Sequencing Batch Reactor Design: Cycles, Volume and SRT

What is a sequencing batch reactor and how does the cycle work?

A sequencing batch reactor (SBR) is an activated-sludge process in which reaction and clarification happen in the same tank, separated in time rather than in space. One tank replaces the aeration basin and secondary clarifier of a continuous-flow plant. Each batch cycle runs through five phases:

• Fill — raw or settled wastewater is admitted to the settled sludge blanket left from the previous cycle. Fill can be static, mixed (anoxic) or aerated depending on the removal targets.
• React — aeration and/or mixing drives substrate oxidation, nitrification and, when phased, denitrification and biological phosphorus release/uptake.
• Settle — aeration stops and the biomass flocculates and settles as a quiescent batch clarifier, forming a distinct sludge blanket.
• Decant — clarified supernatant is drawn off from above the blanket by a moving or floating decanter.
• Idle — a buffer phase between decant and the next fill; waste-activated sludge (WAS) is usually removed here.

Because the reactor is fed-batch, the biology sees a genuinely unsteady environment: concentrations of substrate, ammonia, nitrate and dissolved oxygen all change through the cycle. That transient is what makes SBR design distinct from steady-state continuous design. It is a close cousin of the continuous activated sludge process, but with time-sequenced rather than spatially separated unit operations.

How do you set cycle time and the number of cycles per day?

The cycle time t_c is the sum of the phase durations, and the number of cycles per day per basin is simply 24/t_c. Municipal SBRs commonly run 4–6 cycles per day (cycle times of 4–6 h), split roughly into fill/react, a fixed 0.5–1.0 h settle and a 0.5–1.0 h decant.

Shorter cycles process more volume per day but leave less react time for nitrification, which is slow. Longer cycles give ammonia and nitrate more time to convert but need more working volume for the same daily flow. The settle and decant phases are effectively dead time for reaction, so a design that spends 40% of every cycle settling and decanting has only 60% of its clock doing biological work — a key efficiency trade-off against continuous systems.

What is the volumetric exchange ratio (VER)?

The volumetric exchange ratio is the fraction of the basin volume that is decanted and refilled each cycle. It ties the fill/decant volume to the total (full) basin volume:

VER is bounded at both ends. Too high a VER (deep decant) risks drawing the sludge blanket down toward the decanter and washing solids out; too low a VER wastes tank volume and dilutes the load the biomass sees. The retained fraction (1−VER) also carries seed biomass and, importantly, the settled blanket that the next fill enters — which is what lets the fill phase act as a selector or an anoxic zone.

What kinetics govern the reaction phase?

Within a batch, substrate removal follows unsteady Monod kinetics. Unlike a CSTR at steady state, both biomass X and substrate S evolve in time, so the design is really an integration of coupled ODEs over the react phase:

The practical consequence: early in the react phase the reactor is substrate-rich and removal is fast (zero-order), but the last few mg/L of BOD and the ammonia polishing occur in the first-order tail, which dominates the required react time. Nitrification adds a second, slower Monod term for autotrophs (low μ_max, strong temperature and DO dependence), so cold-weather nitrification usually sets the react duration. The batch nature is an advantage here — the high initial substrate gradient acts as a built-in selector that discourages filamentous bulking.

How does phasing achieve nitrogen and phosphorus removal?

The great strength of the SBR is that a single tank can be driven through anoxic, aerobic and anaerobic conditions in sequence simply by switching aeration and mixing on and off — no internal recycle pumps or separate zones are needed.

• Nitrification (aerobic): NH₄⁺ → NO₃⁻ during aerated react, requiring DO ≥ ~2 mg/L and adequate SRT.
• Denitrification (anoxic): with mixing but no air, heterotrophs reduce NO₃⁻ to N₂ using influent carbon as electron donor. An anoxic fill or an anoxic react sub-phase recovers alkalinity and removes total nitrogen.
• Biological P removal (bio-P): an anaerobic fill (no DO, no NO₃⁻) lets phosphorus-accumulating organisms release P and store VFAs, then take up P in excess during the following aerobic phase. The luxury P uptake leaves the tank in the wasted sludge.

