Krohne vs ABB Clarification Equipment: Comparison & Best Fit

Introduction

The secondary clarifier is often described as the most critical bottleneck in activated sludge systems, yet its performance is frequently limited by the quality of data feeding the control loops. The black-box nature of clarification — settling zones and compression layers hidden beneath the surface — presents a persistent operational challenge. Solids carryover events are commonly triggered not by hydraulic overload but by instrument failure or a simple lack of visibility into sludge blanket dynamics, which delays return activated sludge adjustments until it is too late.

A scope note belongs immediately in any Krohne vs ABB Clarification Equipment: Comparison & Best Fit evaluation. Neither Krohne nor ABB manufactures mechanical clarification equipment — scraper mechanisms, drive cages, suction headers or clarifier bridges. That equipment comes from process equipment manufacturers such as WesTech, Ovivo, Xylem, Monroe Environmental and Walker Process. Both companies here supply the instrumentation and control equipment that serves as the clarifier’s nervous system: electromagnetic flowmeters for sludge lines, turbidity sensors for effluent compliance and sludge blanket level detectors.

That distinction matters because the instrumentation is a genuine specification decision with its own failure modes. Without precise flow measurement on return and waste sludge lines, accurate effluent turbidity monitoring and reliable blanket detection, the mechanical equipment cannot be operated efficiently regardless of how well it was designed.

This technology is used across municipal wastewater plants and industrial effluent treatment. From primary sedimentation through final clarification and gravity thickening, the instrumentation must survive harsh, corrosive, bio-fouling environments. Abrasive sludge destroys standard flowmeter liners in months, and optical sensors without adequate cleaning drift within hours. Broader supplier positioning is covered in our overview of the top clarifier equipment manufacturers.

How to Select and Specify Clarification Instrumentation

Duty Conditions & Operating Envelope

The envelope varies significantly between primary and secondary stages. For sludge flow metering, solids concentration is the primary variable. Primary sludge runs several percent solids and often carries grit and rags, posing abrasion and clogging risks. Return activated sludge runs lower solids but is biologically active, producing rapid biofilm formation on sensor surfaces.

For blanket level monitoring, the operating envelope is really about stratification layers. The sensor must distinguish the fluffy hindered-settling zone from the compacted bed below. Devices relying on a single ultrasonic echo often fail in bulking sludge, whereas optical profilers detect density gradients directly.

Materials & Compatibility

• Flowmeter liners: For abrasive primary sludge or grit lines, soft rubber or polyurethane liners offer resilience. In chemically aggressive industrial clarification, PTFE or PFA is required instead.
• Electrodes: 316L stainless is generally sufficient for municipal wastewater. Where ferric chloride or alum is dosed upstream for phosphorus removal, chloride content can cause pitting, and Hastelloy or titanium electrodes become appropriate.
• Submerged components: Blanket sensors operate underwater continuously. Housings should be 316L stainless or high-grade PVDF. Cable jackets must resist UV where exposed on the bridge and hydrolysis where submerged.

Hydraulics & Process Performance

Magnetic flowmeters generally require several pipe diameters of straight run upstream and downstream for a symmetric flow profile. In retrofits with cramped sludge galleries this is frequently impossible, so selection shifts toward technology tolerant of flow disturbance.

For suspended solids and turbidity sensors, sample velocity across the sensor face is critical. Too low and solids settle and foul the lens; too high and floc shears or cavitation bubbles scatter light, creating false high readings. Specifications should define the installation angle to exploit self-cleaning hydraulic forces.

Installation Environment & Constructability

• Rotating bridges: On circular clarifiers, power and signal must pass through a slip ring assembly. Specify loop-powered devices where possible to minimize slip ring channels. Wireless transmission is becoming a viable alternative to noisy slip rings.
• Mounting location: Blanket sensors must sit away from the feed well turbulence and the scum beach. The optimal location is roughly a third to half of the radius in from the outer wall, where the blanket is most stable.
• Sun and temperature: Bridge-mounted transmitters face direct sunlight. Sunshades are a mandatory specification item to prevent display blackening and electronics overheating.

