Aerobic granular sludge technology offers a possibility to design compact wastewater treatment plants based on simultaneous chemical oxygen demand (COD), nitrogen and phosphate removal in one sequencing batch reactor. In earlier studies, it was shown that aerobic granules, cultivated with an aerobic pulse-feeding pattern, were not stable at low dissolved oxygen concentrations. Selection for slow-growing organisms such as phosphate-accumulating organisms (PAO) was shown to be a measure for improved granule stability, particularly at low oxygen concentrations. Moreover, this allows long feeding periods needed for economically feasible full-scale applications. Simultaneous nutrient removal was possible, because of heterotrophic growth inside the granules (denitrifying PAO). At low oxygen saturation (20%) high removal efficiencies were obtained; 100% COD removal, 94% phosphate (P-) removal and 94% total nitrogen (N-) removal (with 100% ammonium removal). Experimental results strongly suggest that P-removal occurs partly by (biologically induced) precipitation. Monitoring the laboratory scale reactors for a long period showed that N-removal efficiency highly depends on the diameter of the granules.

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Recently, several groups have showed the occurrence of aerobic granular sludge. The excellent settling characteristics of aerobic granular sludge allow the design of very compact wastewater treatment plants. In laboratory experiments, high oxygen concentrations were needed to obtain stable granulation. However, in order to obtain energy efficient aeration and good denitrification low oxygen concentrations would be required. From earlier research on biofilm morphology, it was learned that slow growing organisms influence the density and stability of biofilms positively. To decrease the growth rate of the organisms in the aerobic granules, easily degradable substrate (e.g. acetate) has to be converted to slowly degradable COD like microbial storage polymers (e.g. PHA). Phosphate or glycogen accumulating bacteria perform this conversion step most efficiently. In this paper it is shown that the selection of such bacteria in aerobic granules indeed led to stable granular sludge, even at low oxygen concentrations.

dCurrently, nitritation-anammox (anaerobic ammonium oxidation) bioreactors are designed to treat wastewaters with high ammonium concentrations at mesophilic temperatures (25 to 40°C). The implementation of this technology at ambient temperatures for nitrogen removal from municipal wastewater following carbon removal may lead to more-sustainable technology with energy and cost savings. However, the application of nitritation-anammox bioreactors at low temperatures (characteristic of municipal wastewaters except in tropical and subtropical regions) has not yet been explored. To this end, a laboratory-scale (5-liter) nitritation-anammox sequencing batch reactor was adapted to 12°C in 10 days and operated for more than 300 days to investigate the feasibility of nitrogen removal from synthetic pretreated municipal wastewater by the combination of aerobic ammonium-oxidizing bacteria (AOB) and anammox. The activities of both anammox and AOB were high enough to remove more than 90% of the supplied nitrogen. Multiple aspects, including the presence and activity of anammox, AOB, and aerobic nitrite oxidizers (NOB) and nitrous oxide (N 2 O) emission, were monitored to evaluate the stability of the bioreactor at 12°C. There was no nitrite accumulation throughout the operational period, indicating that anammox bacteria were active at 12°C and that AOB and anammox bacteria outcompeted NOB. Moreover, our results showed that sludge from wastewater treatment plants designed for treating high-ammonium-load wastewaters can be used as seeding sludge for wastewater treatment plants aimed at treating municipal wastewater that has a low temperature and low ammonium concentrations. N itrogen removal from wastewater treatment is necessary because of the significant adverse environmental impact of ammonia/ammonium, such as eutrophication and toxicity to aquatic life, on the receiving bodies. Generally, carbonaceous waste is removed in the first stage of wastewater treatment, which is followed by nitrogen removal systems. Conventionally, the removal of nitrogen (ammonium) is accomplished by the combination of nitrification and denitrification processes. Both of these are energy consuming and are associated with high costs. Moreover, these processes have an additional environmental impact due to high biomass production and greenhouse gas (CO 2 , N 2 O, etc.) emission, which promote global warming.Anaerobic ammonium-oxidizing (anammox) bacteria convert ammonium and nitrite directly to dinitrogen gas (N 2 ) under anoxic conditions. Since they were first detected in a denitrifying pilot plant by Mulder et al. in 1995 (1), anammox bacteria have been found in various oxygen-limited natural (2-4) and manmade ecosystems. The application of the anammox process in wastewater treatment results in significant energy reduction (60%) and greenhouse gas emission (90%) compared to those of traditional biological nitrogen removal processes (5-7). In fullscale nitritation-anammox wastewater treatment plants, ammonium-oxidizing bacteria (AOB) convert approx...