Activated Sludge System Process Control

Not all types of filamentous bacteria can be controlled, so an assessment is required before investing in construction
Activated Sludge System Process Control
Activated Sludge Volume Control
Aeration and Dissolved Oxygen Control
Settling Clarifier Control
Too high
As a result, the load of the secondary settling tank is too high, the sludge layer rises and enters the effluent tank, causing the SS and BOD of the effluent to rise.
Too low
Incomplete treatment, the BOD of the effluent will be higher than the required concentration value
Claim
The amount of activated sludge in the biological treatment reactor should be as low as possible, but it should be based on the low load of the sedimentation clarifier (close to or no sludge layered sedimentation) and the minimum oxygen demand.
Strategy
MCRT control
Features
Regardless of whether the water quality or quantity changes, the same amount of activated sludge is discharged from the treatment system every day
advantage
Compared with F:M or MLSS index control, the mud discharge operation is easier
Operation management personnel have more room to control the activated sludge treatment process, and the process efficiency is more stable
Disadvantage
Not applicable in certain situations where the characteristics of sewage fluctuate greatly
F: M value control
Features
The residual sludge discharge rate depends on the change of the influent BOD
advantage
It is suitable for occasions where the influent BOD fluctuates greatly, such as when the high-load BOD discharged by some factories intermittently enters the sewage treatment plant. This method is the safest way to control residual sludge discharge.
MLSS control
Features
The discharge rate depends on the growth rate of activated sludge.
advantage
For small sewage treatment plants with insufficient laboratory analysis and determination instruments and equipment, this method is often used for simplicity.
F:M control is adopted for situations where the influent BOD fluctuates greatly; municipal sewage often adopts MCRT control, and small sewage treatment plants with insufficient conditions can adopt MLSS
Mud
Usually after the second sedimentation clarification tank is concentrated, the remaining sludge is discharged directly from the sedimentation clarification tank or from the return sludge (RAS) pipeline. Sometimes, part of the activated sludge can also be directly discharged from the bioreactor.
F:M control is used for the situation of large fluctuations in the influent BOD; municipal sewage is often controlled by MCRT, and small sewage treatment plants with insufficient conditions can use MLSS
Usually regular
Aerobic: 2-3mg/L
Hypoxia <0.5mg/l
Anaerobic <0.2mg/L
Insufficient oxygen supply
A large number of filamentous bacteria proliferate under hypoxic conditions
Treatment of turbid effluent
The activated sludge is dark gray or black with a foul smell
DO control
In order to monitor DO indicators in a timely manner, DO probe or DO meter must be set
Some managers try to maintain the highest concentration of DO for safety. However, the aeration system accounts for the largest proportion of the operating cost of the entire sewage treatment plant, so excessive aeration will cause great energy waste.
control parameter
Concentration of MLSS entering the mixed solution of sedimentation clarifier
Sewage flow
Return sludge flow
Surface area of ​​sedimentation clarifier
All related to solid loading rate (SLR)
Settling performance of activated sludge
Sludge layer
Depth range
Generally it can reach 0.15-0.9m above the bottom of the pool, and the optimized depth is 0.3-0.6m above the bottom of the pool
If the depth is too large, the sludge will be rolled up by sudden hydraulic impact and will enter the outlet trough with the water, causing fluctuations in the quality of the outlet water.
A small depth is not conducive to sludge thickening, and because the amount of sludge is small, it is easy to cause the secondary treatment water to flow back to the bioreactor with the sludge.
In-depth control principle
In order to meet the sludge concentration and avoid the secondary treatment water from returning to the biological processor.
The sedimentation clarifier should not have the sludge layer floating or too thick
Sludge return control
importance
If no sludge is returned to the biological treatment reactor, the activated sludge solids in the sedimentation clarifier will continue to accumulate, and eventually will overflow out of the tank and enter the secondary treatment water.
If the return sludge flow is too large, the hydraulic load of the sedimentation clarifier will eventually be too high, the activated sludge settling process will be affected, and the sludge-water separation efficiency will be poor. In extreme cases, activated sludge solids will be lost with the treated water, which will affect the effluent quality.
Primary indicator
Although the return sludge can achieve full mixing of activated sludge solids and wastewater to be treated, the efficiency of the sedimentation clarifier is more sensitive to changes in the return sludge flow than the biological treatment reactor. Therefore, the sludge depth in the sedimentation clarifier should be the primary indicator parameter to optimize the return sludge flow.
Control range
In a treatment system with a short MCRT, the return sludge volume is generally 25%-50% of the reactor inlet water flow rate (maximum 30%-40%)
In the long process of MCRT, the return sludge volume is generally 100%-150% of the reactor inlet flow
Sludge sedimentation performance and foam
Filamentous fungus
Environmental conditions
Low DO environment in bioreactor
Most of the sewage is industrial wastewater with high solubility and high sugar content (lack of nutrients)
Low PH (especially beneficial to filamentous fungi)
solution
Article 1
Enhanced aeration
Article 2
Increase PH (adding chemicals such as lime, caustic soda or bicarbonate, but the PH should be carefully controlled not to exceed 8.5)
Article 3
Add aerobic, anoxic or anaerobic selectors to strengthen the competitiveness of the micelle bacteria
Not all types of filamentous bacteria can be controlled, so an assessment is required before investing in construction
Chlorine dosing in return sludge (special attention is needed to dosing amount)
Foam control
Surfactant
Traits
White, huge wave-like foam
solution
When regulating MCRTs for a long time, some slow growth rate microorganisms can be removed
Nocardia (a kind of filamentous bacteria)
Traits
Light chocolate
solution
Adjust the MCRT to be shorter, so that the discharge rate of sludge is much greater than the growth rate of Nocardia
Physically discharge Nocardia foam from the bioreactor
Once eliminated, never introduce it into the inlet of the sewage treatment plant or the digestion tank. The foam should be placed in a drying bed (the liquid cannot be returned to the sewage treatment plant), and then buried or disposed of by other measures after drying.
If a large amount of sludge solids are sent out during the process of removing foam, it is necessary to check the change in the amount of activated sludge solids after removing foam: the MCRT value may be shorter than the design value.
Add cationic polymer to the return sludge system or mixed liquid return system, and spray high concentration chlorine directly on the foam
Membrane process for solid separation
The membrane separation process is not affected by the settling performance of activated sludge
Membrane system can remove more suspended solids and treat effluent with better water quality
The membrane system can remove bacteria and play a certain disinfection effect, reducing the cost of subsequent disinfection
Membrane system takes up little space
The membrane process allows the SLRs to be long and the mixed solution concentration is high, so the reactor volume can be greatly reduced
CBOD removal process control
Completely mixed flow
Promote the growth of filamentous bacteria under specific conditions
Set up a fixed growth biological treatment system (ie, biological trickling filter, which is more expensive)
Set the selector (less expensive)
Plug flow
advantage
The front end quickly degrades BOD concentration high
The DO index along the reactor can be measured to gradually reduce the air supply along the way and optimize the amount of air entering the reactor
You can simply install a partition wall at the entrance of the reactor to separate the selector space
Nitrification process control
Factors to ensure good nitrification efficiency
Sufficient DO in the bioreactor
Sufficient alkalinity
MCRT is at least 4d, preferably not less than 10d
Suitable pH and temperature in the biological treatment reactor
Denitrification process control
Rapid degradation of BOD is the best carbon source for denitrifying bacteria
If the rapid degradation of BOD is insufficient, it must be added to the hypoxic zone
Phosphorus removal process control
All related to solid loading rate (SLR)
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