CASS Process Design Parameters: 2026 Engineering Reference

What Is the CASS Process and How Does It Differ from Conventional SBR?

A Cyclic Activated Sludge System (CASS) is a time-based variant of the sequencing batch reactor (SBR) that adds a biological selector upstream of the main aeration basin, repeating a fill-aerate-settle-decant sequence inside a single tank. Over 400 small and mid-scale municipal WWTPs in northern China use the configuration, primarily because it combines configuration flexibility, operational simplicity, low construction and maintenance cost, and simultaneous nitrogen and phosphorus removal (Zhang et al., 2021, MDPI Processes Vol. 9 No. 3, p. 527).

The selector is the element that distinguishes CASS from a conventional SBR. At the Zhangjiakou full-scale plant — 20,000 m³/day domestic capacity, Hebei Province — the pre-denitrification anoxic zone measures 4.3 m × 17.6 m × 5.6 m with an effective volume of 423.8 m³, mixed by two submersible agitators on opposite corners (Zhang et al., 2021). The selector's main job is to control filamentous bulking: by exposing return activated sludge to a high substrate gradient under anoxic conditions, floc-forming organisms outcompete filaments for the readily biodegradable COD, which improves settleability in the subsequent settle phase. That is the mechanism that makes the parameter table below — particularly MLSS and SVI — behave predictably at full scale.

CASS Process Design Parameters — Master Reference Table

Consolidated CASS design parameters for domestic-strength municipal wastewater: a 6-h cycle split into 90-min fill, 180–240-min react, 30–60-min settle, and 30–60-min decant, with an HRT of 20–30 h, SRT of 15–25 d, MLSS of 3,000–5,000 mg/L, and aerobic DO of 2.0–3.0 mg/L. Each row below is anchored to either the Zhangjiakou full-scale plant (20,000 m³/day, Hebei) or the lab-scale reactor operated at 20 °C with synthetic low-carbon-source domestic wastewater.

Note: units in the header are explicit because HRT is reported in hours (h) and SRT in days (d) — a frequent source of misread spec sheets. The values above apply to domestic-strength wastewater; industrial C/N adjustments are covered later.

Influent Characterization: COD, BOD, C/N Ratio and How They Set CASS Loading

Six parameters govern discharge compliance for any CASS plant: TSS, COD, BOD₅, Total-N, Total-P, and heavy metals (Alumichem, 2024). The COD/TKN ratio and BOD₅ loading are the two values that most directly drive reactor sizing, because they set the aeration demand, the selector substrate gradient, and the carbon available for denitrification.

Low-carbon-source domestic wastewater is the common CASS design condition, defined as COD <200 mg/L with COD/N <4 (Sun et al., 2015). For that envelope, the food-to-microorganism (F/M) ratio typically lands in the 0.05–0.20 kg BOD/kg MLSS·d range. Two variables dominate the kinetic parameters that calibrate the F/M calculation: influent COD fractions (S_S, S_I, X_S, X_I) and water temperature (Zhang et al., 2021). High-strength industrial effluent (COD 500–5,000 mg/L) sits outside the master table above and almost always needs equalization or pre-acidification before the CASS basin; the load on the selector and the F/M ratio both change by an order of magnitude, and the cycle has to be lengthened to keep the aeration phase from going oxygen-limited.

Biological Selector and Pre-Denitrification Zone Sizing

The pre-anoxic selector at Zhangjiakou occupies 423.8 m³ of the bioreactor, with two submersible agitators on opposite corners to maintain mixed conditions (Zhang et al., 2021). Selector HRT is typically 1–2 h with a return-activated-sludge (RAS) ratio of 20–50% feeding the anoxic zone — sized to give a high F/M contact under anoxic conditions so floc-formers outcompete filaments. CFD simulation of agitator placement is now standard practice in 2026 selector design; the Zhangjiakou team used CFD to optimize the stirrer layout before recommissioning.

Selector geometry alone does not guarantee TN ≤15 mg/L. The full-scale plant could not meet the GB 18918–2002 Grade 1-A TN limit of 15 mg/L in winter until the operational strategy and kinetic parameters were recalibrated (Zhang et al., 2021). The mechanism that gives CASS its nitrogen flexibility is simultaneous nitrification-denitrification (SND) within the floc: an oxygen gradient inside each floc lets the outer layer nitrify while the inner core denitrifies, and a properly sized selector amplifies the denitrification share. The decay coefficient b_H that controls endogenous denitrification is derived from respirometry: b_H = K_d / [1 − Y_H(1 − f_p)] (Zhang et al., 2021), so any change in Y_H or f_p recalibrates the selector's denitrification capacity.

Aeration Control, DO Setpoints and Cycle Time Distribution

DO setpoints in a CASS reactor are phase-specific: hold the anoxic selector below 0.3 mg/L to keep denitrification active, run the aerobic react phase at 2.0–3.0 mg/L, and let settle and decant fall below 0.5 mg/L so the sludge blanket stays undisturbed. A lab CASS reactor fed with low-carbon-source synthetic domestic wastewater at 20 °C held mean aerobic DO of 1.02, 2.60, 2.87 and 3.44 mg/L across four test conditions (Sun et al., 2015); the practical 2026 compromise between N₂O mitigation and denitrification efficiency sits in the 2.0–3.0 mg/L band.

