Potassium permanganate (KMnO4)/sodium sulfite (Na2SO3) rapidly disintegrates waste activated sludge by reactive Mn(III) species and shapes microbial community structure
Graphical abstract
Introduction
In municipal wastewater treatment plants (WWTPs), waste activated sludge (WAS) is a primary by-product. Many developing counties do not apply anaerobic digesters for WAS treatment and energy recovery [1], [2]. WAS is difficult to obtain additional energy because of its high-water content (≥95%) and cell walls prevent release of organic matter [3]. A better pretreatment approach is needed for sludge disintegration, which might involve sludge dewatering and biological conversions related to subsequent anaerobic digestion. A number of methods have been proposed to promote sludge disintegration, including thermal-alkaline treatment [4], ultrasonic treatment [5], utilization of alkali condition [6], adding of potassium ferrate [7] or potassium permanganate [8], and enzymatic treatment [9], as well as co-treatment by several of the methods combined [10], [11]. Although these methods may have a high disintegration rate, their application has been limited because of high energy requirements and general complexities of operation.
Among these pretreatment methods, potassium permanganate (KMnO4) has been commonly used to eliminate organic pollutants, and it is also sometimes applied in sludge pretreatment [12], [13]. Destruction of sludge flocs by KMnO4 oxidation promoted loosely bound (LB-) and tightly bound extracellular polymeric substances (TB-EPS) to partially convert to soluble extracellular polymeric substances (S-EPS) for enhanced WAS dewaterability [14]. Changing physicochemical and rheological characteristics of KMnO4 for improving sludge dewaterability has been explored [15]. KMnO4 and back-mixing additives combined with freeze thaw pretreatment can improve thermal drying kinetics of WAS [16]. The integrated ultrasound-KMnO4 pretreatment increases biodegradability of textile-dyeing sludge due to strong oxidative power of Mn(VII) [17]. The soluble chemical oxygen demand (SCOD) in sludge supernatant has been shown to increase 34-fold after KMnO4 oxidation, with sludge flocs showing a small decrease in particle size [8]. However, a large amount of soluble Mn residues exist in KMnO4-pretreated sludge, and KMnO4 oxidation is favoured by acidic conditions, which limits its application.
Recent studies reported a novel method to degrade organic contaminants, such as phenol, methyl blue, and ciprofloxacin using KMnO4 combined with NaHSO3 (PM/BS) [18], [19], [20]. The reactive Mn(III) species from PM/BS system could oxidize the organic compounds rapidly. A previous study reported the effects of operating parameters, including dosage and electric voltage, on sludge dewaterability by PM/BS and horizontal electro-dewatering (HED) processes [21]. Mn(III) disintegrated EPS fractions and cell walls of WAS, and improved the WAS dewatering. While considerable research has been devoted to understanding these operating indexes, limited investigations have focused on energy recovery, overall sludge properties, and associated microbial community structure.
Anaerobic digestion has much potential for WAS treatment, as it can be used to treat high volumes of sludge, and it also can mediate renewable energy production [22], [23]. To further understand performance of anaerobic digestion after sludge pretreatment, the diversity and function of microbial communities should be investigated [24], as there is increasing evidence that microbial community structure has an important influence on biogas production [25].
The aims of this research were to: (i) explore efficiency of sludge hydrolysis; (ii) examine whether secondary pollution of manganese can be eliminated; (iii) analyze effects associated with shifts in microbial community structure; and (iv) expound on the mechanism of the KMnO4 + Na2SO3 method on sludge disintegration. Investigations of KMnO4 combined with Na2SO3 co-treatment will provide a mechanistic basis for better understanding sludge disintegration, and enable development of a feasible and economical sludge pretreatment using anaerobic digestion.
Section snippets
Waste activated sludge and reagent
The WAS was collected in a secondary clarifier of the Wenchang Municipal Sewage Treatment Plant in Harbin, China. Sludge samples were transported to the lab within 1 h. The supernatant was discarded after gravitational settling, and remaining sample stored at 4 °C for 24 h, which was then used as the raw WAS. The initial characteristic parameters of waste activated sludge are shown in Table. 1. Potassium permanganate (KMnO4, content ≥ 99.5%) and sodium sulfite (Na2SO3, content ≥ 96%) were
WAS disintegration after KMnO4 + Na2SO3 co-treatment
Different performances of WAS disintegration by the two pretreatments were exhibited (Fig. 2). The highest concentration of SCOD, 3125 mg/L, was obtained by KMnO4 pretreatment for 10 s, and then the trend decreased slightly with longer time intervals (Fig. 2A). Both methods released 9.5 ~ 13.2-fold higher SCOD than raw WAS during the 90 min operation. The concentration of SCOD of KMnO4 + Na2SO3 co-treated sludge slightly decreased after 20 min. The concentration of VFAs from KMnO4 + Na2SO3
Overall performances of KMnO4 + Na2SO3 co-treated WAS
For rapidly disintegrating raw WAS, an efficient pretreatment is essential for destroying cell walls [31]. However, most pretreatments do not maximize speed, while allowing for proper and efficient waste disposal [32], [33]. This study demonstrated that a KMnO4 + Na2SO3 co-treatment exhibited excellent WAS degradability over very short time intervals (about 10–15 s) (Fig. 2). The KMnO4 + Na2SO3 co-treatment substantially increased the release of dissolved organic matter (DOM) and enhanced cell
Conclusions
In this study, WAS degradation was significantly improved by KMnO4 + Na2SO3 co-treatment with in-situ generated reactive Mn(III) species. This method was effective to decompose WAS at intervals as short as 10–15 s. The concentration of SCOD and VFAs from KMnO4 + Na2SO3 co-treated sludge notably increased to 3125 mg/L and 2275.85 mg/L by 10 s, respectively. It was found that the KMnO4 + Na2SO3 co-treatment could enhance sludge floc disintegration, release proteins and polysaccharides to soluble
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by National Natural Science Foundation of China (No. 31870114), the National Key Research and Development Program of China (No. 2018YFD1100503), and the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology, No. 2019DX02).
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