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 the data feeding the control loops. For municipal consulting engineers and plant directors, the “black box” nature of clarification—where settling zones and compression layers are hidden beneath the surface—presents a persistent operational challenge. A surprising industry statistic suggests that up to 40% of unintentional solids carryover events are caused not by hydraulic overload, but by instrument failure or lack of visibility into sludge blanket dynamics, leading to delayed Return Activated Sludge (RAS) adjustments.

When engineers evaluate the market for reliable process monitoring, the conversation often centers on Krohne vs ABB Clarification Equipment: Comparison & Best Fit. It is important to clarify a fundamental distinction immediately: neither Krohne nor ABB typically manufactures the heavy mechanical clarification equipment (such as scraper mechanisms, drive cages, or suction headers). Rather, they manufacture the critical instrumentation and control equipment that serves as the nervous system for the clarifier. Without precise electromagnetic flowmeters for RAS/WAS lines, accurate turbidity sensors for effluent compliance, and reliable sludge blanket level detectors, the mechanical equipment cannot operate efficiently.

This technology is utilized across municipal wastewater treatment plants (WWTPs) and industrial effluent treatment systems (ETPs). From primary sedimentation to final clarification and gravity thickening, the instrumentation must survive harsh, corrosive, and bio-fouling environments. The consequences of poor specification in this area are severe: abrasive sludge can destroy standard flowmeter liners in months, and optical sensors without adequate cleaning systems can drift within hours, rendering automation useless.

This article provides a rigorous technical analysis to help engineers accomplish a specific goal: specifying the correct instrumentation package for clarification systems. We will move beyond brochure specifications to analyze the real-world performance, maintenance burdens, and application suitability of the Krohne and ABB portfolios.

How to Select / Specify Clarification Instrumentation

Selecting the right sensor suite requires a deep understanding of the process dynamics within a clarifier. Engineers must evaluate the equipment based on the specific constraints of sludge handling and optical measurement.

Duty Conditions & Operating Envelope

The operating envelope for clarification instrumentation varies significantly between primary and secondary stages. For flow metering (RAS/WAS), the solids concentration is the primary variable. Primary sludge can range from 3% to 6% solids, often containing grit and rags that pose abrasion and clogging risks. Secondary sludge (RAS) is typically 0.5% to 1.5% solids but is biologically active, leading to rapid biofilm formation on sensor surfaces.

When evaluating Krohne vs ABB Clarification Equipment: Comparison & Best Fit regarding duty conditions, engineers must verify the liner material and electrode design of magnetic flowmeters. For sludge blanket level monitoring, the “operating envelope” refers to the distinct stratification layers. The sensor must distinguish between the fluffy “rag layer” (hindered settling zone) and the compacted sludge bed. Devices that rely solely on a single ultrasonic echo often fail in bulking sludge conditions, whereas optical profilers can detect density gradients.

Materials & Compatibility

Corrosion and abrasion are the twin enemies of clarifier instrumentation.

• Flowmeter Liners: For abrasive primary sludge or grit removal lines, soft rubber or polyurethane liners are often specified for their resilience. However, in chemically aggressive industrial clarification (e.g., metal finishing or pulp and paper), PTFE or PFA liners are required.

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology
• Electrodes: Standard 316L Stainless Steel is generally sufficient for municipal wastewater. However, if ferric chloride or alum is dosed upstream for phosphorus removal, the chloride content can cause pitting. In these cases, Hastelloy C or Titanium electrodes should be specified.
• Submerged Components: Sludge blanket sensors operate underwater continuously. Housings should be 316L SS or high-grade PVDF. Cable jackets must be resistant to UV radiation (where exposed on the bridge) and hydrolysis (where submerged).

Hydraulics & Process Performance

Hydraulic conditions significantly impact sensor performance. Magnetic flowmeters generally require 5 diameters (5D) of straight pipe upstream and 2D downstream to ensure a symmetric flow profile. In retrofits, where RAS galleries are cramped, this is often impossible. Here, the selection turns to technology capable of handling flow disturbances.

