Anaerobic co-digestion of waste activated sludge with fats, oil and grease

摘要

Disposal of fat, oil and grease (FOG) waste products is a major problem, particularly for the food processing industry. Due to the inherent large organic content these wastes, however, they show potential as substrate for anaerobic digestion. This biological process is able to reduce the amount of fats over 80% while producing an energy rich biogas (55%-70% CH4) which can be valorized energetically. Direct influent feed of these wastes into a digester is mostly not possible due to the fact that high fat contents in the digester medium tend to adsorb to the biomass, resulting in a decrease of mass transfer. Additionally, long chain fatty acids (LCFA) and the derived volatile fatty acids (VFA) that result from the decomposition of the fats inhibit the acetoclastic and methanogenic bacteria. To overcome these obstacles, FOG wastes are often co-digested with other wastes that exhibit complementary characteristic of which waste activated sludge from waste water treatment plants is the most common. Interesting for this co-substrate is the abundance of buffering components such as bicarbonate and micro-nutrients (e.g. S, P, N...) necessary for cellular activity and growth. In this paper, the potential of co-digestion of FOG waste with thickened waste activated sludge (TWAS) is examined. In a preliminary screening, the influence of the ratio of FOG waste to TWAS is investigated concerning methane yield, inhibition effects, occurrence of lag-phases, drops in pH and possible synergetic digestion effects between FOG waste and TWAS. In a second test, the dynamics of the digestion process are investigated concerning the intermediate products, i.e. LCFA’s, VFA’s, alkalinity, sugars and proteins… The tests are executed for various FOG types: e.g. vegetable oils, animal originating fats, used frying oil and livestock manure. The multivariate data is subsequently analyzed with a partial least squares algorithm to link certain components to methane yield.