Continuous feeding during the react phase reduces N₂O emission relative to batch feeding, because the continuous influent relieves electron competition between denitrification reductases during oxygen-limiting intervals (Sun et al., 2015). The cycle must deliver effluent that meets GB 18918–2002 Grade 1-A: TN ≤15 mg/L, COD ≤50 mg/L, NH₃-N ≤5 mg/L (Zhang et al., 2021).

Temperature Correction for CASS Kinetic Parameters

Water temperature is the single most influential variable for CASS design because nearly all ASM1 kinetic parameters — Y_H, b_H, μ_H,max, k_h, K_S, K_O,H — vary with it (Zhang et al., 2021). Calibrating the model at 20 °C and applying the result to a 10 °C winter basin will under-size aeration and SRT.

Respirometry at the Zhangjiakou plant was performed at 10 °C and 20 °C and yielded two distinct sets of dominant parameters. The summer-sensitive set at 20 °C was k_h, μ_H,max, K_S, K_O,H, μ_A,max. The winter-sensitive set at 10 °C was k_h, K_X, K_NH, K_S, b_A, μ_H,max. Different calibrated values for the same parameter between seasons drove the upgrade strategy (Zhang et al., 2021). For 2026 designs in cold regions, 10 °C should be the winter design temperature when sizing aeration capacity, SRT, and selector HRT. An Arrhenius-type correction with a temperature coefficient θ is applied to endogenous decay b_H; the engineering literature commonly uses θ ≈ 1.04–1.07 for heterotrophic decay, and the same approach extends to μ_H,max and k_h.

Adapting CASS Design Parameters for Industrial Wastewater

The Zhangjiakou reference plant treats 20,000 m³/day of domestic wastewater at COD <200 mg/L and COD/N <4 (Zhang et al., 2021). Industrial CASS applications sit in a different envelope: typical influent is COD 500–5,000 mg/L and COD/N 5–20, and slowly biodegradable fractions require a longer SRT (25–40 d) and a longer cycle (8–12 h total). CASS has been adapted to landfill leachate, food-processing wastewater, coking effluent, and pulp-and-paper condensate, and the 2024 cyclic-activated-sludge review documents ongoing extensions to enzymatic and attached-growth configurations (Mosaferi et al., 2025, Sustainable Chemistry for the Environment Vol. 9, art. 100191).

Pairing CASS with a ZSQ series dissolved air flotation system upstream strips fats, oils, grease, and colloidal matter that would otherwise overload the selector and increase effluent TSS. For sites where polishing effluent quality is critical — typically reuse applications or tight Total-P limits — a polishing step such as an MBR membrane bioreactor system or an automatic chemical dosing system for chemical P precipitation can be added downstream.

CASS vs SBR vs AAO: When to Choose Each Reactor

The procurement question is rarely "should I use CASS" — it is "given my flow, influent, footprint, and discharge limits, which reactor family is the right one to size first." A 2026 decision framework for that meeting is below; see also the AAO process working principle 2026 guide and the oxidation ditch design parameters 2026 reference for adjacent reactor families, and the IFAS process flow diagram 2026 guide for hybrid fixed-film alternatives.

CASS advantages are configuration flexibility, operational simplicity, simultaneous N/P removal, and low construction and maintenance cost (Zhang et al., 2021). CASS disadvantages are limited scalability beyond ~50,000 m³/day without parallel trains, sensitivity to sludge settleability, and reliance on chemical polishing for tight TP limits. Choose AAO when biological P removal is required at large flow and the operator can manage 200–400% RAS and 100–300% nitrate recycle.

Frequently Asked Questions

What is the typical CASS cycle time and how should it be split?

A typical CASS cycle is 6 hours, split into 90 minutes of fill, 180–240 minutes of react (aeration), 30–60 minutes of settle, and 30–60 minutes of decant (Zhang et al., 2021). Idle time is added as needed to keep four parallel CASS trains in continuous-feed alignment.

What MLSS range is normal in a CASS reactor?

CASS reactors typically operate at 3,000–5,000 mg/L MLSS. The lab reactor in the Sun et al. (2015) study was held at approximately 3,000 mg/L, and the Zhangjiakou full-scale plant operates in the 3,000–5,000 mg/L band (Zhang et al., 2021).

Can CASS achieve TN ≤15 mg/L in cold climates?

Yes, but the selector and SRT must be sized for 10 °C winter conditions and the kinetic parameters (k_h, K_X, K_NH, K_S, b_A, μ_H,max) must be recalibrated seasonally (Zhang et al., 2021). Designing at 20 °C and operating at 10 °C is the most common cause of TN non-compliance in northern China.

How does continuous feeding differ from batch feeding in CASS?

Continuous feeding during the react phase reduces N₂O emission and improves TN removal for low C/N domestic wastewater, because the continuous carbon supply relieves electron competition between denitrification reductases during oxygen-limiting intervals (Sun et al., 2015).

Is CASS suitable for industrial wastewater?

Yes, with pre-treatment — typically a DAF system to strip FOG and colloids — and an extended SRT of 25–40 days to handle slowly biodegradable COD. The 2024 cyclic-activated-sludge review documents ongoing extensions to enzymatic and attached-growth configurations for industrial use (Mosaferi et al., 2025).