Test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill

The Evaluation of Indicators Used to Assess the Suitability of Agricultural Waste for Fermentation

To ensure high fermentation efficiency, it is necessary to assess the biodegradability of a substrate. These parameters are most often determined on the basis of the amount of loss on ignition and total organic carbon. We are more and more often using chemical indices. However, these indices do not provide information on how much an organic substance is susceptible to biodegradation. The actual assessment of the content of easily biodegradable matter in substrates that are used for fermentation should be performed on the basis of aerobic (AT4) and anaerobic tests (BMP), which require specialised equipment and are time consuming. The AT4 index is being more and more frequently adopted for the analysis of substrates that are used in the fermentation process, because AT4 takes a much shorter time than BMP and provides information on the biodegradability of substrates. The aim of the article is to answer the question of whether the AT4 parameter can be used to assess the suitability of the substrate from the agricultural sector for the fermentation process. The results show that the AT4 index could be used instead of the BMP parameter.

A comparison of methods for early prediction of anaerobic biogas potential on biologically treated municipal solid waste

Anaerobic gas production tests, generically Biochemical Methane Potential (BMP) or Biogas Potential (BP) tests, are often used to assess biodegradability, though long duration limits their utility. This research investigated whether simple modelling approaches could provide a reliable earlier prediction of total biogas production. Data were assessed from a non-automated biogas test on a large number of both fresh and processed municipal solid waste (MSW) samples, sourced from a mechanical biological treatment (MBT) plant. Non-linear models of biogas production curves were useful in identifying a suitable test endpoint, supporting a test duration of 50 days. Biogas production at 50 days (B₅₀) was predicted using the first 14 days of test data, using (a) linear correlation, (b) a new linearisation process, and (c) non-linear kinetic models. Prediction errors were quantified as relative root mean squared error of prediction (rRMSEP), and bias. Predictions from most models were improved by removing the initial exponential increase phase. Linear correlation gave the most precise and accurate predictions at 14 days (rRMSEP = 2.8%, bias under 0.05%) and allowed acceptable prediction (rRMSEP <10%) both at 8 days, and at 6 days using separate correlations for each sample type. Of the other predictions, the new linearisation process gave the lowest rRMSEP (10.6%) at 14 days. More complex non-linear models conferred no advantage in prediction of B₅₀. These results demonstrate that early prediction of anaerobic gas production is possible for a well-optimised test, using only basic equipment and without recourse to external data sources or complex mathematical modelling.

Characterization of composted sewage sludge during the maturation process: a pilot scale study

This paper determines the impact of the maturation process of composted sewage sludge on the quality of the final product and assesses the stabilization effect. The samples of composted sewage sludge were taken from a wastewater treatment plant located in Pomerania in northern Poland. The sewage sludge was composted in an open windrow composting plant with the addition of straw and wood chips in the turning windrow. The aeration of the sewage sludge mixture was conducted based on two methods. The first phase (intensive degradation phase of 6 to 8 weeks) was characterized by frequently turning; the second phase for maturation used aeration channels (2 to 3 months). In three sampling campaigns samples were taken from the same windrow after 2 (no. 1), 8 (no. 2), and 12 weeks (no. 3) of maturation. Fresh samples were used for analyzing the stabilization parameter as static respiration activity (AT₄). Furthermore, the values of pH, organic matter (OM), total organic carbon (TOC), elementary composition, nutrients, total content, and mobile forms of heavy metals were analyzed in the compost samples. A significant decrease was found in the stabilization parameter (AT₄) during the maturation of tested materials. In turn, no significant differences were found in the elementary composition. The concentration of most metals increased in the final product. The total content of heavy metals in the final product did not exceed the limit values for the agricultural use of sewage sludge, compost from municipal waste, and for organic fertilizers. There were no significant changes in the percentage of bioavailable and mobile forms of heavy metals during compost maturation. Zinc was characterized by the highest level of mobile and bioavailable forms, which may cause bioaccumulation after the fertilization of soil. The study has shown that the process of maturation of compost from sewage sludge not affects changes in the content of heavy metal forms. The scope of this study has been planned on a wider scale for different variants of sewage sludge composting, in order to evaluate the process.

