Long-term stability of aerobic granular sludge for the treatment of very low-strength real domestic wastewater

Aim of the present study was to assess the feasibility of cultivating aerobic granular sludge for the treatment of real very low-strength (COD <120 mg L−1) municipal wastewater by the application of a strict metabolic selective pressure. The feasibility was evaluated in terms of long-term physical stability of the granular biomass as well as COD and phosphate removal efficiencies. A laboratory-scale sequencing batch reactor was inoculated with conventional activated sludge and operated for 175 d. Complete granulation of conventional activated sludge (SVI5/SVI30 = 1) was achieved within 45 d at an average influent COD concentration of 290 ± 44 mg L−1 obtained by the addition of an external source of acetate. At day 51, acetate dosage was stopped and the reactor was then operated for other four months with only real wastewater as carbon source, resulting in an influent COD concentration as low as 115 ± 23 mg L−1. Besides the decrease of the influent COD concentration and of the organic load, the long term stability of the granular sludge was not affected and granular size continuously increased until it reached a maturation phase with an average diameter of 1.5 mm, with more than 30% of the aggregates being larger than 2 mm and marginal presence of floccular biomass (about 5%). The applied metabolic selective pressure based on the complete biodegradable COD uptake under anaerobic conditions (85 ± 7% biodegradable COD removal efficiency obtained) was shown to be able to stimulate the formation of stable and large granules, indicating the other commonly applied selective pressure based on settling velocity (commonly > 8 m h−1 versus 1.9 m h−1 applied in this study), as not strictly required. The identification of the key factors to preserve biomass long-term stability and functionality, poses the basis to allow the application of aerobic granular sludge technology in the context of combined sewers systems and facilitate the retrofitting of existing wastewater treatment plants excluding the need for a plug-flow distribution system for the influent wastewater.