Quazi Hamidul Bari

Effect of air recirculation and reuse on composting of organic solid waste

Abstract Composting has become increasingly popular in the past decade as a biological treatment process of organic solid wastes, generated from different sources, with the purpose of recovery, stabilization and volume reduction of waste material in the form of compost. In this study, the effect of different modes of aeration on composting of solid waste using two heat insulated closed pilot-scale reactors was investigated. The modes of aeration studied were upflow, downflow, alternate upflow/downflow, and internal recirculation in a single-reactor system as well as reuse of spent air in a two-reactor system in series. Composting tests were performed in two stages of 20–30 days duration each. Temperature at different heights of the composting mass as well as air velocity were continuously monitored by a datalogger. Air flow was continuous or intermittent depending on temperature. Oxygen content in the spent air was regularly measured. The results show that (i) the application of unidirectional upflow or downflow aeration creates significant vertical distribution of temperature in the composting mass; and (ii) internal recirculation of air in a single-reactor system and reuse of spent air in a two-reactor system appeared to achieve a more uniform temperature distribution and thereby accelerated degradation of the organic matter. Practical applications in composting plants are discussed.

Kinetic analysis of forced aeration composting- II. Application of multilayer analysis for the prediction of biological degradation

Extensive pilot-scale composting tests for organic solid waste were conducted under different modes of aeration, to investigate vertical temperature distributions and their effect on biological degradation at different heights of the composting mass. The modes of aeration applied were upflow, downflow, alternate upflow/downflow, and internal air recirculation. Temperatures at different heights of the composting mass were continuously monitored. Results show that (i) significant variation in temperature and biodegradable volatile solids (BVS) degradation occur in the composting mass along the vertical direction when unidirectional aeration is applied; (ii) application of alternate upflow/downflow aeration or periodic mixing greatly reduces vertical gradients in temperature and biodegradable volatile solids (BVS) degradation; and (iii) the rate and extent of degradation in different layers of the composting mass can be quantitatively predicted by applying a combination of multilayer analysis and a previously established temperature-dependent first-order reaction model.

Application of self-heating test for indirect estimation of respirometric activity of compost: theory and practice.

The self-heating test is widely adopted at solid waste composting plants as a simple and inexpensive test to determine the biological stability of the produced compost. Among other stability measures, the popularity of the respirometric activity determined by oxygen uptake rate is also increasing. However, the determination of respirometric activity by the respirometer is comparatively complicated and expensive. Therefore, in this study an attempt was made to determine indirectly the respirometric activity, using easily available data from the self-heating test. Experimental and theoretical analysis show that (i) the self-heating test could serve two purposes namely, (a) determination of the stability index of the compost; and (b) indirect estimation of respirometric activity; and (ii) the estimated respirometric activity based on the heat energy balance of the self-heating test compares well with the conventional method and could be used as a stability index for compost.

Application of a simplified mathematical model to estimate the effect of forced aeration on composting in a closed system.

The aeration rate is a key process control parameter in the forced aeration composting process because it greatly affects different physico-chemical parameters such as temperature and moisture content, and indirectly influences the biological degradation rate. In this study, the effect of a constant airflow rate on vertical temperature distribution and organic waste degradation in the composting mass is analyzed using a previously developed mathematical model of the composting process. The model was applied to analyze the effect of two different ambient conditions, namely, hot and cold ambient condition, and four different airflow rates such as 1.5, 3.0, 4.5, and 6.0 m(3) m(-2) h(-1), respectively, on the temperature distribution and organic waste degradation in a given waste mixture. The typical waste mixture had 59% moisture content and 96% volatile solids, however, the proportion could be varied as required. The results suggested that the model could be efficiently used to analyze composting under variable ambient and operating conditions. A lower airflow rate around 1.5-3.0 m(3) m(-2) h(-1) was found to be suitable for cold ambient condition while a higher airflow rate around 4.5-6.0 m(3) m(-2) h(-1) was preferable for hot ambient condition. The engineered way of application of this model is flexible which allows the changes in any input parameters within the realistic range. It can be widely used for conceptual process design, studies on the effect of ambient conditions, optimization studies in existing composting plants, and process control.

Study on the quality and stability of compost through a Demo Compost Plant

This study is concerned with the performance of a Demo Compost Plant for the development of acceptable composting technology in Bangladesh. The Demo Compost Plant was setup at the adjacent area of an existing compost plant located at Khulna city in Bangladesh. Four different composting technologies were considered, where Municipal Solid Waste (MSW) were used as a raw material for composting, collected from the adjacent areas of the plant. Initially the whole composting system was conducted through two experimental setups. In the 1st setup three different types of aerators (horizontal and vertical passively aerator and forced aerator) were selected. For a necessary observation four piles, using only MSW as the input materials in the first three compost pile, the fourth one was the existing Samadhans compost pile. Based on the analysis of the experimental findings, the horizontal passively aerated composting technique is suitable for Bangladesh as it had better performance for reducing composting period than that of the others. It was being observed from the quality parameters of compost in the both 1st and 2nd setup that as the waste directly come from kitchen, degradation rate of waste shows a positive result for reducing this waste and there is no possibility of toxic contamination, when it would be used as a soil conditioner. Though there is no significant improvement in the quality of the final product in the 2nd setup as comparing with the 1st setup but it fulfills one of the main objectives of this study is to reduce the whole composting period as well as immediate management of the increasing amount of waste and reducing load on landfill. Selfheating tests reveal that degree of stability of compost with respect to maturation period was remained in the acceptable level, which was further accelerated due to the use of organic additives.

Scenario of solid waste reuse in Khulna city of Bangladesh

The reuse and recycling of waste materials are now sincerely considered to be an integral part of solid waste management in many parts of the world. In this context, a vast number of options ranging from small scale decentralized to larger scale centralized plants have been adopted. This study aimed at investigating the waste reuse schemes in Khulna city located in the southern part of Bangladesh and ranked third largest city in the country. The shops for reusable material (SRM) were mostly situated around railway, waterway, and truck station markets which provided easy transportation to further locations. For the reuses of waste materials and products, a chain system was found to collect reusable wastes under a total number of 310 identified SRM with 859 persons directly or indirectly involved in the scheme. This was a decentralized waste management system with self sufficient (autonomous) management. According to mass balance, about 38.52 tons d(-1) solid wastes were reused in Khulna city area, accounting for 7.65% of the total generated wastes. This study revealed that apparently a silent, systematic, smooth, and clean reuse chain has been established in Khulna city area under private initiatives, whose sustainability was confirmed over the years in the country without any official or formal funds. However, proper adjustment between the higher and lower chain in the materials flow path, as well as personal hygiene training for the workers, would further improve the achievements of the established reuse scheme.

Characterization of Rice Husk Carbon Produced through Simple Technology

Textile wastewater contains different colors which are harmful to the environment. Activated carbon can be used for the decolorization of textile wastewater. Most of the textile plants in Bangladesh do not use the activated carbon due to its expensive cost and still it is classified as imported item. Low-cost activated carbon produced from locally available materials can solve this problem. This paper describes the color removal of textile wastewater by adsorption process using activated carbon derived from rice husk in a low-cost method. Thermal activation system was applied for the preparation of carbon. The maximum adsorption of color was found at an optimum temperature of 400C with the retention time of 60 minutes. Thus this study demonstrated encouraging performance of activated carbon produced from rice husk compared to the industrial grade activated carbon for decolorization of textile wastewater in the analysis.