A Comparison of Waste Stability Indices for Mechanical⁻Biological Waste Treatment and Composting Plants

Achieving high efficiency of biological waste treatment in mechanical–biological treatment (MBT) plants requires reliable methods for measuring the degree of biodegradation of organic substances. For this purpose, several physical, chemical, and biological indices are used. This paper presents respirometric activity (AT4), biogas potential (GB21), total and dissolved organic carbon (TOC and DOC, respectively), and loss on ignition (LOI) values, as well as the correlations between the indices selected for stabilized waste produced in 18 MBT plants in Poland, which use various technologies for biological processing of the organic fraction of municipal solid waste. The study confirms that there is a linear relationship between AT4 and GB21 for stabilized waste produced in MBT plants, regardless of the waste treatment technology used. It has also been found that there is a linear relationship between AT4 and the concentration of dissolved carbon in water extract from stabilized waste. This indicates that DOC can be used for monitoring the organic matter stabilization process in mechanical–biological waste treatment plants. Its advantage is a shorter time needed for measurements in comparison to AT4 and GB21 tests.

Characterization of selected municipal solid waste components to estimate their biodegradability

Biological treatments of Residual Municipal Solid Waste (RMSW) allow to divert biodegradable materials from landfilling and recover valuable alternative resources. The biodegradability of the waste components needs however to be assessed in order to design the bioprocesses properly. The present study investigated complementary approaches to aerobic and anaerobic biotests for a more rapid evaluation. A representative sample of residual MSW was collected from a Mechanical Biological Treatment (MBT) plant and sorted out into 13 fractions according to the French standard procedure MODECOM™. The different fractions were analyzed for organic matter content, leaching behavior, contents in biochemical constituents (determined by Van Soest's acid detergent fiber method), Biochemical Oxygen Demand (BOD) and Bio-Methane Potential (BMP). Experimental data were statistically treated by Principal Components Analysis (PCA). Cumulative oxygen consumption from BOD tests and cumulative methane production from BMP tests were found to be positively correlated in all waste fractions. No correlation was observed between the results from BOD or BMP bioassays and the contents in cellulose-like, hemicelluloses-like or labile organic compounds. No correlation was observed either with the results from leaching tests (Soluble COD). The contents in lignin-like compounds, evaluated as the non-extracted RES fraction in Van Soest's method, was found however to impact negatively the biodegradability assessed by BOD or BMP tests. Since cellulose, hemicelluloses and lignin are the polymers responsible for the structuration of lignocellulosic complexes, it was concluded that the structural organization of the organic matter in the different waste fractions was more determinant on biodegradability than the respective contents in individual biopolymers.

Case study comparison of functional vs. organic stability approaches for assessing threat potential at closed landfills in the USA

Municipal solid waste (MSW) landfills in the USA are regulated under Subtitle D of the Resource Conservation and Recovery Act (RCRA), which includes the requirement to protect human health and the environment (HHE) during the post-closure care (PCC) period. Several approaches have been published for assessment of potential threats to HHE. These approaches can be broadly divided into organic stabilization, which establishes an inert waste mass as the ultimate objective, and functional stability, which considers long-term emissions in the context of minimizing threats to HHE in the absence of active controls. The objective of this research was to conduct a case study evaluation of a closed MSW landfill using long-term data on landfill gas (LFG) production, leachate quality, site geology, and solids decomposition. Evaluations based on both functional and organic stability criteria were compared. The results showed that longer periods of LFG and leachate management would be required using organic stability criteria relative to an approach based on functional stability. These findings highlight the somewhat arbitrary and overly stringent nature of assigning universal stability criteria without due consideration of the landfill's hydrogeologic setting and potential environmental receptors. This supports previous studies that advocated for transition to a passive or inactive control stage based on a performance-based functional stability framework as a defensible mechanism for optimizing and ending regulatory PCC.

Towards developing a representative biochemical methane potential (BMP) assay for landfilled municipal solid waste - A review

The applicability of slurry-based (semi-liquids) BMP assay in determining biodegradation kinetic parameters of landfilled waste is critically reviewed. Factors affecting the amount and rate of methane (CH₄) production during anaerobic degradation of municipal solid waste (MSW) and optimal values of these factors specific to landfill conditions are presented. The history of conventional BMP, and some existing procedures are reviewed. A landfill BMP (LBMP) assay is proposed that manipulates some of the key factors, such as moisture content, particle and sample size, that affects the rate of CH₄ production and the CH₄ generation potential of landfilled MSW (LMSW). By selecting proper conditions for these factors, a representative BMP assay could be conducted to ensure accurate determinations of CH₄ potential and the kinetic parameters k; first order rate coefficient and L_o; methane generation potential.