For suspended solids and turbidity sensors, hydraulic velocity is critical. If the sample flow is too low across the sensor face, solids will settle and foul the lens. If too high, it may shear flocs or cause cavitation bubbles that scatter light, creating false high readings. Specifications must define the installation angle (typically 45 degrees downstream) to utilize self-cleaning hydraulic forces.

Installation Environment & Constructability

The physical installation environment on a clarifier bridge presents unique challenges.

• Rotating Bridges: For circular clarifiers with rotating bridges, power and signal transmission must pass through a slip ring assembly. Engineers must specify 2-wire (loop-powered) devices where possible to minimize the number of slip ring channels required. Wireless signal transmission is becoming a viable alternative to noisy slip rings.
• Mounting Locations: Sludge blanket sensors must be mounted away from the feed well (turbulence) and the scum beach. The optimal location is typically 1/3 to 1/2 of the radius in from the outer wall, where the blanket is most stable.
• Sun and Temperature: Transmitters mounted on the bridge are exposed to direct sunlight. UV shields or sunshades are mandatory specification items to prevent LCD screen blackening and internal electronics overheating.

Reliability, Redundancy & Failure Modes

In the context of Krohne vs ABB Clarification Equipment: Comparison & Best Fit, reliability is defined by the Mean Time Between Maintenance (MTBM). The most common failure mode for analytical sensors is fouling. An optical sensor without a wiper or air-blast cleaning system will fail within 24-48 hours in wastewater.

Redundancy strategies often involve “voting logic.” For critical RAS flow, plants may rely on a magmeter as the primary instrument, with a V-notch weir measurement or pump speed curve calculation as a backup validation variable in the SCADA system.

Controls & Automation Interfaces

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Integration with the plant SCADA is paramount.

• Protocols: While 4-20mA HART is the industry standard, modern specifications increasingly call for EtherNet/IP or Profibus for deeper diagnostics. This allows the instrument to report not just the process variable (e.g., Flow), but also its health status (e.g., “Electrode Coating Detected”).
• Control Strategies: Sludge blanket level data is used to control the RAS pump speed. If the blanket rises, the RAS rate increases. This loop requires a damped signal; raw data is often too noisy. The instrument transmitter should have adjustable damping settings (0-100 seconds).

Maintainability, Safety & Access

Clarifier bridges can be slippery and dangerous in winter. Operators should not have to lean over handrails to clean sensors.

• Retraction Assemblies: Specify rail-mounting kits with pivot arms or chain-hoist retrievers that allow the operator to pull the sensor onto the walkway for cleaning without breaking the plane of the handrail.
• Remote Electronics: Whenever possible, specify remote-mounted transmitters located at the end of the bridge or in a gallery, rather than compact versions mounted on the sensor itself. This keeps the operator interface accessible.

Lifecycle Cost Drivers

The Total Cost of Ownership (TCO) analysis typically favors higher-spec instruments in clarification.

• CAPEX: A motorized optical sludge profiler costs significantly more ($8k-$12k) than a simple ultrasonic gap switch ($2k-$4k).
• OPEX: The ultrasonic switch requires manual calibration and frequently gives false readings during upset conditions, requiring operator intervention. The motorized profiler provides a true depth profile and self-cleans. The labor savings over 10 years often justify the higher CAPEX.
• Consumables: Consider the cost of wiper blades, reagents (if applicable for other analytes), and backup batteries for wireless units.

Comparison Tables: Krohne vs ABB

The following tables provide a direct comparison of the instrumentation portfolios relevant to clarification. Table 1 focuses on the specific technology strengths of each manufacturer, while Table 2 assists engineers in selecting the best fit based on application constraints.

Table 1: Manufacturer Technology Profile (Clarification Scope)

Table 2: Application Fit Matrix

Engineer & Operator Field Notes

The difference between a successful installation and a maintenance nightmare often lies in the details of commissioning and daily operation. The following notes are compiled from field experiences with Krohne vs ABB Clarification Equipment: Comparison & Best Fit implementations.

Commissioning & Acceptance Testing

Commissioning clarification instrumentation requires more than just powering up the device.

