Sequential Batch Reactor | Environmental Engineering - Civil Engineering (CE) PDF Download

 keeping on/off oxygen supply to the reactor. During the aerobic condition nitrification takes place. Aerated Fill can reduce the aeration time required in the react step.

React: Depending on the conditions applied: anaerobic, anoxic or aerobic reactions, substrate present in the waste water are consumed by the biomass.  

Settle: After sufficient time of reaction, aeration and mixing is stopped and biomass is allowed to settle from the liquid resulting in clear supernatant.  

Decant: Clear supernatant (treated waste water) is removed from the reactor. 

Idle: This is the time between cycles which is used to prepare the SBR for next cycle. It is also used to adjust the cycle time between the SBR reactors. Sludge wasting is also performed during this phase. 

OPERATING PARAMETERS IN SBR PROCESS 

The treatment efficiency of SBR depends on the operating parameters such as phase duration, hydraulic retention time (HRT) and organic loading, Sludge retention time (SRT), temperature, mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), dissolved oxygen (DO) concentration and the strength of wastewater. Cycle time: A cycle in SBR comprises of fill, react, settle, decant and idle phase. The total cycle time (tC) is the sum of all these phases.

t_C = t_F + t_R +t_S +t_D + t_I        (4.5.1)

Where, tF is the fill time (h), tR is the react time (h), tS is the settle time (h), t_D is the decant time (h), and tI is the idle time (h). Moreover during the react phase, organic matters, nitrogen or phosphorus removal may be achieved by arresting aerobic, anoxic or anaerobic condition, respectively. Therefore, aerobic, anoxic or anaerobic time can be found in react time (t_R). 

Hence t_R = t_AE + t_AX + t_AN         (4.5.2) 

Where, t_AE is the aerobic react time (h), t_AX is the anoxic react time (h), and t_AN is the anaerobic react time (h). 

Volume exchange ratio (VER) and hydraulic retention time (HRT): Due to filling and decanting phase during a cycle, SBR operate with varying volume. Volume exchange ratio (VER) for a cycle is defined as V_F/V_T, Where, V_F is the filled volume of wastewater and decanted effluent for a cycle and V_T is the total working volume of the reactor . 

HRT for the continuous system is defined as   

(4.5.3)
 Where, Q is the daily waste water flow rate. For SBR systems;  

 (4.5.4) 
  Where, N_C is t he number of cycles per day and defined as: 

 (4.5.5) 
 Therefore, HRT for the SBR systems may be given as: 

 (4.5.6) 

Solid Retention Time (SRT): In biological treatment of wastewater, excess sludge is withdrawn from the reactor to control the sludge age (SRT). SRT determines the time (d) for which the biomass is retained in the reactor. 

 (4.5.7) 
 Where, X is the MLSS in the reactor with full filled (mg/l), X_W is the MLSS in waste stream (mg/l), and V_W is the waste sludge volume (l). 

NITRIFICATION AND DENITRIFICATION 

Nitrogen is the main source of eutrophication. In this regard, the complete oxidation of nitrogen during the treatment is favorable. Biological nitrogen is removed in two stages: aerobic nitrification and anoxic denitrification. In the nitrification process, ammonia (N-NH₄⁺) is oxidized to nitrite (N-NO₂^-) (equation 3.4.8) by autotrophic bacteria called Nitroso-bacteria and generated nitrite is oxidized to nitrate (N-NO₃^-) (equation 3.4.9) by another group of autotrophic bacteria called Nitro-bacteria under aerobic conditions and using oxygen as the electron acceptor. 

The autotrophic bacteria produce energy for their multiplication from the oxidation of inorganic nitrogen compounds, using inorganic carbon as their source of cellular carbon. During the nitrification, alkalinity of wastewater is used which reduces the pH of influent wastewater and required amount of alkalinity to carry out the reaction (equation 3.4.8, 3.4.9) in the CaCO₃ form, can be calculated by the following equation;  

 Biological denitrification involves the biological oxidation of many organic substrates in wastewater treatment using nitrate or nitrite as the electron acceptor under the anoxic condition or limited dissolved oxygen (DO) concentrations and nitrate is degraded to nitric oxide, nitrous oxide, and nitrogen gas [4-6] by following any of the two different routes. One of these routes predominates depending on the dissolved oxygen concentration .

During the denitrification process, pH of influent wastewater increases because of increase of alkalinity. Both heterotrophic and autotrophic bacteria are capable of denitrification. Most of these heterotrophic bacteria are facultative aerobic organisms with the ability to use oxygen as well as nitrate or nitrite, and some can also carry out fermentation in the absence of nitrate or oxygen . 

ADVANTAGES AND DISADVANTAGES OF SBR 

 Advantages 

• Equalization, primary clarification (in most cases), biological treatment, and secondary clarification can be achieved in a single reactor vessel.  
• Operating flexibility and control.  
• Potential capital cost savings by eliminating clarifiers and other equipments. 

Disadvantage

• A higher level of sophistication, (compared to conventional systems), especially for larger systems, of timing units and controls is required. 
• Higher level of maintenance (compared to conventional systems) associated with more sophisticated controls, automated switches and automated valves.
• Potential of discharging floating or settled sludge during the draw or decant phases with some SBR configurations.
• Potential plugging of aeration devices during selected operating cycles, depending on the aeration system used by the manufacturer.