The Sequencing Batch Reactor (SBR) process has been extensively used in Europe and the United States in the past two decades. Its use in Canada has been limited to date, although within the last three years approximately seven treatment plants using this technology were constructed in Nova Scotia. One of the obstacles in the acceptability of SBR process has traditionally been the need for precise, automated and reliable control of various stages of the process. Recent developments in the programmable logic controller (PLC) technology, however, have made the control of an SBR process readily achievable.
The SBR process is an activated sludge process in which the sewage is introduced into a Reaction Tank (or SBR Tank), one batch at a time. Wastewater treatment is achieved by a timed sequence of operations which occur in the same SBR Tank, consisting of filling, reaction (aeration), settling, decanting, idling, and sludge wasting. The various stages in the sequence are as follows:
Stage 1: Filling
During this stage the SBR Tank is filled with the influent wastewater. In order to maintain suitable F/M (food to microorganism) ratios, the wastewater should be admitted into the tank in a rapid, controlled manner. This method functions similarly to a selector, which encourages the growth of certain microorganisms with better settling characteristics.
Stage 2: Reaction
This stage involves the utilization of biochemical oxygen demand (BOD) and ammonia
nitrogen, where applicable, by microorganisms. The length of the aeration period and the sludge mass determines the degree of treatment. The length of the aeration period depends on the strength of the wastewater and the degree of nitrification (conversion of the ammonia to a less toxic form of nitrate or nitrite) provided for in the treatment.
Stage 3: Settling
During this stage, aeration is stopped and the sludge settles leaving clear, treated effluent above the sludge blanket. Duration for settling varies from 45 to 60 minutes depending on the number of cycles per day.
Stage 4: Decanting
At this stage of the process effluent is removed from the tank through the decanter, without disturbing the settled sludge.
Stage 5: Idling
The SBR Tank waits idle until it is time to commence a new cycle with the filling stage.
Stage 6: Sludge Wasting
Excess activated sludge is wasted periodically during the SBR operation. As with any activated sludge treatment process, sludge wasting is the main control of the effluent quality and microorganism population size. This is how the operator exerts control over the effluent quality by adjusting the mixed liquor suspended solids (MLSS) concentration and the Mean Cell Residence Time (MCRT).
In this process, the SBR Tank acts as the equivalent of several components in the conventional
activated sludge treatment process, as follows:
1. Aeration Tank: the SBR Tank acts as an aeration tank during the reaction stage where the
activated sludge is mixed with the influent under aerated conditions.
2. Secondary Clarifier: the SBR Tank acts as a secondary clarifier during the settling and
decanting stages where the mixed liquor is allowed to settle under quiescent conditions,
and the overflow is discharged to the next stage of treatment.
3. Sludge Return System: the activated sludge, following settling in the SBR Tank, is
mixed with the influent similar to the sludge return system, except that the feed is transferred to the sludge rather than the sludge being transferred to the front end of the