CASS Process Design Guide 2026: Reactor Sizing, Cycle Timing & Selector Ratios

What a CASS Reactor Is and Why It Is Designed This Way

A CASS (Cyclic Activated Sludge System) reactor is a variable-volume sequencing batch process divided by baffle walls into a Selector zone (≈5% of volume), Secondary Aeration zone (≈10%), and Main Aeration zone (≈85%), with continuous sludge recycle from the main zone back to the selector. Each cycle runs Fill-Aeration, Fill-Settle, Decant and Idle so that biological reaction, solids separation and effluent withdrawal all occur in a single basin. The selector geometry is what differentiates CASS from a generic SBR and is the key control point for nitrification, denitrification, biological phosphorus removal and filamentous bulking control (per the DEU CASS reference, web.deu.edu.tr/atiksu/ana58/cass.html).

The SBR concept was already well established by the late 1970s, with most installations classified as small or medium scale. A 1978 modification — adding a pre-react (selector) zone to control filamentous sludge bulking — was the breakthrough that made the modern CASS configuration possible. CASS has since been applied at flows up to 50 MGD (≈400,000 population equivalent), breaking the historical SBR ceiling and extending the technology to large municipal works and retrofits (per S1, DEU CASS reference).

The design intent is straightforward: a single basin performs biological reaction, solids-liquid separation, and effluent removal, while alternating aerobic, anoxic, and anaerobic conditions deliver BOD removal, nitrification-denitrification and biological phosphorus removal simultaneously. For the engineer writing a P&ID legend or basis-of-design narrative, the 40-second definition above is the version to quote.

Influent and Effluent Data the Designer Must Collect First

Before any sizing math, the designer must lock down the influent envelope; every downstream parameter — F/M, SRT, basin volume, decanter flow — is set by the numbers in this list. Minimum inputs are: average dry weather flow (ADWF), peak wet weather flow (PWWF), peak factor (PWWF/ADWF), BOD5, COD, TSS, VSS, TKN, NH3-N, total P, alkalinity, influent temperature (winter minimum and summer maximum), and the target effluent BOD, TSS, NH3-N, TN, and TP. For industrial projects, add pH, salinity, and any inhibitory compounds (phenols, cyanides, solvents) that depress nitrification rates.

The UASB-CASS brewery case is a useful industrial benchmark (per S3, IEEE doi:10.1109/CDCIEM.2011.521): influent COD 2,500 mg/L, BOD5 1,100 mg/L, SS 300 mg/L. Industrial-specific watch items: wide temperature swings that shift nitrification kinetics, low C:N:P ratio that limits biological phosphorus removal, and inhibitory compounds that force a side-stream equalization tank ahead of the basin.

Default effluent design envelope for combined carbon oxidation + nitrification: BOD ≤20 mg/L, TSS ≤30 mg/L, NH3-N ≤5 mg/L. Treat these as a starting target; local discharge permits may be tighter or looser.

Three-Zone Geometry: Converting the 5 / 10 / 85 Split into Real Volumes

For typical domestic wastewater the CASS basin is split Selector 5% / Secondary Aeration 10% / Main Aeration 85% (per S1, DEU CASS reference). The split is not arbitrary — the selector must be small enough to maintain a high local F/M (typically 3–5 kg BOD/kg MLSS·d) that floc-loads soluble substrate and suppresses filamentous growth, and the main aeration zone must be large enough to absorb the diurnal load swing and provide plug-flow-to-complete-mix transition for BOD oxidation and nitrification.

The sizing logic: total basin volume is set by the Main Aeration HRT, typically 6–10 hours at average flow. The selector and secondary aeration zones are then sized at 5% and 10% of the main aeration volume to preserve the plug-flow initial reaction conditions. Worked example for Q = 5,000 m³/d at HRT = 8 h: main aeration volume = 5,000 × 8/24 = 1,667 m³, selector ≈83 m³, secondary aeration ≈167 m³, total basin ≈1,917 m³ per basin.

Number of basins: minimum two for continuous inflow, since each basin must accept flow during every phase of the cycle including Fill-Settle. Most full-scale CASS plants are dual-basin; N+1 redundancy is used for peak-flow or High Flow Cycle operation as at the Portage-Catawba Island plant (1.34 MGD average / 3.8 MGD peak, per S5 Ottawa County). Design sidewater depth: 4–6 m is typical; deeper basins reduce footprint but require stronger aeration mixing and longer settle times because the sludge blanket has farther to fall.

