Types of Activated Sludge Plants | Activated Sludge Process | Wastewater Treatment Plant

 Activated Sludge Plants

Based on the sludge age maintained in the process, there are three basic operational modes for activated sludge plants.

1. High Rate (Modified Aeration),
2. Conventional, and
3. Extended Aeration. 

High Rate

When the discharge quality must be higher than that of raw wastewater or wastewater that has only undergone primary treatment, but not as high as that obtained by a conventional activated sludge process, a high rate process is used.

The amount of food available in a high rate activated sludge treatment plant exceeds the capacity of the organisms to stabilize it. It is distinguished by:

• High F/M ratio (0.4 to 1.5).
• Short sludge age (0.5 – 5 days).
• High organism growth rate.
• 1 to 3 hour hydraulic retention time.
• The quality of the effluent is not as good as it should be (because some food passes through the process without being stabilized by the organisms).

The primary advantage of a high rate plant is the rapid incorporation of food from the wastewater into the organism biomass.

Detriments of a high rate plant include: 

• The effluent contains a high level of BOD and suspended solids.
• A high rate of sludge production.
• More operator attention and monitoring of process parameters are required.
• Processes that are more easily disrupted than others.

Conventional 

The amount of food available for the organisms in a conventional activated sludge treatment plant is limited, requiring the organisms to compete for the available food. It is distinguished by:

• F/M ratio is moderate (0.2 to 0.4).
• A 3.5-10 day sludge age is considered average.
• A slow rate of organism growth.
• The hydraulic retention time ranges from 6 to 8 hours.
• Because the organisms process any available food, the effluent is of high quality.

The following are the primary advantages of a conventional activated sludge treatment plant:

• The high quality of the effluent.
• Its resistance to certain shock loads.

Traditional activated sludge plants have trade-offs and drawbacks when compared to a high-rate process. A conventional procedure requires more tank volume and oxygen per pound of BOD treated than a high rate process. As a result, while the activated sludge plant is more expensive, ancillary treatment procedures (such as sludge processing) are less costly, and process control needs are less strict.

Extended Aeration 

The amount of food introduced into the process in an extended aeration treatment plant is insufficient to support net organism growth (i.e. Organisms perish at the same rate as they grow.). The organisms in this mode must obtain some of their food by breaking down their own cellular material. It is distinguished by:

• F/M ratio is low (0.05 to 0.15).
• A relatively long sludge age, typically greater than 10 days and frequently 20 to 30 days.
• A net organism growth rate of zero.
• The hydraulic retention time is relatively long, ranging from 18 to 24 hours.

The following are the primary advantages of extended aeration:

• The process's stability.
• Operational requirements are less stringent.
• Effluent of high quality (because complete oxidation of food occurs).
• Sludge production rate is low.
• If the sludge age is on the higher end of the range, nitrification is more likely to occur.

The cost of installing an extended aeration plant is the biggest downside of prolonged aeration (because of the large size of the tank needed to provide the requisite hydraulic retention time of 18 to 24 hours.)