22 Apr 2016

Determining Features of Aerobic Granular Sludge Formation, Stability and Performance in Sequencing Batch Reactors

Marieska Verawaty, May 2013
The University of Queensland

Abstract
Aerobic granular sludge is a promising technology for secondary wastewater treatment systems. It offers high rate, low cost and a small footprint treatment system; however, few aerobic full-scale systems have developed globally. This lack of use reflects the obstacles relating to implementing aerobic granule technology; namely, long start-up periods for the development of granules, poor nutrient removal during granulation selection pressure, difficulty in maintaining stability of aerobic granular sludge technology, and limited understanding of the microbial community structure for granule formation and its optimal biological activity. This thesis aims to address these obstacles by understanding and investigating important aspects of aerobic granulation.

Producing aerobic granules in low-strength wastewater is challenging, as low substrate concentration may cause a long start-up period. One study in this thesis examined strategies for cultivating aerobic granules for nitrogen and phosphorus removal using domestic wastewater in a sequencing batch reactor (SBR) seeded with floccular sludge. Aerobic granules were formed after 60 days of operation and achieved a fully granular system after 130 days of operation. The key strategies applied in this study for the successful granule formation were the stepwise reduction of both the settling time and hydraulic retention time (HRT). This system achieved an average of 99% and 98% phosphorous and nitrogen removal respectively during the 160 days operation.

Mixing crushed granules and floccular sludge as the seed was used to improve aerobic granule formation with biological nutrient removal (BNR) while treating domestic wastewater. SBRs were inoculated with either floccular sludge only (100%-floc SBR) or with floccular sludge and 10% crushed granules (90%-floc SBR). Granules successfully developed in both reactors. The 90%-floc SBR had an enhanced start-up period, and during the granule formation nutrient removal disturbances and biomass loss was avoided. The 90%-floc SBR achieved a fully-granular system with 84% and 99% of nitrogen and phosphorus removals respectively, while the 100%-floc SBR achieved 75% and 93% nitrogen and phosphorus removal respectively without achieving a fully granular system.

The mechanism for the acceleration of granule formation using a mixture of floccular and crushed granular sludge was investigated by fluorescent microbead labelling method. Flocs were observed to attach to the surface of the crushed granules seeds, resulting in a reduced biomass wash-out during the early stage of granulation, and maintain nutrient removal performance. Crushed granules acted as nuclei for floccular particle attachment in the early stage of granulation (first 20 days) when the highest aggregation of flocs to granules occurred. This demonstrated the importance of limiting the wash-out of flocs in this period. It is recommended to apply a less aggressive selection pressure at this initial stage to allow the flocs to bind; a more aggressive selection pressure can be applied after this stage as a strategy for granule formation.

Understanding the dynamics of particle size is important for system performance as a direct parameter to show the growth and aging process in the microbial system, which would impact on the effluent’s final quality. Granule particle size will be dictated by the balance between granule growth and breakage or attrition. The particle size of mature aerobic granules was fractionated into three different sizes (large, medium and small) and their dynamics were monitored for 50 days wastewater treatment period to understand the particle size changes. Granule size was noted to migrate towards a certain particle size, termed the “Critical Size”, approximately 600-800 μm, by reduction and breakage from a larger granule size (median of 1125 μm) or by granules growing from a smaller granule size (median of 235 μm). This study recommends to avoiding biomass wash-out during selection for granules (decreasing settling times) at the initial stages of granulation.

Aerobic granular systems typically are operated in SBRs using a rapid feed process during wastewater addition. Consequently, it is of interest to study the system with slower feed stages, which would be more relevant to the operation of a full-scale system. Cultivating aerobic granules in a system with a slow-feed rate and treating domestic wastewater treatment may be predicted to be difficult and, to date no successful study has been reported. This study compared granulation and BNR in SBRs operating with slow- and fast-feed conditions. Feast and famine conditions occurred in the fast-feed system and certain species were enriched (i.e. polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs)) and large, dense, and smooth aerobic granules developed. In contrast, in the slow-feed system it was difficult to form smooth and dense aerobic granules; filamentous overgrowth occurred and resulted in fluffy and large appendages. A substantial increase of Competibacter spp. occurred in the fast-feed reactor during granule formation. A noticeable increase of Flavobacteria sp and Commamonas sp occurred in the slow-feed reactor, when large loosely aggregated biofilms formed. Interestingly, when the aggregates in the slow-feed reactor evolved to a granular-like formation, Competibacter spp. increased in abundance, implicating their possible importance for granule formation.

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