Cycle Timing: Designing Fill-Aeration, Fill-Settle, Decant and Idle

A CASS cycle is a time-based sequence of four phases — Fill-Aeration (air on, biological reaction), Fill-Settle (air off, quiescent settling), Decant (effluent withdrawal via moving weir), and Idle (residual time, optional mix or react) — per S1 (DEU CASS reference). A typical 4-hour municipal cycle allocates roughly: 120 min Fill-Aeration, 45–60 min Fill-Settle, 30–45 min Decant, and 15–30 min Idle. The exact split is tuned to influent F/M and temperature; at lower temperatures the FILL-AERATION phase lengthens to compensate for slower nitrification kinetics.

During Fill-Settle, incoming flow is routed to the second basin in a dual-basin configuration, or to a pump well in a single-basin configuration. The weir trough of the moving-weir decanter sits above top water level during aeration and settling so that no mixed liquor suspended solids can be drawn off accidentally. Only during the Decant phase does the weir descend to draw treated supernatant (per S1).

Decanter flow rate calculation: required decanter flow = peak hourly flow × (cycle time / decant time). For the 5,000 m³/d example with Qpeak = 2.5 × Qavg = 12,500 m³/d, peak hourly flow ≈521 m³/h, and a 4-h cycle with 45 min Decant phase, the decanter must deliver 521 × (240/45) ≈ 2,778 m³/h. The High Flow Cycle option at peak wet weather shortens the cycle and lengthens the decant window so a single basin passes 2–3× the average flow without solids washout — the operating mode at Portage-Catawba Island (1.34 MGD average / 3.8 MGD peak, per S5).

MLSS, F/M, SRT and Aeration Intensity: Setting the Operating Targets

Design MLSS in the main aeration zone runs 3,500–5,000 mg/L. During the Fill-Settle phase the sludge blanket forms at the top water level concentration of ≈3,500 mg/L and settles to ≈10,000 mg/L by the end of the settle period — the high-concentration waste sludge stream that gets wasted from the bottom of the basin (per S1, DEU CASS reference). Operating above ≈5,000 mg/L risks oxygen-transfer limitations and poor settling; operating below 3,000 mg/L starves the selector of return sludge and degrades BOD removal.

Target F/M ratio 0.05–0.15 kg BOD/kg MLSS·d for combined carbon oxidation and nitrification in CASS — the extended-aeration end of the activated-sludge spectrum. Target SRT 10–25 days at 15–25 °C for combined nitrification-denitrification; the winter design temperature usually sets the upper bound because colder mixed liquor requires longer SRT to hold nitrifier inventory. Aeration intensity: design for ≈1.5–2.0 kg O2/kg BOD applied when nitrification is required; convert standard oxygen demand to standard air (Nm³/h) with a typical transfer efficiency of 20–25% for fine-bubble diffusers at 4–5 m submergence.

The CASS Respiration Rate Control (RRC) logic throttles blowers to actual oxygen demand using an in-basin DO probe as a respirometer — low load lowers air supply, high load raises it, so DO stays in the 1.5–2.5 mg/L band typical for combined nutrient removal (per S1). The reason to avoid over-aeration is specific to CASS: excessive DO carryover into the selector inhibits biological P uptake and defeats the anoxic/anaerobic contact that suppresses filaments.

Selector Tuning and Filamentous Bulking Control

The selector exists for one reason: to suppress filamentous bacteria before they reach the main aeration zone. The mechanism is substrate concentration gradient. At the selector inlet, the local F/M is high — typically 3–5 kg BOD/kg MLSS·d — so floc-formers (which can store soluble substrate as intracellular polyhydroxyalkanoates) out-compete filaments (which cannot), and the sludge exits the selector with good settling properties (per S1, DEU CASS reference). If selector F/M is too low, filaments dominate and the sludge volume index (SVI) climbs above 150 mL/g — the visible signature of bulking.

