A new liquid-phase method and its comparison to two solid-phase microbial respiration activity methods to assess organic waste stability.

Goal of the work was to compare the respiration activities, as measured via oxygen consumption with three different organic waste stability methods so that to propose the optimal one. The novelty of the work is that there exists no comparison of solid-phase with liquid-phase stability assessment techniques in the literature. The respiration activities were assessed using two solid-phase methods and a manometric liquid-phase method (MANLIQ) performed on twenty-seven organic substrates. The methods rely on measuring oxygen consumption (uptake) via pressure drops (liquid-phase test, static solid-phase test) or via direct O2 measurements on the gaseous phases at the inlet and outlet of the respirometer (solid-phase dynamic test). A positive statistically significant correlation was calculated between the MANLIQ and the static solid-phase indices. The maximum rate MANLIQ index for the raw substrates was 2900 mg O2 kg-1 VS h-1, while most of the processed substrates had cumulative MANLIQ indices below 160 g O2 kg-1 VS. The ratio of the liquid indices to the static solid-phase indices ranged from 1.6 to 2.7 and the ratio of the liquid indices to the dynamic solid-phase indices ranged from 0.2 to 0.4. The MANLIQ method failed to result in a good correlation of the processing time with the respiration indices. On the other hand, a correlation was more visible in the two solid-phase tests, despite the large variability of the types and sources of the substrates. Therefore, the solid-phase methods should be preferred over the liquid-phase method to assess stability for various organic substrates.

Effect of air flowrate on the dynamic respiration activity of the raw organic fraction of municipal solid wastes.

Scope of this work was to study the effect of the aeration rate on the respiration activity of the fresh organic fraction of MSW and to compare the resulting dynamic respiration indices with those of MSW derived compost. Thus, a categorization of the dynamic respiration activity of MSW throughout a composting facility is provided. A simulated organic fraction of MSW was used as a substrate and four experimental runs were performed to achieve unit airflow rates (UAF) from around 6 to 30Lairkg-1VSh-1. Six dynamic respiration activity indices were calculated and compared to the corresponding indices of stable MSW compost from a previous work. Findings indicate that the increase of the UAF results in a corresponding increase of the dynamic stability indices. Dynamic respiration activity indices above 1500 and below 520mgO2kg-1VSh-1 indicate fresh and very stable MSW materials, respectively.

Monitoring the performances of a real scale municipal solid waste composting and a biodrying facility using respiration activity indices.

Objective of the work was to monitor two full-scale commingled municipal solid waste (MSW) mechanical and biological pretreatment (MBT) facilities in Greece, namely a biodrying and a composting facility. Monitoring data from a 1.5-year sampling period is presented, whilst microbial respiration indices were used to monitor the decomposition process and the stability status of the wastes in both facilities during the process. Results showed that in the composting facility, the organic matter reduced by 35 % after 8 weeks of combined composting/curing. Material exiting the biocells had a moisture content of less than 30 % (wb) indicating a moisture limitation during the active composting process. The static respiration indexes indicated that some stabilization occurred during the process, but the final material could not be characterized as stable compost. In the biodrying facility, the initial and final moisture contents were 50 % and less than 20 % wb, respectively, and the biodrying index was equal to 4.1 indicating effective biodrying. Lower heating values at the inlet and outlet were approximately 5.5 and 10 MJ/wet kg, respectively. The organic matter was reduced by 20 % during the process and specifically from a range of 63-77 % dw (inlet) to a range of 61-70 % dw. A significant respiration activity reduction was observed for some of the biodrying samples. A statistically significant correlation among all three respiration activity indices was recorded, with the two oxygen related activity indices (CRI7 and SRI24) observing the highest correlation.

Anaerobic digestion of tomato processing waste: Effect of alkaline pretreatment.

The objective of the work was to assess the effect of mild alkaline pretreatment on the anaerobic biodegradability of tomato processing waste (TPW). Experiments were carried out in duplicate BMP bottles using a pretreatment contact time of 4 and 24 h and a 1% and 5% NaOH dosage. The cumulative methane production during a 30 d period was recorded and modelled. The alkaline pretreatment did not significantly affect methane production in any of the treatments in comparison to the control. The average methane production for all runs was 320 NmL/gVS. Based on first order kinetic modelling, the alkaline pretreatment was found to slow down the rate of methanogenesis, mainly in the two reactors with the highest NaOH dosage. The biodegradability of the substrates ranged from 0.75 to 0.82 and from 0.66 to 0.72 based on two different approaches.

Aerobic biological pretreatment of municipal solid waste with a high content of putrescibles: effect on landfill emissions.

