The use of respiration indices in the composting process: a review

Respiration is directly related to the metabolic activity of a microbial population. Micro-organisms respire at higher rates in the presence of large amounts of bioavailable organic matter while respiration rate is slower if this type of material is scarce. In the composting process respiration activity has become an important parameter for the determination of the stability of compost. It is also used for the monitoring of the composting process and it is considered an important factor for the estimation of the maturity of the material. A wide range of respirometric protocols has been reported based either on CO2 production, O2 uptake or release of heat. The most common methods are those based on O2 uptake. Respirometric assays are affected by a number of parameters including temperature, humidity, and both incubation and pre-incubation conditions. Results from respirometries are generally expressed as ‘respiration indices’, most of them with their own units and basis. In consequence, some confusion exists when referring and comparing respiration indices. This is particularly important because current and future legislations define and measure the biological stability of waste on the basis of respiration activity of the material. This paper discusses and compares most common respiration indices currently used.


Introduction
Composting is a natural aerobic process by which microorganisms transform putrescible organic matter into CO 2 , H 2 O and complex metastable compounds (e.g. humic substances). The final product, the compost, is a stable, sanitized and humus-like material.
Maturity and stability are important parameters for compost quality assessment. Maturity is a general term describing the fitness of a compost for a particular end use (Brewer & Sullivan 2001). It is commonly associated with plant-growth potential or phytotoxicity (Iannotti et al. 1993). Mature composts are ready to use; they contain negligible or acceptable concentrations of phytotoxic compounds such as NH 3 or short-chain organic acids (Brewer & Sullivan 2003).
Stability can be defined as the extent to which readily biodegradable material has decomposed. A material is considered unstable if it contains a high proportion of biodegradable matter that may sustain high microbial activity. If the material contains mainly recalcitrant or humus-like matter, it is not able to sustain microbial activity and therefore, it is considered stable. Stability is not only an important compost quality characteristic but it can also be used for process performance monitoring and comparative evaluation of different composting systems (Lasaridi & Stentiford 1998).
Respiration can be considered as a general measure of the microbial activity. It can provide a reliable, repeatable and sci-Waste Management & Research entifically sound assessment of microbial activity. For this reason, respirometry (CO 2 evolution rate and/or O 2 uptake rate) has been widely used to evaluate microbial activity and therefore, stability of a compost sample. Different respiration indices, obtained from different respirometry techniques, are currently used to determine the level of microbial activity in a sample of compost as determined by a respiration test. In general, a respiration index (RI) can be defined as the rate of O 2 uptake or CO 2 evolution of a sample under specific conditions. All indices use their own units and nomenclature. Furthermore, some of them have a threshold value, below which will determine if a compost is stable or not. This has produced a certain degree of confusion when referring to respirometric techniques and stability limits.
On this basis, the objective of this paper is to review and discuss the different respirometric techniques currently available and the different stability limits that have been proposed based on respiration indices. A detailed description of the analytical procedures used in the respiration measurements is also presented.

Methods for determining respirometric activity
As mentioned above, respirometric activity of a material can be directly determined either from the O 2 uptake or the CO 2 production. It can also be indirectly estimated from the released heat during the process. Figure 1 shows a general diagram of the general procedure to obtain the respiration index of a compost sample. The following methods have been described for the determination of the respirometric activity.