Rule of thumb for selector HRT: keep it short, ≈5–15 min at average flow, so that readily degradable substrate is removed before the mixed liquor reaches the main aeration zone. The internal sludge recycle from Main Aeration back to Selector should cycle the main-zone biomass through the selector about once per day — the design intent stated directly in the DEU reference ("the sludge return rate causes an approximate daily cycling of biomass in the main aeration zone through the selector zone," per S1).

If bulking appears in operation, the first checks are: (1) selector F/M — confirm return sludge rate is delivering the targeted daily cycling; (2) selector DO — confirm the selector is operating under anoxic/anaerobic conditions during non-aerated periods; (3) F:M ratio at the basin level — F/M below 0.05 in the main zone encourages filaments throughout. A 20% bump in return sludge rate, or a temporary switch to fully aerobic selector operation, is the typical operator response.

Industrial CASS: When to Use It and How Loads Change the Design

CASS is the right pick when the wastewater has high BOD/COD, moderate temperature variability, and a need for biological nutrient removal in a small footprint. The UASB-CASS brewery case demonstrates the envelope (per S3, IEEE doi:10.1109/CDCIEM.2011.521): COD 2,500 → 75 mg/L (97% removal), BOD5 1,100 → 16.5 mg/L (98.5% removal), SS 300 → 9 mg/L (97% removal) at steady state, meeting the Chinese GB8978-96 integrated discharge standard. The UASB front end strips the high organic load anaerobically; the CASS polishes and nitrifies. For pure industrial streams without the UASB buffer, the CASS basin alone has to absorb the full BOD load, so the main aeration HRT moves toward the 10-hour end of the range and the F/M check at start-up is critical.

For very tight effluent targets — TSS <10 mg/L, reuse-grade polish — add an MBR membrane bioreactor system for reuse-grade polish downstream of the CASS basin; the MBR is a downstream polish, not a CASS replacement, and the combination gives both biological nutrient removal and an absolute solids barrier. Where the question is retrofitting an existing SBR or CASS basin with a membrane cassette, the relevant MBR retrofit of an existing SBR or CASS basin workflow covers tank-conversion criteria, flux targets, and aeration-demand changes. For a head-to-head with conventional activated sludge on a heavy-metal-bearing industrial stream, the MBR vs conventional activated sludge comparison gives the side-by-side capex/footprint/energy numbers. For a packaged or buried alternative at low flow, see the SBR design guide 2026 for the parameters the engineer would feed into that decision. At the headworks, a rotary mechanical bar screen at CASS headworks protects the selector and decanter from rags and debris, and an automatic chemical dosing system for nutrient polishing is typically specified for alum or metal-salt addition when biological phosphorus removal alone cannot meet the TP target.

Frequently Asked Questions

What is the standard CASS selector to main aeration ratio?

The standard CASS zone split for typical domestic wastewater is Selector 5% / Secondary Aeration 10% / Main Aeration 85%, with continuous sludge recycle from the main zone back to the selector (per S1, DEU CASS reference).

How many basins does a CASS plant need?

A minimum of two basins is required for continuous inflow, since each basin must accept flow during every phase of the cycle including Fill-Settle. Dual-basin is the most common layout, with N+1 redundancy used for peak-flow operation as at the Portage-Catawba Island plant (per S5).

What design MLSS should be used for a CASS basin?

Design MLSS in the main aeration zone runs 3,500–5,000 mg/L. The settled sludge blanket at the end of the Fill-Settle phase reaches approximately 10,000 mg/L, which is also the typical waste sludge concentration drawn from the bottom of the basin (per S1).

Can CASS handle industrial wastewater?

Yes. CASS treats both municipal sewage and a wide range of industrial wastewaters. The UASB-CASS brewery case achieved 97% COD removal, 98.5% BOD5 removal, and 97% SS removal on a stream running 2,500 mg/L COD, 1,100 mg/L BOD5, and 300 mg/L SS (per S3, IEEE doi:10.1109/CDCIEM.2011.521).

What is a High Flow Cycle in a CASS plant?

A High Flow Cycle is a shortened operating mode used at peak wet weather flows that lets a single basin pass 2–3× the average flow without solids washout. The Portage-Catawba Island plant is designed for 1.34 MGD average and 3.8 MGD peak, roughly 2.8× the average, and switches to High Flow Cycle as inflows approach peak (per S5, Ottawa County).

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