A comparison of SBR versus MBBR/IFAS
|Type of biological process
|Combination of both suspended growth and fixed film /attached growth process
|Mode of Operation
|Continuous mode of operation
|Treatment Configuration Feasible
| Sample Example of SBR volumes: 1 MLD If total cycle time is 4 hours and fill cycle is 1 hour which means fill volume would be average flow of 41.66 m3/hr.
No. of SBR basins , n = Total Cycle time / Fill Cycle time
N = 4/1 = 4. For C-tech designs, they practice Fill Volume to total volume ratio of 24% appx. Thus, volume of each reactor = 41.66 / 0.24 = 173.58 m3 And thus total SBR plant volume would be 694 m3 minimum which corresponds to 17 hours of liquid retention time at average flow rate
|An IFAS process with Bio-P removal, Anoxic Tank, Aerobic Reactor , Clarifier would be of 1.5 + 1 + 4.5 + 2 = 9 hours of liquid retention time at average flow rates which can be further optimized with different engineering combinations.
|Scalability or Modularity
|New Designs can be modularised based on the requirement
|Can also be modularised
|Shock load susceptibility
|Offers same susceptibility to shock loads as conventional activated sludge due to being suspended growth reactor. However, temporal shock loads can be contained within a single reactor or two which is receiving the shock loads and thus other reactors are prevented.
|Due to enmeshment of biomass on carrier materials within EPS matrix which acts as a glue for the microbes on carrier and also as a protective mechanism to prevent them against toxic shock loads, a Hybrid MBBR/IFAS process offers better protection against toxic shock loads and also under high hydraulic loading conditions due to presence of significant amount of biomass on carriers/media , a wash out of microbes and eventually performance can be prevented.
|Settling Properties of Biomass
|Due to Feast-Famine configuration and associated kinetic selectors configuration practiced in SBRs, sludge/biomass has better settling properties compared to conventional activated sludge. However, still under stress conditions, the biomass has susceptibility of the same bulking, foaming and filamentous growth problems associated with CAS and there are numerous examples of SBR failures due to bulking, filamentous growth world wide.
|Hybrid process configuration minimizes many of the causes related to bulking and filamentous growth due to association of poor floc formers within EPS matrix of the biofilms especially Nitrifiers which have poor floc forming ability and thus settling properties of the sludge are improved a lot. Also due to a lot of biomass remain on carriers within the reactor it self , for the same MCRT, the solids loading to clarifier is lower compared to activated sludge and thus clarifier performance improvement is feasible and also better settling properties due floc formation of detached biomass from the EPS glued biomass.
|Feast –Famine Conditions
|It is one of the unique feature of SBR due to its cyclic mode of operation in which during fill cycle high F/M ratio or Feast condition is created which improves the floc formation while during the end of cycle a lack of substrate creates a Famine conditions which is called starvation phase and thus develop a specific type of biomass which has high adsorbent capacity and thus floc formation ability which settles well.
|In a Hybrid Reactor with MBBR media, MLSS is maintained in the suspended phase at higher F/M ratios which replicates the Feast conditions of the SBR while on the carrier material due to biofilm thickness and substrate depletion in the suspended phase, a Famine condition exist which thus help to develop or achieve the same floc formation ability and settling properties as comparable to SBRs.
|Simultaneous nitrification and Denitrification
|Due to ON/OFF and OUR /DO based control of the reactor, SNDN is feasible in SBR
|Due to microbes growing on the biofilm which experiences a substrate gradient of low DO and BOD conditions, Simultaneous nitrification , denitrification is also observed in MBBR/IFAS process even at higher DO concentration in bulk liquid . Also outer layers of biofilms are nitrifying while inner layers are denitrifying in MBBR/IFAS process.
|Nitrate and Sludge Recycle
|Conceptually no active RAS is required for SBR but the total SBR active volume to Fill Volume (Vt/Vf) ratio is equivalent to RAS flow. Thus by providing larger volumes to retain sludge within the reactor, the same theoretical effect of RAS is achieved with SBR. Anoxic fill cycles in same SBR basins is seldom used in practiced due to very high amount of mixing energy required to mix the influent with the sludge/biomass within the reactor and thus separate Anoxic selectors are provided in most of the SBRs and a part of the sludge is also recycled to provide Nitrates during Anoxic fill in the Selector. Due to the higher Vt/Vf ratio designs practiced, the amount of nitrate carry over and thus recycle flow to selectors are lower.