Self-heating test
This method measures the temperature increase due to the heat released from the biological and chemical activity of a compost sample. It is a handy and suitable method for everyday operations. It is simple to implement and results are easy to understand. It is widely used in Europe and North America (Brinton et al. 1995, ADAS 2003. However, it could be argued that this test cannot be directly correlated to respiration since many chemical and biochemical reactions not related to respiration are also exothermal. Moreover, biomass heating is also influenced by other factors such as porosity or moisture content. Nevertheless, Koening & Bari (2000) indirectly determined the respirometric activity of a compost sample from results obtained in a self-heating test using a bioenergy approach to estimate the heat generated along the process. The maximum respirometric activity was obtained on the basis that the generation of 14 000 J of biological heat consumes 1 g of O 2 .
Methods based on CO 2 production These methods are widely used in commercial laboratories. Their equipment is generally very simple and easy to use. CO 2 production is directly correlated with the aerobic respiration. Among the most commonly used are those that use alkaline traps to fix the CO 2 . These methods include the commercial kit Solvita ® , widely used for the determination of the respirometric activity and ammonia production of volumetric compost samples. There are also more complex methods based on colorimetric techniques and gas chromatography. More sophisticated methods such as microtitre plate methods (Biolog) have also been reported for the monitoring of CO 2 evolution (Campbell et al. 2003). Many authors have proposed new versions and modifications of the original methods (California Compost Quality Council CCQC 2001, Brewer & Sullivan 2003. The main disadvantage of these methods is that they are unable to distinguish between CO 2 produced aerobically from that produced anaerobically. Moreover, these methods assume that the CO 2 /O 2 ratio is always 1. However, it can vary depending on the oxidation degree of the organic carbon. On this basis, some authors argue that they cannot be used to estimate the RI of a material (Lasaridi & Stentiford 1998). Conversely, it has been indicated that if the assay is carried out under controlled aerobic conditions, all CO 2 will be produced under aerobic respiration (ADAS 2003). However, monitoring of CO 2 evolution presents two major drawbacks: (1) the solubility of CO 2 in aqueous solutions; and (2) this solubility is pH-dependent. This is particularly important when com- paring respiration activities of different residues since their pH can vary over a wide range. For instance, the pH of organic fraction of municipal solid wastes is often near acidic conditions (5.5-6.5), whereas pH of sewage sludge is in the alkaline range (7.5-8.5) (Gea et al. 2004). Since the pKa of CO 2 is 6.37, a difference of 2-3 units in the pH of two different residues may not permit a comparison between respiration indices obtained measuring CO 2 production.
Methods based on O 2 uptake They are the most accepted methods for the determination of the biological activity of a material (Iannotti et al. 1994, Adani et al. 2001, 2003, Gea et al. 2004, Barrena et al. 2005. Respirometries provide accurate information on the activity of a compost sample. Their main disadvantage is that they need more specific instrumentation and more skilled labour. Furthermore, the equipment needs constant maintenance and frequent calibration. Different commercial equipments are currently available (Costech, Oxytop, Micro-Oxymax, etc.); however, they are expensive and troublesome. The rate of O 2 uptake can be quantitatively measured using manometric or electrolytic respirometers, by measuring changes in O 2 concentrations with gas chromatography or O 2 electrodes. O 2 can be measured either directly or as dissolved O 2 in aqueous suspensions. The expression of the RI and the assay conditions depend on the method used for its determination. This will be reviewed in detail later on this paper.
Methods based on O 2 uptake rate have been classified into two different classes: statics and dynamics (Adani et al. 2001). Dynamic methods are those where a continuous supply of air is used throughout the assay minimizing thus O 2 diffusion limitations. This is particularly important since it is well known that biological reactions that take place within solid substrates are often limited by the O 2 transfer rate (Paletski & Young 1995). Several authors have described the use of dynamic methods (Paletski & Young 1995, Scaglia et al. 2000, Adani et al. 2002a, Gea et al. 2004. Static methods do not include a continuous O 2 supply during the assay. They can be performed either with solid or liquid samples (Pressel & Bidlingmaier 1981, Usui et al. 1983, Wilson &Dalmat 1986, Haug & Ellsworth 1991, Iannotti et al. 1993, Lasaridi & Stentiford, 1998. Table 1 summarizes the characteristics of the main respirometry methods including the type of respirometry, assay conditions and nomenclature used. A brief description of some of them is given below.

