European countries, such as Italy, Spain and Portugal set limit values within the same high range, so most composts could also gain access to these markets. More stringent values are set in the EC decision on the eco-label award, which nevertheless are still met by most composts, with the exception of zinc. None of the MSW composts, derived from mechanically separated waste, met the eco-label standards for Pb, Zn and Cu. Many northern EU countries have set particularly stringent heavy metal limits, especially for high class composts whose unrestricted use is permitted. These limits are considerably stricter than
those for the eco-label award for soil improvers and growing media. In Table 2 the limits set for the AC class composts in Austria are included, although still stricter limits exist in other countries. Only one compost, F5, of exclusively plant origin, met these limits, most composts failing the limit for Ni and to a lesser extent, Zn. The wide discrepancy among the heavy metal limits in the different EU countries, with a distinct north–south division, leads to very different soil conservation actions, seems to reflect different attitudes toward environmental protection and certainly does not contribute to the establishment of a common market for this type of product. It is also worth noticing that a product which qualifies for the eco-label award may not be eligible for unrestricted use in many EU countries. The development of a compost directive, with different foci varying from a ‘product directive’ that simply aims at setting quality standards for composts, to a ‘strategy directive’ that promotes source separation of biowaste, has had a long, rather unsuccessful history over the last 20 years and its future is still unknown. This would be an important contribution to leveling the field in the compost markets and biowaste treatment, while at the same time ensuring soil and environment protection, presumably based on the best available scientific knowledge, risk assessment and the precautionary principle. The population of total mesophilic bacteria reached very high levels (O108 cfu/g of compost) in most composts, with the exception of K6, F1, F2, F3, V2 and V3, where the population varied within the range of 106–108 cfu/g of compost (Fig. 3). Composts constitute a suitable substrate for the microorganisms, as they are characterized by high OM and moisture content and appropriate pH levels. The mesophilic bacteria population of the MSW composts was approximately 1010 cfu/g of compost, a high value that could be attributed to the presence of easily biodegradable OM in this type of material. However, the values measured in this study were considerably higher than those reported elsewhere (Hassen et al., 2001; Deportes et al., 1998). Spore forming bacteria were detected in all composts at levels higher than 106 cfu/g of compost and in several samples (F1, F2, F4, F5, V1, M1, M3 and W3) they exceeded the level of 108 cfu/g of compost
Fig. 3. Populations of total aerobic and spore forming bacteria in the products analyzed. Bars represent standard deviation (nZ3).
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Fig. 4. Populations of fungi and yeasts in the composts analyzed. Bars represent standard deviation (nZ3).
Fig. 5. Populations of selected pathogen categories in the composts analyzed. Bars represent standard deviation (nZ3).
(Fig. 3). High populations of spore forming bacteria are common in composted materials, as these bacteria survive the high temperatures of the process through their transformation into a very resistant form, the endospore. Yeasts were detected in only eight samples (F3, V1, V3, M1, M3, M4, W1 and W3) and their population varied from 102 to 106 cfu/g of compost. In contrast, filamentous fungi were detected in all composts examined with the exception of M2 (Fig. 4). Fecal coliforms were found at high levels, although the Greek statutory standards require the absence of coliforms in compost (Anon, 1997). It is remarkable that fecal coliforms were not detected in MSW composts and
neither in K5 and K6 samples, although they are derived from animal manure (Fig. 5). The population of fecal streptococci is another indicator of product sanitization during composting and is directly related to the high temperatures attained. Some bacterial limit values for sanitized compost have been proposed as 5!102 cfu/g of dry weight for fecal coliforms, 5!103 cfu/g of dry weight for fecal streptococci and 0 Salmonella in 100 g (Zucconi and de Bertoldi, 1987) although Deportes et al. (1998) observed the disappearance of Salmonella and Ascaris eggs at higher fecal streptococci concentrations. In this study all the composts examined exceeded this limit for fecal
Fig. 6. Population levels of Clostridium perfringens.
