Operation performance of an A/A/O process coupled with excess sludge ozonation and phosphorus recovery: A pilot-scale study
Graphical abstract
Introduction
The large amount of excess sludge produced in activated sludge processes has long been a serious problem for wastewater treatment plants (WWTPs) worldwide. In China, the annual production of excess sludge is expected to reach 30 million tons in 2015, given an increasing amount of wastewater to be treated and more stringent standards to be enforced for effluent discharge [1]. The disposal of excess sludge not only increases significantly the total operating cost of WWTPs [2], [3], but also poses a great threat to the environment.
Physical, chemical, and biological methods have been developed for sludge reduction based on lysis–cryptic growth, uncoupling metabolism, maintenance metabolism, and predation of microbes [4], [5], [6]. Compared with other sludge solubilization techniques such as thermal, ultrasonic and alkaline treatments, ozonation is considered as a promising technology for sludge reduction [4], [7]. The sludge solubilization efficiency, affected principally by ozone dose and sludge properties, could vary from 20% to 70% [6]. Some sludge characteristics (e.g., pH, zeta-potential, and particle size) were altered significantly during ozonation [8], [9], [10]. Excess sludge reduction could be achieved within the activated sludge system by combining activated sludge process with sludge ozonation based on lysis–cryptic growth [11], [12], [13]. Different combined systems (e.g., anoxic/oxic process coupled with sludge ozonation, sequenced batch reactor coupled with sludge ozonation) have been tested with sludge reduction efficiency of 40–100% [6]. In our previous work, a conventional activated sludge process coupled with sludge ozonation was operated continuously for 45 days at bench scale without excess sludge discharge at an ozone dose of 100 mg g−1 MLSS (mixed liquor suspended solids) [14]. Moreover, the microbial activity in the bioreactor may be affected by ozonated sludge recycle (OSR) because of the change in the sludge loads (Ns) of the main wastewater quality indicators and the possible inactivation of microbes by ozone. The effluent concentrations of chemical oxygen demand (COD) and total nitrogen (TN) could increase to some extent, whereas that of total phosphorus (TP) could increase significantly, as a result of the reduced discharge of excess sludge [14], [15], [16]. In addition, the organic substances released upon sludge solubilization could be used as an internal carbon source for denitrification [17], [18], thus reducing wastewater treatment costs.
The large amount of phosphorus (P) rich excess sludge presents a serious challenge to the anaerobic/anoxic/oxic (A/A/O) process, which has been widely employed to reduce nitrogen (N) and P simultaneously in WWTPs. On the one hand, sludge ozonation leads to a reduced discharge of excess sludge. Since P can only be removed through excess sludge discharge in biological treatment processes, it will continuously accumulate in the bioreactor until the treatment process collapses eventually. On the other hand, the gradual depletion of P resources [19] and the undesirable disturbance on aquatic environment (e.g., eutrophication) induced by excessive input of P to water bodies have called for the recovery of P in WWTPs. Therefore, P recovery is necessary for the A/A/O process if excess sludge is reduced by ozonation.
In this study, an A/A/O process coupled with sludge ozonation and P recovery was operated to investigate sludge reduction efficiency and wastewater treatment performance. Batch experiments of sludge ozonation were first carried out to determine the optimal ozone dose and the change of sludge characteristics post ozonation. A pilot-scale treatment system was then run for 141 days with different operation phases to examine the sludge reduction efficiency and the fluctuation of effluent quality. With mass balance model calculations, this study demonstrates the feasibility of excess sludge ozonation and P recovery in the A/A/O process at pilot scale.
Section snippets
Mass balance model
An overall mass balance model was developed for a bioreactor coupled with sludge reduction by ozone according to Yasui and Shibata [11] after certain modifications:
Under steady-state conditions, Eq. (2) can be derived (see Text S1 for detailed information):
Hence, the sludge reduction efficiency (σ) and the net observed sludge yield coefficient (Yobs,N) of the combined treatment process can be calculated as
Excess sludge ozonation
The change of sludge solubilization efficiency (η) and the release of COD, N and P as a function of ozone dose are shown in Fig. 2. The results indicate that the η increased quickly to 35.5% as the ozone dose increased to 101 mg g−1 MLSS; afterwards, a slow increase was observed as the ozone dose further increased to 207 mg g−1 MLSS (Fig. 2a), which was attributed to the enhanced competition of soluble components for ozone against particulate matter [22] and the inhibition on indirect oxidations by
Conclusions
This study investigated the long-term operation performance of an A/A/O process coupled with excess sludge ozonation and P recovery at pilot scale. Based on the experimental results obtained, the following conclusions can be drawn:
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The release of COD, N and P from cell lysis during sludge ozonation was enhanced as ozone dose increased. The optimal ozone dose was about 100 mg g−1 MLSS, at which a sludge solubilization efficiency of 35.5% was achieved.
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The pilot-scale combined treatment system
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (51221892, 21277160) and Siemens Ltd., China (Technical study on sludge source reduction).
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