Environ. Sci. Technol. 2007, 41, 6808-6814
Start-up of Anaerobic Digestion of Source-Sorted Organic Municipal Solid Waste in the Absence of Classical Inocula RANIA MAROUN AND MUTASEM EL FADEL* Department of Civil and Environmental Engineering, American University of Beirut, Lebanon
This study investigates the start-up and operation of bench-scale anaerobic digesters treating the sourcesorted organic fraction of municipal solid waste in the absence of mesophilic inocula. The experimental program consisted of starting up a digester with two consecutive interventions implemented to approach steady state. While start-up was relatively slow, re-inoculation with cattle manure together with effluent dilution reduced the acclimation period and achieved better stability and effectiveness, accommodating a feeding rate at an OLR ) 2.03 kg total volatile solids (TVS) m-3 day-1 and an equivalent hydraulic retention time (HRT) ) 148 days. The latter highlights the long duration that should be taken into consideration in the design and start-up of anaerobic digestion systems for municipal solid waste, in countries where this system is not established, before reaching optimal steady-state conditions.
Introduction The anaerobic digestion (AD) of solid substrate is still an evolving technology, having attracted comparatively less attention than the digestion of liquid waste. It offers the advantage of rapid stabilization of organic matter, reduction in waste volume, and subsequent land requirement for disposal, generation of energy from the produced biogas, production of stabilized sludge that can be used as a soil conditioner, and preservation of environmental quality in comparison to landfilling and incineration (1, 2). Recent years have witnessed a surge in work on various aspects of AD of the organic fraction of municipal solid waste (OFMSW), ranging from the fundamentals (including kinetic modeling) to process characteristics (including digester performance, inhibition factors, substrate, and temperature influence, twophase systems, etc.) (3). In this context, the fast and reliable process start-up in terms of type of inocula and overall startup strategies continues to lag at the research and development levels (4). In fact, AD processes for waste treatment have been traditionally hampered because of the time requirements of the start-up period, which can reach a year, a condition attributed to the slow growth rates of anaerobic bacteria (5-7). The successful start-up and operation of an anaerobic system requires that a proper balance be maintained between the hydrolytic and fermentative organisms * Corresponding author phone: 961-3-228-338; E-mail: mfadel@ aub.edu.lb; address: American University of Beirut, Faculty of Engineering and Architecture, Bliss Street, P.O. Box 11-0236, Beirut, Lebanon. 6808
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 19, 2007
involved in the first step and the methanogenic organisms responsible for the second step of the digestion process. This balance is accomplished through proper seeding, as well as through control of organic acid production and pH during start-up, when the microbial populations are establishing themselves (6). Various start-up strategies have been reported in the literature for both mesophilic and thermophilic systems using inocula from a wide range of sources and showing varying and at times inconclusive or contradictory results (5, 7-11). Laboratory and field-scale applications indicate startup periods ranging from three weeks to a year (4, 12, 13). Mesophilic anaerobic digesters (35-37 °C) reportedly exhibited successful start-up by relying on aerobic and anaerobic wastewater sludge and cattle manure as inocula. The current widespread presence of mesophilic AD plants for the OFMSW in Europe allowed faster start-up of new plants using already existing inocula. In contrast, the absence of an adequate mesophilic inoculum remains a serious constraint for countries that are newly introducing this technology whereby the effectiveness of other types of inocula has not been widely documented. In developing countries in particular, where technical and financial resources are limited, appropriate and fast process start-up has been the bottleneck for the dissemination of AD of MSW technology due to the lack of bulk anaerobic inocula in these countries (4). In general, the experience reported in the literature is still evolving and considered relatively insufficient or sometimes contradictory, making the comparison of applied startup strategies difficult and often impossible due to lack of uniformity in the presentation of the reported data, thus necessitating the performance of laboratory or pilot studies prior to constructing full-scale plants. The present study provides valuable insights on start-up, the most critical stage in the AD of MSW that represents a treatment technology with the potential of providing at least a contribution to energy savings and a partial solution to the environmental impact of MSW disposal, particularly in developing countries with limited energy resources and lack of proper solid-waste disposal methods. It examines the feasibility of starting up AD of the source-sorted organic fraction of municipal solid waste (SS-OFMSW) where classical anaerobic inocula are nonexistent and where MSW is dominated by a high fraction of food waste often exceeding 60% of the waste stream, increasing the attractiveness of AD. For this purpose, two laboratory-scale anaerobic reactors were set up and operated under varying start-up conditions, and their performance was compared in terms of acclimation time, treatment efficiency, and methane yield by using the SS-OFMSW as substrate. The general objective is to provide information on the application of AD of MSW at local and international levels where similar characteristics prevail.
Materials and Methods Reactors. Two laboratory-scale bioreactors of the type “BioFlo 110” from New Brunswick Scientific Co., Inc., with a working volume of 10.4 L each were used for the anaerobic digestion of biowaste (See Figure S1, Supporting Information). The air-tight bioreactors are connected to a customized 16.5 L water-displacement gasometer. Inocula. The inoculum during the initial start-up phase of the experiment was sewage sludge obtained from a severalmonths-old small wastewater treatment plant located in a rural area and whose characteristics are listed in Table 1. Another inoculum type used in the last run of the experiment consisted of fresh manure obtained from a cattle-rearing farm. The manure was enriched to increase the concentration 10.1021/es070686w CCC: $37.00
2007 American Chemical Society Published on Web 09/01/2007
organic matter (lignin content