Further steps in the standardization of BOD5/COD ratio as a biological stability index for MSW

In recent decades the definition of standard test methodologies suitable for use in assessing the biological stability of solid waste has become increasingly imperative. To meet this requirement, the BOD₅/COD ratio, measured on waste eluate, has been proposed by Cossu et al. (2012) as a more appropriate parameter than the traditional respirometric indices and biogas production measured directly on solid samples. However, to ensure reproducibility, the parameter should undergo standardization of operational conditions. Previous studies have demonstrated that: the testing mode (static or dynamic) does not influence test representativeness; the long testing time (>6h) does not influence BOD₅/COD ratio; COD measured on unfiltered or filtered samples is consistent and significant in both cases. The main aim of this study was to further promote the standardization of this parameter. A series of static leaching tests on representative samples of five types of waste was carried out under different operative conditions: contact time, liquid to solid ratio and pretreatment. The results obtained demonstrate: the apparent adequacy of a short contact time (2h), which is highly preferable and would speed up the procedure; a low liquid to solid ratio (5 l/kgTS) which is recommended as a water saving strategy; the applicability of centrifugation of the eluate prior to analysis which is faster that filtration method.

Characterisation of the biodegradability of post-treated digestates via the chemical accessibility and complexity of organic matter

The stability of digestate organic matter is a key parameter for its use in agriculture. Here, the organic matter stability was compared between 14 post-treated digestates and the relationship between organic matter complexity and biodegradability was highlighted. Respirometric activity and CH₄ yields in batch tests showed a positive linear correlation between both types of biodegradability (R²=0.8). The accessibility and complexity of organic matter were assessed using chemical extractions combined with fluorescence spectroscopy, and biodegradability was mostly anti-correlated with complexity of organic matter. Post-treatments presented a significant effect on the biodegradability and complexity of organic matter. Biodegradability was low for composted digestates which comprised slowly accessible complex molecules. Inversely, solid fractions obtained after phase separation contained a substantial part of remaining biodegradable organic matter with a significant easily accessible fraction comprising simpler molecules. Understanding the effect of post-treatment on the biodegradability of digestates should help to optimize their valorization.

Compositional and physicochemical changes in waste materials and biogas production across 7 landfill sites in UK

The aim of this study was to evaluate the spatial distribution of the paper and fines across seven landfill sites (LFS) and assess the relationship between waste physicochemical properties and biogas production. Physicochemical analysis of the waste samples demonstrated that there were no clear trends in the spatial distribution of total solids (TS), moisture content (MC) and waste organic strength (VS) across all LFS. There was however noticeable difference between samples from the same landfill site. The effect of landfill age on waste physicochemical properties showed no clear relationship, thus, providing evidence that waste remains dormant and non-degraded for long periods of time. Landfill age was however directly correlated with the biochemical methane potential (BMP) of waste; with the highest BMP obtained from the most recent LFS. BMP was also correlated with depth as the average methane production decreased linearly with increasing depth. There was also a high degree of correlation between the Enzymatic Hydrolysis Test (EHT) and BMP test results, which motivates its potential use as an alternative to the BMP test method. Further to this, there were also positive correlations between MC and VS, VS and biogas volume and biogas volume and CH₄ content. Outcomes of this work can be used to inform waste degradation and methane enhancement strategies for improving recovery of methane from landfills.

Case study on prediction of remaining methane potential of landfilled municipal solid waste by statistical analysis of waste composition data

Main objective of this study was to develop a statistical model for easier and faster Biochemical Methane Potential (BMP) prediction of landfilled municipal solid waste by analyzing waste composition of excavated samples from 12 sampling points and three waste depths representing different landfilling ages of closed and active sections of a sanitary landfill site located in İstanbul, Turkey. Results of Principal Component Analysis (PCA) were used as a decision support tool to evaluation and describe the waste composition variables. Four principal component were extracted describing 76% of data set variance. The most effective components were determined as PCB, PO, T, D, W, FM, moisture and BMP for the data set. Multiple Linear Regression (MLR) models were built by original compositional data and transformed data to determine differences. It was observed that even residual plots were better for transformed data the R(2) and Adjusted R(2) values were not improved significantly. The best preliminary BMP prediction models consisted of D, W, T and FM waste fractions for both versions of regressions. Adjusted R(2) values of the raw and transformed models were determined as 0.69 and 0.57, respectively.

Treatment of mechanically sorted organic waste by bioreactor landfill: Experimental results and preliminary comparative impact assessment with biostabilization and conventional landfill

Treatment and disposal of the mechanically sorted organic fraction (MSOF) of municipal solid waste using a full-scale hybrid bioreactor landfill was experimentally analyzed. A preliminary life cycle assessment was used to compare the hybrid bioreactor landfill with the conventional scheme based on aerobic biostabilization plus landfill. The main findings showed that hybrid bioreactor landfill was able to achieve a dynamic respiration index (DRI)<1000 mgO2/(kgVSh) in 20weeks, on average. Landfill gas (LFG) generation with CH4 concentration >55% v/v started within 140days from MSOF disposal, allowing prompt energy recovery and higher collection efficiency. With the exception of fresh water eutrophication with the bioreactor scenario there was a reduction of the impact categories by about 30% compared to the conventional scheme. Such environmental improvement was mainly a consequence of the reduction of direct and indirect emissions from conventional aerobic biostabilization and of the lower amount of gaseous loses from the bioreactor landfill.

