Production of polyhydroxyalkanoates (PHA) by activated sludge treating municipal wastewater: effect of pH, sludge retention time (SRT), and acetate concentration in influent
Introduction
The development of biodegradable plastics is one of the major concerns in the present society because the conventional plastics have many faults. They are produced from non-renewable resources such as petrochemicals, and are not compatible with natural carbon cycles because of their non-degradable characteristics. They are also causing serious problems of damaging beautiful natural scenery and wild lives due to their persistence in natural environment. In abating these problems, the development of biodegradable plastics has become one of the potential counter-measures. Polyhydroxyalkanoates (PHA) is one of the biodegradable plastics produced mainly by bacteria. In the last three decades, PHA have attracted industrial interest as biodegradable plastics not only because of their compatible material properties like synthetic thermoplastics but also could PHA be synthesized from renewable carbon resources, based on agriculture or even on industrial wastes [1]. Due to these unique characteristics of PHA, various kinds of bacterial strains have been tested for their PHA production capability. To date, there are more than 300 different microorganisms, which can synthesize PHA [2]. Several of these, such as Ralstonia eutropha, Alcaligenes latus, Azotobacter vinelandii, and several strains of methylotrophs and recombinant Escherichia coli are being intensively studied because of higher productivity [2]. For example, P(3HB-co-3HV), the copolymer of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) has been commercially produced by pure culture fermentation process using Ralstonia eutropha and the PHA content achieved is more than 80% of cell dry weight [3]. The PHA content achieved by Alcaligenes latus and recombinant Escherichia coli have been reported to reach 88% [4] and 76% [5] of cell dry weight, respectively. Although high PHA content could be achieved by using pure culture fermentation process, the cost of PHA production is still too high for PHA to become a competitive commodity plastic material. As to reduce the expensiveness of PHA, a novel PHA production strategy, which is to utilize the mixed bacterial culture in activated sludge for PHA production has been proposed in the last decade. Considerable efforts have been devoted to this direction, and the studies conducted are reviewed by Satoh et al. [6], [7].
The idea of PHA production by using activated sludge was ignited owing to PHA's function as an intermediate metabolic product in activated sludge process. It has been recognized that PHA is one of the most important carbon storage materials especially in the anaerobic–aerobic activated sludge process or the Enhanced Biological Phosphorous Removal (EBPR) process [8]. In EBPR process, microorganisms in activated sludge consume polyphosphate as an energy source for anaerobic uptake of carbon substrates. The carbon substrates taken up are temporarily stored as PHA. When the condition turns aerobic, PHA is utilized for growth and polyphosphate regeneration. The microorganisms in EBPR process should therefore possess the characteristic of phosphate removal and PHA accumulation. For that reason, anaerobic–aerobic activated sludge process was employed in this study to acclimatize activated sludge for PHA production.
When compared with pure culture fermentation processes, the merits of PHA production system by activated sludge will be the cost reduction in cultivating PHA producing bacterial cultures, simpler facility construction, and material recovery from wastes [9].
In this study, a two-stage system shown in Fig. 1 was proposed. The first stage is the activated sludge process for wastewater treatment, and the second stage is PHA production process by using the excess sludge from the wastewater treatment process. In the first stage, it is essential to optimize the operational conditions for sludge acclimatization or for the enrichment of PHA accumulating microorganisms so that the PHA production capability of activated sludge could be improved. In the second stage, carbon substrate such as acetate was fed to the acclimatized sludge for PHA production.
In most of the studies of PHA production by activated sludge, synthetic wastewaters were used to cultivate PHA producing sludge, such as in Ueno et al. [10], Iwamoto et al. [11], Saito et al. [12], Hu et al. [13], Chua et al. [14], [15], [16], Tsunemasa [17], Satoh et al. [9], Lemos et al. [18], Fang et al. [19], and Ma et al. [20]. Furthermore, very little work had been done on how operational conditions of activated sludge process could enhance the PHA production capability of sludge. Satoh et al. [9] reported a high accumulation of 62% of cell dry weight by using activated sludge acclimatized in anaerobic–aerobic process with very limited oxygen supply to the anaerobic condition. The effect of carbon–nitrogen (C:N) ratio in reactor liquor on PHA productivity was examined in Chua et al. [14], [15], [16], Fang et al. [19], and Ma et al. [20].
In the present work, attention was devoted to the two lacking aspects mentioned above. Real municipal wastewater was used to cultivate activated sludge for PHA production, and our focus was on how the operational conditions in activated sludge process could influence the PHA production capability of sludge. The operational conditions being investigated were the acetate concentration in influent, pH, and sludge retention time. As known well, acetate is the most easily assimilated carbon substrate in producing PHA. It is essential to know how significant its concentration can enhance the PHA production capability. This will then enable us to select the suitable wastewater for sludge acclimatization. As mentioned before, optimization of operational conditions in activated sludge process is essential for PHA production capability enhancement as well as for satisfactory effluent quality. pH condition and sludge retention time were investigated in this study because they are important and easily manipulated parameters in activated sludge process. Besides, we also studied the effect of pH on PHA production process. At last, feasibility of using activated sludge treating municipal wastewater for PHA production is discussed.
Section snippets
Operation of anaerobic–aerobic activated sludge processes under different operational conditions
Two bench-scale sequential batch activated sludge reactors (SBRs) served as the wastewater treatment process, the first stage of the proposed PHA production system (Fig. 1). They were operated with municipal wastewater as influent. The municipal wastewater was supplied from a municipal wastewater treatment plant located in the central of Tokyo, which receives wastewater from the commercial, residential and industrial areas. While the influent wastewater quality fluctuated, the typical influent
Performance of the wastewater treatment process
In this study, despite the differences of operational conditions, all sludge acclimatized under anaerobic–aerobic sequence showed metabolic characteristics typically observed in EBPR sludge [8]. Typical characteristics of activated sludge observed are shown in Fig. 4. All acetate present in the influent was taken up during anaerobic phase of the activated sludge process. Anaerobic phosphate release and aerobic phosphate uptake, coupled with anaerobic PHA accumulation and aerobic PHA
Conclusion
The authors have proposed a PHA production system in which excess sludge of the wastewater treatment process was utilized as PHA production bacterial cultures. Main focus of this research was to investigate the optimum operating conditions of activated sludge process for enhancing the PHA production capability of sludge. Although the PHA content achieved (30%) in present study is much lower than that by pure culture, the proposed method may still serve well as an environment-friendly means to
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