Abstract
This study investigated the potential of aeration control for the achievement of N-removal over nitrite with aerobic granular sludge in sequencing batch reactors. N-removal over nitrite requires less COD, which is particularly interesting if COD is the limiting parameter for nutrient removal. The nutrient removal performances for COD, N and P have been analyzed as well as the concentration of nitrite-oxidizing bacteria in the granular sludge. Aeration phase length control combined with intermittent aeration or alternate high-low DO, has proven to be an efficient way to reduce the nitrite-oxidizing bacteria population and hence achieve N-removal over nitrite. N-removal efficiencies of up to 95% were achieved for an influent wastewater with COD:N:P ratios of 20:2.5:1. The total N-removal rate was 0.18 kgN·m−3·d−1. With N-removal over nitrate the N-removal was only 74%. At 20 °C, the nitrite-oxidizing bacteria concentration decreased by over 95% in 60 days and it was possible to switch from N-removal over nitrite to N-removal over nitrate and back again. At 15 °C, the nitrite-oxidizing bacteria concentration decreased too but less, and nitrite oxidation could not be completely suppressed. However, the combination of aeration phase length control and high-low DO was also at 15 °C successful to maintain the nitrite pathway despite the fact that the maximum growth rate of nitrite-oxidizing bacteria at temperatures below 20 °C is in general higher than the one of ammonium-oxidizing bacteria.
Highlights
Biological wastewater treatment by aerobic granular sludge sequencing batch reactor (AGS-sequencing batch reactors (SBR))technology offers the possibility to remove carbon, nitrogen (N) and phosphorus (P) in one single reactor [1,2,3]
Part of the COD is consumed by denitrifying bacteria, leaving less carbon for polyphosphate-accumulating organisms (PAO) to be stored in the form of polyhydroxyalkanoates during the anaerobic phase resulting in lower P-removal overall [11]
The automated aeration phase length control was implemented based on the dissolved oxygen (DO) signal, with the aim to stop aeration once ammonium oxidation was upon completion
Summary
Biological wastewater treatment by aerobic granular sludge sequencing batch reactor (AGS-SBR)technology offers the possibility to remove carbon (hereafter referred to as chemical oxygen demand, COD), nitrogen (N) and phosphorus (P) in one single reactor [1,2,3]. It has been reported that the presence of nitrate (NO3−) and/or nitrite (NO2−) can disturb P-removal in sequencing batch mode [10,11]. Presence of nitrate and/or nitrite (nitrate and nitrite = NOx−) at the end of the sequencing batch cycle leads to anoxic conditions in the subsequent feeding phase. Under such conditions, part of the COD is consumed by denitrifying bacteria, leaving less carbon for PAO to be stored in the form of polyhydroxyalkanoates during the anaerobic phase resulting in lower P-removal overall [11]
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