Evaluation of Optimum Moisture Content for Composting of Beef Manure and Bedding Material Mixtures Using Oxygen Uptake Measurement

Moisture content influences physiological characteristics of microbes and physical structure of solid matrices during composting of animal manure. If moisture content is maintained at a proper level, aerobic microorganisms show more active oxygen consumption during composting due to increased microbial activity. In this study, optimum moisture levels for composting of two bedding materials (sawdust, rice hull) and two different mixtures of bedding and beef manure (BS, Beef cattle manure+sawdust; BR, Beef cattle manure+rice hull) were determined based on oxygen uptake rate measured by a pressure sensor method. A broad range of oxygen uptake rates (0.3 to 33.3 mg O₂/g VS d) were monitored as a function of moisture level and composting feedstock type. The maximum oxygen consumption of each material was observed near the saturated condition, which ranged from 75% to 98% of water holding capacity. The optimum moisture content of BS and BR were 70% and 57% on a wet basis, respectively. Although BS’s optimum moisture content was near saturated state, its free air space kept a favorable level (above 30%) for aerobic composting due to the sawdust’s coarse particle size and bulking effect.

Static, dynamic and inoculum augmented respiration based test assessment for determining in-vessel compost stability

The purpose of this work was to evaluate compost (and related industry) stability tests given recent large-scale changes to feedstock, processing techniques and compost market requirements. Five stability tests (ORG0020, DR4, Dewar self-heating, oxygen update rate (OUR) and static respiration) were evaluated on composts from ten in-vessel composting sites. Spearman rank correlation coefficients were strong for the ORG0020, OUR and DR4 (both CO2 and O2 measurement), however, OUR results required data extrapolation for highly active compost samples. By comparison the Dewar self-heating and static respiration tests had weaker correlations, in part the result of under reporting highly active, low pH samples. The findings suggest that despite differences in pre-incubation period both dynamic respiration tests (ORG0020 and DR4) are best suited to deal with the wide range of compost stabilities found.

What is the acceptable margin of error for the oxygen uptake method in evaluating the reactivity of organic waste?

The acceptable margin of error for the organic waste reactivity measured by the oxygen uptake method was assessed. Oxygen uptake was determined by the Dynamic Respiration Index (DRI) (mgO2/kgVS h). The composed uncertainty (uC) of the experimental set up used for the DRI test was evaluated and the uncertainty (u) of all the components of the apparatus was evaluated. A procedure for calculating the uC of the apparatus is proposed. The components affecting the uC of the DRI to a more significant extent were the one of the oxygen mass rate and the u of the amount of VS in the sample analyzed. For a confidence level of 99.73%, the extended uC (UC) interval for a DRI = 1024 mgO2/kgVS h was ± 440 mgO2/kgVS h, whereas for a DRI = 3,489 mgO2/kgVS h, the UC interval was ± 1288 mgO2/kgVS h. When oxygen consumption and VS content become lower than 600 mgO2/h and 0.9 kg, respectively, the UC interval is similar to the measured DRI.

Home composting versus industrial composting: influence of composting system on compost quality with focus on compost stability

Stability is one of the most important properties of compost obtained from the organic fraction of municipal solid wastes. This property is essential for the application of compost to land to avoid further field degradation and emissions of odors, among others. In this study, a massive characterization of compost samples from both home producers and industrial facilities is presented. Results are analyzed in terms of chemical and respiration characterizations, the latter representing the stability of the compost. Results are also analyzed in terms of statistical validation. The main conclusion from this work is that home composting, when properly conducted, can achieve excellent levels of stability, whereas industrial compost produced in the studied facilities can also present a high stability, although an important dispersion is found in these composts. The study also highlights the importance of respiration techniques to have a reliable characterization of compost quality, while the chemical characterization does not provide enough information to have a complete picture of a compost sample.

Biochemical methane potential (BMP) of artichoke waste: the inoculum effect

The aim of this work was to investigate anaerobic digestibility of artichoke waste resulting from industrial transformation. A series of batch anaerobic digestion tests was performed in order to evaluate the biochemical methane potential of the matrix in respect of the process. A comparison of the different performances of the laboratory-scale reactors operating in mesophilic conditions and utilizing three different values of the inoculum/substrate ratio was carried out. The best performance was achieved with an inoculum/substrate ratio of 2. Artichoke-processing byproducts showed a classical organic waste decomposition behaviour: a fast start-up phase, an acclimation stage, and a final stabilization phase. Following this approach, artichoke waste reached chemical oxygen demand removal of about 90% in 40 days. The high methane yield (average 408.62 mL CH4 gvs (-1) voltatile solids), makes artichoke waste a good product to be utilized in anaerobic digestion plants for biogas production.

