The Effect of Slowly Biodegradable Carbon on the Morphology, Integrity and Performance of Aerobic Granular Sludge

Abstract

In the last two decades, a new biofilm process, aerobic granular sludge technology, has been applied by researchers for organic and nutrient removal from municipal wastewater. Most studies have been performed with high strength wastewater and reported granules with irregular shape and incomplete organic and nutrients removal. Slowly biodegradable organic compounds, including the particulate and colloidal fraction, constitute an essential fraction of municipal wastewater. The objective of this study was to investigate the influence of slowly biodegradable organic matters, including particulate compounds, on the morphology, integrity and performance of aerobic granular sludge. Three identical lab-scale aerobic granular reactors (AGRs) were operated for two experimental phases in a sequencing batch reactor (SBR) regime. The objective of the first experimental phase was to develop stable, compact and regular aerobic granules that achieve biological nutrient removal (BNR) of both nitrogen and phosphorus. For both experimental phases, the hydraulic loading rate was 1.32 kg COD/m3/day. For the 1st phase, the three reactors were fed soluble organic matter (sodium acetate) as the sole carbon source with an anaerobic-aerobic SBR cycle. To achieve complete BNR, a post anoxic cycle was included in the SBR cycle. During the second experimental phase, the influence of particulate COD was studied with particulate potato starch being added with and without pretreatment. In the first reactor, particulate starch was included in a 1:1 soluble COD: particulate COD ratio. For the other AGRs, the potato starch was pretreated either with heat hydrolysis or biological fermentation, an anaerobic process. Both pretreatments were applied to increase the bioavailable specific substrate for diffusion into the granular structure and degradation. For both experimental phases, results are presented for granule morphology, integrity and reactor treatment performance. The findings of this study demonstrated that implementing a post anoxic phase remarkably increased both the nitrogen and phosphorous removal performance of the aerobic granules. 96% COD, 94% N and 86% P removal was accomplished after applying the post anoxic period in the SBR cycle. Comparable to previous studies, the free nitrous acid (FNA) inhibited the selection of PAOs when nitrite accumulated in the AGRs. The microscopic investigations and the EPS extraction showed that the presence of particulate starch in the feeding solution influenced the morphology and the structure of the granules. The granule surface had more filamentous growth and a more porous structure. AGR fed with a 1:1 particulate:soluble COD feed also demonstrated reduced nitrogen and phosphorous removal (78 and 81%, respectively). Moreover, only 75% COD removal was accomplished. Microscopy suggests that particulate removal was achieved by surface adsorption, followed by hydrolysis and degradation by microorganisms. Both pretreatments were capable of solubilizing most of the particulate starch. The heat hydrolysis broke down the starch polymers to produce mostly soluble starch, slowly biodegradable compound. This pretreatment also lead to filamentous growth, indicating that both the size and nature of the organic matter affected the aerobic granular sludge. On the other hand, most of the fermentation pretreatment products were VFAs. The anaerobic fermentation pretreatment enhanced and maintained morphology, integrity and performance of the aerobic granular sludge. Therefore, aerobic granular sludge can be successfully applied for BNR from municipal wastewater. However, if high particulate COD fractions are present, a fermentation pretreatment may be necessary.