CASS Process Retrofit and Upgrade: 2026 Engineering Guide
O&M Services & Cost Optimization
Zhongsheng Engineering Team
What a CASS Process Retrofit Actually Changes
A CASS process retrofit involves targeted modifications within the existing Cyclic Activated Sludge System reactor, including the selector zone, secondary aeration, and main aeration, along with ancillary units such as decanters, recycle pumps, and blowers, avoiding new tankage construction. This approach leverages the inherent flexibility of the CASS design, which typically divides the reactor basin into three sections: a selector zone (approximately 5% of the volume), a secondary aeration zone (around 10%), and a main aeration zone (about 85%) (per CASS™ process documentation [S4]). Sludge biomass is continuously recycled from the main aeration zone to the selector, ensuring biomass cycles through the selector roughly once per day (per CASS™ process documentation [S4]).
Retrofits pull four primary CASS-specific levers to enhance performance:
Selector Reconfiguration: Adjusting hydraulic flow patterns and agitator placement within the selector zone to optimize anaerobic/anoxic conditions and promote floc-forming microorganisms, thereby controlling filamentous bulking (per CASS™ process documentation [S4]).
Cycle-Phase Reprogramming: Fine-tuning the FILL-AERATION, FILL-SETTLE, and DECANT phases to maximize biological reaction times, improve solids separation, and manage nutrient removal efficiency.
Decanter Hydraulics: Upgrading or optimizing decanter operation and utilizing the IDLE phase for additional anoxic/anaerobic reaction time, preventing accidental discharge of mixed liquor suspended solids by keeping the weir above top water level during aeration and settling (per CASS™ process documentation [S4]).
Aeration Control: Implementing Respiration Rate Control (RRC™) or similar DO-based aeration strategies to match oxygen supply with demand, which can significantly reduce energy consumption and improve nutrient removal by optimizing aerobic and anoxic periods (per CASS™ process documentation [S4]).
A full-scale CASS plant in a cold region of China demonstrated that a model-led retrofit drove its effluent to GB 18918-2002 Grade 1A (COD 50, TN 15, TP 0.5 mg/L) while simultaneously cutting energy consumption by 25% in summer and 16.67% in winter (Liu et al., 2021 [S3]). This validates the efficacy of in-place CASS upgrades. in-basin biofilm hybridization, such as integrating MBBR carriers or MABR modules into the main aeration zone, represents a specialized class of CASS retrofit that introduces a fixed-growth biomass population to enhance nitrification and denitrification capacities, often without requiring new civil infrastructure. For more on core treatment distinctions, refer to our guide on primary vs. secondary wastewater treatment.
Why CASS Plants Get Retrofitted in 2026: Drivers and Constraints
The primary driver for CASS plant retrofits in China is the Action Plan for Water Pollution Prevention and Control (2015), which mandates all wastewater treatment plants (WWTPs) meet GB 18918-2002 Grade 1A effluent standards (Liu et al., 2021 [S3]). This translates to stringent limits of COD 50 mg/L, TN 15 mg/L, and TP 0.5 mg/L, often requiring significant upgrades from previous Grade 1B standards (Liu et al., 2021 [S3]).
Beyond regulatory compliance, hydraulic capacity is another critical driver. As of 2017, China's municipal wastewater sector included over 4,000 WWTPs, with 83.3% classified as small to mid-scale (less than 50,000 m³/d capacity), many of which are now operating beyond their original design flow rates (Liu et al., 2021 [S3]). This necessitates increasing treatment capacity within existing footprints.
Energy consumption is a third significant factor. With rising electricity tariffs, aeration has become the dominant operational expenditure (OpEx) line item for most biological treatment plants. Many older CASS plants lack sophisticated dissolved oxygen (DO) control, leading to wasted air during periods of low organic load and contributing to unnecessarily high energy bills (per CASS™ process documentation [S4]). Retrofits incorporating advanced aeration control can directly address this.
