ABSTRACT
Dry anaerobic digestion of solid wastes (DASS for the Spanish abbreviation of solid substrate anaerobic digestion) is receiving much attention worldwide as an alternative for industrial and municipal waste stabilization and reclaiming. Although some efforts were made in DASS process modeling, they were concentrated on the thermophilic DASS and only addressed empirical or semiempirical models. This chapter, then, aimed at modeling the mesophilic steady-state DASS process with both empirical and kinetic models.
Bench-scale, semicontinuous, mesophilic reactors were operated at six mass retention times (MRT, 15, 18, 21, 25, 30, and 40 days) by duplicate. Process performance was evaluated in terms of volatile solids efficiency removal, biogas productivity, methane content in biogas, volatile organic acids contents in mixed solids, etc. Methanogenic biomass was quantitated by coenzyme F 420 which is a factor practically specific of methanogenic bacteria. The feedstock was a mixture of lignocellulosic wastes, food wastes, and biosolids intended to simulate the codigestion of industrial and municipal wastes. Empirical models based on Levenspiel kinetics were applied to the efficiency and the unit removal rate of volatile solids. Another kinetic model based on the conventional anaerobic digestion concept (as a series process hydrolysis/acidogenesis, volatile organic acids uptake rate, and methanogenesis) was adapted and fitted to data.
The efficiency increased in the range 60% to 83% (biodegradable volatile solids base) with increasing MRT (18 to 40 days MRT). However, the efficiency was very poor (27%) at 15 days MRT. Unit removal rate and biogas productivity increased steadily from 40 to 18 days MRT, with a sudden drop of both responses at 15 days MRT. The overall performance indicated a partial process inhibition at 15 days MRT.
320Empirical models of efficiency vs. MRT and unit removal rate vs. loading rate fitted adequately the data and predicted process cessation for MRT between 13.3 to 14.5 days. The kinetic model based on the conventional anaerobic digestion concept, modified for inhibition of the hydrolysis rate and volatile acids net uptake rate, also fitted the experimental results. A model with fractional order in volatile solids (approx. 0.4) and a Levenspiel-like inhibition factor gave the best fitting of the hydrolysis rate. For the volatile organic acids uptake rate, a Haldane model was the best representation. The kinetic coefficient values were lower than those reported for similar kinetic models of low total solids anaerobic digestion (suggesting the influence of mass transfer retardation effects), while the stoichiometric coefficients (i.e., the yield synthesis) gave comparable values.
