Sequencing Batch Reactor, Simulation Modeling, ASM1, Flow Routing
Many small municipal wastewater treatment plants in North America use sequencing batch reactors (SBR) to provide secondary treatment. SBRs are a fill-and-draw variant of the activated sludge process in which a single reactor is used to mix raw wastewater with mixed liquor, biologically treat the wastewater, gravity-separate the activated sludge from the treated liquid, and then discharge the treated effluent. At least two reactor cells are provided in most plants so that raw wastewater feed may alternate among the reactor cells to provide time in a particular reactor to allow for biological reaction, clarification and decant without raw wastewater feed.
Specifically, each SBR cell typically operates in the following modes:
- Anoxic Fill (screened wastewater enters decanted tank; mixing but no aeration)
- Aerated Fill (wastewater is fed while the contents are being aerated)
- React (tank is aerated while ammonia and BOD are removed)
- Settle (activated sludge settles in tank)
- Decant (secondary effluent is removed from the top of the tank)
- Wasting (excess activated sludge is removed from the system)
- Idle (concentrated activated sludge is aerated, awaiting the next fill mode)
Anaerobic modes may also be added to provide for biological phosphorus removal. Refer to the USEPA Wastewater Technology Fact Sheet (USEPA, 1999) for more background on the SBR process.
In most cases, the process designs for those plants are performed by the SBR vendors. In the author’s experience, the process design calculations performed by many of those vendors are based on conceptual, time-averaged models of the activated sludge process and do not take advantage of modern modeling tools. The end result of this shortfall is the potential undersizing of the oxygen transfer systems of the SBRs and/or an overestimate of the performance.
The use of the activated sludge models developed by the International Water Association (IWA) for evaluation and design of activated sludge process units has become the defacto standard of the wastewater treatment industry. These models have been published by the IWA (International Water Association, 2000) as a series of simultaneous equations, but the IWA has left the development and implementation of computerized solvers of the equations to others. Several commercial software products that solve these equations are currently available and in use by practicing sanitary engineers, however their high cost is a deterrent for many to their use.