Thickness of the buffer layer in the 'reaction front' model of solid-state digestion

Conventional models of anaerobic digestion assume dispersed reaction, in the aqueous phase and at solid-liquid interfaces. However, a second, localized mechanism might operate in a bed of nutrient-rich wastes, such as occurs in many landfills and some waste digesters. A 'reaction front' (comprising two distinct reaction zones separated by a buffer zone) might develop around 'seed bodies' (local concentrations of methanogenic inoculum) then propagate through the waste. The initial conditions, notably the mode of seeding, would determine whether which mechanism is dominant. This paper presents a method of predicting the thickness of such a front and indicates a thickness of the order of 3.5 cm. This would suggest that the seed material must be present in the form of 'seed bodies' of over 7 cm diameter or layers over 7 cm thick. The probability of the formation of a reaction front is discussed, as are the implications for operational practice.