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coliforms. In the group of manure-derived composts (group K) the highest populations of fecal streptococci were measured, but the abovementioned limit value was also exceeded by spent mushroom and MSW composts (W2, W3, M1 and M2) as well as by V1 and V5 (Fig. 5). Nevertheless, Salmonella spp. was not detected in any case. The human pathogen Staphylococcus aureus is transmitted with food consumption and its presence in compost used for vegetable production can cause serious epidemiological problems (Beuchat, 1998; Michino and Otsuki, 2000; Nguz et al., 2005). The pathogen has been detected in three manure composts (K1, K3 and K4) and one of exclusively plant origin (F4). The population of the pathogen in all these cases exceeded 105 cfu/g of compost (Fig. 5). C. perfringens constitutes a very important fecal indicator, as it is a member of human enteric flora but also a human pathogen. It was detected in all composts with the exception of M1 (Fig. 6). This bacterium can survive even in high temperatures because it transforms into endospores.
4. Conclusions Today the quality of compost is the most essential criterion in recycling organic waste, as well as its marketing and utilization in agriculture. Environmentally safe recycling of organic waste to agricultural land could be crucial to sustaining soil productivity in Mediterranean areas, where soil organic matter content is very low. This seems to be well understood by farmers, as reflected in the high prices of composted products on the Greek market. Analysis of the 28 composts that were identified on the Greek market revealed broad variations for all parameters examined, even within the same group of products. The heavy metal content ranged from levels exceeding the fairly lenient Greek standards to below the stringent limits for AC class compost in Austria. About 25% of the composts examined met the heavy metal limits for the EU eco-label award. Salmonella spp. was not detected in any of the composts but S. aureus and C. perfringens were found in 17 and 96% of the composts respectively. Pathogen indicator microorganisms were present at values above suggested limits in all the composts. The high variability of such important parameters in composts available on the Greek market suggests an urgent need for establishing quality assurance procedures and mechanisms in the country, in order to build customer confidence and increase the reliability of composting itself as a waste treatment process, which can result in useful products. Moreover, the wide range of limit values within EU member states suggests the need for developing EU compost quality standards, in order to harmonize the compost markets. Such standards should be based on commonly accepted sound science, risk assessment and the precautionary principle, and should be no more stringent than those for the eco-label award for soil conditioners and growing media.
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Quality assessment of composts in the Greek market: The need for standards and quality assurance Katia Lasaridi a,*, Ioanna Protopapa a, Maria Kotsou a, George Pilidis b, Thakis Manios c, Adamantini Kyriacou a a
Harokopio University, 70 El. Venizelou, 17671 Kallithea, Athens, Greece b University of Ioannina, Dourouti, 451 10 Ioannina, Greece c Technological Education Institute of Crete, School of Agricultural Technology, Heraklion, Greece Received 28 February 2005; received in revised form 18 July 2005; accepted 18 August 2005 Available online 5 December 2005
Abstract An extensive data survey and study of the Greek market for composts or products marketed as such was carried out in order to acquire a comprehensive image of the local situation, in view of the proposed operation of large municipal solid waste (MSW) composting facilities and EU legislation changes. Physical and chemical parameters (moisture, organic matter, electrical conductivity, pH and heavy metals), stability indicators (self-heating potential, germination index) and biological indicators (microbial population, pathogen indicators and selected pathogens) were analyzed for the assessment of product quality. Results revealed wide variations even within the same group of products, which is particularly significant for parameters directly related to environmental protection and public health. The heavy metal content ranged from levels exceeding the fairly lenient Greek standards to below the stringent limits for AC class compost in Austria. About 25% of the composts examined met the heavy metal limits for the EU eco-label award. Salmonella spp. was not detected in any of the composts but Staphylococcus aureus and Clostridium perfringens were found in 17 and 96% of the composts respectively. Pathogen indicator microorganisms were present at levels above suggested limits in all the composts. The high variability of such important parameters in composts available on the Greek market suggests an urgent need for establishing quality assurance procedures and mechanisms in the country. Moreover, the wide range of limit values within EU member states suggests the need for developing EU compost quality standards, in order to harmonize the compost markets. q 2005 Elsevier Ltd. All rights reserved. Keywords: Compost quality; Pathogens; Stability; Heavy metals; Compost microbial community
1. Introduction Compost quality is a difficult to define and often elusive term, meaning different things to different people according to their professional background and national legislations. Nevertheless, compost quality lies at the core of the issue of composting and biological treatment in general, as it defines the marketing potential and the outlets of the product and in most cases, the viability of the treatment plant, but also the long-term acceptability of biological treatment as a valuable option in the waste hierarchy (Lasaridi, 1998).