For industrial sites, such as petrochemical, food & beverage, and pharmaceutical facilities utilizing CASS, the challenge often involves handling higher-strength influent, increased variability, or more toxic components. In these scenarios, the inherent complete-mix nature of the CASS main reactor offers some tolerance to shock loads (per CASS™ process documentation [S4]); however, biofilm-based retrofits, while highly efficient for nutrient removal, typically require influent equalization as a prerequisite to protect the sensitive fixed-film biomass from toxic spikes (per MABR retrofit guide [S2]).
The CASS Retrofit Diagnostic: Modelling the Existing Reactor
Accurate pre-retrofit diagnostics, beginning with respirometry, are crucial for determining influent COD fractions and key ASM1 parameters like YH and bH, essential for model-based upgrade designs (Liu et al., 2021 [S3]). This foundational data ensures that any proposed modifications are based on the actual biochemical characteristics of the wastewater.
The diagnostic process typically follows these steps:
Respirometry: Conduct respirometric tests at both 10 °C and 20 °C to accurately characterize influent COD fractions and calibrate key Activated Sludge Model No. 1 (ASM1) parameters such as heterotrophic yield (YH) and heterotrophic decay rate (bH). These parameters are critical for simulating biological nutrient removal under varying temperature conditions, particularly in cold regions of China (Liu et al., 2021 [S3]).
ASM1 Simulation and Validation: Develop an ASM1 simulation model of the existing CASS cycle. This model must be rigorously validated against historical and real-time operating data, aiming for an average relative deviation of less than 20% between simulated and actual effluent parameters before any upgrade design is finalized (Liu et al., 2021 [S3]).
CFD Simulation: Perform Computational Fluid Dynamics (CFD) simulations to analyze the submerged agitator layout within both the selector and main aeration zones. This identifies dead zones, short-circuiting, and suboptimal mixing patterns. Agitator repositioning or minor modifications, guided by CFD, can increase the effective reactor volume and improve mass transfer efficiency without costly civil work (Liu et al., 2021 [S3]).
Diffuser and Aeration Audit: Conduct a comprehensive audit of the existing aeration system, evaluating diffuser type (fine-bubble vs. coarse-bubble), material (EPDM vs. silicone), aeration pattern uniformity, and estimated remaining operational lifespan (per aeration retrofit best practice [S5]). This informs decisions on diffuser replacement or upgrades to enhance oxygen transfer efficiency.
The output of this diagnostic phase is a robust set of influent fraction data, a fully validated ASM1 model capable of predicting performance under dynamic loads, a CFD-optimized agitator and mixing plan, and a prioritized, data-backed scope for the CASS process retrofit and upgrade. This systematic approach avoids over-engineering and ensures that capital investments are targeted effectively. For related troubleshooting insights, consider our lime dosing system troubleshooting guide.
Four CASS Retrofit Paths Compared
Evaluating four distinct CASS retrofit paths provides engineers with a structured decision framework, balancing CapEx, effluent quality, and operational impact based on specific plant constraints. The optimal choice depends heavily on the binding constraint—be it footprint, energy cost, specific nutrient limits, or desired effluent reuse quality.
Path A: Pure In-Basin Optimization
This path focuses on reprogramming the CASS cycle, implementing Respiration Rate Control (RRC) for aeration, optimizing selector agitator placement via CFD, and upgrading decanters. It represents the lowest capital expenditure option. A full-scale Chinese CASS retrofit achieved GB 18918-2002 Grade 1A effluent standards while documenting a 25% energy reduction in summer and 16.67% in winter through this approach (Liu et al., 2021 [S3]).
Path B: MBBR Carriers in the Main Aeration Zone
This involves adding Moving Bed Biofilm Reactor (MBBR) carriers to the existing main aeration zone. The carriers provide a fixed surface for biofilm growth, significantly lifting nitrification and BOD removal capacity without requiring new tanks. It is particularly effective for small-flow upgrades focused on BOD and nitrification. However, it incurs a modest energy penalty due to the need for membrane air scour to keep carriers mixed and prevent fouling (per MABR retrofit guide [S2]).