Effect of airflow on biodrying of gardening wastes in reactors

Biodrying consists of reducing moisture by using the heat from aerobic bio-degradation. The parameters that control the process are: aeration, temperature during the process, initial moisture of biowaste, and temperature and relative humidity of the input air. Lawn mowing and garden waste from the gardens of the University Jaume I, Castellón (Spain) were used as a substrate. Biodrying was performed in 10 reactors with known air volumes from 0.88 to 6.42 L/(min x kg dry weight). To promote aeration, 5 of the reactors had 15% of a bulking agent added. The experiment lasted 20 days. After the experiments it was found that the bulking agent led to greater weight loss. However, the increased airflow rate was not linearly proportional to the weight loss.

Is biodegradable waste a porous environment? A review

This article presents a review of the porous physical characteristics, phenomena and simulation models so far investigated and applied in the management of biodegradable wastes (BW), summarising the main properties of porous media and the dynamics of fluids within its voids. The aim is to highlight how the description of biodegradable wastes as porous media and the use of porous media models can facilitate the development of new sustainable and affordable technologies for BW recycling. However, it is pointed out how the lack of physical experimental data and of tailored modelling tools has so far hampered the use of this approach. Therefore, it is suggested that a simpler way to design and implement modelling tools simulating BW treatment technologies is by modifying available models designed originally for other porous media, such as soil and rock.

Stabilisation of biodried municipal solid waste fine fraction in landfill bioreactor

The biodrying process of solid waste is a pre-treatment for the bio-stabilisation of the municipal solid waste. This study aims to investigate the fate of the municipal solid waste fine fraction (MSWFF) resulting from a biodrying treatment when disposed in landfills that are operated as bioreactors. Biodried MSWFF was apparently stable due to its low moisture content that slows down the microbial activity. The lab-scale anaerobic bioreactors demonstrated that a proper moisture content leads to a complete biodegradation of the organic matter contained in the biodried MSWFF. Using a pilot-scale landfill bioreactor (LBR), MSWFF stabilisation was achieved, suggesting that the leachate recirculation could be an effective approach to accomplish the anaerobic biodegradation and biostabilisation of biodried MSWFF after landfilling. The biostabilisation of the material resulting from the LBR treatment was confirmed using anaerobic and aerobic stability indices. All anaerobic and aerobic indices showed a stability increase of approximately 80% of the MSWFF after treatment in the LBR. The similar values of OD7 and BMP stability indices well agree with the relationship between the aerobic and anaerobic indices reported in literature.

Standardization of BOD₅/COD ratio as a biological stability index for MSW

The control of biodegradable substances is the key issue in evaluating the short and long-term emission potential and environmental impact of a landfill. Aerobic and anaerobic indices, such as respirometric index (RI) and biomethane potential production (GB21), can be used in the estimation of the stability of solid waste samples. Previous studies showed different degrees of relationship between BOD₅/COD ratio compared with RI4. Aim of this study is to standardize the parameter BOD₅/COD ratio and to test the methodology under different operating conditions (dynamic or static leaching and leaching duration, 6 and 24-h) keeping constant temperature and liquid/solid ratio (L/S=10 l/kg(TS)), with the introduction of a COD fractioning method. The COD fractioning is based on the differentiation between the soluble fraction (COD(sol)) and the colloidal fraction (COD(coll)) using a flocculation method. The BOD₅/COD and the BOD₅/COD(sol) indices are both consistent and significant and can be used as stability indices. The BOD₅/COD ratio does not seem to be influenced, for the same test duration, by the type of test, static or dynamic. In the same way the longer test duration (24-h) does not influence significantly the values of BOD₅/COD ratio. As a consequence a leaching test duration of 6-h is preferable to avoid the beginning of the hydrolysis and oxidation processes.

A modified dynamic respiration test to assess compost stability: effect of sample size and air flowrate

Goal of this work was to study the effect of the unit air flowrate on dynamic respiration activity indexes during the assessment of compost stability. A MSW compost was used and six experimental runs were performed with variable compost masses and variable air flowrates, so that to achieve six unit air flowrates (6, 9, 16, 17, 23 and 30 L air kg(-1) organic matter h(-1)). Six respiration activity indexes were quantified, namely a dynamic respiration index (DRI24), the cumulative O2 consumption at 4 and 7 days (DCRI4, DCRI7), a CO2 index, the cumulative CO2 generation after 7 days (Total CO2) and the respiratory quotient. Results indicate that the CO2 related indexes and the respiratory quotients had a strong negative correlation with the unit air flowrate, whilst the DRI24 and both DCRIs slightly increased with increasing unit air flowrates.

