Effect of freezing on the conservation of the biological activity of organic solid wastes

To assess the effect of freezing on the indigenous biological activity of an organic waste, five types of organic wastes (raw sludge [RS], municipal solid waste [MSW], partially processed municipal solid waste [MSWpp], digested sludge [DS] and composted organic fraction of municipal solid waste [OFMSWc]) were frozen and stored during different times to identify if the interruption of the native biological activity was recovered. Respiration indices (DRI₂₄h and AT₄) were used to determine the biological activity expressed as oxygen consumption. ANOVA analysis was used to compare the results. Respiration indices of RS, DS, MSWpp and OFMSWc were not affected by freezing storage during 1 year. Contrarily, respiration indices of MSW samples were statistically different after 52 and 20 weeks of freezing storage (DRI₂₄h and AT₄, respectively). Regarding the lag phase and the time to reach maximum respiration activity, frozen samples induced a significant change in the organic samples analyzed except for OFMSWc

A complete mass balance of a complex combined anaerobic/aerobic municipal source-separated waste treatment plant

In this study a combined anaerobic/aerobic full-scale treatment plant designed for the treatment of the source-separated organic fraction of municipal solid waste (OFMSW) was monitored over a period of one year. During this period, full information was collected about the waste input material, the biogas production, the main rejects and the compost characteristics. The plant includes mechanical pre-treatment, dry thermophilic anaerobic digestion, tunnel composting system and a curing phase to produce compost. To perform the monitoring of the entire plant and the individual steps, traditional chemical methods were used but they present important limitations in determining the critical points and the efficiency of the stabilization of the organic matter. Respiration indices (dynamic and cumulative) allowed for the quantitative calculation of the efficiency of each treatment unit. The mass balance was calculated and expressed in terms of Mg y⁻¹ of wet (total) matter, carbon, nitrogen and phosphorus. Results show that during the pre-treatment step about 32% of the initial wet matter is rejected without any treatment. This also reduces the biodegradability of the organic matter that continues to the treatment process. About 50% of the initial nitrogen and 86.4% of the initial phosphorus are found in the final compost. The final compost also achieves a high level of stabilization with a dynamic respiration index of 0.3 ± 0.1 g O₂ per kg of total solids per hour, which implies a reduction of 93% from that of the raw OFMSW, without considering the losses of biodegradable organic matter in the refuse (32% of the total input). The anaerobic digestion process is the main contributor to this stabilization

Evolution of organic matter in a full-scale composting plant for the treatment of sewage sludge and biowaste by respiration techniques and pyrolysis-GC/MS

A full-scale composting plant treating in two parallel lines sewage sludge and the source-selected organic fraction of municipal solid waste (OFMSW or biowaste) has been completely monitored. Chemical routine analysis proved not to be suitable for an adequate plant monitoring in terms of stabilization and characterization of the process and final compost properties. However, the dynamic respiration index demonstrated to be the most feasible tool to determine the progression of the degradation and stabilization of organic matter for both sewage sludge and OFMSW lines. Both lines exhibited an important degree of stabilization of organic matter using rapid and cumulative respiration indices. Pyrolysis-GC/MS was applied to the most important inputs, outputs, and intermediate points of the plant. It proved to be a powerful tool for the qualitative characterization of molecular composition of organic matter present in solid samples. A full characterization of the samples considered is also presented

Anaerobic degradation of PAHs in soil : impacts of concentration and amendment stability on the PAHs degradation and biogas production

In this study, the bioremediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil under strict anaerobic-methanogenic conditions was systematically studied applying the central composite design approach. The effect of PAHs concentration and the stability of the compost as an organic amendment for anaerobic digestion were examined. In all assays, the used methanogenic consortium was able to degrade the PAHs although some inhibition effects were observed during the initial stage in some cases. The degradation rates varied between 31.4 and 90.6% during 50 days incubation period. The study demonstrated that the PAHs concentration influences the degradation rate where more degradation was observed by increasing the concentration of PAHs. However, the biogas production as a result of the digestion process was more influenced by the compost stability which also has its effect on the degradation rates as more degradation occurred with more stable compost, but more biogas was produced with less stable compost, which indicates that the biogas is mainly produced by the anaerobic digestion of the amended compost. Finally, it seems that compost addition is required to improve the process in some cases but in other circumstances it does not greatly improve the bioremediation of PAHs

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 DRI24h 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 (0.3 g O₂ 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

Determination of the energy and environmental burdens associated to the biological treatment of source-separated municipal solid wastes

Environmental burdens of four different full-scale facilities treating source-separated organic fraction of Municipal Solid Wastes (OFMSW) have been experimentally evaluated. The studied facilities include different composting technologies and also anaerobic digestion plus composting. Home composting, as an alternative to OFMSW management, was also included in the study. Energy (electricity and diesel), water consumption and emissions of volatile organic compounds (VOC), ammonia, methane and nitrous oxide have been measured for each process. Energy consumption ranged between 235 and 870 MJ Mg OFMSW⁻¹ while the emissions of the different contaminants considered per Mg OFMSW were in the range of 0.36-8.9 kg VOC, 0.23-8.63 kg NH₃, 0.34-4.37 kg CH₄ and 0.035-0.251 kg N₂O, respectively. Environmental burdens of each facility are also analyzed from the point of view of process efficiency (i.e. organic matter stabilization degree achieved, calculated as the reduction of the Dynamic Respiration Index (DRI) of the waste treated). This study is performed through two new indices: Respiration Index Efficiency (RIE), which includes the reduction in the DRI achieved by the treatment process and Quality and Respiration Index Efficiency (QRIE), which also includes the quality of the end product. Finally, a Life Cycle Assessment is performed using the Respiration Index Efficiency (RIE) as the novel functional unit instead of the classical LCA approach based on the total mass treated

