## 1 Introduction  
Anaerobic granular sludge is the major success factor of high-rate anaerobic processes [1]. The formation of granular sludge causes efficient decoupling of solids retention time (SRT) and hydraulic retention time (HRT), thereby creating the conditions for the anaerobic reactor of biomass interception and stable operation. The size and structure of granular sludge are considered fundamental to the efficient operation of water treatment.  
Granulation, defined as the formation of well settleable microbial aggregates with various functionalities, depends on several aspects, such as hydraulic conditions, wastewater characteristics, physico-chemical parameters, and so on [2]. The formation of mechanisms for anaerobic granulation reveals that there are several phases of sludge flocs growth [3]. At the beginning, anaerobic bacteria could attach to the inert nuclei particle surfaces to form the initial biofilms, namely embryonic granule for anaerobic granulation [4]. During this period, embryonic granules are aggregated into large sludge flocs due to winding by a large number of filamentous bacteria, then the internal parts of the sludge flocs form a stable environment for anaerobic microbial growth and the structure is tighter as the flocs grow and become larger. After a certain time, granular sludges step into a maturity stage resulting in a steady-state micro ecosystem.  
Image analysis has become a common methodology in many research areas [5], such as chemical coagulation-flocculation [6; 7; 8] and electrocoagulation [9; 10; 11]. Moreover, the structure of the sludge aggregation flocs can be seen in the image as well. The observedstructures, regardless of the applied scale of magnification (or reduction), are generally indicative of their fractal character [12]. Statistics have documented that a given group of sludge flocs usually supports determinations of their fractal dimension \(D_{f}\)[13; 14; 15]. The value of \(D_{f}\) can define the compactness and the spatial structure of sludge flocs, which increased as the sludge flocs' density increased.  
Whether the start-up process is successful or not is a prerequisite for the operation of the anaerobic reactor. Over the years, various works representing the start-up performance of anaerobic reactors have been developed, however, there is no previous work focusing on sludge flocs' size and structure characteristics during the start-up process. In the present study, a start-up experiment was carried out to investigate the sludge flocs in terms of perimeter \(P\) and area \(A\) contrast-enhanced images derived from a non-intrusive optical sampling and digital image analysis technique, and then the \(D_{f}\) analyzed the sludge aggregates-flocs with fractal characteristics. The aim of this study is to identify the diversification of sludge flocs' characteristics during the start-up process in an efficient anaerobic baffled reactor. This demonstrates the mechanism of anaerobic granulation in wastewater treatment. The results obtained in this study are expected to provide a better understanding of the size, sedimentation, and structure of granular sludge in high-efficiency reactors and further contribute to establishing a unified theory of anaerobic granulation and the accelerated formation technology of granular sludge.