Modeling of Anaerobic Digestion of Sludge

Abstract

Anaerobic treatments have been successfully employed for more than one hundred years to stabilize sewage sludge. The main aim of sludge anaerobic digestion is the degradation and destruction of organic matter, with consequent sludge stabilization and pathogen reduction. One of the advantages of the fermentative system over aerobic treatment is the production of biogas (i.e., an available source of energy), which in some cases can be exploited to satisfy part of the energy requirements of the treatment plant. Nevertheless, inadequate knowledge of the principles governing the process often prevents satisfactory digester management, and thus there is an evident need for reliable kinetic models in order to improve performance and optimize the control strategies. In the present review, an overview of the anaerobic digestion models proposed in the specialized literature is presented. Models are grouped according to the degree of substrate characterization and thus of complexity (in terms of number of equations and parameters). This criterion is practically equivalent to considering that a chronological order exists in their evolution. To make the model comparison easier to use, a representation in matrix form is also given. The potential and limits of available models are also discussed with a view to defining both fundamental research needs and critical aspects for practical application.

KEY WORDS:

• anaerobic process models
• sludge digestion
• substrate characterization
• anaerobic metabolic pathways
• hydrolysis kinetics
• acidogenesis kinetics
• methanogenesis kinetics

Notes

∗ Substrate: primary sludge, T = 35°C, pH = 5.15, CSTR

∗ Estimated values from data published in Angelidaki et al. (1993), Angelidaki and Ahring (1994), Hashimoto (1983), and Hashimoto et al. (1981)

^†VFA = acetate

^‡Inhibition constant evaluated from experimental data of anaerobic digestion inoculated with manure Evaluated at the optimal temperature for growth

∗ pHUL and pHLL are respectively the upper and lower pH limit causing 50% inhibition. As an example, the acetoclastic methanogens have a pHUL and a pHLL equal to 7.5 and 6.5, respectively; their optimal pH is 7.

^†pHUL is the pH value of no–inhibition, while pHLL is instead the pH value giving complete inhibition. As an example, the acetoclastic methanogens have a pHUL and a pHLL equal to 7 and 6, respectively, so they will be completely inhibited for pH inferior to 6, but they will not be above pH 7.