Sequencing gives fine control: a designer can, for example, run anoxic fill → aerobic react → brief anoxic polish → short re-aeration → settle, tuning each sub-phase to hit a total-N or total-P consent. Achieving the same on a continuous plant needs multiple compartments and recycle streams. For membrane-based nutrient plants, an MBR or MBBR configuration is the usual alternative; see also MBR vs MBBR.

Worked example: sizing an SBR basin from flow, cycles and VER

Size the basins for a municipal works with an average dry-weather flow of 4,800 m³/d. Choose n = 6 cycles/day per basin, a volumetric exchange ratio VER = 0.30, and provide two identical basins operating in parallel and out of phase.

• Flow per basin: with two basins sharing flow equally, Q_basin = 4,800 / 2 = 2,400 m³/d.
• Decant volume per cycle: V_D = Q_basin / n = 2,400 / 6 = 400 m³ per cycle.
• Full basin volume from VER: V_T = V_D / VER = 400 / 0.30 = 1,333 m³ per basin.
• Settled (retained) volume: V_min = (1−VER) · V_T = 0.70 × 1,333 = 933 m³, holding the sludge blanket between cycles.
• Decant depth: for a 1,333 m³ basin at, say, 400 m² plan area (5.0 m side water depth), the decant draws V_D/A = 400 / 400 = 1.0 m off the top — comfortably above a blanket sitting at ~2.3 m of settled depth.

So each basin is about 1,333 m³ (two provide ~2,667 m³ of working volume). Check the SRT next: if the design MLSS is 3,500 mg/L, the aerobic mass of solids and the daily WAS rate must give an SRT long enough (typically 15–25 d for nitrification at UK temperatures). If not, either lengthen the react phase, raise MLSS, or add basin volume. Peak wet-weather flow is handled by shortening the cycle (raising n) and/or bringing the second basin fully online — the multi-basin arrangement is what lets an SBR accept peaks without a separate balancing tank.

How is SRT controlled and how do multiple basins handle peak flow?

Solids retention time is controlled by sludge wastage, exactly as in continuous activated sludge, but the wasting is done as a batch draw — usually during the react or idle phase when the tank is fully mixed, so the wasted stream is at the reactor MLSS.

Because the tank is mixed at wastage, the SBR avoids the thickened-underflow bookkeeping of a clarifier — the wasted concentration is simply the MLSS. Peak flows are absorbed three ways: (1) increasing the number of cycles per day (shorter cycles, larger V_D throughput), (2) raising the VER within the blanket-stability limit, and (3) staggering multiple basins so that while one is settling/decanting another is filling, giving the plant a continuous inlet even though each tank is batch. Storm design typically fixes settle and decant times and flexes only the react/fill split.

SBR vs continuous activated sludge vs MBBR

The table compares the three common suspended/attached-growth options on the parameters that drive capital and operating cost.

For industrial effluents with fluctuating strength, the SBR's ability to ride out shock loads and hold sludge in-tank is often decisive; MCBA can help scope the right biological wastewater treatment process against your flow and consent data.

Sequencing batch reactor design sequence

1. Fix flows and loads. Establish average and peak flow and the BOD/COD, ammonia and phosphorus loads. Decide the number of parallel basins so one can fill while another settles.
2. Choose cycles per day and cycle structure. Select n (typically 4–6/day) and allocate fill, react, settle (~0.5–1 h), decant (~0.5–1 h) and idle. Reserve enough react time for cold-weather nitrification.
3. Set the volumetric exchange ratio. Pick VER = 0.20–0.40 so the decant stays clear of the sludge blanket. Compute decant volume V_D = Q/(n·basins).
4. Size the basin volume. V_T = V_D / VER. Set plan area and side water depth so decant depth (V_D/A) sits above the settled blanket.
5. Design the phasing for N and P. Insert anoxic fill/react for denitrification and, if required, anaerobic fill for bio-P. Verify DO and alkalinity through the cycle.
6. Set SRT by wastage. Choose MLSS and waste sludge from the mixed tank to hold a nitrifying SRT (~15–25 d in temperate climates). Confirm aeration and blower duty for the peak oxygen demand within the cycle.