Recovery of solid fuel from municipal solid waste by hydrothermal treatment using subcritical water

Hydrothermal treatments using subcritical water (HTSW) such as that at 234 °C and 3 MPa (LT condition) and 295 °C and 8 MPa (HT condition) were investigated to recover solid fuel from municipal solid waste (MSW). Printing paper, dog food (DF), wooden chopsticks, and mixed plastic film and sheets of polyethylene, polypropylene, and polystyrene were prepared as model MSW components, in which polyvinylchloride (PVC) powder and sodium chloride were used to simulate Cl sources.While more than 75% of carbon in paper, DF, and wood was recovered as char under both LT and HT conditions, plastics did not degrade under either LT or HT conditions. The heating value (HV) of obtained char was 13,886-27,544 kJ/kg and was comparable to that of brown coal and lignite. Higher formation of fixed carbon and greater oxygen dissociation during HTSW were thought to improve the HV of char.Cl atoms added as PVC powder and sodium chloride to raw material remained in char after HTSW. However, most Cl originating from PVC was found to converse into soluble Cl compounds during HTSW under the HT condition and could be removed by washing.From these results, the merit of HTSW as a method of recovering solid fuel from MSW is considered to produce char with minimal carbon loss without a drying process prior to HTSW. In addition, Cl originating from PVC decomposes into soluble Cl compound under the HT condition. The combination of HTSW under the HT condition and char washing might improve the quality of char as alternative fuel.     

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.     

Comparison of leaching tests to determine and quantify the release of inorganic contaminants in demolition waste

The changes in waste management policy caused by the massive generation of waste materials (e.g. construction and demolition waste material, municipal waste incineration products) has led to an increase in the reuse and recycling of waste materials. For environmental risk assessment, test procedures are necessary to examine waste materials before they can be reused. In this article, results of column and lysimeter leaching tests having been applied to inorganic compounds in a reference demolition waste material are presented. The results show a good agreement between the leaching behaviour determined with the lysimeter unit and the column units used in the laboratory. In view of less time and system requirements compared to lysimeter systems, laboratory column units can be considered as a practicable instrument to assess the time-dependent release of inorganic compounds under conditions similar to those encountered in a natural environment. The high concentrations of elements in the seepage water at the initial stage of elution are reflected by the laboratory column leaching tests. In particular, authorities or laboratories might benefit and have an easy-to-use, but nevertheless reliable, method to serve as a basis for decision-making.     

Performance of a low cost MBT prior to landfilling: study of the biological treatment of size reduced MSW without mechanical sorting

In France, the interest in Mechanical Biological Treatment (MBT) prior to landfilling is actually growing. In the absence of acceptance criteria for the waste to be landfilled, an alternative to the intensive, high-technology MBT can only find its place in the French context if it shows substantial benefits from an environmental, economic or operational point of view. This paper presents an experiment of low-cost MBT of size reduced MSW without material splitting. The performance of an experimental, pilot-scale mechanical and biological treatment process has been studied on 37.5 Mg of raw municipal solid waste. The mechanical process has been kept simple with only coarse shredding and no material recovery. The biological treatment, which was a low-cost forced aeration process, was monitored for 25 weeks. The biogas production potential of the waste was reduced by 90% to 19 NL kgDM(-1). The initial AT4 index of 82.9 mg O2 gDM(-1) decreased to 16.0 mg O2 gDM(-1). After 25 weeks of aerobic treatment, the dry mass loss reached 37%, while the mass of waste going to landfill was reduced by 28%. The average performances of the process were explained by the biological process itself, which was not optimal, and also by the characteristics of the input waste. The high particle size of the treated waste and the high content of slowly biodegradable matter (such as paper and cardboard) may both be significant drawbacks for the biological stabilisation of waste.     

