Key points for sludge addition and start-up in anaerobic reactors

The start-up of anaerobic reactors usually requires the addition of anaerobic granular sludge, with an inoculation amount of approximately 10-15%. However, many environmental protection companies, in order to save costs, will require on-site debugging personnel to use activated sludge to domesticate granular sludge.
1. Calculation of sludge dosage
When starting the anaerobic reactor for biological treatment, if the treated organic load is lower than the maximum treatment load of the reactor, the corresponding amount of anaerobic sludge inoculation can be calculated based on the total amount of organic matter to be treated, without the need for full inoculation, thereby reducing the procurement cost of anaerobic sludge.
Sludge load is a basic concept that represents the amount of organic matter applied per unit mass of effective anaerobic sludge per day. The calculation formula is as follows:
Sludge load (kgSCOD/kgVS. d)=Q (m3/d) × SCOD (mg/L)/VS (kg)
Among them:
Q is the daily processing capacity of the anaerobic reactor
SCOD is the dissolved COD concentration of wastewater
VS is the total volatile solid content of anaerobic sludge in the anaerobic reactor
2. Start of sludge inoculation
The start-up process is divided into three stages: increasing sludge activity stage, forming granular sludge stage, and forming granular sludge bed stage.
(1) The initial stage of start-up: This stage refers to the stage where the reactor load is below 2KgCOD/(m3. d). Starting from 0.5-1.5kgCOD/(m3 · d) or sludge load of 0.05-0.1kgCOD/(kgVSS · d). The washed out sludge is limited to the small dispersed sludge in the seed sludge, mainly due to the upstream speed of water and the small amount of biogas produced. The concentration of the mixed solution entering the anaerobic tank for digestion and degradation of wastewater should not exceed COD 5000mg/L, and the inflow should be controlled according to requirements, with a minimum COD load of 1000mg/L. When entering the liquid, if it does not meet the requirements, it should be diluted.
The liquid inlet adopts an intermittent impact form, which is every 3-4 hours for 5-10 minutes each time, and then gradually reduces the interval time to 1 hour, gradually increasing the liquid inlet time by 20-30 minutes each time. In the initial stage of start-up, if the interval between inlet water is too long, the pump should be started every hour to stir the sludge for 3-5 minutes each time.
(2) Start the second stage: when the volumetric load of the reactor increases to 2-5kgCOD/m3 · d. At this stage, the amount of washed sludge increases, most of which is flocculent sludge. The reason for washing out is the expansion of the sludge bed caused by the increase in gas production and upstream velocity. At the end of this stage, due to the formation of granular sludge, the activity of the sludge increases. During this stage, the sludge concentration in the reactor decreases to the lowest level due to the washing out of flocculent sludge. In fact, heavy granular sludge and dispersed, flocculent sludge were selected in the reactor.
(3) Start the third stage: This stage refers to the stage where the reactor load exceeds 5KgCOD/(m3. d). During this stage, the flocculent sludge rapidly decreases while the granular sludge accelerates its formation until there is no longer flocculent sludge present in the reactor. At this stage, the reactor load can be increased to a very high level, and when most of the reactor is filled with granular sludge, its maximum load can exceed 50 KgCOD/(m3. d). At this stage, it is necessary to control the concentration of volatile fatty acids (VFA) to not exceed 500mg/L. When VFA exceeds 500-1000mg/L, the anaerobic reactor presents an acidification state; If it exceeds 1000mg/L, it indicates acidification and immediate measures need to be taken to stop feeding and domesticate the bacterial strains. Generally speaking, the second to third stages also take 30-40 days.
3. Key points for startup
① The start-up must be carried out step by step, leaving ample time and not expecting to enter the feeding operation for a short period of time to achieve the goal of anaerobic degradation. Because initiation is actually the process of restoring bacteria from a dormant state. During the start-up process, bacterial selection, domestication, and proliferation are all ongoing, and the growth rate of methane producing bacteria with lower concentrations in the original anaerobic sludge is much slower than that of acid producing bacteria. Therefore, at this time, the load should generally not be high, the time should not be short, the amount of feed should be small, and the interval time should be long.
② The mixed inlet concentration must be controlled at a lower level, with a general COD concentration of 1000-5000mg/L. When it exceeds 5000mg/L, effluent circulation and water dilution should be carried out to meet the requirements. If the concentration of sulfite in the mixed solution is greater than 200mg/L, it should also be diluted to below 100mg/L before entering the liquid.
③ If the concentration of sulfite in the mixed solution is greater than 200mg/L, it should also be diluted to below 100mg/L before entering the liquid.
④ Load increase operation mode: Can the volume load during the initial start-up range from 0.2 to 0.5 kgCOD/m3? At the beginning, when the biodegradability reaches over 80%, it gradually increases. If the anaerobic process is still abnormal and COD cannot be digested despite the lowest load feeding, the feeding interruption time should be extended for 24 hours or 2-3d. The main indicators of digestion and degradation should be checked to measure the concentration of VFA. During the start-up stage, VFA should be kept below 3mmol/L.
⑤ When the volumetric load reaches 2.0kgCOD/m3? After d, the feed load can be increased each time, but the maximum does not exceed 20%. Only when the feed increases and the VFA concentration remains unchanged, or is still maintained at a level of<3mmol/L, can the feed volume continue to increase and the feed interval continue to decrease.