Path C: MABR Drop-in Modules in the Main Aeration or Selector
Membrane Aerated Biofilm Reactor (MABR) modules utilize bubble-less oxygen transfer, delivering oxygen directly to a nitrifying biofilm on the membrane surface. This can achieve up to a 90% reduction in aeration energy consumption compared to legacy fine-bubble systems, with overall plant energy savings of up to 50% (Fluence SUBRE documentation [S2]). MABR modules are designed for basins ranging from 2,000–100,000 m³/d with depths between 1.5–6 m (per MABR retrofit guide [S2]). Pilot projects have documented effluent total nitrogen (TN) as low as 4.1 mg/L and total phosphorus (TP) as low as 0.4 mg/L (Fluence pilot data [S2]). A key consideration for MABR is the biofilm's sensitivity to toxic shock, making influent equalization upstream mandatory for industrial sites (per MABR retrofit guide [S2]). For detailed applications, consult our MABR for airport wastewater guide.
Path D: MBR Polish on Existing CASS
Implementing a Membrane Bioreactor (MBR) as a tertiary polishing step downstream of the existing CASS system is chosen when the binding effluent parameter is reuse-grade suspended solids and turbidity, rather than solely nutrient removal. MBR systems deliver superior effluent clarity suitable for various reuse applications. However, this path typically involves higher membrane operational expenditures (OpEx) due to cleaning, maintenance, and replacement costs (per MABR retrofit guide [S2]).
A fourth fallback option is converting the CASS to a continuous-flow A²/O process. This is generally considered only when basin depth, consistently high toxic loads, or operator capability rules out biofilm-based retrofits or in-basin optimization.
Retrofit Path
Primary Benefit
Key Constraint/Consideration
Typical CapEx (Relative)
Effluent Target (TN/TP)
Energy Impact
Downtime (Relative)
A. Pure In-basin Optimization
Energy reduction, Grade 1A
Requires existing basin volume, operator skill
Lowest
GB 18918-2002 Grade 1A (TN 15, TP 0.5)
16.67–25% reduction (plant)
Minimal (reprogramming)
B. MBBR Carriers
Nitrification capacity lift, BOD removal
Modest energy penalty (air scour), carrier retention
Low-Moderate
Enhanced nitrification, BOD <10
Slight increase (air scour)
Moderate (carrier installation)
C. MABR Drop-in Modules
High TN/TP removal, massive energy savings
Influent equalization for toxic shock, basin depth 1.5–6m
Moderate
TN <5, TP <0.5 (reuse-grade)
Up to 90% aeration reduction
Moderate (staged installation)
D. MBR Polish
Reuse-grade SS/turbidity, pathogen barrier
High membrane OpEx, separate footprint often needed
High
SS <1, Turbidity <0.5 NTU (reuse)
Higher (membrane operation)
Moderate (new unit integration)
CASS Retrofit Engineering Envelope and Parameter Table
Defining the CASS retrofit engineering envelope with precise flow ranges, basin depths, and effluent targets is critical for accurate technology selection and project scoping. For MABR retrofits, the operational envelope typically covers plants with design flows from 2,000–100,000 m³/d (0.5–25 MGD) and existing basin depths between 1.5–6 m (per MABR retrofit guide [S2]). Plants with flows below approximately 20 m³/d are usually better served by containerized MABR units rather than in-basin retrofits (per MABR retrofit guide [S2]).
Effluent benchmarks are stringent. For China, the primary target is GB 18918-2002 Grade 1A, demanding COD 50 mg/L, TN 15 mg/L, and TP 0.5 mg/L (Liu et al., 2021 [S3]). For advanced reuse applications, MABR pilot projects have demonstrated the capability to achieve TN below 3 mg/L and TP below 0.3 mg/L, meeting criteria for California Title 22 reuse-grade polish (Fluence pilot data [S2]).
Energy benchmarks offer significant motivation for retrofits. Pure in-basin optimization has been documented to achieve 16.67–25% overall plant energy reduction (Liu et al., 2021 [S3]). MABR technology can deliver even greater savings, with up to a 90% reduction in aeration energy and up to a 50% reduction in overall plant energy consumption compared to conventional activated sludge baselines (Fluence SUBRE documentation [S2]).