* Corresponding author. Tel.: C30 210 9549164/9549150; fax: C30 210 9514759. E-mail address: [email protected] (K. Lasaridi).
0301-4797/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2005.08.011
Compost quality refers to the overall state of the compost in regard to physical, chemical and biological characteristics, which indicate the ultimate impact of the compost on the environment. The quality of compost is determined by the sum of its different features and properties; specifications are usually determined by minimum admissible levels of required substances or maximum tolerable limits for unwanted ones (de Bertoldi, 1993; Hogg et al., 2002). The criteria that are relevant to the evaluation of quality depend on what purpose the compost is used for, the relevant environmental protection policies and the market requirements (Gillett, 1992; Kehres, 1992). For example, composts intended as growing media should meet more stringent quality criteria compared to composts that will be used as landfill cover. A number of characteristics determine compost quality, such as particle size distribution, moisture, organic matter and carbon content, concentration and composition of humus-like substances, nitrogen content and forms of N, phosphorus and
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potassium, heavy metals, salinity and the nature of ions responsible for it, cation exchange capacity, water holding capacity, porosity and bulk density, inert contaminants, pathogens, and state of maturity or stability (Lasaridi, 1998). However, the most important, from the point of view of standards for the protection of public health, the soil and the environment in general, are those relating to pathogens, inorganic and organic potentially toxic compounds (heavy metals, PCBs, PAHs, phthalates etc.) and stability, the latter determining compost nuisance potential, nitrogen immobilization and leaching and phytotoxicity (Brinton, 2000; Deportes et al., 1995; Hogg et al., 2002). Currently, compulsory and voluntary compost standards in different countries are characterized by a great degree of heterogeneity, stemming from the effort to combine two often contradicting targets: maximum environmental and public health protection on the one hand and maximum organic matter recycling on the other. Moreover, the precautionary approach adopted in the EU and the risk assessment approach prevailing in the USA, may lead to broad differences in the accepted limit values for a number of critical parameters, such as heavy metals (Hogg et al., 2002). Even within the EU there is a wide variation among the limit values adopted by the member countries, with the north being usually more stringent than the south, reflecting mainly the varying level of progress on source separation of the biodegradable fraction of MSW, but also the different needs in soil organic matter. The adoption of the long debated ‘biowaste directive’ would alter this situation, leading to a more homogeneous European market for composted products. However, the delivery of a stand-alone ‘biowaste directive’ is currently uncertain and the discussion for including relevant standards and targets in a framework directive, such as a ‘soil framework directive’ or the revision of the waste framework directive is still at a preliminary level. All national compost standards include compost sanitization criteria for human pathogens and occasionally for plant pathogens. These criteria may refer to the product (absence of Salmonella, absence or low levels of fecal coliforms and fecal streptococci), the process (setting a minimum period for which the compost should maintain a temperature higher than a designated level) or both. Maximum permissible values are set for heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) although the limits vary widely (Deportes et al., 1995; Hogg et al., 2002). Similar values are set for foreign matter (glass, plastics and stones) in most national specifications, usually defined as maximum allowed content on a dry weight basis and in reference to their particle size (Hogg et al., 2002; Brinton, 2000). The degree of compost stability and its nitrogen content are particularly important for its agronomic use and are increasingly more often defined in compost specifications. Compost stability is no longer defined as the C/N ratio, as was often the case in older standards, but usually on the basis of compost microbial activity measured through the respiration activity (e.g. AT4, Dynamic Respiration Index, SOUR) or the self-heating potential (Rottegrade) (FCQAO, 1994; Hogg et al., 2002; Lasaridi and Stentiford, 1998).