Fluorescence-based rapid assessment of the biological stability of landfilled municipal solid waste

Fluorescence Excitation-Emission Matrix (EEM) combined with fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC) was employed to tracing the properties and behavior of the water-execrable organic matter (WEOM) from landfilled municipal solid waste (MSW) for assessing the biological stability. The 3-components PARAFAC model developed showed the WEOM dominated by two humic-like materials (components C1 and C2) which were highly correlated and behave similarly in this work, and a protein-like material (component C3). The percent fluorescence response P(i,n) from FRI and maximum fluorescence intensity F(max) of the components from PARAFAC proved to be sensitive indicators of the bulk properties and transformation of WEOM during landfill stabilization. C1/C3 F(max) ratio was found to be the most sensitive indicator of the biostablization state of the landfilled MSW and can be considered a reliable parameter. These results reveal that EEM-PARAFAC/FRI enabled a rapid and accurate assessment of biological stability of landfilled MSW.

Respiration indices and stability measurements of compost through electrolytic respirometry

An experimental technique for compost stability measurements based on Sapromat electrolytic respirometry was optimised and subsequently applied to a sludge composting process. Anaerobically digested sewage sludge mixed with reed was composted during 90 days in a pilot-scale rotary drum with forced aeration. Periodic solid samples were taken, and a previously optimised respirometric procedure was applied in order to measure the oxygen consumption. The respirometric experiments were made directly with a few grams of solid samples, optimum moisture and 37 °C over a period of 96 h. The results obtained showed how the respiration activity of the sludge decreased during the composting experiment under the specific operating conditions. The specific oxygen uptake rate (SOUR) instant values from the oxygen consumption curves were obtained, and two commonly used respirometric indexes (RI(24) and AT(4)) were calculated for all samples. Both RI(24) (a mean of the SOUR values during the 24 h maximum activity period) and AT(4) (total oxygen consumption after 4 days) were the recommended parameters for the estimation of compost stability by the European Union in the second draft of the Working Document on the Biological Treatment of Biowaste in 2001. Both indexes exponentially decreased with the composting time, and a good linear correlation between them was observed. Final values of RI(24) and AT(4) after 90 days were 600 mg O(2) kg VS(-1) h(-1) and 26 mg O(2) gTS(-1), respectively. We also considered if this technique could be classified as a Dynamic or Static method, the two primary respirometric techniques for measuring compost stability. Supposing that the proposed procedure is considered a dynamic method (no limitations on the amount of oxygen supply), the final RI(24) obtained was compared with the dynamic respiration index (DRI) proposed by the EU (1000 mg O(2) kg VS(-1) h(-1)). Our result indicated that stable compost was obtained after 90 d. However, if a static limit was considered (AT(4) lower than 10 mg O(2) gTS(-1) as proposed by the EU), our result would indicate that more residence composting time would be needed. Taking into account these results, the advantages and disadvantages and the validity of the proposed method are discussed.

Investigation of the application of an enzyme-based biodegradability test method to a municipal solid waste biodrying process

This paper presents a study to evaluate the recently developed enzymatic hydrolysis test (EHT) through its repeated application to a waste treatment process. A single waste treatment facility, involving a biodrying process, has been monitored using three different methods to assess the biodegradable content of the organic waste fractions. These test methods were the anaerobic BMc, aerobic DR4 and the EHT, which is a method based on the enzymatic hydrolysis of the cellulosic content of waste materials. The input municipal solid waste (MSW) and the output solid recovered fuel (SRF) and organic fines streams were sampled over a period of nine months from a single mechanical biological treatment (MBT) facility. The EHT was applied to each stream following grinding to <10 mm and <2 mm, in order to investigate the effect of particle size on the release of dissolved organic carbon (DOC) from enzyme hydrolysis. The output organic fines were found to more biodegradable than the MSW input and SRF output samples in each of the test methods, significantly (p<0.05) for the EHT and DR4 methods, on the basis of DOC released and oxygen consumed, respectively. The variation between sample replicates for the EHT was higher where sample sizes of <2 mm were analysed compared to sizes of <10 mm, and the DOC release at each phase of the EHT was observed to be higher when using particle sizes of <2 mm. Despite this, additional sample grinding from the <10 mm to a smaller particle size of <2 mm is not sufficiently beneficial to the analysis of organic waste fractions in the EHT method. Finally, it was concluded that as similar trends were observed for each test method, this trial confirms that EHT has the potential to be deployed as a practical operational biodegradability monitoring tool.