• Zero-Point Calibration (Magmeters): Never assume the factory zero is perfect for your installation. Fill the RAS pipe, ensure the liquid is static (zero flow), and perform a zero calibration in the field. This accounts for local electrical noise and grounding conditions.
• Blanket Profiling Setup: For the Krohne OPTISYS or similar profiling units, you must define the “bottom” and the “surface.” Commissioning must include a “teach” cycle where the probe lowers to the floor (or a preset stop) to map the tank depth. You must also configure the sensitivity threshold—at what TSS concentration does the instrument decide it has hit the “blanket”? A common error is setting this too low, causing the sensor to report the fluff layer as the blanket.
• Interference Mapping: If using ultrasonic/sonar technologies (common in some ABB configurations), you must map out false echoes caused by skimmer arms, launder troughs, or internal piping.

Common Specification Mistakes

One of the most frequent errors in clarifying specifications is over-specification of accuracy and under-specification of cleaning.

• Accuracy vs. Repeatability: In sludge blanket monitoring, ±1 cm accuracy is irrelevant if the blanket is undulating by ±10 cm. Repeatability is more important. Do not pay for lab-grade accuracy where process noise dominates.
• Missing Flush Ports: Specifying flowmeters on sludge lines without flushing rings or ports is a critical mistake. When (not if) the line plugs or needs maintenance, operators need a way to inject high-pressure water or air to clear the sensor without dismantling the flange.
• Liner Selection: Specifying PTFE (Teflon) liners for vacuum service lines (suction side of RAS pumps) can lead to liner collapse if the pipe goes into deep vacuum. Use polyurethane or hard rubber liners, or ensuring the PTFE is bonded/anchored for vacuum service.

O&M Burden & Strategy

Maintenance strategies differ between the technologies.

• Wiper Maintenance: Optical sensors (turbidity/TSS) usually have mechanical wipers. These rubber wipers wear out. A preventive maintenance (PM) task should be scheduled every 3-6 months to inspect and replace the wiper blade. A worn wiper smears the algae rather than removing it, causing signal drift.
• Desiccant Packs: Both Krohne and ABB transmitters (if not fully potted) often contain desiccant packs to manage humidity. These must be checked annually. Moisture ingress is the leading cause of electronics failure in outdoor bridge-mounted units.
• Magmeter Electrodes: In high-grease applications (scum pumping), electrodes can become insulated. While modern meters (Krohne’s Virtual Reference, ABB’s Coating Detection) can diagnose this, they cannot fix it. Periodic mechanical cleaning or chemical flushing may be required.

Troubleshooting Guide

• Symptom: Sludge blanket level reads “0” or “Full” continuously.
  Root Cause: For optical profilers, the cable may be tangled or the sensor stuck in the cleaning station. For sonar, the signal may be absorbed by a heavy “fluff” layer, causing loss of echo.
  Fix: Check the cable winding mechanism. Increase gain on sonar units (carefully).
• Symptom: Flowmeter reading wanders when pump is off.
  Root Cause: Ground loop or poor grounding rings.
  Fix: Ensure grounding rings are installed (especially on plastic pipe) and bonded to the sensor housing and earth ground.

Design Details & Calculations

Sizing Logic & Methodology

Proper sizing of flowmeters for clarifier service involves balancing velocity constraints.

The Velocity Sweet Spot:
Flowmeters on RAS/WAS lines should be sized to maintain a velocity between 3 ft/s and 10 ft/s (1 m/s to 3 m/s).

• < 3 ft/s: Risk of solids settling in the meter tube and grease coating the electrodes.
• > 10 ft/s: Excessive wear on the liner (abrasion) and potential for hydraulic noise.

Example Calculation:
Assume a WAS flow rate of 300 GPM.
Using a 4-inch meter: Area = 0.087 ft². Velocity = (300 / 448.8) / 0.087 ≈ 7.7 ft/s. (Acceptable)
Using a 6-inch meter: Area = 0.196 ft². Velocity = (300 / 448.8) / 0.196 ≈ 3.4 ft/s. (Marginal – risk of settling)
Decision: Select the 4-inch meter, even if the connecting piping is 6-inch, and use concentric reducers.

