Elsevier

Chemical Engineering Journal

Volume 268, 15 May 2015, Pages 162-169
Chemical Engineering Journal

Operation performance of an A/A/O process coupled with excess sludge ozonation and phosphorus recovery: A pilot-scale study

https://doi.org/10.1016/j.cej.2015.01.054Get rights and content

Highlights

  • An A/A/O process coupled with sludge ozonation and P recovery was developed.

  • The sludge solubilization efficiency was 35.5% at about 100 mg O3 g1 MLSS.

  • The maximum sludge reduction efficiency reached 85% in the combined treatment system.

  • Ozonated sludge recycle impacted slightly the removal of pollutants (except P).

  • The influent P was recovered by 29% from the supernatant of ozonated sludge.

Abstract

This study investigated the long-term operation performance of an anaerobic/anoxic/oxic (A/A/O) process coupled with sludge ozonation and phosphorus (P) recovery at pilot scale with a treatment capacity of 1.0 m3 d1. Batch experiments of sludge ozonation showed that the release of organic substances (expressed as chemical oxygen demand (COD)), nitrogen (N), and P from disintegrated microbial cells was enhanced with an increasing ozone dose. The optimal ozone dose was about 100 mg g1 MLSS (mixed liquor suspended solids), at which a sludge solubilization efficiency of 35.5% was achieved. The pilot-scale treatment system was continuously operated for 141 days and a maximum sludge reduction efficiency of 85% was reached. Ozonated sludge recycle (OSR) affected slightly the COD and N removal, mixed liquor volatile suspended solids to MLSS ratio, and sludge activity in the bioreactor, and significantly improved the sludge settleability. However, P removal was obviously inhibited because of the reduced discharge of excess sludge. The mass balances of COD and N indicated that OSR improved both organic mineralization and denitrification. The influent P could be recovered by 29% from the supernatant of ozonated sludge, thus notably enhancing its removal in the pilot-scale treatment system. This study demonstrates the operational feasibility of the A/A/O process coupled with excess sludge reduction by ozone and P recovery.

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 g1 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:VbrdXbrdt=Yobs,br(QinCin-QefCef)+kQorXor-QdXd-QorXor-bXbrVbr-QefXef

Under steady-state conditions, Eq. (2) can be derived (see Text S1 for detailed information):Yobs,brLs=b+D+ηE

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 g1 MLSS; afterwards, a slow increase was observed as the ozone dose further increased to 207 mg g1 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:

  • 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 g1 MLSS, at which a sludge solubilization efficiency of 35.5% was achieved.

  • 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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