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The static respirometry proposed by Iannotti et al. (1993), measures changes in O 2 concentration in the head space of a closed flask containing a moist compost sample of known volume and mass, at known temperature and barometric pressure. The decline in O 2 concentration over time is monitored with an O 2 electrode.
In the DiProVe method proposed by Adani et al. (2001), the 'dynamic respiration index' (DRI) is determined measuring the difference in O 2 concentration (mL L -1 ) between the inlet and outlet of an air flow passing throughout a compost reactor. The DRI is calculated from the average of 12 measurements taken every 2 h, representing 24 h of the maximum activity during 4 days. According to the assay conditions, authors distinguish between a real dynamic respiration index (RDRI) carried out with no moisture adjustment of the sample and a potential dynamic respiration index (PDRI) for samples adjusted to optimal moisture. The static respiration index (SRI) can also be estimated in the same reactor. For this case, aeration is stopped and an O 2 electrode is placed in the headspace on top of the solid material. O 2 uptake rate is calculated from the decline in O 2 concentration. Readings are made every 5 min during 3 h. SRI is calculated according to Iannotti et al. (1993) requiring also the measurement of the free air space (FAS). Since all measures are obtained in an adiabatic reactor respirometries are then recorded at the process temperature of the material at the moment of the assay.
A protocol based on the biological oxygen demand (BOD), method customarily used in wastewater treatment has also been suggested (Lasaridi & Stentiford 1998). Two indices are obtained: the specific oxygen uptake rate (SOUR) and the cumulative oxygen demand in 20 h (OD 20 ). For the SOUR determination a dissolved O 2 probe is used to measure changes in O 2 concentration of a sample suspended in water under optimal conditions for microbial activity and O 2 uptake at a temperature of 30°C. The OD 20 is calculated from the integration of the oxygen uptake curve from 0 to 20 h. The two methods can be used to determine the stability of a compost sample. However, SOUR determination is faster (Chica et al. 2003). Furthermore, it only needs a single reading from the curve O 2 concentration over time whereas the OD 20 requires a graphical integration. The specific oxygen uptake rate for a solid sample (DSOUR) is calculated as described by Iannotti et al. (1993), but in this case the assay is performed at 30°C for its comparison with SOUR results.
In Europe, the respiration activity after 4 days (AT 4 ) and the DRI are recommended in the Second Draft of the Working Document on the Biological Treatment of Biowaste as parameters for the estimation of the stability of compost (European Union 2001). This working document was supposed to be included in a new Directive on Compost. However, the European Commission abandoned this initiative very recently (European Compost Network 2005). Nevertheless, this document is widely used as guidelines in the design of treatment plants all over Europe.
Meanwhile other official bodies (US Department of Agriculture and US Composting Council 2001) recommend the use of the static respirometer proposed by Iannotti et al. (1993), for the determination of compost stability.
Moreover, another significant impeding use of respiration indices will be the assessment the degree of biological stability of end-products from combined mechanical biological waste processing plants (MBT) (Adani et al. 2002b(Adani et al. , 2004. This is important since European legislation states that only stabilized waste can be disposed in landfill according to the Landfill Directive (European Union 1999).

Comparison between the different respirometric methods
Several studies have compared the different respirometric techniques both among themselves and with other protocols used either for the monitoring of the composting process or for the evaluation of the stability of the end product. These studies include the following.
• Koening & Bari (2000) compared the self-heating test with a respirometry based on O 2 consumption. They concluded that the former is a simpler, cheaper and more suitable method than the latter. Furthermore, as self-heating tests use a higher amount of sample (1.5 L) the results are more representative of the process. • Lasaridi et al. (2000) indicated that during the first stages of the composting process, the self-heating test is not accurate enough. Therefore, they suggest that during the first 2-3 weeks of the process, respirometries are more useful for monitoring it. However, the self-heating test, in conjunction with germination tests, is more appropriate for the determination of the stability/maturity of the end product. • Brinton (2001) also compared the information provided by the self-heating test with that from respirometries. The author argues that the former gives more comprehensive information about the composting process but respirometries include a larger number of factors related to the composting process. Moreover, it is considered that the self-heating test is not able to distinguish between different curing stages during the late stages of the process. This information is particularly important when the final product is intended for land application. The author also emphasizes that a single method should not be used. • Butler et al. (2001) indicated that the self-heating test is more appropriate for the monitoring of the process and the determination of the stability of the material than the respirometric techniques. They observed that respirometric values obtained from day 29 of the process did not change; however, the self-heating values varied until day 57. • Brewer & Sullivan (2003) compared different respirometry methods: self-heating test, colorimetric CO 2 (Solvita ® ), alkaline trap and the CO 2 evolution via a Dräger tube method. According to these authors, all methods provided similar information; however, they consider that the selfheating test took substantially longer to provide such information. • Brinton et al. (1995) proposed a standardized protocol for the self-heating test. They also found a correlation between this test and the production of CO 2 . This equivalency is shown in Table 2. This table shows that this test is unable to distinguish between active and very active samples. • Lasaridi et al. (2000) consider that respirometries based on O 2 uptake are the best method for the evaluation of microbial activity during the composting process. • The CCQC (2001) compared different respirometry techniques and concluded that measurement of O 2 uptake took longer and required more control and more sophisticated equipment than methods based on the measurement of CO 2 evolution. • Adani et al. (2002a) agree with Haug (1986) in indicating that methods based on the monitoring of O 2 uptake are better than those that monitor the production of CO 2 , since O 2 uptake is directly related to the oxidation of organic matter. It is argued that in the case of CO 2 production, oxidation of organic matter not related to microbial respiration, may interfere with the measurement. Methods based on O 2 are not affected by this interference. Nevertheless no practical comparison has been reported so far on the monitoring of composting following O 2 uptake and CO 2 evolution. • According to ADAS Consulting Ltd. (2003), the composting process is better monitored by a combination of the self-heating test and respirometries based on O 2 uptake. It is also mentioned that there are no references regarding the use of CO 2 measurements for the monitoring of the process although they are very useful for the determination of the stability of the material.
• Palestski & Young (1995) consider that respirometries based on O 2 uptake are the best method for the determination of the stability of a compost sample since they directly provide information about the metabolic activity of the aerobic microbial population.
From these studies, it can be seen that at present, there is no general consensus on the use of a common respirometric technique.

