The study of filamentous foaming control in activated sludge process

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The study of filamentous foaming control in activated sludge process


Author: Yang, Yang
Title: The study of filamentous foaming control in activated sludge process
Degree: M.Phil.
Year: 2013
Subject: Sewage -- Purification -- Activated sludge process.
Sludge bulking.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Biology and Chemical Technology
Pages: xvii, 110 leaves : ill. ; 30 cm.
Language: English
Abstract: In Hong Kong, communal municipal sewage treatment plants have been extensively surveyed and were found to have severe occasional foaming problems. Foaming problems and associated sludge bulking problems in secondary sedimentary tanks have continuously led to operating and control problems in plants, and have adversely affected process efficiency. The predominant foaming and bulking problems have been identified to be primarily caused by the excessive growth of filamentous microorganisms, which are the predominant microbial species in the activated sludge process of the sewage treatment system. Based on morphological and physiological studies, the causative microorganism in foaming sludge was determined to be Gordonia amarae, a recently discovered filamentous bacterial genus. Several bacterial physiological and morphological evidences have helped in the specific identification of this newly discovered and taxonomized genus. The bacterial growth kinetics of this novel filament, G. amarae, were studied and used as a theoretical foundation to develop an effective operational strategy in controlling filamentous foaming. In the kinetics study, the food to microorganism ratio, which is a critical process operating parameter, is an important factor for the overgrowth of G. amarae. The results from the microbial kinetic studies also indicated a strong affinity between G. amarae and fatty acids, which are non-readily biodegradable organics. Based on this specific characteristic, the Feasting-Fasting Operation (FFO) was developed for foaming control as process operational strategy during the biological wastewater treatment process, in which the microorganisms grew in changing conditions with high and low F/M ratios. The Sludge Volume Index (SVI) was reduced to 80 ml/g and was then stabilized at approximately 70 ml/g. The resulting activated sludge system gradually dissociated from the stable foam, while the BOD removal rate was kept at a particular level to meet the discharge standard.
Excessive growth of filaments could be sufficiently avoided and the activated sludge settleability could be increased without any adverse effects on the treatment performance and process stability with this control technology. The Sequencing Batch Reactor (SBR) with FFO operation mode, which is the activated sludge process simulator used in this study, was modified to treat the paper mill wastewater. At optimal operation conditions, the SVI was successfully reduced without affecting the removal efficiency. The first stage of this work involves the identification of the predominant filamentous microorganism that caused problems of foaming and bulking in the activated sludge from the paper mill wastewater treatment plant. Different diagnostic and examination methods were used to identify the particular microorganism. The predominant filamentous species was identified as Actinomycetes spp., namely, Gordonia amarae. Based on the specifically developed novel FFO technique, the filamentous overgrowth, as well as the related foaming and bulking problems, were effectively controlled. In the FFO process, the sludge microbes were subjected to repetitive switches between high and low F/M ratio conditions during the wastewater treatment process. Thus, the exposure time of the sludge microbes to a fixed F/M ratio was minimized. In the optimized implementation of FFO, in which the F/M ratio of the ‘feast tank’ was maintained at 0.89 d-1 compared with that in the ‘fast tank’ at 0.15 d-1, of the filamentous growth was quickly suppressed as shown in the improved SVI and in preventing foaming in the system. The treatment system achieved not only an effective control of foaming but also a satisfactory BOD removal efficiency. Based on previous kinetic findings and the novel FFO strategy, the second stage of this work involves the setting up of a lab-scale SBR system with the FFO to treat paper mill wastewater. The operating conditions, including Hydraulic Retention Time (HRT), aeration time in the SBR cycle, Volumetric Exchange Rate (VER), Mixed Liquor Suspended Solids (MLSS) concentrations and temperatures, were optimized to attain a stable and efficient treatment process. The experimental results show that the optimal MLSS, aeration time, VER, temperature, and operation cycle for the SBR system were 4500 mg/l, 4 h, 0.50, 30°C, and 2 cycles per day. With the implementation of FFO, the SVI was reduced from as high as 260 ml/g in conventional activated sludge process to a desirable and stable level of 52.7 ± 1.3 ml/g.

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