Mathematical model of organic substrate degradation in solid waste windrow composting.

Organic solid waste composting is a complex process that involves many coupled physical, chemical and biological mechanisms. To understand this complexity and to ease in planning, design and management of the composting plant, mathematical model for simulation is usually applied. The aim of this paper is to develop a mathematical model of organic substrate degradation and its performance evaluation in solid waste windrow composting system. The present model is a biomass-dependent model, considering biological growth processes under the limitation of moisture, oxygen and substrate contents, and temperature. The main output of this model is substrate content which was divided into two categories: slowly and rapidly degradable substrates. To validate the model, it was applied to a laboratory scale windrow composting of a mixture of wood chips and dog food. The wastes were filled into a cylindrical reactor of 6 cm diameter and 1 m height. The simulation program was run for 3 weeks with 1 s stepwise. The simulated results were in reasonably good agreement with the experimental results. The MC and temperature of model simulation were found to be matched with those of experiment, but limited for rapidly degradable substrates. Under anaerobic zone, the degradation of rapidly degradable substrate needs to be incorporated into the model to achieve full simulation of a long period static pile composting. This model is a useful tool to estimate the changes of substrate content during composting period, and acts as a basic model for further development of a sophisticated model.

Degradation of dibromophenols by UV irradiation.

We examined the degradation of dibromophenols (DBPs), i.e. 2,4-DBP, 2,6-DBP and 3,5-DBP by ultraviolet (UV) irradiation and estimated the relationship between degradability and molecular orbital properties of each dibromophenol. The removal of DBPs under a UV lamp system was successfully performed in an aqueous solution. After 5 min of irradiation, the initial DBPs concentration of 20 mg/L was decreased to below 1 mg/L, and about 60% of bromide ion was released. A decrease in the concentration of dissolved organic carbon (DOC) suggested the mineralization of DBPs. The mineralization may occur after release of bromide ions because the decrease of DOC was slower than the release of bromide ions. The degradability of 3,5-DBP was slightly lower than 2,6-DBP and 2,4-DBP. Molecular orbital calculation suggested that the electrophilic frontier density and the highest occupied molecular orbital (HOMO) energy may be related to the degradability of DBPs.

Scenario analysis of the benefit of municipal organic-waste composting over landfill, Cambodia.

This paper presents insight into the benefits of organic waste recycling through composting over landfill, in terms of landfill life extension, compost product, and mitigation of greenhouse gases (GHGs). Future waste generation from 2003 to 2020 was forecast, and five scenarios of organic waste recycling in the municipality of Phnom Penh (MPP), Cambodia, were carried out. Organic waste-specifically food and garden waste-was used for composting, and the remaining waste was landfilled. The recycling scenarios were set based on organic waste generated from difference sources: households, restaurants, shops, markets, schools, hotels, offices, and street sweeping. Through the five scenarios, the minimum volume reductions of waste disposal were about 56, 123, and 219 m(3) d(-1) in 2003, 2012, and 2020, respectively, whereas the maximum volume reductions in these years were about 325, 643, and 1025 m(3) d(-1). These volume reductions reflect a landfill life extension of a minimum of half a year and a maximum of about four years. Compost product could be produced at a minimum of 14, 30, and 54 tons d(-1) in 2003, 2012, and 2020, respectively, and at a maximum in those years of about 80, 158, and 252 tons d(-1). At the same time benefit is gained in compost product, GHG emissions could be reduced by a minimum of 12.8% and a maximum of 65.0% from 2003 to 2020. This means about 3.23 (minimum) and 5.79 million tons CO(2)eq (maximum) contributed to GHG mitigation. In this regard, it is strongly recommended that MPP should try to initiate an organic-waste recycling strategy in a best fit scenario.

Development of water movement model as a module of moisture content simulation in static pile composting.