Reliability & Failure Modes

Reliability here is defined by mean time between maintenance, and the most common failure mode for analytical sensors is fouling. An optical sensor without a wiper or air-blast cleaning system will fail within a day or two in wastewater.

Redundancy strategies typically involve voting logic. For critical sludge flow, plants use a mag meter as the primary instrument with a weir measurement or pump speed curve calculation as a backup validation variable in SCADA.

Controls & Automation

• Protocols: Analog with HART remains the industry baseline, but modern specifications increasingly call for industrial Ethernet protocols to expose deeper diagnostics. That allows the instrument to report not just flow but its own health status, such as electrode coating detected.
• Control strategies: Blanket level data drives return sludge pump speed. As the blanket rises, return rate increases. This loop needs a damped signal since raw data is too noisy, so the transmitter should offer adjustable damping across a wide range.

Maintainability, Safety & Access

• Retraction assemblies: Specify rail mounting with pivot arms or chain-hoist retrievers so operators can pull the sensor onto the walkway without leaning over the handrail.
• Remote electronics: Specify remote-mounted transmitters at the end of the bridge or in a gallery rather than compact versions mounted on the sensor, keeping the operator interface accessible.

Lifecycle Cost Drivers

• Capital: A motorized optical sludge profiler costs substantially more than a simple ultrasonic gap switch.
• Operating: The ultrasonic switch requires manual calibration and gives false readings during upset conditions, demanding operator intervention. The motorized profiler provides a true depth profile and self-cleans. Labor savings over a decade frequently justify the higher capital cost.
• Consumables: Account for wiper blades, reagents where applicable, and backup batteries on wireless units.

Comparison Tables

Engineer & Operator Field Notes

Commissioning & Acceptance Testing

• Zero-point calibration: Never assume the factory zero suits your installation. Fill the sludge line, ensure the liquid is static, and zero in the field to account for local electrical noise and grounding conditions.
• Blanket profiling setup: Profiling units require a teach cycle where the probe descends to the floor or a preset stop to map tank depth. Configure the sensitivity threshold deliberately — at what solids concentration does the instrument decide it has found the blanket? Setting this too low causes the sensor to report the fluff layer as the blanket.
• Interference mapping: Where ultrasonic technology is used, map out false echoes from skimmer arms, launder troughs and internal piping during commissioning.

Common Specification Mistakes

• Accuracy against repeatability: In blanket monitoring, centimeter-level accuracy is irrelevant when the blanket itself undulates by an order of magnitude more. Repeatability matters more. Do not pay for laboratory-grade accuracy where process noise dominates.
• Missing flush ports: Specifying flowmeters on sludge lines without flushing rings or ports is a critical error. When the line plugs, operators need a way to inject high-pressure water or air without dismantling the flange.
• Liner selection: Specifying PTFE for vacuum service on the suction side of sludge pumps can lead to liner collapse. Use polyurethane or hard rubber, or ensure the PTFE is anchored for vacuum service.

O&M Burden & Strategy

• Wiper maintenance: Optical sensor wipers wear out. Schedule inspection and replacement every few months. A worn wiper smears algae rather than removing it, causing signal drift that looks like a process change.
• Desiccant packs: Transmitters that are not fully potted contain desiccant packs to manage humidity. Check these annually — moisture ingress is the leading cause of electronics failure in outdoor bridge-mounted units.
• Electrode coating: In high-grease applications, electrodes become insulated. Modern meters can diagnose this but cannot fix it. Periodic mechanical cleaning or chemical flushing remains necessary.

Troubleshooting

• Blanket level reads zero or full continuously. On optical profilers, the cable may be tangled or the sensor stuck in its cleaning station. On echo-based sensors, a heavy fluff layer may absorb the signal, causing loss of echo. Check the cable winding mechanism first.
• Flowmeter reading wanders with the pump off. Usually a ground loop or poor grounding. Ensure grounding rings are installed, particularly on plastic pipe, and bonded to the sensor housing and earth ground.

Design Details & Calculations

Sizing Methodology

Flowmeters on sludge lines should be sized to maintain velocity between roughly 3 and 10 ft/s (1 to 3 m/s).