Specification Checklist

When writing the spec for Krohne vs ABB Clarification Equipment: Comparison & Best Fit, ensure the following items are explicitly requested:

1. Transmitter Enclosure: NEMA 4X / IP66 minimum; Stainless Steel or Polycarbonate (avoid painted aluminum in corrosive salt air).
2. Cable Length: Specify factory-potted cables of sufficient length to reach the junction box without field splicing (splicing 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: CSA/UL approval for the electrical components. Verification certificate (factory calibration) included in O&M manual.

Standards & Compliance

• AWWA M33: Flowmeters in Water Supply (Guidelines for magnetic flowmeter selection).
• ISO 1554: Wastewater treatment plant instrumentation standards.
• NEC (NFPA 70): Wiring methods for wet and corrosive locations (Article 500 if explosion-proof is required, though rare in open clarifiers).

Frequently Asked Questions

What is the main difference between Krohne OPTISYS and ABB Aztec for clarification?

The primary difference lies in the technology application. The Krohne OPTISYS SLM 2100 is widely recognized as a “sludge blanket profiler,” meaning it physically lowers a sensor on a cable to map the stratification layers of the clarifier. The ABB Aztec series (specifically ATS430) is primarily an optical turbidity/TSS sensor used for fixed-point monitoring (e.g., effluent quality or mixed liquor). While both measure solids, Krohne’s profiling capability is more specific to blanket level control.

Which flowmeter liner material is best for RAS/WAS applications?

For Return Activated Sludge (RAS) and Waste Activated Sludge (WAS), Polyurethane or Hard Rubber are typically the best fit. They offer excellent abrasion resistance against grit and sand found in sludge. PTFE (Teflon) is often overkill and can be susceptible to damage if vacuum conditions occur or if the liner is not bonded. PFA is a high-end alternative for high-temperature or chemically aggressive industrial sludge.

How often should clarifier sensors be calibrated?

Magnetic flowmeters (Krohne or ABB) are inherently stable and typically do not require “calibration” (adjustment) for years. However, they should undergo verification annually using the manufacturer’s verification tool (e.g., ABB VeriMaster or Krohne OPTICHECK) to confirm electronics health. Optical sensors (turbidity/sludge level) require more frequent attention; zero checks should be performed monthly, and comparison against grab samples (lab analysis) should be done weekly or bi-weekly to adjust the slope/gain.

Why do my sludge blanket readings fluctuate wildly?

Wild fluctuations are usually caused by either: 1) The rake arm passing under the sensor (causing a physical disturbance or echo interference), or 2) The sensor mistaking the “rag layer” (fluff) for the compact bed. To fix this, increase the signal damping (time constant) in the transmitter to smooth out the rake arm interference. If using an optical profiler, adjust the TSS threshold setting higher so it ignores the light fluff layer.

Is it better to use ultrasonic or optical sensors for sludge level?

For clarifiers with good settling characteristics, ultrasonic (sonar) is cheaper and requires no maintenance (no moving parts). However, for plants with bulking sludge, high fluff layers, or frequent process upsets, optical profiling (like the Krohne SLM) is superior. Optical sensors physically detect the density change, whereas sonar can be “fooled” by soft layers that absorb the sound wave, leading to “loss of echo” errors.

Conclusion

In the evaluation of Krohne vs ABB Clarification Equipment: Comparison & Best Fit, the decision rarely comes down to a single “winner” for the entire plant. A nuanced engineering approach often yields the best results. For Sludge Blanket Level Profiling, Krohne holds a technical edge with the OPTISYS SLM 2100 due to its direct-measurement profiling capability, which is critical for automating RAS pumps in plants with variable sludge settling characteristics.

However, for Effluent Compliance and Flow Metering, ABB provides a highly competitive offering. The WaterMaster flowmeter’s onboard diagnostics and the Aztec turbidity system’s reliability make them favorites for regulatory reporting points. Engineers should focus less on brand loyalty and more on the specific physics of the measurement point: use optical profiling for the blanket, robust magmeters with proper liners for the sludge lines, and wiper-equipped optical sensors for the clear well.

Ultimately, the “best fit” is an instrument that survives the environment. By specifying NEMA 4X/IP68 enclosures, proper cleaning mechanisms, and correct hydraulic sizing, engineers can ensure that whichever manufacturer is selected, the clarifier operates with the visibility required for stable process control.