Cycle-time parameters are fundamental to CASS operation and require careful redesign during a retrofit. These include the FILL-AERATION, FILL-SETTLE, DECANT, and IDLE phases (per CASS™ process documentation [S4]). The moving-weir decanter must be accurately programmed to remain above the top water level during aeration and settling to prevent solids carryover (per CASS™ process documentation [S4]). Sludge wasting protocols are also critical; maintaining stable sludge inventory and wasting sludge at concentrations exceeding 10,000 mg/L after settling helps manage the sludge line through the retrofit cut-over (per CASS™ process documentation [S4]).
Parameter
Typical Range/Target
Notes
MABR Retrofit Flow Envelope
2,000–100,000 m³/d (0.5–25 MGD)
Plants <20 m³/d often use containerized MABR units.
Basin Depth (MABR)
1.5–6 m (5–20 ft)
Critical for module stack height and cost-effectiveness.
Effluent Target (China Grade 1A)
COD 50, TN 15, TP 0.5 mg/L
Mandated by GB 18918-2002 (Liu et al., 2021 [S3]).
Effluent Target (MABR Pilot, reuse)
TN <3, TP <0.3 mg/L
Achieved for California Title 22 reuse (Fluence pilot data [S2]).
Plant Energy Reduction (In-basin opt.)
16.67–25%
Documented for full-scale CASS in China (Liu et al., 2021 [S3]).
Aeration Energy Reduction (MABR)
Up to 90%
Vs. conventional fine-bubble CAS (Fluence SUBRE documentation [S2]).
Overall Plant Energy Reduction (MABR)
Up to 50%
Vs. legacy CAS baseline (Fluence SUBRE documentation [S2]).
Sludge Wasting Concentration
>10,000 mg/L (after settling)
Maintains stable sludge line during retrofit.
CASS Retrofit Execution: 5-Step Sequence with the Plant Live
A CASS retrofit project can be executed as a 5-step sequence, minimizing plant downtime by staging installation basin-by-basin while the rest of the facility remains in service (per MABR retrofit guide [S2]). This approach is crucial for maintaining continuous treatment compliance.
Step 1: Diagnostic & Model (8–12 weeks)
This initial phase involves comprehensive respirometry, ASM1 model calibration against historical data, and CFD studies for agitator optimization. This typically takes 8–12 weeks to complete, providing the engineering basis for the retrofit design (per MABR retrofit guide [S2], Liu et al., 2021 [S3]).
Step 2: Pilot Study (Mandatory for Industrial CASS)
For industrial CASS plants, especially those handling petrochemical, food & beverage, or pharmaceutical wastewater, a pilot study is mandatory. This phase validates the chosen technology under site-specific conditions. It is crucial to apply a winter temperature factor to the ammonia load before sizing biofilm modules to account for seasonal performance variations (per MABR retrofit guide [S2]).
Step 3: Detailed Design & Procurement
Based on diagnostic and pilot results, detailed engineering designs are developed. This includes specifications for new blower skids (especially low-pressure for MABR), biofilm modules or carriers, mixing diffusers, an optional internal dividing wall (if required to create an anoxic zone), and tie-ins for chemical dosing systems.
Step 4: Staged Installation Basin-by-Basin
Installation is meticulously staged, taking one CASS basin offline at a time while the remaining basins continue to treat wastewater. This limits hydraulic disruption to a single aeration basin during the cut-over window, ensuring ongoing plant operations (per MABR retrofit guide [S2]).
Step 5: Commissioning & Biofilm Establishment
Following installation, the new system is commissioned. For biofilm-based retrofits, 2–4 weeks are typically required for the biofilm to establish itself, with full stabilization achieved by week 6–8 (per MABR retrofit guide [S2]). The total elapsed time from pilot start to full-scale performance verification for biofilm retrofits is typically 6–9 months, significantly shorter than the 18–36 months required for new concrete construction (per MABR retrofit guide [S2]). This rapid deployment minimizes both project risk and capital holding costs. For maintaining system integrity, effective foam control in wastewater treatment is also essential.
CASS Retrofit Economics: CapEx, Energy Payback, and Grade 1A ROI
Pure in-basin CASS optimization retrofits typically present the lowest capital expenditure, with return on investment primarily driven by aeration energy reductions of 16.67–25% and avoided new tankage construction (Liu et al., 2021 [S3]). This makes it an attractive option when the existing hydraulic capacity is sufficient, and the primary goal is Grade 1A compliance and energy efficiency.