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In Greece the existing compost quality specifications refer to composts from MSW and sewage sludge and require that Salmonella and enterobacteriaceae be absent with no further reference to plant pathogens or process specifications. As enterobacteriaceae are commonly encountered in various environmental media, such as soil, the requirement for their absence in composts is particularly stringent and difficult to meet. On the other hand, rather lax limits for heavy metal content are permitted, following the 86/278/EEC Directive on the use of sewage sludge in agriculture. Maximum allowable limits for compost foreign matter are also set. Parameters for the determination of compost stability, nitrogen content and/or phytotoxicity are not defined (Anon, 1997). The EU eco-label criteria for soil improvers and growing media (Decision 2001/688/EC) are also applicable in Greece, but they are rather demanding in terms of quality requirements and application rates and thus relevant only for the top products on the market. For the remaining compost products available on the Greek market, whether produced locally or imported, it is presumed that they should at least conform to the requirements for the MSW composts (Anon, 1997). However, currently there are neither mechanisms nor organizations responsible for the analysis, control and certification of the composts available on the market. This results in low quality products making their way onto the market, creating consumer distrust of compost products in general. It also distorts competition between high and low quality compost and may allow the use of products that are harmful to public health, plants and the environment. The aim of this study was to identify, register and analyze all compost products available on the Greek market, in order to derive their quality profile and examine their compliance with Greek standards. Moreover, as more stringent EU-wide standards are forthcoming, compost compliance with more stringent limit values established in different EU countries was also examined.
2. Materials and methods 2.1. Market research The different compost materials available on the Greek market at the time of this study (January–June 2003) were identified through both desk and field research. Sources for the desk study included the Internet, agricultural magazines, environmental and agricultural exhibition guides, associations for the certification of biological agriculture products and personal contacts with compost producers. This was followed by a three months on-site market research in the wider Athens area, in super-markets, garden centers and agricultural provisions stores. A total of 28 products were identified and acquired in their original packaging. The products were classified into six groups according to their main substrates, as specified in the product description on the packaging and a code system was used instead of brand names to ensure confidentiality (Table 1). After closer examination it was
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Table 1 Description of the different groups of composts and soil improvers identified on the Greek market in the period from January to June 2003 Group code
Nunber of products
Description
K1–K7 F1–F5 P1–P4
7 5 4
V1–V5
5
M1–M4
4
W1–W3
3
Composts containing manures Composts of exclusively plant origin (withdrawn fruits, olive mill press-cake, seaweed, etc) Various peat products, marketed as composts. Biological indicators were not determined for these products as they did not contain waste materials Various products, the composition of which could not be clearly identified from the label information, marketed as composts and believed to be such, with the exception of V4, which was leonardite and was not analyzed for biological indicators Fresh and spent mushroom compost-not available in the market. The spent mushroom compost is utilized as a component in potting mixtures sold in bulk by various local producers Compost from mechanically separated MSW-not sold on the market but used as landfill cover and for coalmine restoration
proven that five of them (P1–P4 and V4) were not composts despite being marketed as such. 2.2. Analytical methods Moisture and organic matter (OM) content were determined through the weight loss at 105 and 550 8C, respectively. Electrical conductivity (EC), the pH value and the self-heating potential (Rottegrade) were determined according to FCQAO (1994). Phytotoxicity, a parameter partly related to stability, was calculated as the Germination Index (GI) of Lepidium sativum seeds (Agrocementi Ltd, Athens), using a modified version of the method proposed by Zucconi et al. (1985). Twenty five seeds were placed on five layers of filter paper pads wetted with 5 ml of 1:10 compost aqueous extract in petri dishes incubated in the dark at 25 8C for 48 h. Tap water was used as control. All aforementioned analyses were performed in triplicate. Heavy metals (Pb, Ni, Zn and Cu) were determined after digestion of 1 g (dw) of pulverized sample with pure nitric acid, using AAS (Shimazu 6800 Flame Atomic Absorption Spectrophotometer) (Sposito et al., 1983). The total mesophilic bacteria, the spore forming bacteria, the staphylococci, the yeasts and the filamentous fungi were determined by the dilution plate method as described by Hassen et al. (2001). For the fecal coliforms the eosin-methylen blue-lactose sucrose agar was used as described by Ichida et al. (2001). Colonies were randomly isolated and microscopically examined (Gram-bacilli, oxidase negative). For the fecal streptococci the Slanetz and Bartley medium was used as described by Vuorinen and Saharinen (1999). Colonies were randomly isolated and microscopically examined (GramC cocci, catalase negative). The population of Clostridium perfringens was determined by the Most Probable Number method, as described by Bezirtzoglou and Romond (1990). The incidence of Salmonella spp. was determined in 25 g compost samples (25 g compost sample C225 ml peptone water for pre-enrichment, 37 8C, 16–20 h). Enrichment was operated in both Rappaport– Vassiliadis and tetrathionate broth (42 8C, 24 h). Streaking onto SS agar, XLD agar and BS agar petri dishes, resulted in several suspected colonies, which were identified with the API 20E system (Biomerieux) (BAM, 2001).