|Theoretically Nitrate Recycle and RAS required for MBBR/IFAS hybrid systems. However, there are process configurations like Step Feed Process in which Alternate trains of Anoxic-Aerobic reactors are incorporated, can completely eliminate the nitrate recycle flow requirement and thus reduce the associated operational costs.
|Solids Separation and sludge Inventory
|Due to ideal settling conditions during settling phase of SBR, solids separation can be improved compared to CAS for SBRs. But depletion of DO at the end of the settling and decanting phase and associated Anoxic/anaerobic conditions can cause severe secondary P release and rising bubble phenomenon due to denitrification occurring during settling phase. Each SBR basin requires separate sludge wasting pump/mechanism and monitoring to maintain desired level of MLSS and MCRT within the reactor which increases amount of monitoring lab work required.
|Due to separation of each unit operation and processes, the solids separation and sludge management is much easier to control for a specific process train with MBBR/IFAS technology. With effective clarifier operation and maintaining right amount of DO concentration in the effluent leaving the aerobic reactor , the secondary release of P and rising bubble Phenomenon due to Denitrification and thus associated TSS carryover in the treated effluent can be avoided.
|Upgradation of Existing CAS facilities
|Incorporating SBR for the upgradation of existing CAS facilities would be much difficult due to lot of change required in flow regime and process control.
|Within the same process flow sheet by simply adding MBBR media/carriers in the Aerobic Reactor , existing CAS process can be upgraded for higher flow rates or for BNR without requiring much changes in the plant. Also same clarifiers can be utilised due to reduction in solids loading to the clarifier as majority of the biomass is retained on the carrier material.
|Sludge Production and Dewatering Properties
|Sludge production is in the range of 0.477 kg.VSS/Kg.COD removed and based on operational cycles, dewatering properties are good for SBR based plants
|Same or better dewatering properties can be achieved with IFAS due to hybrid process configuration.
|Ease of Operation
|Automation is a must due to cyclic operation and for distribution of flows between different SBR basins. At times in a multi basin SBR plant where two or more reactors are idle at the same time, assigning incoming flow to a particular SBR is very difficult and this issue has caused severe problems in certain plants where some basins are always loaded while other basins are not in use at all and thus the design advantage is lost due to poor biomass present in that specific reactors. Also a specific automation algorithm is required for inflow management which creates issues of its own.
|Automation may or may not be necessary. But can be incorporated in the plant process control. Due to simplicity of the process flow sheet and similarity with conventional activated sludge process, operation and process control is much easier and efficient and also an experienced operator for a CAS can be trained and deployed for the plant operation.
|Further points on Ease of Operation
|Fixed Cycle based SBR operation takes out or nullify the operational flexibility and advantage of automatic SBRs and if not monitored carefully could create various operational issues. Under Varying load conditions, fixed aeration time even with OUR control can result in over or under aeration of the biomass and thus effectively keeps it under stress conditions which eventually loss its resiliency to shock loads.Failure of Decanting Mechanism is added problem in SBRs As mentioned earlier, if DO levels are not maintained very well at the end of Settling and Decanting phase then secondary P release and rising bubble phenomenon due to denitrification during settling phase cause severe problems of higher P concentration in treated effluent and TSS carryover. If filamentous growth and bulking is not identified at early stage and if settling times are not varied according to the sludge properties , TSS carryover may become routine in fixed cycle based SBRs.
|Even with human interface, due to separation of various unit operations and processes, in fact the process operation and control is much easier and efficient as each step can be monitored and rectified individually. This separation of unit processes and operations also allows for better trouble shooting and problem identification associated with them and thus overall plant trouble shooting becomes much easier. While in SBR due to all the processes are taking place in the same reactor in time sequence trouble shooting and problem rectification is a very complex task.
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Amit Christian, having MSc degree in Environmental Science from UK university has more than 20 years of experience in the field of water and wastewater treatment. He has expertise in MBBR and IFAS process design, engineering and process start up /commissioning.