Respirometric screening of several types of manure and mixtures intended for composting

The viability of mixtures from manure and agricultural wastes as composting sources were systematically studied using a physicochemical and biological characterization. The combination of different parameters such as C:N ratio, free air space (FAS) and moisture content can help in the formulation of the mixtures. Nevertheless, the composting process may be challenging, particularly at industrial scales. The results of this study suggest that if the respirometric potential is known, it is possible to predict the behaviour of a full scale composting process. Respiration indices can be used as a tool for determining the suitability of composting as applied to manure and complementary wastes. Accordingly, manure and agricultural wastes with a high potential for composting and some proposed mixtures have been characterized in terms of respiration activity. Specifically, the potential of samples to be composted has been determined by means of the oxygen uptake rate (OUR) and the dynamic respirometric index (DRI). During this study, four of these mixtures were composted at full scale in a system consisting of a confined pile with forced aeration. The biological activity was monitored by means of the oxygen uptake rate inside the material (OURinsitu). This new parameter represents the real activity of the process. The comparison between the potential respirometric activities at laboratory scale with the in situ respirometric activity observed at full scale may be a useful tool in the design and optimization of composting systems for manure and other organic agricultural wastes.

Monitoring the organic matter properties in a combined anaerobic/aerobic full-scale municipal source-separated waste treatment plant

Respiration indices (dynamic and cumulative) and the anaerobic biogasification potential are applied to the quantitative calculation of the biodegradation efficiency in a combined anaerobic/aerobic treatment for the organic fraction of municipal solid waste (OFMSW). They also permit to observe possible deficiencies in some parts of the entire sequence of organic matter decomposition. On the contrary, chemical methods presented a limited utility. Dynamic respiration indices highlighted that anaerobic digestion was the most efficient step to reduce the respiration activity of the waste (61% calculated on a DRI(24h) basis). Respirometric activity of final compost was 93% lower than initial OFMSW confirming the overall efficiency of the plant studied and the stability of the final product (0.3g O(2) kg TS(-1)h(-1)). Finally, the use of an advanced methodology such as the Diffuse Reflectance Infrared Fourier Transformed (DRIFT) allows the determination of the main functional groups of organic matter, which significantly change during the biological treatment of organic matter.

Test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill

This response follows on from a recent discussion by Sánchez (2009) on test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill. Test methods to assess the biodegradability/biodegradable content of organic waste are of great interest across Europe for different purposes, such as landfill acceptance criteria, monitoring treatment facility performance and for monitoring the diversion of biodegradable municipal waste (BMW) from landfill. Many studies have recently attempted to correlate short-term test methods with long-term anaerobic test methods. This response discusses recent findings and conclusions made by Sánchez (2009) and describes recent work undertaken at Cranfield University to develop the enzymatic hydrolysis test (EHT) method. The EHT has previously shown potential as a short-term, non-biological, biodegradability assessment tool, however there is a requirement to further develop this test method. We conclude that aerobic and anaerobic biological test methods are not the only suitable methods of assessing waste treatment process performance; and that alternative methods such as EHT are feasible and potentially suitable.

Assessment of the effectiveness of an industrial unit of mechanical-biological treatment of municipal solid waste

An assessment of the French municipal solid waste (MSW) mechanical-biological treatment (MBT) unit of Mende was performed in terms of mass reduction, biogas emissions reduction and biostability of the biologically treated waste. The MBT unit consists of mechanical sorting operations, an aerobic rotating bioreactor, forced-aeration process in open-air tunnels (stabilization), ripening platforms and a sanitary landfill site for waste disposal in separated cells. On the overall plant, results showed a dry matter reduction of 18.9% and an oxidative organic matter reduction of 39.0%. A 46.2% biogas production decrease could also be observed. Concerning the biotreatment steps, high reductions were observed: 88.1% decrease of biogas potential and 57.7% decrease of oxidative organic matter content. Nevertheless, the usually considered stabilization indices (biogas potential, respirometric index) remained higher than recommended by the German or Austrian regulation for landfilling. Mass balance performed on each step of the treatment line showed that several stages needed improvement (especially mechanical sorting operations) as several waste fractions containing potentially biodegradable matter were landfilled with very few or no biological treatment.

