What the CASS Process Is and Why It Exists
The CASS (Cyclic Activated Sludge System) process is an SBR variant that runs fill, react, settle, decant, and idle phases sequentially in a single tank with a pre-react (selector) zone at the head of the basin. For municipal sewage, MLSS is held in the mid-thousands of mg/L and effluent targets of COD ≤50 mg/L, TN ≤15 mg/L, and TP ≤0.5 mg/L per China GB 18918-2002 Grade I-A are achievable when the cycle is time-controlled by the PLC and the decanter weir is levelled to ±2 mm across the full crest.
CASS provides the biological performance of a continuous-flow activated sludge plant without the civil footprint of separate aeration, clarification, and return-sludge compartments. By collapsing the entire reaction train into one basin, the design eliminates the final clarifier and most return-activated-sludge pumping. The pre-react zone — typically 5–10% of the basin volume — receives raw influent first and exposes the mixed liquor to a high food-to-microorganism (F/M) ratio. That environment selects floc-forming bacteria over filamentous organisms, which is the standard lever for suppressing bulking in cyclic systems. A 2021 full-scale study upgraded a Chinese CASS plant to meet GB 18918-2002 Grade I-A and reduced energy consumption by 25% in summer and 16.67% in winter, with an average relative deviation between ASM1 simulation and operating data under 20% (Liu et al., MDPI Processes, 2021-03-15). Engineers can specify a CASS basin where the project requires nutrient removal in a small-to-mid plant and the operator is willing to run a time-based cycle rather than continuous flow.
The Five CASS Phases in Sequence
One complete CASS cycle runs five distinct phases in fixed sequence, and every phase is governed by a PLC timer plus discrete instrument feedback.
- Fill. The influent valve opens and the basin receives raw sewage. The aerator is typically off or running on intermittent demand so the selector zone creates high-F/M contact at the head of the tank. Fill duration is set by influent flow rate and the target mixed-liquor volume at the end of fill — for a 4–6 h cycle on municipal sewage, fill usually runs 1–2 h.
- React. The aerator turns on under DO control, holding a setpoint of 1.5–2.5 mg/L through a PLC loop. Carbon oxidation, nitrification, and (during any anoxic sub-phase) denitrification consume the substrate load. React duration drives the effective MLSS contact time and is the longest phase in a typical cycle.
- Settle. The aerator switches off and the decanter is mechanically locked. With mixing stopped, the sludge blanket forms under quiescent conditions. Minimum settle time is set by the designer's SVI window — a high SVI needs more settle time to reach the same clarity.
- Decant. The decanter valve opens and treated supernatant is drawn off the top through a floating or fixed-weir decanter. The weir must be level to ±2 mm across the full crest; a misaligned weir pulls sludge and destroys effluent clarity (per the CASS process installation and commissioning guide).
- Idle. All outputs rest. The short idle buffer between cycles is also when sludge wasting is triggered to keep SRT and MLSS inside the operating window.
One safety interlock defines the whole cycle: the decanter must not open if the aerator is running, and the aerator must not restart until the decanter is parked and idle time has elapsed. Skipping that check during commissioning is the single most common cause of washed-out sludge on a CASS plant.
Key CASS Design and Operating Parameters
The numbers below represent the engineering envelope a process designer provides to the mechanical and electrical teams.
| Parameter | Typical Range / Value | Notes |
|---|---|---|
| MLSS (municipal sewage) | Mid-thousands of mg/L (e.g. 3,000–5,000) | Set by process designer for the specific load |
| SRT | Held at design value (commonly 10–25 d) | Controlled by WAS phase and cycle count |
| HRT | Equal to one full cycle | 4–6 h typical for municipal; longer for high-strength industrial |
| DO setpoint (react) | 1.5–2.5 mg/L | Combined carbon oxidation + nitrification |
| Decanter type | Floating (free-movement + tensioned stop cables) or fixed-weir (level survey ±2 mm) | Weir tolerance is non-negotiable |
| Cycle time | 4–6 h typical | Lengthen for industrial / high-strength loads |
| SVI target | Healthy well-settled band (typically <150 mL/g) | Rising SVI is the first warning of bulking |
| Selector zone volume | ~5–10% of basin | Drives floc-former selection |
Effluent target on a well-tuned municipal CASS plant: COD ≤50 mg/L, TN ≤15 mg/L, TP ≤0.5 mg/L, against China GB 18918-2002 Grade I-A. SVI is the parameter an operator watches daily — a creeping SVI is the early indicator that bulking will bleed into the decant and break effluent clarity (per the CASS process installation and commissioning guide).
How the Cycle Is Controlled in 2026
CASS plants fail differently than continuous-flow plants. Because decant is batch-time-controlled, any PLC logic error, level-switch miscalibration, or decanter misalignment breaks the cycle outright instead of degrading effluent slowly. The 2026 control package is built around a small, disciplined instrument list rather than a sprawling SCADA network.
