Operational strategies for nitrogen removal in granular sequencing batch reactor
Introduction
Aerobic granule technology has been studied for over 10 years for wastewater treatment. Compared to conventional activated sludge, aerobic granule has regular and compact physical structure, diverse microbial species, good settling property, high biomass retention, and great ability to withstand shock load or shock of toxic compound. Therefore, aerobic granule is becoming a very promising technology for wastewater treatment [1], [2], [3], [4], [5], [6], [7], [8], [9].
Nutrient removal is the basic requirement for wastewater treatment with the implementation of high discharge standard of treated water. Nitrogen is one of the key nutrients causing eutrophication in water body, which is required to be removed from water resources in many countries. Generally, biological nitrogen removal consists of two steps, i.e., nitrification, with which ammonia is converted into nitrite and finally nitrate aerobically, and denitrification, with which nitrite or nitrate is converted into gaseous nitrogen anoxically. Therefore, anoxic and oxic phases are normally exerted in different compartments for nitrification and denitrification with traditional activated sludge, which thus compromises the compactness of the whole system. However, the presence of anaerobic zone in dense aerobic activated sludge due to low aeration makes it possible for the integrated nitrogen removal in single aeration basins, i.e., simultaneous occurrence of nitrification and denitrification (SND). Since an anoxic zone exists in aerobic granules due to mass transfer resistance from compact structure and big size [4], [10], [11], it is generally believed that nitrogen could be removed efficiently in single granules. As the nitrogen removal depending on aerobic and anoxic zones of granules, DO concentration in the bulk liquid is usually required to be precisely controlled. In detail, aerobic and anoxic volume created by DO penetration in the single granules may influence nitrification and denitrification rates and further total nitrogen removal rate. In addition, the practically uncontrollable factors, such as granule size, granule density, biomass spatial distribution and activity of diverse bacteria species, are associated with DO diffusion in granules, which may also influence nitrogen removal of single granules [12]. Among these factors, granule size has been reported to determine the volume of anoxic zones in the granules at certain DO concentrations in the bulk liquid, which is thus closely correlated with nutrient removal [12], [13], [14], [15]. However, there is little report on the effects of granule size on nitrogen removal under different operating modes.
So far, a few operating modes of granular sludge systems for nitrogen removal have been reported on the basis of lab-scale experiments, such as alternating oxic–anoxic mode (OA) [16], [17], continuous or on/off aeration with controlled DO for simultaneous nitrification and denitrification (SND) [13], [18], anaerobicly-controlled oxic mode (AO) [19] and alternating anaerobic–oxic or anaerobic–oxic–anoxic mode (AO or AOA) using denitrifying polyphosphate accumulating organisms (DNPAOs) [11], [14], [15], [20]. During granular N-removal process, nitrification is easy to be achieved when aeration is sufficiently supplied, while denitrification is normally a rate-limiting step due to deficient anoxic condition and/or carbon source supply. As for these modes, denitrification occurs either in anoxic zone inside single granules or anoxic condition (phase) provided in some parts of an entire cycle. For OA process, extra carbon addition is normally needed because of insufficient carbon source after oxic period. For SND at controlled DO, long retention time is normally required due to low reaction rate at low DO. In addition, the variable granule size or density will affect aerobic and anoxic volumes in granules, which could probably result in unstable nitrogen removal. For SND and AO or AOA (using DNPAOs) mode, it is difficult to achieve optimal microbial community only by regulating operation conditions. Ammonia or oxidized nitrogen may thus be present in the effluent [19].
Step-feed mode in activated sludge system has been reported to be effective in making good use of carbon source in influent, which increases denitrification rate and further total nitrogen removal efficiency. In addition, it allows nitrification to occur at a lower organic loading in the aerobic phase, which accelerates nitrification rate and saves aeration consumption to oxidize organic matters in influent. Some studies have been documented on alternating aerobic/anoxic and step-feed mode in activated sludge system [21], [22], [23], [24], [25], [26], which shows that it is both technologically and economically effective in enhancing nitrogen removal efficiency. With more compact structure and bigger size than activated sludge, whether granular sludge can achieve effective nitrogen removal under this operational strategy is to be investigated.
In this study, nitrogen removal rates and efficiencies in granular sequencing batch reactor with AO mode with or without DO control were compared on the basis of two different granule sizes. In addition, a novel operational strategy with alternating anoxic/oxic combined with step-feed was developed for nitrogen removal by aerobic granules.
Section snippets
Reactor set-up and operation
Two columns (a diameter of 5 cm and a height/diameter ratio of 20) with a working volume of 2 l were inoculated by granules with a mean size of 1.5 mm (R1) and 0.7 mm (R2). Since the inoculated granular sludge was stored in a refrigerator for months, revival of granule activity was conducted in the two reactors under a condition of non-aeration followed by aeration. For the both reactors, cycle time of 4 h was used which consisted of 10 min influent filling phase, 10 min of non-aerobic phase, 213 min
Results
After about one-month acclimation, granules seeded to the reactors became stable in terms of physical characteristics and COD and ammonia removal. MLVSS maintained at 5–8 g/l with sludge SVI of 41–48 ml/g in R1 and 5–6.4 g/l with sludge SVI of 20–30 ml/g in R2. At the steady state, different operating modes were applied in the both reactors. The COD and nitrogen removal rates were monitored in the batch cycles.
Influence of size and density of granules on reaction rates
Aerobic granules display a wide range of sizes, approximately 0.3–5.0 mm in diameter, which causes very different morphological and physical characteristics of granules [27]. Liu et al. reported that large-size aerobic granules would not favor biological removal of substrate due to potential mass transfer limitation. The diffusion resistance would thus be a limiting factor to the reaction in aerobic granules with a mean size larger than 0.7 mm [28]. Therefore, small-size granules are more
Conclusions
Apart from granule size, granule density is another important factor to influence nitrification rate and denitrification rate, which would further affect nitrogen removal efficiency. However, granule size and density may fluctuate even at the steady state of the reactor operation, which may cause shifting volumes of aerobic and anoxic zone inside granules and thus unstable nitrogen removal. Therefore, simply depending on single granules and their aerobic and anoxic zone inside for nitrogen
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