Optimization of solid state anaerobic digestion of the OFMSW by digestate recirculation: A new approach

Dry anaerobic digestion (AD) of OFMSW was optimized in order to produce biogas avoiding the use of solid inoculum. Doing so the dry AD was performed irrigating the solid waste with liquid digestate (flow rate of 1:1.18-1:0.9 w/w waste/digestate; 21. d of hydraulic retention time - HRT) in order to remove fermentation products inhibiting AD process. Results indicated that a high hydrolysis rate of organic matter (OM) and partial biogas production were obtained directly during the dry AD. Hydrolysate OM was removed from digester by the percolate flow and it was subsequently used to feed a liquid anaerobic digester. During dry AD a total loss of 36.9% of total solids was recorded. Methane balance indicated that 18.4% of potential methane can be produced during dry AD and 49.7% by the percolate. Nevertheless results obtained for liquid AD digestion indicated that only 20.4% and 25.7% of potential producible methane was generated by adopting 15 and 20. days of HRT, probably due to the AD inhibition due to high presence of toxic ammonia forms in the liquid medium

Effect of freezing on the conservation of the biological activity of organic solid wastes

To assess the effect of freezing on the indigenous biological activity of an organic waste, five types of organic wastes (raw sludge [RS], municipal solid waste [MSW], partially processed municipal solid waste [MSWpp], digested sludge [DS] and composted organic fraction of municipal solid waste [OFMSWc]) were frozen and stored during different times to identify if the interruption of the native biological activity was recovered. Respiration indices (DRI₂₄h and AT₄) were used to determine the biological activity expressed as oxygen consumption. ANOVA analysis was used to compare the results. Respiration indices of RS, DS, MSWpp and OFMSWc were not affected by freezing storage during 1 year. Contrarily, respiration indices of MSW samples were statistically different after 52 and 20 weeks of freezing storage (DRI₂₄h and AT₄, respectively). Regarding the lag phase and the time to reach maximum respiration activity, frozen samples induced a significant change in the organic samples analyzed except for OFMSWc

A complete mass balance of a complex combined anaerobic/aerobic municipal source-separated waste treatment plant

In this study a combined anaerobic/aerobic full-scale treatment plant designed for the treatment of the source-separated organic fraction of municipal solid waste (OFMSW) was monitored over a period of one year. During this period, full information was collected about the waste input material, the biogas production, the main rejects and the compost characteristics. The plant includes mechanical pre-treatment, dry thermophilic anaerobic digestion, tunnel composting system and a curing phase to produce compost. To perform the monitoring of the entire plant and the individual steps, traditional chemical methods were used but they present important limitations in determining the critical points and the efficiency of the stabilization of the organic matter. Respiration indices (dynamic and cumulative) allowed for the quantitative calculation of the efficiency of each treatment unit. The mass balance was calculated and expressed in terms of Mg y⁻¹ of wet (total) matter, carbon, nitrogen and phosphorus. Results show that during the pre-treatment step about 32% of the initial wet matter is rejected without any treatment. This also reduces the biodegradability of the organic matter that continues to the treatment process. About 50% of the initial nitrogen and 86.4% of the initial phosphorus are found in the final compost. The final compost also achieves a high level of stabilization with a dynamic respiration index of 0.3 ± 0.1 g O₂ per kg of total solids per hour, which implies a reduction of 93% from that of the raw OFMSW, without considering the losses of biodegradable organic matter in the refuse (32% of the total input). The anaerobic digestion process is the main contributor to this stabilization

Anaerobic degradation of PAHs in soil : impacts of concentration and amendment stability on the PAHs degradation and biogas production

In this study, the bioremediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil under strict anaerobic-methanogenic conditions was systematically studied applying the central composite design approach. The effect of PAHs concentration and the stability of the compost as an organic amendment for anaerobic digestion were examined. In all assays, the used methanogenic consortium was able to degrade the PAHs although some inhibition effects were observed during the initial stage in some cases. The degradation rates varied between 31.4 and 90.6% during 50 days incubation period. The study demonstrated that the PAHs concentration influences the degradation rate where more degradation was observed by increasing the concentration of PAHs. However, the biogas production as a result of the digestion process was more influenced by the compost stability which also has its effect on the degradation rates as more degradation occurred with more stable compost, but more biogas was produced with less stable compost, which indicates that the biogas is mainly produced by the anaerobic digestion of the amended compost. Finally, it seems that compost addition is required to improve the process in some cases but in other circumstances it does not greatly improve the bioremediation of PAHs