Comparison amongst methods based on O 2 uptake
In static solid methods, the potential O 2 uptake rate is underestimated. The actual O 2 uptake rate is lower than in dynamic and/or soluble methods, and it is the actual O 2 uptake rate that is measured. Methods using liquid suspensions do not have these problems as the sample is continuously stirred, and so in the determination of SOUR there are no O 2 transfer limitations as with solid samples. The results obtained with liquid samples are also more reproducible since for solid samples they depend on the material structure and moisture. A liquid suspension obviates limitations related to the structure and moisture of the sample and O 2 transfer limitations.
Nevertheless, liquid respirometries are limited by the small quantity of sample used for the assay (3-8 g). Samples from the organic fraction of municipal solid wastes are highly heterogeneous mainly during the early stages of the composting process. Therefore, larger samples are required to improve their representation of the waste stream.
The length of the assays can also vary. Dynamic respirometries can be either carried out on-line (Gea et al. 2004) or measured within 2 days (Adani et al. 2003). On the other hand, static assays usually last up to 2 days.
The main advantage of the DRI is that the assay is carried out under conditions similar to those of real scale. However, a more important advantage of DRI is that it may be used in production scale composters for the determination of the respiration index on-line, although no reference to this use has been reported so far. On the contrary, one of the main disadvantages of the SOUR index is that it does not really represent the actual conditions of the material. SOUR measurements are made in aqueous suspension where O 2 transfer limitations  Brinton et al. (1995) and Körner et al. (2003 (Lasaridi et al. 2000) for the monitoring of a composting process. The results showed that both indices were fairly similar during the curing stage. The correlation coefficient between the two parameters was 0.94 with a 0.01 significance level indicating a good correlation between them. However, the DSOUR values were somewhat erratic during the initial thermophilic stage; therefore, they could not clearly represent this phase. This could be attributed to experimental errors to which the DSOUR test is more susceptible, and to the inherent limitations of respirometric tests using solid samples. Adani et al. (2003) compared three different methods with the aim of finding their similarities. Two of the methods used solid samples, one in static conditions (SRI) and the other under dynamic conditions (DRI), whereas the third was carried out in liquid samples (SOUR). The results indicated that there was a good correlation among them and all could be used to describe the biological stability of the samples. However, they were affected by different factors that in some cases could influence the results. For instance, it seems that soluble organic matter content might affect the SOUR index. Hence, depending on the material, stability values estimated using this method may be different from those obtained with methods using solid samples. The authors recommend more research on the relationship between SOUR and soluble organic matter. When the SRI and DRI were compared it was shown that former were lower. This is probably because of mass transfer limitations in O 2 diffusion in static methods whereas the continuous supply of O 2 in dynamic methods prevents these limitations. Another possible drawback of the static method is the systematic error when measuring the FAS of the sample. This is because accurate measurement of FAS is complicated. The equipment is generally expensive and complicated to use (Oppenheimer et al. 1997, Agnew et al. 2003. Nevertheless, stability values obtained with the three methods are reliable. DRI, SRI and respiratory quotient (RQ) have been used for the monitoring of the composting of different materials (Gea et al. 2004). RQ is the ratio between CO 2 produced and O 2 consumed. It is assumed that under aerobic conditions, the RQ value is close to one although it depends on the biochemical composition of the material (Atkinson & Mavituna 1983). The results indicated that DRI values were the most reliable to evaluate the microbial activity in the process. SRI was evaluated at 37°C and at the process temperature. It was found that during the first stages of the process SRI at 37°C were significantly lower than DRI probably due to O 2 diffusion limitations whereas at latter stages both DRI and SRI were similar. The RQ did not show any significant change throughout the process.
Other studies have shown that the SOUR index is a good indicator of the stability of the material (Lasaridi & Stentiford 1998). However, it cannot be used for the monitoring of the first stages of the process. Conversely, SRI and DRI are useful for both stability determination and monitoring of the process.
Equipment required for the determination of SRI described by Iannotti et al. (1993) and the US Department of Agriculture and US Composting Council (2001) is cheaper and easier to use than that required for the SRI and DRI determination proposed by Adani et al. (2003). Respiration indices obtained at conditions closer to the actual process conditions are more realistic than those obtained at less representative conditions such as SOUR.