3. Results and discussion Most products were available in 20 l bags and their price ranged from 20 to 35 V. Some products, mainly derived from manures, were available in bulk at prices ranging from 25 to 40 V/m3. The MSW composts were not marketed but were used for landfill and coal-mine restoration, with the transport costs covered by the municipality operating the plant. The spent mushroom composts were offered at prices ranging from 0 to 10 V/m3 to form the basis for various soil improvers, empirically formulated and available locally in bulk. Moisture and organic matter content varied widely, even among composts of the same group. This may be confusing or misleading for consumers, especially when materials are marketed on a weight basis. Moisture content varied from 6% for an MSW compost (W1) to 70% for a peat-based product (P3). Several national standards set an upper limit value for moisture content to prevent ‘selling water’ and the development of anaerobic conditions during storage. Eighteen samples (64%) exceeded the Greek limit of 40% moisture, while 13 (46%) exceeded the most commonly met limit value of 45% (Italy, Luxembourg and Germany). However, all samples satisfied the minimum requirement of 25% dry matter of the eco-label standard for soil improvers and growing media (EC, 2001). All composts contained more than 40% organic matter, aside from two samples from the group of manures (K1, K2) and two from the group of products of exclusively plant origin (F2, F3). To receive the EU eco-label, products should contain no less than 20% OM, which was met by all samples examined. However, the parameter does not provide any insight on the quality, type and stability of the organic matter, which is why it is not specified in any of the statutory national standards. It is believed that a statutory minimum value for OM content could be useful, to prevent blends with e.g. soil from being offered as composts, but this should be defined in parallel with product stability (Hogg et al., 2002). The high OM content of the Greek composts indicates that such blending is probably not practiced. All samples tested from almost neutral to slightly alkaline, with pH values ranging from 6.3 to 8.9, apart from one product containing peat, which was more acidic (Fig. 1). Greek
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Fig. 1. pH values and electrical conductivity of the products analyzed.
statutory standards for MSW and sludge derived composts (Anon, 1997) require that their pH lies within the range of 6.0–8.0, which was not met by 12 samples (43%) mainly from the K and V groups. In most national standards that define pH limits (e.g. Italy, Belgium), composts should have a pH value within the range of 6.0–8.5 to ensure compatibility with most plants (Hogg et al., 2002), a condition not met by seven samples (25%). The EC decision on awarding the eco-label requires that information is provided on the product pH but does not set limit values. The Greek standards’ upper limit of 4.0 dS/m for electrical conductivity, a level considered tolerable by plants of medium sensitivity, was surpassed by eight samples (29%). Five of these (K5, K6, F5, V2 and V4) showed values from 6 to over 12 dS/m (Fig. 1), indicating toxicity due to salts for most plants, if used undiluted in potting mixtures. Such usage restrictions were usually not indicated on the label. The high phytotoxicity (Fig. 2) exhibited by these samples may be, at least in part, attributed to their high EC values. Iannotti et al. (1994) found a strong inhibition of cress seed germination by MSW compost water extracts, at all maturity levels, which was mainly attributed to the high salinity of the compost. The EC decision on the eco-label award sets an upper conductivity limit of 1.5 dS/m for growing media only, i.e. materials other than soil in situ, in which plants are grown. This was met by six products (21%), half of which were peat-based. Stability of the composts was estimated by the self-heating potential and GI (Fig. 2). According to the former, as used in the Rottegrad definition in Germany, the Netherlands,
Luxembourg and elsewhere, all samples would be classified as mature composts (class V for all samples except for W3, which was class IV). Results were more diversified for GI, which varied from 25% for K6, to 151% for K7. Five samples (18%), mostly from the class of manure composts, exhibited a GI below 80%, indicating phytotoxicity. There was a significant correlation between GI and electrical conductivity (p!0.001), indicating that for composts with a wide range of conductivity values, GI is mainly governed by this parameter rather than compost stability. The EC decision on the eco-label award demands that products shall not adversely affect plant emergence or subsequent growth, but does not specify testing methods and limit values. Its only other reference to stability is indirect, requiring that products do not give off offensive odors. Stability is increasingly recognized as an important compost quality characteristic, as poorly stabilized composts may be unsuitable, even harmful, for some applications. Plant damage due to the presence of phytotoxic compounds, reduced nitrogen availability or soil oxygen depletion, as well as odor nuisances and potential regrowth of pathogens are among the main problems related to unstable composts (Lasaridi, 1998; Saviozzi et al., 2004). There is still no clear agreement on methods for determining compost stability and many believe that it may not be possible to accurately assess stability on the basis of a single parameter. Nevertheless, several countries (Austria, Belgium-Flanders, Denmark, Germany, Luxembourg, Netherlands, Sweden, UK, Canada, Australia, New Zealand, states in the USA) have implemented some form of measurement for stability in their statutory or voluntary
Fig. 2. Range of the germination index values and the self-heating potential, measured as maximum temperature above ambient, for the samples analyzed.
