Secondary Treatment (Biological) | Activated Sludge Process | Wastewater Treatment Plant

 Secondary Treatment (Biological)

Secondary treatment is a biological procedure for removing dissolved and suspended organic matter from treated water.

Purpose

• After primary treatment, remove any dissolved and nonsettleable (also known as colloidal) organic material that remains in the waste.

Impacts

A biological treatment process is a living process that is vulnerable to a variety of external influences, including:

• Temperature effects that may be caused by industrial sources, wet weather events, or seasonal effects.
• Significant increases in organic loadings (shock loads), which frequently originate from industrial sources.

Fixed Film Process Descriptions

A Fixed Film Process is a biological treatment method in which microorganisms attach to media structures. As the wastewater flows past and over the media containing the attached microorganisms, the biodegradable organics are removed.

The material on which slime accumulates and organisms grow in a trickling filter is known as the media.

Trickling Filter (TF)

• Water cascades downward through stone or plastic media.
• Plastic media can be found in a variety of shapes and sizes.
• Aerobic bacteria can attach, multiply, and feed on the passing wastewater in the media.

Rotating Biological Contactor (RBC)

• A rotating shaft is surrounded by plastic discs (media) that allow microorganisms to grow in Rotating Biological Contractors.
• To provide oxygen for organisms that feed on wastewater, media is rotated in and out of the wastewater.

Suspended Growth Process Description

Suspended Growth Processes (SGPs) are biological treatment methods in which microorganisms are suspended in wastewater rather than physically attached to media. To keep the biomass suspended and maintain aerobic conditions, oxygen is usually added to the suspended growth process.

Activated Sludge

• The activated sludge process introduces oxygen into the environment, allowing aerobic bacteria and other organisms to thrive and multiply.
• Biological treatment occurs in the aeration tanks, which add oxygen and wastewater to the biomass.
• Bacteria use biodegradable organic matter as a food source to create new cells.
• The contents of the aeration tank, known as the Mixed Liquor Suspended Solids (MLSS), flow to the secondary clarifier, which separates the MLSS from the treated effluent.

• A portion of the microorganism-containing settled MLSS is returned to the aeration tank to continue the treatment process (Returned Activated Sludge).
• If left unchecked, the organisms proliferate to such an extent that some must be removed on a regular basis (Waste Activated Sludge).

Secondary Clarifiers

• Both fixed film and suspended growth technologies are used.
• Sludge containing live microorganisms separates from the liquid and settles to the tank's bottom.
• A density current baffle can be installed to ensure that currents are minimized, thereby improving the clarifier's performance. The baffle cancels out the effect of density currents flowing up the clarifier wall. Solids will not settle properly if density currents are present, and solids may flow directly into the effluent trough.

1. The baffle is installed on the clarifier's wall beneath the effluent launder and scum baffle.
2. The baffle is designed to slope downward at an angle and extend beyond the scum baffle to the tank's center.
3. The flow is redirected inward and downward toward the tank's centre, improving settleability and preventing solids from entering the effluent trough.

• A portion of the settled sludge is returned to the aeration tank as seed to maintain the biological treatment process in an activated sludge process (Return Activated Sludge).
• Excess sludge is transported to the sludge handling processes for further treatment and eventual disposal.
• Prior to discharge into the stream, the clarified effluent is disinfected.