The effect of storage and mechanical pretreatment on the biological stability of municipal solid wastes

Modern mechanical-biological waste treatment plants for the stabilization of both the source-separated organic fraction of municipal solid wastes (OFMSW) and the mixed stream of municipal solid wastes (MSW) include a mechanical pretreatment step to separate recyclable materials such as plastics, glass or metals, before biological treatment of the resulting organic material. In this work, the role of storage and mechanical pretreatment steps in the stabilization of organic matter has been studied by means of respiration techniques. Results have shown that a progressive stabilization of organic matter occurs during the pretreatment of the source-separated OFMSW, which is approximately 30% measured by the dynamic respiration index. In the case of mixed MSW, the stabilization occurring during the reception and storage of MSW is compensated by the effect of concentration of organic matter that the pretreatment step provokes on this material. Both results are crucial for the operation of the succeeding biological process. Finally, respiration indices have been shown to be suitable for the monitoring of the pretreatment steps in mechanical-biological waste treatment plants, with a strong positive correlation between the dynamic respiration index and the cumulative respiration index across all samples tested.

Different indices to express biodegradability in organic solid wastes

Respiration indices are suggested in literature as the most suitable stability determination and are proposed as a biodegradability measure in this work. An improved dynamic respiration index methodology is described in this work. This methodology was applied to 58 samples of different types of waste including municipal solid wastes and wastewater sludge, both raw materials and samples collected in a mechanical-biological treatment plant at different stages of biodegradation. The information obtained allowed to establish a qualitative classification of wastes in three categories: highly biodegradable, moderately biodegradable, and wastes of low biodegradability. Results were analyzed in terms of long and short-term indices and index expression: dynamic respiration indices expressed as average oxygen uptake rate (mg O(2) g(-1) dry matter [DM] h(-1)) at 1 and 24 h of maximum activity (DRI(1h), DRI(24h)); and cumulative oxygen consumption in 24 h of maximum activity and 4 d (AT(24h), AT(4)). The statistical comparison of indices and wastes is also presented. Raw sludge presented the highest biodegradability followed by the organic fraction of municipal solid waste and anaerobically digested sludge. All indices correlated well but different correlations were found for the different wastes analyzed. The information in the dynamic respiration profile allows for the calculation of different indices that provide complementary information. The combined analysis of DRI(24h) and AT(4) is presented here as the best tool for biodegradable organic matter content characterization and process requirements estimation.

Estimating biogas production of biologically treated municipal solid waste

In this work, a respirometric approach, i.e., Dynamic Respiration Index (DRI), was used to predict the anaerobic biogas potential (ABP), studying 46 waste samples coming directly from MBT full-scale plants. A significant linear regression model was obtained by a jackknife approach: ABP=(34.4+/-2.5)+(0.109+/-0.003).DRI. The comparison of the model of this work with those of the previous works using a different respirometric approach (Sapromat-AT(4)), allowed obtaining similar results and carrying out direct comparison of different limits to accept treated waste in landfill, proposed in the literature. The results indicated that on an average, MBT treatment allowed 56% of ABP reduction after 4weeks of treatment, and 79% reduction after 12weeks of treatment. The obtainment of another regression model allowed transforming Sapromat-AT(4) limit in DRI units, and achieving a description of the kinetics of DRI and the corresponding ABP reductions vs. MBT treatment-time.

CIELAB color variables as indicators of compost stability

The composting process of different organic wastes both in laboratory and on a large-scale was characterized using CIELAB color variables to evaluate compost stability for the better application in agriculture. The time courses of the CIELAB variables of composting materials were determined directly from the bottom of a glass petri dish filled with dried and ground samples using a Minolta Color Reader (CR-13) calibrated with clean empty petri dishes placed on a white tile. To compare the proposed method with conventional methods, the same materials were also evaluated using commonly used compost stability evaluation indices. Most of the CIELAB variables of a compost made from a mixture of green tea waste and rice bran reached a plateau after 84 days of composting and showed strong relationships with the commonly used compost stability evaluation indices. The time needed for CIELAB variables, especially the L*and b* values, to stabilize at large-scale composting plants of cattle litter, farmyard manure, kitchen garbage and bark compost, were more or less similar to the times of maturation evaluated by the respective compost producers. The CIELAB color variable offers a new, simple, rapid and inexpensive means of evaluating compost stability and its quality prior to agricultural use.

Test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill

This discussion explores one crucial point about the use of biodegradability indicators to monitor biological processes in organic solid waste treatment plants. Today, some different measures are being used for the determination of biodegradable organic matter and most of them are based on respiration indices (oxygen consumption or carbon dioxide production under aerobic conditions) or biogas production tests (under strict anaerobic conditions). However, it is not evident from scientific literature that both tests may be equivalent or comparable. This discussion includes the results obtained when trying to correlate both anaerobic and aerobic tests to complement the recent work published by Wagland et al. [Wagland, S.T., Tyrrel, S.F., Godley, A.R., Smith, R., 2009. Test methods to aid in the evaluation of the diversion of biodegradable municipal waste (BMW) from landfill. Waste Management 29, 1218-1226].