This paper presents a mathematical model of vertical water movement and a performance evaluation of the model in static pile composting operated with neither air supply nor turning. The vertical moisture content (MC) model was developed with consideration of evaporation (internal and external evaporation), diffusion (liquid and vapour diffusion) and percolation, whereas additional water from substrate decomposition and irrigation was not taken into account. The evaporation term in the model was established on the basis of reference evaporation of the materials at known temperature, MC and relative humidity of the air. Diffusion of water vapour was estimated as functions of relative humidity and temperature, whereas diffusion of liquid water was empirically obtained from experiment by adopting Fick's law. Percolation was estimated by following Darcy's law. The model was applied to a column of composting wood chips with an initial MC of 60%. The simulation program was run for four weeks with calculation span of 1 s. The simulated results were in reasonably good agreement with the experimental results. Only a top layer (less than 20 cm) had a considerable MC reduction; the deeper layers were comparable to the initial MC, and the bottom layer was higher than the initial MC. This model is a useful tool to estimate the MC profile throughout the composting period, and could be incorporated into biodegradation kinetic simulation of composting.

Municipal solid waste management in Phnom Penh, capital city of Cambodia.

This paper presents an overview of municipal solid waste management (MSWM) for both technical and regulatory arrangements in the municipality of Phnom Penh (MPP), Cambodia. Problems with the current MSWM are identified, and challenges and recommendations for future improvement are also given in this paper. MPP is a small city with a total area of approximately 374 km(2) and an urban population of about 1.3 million in 2008. For the last 14 years, average annual municipal solid waste (MSW) generated in MPP has increased rapidly from 0.136 million tons in 1995 to 0.361 million tons in 2008. The gross generation rate of MSW per capita was 0.74 kg day(-1). However, the per capita household waste generation was 0.487 kg day(- 1). At 63.3%, food waste is the predominant portion of generated waste, followed by plastics (15.5%), grass and wood (6.8%), and paper and cardboard (6.4%). The remaining waste, including metals, glass, rubber/leather, textiles, and ceramic/ stone, accounted for less than 3%. Waste recycling through informal sectors is very active; recycled waste accounted for about 9.3% of all waste generated in 2003. Currently, the overall technical arrangement, including storage and discharge, collection and transport, and disposal, is still in poor condition, which leads to environmental and health risks. These problems should be solved by improving legislation, environmental education, solid waste management facilities, and management of the waste scavengers.

Hydrolytic dehalogenation of 4-chlorobenzoic acid by an Acinetobacter sp.

Acinetobacter sp. strain ST-1, isolated from garden soil, can mineralize 4-chlorobenzoic acid (4-CBA). The bacterium degrades 4-CBA, starting with dehalogenation to yield 4-hydroxybenzoic acid (4-HBA) under both aerobic and anaerobic conditions, suggesting that the dehalogenating enzyme in the strain is not an oxygenase; the enzyme may catalyze halide hydrolysis. To identify the oxygen source of the C(4)-hydroxy groups in the dehalogenation step, we used H(2)(18)O as the solvent under anaerobic conditions. When resting cells were incubated in the presence of 4-CBA and H(2)(18)O under a nitrogen gas stream, the hydroxy group on the aromatic nucleus of the 4-HBA produced was derived from water, not from molecular oxygen. This dehalogenation was hydrolytic, because analysis of the mass spectrum of the trimethylsilyl derivative of one of the metabolites, (18)O-labeled 4-HBA, showed that 80% of the C4-hydroxy groups were labeled with (18)O. Hydrolytic dehalogenation of 4-CBA in intact cells has not been reported earlier. To identify substrate specificity, we next examined the ability of the strain to dehalogenate 4-CBA analogues and dichlorobenzoic acids. The results of metabolite analysis by high-pressure liquid chromatography showed that the strain dehalogenated 4-bromobenzoic acid and 4-iodobenzoic acid, yielding 4-HBA, suggesting that these compounds could be further degraded and mineralized by the strain via the beta-ketoadipate pathway, as occurs with 4-CBA. This strain, however, did not dehalogenate 4-fluorobenzoic acid, 2- and 3-chlorobenzoic acids, or 2,4-, 3,4-, and 3,5-dichlorobenzoic acids during 4 days of incubation, implying that the dehalogenating enzyme of the strain has high substrate specificity.