Microbial activity in a combined UASB–activated sludge reactor system

A combined upflow anaerobic sludge bed–activated sludge (UASB–AS) reactor system with consistently wasting of excess biomass was used to treat suspended-solids pre-settled piggery wastewater (COD = 2000 mg l−1, total Kjeldahl nitrogen TKN = 400 mg l−1, suspended solids = 250–400 mg l−1). Thus, the activity of nitrogen-related microbial groups in each individual bioreactor was investigated. When the granules retention time (GRT) of 20–50 d in the UASB reactor, the solids retention time (SRT) of 10–25 d in the AS reactor and the recycle-to-influent ratio (Re) of 1 were maintained, the combined system removed 95–97% of chemical oxygen demand (COD), 100% of TKN and 54–55% of total nitrogen (TN). Denitrification and methanogenesis occurred in the UASB reactor so that both biochemical processes contributed to most of the COD removal and, complete nitrification (most of the TKN removal) occurred in the AS reactor. Compact granules with good settling abilities developed in the UASB reactor, and rapid rates of granulation of break-up granules in the UASB reactor were confirmed by experiments. The activity of nitrifiers and denitrifiers (an = 0.68–0.87; adn = 0.55–0.70) and the calculated specific nitrification and denitrification rates (qn = 0.26–0.47 mg image–N mg VSS−1 d−1; qdn = 0.046–0.076 mg image–N mg VSS−1 d−1) significantly increased with decreasing SRT and GRT, respectively. Accordingly, the combined UASB–AS reactor system should be regarded a promising alternative for the removal of organic carbon and nitrogen from piggery wastewater.