Enhancing stability of aerobic granules by microbial selection pressure using height-adjustable influent strategy

Highlights

• Optimal size range was proposed regarding mass transfer and granules stability.

• Height-adjustable influent strategy was used to optimize size distribution.

• Propagation of Thiothrix sp. in oversized granules caused filamentous bulking.

• Exceeded hydraulic shear stress decreased abundance of SND bacterium.

• Optimize size distribution facilitated enrichment of SND bacterium and stability.

Abstract

Optimizing granules size distribution is critical for both reactor performance and stability. In this research, an optimal size range of 1800 to 3000 μm was proposed regarding mass transfer and granules stability based on granules developed at DO around 8.0 mg L⁻¹ with the feed COD:N:P at 100:5:1. A height-adjustable influent strategy was applied to facilitate the nutrient storage of granules at optimum size range via microbial selective pressure. Results suggested insufficient hydraulic shear stress led to overgrowth of granules size. High abundance of filamentous bacteria (Thiothrix sp.) was observed in oversized granules, which detached and affected the remaining granules, resulting in severe sludge bulking. Strong hydraulic shear stress suppressed uncontrolled growth of granules. However, fewer abundance of simultaneous nitrification and denitrification (SND) bacterium was acquired, which led to unfavored SND effect and total nitrogen (TN) removal efficiency. The height-adjustable influent strategy facilitated the poly-β-hydroxybutyrate (PHB) storage of granules at optimum size range, while limiting the overgrowth of granules size. Additionally, more than 87.51% of total granules situated in optimal sizes range, which led to higher abundance of SND bacterium and higher TN removal efficiency.

Introduction

Aerobic granular sludge is regarded as one of the most promising biological methods of wastewater treatment (Pishgar et al., 2020). It is advantageous in compact microbial structure, settling ability, biomass retention, allowing occurrence of simultaneous nitrification and denitrification (SND) process in a single granule (Coma et al., 2012; Lochmatter et al., 2013). However, the overgrowth of large granules would weaken granule stability, leading to reactor failure, which limited the application of this method (Franca et al., 2018).

Limited oxygen concentration in oversized granules has been a significant obstacle for stable operation (Li et al., 2020). Low oxygen concentration might stimulate the propagation of filamentous bacteria (Wu et al., 2019). Uncontrolled growth of filamentous bacteria decreases the structure intensity, and turns the round-shape granules into fluffy surface (Wan et al., 2014).

Meanwhile, the limited oxygen concentrations in granules favors formation of anaerobic microbes, which reduces the pH by producing acidic products (Luo et al., 2020). The framework responsible for granule formation and structural stability would then be dissolved, causing decrease of granules strength. Moreover, the acidic pH conditions might stimulate the growth of filamentous microbe, which leads to an unexpected positive regeneration (Rollemberg et al., 2019). Consequently, it is necessary to avoid overgrowth of large granules during the operation.

Hydraulic shear stress has been proved to be an effective way on controlling overgrowth of granules by enhancing erosion and detachment on granules surface. (Zhou et al., 2019a). The strength of hydraulic shear stress is conventionally achieved by a stronger aeration intensity or higher height to diameter ratio (H/D) (Devlin et al., 2017; Zhou et al., 2019a). However, high H/D ratio is incapable on full-scale plants, while enhancing the aeration intensity resulted in extra operational costs (Rollemberg et al., 2018). Moreover, strong hydraulic shear stress leads to smaller granule size distribution (Liu et al., 2010), resulting in insufficient SND effects and TN removal efficiency. Strategies to optimizing granule size regarding both reactor performance and stability are worth studying (Long et al., 2019).

Previous studies suggested microbial selective pressure could improve stability of aerobic granular sludge (Derlon et al., 2016). Reducing sludge retention time (SRT) promotes the competitive advantage of phosphorus accumulating organisms (PAOs) over filamentous bacteria in SBRs (Moura et al., 2018). Li et al. (2016) suppressed filamentous bulking via removing the biomass in upper part of SBR. Sodium acetate mixed with succinate was used to balance the competition of phosphorus/glycogen accumulating organisms (PAOs/GAOs) (He et al., 2020). However, a valid strategy to optimize granules size distribution is yet unknown.

In this study, a height-adjustable influent strategy was applied to optimize granule sizes distribution via microbial selective pressure. The main purpose were to i) determine granule optimal size range through oxygen mass transfer and granule strength analysis, ii) investigate the effect of height-adjustable influent strategy on granule size distribution and microbial community structure, and iii) explore the selective pressure mechanism on size optimization and granule stability.