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Table 2 Heavy metals concentration in the composts examined and selected limit values (in mg/kg dm) Compost code
Ni
Pb
Zn
Cu
K1 K2 K3 K5 K7 F1 F2 F3 F4 F5 V3 M4 W1 W2 W3 Eco-label limits Greek limits Austrian limits (AC)
18.4 46.2 48.5 266.5 65.8 123.3 49.4 72.2 337.7 2.3 59.6 70.6 28.4 29.4 37.9 50.0 200.0 25.0
9.2 28.5 8.1 24.1 7.2 8.6 14.1 9.0 44.9 6.2 3.7 11.5 479.5 642.7 770.1 100.0 500.0 45.0
234 569 73 3984 269 118 190 309 721 99 260 109 732 497 565 300 2000 200
27.5 68.8 11.2 45.3 25.0 25.9 24.8 26.0 28.6 2.0 16.4 14.3 346.4 271.9 286.8 100.0 500.0 70.0
standards, most often based on respiration activity (including the self-heating test) or phytotoxicity assessment (Hogg et al., 2002). With the exception of nickel, the concentration of the heavy metals determined was higher for the MSW composts (Table 2), although some manure and plant residue composts also exhibited particularly high levels of nickel and zinc. Few composts did not comply with the rather lenient limits defined in Greek legislation (two for Ni, two for Pb and one for Zn). Other southern European countries, such as Italy, Spain and Portugal set limit values within the same high range, so most composts could also gain access to these markets. More stringent values are set in the EC decision on the eco-label award, which nevertheless are still met by most composts, with the exception of zinc. None of the MSW composts, derived from mechanically separated waste, met the eco-label standards for Pb, Zn and Cu. Many northern EU countries have set particularly stringent heavy metal limits, especially for high class composts whose unrestricted use is permitted. These limits are considerably stricter than
those for the eco-label award for soil improvers and growing media. In Table 2 the limits set for the AC class composts in Austria are included, although still stricter limits exist in other countries. Only one compost, F5, of exclusively plant origin, met these limits, most composts failing the limit for Ni and to a lesser extent, Zn. The wide discrepancy among the heavy metal limits in the different EU countries, with a distinct north–south division, leads to very different soil conservation actions, seems to reflect different attitudes toward environmental protection and certainly does not contribute to the establishment of a common market for this type of product. It is also worth noticing that a product which qualifies for the eco-label award may not be eligible for unrestricted use in many EU countries. The development of a compost directive, with different foci varying from a ‘product directive’ that simply aims at setting quality standards for composts, to a ‘strategy directive’ that promotes source separation of biowaste, has had a long, rather unsuccessful history over the last 20 years and its future is still unknown. This would be an important contribution to leveling the field in the compost markets and biowaste treatment, while at the same time ensuring soil and environment protection, presumably based on the best available scientific knowledge, risk assessment and the precautionary principle. The population of total mesophilic bacteria reached very high levels (O108 cfu/g of compost) in most composts, with the exception of K6, F1, F2, F3, V2 and V3, where the population varied within the range of 106–108 cfu/g of compost (Fig. 3). Composts constitute a suitable substrate for the microorganisms, as they are characterized by high OM and moisture content and appropriate pH levels. The mesophilic bacteria population of the MSW composts was approximately 1010 cfu/g of compost, a high value that could be attributed to the presence of easily biodegradable OM in this type of material. However, the values measured in this study were considerably higher than those reported elsewhere (Hassen et al., 2001; Deportes et al., 1998). Spore forming bacteria were detected in all composts at levels higher than 106 cfu/g of compost and in several samples (F1, F2, F4, F5, V1, M1, M3 and W3) they exceeded the level of 108 cfu/g of compost
Fig. 3. Populations of total aerobic and spore forming bacteria in the products analyzed. Bars represent standard deviation (nZ3).
