Elsevier

Chemical Engineering Journal

Volume 240, 15 March 2014, Pages 420-425
Chemical Engineering Journal

Disintegration of excess activated sludge with potassium permanganate: Feasibility, mechanisms and parameter optimization

https://doi.org/10.1016/j.cej.2013.12.011Get rights and content

Highlights

  • Excess sludge was effectively disintegrated with KMnO4 oxidation.

  • Supernatant proteins and polysaccharides increased by 490% and 141% respectively.

  • Loosely bound EPS increased by 498% due to efficient release.

  • KMnO4/TS mass ratio was the most important factor for sludge disintegration.

Abstract

Sludge disintegration destroys the sludge floc structure and releases the extracellular polymeric substances (EPS) and cell contents into the liquid phase, to enhance the sludge anaerobic digestion. Potential benefits of potassium permanganate (KMnO4) disintegrating excess sludge were investigated in this paper. Sludge disintegration feasibility was analyzed with soluble chemical oxygen demand (SCOD) increase in supernatant. Sludge disintegration mechanisms were explored with analyzing the change of particle size, EPS, oxidation reduction potential (ORP) and Mn state. Sludge disintegration process was optimized through analyzing disintegration degree (DDCOD). Results showed that KMnO4 effectively disintegrated the excess sludge with a SCOD increase of 3473% and more than 97% of Mn transferred into the solid phase. A slow decrease in particle size was observed. EPS was efficiently released into supernatant, and supernatant proteins and polysaccharides increased by 490% and 141%, respectively. Loosely bound EPS increased by 498%, which was much higher than slime EPS and tightly bound EPS. The main mechanism of sludge disintegration was KMnO4 oxidation. After sludge oxidation, KMnO4 mainly transferred to the sludge solids with the states of MnO2 and Mn3O4, and a state of KxMnO4 also existed in the solids. Optimization experiments showed that suitable reaction time was 30 min, and KMnO4/sludge solid mass ratio was the most important factor and its optimal value was 0.1 with a stable DDCOD of about 34%.

Introduction

Treatment and disposal of excess activated sludge is a difficult problem in urban wastewater biological treatment plants. Cost for treatment and disposal of excess sludge accounted for 25–40% of the total cost of the sewage treatment plant, sometimes even up to 60% [1]. In China, an increase in the amount of wastewater has resulted in a significant increase in excess sludge production. According to the State Environmental Protection Administration of China, the amount of wastewater discharged is estimated to reach 66 billion tons per year, producing about 3.2 billion tons of dry solids per year [2]. To minimize the excess sludge production is one of the most challenging problems in the field of wastewater treatment. Nowadays, anaerobic digestion is mostly used to degrade organic ingredient, recover energy substances, and kill the pathogen. However, anaerobic digestion shows several problems such as too slow reaction, huge sludge tank, and complicated operation and management. The main reason is that the hydrolytic stage of anaerobic digestion usually lasts at least 10 d and becomes the rate-limiting stage [3].

In biological wastewater treatment systems, microorganisms are in the form of microbial aggregates and flocs. Extracellular polymeric substances (EPS) present both outside of cells and in the interior of microbial aggregates [4]. Researchers used the abbreviation “EPS” as a more general and comprehensive term to represent different classes of macromolecules such as polysaccharides, proteins, nucleic acids, lipids and other polymeric compounds presented in the interior of various microbial aggregates [4]. Nielsen and Jahn [5] proposed that all polymers outside the cell wall, which are not directly anchored to the outer membrane or murein–protein layer, should be EPS. The macromolecules in EPS can cause the retention of much water in sludge flocs and the increase interstitial water in such flocs [6]. EPS can also form a stable gel that prevents water seepage from the pores of flocs, which increases the difficulty of sludge disposal [7].

The forms of EPS that exist outside of cells can be subdivided into bound EPS (sheaths, capsular polymers, condensed gels, loosely bound polymers, and attached organic materials) and soluble EPS (soluble macromolecules, colloids, and slimes) [5], [8]. Bound EPS are closely bound with cells, while soluble EPS are weakly bound with cells or dissolved into the solution. Generally, these two types of EPS can be separated by centrifugation, and those remaining in the supernatant are soluble EPS and those forming microbial pellets are bound EPS. Although the interaction between soluble EPS and cells is very weak, previous study showed that soluble EPS also have a crucial effect on the microbial activity and surface characteristics of the sludge [9]. The structure of bound EPS is generally depicted by a two layer model [5]. The inner layer consists of tightly bound EPS (TB-EPS), which has a certain shape and is tightly and stably bound with the cell surface. The outer layer, which consists of loosely bound EPS (LB-EPS), is a loose and dispersible slime layer without an obvious edge. The content of the LB-EPS in microbial aggregates is always less than that of the TB-EPS, and thus the TB-EPS may have more significant influence on the characteristics of microbial aggregates [10], [11]. A portion of slime EPS, LB-EPS and TB-EPS can be extracted by centrifuged and ultrasound, so slime EPS, LB-EPS and TB-EPS are totally named extractable EPS.

