University of the Philippines at Diliman

Treatment of Alkali-Pretreated Slurry of Organic Fractions of Municipal Solid Waste using Upflow Anaerobic Sludge Blanket (UASB) Reactor

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Courtesy of University of the Philippines at Diliman

[Master's Thesis in Environmental Engineering] As an alternative technology for use of local governments, we propose to demonstrate the suitability of anaerobic digestion (AD) system to treat the organic fractions of municipal solid waste (OFMSW). To enhance the process, we applied several pre-treatment steps: comminution, extraction, and addition of sodium hydroxide to achieve the desired pH 8.0. The pre-treated kitchen waste slurry was continuously pumped to an upflow anaerobic sludge blanket reactor (UASB) at an organic loading rate (OLR) of 5 to 12.5 kg TCOD/m3-d during a 69-day period. During the first 16 days when OLR was set at 9.0 kg TCOD/m3-d. From days 16 to 31, OLR was increased to 12.5 kg TCOD/m3-d but resulted in higher effluent TCOD and hampered methane generation rates. Starting day 31, OLR was reduced to 7.0 but was gradually increased to 11.1 kg TCOD/m3-d at the last feeding period. This brought about 70 to 95% TCOD removal and an increase of methane generation from 2.25 to 3.70 g CH4-COD/l-d. The specific gas production rate during days 58-69 was 0.087 m3/kg-TCOD (28.90C, 1 atm). The gas production rate at day 69 was 1.425 m3/m3reactor-d, which was relatively high as compared to other AD systems used to treat OFMSW. This means that a relatively smaller volume of reactor would be required for the same amount of methane recovered.

1. Introduction
The Philippine Ecological Solid Waste Management Act of 2001 mandates local governments to implement source segregation and diversion of refuse from disposal sites. About 45% of the refuse generated in the country is food and kitchen wastes [1]. Even before the passage of the law, majority have chosen to adopt aerobic composting technologies in managing the organic fractions of municipal solid waste (OFMSW) because of its popularity and operational simplicity. However, the need to add bulky cellulosic materials and aeration systems and the limitations associated with space, capacity and seasonal variations motivate them to consider anaerobic digestion as a sustainable alternative [2]. In addition, anaerobic systems offer the advantage of both a net energy gain by producing methane as well as the production of a fertilizer from the residuals [3,4]. Hydrolysis is known to be the rate-limiting step in anaerobic degradation of complex organic matter [5] but for substrates such as exclusively sorted OFMSW or cellulosepoor kitchen wastes, methanogenesis becomes the limiting factor [6,7]. Kitchen wastes, being rapidly acidified, tend to inhibit the methanogenesis in one-stage reactors when the feedstock is not adequately mixed, buffered and dosed. Still, one-stage systems have proven to be practical for large-scale operations because of lower operational and capital requirements.

Modifications in reactor design and operating conditions are based not only on the type of waste but also in consideration of the varying amount of solids and water present. Palmowsky and Müller (1999) observed that reducing the particle size of substrates with high fiber content and low degradability lead to a more rapid digestion and improved gas production [8]. This was supported by a local study (Olairez, 2006), which found that methane generation from the slurried waste was highest at lower total solids (TS) and longer comminution time [9]. ‘Wet’ systems, or those operating at low TS content of 4 to 15%, also have the advantage of diluting potential inhibitors in the feed.

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