Operative conditions for respirometry techniques
Respirometries should be done under conditions that allow the optimum development of micro-organisms.
Respirometries can be used to determine the biological activity in a sample if the assay is performed under optimal and controlled conditions (Adani et al. 2001). A respirometry assay requires optimal moisture content, oxygen content, appropriate temperature and, a nutrient balance that favours microbial activity.
Microbial activity in a compost process and -by consequence -in a respirometry assay, is affected by many different factors such as moisture content and temperature of the sample, microbial population, nutrients equilibrium, or occurrence of toxic compounds. Composting Council 2001, ADAS 2003, Adani et al. 2003 this is the most influential parameter in a respirometry assay. Palentski & Young (1995) have shown that O 2 uptake is directly related to the moisture content of a solid matrix. Reliable results require a sample with an optimal moisture content since microbial activity can be limited either in too wet samples (anaerobic conditions are favoured) or too dry (lower potential microbial activity). In general, compost samples with moisture below 35% on a wet weigh basis, will be biologically dormant and in consequence its respiration index will be falsely low.

Moisture content For many authors (US Department of Agriculture and US
Some debate exists on the way moisture content is expressed, according to the US Department of Agriculture and US Composting Council (2001) it should be referred to the waterholding capacity of the material rather than based upon its total wet weight. For instance, samples with high bulk density (0.75 kg m -3 ) and low organic matter content are generally over-saturated at moisture contents between 40 and 50%. Conversely, samples with low bulk density and very high waterholding capacity may be too dry at these moisture levels. However, it has also been pointed out (US Department of Agriculture and US Composting Council 2001) that appropriate moisture content should be between 70 and 85% of water-holding capacity which, for most samples, corresponds to 40-50% moisture (wet weight basis). Moreover, overmoist samples, tightly packed in a sealed container may reach an anaerobic state that is unrepresentative of the sample source and therefore would not be suitable for respirometry analysis.

