Anaerobic degradation of butyrate in a UASB reactor

Wastewater with concentrated butyrate was treated in a 2•8 l UASB (Upflow Anaerobic Sludge Blanket) reactor at 37°C and pH 7•1–7•7. The process consistently removed 97–99% of COD for loading rates up to 31 g COD/l/day. Of all the COD removed, 94•5% was converted to methane; the average sludge yield was 0•037 g VSS/g COD. Conversion of acetate to methane appeared to be the rate-limiting step. Sludge granules had a maximum specific methane production rate of 1•57 g methane COD/g VSS/day, but were unable to degrade propionate. The granules were 1–2 mm in size and had a densely-packed skin layer which comprised two types of microcolony: one was composed of cocci with abundant extracellular polymer and the other was composed of two bacterial species in juxtapositioned syntrophic association. The interior was mainly composed of Methanothrix-like bacteria, a large number of which were entwined into rope-shaped aggregates.