Destruction of PAHS from soil by using pressurized hot water extraction coupled with supercritical water oxidation

Chemical processes utilizing water both as extraction solvent and reaction medium are promising "Green Chemistry" alternatives to conventional techniques. Equipment for on-line coupled hot water extraction and supercritical water oxidation was constructed to extract polyaromatic hydrocarbons and toluene from sea sand followed by oxidation using hydrogen peroxide. The effectiveness of the technique is based on the physico-chemical properties of heated and pressurized water. Extraction efficiency increased with temperature and time; the best results were obtained at 300 degrees C with 40 min extraction time. In the oxidation stage, conversion of the PAHs increased with reaction time and oxidant concentration and the best conversion (97.0-99.9%, depending on the compound) was obtained at 425 degrees C with 43 s reaction time. Benzaldehyde and benzoic acid were the most abundant reaction intermediates in the oxidation process. In addition, phenol, p-cresol, and benzyl alcohol were found as intermediates. The intermediates originated mainly from toluene, which was present in much greater concentration than PAHs in the reaction medium.     

Recovery of zinc and lead from fly ash from ash-melting and gasification-melting processes of MSW--comparison and applicability of chemical leaching methods

Fly ash generated from MSW ash-melting and gasification-melting plants, known as Melting Furnace Fly Ash (MFA), contains considerable amounts of heavy metals such as Pb and Zn. These metals can be recovered using a smelting furnace after "pre-treatment" for removal of unnecessary elements such as Cl, Sn and Si. Chemical methods have been studied for pretreatment in the past. However, they have been discussed only with regard to treatment cost and the concentration of Pb and Zn recovered, but neither applicability to various types of MFA nor the environmental impact have been considered. In this study, acid, alkaline and ammonia/chloride leaching methods were compared from the standpoints of: (1) applicability to MFA, (2) concentration of Pb and Zn recovered, (3) treatment cost, and (4) environmental impact. Twenty-three samples of MFAs were collected and classified into 4 types based on element contents. A Pb and Zn recovery experiment was conducted for the representative MFA of those types. The results showed: (1) MFA from gasification-melting plants cannot be treated by chemical methods; (2) the other MFA can be treated to an acceptable quality by existing smelting furnaces; (3) only MFA from electric resistance ash-melting plants can be treated easily by the water washing method; and (4) alkaline and ammonia/chloride leaching methods were more effective than acid leaching.     

Municipal solid waste management in Nepal: practices and challenges

Solid waste management in Kathmandu valley of Nepal, especially concerning the siting of landfills, has been a challenge for over a decade. The current practice of the illegal dumping of solid waste on the river banks has created a serious environmental and public health problem. The focus of this study was to carry out an evaluation of solid waste management in Nepal based on published information. The data showed that > or =70% of the solid wastes generated in Nepal are of organic origin. As such, composting of the solid waste and using it on the land is the best way of solid waste disposal. This will reduce the waste volume transported to the landfill and will increase its life.     

Microbiological quality of roof-harvested rainwater and health risks: a review

Roof-harvested rainwater (RHRW) has been considered an effective alternative water source for drinking and various nonpotable uses in a number of countries throughout the world. The most significant issue in relation to using untreated RHRW for drinking or other potable uses, however, is the potential public health risks associated with microbial pathogens. This paper reviews the available research reporting on the microbial quality of RHRW and provides insight on the capacity of fecal indicator bacteria to monitor health risks and disease outbreaks associated with the consumption of untreated RHRW. Several zoonotic bacterial and protozoan pathogens were detected in individual and communal rainwater systems. The majority of the studies reported in the literature assessed the quality of rainwater on the basis of the presence or absence of specific pathogens, with little information available regarding the actual numbers of such pathogens. In addition, no information is available concerning the ongoing prevalence of different pathogens in RHRW over time. The published data suggest that the microbial quality of RHRW should be considered less than that expected for potable water and that the commonly used indicators may not be suitable to indicate the presence of pathogens in RHRW. Several case control studies established potential links between gastroenteritis and consumption of untreated RHRW. Therefore, health risks assessment models, such as those using Quantitative Microbial Risk Assessment, should be used to manage and mitigate health risks associated with drinking and nonpotable uses of RHRW.