Temperature influence on microbial population
Temperature is considered a critical parameter for the determination of respiration indices since biological activity is a function of temperature (Iannotti et al. 1993, Lasaridi et al. 2000, ADAS 2003, Mari et al. 2003, Liang et al. 2003, Cronjé et al. 2004. There is no agreement about an optimal temperature range for the respirometry assays. Most of them are performed at a standard temperature, normally set between 30 and 37°C (Pressel & Bidlingmaier 1981, Iannotti et al. 1993, Paletski & Young 1995, Lasaridi & Stentiford 1998. American procedures generally use 35°C as standard temperature, whereas in other countries a temperature of 30°C is used (Stentiford 2002). It is considered that respirometries carried out at these temperatures are a good indicator of the metabolic potential of the sample once the compost is incorporated into the soil. Stentiford (2002) carried out respirometries at 30°C and argued that working at higher temperatures, for instance 35°C, produced higher uptakes. The author proposes an equation to convert the SOUR obtained at 30°C to any given temperature based on empirical data (1) Meanwhile, Cronjé et al. (2003) have related OUR to the process temperature according to (2) where OUR is based on the volatile solid (VS) content at temperature T, expressed in centigrade degrees, and expressed in g O 2 kg -1 VS h -1 . Mari et al. (2003) used a 'thermogradient' respirometer to carry out respirometries at different temperatures. They concluded that assays at 35°C are good indicators of the metabolic potential of the compost. However, assays at 48.5°C were better indicators of the respiration activity occurring in situ.
Another advantage of carrying out respirometries at working temperature when in the thermophilic stage is that nitryfing bacteria are inhibited. Haug & Ellsworth (1991) working with suspended samples, recommended a temperature of 45°C to avoid the effect of nitrifying bacteria; otherwise they suggest the use of nitrifying inhibitors.
The influence of temperature on SRI has also been studied by Barrena et al. (2005) using sludge. SRI indices of composting samples were determined at 37°C and at process temperature at sampling. The results indicated that both indices correlated well with the temperature profile in the composter.
The SRI values at process temperature were more representative of the metabolic activity in the composter and more sensitive to temperature and composition variations of the composting material; however, they could not indicate the stability of the material at later stages of the process. Therefore, the SRI values at process temperature are more useful for the monitoring of the process whereas the SRI at 37°C are more suitable for the determination of the stability of the material.

Pre-incubation requirements
When samples are moistened either because their moisture content is very low or because they have been previously dried, a pre-incubation is required to restore the metabolic equilibrium of the population. However, no uniform criteria exist about the conditions under which this should be carried out. The US Department of Agriculture and US Composting Council (2001) recommend adjusting the moisture content directly in the pile or reactor. However, in cases where this is not possible, a 24-h pre-incubation is proposed at the specified temperature of the assay. Some samples may require up to 3 days of pre-incubation at temperatures between 25 and 28°C. Iannotti et al. (1993) emphasized the importance of using an appropriate temperature and thus avoiding a thermal shock for thermophillic micro-organisms. In consequence, samples should be incubated prior to the assays at the corresponding temperature. Recommended incubation times vary between 16 h (Iannotti et al. 1993) and 25 h (US Department of Agriculture and US Composting Council 2001), although in some instances incubation times may be as long as 3 days. If short incubation times are used false respiration indices maybe obtained. Respiration indices of samples from early stages of decomposition may be too low if incubation time has not been long enough. According to Iannotti et al. (1993), respirometries should be carried out at 37°C. However, it has also been argued that using such a temperature may be selectively testing for organisms in the upper range of the mesophillic organisms and may not be indicative of what happen in the soil after the compost is incorporated (US Department of Agriculture and US Composting Council 2001).
Thus, a pre-incubation at 25-28°C and testing at 34°C is suggested as more representative of the actual compost metabolic activity potential.
Sometimes it is necessary to pre-incubate the samples prior to their assay, especially those that had been previously dried. Once the samples are moistened they need to be pre-incubated as there is a lag phase while the metabolic activity is reestablished. This may require from 3 to 5 days. Nevertheless, more work is needed to determine optimal conditions for preincubations (temperature, moisture, time).
Nutrients equilibrium and occurrence of toxic compounds Respirometry assays require an appropriate balance of nutrients and the absence of toxins and other compounds that may inhibit microbial respiration.
Low microbial activity may be a consequence of lack of nutrients. For instance, sludge from the paper industry has enough carbon but low levels of nitrogen for microbial growth. As a result, respiration indices could be low. Nevertheless, nitrogen and/or phosphate can be added to make good such deficiencies.
Conversely, problems can also arise from an excess of nutrients. For instance, in samples with very high organic N content such as fish waste, this nitrogen can be transformed to produce very high levels of NH 4 , (above 500 mg kg -1 ). As a result, these samples may be colonized by saprophytic fungi (US Department of Agriculture and US Composting Council 2001). Fungal mycelium serves as a food source for bacteria and will induce an abundant bacterial activity during incubation and upon aeration. If the presence of fungi is not diminished through incubation prior to respirometry measurements, respiration measures will indicate high O 2 uptake rates. Figure 2 shows a compost sample in which fungi have proliferated during incubation.