The objective of this work was to study the effect of aerobic biological pretreatment on the emissions of municipal solid waste (MSW) with a high content of putrescibles after landfilling. For this purpose, the organic fraction of MSW was simulated by a mixture of food waste and office paper at a 2.4:1 wet weight ratio. MSW was first pretreated aerobically for three different time periods (8, 45 and 90 days) resulting in organic matter reductions equal to 15%, 45% and 81% respectively. MSW were then placed in 160-L air-tight anaerobic bioreactors. The control anaerobic bioreactors contained untreated MSW. Anaerobic experiments lasted from 300 to 550 days. Results showed that the biogas production from untreated MSW was 372 NL dry kg(-1) (average of two replications) after 530 days. The MSW that was pretreated aerobically for 45 days and 90 days yielded 130 and 62 NL dry kg(-1) of biogas after 310 days and 230 days respectively. However, the 8-day (very short-term) pretreatment period led to an increase of the biogas yield (550 NL dry kg(-1) after 340 days) compared with that of raw refuse. All three runs with aerobically pretreated MSW reached the steady methanogenic phase faster than raw MSW. Leachate emissions were significantly lower in the aerobically-pretreated MSWthan the untreated ones. The leachate ammonium concentrations had an increasing trend in all anaerobic reactors and reached a plateau of between 2 and 3.5 g L(-1) at the end of the process.

Use of municipal solid waste compost as a growth media for an energy plant (rapeseed).

Objective of this work was to study the effect of mixtures of municipal solid waste (MSW) compost with a peat based growth media on the yields of rapeseed. The MSW compost and the peat were respectively mixed at ratios equal to 10%:90%, 20%:80% and 50%:50% (wet weight basis). A randomized block design was adopted with 5 treatments and 3 blocks per treatment. The 50%:50% mixture achieved a statistically higher yield (dry weight and average leaf area) compared to all other treatments, whilst compost alone led to the lowest shoot growth. Negative correlations were calculated between the total contents of Pb, Ni and Cd of the plant tissues and the corresponding dry weights of the shoots. Regression equations were developed that described shoot yields as a function of the water holding capacity, pH, electrical conductivity, C/N ratio and the total and leachable contents of certain metals in the growth media.

Revisiting the elemental composition and the calorific value of the organic fraction of municipal solid wastes.

In this work, the elemental content (C, N, H, S, O), the organic matter content and the calorific value of various organic components that are commonly found in the municipal solid waste stream were measured. The objective of this work was to develop an empirical equation to describe the calorific value of the organic fraction of municipal solid waste as a function of its elemental composition. The MSW components were grouped into paper wastes, food wastes, yard wastes and plastics. Sample sizes ranged from 0.2 to 0.5 kg. In addition to the above individual components, commingled municipal solid wastes were sampled from a bio-drying facility located in Crete (sample sizes ranged from 8 to 15 kg) and were analyzed for the same parameters. Based on the results of this work, an improved empirical model was developed that revealed that carbon, hydrogen and oxygen were the only statistically significant predictors of calorific value. Total organic carbon was statistically similar to total carbon for most materials in this work. The carbon to organic matter ratio of 26 municipal solid waste substrates and of 18 organic composts varied from 0.40 to 0.99. An approximate chemical empirical formula calculated for the organic fraction of commingled municipal solid wastes was C(32)NH(55)O(16).

Monitoring and optimizing the co-composting of dewatered sludge: a mixture experimental design approach.

The management of dewatered wastewater sludge is a major issue worldwide. Sludge disposal to landfills is not sustainable and thus alternative treatment techniques are being sought. The objective of this work was to determine optimal mixing ratios of dewatered sludge with other organic amendments in order to maximize the degradability of the mixtures during composting. This objective was achieved using mixture experimental design principles. An additional objective was to study the impact of the initial C/N ratio and moisture contents on the co-composting process of dewatered sludge. The composting process was monitored through measurements of O(2) uptake rates, CO(2) evolution, temperature profile and solids reduction. Eight (8) runs were performed in 100 L insulated air-tight bioreactors under a dynamic air flow regime. The initial mixtures were prepared using dewatered wastewater sludge, mixed paper wastes, food wastes, tree branches and sawdust at various initial C/N ratios and moisture contents. According to empirical modeling, mixtures of sludge and food waste mixtures at 1:1 ratio (ww, wet weight) maximize degradability. Structural amendments should be maintained below 30% to reach thermophilic temperatures. The initial C/N ratio and initial moisture content of the mixture were not found to influence the decomposition process. The bio C/bio N ratio started from around 10, for all runs, decreased during the middle of the process and increased to up to 20 at the end of the process. The solid carbon reduction of the mixtures without the branches ranged from 28% to 62%, whilst solid N reductions ranged from 30% to 63%. Respiratory quotients had a decreasing trend throughout the composting process.