The standard instrument package includes a DO probe (air-calibrated in saturated clean water), an MLSS probe (zeroed in clean water and verified in mixed liquor), level switches or an ultrasonic sensor for the decant trigger, and an influent flow meter. Electrical pre-commissioning starts with a 500 V DC megger test on every motor feeder, phase-rotation verification on each pump and blower, an earth-continuity check, and VFD parameter loading matched against the motor nameplate. The PLC logic is then walked phase by phase in manual mode — fill, react, settle, decant, idle — and every I/O is signed off on a loop-check sheet by the commissioning engineer and the client's instrument technician.
For nutrient removal stages, a packaged chemical-dosing skid is loop-checked into the same PLC so the dosing interlocks ride on the same react-phase timer. That keeps coagulant or carbon-source dosing synchronized with the biological phase instead of running on a separate timer that can drift against the cycle. The full sequence and loop-check sheet format is documented in the CASS process installation and commissioning guide for 2026 plant handovers.
CASS vs SBR vs MBR: How to Choose
Engineers evaluate the CASS process after considering conventional SBR or MBR options based on settleability risk, footprint, and effluent polish.
| Criterion | CASS | Conventional SBR | Submerged MBR |
|---|---|---|---|
| Reactor count | Single basin with selector zone | Single basin, no selector | Biological tank + membrane tank |
| Sludge settleability | Selector zone resists bulking | Vulnerable to bulking | Membrane decouples clarity from settleability |
| Effluent TSS / turbidity | Decanter-limited, typically <30 mg/L TSS | Decanter-limited, similar | <1 NTU typical; near-reuse quality |
| Footprint | Small (no separate clarifier) | Small | Larger (membrane skid + tankage) |
| Mechanical complexity | Low–moderate (no RAS pumping) | Low | High (membrane cleaning, aeration scour) |
| Energy use | Lower than cross-flow MBR; case-study 25% summer / 16.67% winter reduction (MDPI 2021) | Comparable to CASS | Higher due to membrane air scour |
| Best fit | Small-to-mid municipal, packaged industrial | Variable loads with operator oversight | Reuse-quality effluent, tight footprint |
The CASS selector zone is the engineering answer to the chronic SBR weakness of bulking under variable F/M. Against continuous-flow ASP, CASS removes the separate final clarifier and most return-sludge pumping, at the cost of more cycle-control discipline. Against an MBR, CASS has lower membrane cost and lower energy, but cannot reach the <1 μm filtration quality a submerged MBR module delivers. For projects where the next step is reuse-grade polishing, a downstream integrated MBR membrane bioreactor system is the typical pairing; for projects where settleability is the dominant risk and polishing is secondary, CASS alone is the right answer. Plants integrating CASS upstream of biological processes with biofilm carriers can also review the IFAS for aquaculture wastewater 2026 engineering guide for an adjacent design case, and an MBR-specific design context is available in the MBR systems engineering guide.
Frequently Asked Questions
What is the difference between CASS and a conventional SBR?
CASS adds a pre-react selector zone at the head of the basin — typically 5–10% of the volume — that subjects raw influent to a high F/M gradient. That gradient selects floc-forming bacteria over filamentous organisms, which is the standard mechanism for resisting bulking in cyclic systems. A conventional SBR runs the full basin as a single mixed reactor and is more vulnerable to settleability excursions when the influent load swings.
What effluent quality can a CASS plant achieve?
A well-designed municipal CASS plant can meet China GB 18918-2002 Grade I-A, i.e. COD ≤50 mg/L, TN ≤15 mg/L, and TP ≤0.5 mg/L. A 2021 full-scale Chinese upgrade study (Liu et al., MDPI Processes, 2021-03-15) confirmed those targets using an ASM1 + CFD model-based redesign, and reported a 25% summer / 16.67% winter energy reduction on the same plant.
How long does a CASS cycle take?
Cycle time is design-specific, but municipal plants typically run a 4–6 h cycle: 1–2 h fill, 1.5–3 h react, 0.5–1 h settle, 0.5–1 h decant, and a short idle buffer. High-strength industrial loads need longer react and settle phases, which pushes total cycle time toward 8 h or more. The cycle is the HRT unit, so lengthening the cycle directly extends hydraulic residence time.
What is the most common CASS failure mode?
A decanter that is not level to ±2 mm pulls settled sludge during the decant phase and destroys effluent clarity; an unverified PLC interlock that lets the aerator restart before the decanter is parked washes out the sludge blanket. Both are caught by clean-water commissioning in 2026 if the megger test, decanter level survey, and phase-by-phase PLC walk-through are all signed off before live sewage enters the basin.
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