Established limits for biological stability
Different limits have been established for the respiration indices for their use as a biological stability parameter. Table 3 shows   (2001), are based on the static model described by Iannotti et al. (1993); howeve, nomenclature and limits used to express the indices are different from the original. Table 4 shows how SOUR nomenclature is used to define a static method with solid samples. As can be seen, some references refer the respiration index to the amount of organic sample whereas others utilize the volatile solids content. The use of this nomenclature can cause some confusion if results are compared with those obtained using the method proposed by Lasaridi & Stentiford (1998).
The maturity test recommended by the CCQC (2001) differentiates between the SOUR and OUR indexes. The difference is given by the way the results are expressed: SOUR is referred to the volatile solid content of the sample whereas OUR is referred to the total solid content.
Moreover, different limits have been proposed for the respiration indices to determine the stability of a material as described below.
In general, it can be said that some confusion exists when applying respirometry protocols probably because of lack of scientific assessment. For instance, some regulations have recommended the use of determined methods but using stability limits derived from different ones. In Italy, the UNI methods recommend that respirometry assays should be carried out at 20°C but the proposed limits (UNI U53001080 2005) derive from the DiProVe method, in which the respirometries are carried out at process temperature (Adani et al. 2003).
Furthermore, the way results are expressed can also be a source of confusion. For instance, respiration indices can be determined either from maximum values or as average of measurements made over 24 h, they can also be referred either to dry weight or to organic matter content. Table 5 shows the different ways used to determine respiration indexes.
Moreover, there is no general interpretation to the biological stability of a material. In Germany and Austria, threshold values are much lower than in Italy. As shown in Table 3, the AT 4 proposed by Germany and Austria is lower, 5 mg O 2 g -1 VS 96 h -1 , than that proposed by the European Union, 10 mg O 2 g -1 VS h -1 . In Austria and Germany, compost is considered mature after 4 to 6 months of processing whereas in Italy the index is referred to a 15 to 30 day process.
Equivalences amongst the most commonly used indices have been proposed (Adani et al. 2003) as shown in Table 6. These equivalences have been obtained from the DRI proposed as the stability threshold value in the second draft of the European Union working document. The DRI has also been compared with the Solvita ® test (Adani et al. 2003). Results indicate that a stable material according to the Solvita ® would have very low DRI values, around 0.2-0.3 mg O 2 g -1 VS h -1 , whereas the stability limit for this index is 1; therefore this test does not have enough resolution for the determination of compost stability.

Future trends
Although respirometry methodologies are established there are still different aspects that need more detailed investigation. Amongt these it is worth mentioning: (1) the relationship between pH and CO 2 and its influence on respirometries based on CO 2 production; (2) the effect of optimizing the porosity of the material on static respirometries and the comparison of results with those obtained from dynamic methods;

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(3) the effect of humidity when highly energetic residues are composted, namely, when temperatures above 70°C are reached during composting; (4) the influence of microbial population on respiration indices since respìrometries are currently performed based on the microbial activity of native microbial populations -no optimum population has been considered so far; (5) the effect of toxins contained in the material; and (6) a collective effort aimed at the unification of criteria in the selection of most suitable methodologies depending on the final application of the compost.

Conclusions
From the information found in the literature, it is evident that there is no single respirometric method that can be used for both the monitoring of the process and the determination of the stability of a compost sample. The most appropriate method will depend on the aim of the assay. Moreover, although respirometries are routinely carried out further investigation is needed in aspects such as those mentioned above to obtain a better understanding of the metabolic activity of a composting material and hence, how this affects the degree of stability of the compost.
Furthermore, it is clear that more work needs to be done to correlate all the different methods and indices that are cur-rently used. This is particularly important since respiration indices are now considered as key parameters in the determination of the stability of a compost and hence, its quality.