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MindMap Gallery Activated Sludge System Process Control

Activated Sludge System Process Control

To control activated sludge, you need to "have a handle" either on aeration, sludge-wasting or return-sludge flow.

Edited at 2020-10-10 07:50:02

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Activated Sludge System Process Control

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Activated Sludge System Process Control

Activated Sludge Volume 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

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

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.

Aeration and Dissolved Oxygen Control

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.

Settling Clarifier Control

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

All related to solid loading rate (SLR)

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