Granular sludge technology has many advantages, including large sludge volume, fast sedimentation rate, abundant microbial species, strong resistance to organic load shock, and good removal of toxic and heavy metal pollutants. It has received extensive attention and research in recent years. Currently, the basic conditions for cultivating granular sludge have been initially mastered, but the mechanism of its formation is still unclear. This article discusses the factors affecting sludge granulation, hoping to provide a reference for in-depth research on sludge granulation.
 
　　1. Substrate
 
　　Cultivating granular sludge requires certain substrate requirements. Generally, in the substrate for cultivating granular sludge, COD:N:P = 110-200:5:1. The substrates of organic wastewater can be divided into carbohydrate-rich and protein-rich types. To successfully cultivate granular sludge, N and P need to be added to carbohydrate-rich wastewater, while carbon sources (such as glucose) need to be added to protein-rich wastewater. Studies have shown that the formation of granular sludge is more difficult without adding carbon sources. Therefore, a suitable proportion of carbon sources is necessary for the formation of granular sludge.
 
　　2. Temperature
 
　　Anaerobic treatment of wastewater mainly relies on the life activities of microorganisms to achieve the treatment purpose. Different microorganisms require different temperature ranges for growth. A temperature difference of a few degrees can cause an imbalance between the two main populations. Therefore, temperature is important for the cultivation of granular sludge. Granular sludge has been successfully cultivated at low temperatures (15-25℃), medium temperatures (30-40℃), and high temperatures (50-60℃). Generally, high-temperature cultivation has a shorter cultivation time than medium-temperature cultivation, but due to the increased toxicity of the mixture of NH3 and certain compounds at high temperatures, its application is limited; medium temperature is generally controlled at around 35℃. Under appropriate conditions, granular sludge can be successfully cultivated in 1-3 months; research on low-temperature cultivation of granular sludge is less common, but literature reports show that in experiments using low-temperature acclimatized granular sludge to treat high-concentration pharmaceutical wastewater, the COD removal rate reached 90%, achieving good results. Therefore, low-temperature cultivation of granular sludge will be one of the future research focuses.
 
　　3. pH Value
 
　　In the anaerobic treatment process, acidogenic bacteria have a wide range of pH adaptability, while methanogenic bacteria are sensitive to pH changes. Their optimal pH range is 6.8-7.2. If the pH value in the reactor exceeds this range, methanogenic bacteria will be inhibited, acid accumulation will occur, and the entire reactor will become acidic. Therefore, the pH range in the reactor should be controlled within the optimal range for methanogenic bacteria. Due to the different pH values of wastewaters of different properties, in order to ensure the stability of the pH value in the reactor and prevent acid accumulation and the resulting inhibition of methanogenic bacteria, chemical substances such as NaHCO3, Na2CO3, and Ca(OH)2 can be added to the wastewater.
 
　　4. Alkalinity
 
　　It is generally believed that the alkalinity in the influent water quality should generally be around 1000 mg/L (calculated as CaCO3), and for wastewater mainly composed of carbohydrates, the influent alkalinity: COD > 1:3 is necessary. Some scholars have shown that in the early stage of granular sludge cultivation, controlling the effluent alkalinity above 1000 mg/L (calculated as CaCO3) can successfully cultivate granular sludge. After the granular sludge matures, the requirement for influent alkalinity is not high [2]. This has positive significance for reducing treatment costs.
 
　　5. Trace Elements and Inert Particles
 
　　Trace elements also play an important role in the good growth of microorganisms. Among them, Fe, Co, Ni, and Zn are beneficial for improving sludge activity and promoting the formation of granular sludge.
 
　　In addition, inert particles, as the core for bacterial attachment, play a positive role in granulation. Furthermore, studies have shown that adding activated carbon can significantly shorten the granulation time of sludge; after adding activated carbon, the granular sludge has a larger particle size, and the reactor operation becomes more stable.
 
　　6. SO42-
 
　　The effect of SO42- on the formation of granular sludge is still under discussion. According to Sam-Soon's extracellular polymeric substance hypothesis, the high partial pressure of local hydrogen is a necessary condition for inducing microorganisms to produce extracellular polymeric substances, which interact with the bacterial surface through electrostatic attraction of charged groups and bridging action through physical contact, forming a biofilm containing various components, thereby forming granular sludge. However, in the presence of sulfate, due to the rapid utilization of hydrogen by sulfate-reducing bacteria, the reactor cannot establish a high hydrogen partial pressure, thus hindering the formation of granular sludge. However, some domestic and foreign scholars have found that when treating wastewater containing high sulfate, very thin filaments are produced, which can serve as the initial core for the attachment of methanogenic filaments, initiating granule formation; the sulfide produced by sulfate reduction combines with some metal ions to form insoluble particles, which may become secondary nuclei for granular sludge growth.
 
　　7. Inoculum Sludge and Inoculum Amount
 
　　Generally, there are no special requirements for inoculum sludge, but different inoculum sludge directly affects the speed of granular sludge formation. Therefore, ensuring good sludge sedimentation performance, abundant anaerobic microbial species, and high activity is very beneficial for accelerating the formation of granular sludge.
 
　　Regarding the amount of inoculum sludge, some scholars believe that when the anaerobic sludge inoculation amount is around 11.5 kgVSS/m3 (calculated based on the reactor volume), it is suitable for rapidly cultivating anaerobic granular sludge. This is consistent with the concentration range of 10-20 kgVSS/m3 recommended by foreign scholars.
 
　　8. Startup Method
 
　　Using low-concentration influent and gradually increasing the hydraulic load is beneficial for sludge granulation. This is because low-concentration influent can effectively avoid the excessive accumulation of inhibitory biochemical substances, and a higher hydraulic load can enhance the hydraulic screening effect.
 
　　9. Hydraulic Loading
 
　　Too low a hydraulic loading will cause excessive growth of dispersed sludge, affecting sludge settling performance and even causing sludge bulking; however, excessive hydraulic loading will shear the granular sludge and peel off the extracellular polysaccharide sticky layer of unaggregated cells, hindering adhesion and aggregation. Therefore, in the initial stage, a smaller hydraulic loading (0.05-0.1 m3/m2·h) should be used to allow the flocculent sludge to bond with each other and grow into groups, which is conducive to the formation of the primary body of granular sludge. When a certain amount of sludge appears, increasing the hydraulic loading to 0.25 m3/m2·h or more can wash away some of the flocculent sludge, allowing the denser granular sludge to settle to the bottom of the reactor, forming a granular sludge layer. In order to achieve sludge granulation as quickly as possible, increasing the hydraulic loading to 0.6 m3/m2·h can wash away most of the flocculent sludge. However, the hydraulic loading cannot be increased too quickly; otherwise, the premature elimination of a large amount of flocculent sludge will lead to excessive sludge loading, affecting the stable operation of the reactor.