For MABR retrofits, the capital expenditure stack includes drop-in modules, a low-pressure blower skid, mixing diffusers, and an optional internal dividing wall (if converting an aerobic zone to an anoxic/aerobic hybrid) (per MABR retrofit guide [S2]). This CapEx is typically a fraction of the cost of building a new concrete aeration basin for the same flow capacity (per MABR retrofit guide [S2]).
Operating savings are a major driver for MABR retrofits, with up to a 90% reduction in aeration energy consumption and up to a 50% reduction in overall plant energy usage compared to legacy fine-bubble activated sludge baselines (Fluence SUBRE documentation [S2]). At current industrial electricity tariffs, this represents the dominant payback lever.
Beyond direct energy savings, CASS retrofits offer substantial avoided CapEx. This includes deferring or outright canceling the need for a new aeration tank, secondary clarifier expansion, or external carbon (e.g., methanol) dosing for denitrification, which would otherwise be necessary to meet tightening nutrient limits (per MABR retrofit guide [S2]).
achieving effluent quality at TN below 5 mg/L and TP below 0.5 mg/L through advanced retrofits (like MABR) opens up various reuse optionality streams, such as irrigation, dust suppression, and toilet-flushing (Fluence pilot data [S2]). This transforms a compliance cost into a cost-avoidance or even a potential revenue stream.
A critical risk note for biofilm-based retrofits is the heightened sensitivity of fixed biofilms to toxic shock. Therefore, influent equalization and toxicity screening are not optional but mandatory prerequisites for industrial sites, and budgeting for these pretreatment steps alongside the modules is essential (per MABR retrofit guide [S2]). An automatic chemical dosing system can support pH correction and alkalinity supplementation for stable nitrification.
Achieved through RRC and process optimization vs. bubble-less oxygen transfer.
Overall Plant Energy Reduction
16.67–25%
Up to 50%
MABR significantly impacts total electricity demand.
Avoided CapEx
New tankage, clarifier expansion
New aeration tank, clarifier, external carbon dosing
Defers significant civil investment.
Payback Drivers
Energy savings, compliance
Energy savings, compliance, potential reuse revenue
MABR offers higher potential for cost conversion.
Key Risk Mitigation
Process control expertise
Influent equalization, toxicity screening
Biofilm sensitive to shock loads.
Frequently Asked Questions
What are the typical effluent targets for a CASS retrofit in China?
For municipal and industrial wastewater treatment plants in China, a CASS retrofit primarily targets GB 18918-2002 Grade 1A effluent standards, which specify limits of COD 50 mg/L, TN 15 mg/L, and TP 0.5 mg/L (Liu et al., 2021 [S3]).
What energy savings can be expected from a CASS retrofit?
Pure in-basin optimization of a CASS process can achieve plant-wide energy reductions of 25% in summer and 16.67% in winter (Liu et al., 2021 [S3]). With MABR drop-in modules, aeration energy consumption can be reduced by up to 90%, leading to an overall plant energy reduction of up to 50% (Fluence SUBRE documentation [S2]).
What is the typical project timeline for a biofilm-based CASS retrofit?
The total elapsed time from the start of a pilot study to full-scale performance verification for a biofilm-based CASS retrofit, such as MABR, is typically 6–9 months. This includes 8–12 weeks for diagnostics and modeling, pilot operations, detailed design, procurement, staged installation, and 2–4 weeks for biofilm establishment (per MABR retrofit guide [S2]). This is considerably faster than the 18–36 months for new concrete construction.
What are the key operational parameters for an MABR CASS retrofit?
MABR retrofits are engineered for existing activated sludge basins handling 2,000–100,000 m³/d (0.5–25 MGD) with water depths between 1.5–6 m (per MABR retrofit guide [S2]). For industrial sites, influent equalization and toxicity screening are prerequisites due to the fixed biofilm's sensitivity to toxic shock (per MABR retrofit guide [S2]). For more on MABR applications, refer to our MABR for hospital wastewater guide.
Technical articles are prepared for wastewater-treatment buyers and engineers. Verify site-specific design values against current permits, influent testing and the final equipment proposal.
