Microbubble aeration impact on polyhydroxyalkanoate production in aerobic granules

Abstract

Escalating plastic pollution has belted out the demand for polyhydroxyalkanoate (PHA), a type of biodegradable plastic. PHA could be synthesized within the cells of bacteria in aerobic granules which primarily functions to degrade wastewater. As the distribution of PHA producing bacteria is dependable on diffusion of oxygen, it is the principal interest in this study. Aerobic dynamic feeding (ADF) strategy was used for the sequential batch reactor (SBR). Microbubble aerator has been used to produce air bubbles (φ < 10 μm) in SBR. Aeration rate was regulated at 1.0, 1.5 and 2.0 L/min in mixed aerobic granules sizes while in another set of experiment, the aerobic granules' diameter was segregated into three different ranges (1.0-1.4; 1.4-1.7; >1.7 mm). Total PHA accumulation, localization of PHA producing bacteria, and COD removal were studied for both set of experiments. Aerobic granules' cross sections were examined under trinocular stereozoom microscope. It was found that, the highest PHA accumulation was at 1.5 L/min (3.9 mg PHA /g of wet aerobic granules) for mixed aerobic granules size. For 1.5 L/min of aeration rate, more PHA producing bacteria was spotted and they were located within 1 μm from the core of the granules as compared to only at 2 μm in 1.0 L/min aeration rate. Meanwhile, COD removal was 90% once the system has achieved steady state irrespective of the aeration rates and granules sizes. On the other hand, it was discovered that aerobic granules diameter of 1.0-1.4 mm accumulated the highest amount of PHA (7.9 mg PHA /g of wet aerobic granules). For aerobic granules at 1.0-1.4 mm diameter, almost 80% of granules were dominated by PHA accumulating bacteria. In short, microbubble aerator found to increase the oxygen diffusivity and smaller diameter aerobic granules could accumulate more PHA.