The dynamics of volatile fatty acids depletion in enhanced anaerobic digestion system

Field: Environmental Science (Physical Sciences)

Sludge treatment is often at a challenging issue in the wastewater treatment plant. Anaerobic digestion has been reported to be a cost-effective technology for sludge treatment. In order to achieve improved biosolids reduction and biogas generation, several enhanced sludge digestion systems have been reported, such as two-phase and two-stage anaerobic systems. Typically, the two-phase anaerobic system out-performs the conventional single-stage anaerobic system for sludge treatment by phase separation to provide growth optima for acid- and methane- formers. Despite it being effective in biosolids removal, the two-phase anaerobic system suffers from instability and limited hydrolysis efficiency. Volatile fatty acids (VFAs) are important intermediates in the anaerobic process. VFAs accumulation could inhibit the activities of acid- and methane- formers, and make the whole process less stable and difficult to maintain. However, there are fewer concerns about the effects of VFAs, especially acetic acid (HAc) and propionic acid (HPr), on the two-phase anaerobic system. To improve the hydrolysis efficiency of the two-phase anaerobic system, the two-stage anaerobic system with thermal-alkaline pretreatment had been proposed. However, instability of the second-stage reactor in the two-stage anaerobic system could occur as the first-stage reactor became more effective at hydrolysis and acidogenesis. The tolerance of methanogens in the second-stage reactor to HAc and sodium (introduced by alkaline treatment) inhibition become important to maintain stability of the two-stage anaerobic system. The objectives of this study are to understand VFAs depletion dynamics in the enhanced (two-phase and two-stage) anaerobic systems, and use the knowledge gained to optimize design, development, and operation of the two-phase and two-stage anaerobic systems for sludge treatment. Results from this study showed methanogens occurred in the acidogenic (1%) and methanogenic reactors (9.6%). The acidogenic phase methanogens showed higher HAc tolerance compared to the methanogenic phase methanogens. High amounts of undissociated acetic acid (unHAc) could inhibit both acidogenic phase and methanogenic phase methanogens. Although being capable of degrading HAc, biomass in the acidogenic reactor showed slow HPr degradation and HPr was accumulated thereby. In comparison, biomass in the methanogenic reactor could degrade HPr at initial concentrations of up to 4585 mg HPr L-1 at pH 6.40 to 7.30, but at expense of higher adenosine triphosphate (ATP) reduction rates. Microbial screening results showed low abundance of propionic acid oxidizing bacteria (POB) in the acidogenic reactor, and relatively higher abundance of POB in the methanogenic reactor and hence confirmed the difference in HPr degradation capability. In terms of long term operation, this two-phase anaerobic system could not be stable for HPr degradation given imbalance in ATP depletion and generation. The performance of the two-stage anaerobic system with thermal-alkaline treatment in the first-stage reactor, further confirmed its higher efficiency in terms of higher organic solubilisation, volatile solids (VS) removal efficiency, and methane generation when compared against the two-phase ananerobic system at same hydraulic retention time. The biomass in the second-stage reactor degraded HAc of up to 4200 mg HAc L-1 without observable lag phase. However, at HAc-shock loading of 7400 mg HAc L-1, it showed a one day lag phase associated with decreased biomass activity. After stepwise HAc-acclimation over 27 d, the biomass degraded HAc concentration of up to 8200 mg HAc L-1 without observable lag phase. Although sodium ion was introduced into the second-stage reactor with alkaline treamtent in the first-stage reactor, its ability to degrade HAc was not affected with the presence of sodium (2.4 g Na+ L-1). Microbial community analysis showed that Methanomicrobiales was the most abundant population. The results from this study implied possibility to reconceptualize the acidogenic phase in the two-phase anaerobic system in that it is not necessary to completely exclude methanogens from the acidogenic phase. The acidogenic phase methanogens can be seen as an integral part of this phase and even showed higher HAc tolerance compared to the methanogenic phase methanogens. However, the two-phase anaerobic system was susceptible to HPr inhibition, and operation conditions should be properly maintained to avoid HPr inhibition and improve POB abundance. After applying thermal-alkaline treatment in the first-stage reactor, and resulting in higher biosolids solubilization efficiency, HAc tolerance (4200 mg HAc L-1), and sodium tolerance (2.4 g Na+ L-1) indeed confirmed the stability and efficiency of this two-stage anaerobic system.