SBR

1. Oxidation and synthesis

COHNS + O₂ + nutrients + bacteria → CO₂ + NH₃ + C₅H₇NO₂ (new cells) + other end products

2. Endogenous respiration

C₅H₇NO₂ (cells) + 5O₂ + bacteria → 5CO₂ + 2H₂O + NH₃ + energy

3. Nitrification is two step reaction

NH4+ + 1.5O2 + Nitrosomonas → NO2– + 2H+ + H2O

NO2– + 0.5O2 + Nitrobactor → NO3–

4. Nitrification reaction

• 4.57 g O2 consumed per g ammonia oxidized to nitrate.
• The nitrification reaction releases hydrogen ions that will lower the pH value, consumes alkalinity at 7.14g alkalinity per g NO3

Optimum growth of nitrifying organisms require –

• Adequate solids retention time (SRT)
• Suitably aerated basin
• Sufficient dissolved oxygen, nitrifier inhibition at low DO levels.
• Optimal pH between 6.5 to 9.0, and pH less than 6 can lead to inhibition.

PROCESS DESCRIPTION

The SBR sequence shall consist of a 2-stream, 3-stream, or 4-stream SBR configuration depending on the quantity of effluent or sewage to be treated.

Major Operations of SBR

Static Fill:

This non-aerated step encourages the formation of floc-forming bacteria and reduces the production of filamentous bacteria. Typically, a minimum of 30 minutes of anoxic fill is required. At higher flows (up to twice the average flow), when the cycle starts with the Fill-Decant step, the time spent in Fill-Decant is considered as static fill time. Accordingly, the Static Fill time will be reduced, and at very high flows, this step may be skipped.

Aerated Fill:

This phase involves simultaneous filling and aeration. The step ends either when the maximum fill time is completed or when the Top Water Level (TWL) is reached in the reactor. At flows higher than the average design flow, filling is allowed up to the Top Water Level (TTWL).

React:

This step completes the reaction process without filling wastewater and reduces the soluble BOD remaining in the SBR at the end of the Aerated Fill step. Nitrification is also resumed at this stage. Waste Activated Sludge (WAS) may be extracted in mixed liquor state from the SBR during this step.

Settle:

Aeration is stopped to prevent mixing within the SBR and to allow quiescent settling of biological flocs to the bottom section. Typically, 45 to 60 minutes are required for this step.

Decant:

Using surface floating decanters, the SBR supernatant (free of solids) is extracted and constitutes the treated water effluent. The Decant step ends when the Bottom Water Level (BWL) is reached in the SBR.

Other Modes of SBR Operation

Additional modes of operation are determined after reviewing the process and technical aspects of waste treatment, and may include:

• Idle phase
• Fill-Decant phase
• Aeration sharing
• Flow distribution between SBRs
• Decant interlock
• Control logic