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Fig. 4. Populations of fungi and yeasts in the composts analyzed. Bars represent standard deviation (nZ3).
Fig. 5. Populations of selected pathogen categories in the composts analyzed. Bars represent standard deviation (nZ3).
(Fig. 3). High populations of spore forming bacteria are common in composted materials, as these bacteria survive the high temperatures of the process through their transformation into a very resistant form, the endospore. Yeasts were detected in only eight samples (F3, V1, V3, M1, M3, M4, W1 and W3) and their population varied from 102 to 106 cfu/g of compost. In contrast, filamentous fungi were detected in all composts examined with the exception of M2 (Fig. 4). Fecal coliforms were found at high levels, although the Greek statutory standards require the absence of coliforms in compost (Anon, 1997). It is remarkable that fecal coliforms were not detected in MSW composts and
neither in K5 and K6 samples, although they are derived from animal manure (Fig. 5). The population of fecal streptococci is another indicator of product sanitization during composting and is directly related to the high temperatures attained. Some bacterial limit values for sanitized compost have been proposed as 5!102 cfu/g of dry weight for fecal coliforms, 5!103 cfu/g of dry weight for fecal streptococci and 0 Salmonella in 100 g (Zucconi and de Bertoldi, 1987) although Deportes et al. (1998) observed the disappearance of Salmonella and Ascaris eggs at higher fecal streptococci concentrations. In this study all the composts examined exceeded this limit for fecal
Fig. 6. Population levels of Clostridium perfringens.
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coliforms. In the group of manure-derived composts (group K) the highest populations of fecal streptococci were measured, but the abovementioned limit value was also exceeded by spent mushroom and MSW composts (W2, W3, M1 and M2) as well as by V1 and V5 (Fig. 5). Nevertheless, Salmonella spp. was not detected in any case. The human pathogen Staphylococcus aureus is transmitted with food consumption and its presence in compost used for vegetable production can cause serious epidemiological problems (Beuchat, 1998; Michino and Otsuki, 2000; Nguz et al., 2005). The pathogen has been detected in three manure composts (K1, K3 and K4) and one of exclusively plant origin (F4). The population of the pathogen in all these cases exceeded 105 cfu/g of compost (Fig. 5). C. perfringens constitutes a very important fecal indicator, as it is a member of human enteric flora but also a human pathogen. It was detected in all composts with the exception of M1 (Fig. 6). This bacterium can survive even in high temperatures because it transforms into endospores.
4. Conclusions Today the quality of compost is the most essential criterion in recycling organic waste, as well as its marketing and utilization in agriculture. Environmentally safe recycling of organic waste to agricultural land could be crucial to sustaining soil productivity in Mediterranean areas, where soil organic matter content is very low. This seems to be well understood by farmers, as reflected in the high prices of composted products on the Greek market. Analysis of the 28 composts that were identified on the Greek market revealed broad variations for all parameters examined, even within the same group of products. The heavy metal content ranged from levels exceeding the fairly lenient Greek standards to below the stringent limits for AC class compost in Austria. About 25% of the composts examined met the heavy metal limits for the EU eco-label award. Salmonella spp. was not detected in any of the composts but S. aureus and C. perfringens were found in 17 and 96% of the composts respectively. Pathogen indicator microorganisms were present at values above suggested limits in all the composts. The high variability of such important parameters in composts available on the Greek market suggests an urgent need for establishing quality assurance procedures and mechanisms in the country, in order to build customer confidence and increase the reliability of composting itself as a waste treatment process, which can result in useful products. Moreover, the wide range of limit values within EU member states suggests the need for developing EU compost quality standards, in order to harmonize the compost markets. Such standards should be based on commonly accepted sound science, risk assessment and the precautionary principle, and should be no more stringent than those for the eco-label award for soil conditioners and growing media.
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