• Below 3 ft/s: Risk of solids settling in the meter tube and grease coating the electrodes
• Above 10 ft/s: Excessive liner abrasion and hydraulic noise

Worked example: A waste sludge flow of 300 gpm through a 4-inch meter (area 0.087 ft²) gives roughly 7.7 ft/s — acceptable. The same flow through a 6-inch meter (area 0.196 ft²) gives roughly 3.4 ft/s, which is marginal and risks settling. Select the 4-inch meter even if the connecting piping is 6-inch, and use concentric reducers.

Specification Checklist

1. Transmitter enclosure: NEMA 4X / IP66 minimum, stainless or polycarbonate rather than painted aluminum in corrosive environments
2. Cable length: Factory-potted cables of sufficient length to reach the junction box without field splicing, which introduces signal noise
3. Sunshades: Mandatory for all outdoor transmitters
4. Cleaning system: Mandatory automatic mechanical wiper or air purge for optical sensors
5. Certifications: Electrical listing for all components; factory calibration certificate included in the manuals

Standards

• AWWA M33: Guidelines for magnetic flowmeter selection in water supply
• NEC (NFPA 70): Wiring methods for wet and corrosive locations
• Manufacturer verification protocols: Specify the in-situ verification tool and require a documented annual verification report

Frequently Asked Questions

Do Krohne or ABB manufacture clarifiers?

No. Neither supplies clarifier mechanisms, scraper drives, bridges or suction headers. That equipment comes from process equipment manufacturers including WesTech, Ovivo, Xylem, Monroe Environmental and Walker Process. Krohne and ABB supply the flow, level and analytical instrumentation that makes a clarifier controllable. Engineers procuring clarifier mechanisms should be working from a different bid list entirely.

What is the main difference between the two portfolios for clarification?

The difference is where each is strongest. Krohne’s sludge blanket profiler physically lowers a sensor on a cable to map stratification through the tank depth, which is a genuine differentiator in difficult settling conditions. ABB’s Aztec series is primarily a fixed-point optical turbidity and suspended solids sensor used for effluent quality or mixed liquor monitoring. Both make capable magnetic flowmeters.

Which flowmeter liner is best for sludge applications?

For return and waste activated sludge, polyurethane or hard rubber are typically the best fit, offering abrasion resistance against grit and sand. PTFE is often unnecessary and can be damaged under vacuum conditions or where the liner is not properly bonded. PFA is the higher-end alternative for high-temperature or chemically aggressive industrial sludge.

How often should clarifier sensors be calibrated?

Magnetic flowmeters are inherently stable and typically do not require adjustment for years, but should undergo verification annually using the manufacturer’s tool to confirm electronics health. Optical sensors need more attention: zero checks monthly, and comparison against grab samples analyzed in the lab every week or two to adjust the slope.

Why do sludge blanket readings fluctuate wildly?

Usually one of two causes. Either the rake arm is passing under the sensor, creating a physical disturbance or echo interference, or the sensor is mistaking the fluff layer for the compact bed. Increase signal damping to smooth rake arm interference. On an optical profiler, raise the solids threshold so it ignores the light fluff layer.

Is ultrasonic or optical better for sludge level?

For clarifiers with good settling, ultrasonic is cheaper and has no moving parts to maintain. For plants with bulking sludge, heavy fluff layers or frequent upsets, optical profiling is superior because it physically detects the density change, whereas echo-based sensors can be fooled by soft layers that absorb the signal.

Conclusion

There is rarely a single winner for an entire plant. For sludge blanket profiling, Krohne holds a technical edge through direct-measurement profiling, which is what makes automated return sludge control workable in plants with variable settling characteristics.

For effluent compliance monitoring and flow metering, ABB provides a highly competitive offering, with onboard diagnostics and reliable turbidity measurement that suit regulatory reporting points. Focus less on brand and more on the physics of the measurement point. Related evaluations appear in our comparison of Badger Meter and Siemens clarification instrumentation and Evoqua and ClearStream Environmental for clarification.