1. Matrix
The cultivation of granular sludge first requires certain requirements for the substrate. Generally, in the substrate for cultivating granular sludge, COD: N：P=110——200： 5: The matrix of organic waste liquid can be divided into carbohydrates and proteins. In order to successfully cultivate granular sludge, N and P need to be added to wastewater containing carbohydrates For wastewater with a preference for proteins, carbon sources such as glucose need to be added. Studies by scholars have shown that the formation of granular sludge is difficult without the addition of carbon sources, and an appropriate proportion of carbon sources is necessary to promote the formation of granular sludge.
2. Temperature
The anaerobic treatment of wastewater mainly relies on the life activities of microorganisms to achieve the purpose of treatment, and the growth of different microorganisms requires different temperature ranges. A slight difference in temperature can cause an imbalance between the two main populations. Therefore, temperature is crucial for the cultivation of granular sludge. There have been successful experiences with granular sludge at low temperatures (15-25 ℃), medium temperatures (30-40 ℃), and high temperatures (50-60 ℃). Generally, the cultivation time at high temperatures is shorter than that at medium temperatures, but due to the increased toxicity of NH3 mixed with certain compounds at high temperatures, its application is limited to some extent; The medium temperature is generally controlled at around 35 ℃, and under appropriate other conditions, granular sludge can be successfully cultured after 1-3 months; There is relatively little research on the cultivation of granular sludge at low temperatures, but there are literature reports that in experiments using low-temperature domestication of granular sludge to treat bottom concentration pharmaceutical wastewater, the COD removal rate reached 90%, achieving good results. Therefore, low-temperature cultivation of granular sludge will be one of the key research areas in the future.
3. PH value
During anaerobic treatment, hydrolytic acid producing bacteria have a large adaptability range to pH values, while methanogenic bacteria are sensitive to changes in pH values, with an optimal pH range of 6.8-7.2. If the pH value in the reactor exceeds this range. This will lead to the inhibition of methane producing bacteria and the accumulation of acid, thereby acidifying the entire reactor. Therefore, the pH range inside the reactor should be controlled within the optimal range for methane producing bacteria. Due to the different pH values of wastewater with different properties, in order to ensure the stability of the pH value in the reactor and prevent the inhibition of methanogenic bacteria caused by acid accumulation, chemicals such as NaHCO3, Na2CO3, Ca (OH) 2, etc. can be added to the wastewater.
4. Alkalinity
It is generally believed that the alkalinity of the incoming water should be around 1000mg/L (calculated as CaCO3), while for wastewater mainly composed of carbohydrates, the alkalinity of the incoming water is: COD>1： 3 is necessary. Studies by scholars have shown that controlling the alkalinity of effluent above 1000mg/L (calculated as CaCO3) during the initial stage of granular sludge cultivation can successfully cultivate granular sludge. After the granular sludge matures, the requirement for alkalinity of the influent is not high [2] This has a positive impact on reducing processing costs.
5. Trace elements and inert particulate trace elements also play an important role in the healthy growth of microorganisms. Among them, Fe, Co, Ni, Zn, etc. are beneficial for improving sludge activity and promoting the formation of granular sludge. In addition, inert particles act as nuclei for bacterial attachment and play a positive role in granulation. In addition, studies have shown that adding activated carbon can greatly shorten the time for sludge granulation; After adding activated carbon, the particle size of the granular sludge is larger, which makes the reactor operation more stable.
6. The formation of granular sludge by SO42- is currently under discussion. According to Sam Soon's extracellular polymeric hypothesis, the high partial pressure of local hydrogen induces microorganisms to produce extracellular polymeric substances and interact with bacterial surfaces. Through bridging effects such as electrostatic attraction and physical contact of charged groups, a biological floc containing multiple components is formed, which is a necessary condition for the formation of 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, which is unfavorable for the formation of granular sludge. However, some domestic and foreign scholars have found that when treating wastewater containing high sulfate, very thin filamentous bodies are produced, which can serve as the original nucleus for attachment of methanogenic filamentous bacteria, and thus the formation of particles begins; Sulfides produced by sulfate reduction combine with some metal ions to form insoluble particles, which may become secondary nuclei for the growth of granular sludge.
7. Inoculation of sludge and inoculation amount
Generally speaking, there are no special requirements for inoculating sludge, but the different types of inoculated sludge have a direct impact on the speed of forming granular sludge. Therefore, ensuring good settling performance of sludge, rich variety of anaerobic microorganisms, and high activity is very beneficial for accelerating the formation of granular sludge. Regarding the amount of inoculated sludge, some scholars believe that an anaerobic sludge inoculation amount of around 11.5kgVSS/m3 (calculated based on the reaction zone volume) is appropriate for rapidly cultivating anaerobic granular sludge. This is consistent with the recommended concentration range of 10-20kgVSS/m3 by foreign scholars.
8. Startup method
The use of low concentration influent combined with gradually increasing hydraulic load is beneficial for sludge granulation. This is because low concentration influent can effectively avoid excessive accumulation of inhibitory biochemical substances, while higher hydraulic loads can enhance hydraulic screening.
9. The hydraulic load is too low, which can lead to excessive growth of a large amount of dispersed sludge, thereby affecting the settling performance of sludge and even causing sludge swelling; However, excessive hydraulic load can cause shear on granular sludge and peel off the extracellular polysaccharide viscous layer of non aggregated cell bodies, hindering adhesion and aggregation. Therefore, in the initial stage of start-up, a smaller hydraulic load (0.05-0.1m3/m2˙ h) should be used to enable floc sludge to bond with each other and grow into clusters, which is conducive to the formation of primary granular sludge. When a certain amount of sludge appears, increase the hydraulic load to 0.25m3/m2; Above h, some floc sludge can be washed away, causing high-density granular sludge to settle to the bottom of the reactor, forming a granular sludge layer. In order to achieve sludge granulation as soon as possible, increase the hydraulic load to 0.6m3/m2; At h, most of the floc sludge can be washed away. However, the increase in hydraulic load cannot be too fast, otherwise the premature elimination of a large amount of floc sludge will lead to excessive sludge load, affecting the stable operation of the reactor.