In order to shorten the duration of anaerobic digestion and reduce the volume of digestion tank, scholars have undertaken extensive researches on sludge disintegration. The sludge disintegration destroys the sludge floc structure and releases the EPS and cell contents into the liquid phase. The organic autochthonous substrate might be used for microbial metabolism, which results in a reduction of the overall biomass. In this way the time of hydrolytic stage can be shortened and the dewaterability of sludge might be improved [12], [13], [14].

Various methods for sludge disintegration were studied, including physical, chemical, biological, and hybrid disintegration. Physical disintegration method includes high pressure jet method, ball mill method, ultrasonic method, heating, etc. Ultrasound is well known to disrupt the sludge flocs and microbial cell walls, inducing the release of soluble substances into the aqueous phase, further more biogas production and short anaerobic digestion time [15]. The chemical oxygen demand (COD) in supernatant, floc size, biodegradability, kinetic model and mechanisms with ultrasound treatment have been extensively studied [16]. Studies showed as much as 70% improvement in solubilization of the waste activated sludge following ultrasonic pretreatment [17]. Thermal treatment destructs the basic composition of microorganisms in sludge, such as protein and fat. The proteins denature under high temperature and the fat of cell membrane can be dissolved, which lead to the leakage of cell contents [18]. Physical method shows good treatment efficiency but always consumes high energy [15], [16], [17], [18]. The biological methods show a wide application prospect but are still under investigation [19]. Chemical method includes ozone oxidation, chlorination oxidation, wet oxidation and alkali treatment [20]. Ozone oxidation is one of the commonly used advanced oxidation techniques. After sludge ozonation, cell walls are fragmented and intracellular compounds are released, and refractory organic structures are oxidized and converted into biodegradable low-molecular compounds [20]. Chlorination oxidation is another commonly used oxidation techniques. The chlorinated sludge could be recycled to wastewater treatment system to reduce the sludge production [21]. All these chemical methods make the organic compounds in sludge be oxidized and dissolved [22], [23].

Potassium permanganate (KMnO4) is a strong oxidant and has been used for water disinfection, toxic matter oxidation, inhibiting the growth of algae [24], [25]. Using KMnO4 shows some advantages than ozonation and chlorination, such as safe, non-toxic and conveniently dosed. However, very few previous works described the disintegration of waste activated sludge with KMnO4. Therefore, in this paper KMnO4 was used to oxide the extracellular polymeric substances and cleave the linkages in polymeric backbone to achieve sludge disintegration and the feasibility, operation optimization, and potential mechanisms of sludge oxidation with KMnO4 were studied.

Section snippets

Excess activated sludge

The excess activated sludge used in this investigation was collected directly from the end of a thickening tank of Qinghe sewage treatment plant in Beijing, China. About 75,000 m3/d of wastewater (93% domestic and 7% industrial sewage) is treated using anaerobic/anoxic/oxic process. The collected samples were transferred to the laboratory within 60 min after sampling and then stored in a refrigerator at 4 °C. All experiments were completed within 48 h and the main characteristics of sludge sample

Feasibility of sludge disintegration with KMnO4

The SCOD and Mn concentration in the supernatant were employed to explore the feasibility of sludge disintegration with KMnO4. The changes of SCOD and Mn2+ concentration are showed in Fig. 1. The SCOD rose rapidly in the beginning of 30 min followed by a slow increase in the prolonging time. After the oxidation of 120 min, the SCOD reached 2084 mg/L and was 3473% more than that before the oxidation, and the solubilization quantity in the first 30 min was 89.6% of total solubilized organic matters

Conclusions

KMnO4 treatment was effective for sludge disintegration. The SCOD in sludge supernatant increased by 3473% after the KMnO4 oxidation. The mechanism of sludge disintegration was that the proteins and polysaccharides were-released into the liquid phase with KMnO4 oxidation, especially the proteins. More loosely bound EPS was stripped by KMnO4 oxidation than the slime EPS and tightly bound EPS. The sludge disintegration was attributed to the high oxidation potential of KMnO4 and reductive sludge

Acknowledgements

This research was funded by the National Natural Science Foundation of China (51178047), and Ministry of Science and Technology of China (China-Israel Joint Research Program).

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