Table of Contents
Key Takeaways
- Total Suspended Solids (TSS) and chemical oxygen demand (COD) track each other closely in municipal wastewater — though the exact ratio shifts with the catchment.
- Online TSS monitoring lets operators tighten settling basin control instead of running conservative margins.
- Optical backscatter sensors hold ±5% accuracy across 0-5000 mg/L ranges.
- Real-time control of dewatering chemistry cuts polymer consumption meaningfully — over-dosing stops being the default insurance policy.
Suspended solids measurement provides essential process control information for municipal wastewater treatment and industrial effluent management. Operators who can see solids loading in real time — across primary treatment, biology, and solids handling — run tighter, cheaper plants than those working from daily lab samples. Continuous TSS monitoring is the foundation for optimizing biological treatment, chemical dosing, and solids handling.
Measurement Technologies
Gravimetric Reference Method
Standard Methods 2540D establishes the reference for TSS determination:
– Glass fiber filter retention of particles >1.2 μm
– 103-105°C drying to constant weight
– Results expressed as mg/L suspended solids
– Laboratory precision on the order of ±2%
Laboratory gravimetric analysis provides definitive measurements but can’t support real-time process control — by the time the filter is dried and weighed, the water that made the sample is long gone. Hence online monitoring.
Optical Backscatter Technology
Near-infrared (NIR) backscatter sensors illuminate particles and measure reflected light intensity:
Working Principle:
– Emitter: 880 nm infrared LED
– Detector: 90° backscatter detection
– Signal correlates with particle concentration and size distribution
Specifications:
– Range: 0-5000 mg/L (configurable)
– Accuracy: ±5% of reading or ±10 mg/L
– Response time: <2 seconds
– Self-cleaning wiper options available
ChiMay SS sensors using optical backscatter technology provide continuous measurement suitable for primary clarifiers, activated sludge basins, and tertiary filtration applications.
Ultrasonic Attenuation
High-frequency ultrasound measures solids concentration through signal attenuation:
Advantages:
– Insensitive to color and optical interferences
– Measures all particle sizes including sub-micron
– Better performance in dark or colored streams
Limitations:
– Higher cost than optical methods
– More complex calibration requirements
– Temperature sensitive requiring compensation
Comparative Technology Assessment
| Technology | Range (mg/L) | Interference | Accuracy | Maintenance | Cost |
|---|---|---|---|---|---|
| Optical Backscatter | 0-5000 | High color, air bubbles | ±5% | Weekly cleaning | Low |
| Ultrasonic Attenuation | 0-10000 | Low | ±3% | Monthly calibration | Medium |
| Microwave Attenuation | 0-50000 | Low (metals) | ±4% | Quarterly | High |
| Capacitance | 100-50000 | High (conductivity) | ±10% | Monthly | Low |
The technology split follows the application: optical backscatter is the workhorse for municipal wastewater, where streams are relatively benign and cost matters; ultrasonic and microwave methods earn their premium in industrial streams with heavy color or extreme solids loading that would blind an optical sensor.
Municipal Wastewater Applications
Primary Clarifier Optimization
Primary clarifiers remove 40-60% of influent TSS through gravity settling. Online monitoring enables:
Control Parameters:
– Influent TSS monitoring for load tracking
– Underflow solids concentration for digester optimization
– Overflow turbidity for effluent quality assurance
– Sludge blanket detection at clarifier bottom
Plants that track primary clarifier solids in real time dose coagulants against actual load instead of worst-case assumptions — and the chemical savings are real, because coagulant overdosing to “be safe” is one of the quietest budget drains in the plant.
Activated Sludge Process Control
Mixed liquor suspended solids (MLSS) concentration directly impacts biological treatment efficiency:
Optimal Ranges:
– Conventional activated sludge: 2,000-4,000 mg/L MLSS
– Extended aeration: 3,000-6,000 mg/L MLSS
– Membrane bioreactors (MBR): 8,000-15,000 mg/L MLSS
Real-time MLSS control enables:
– Food-to-Microorganism (F/M) ratio optimization
– Aeration energy reduction of 10-20% through appropriate MLSS targeting
– Sludge settleability improvement through MLSS/DO coordination
Aeration is the biggest power bill in an activated sludge plant. Running MLSS at the right setpoint instead of “high, just in case” is one of the few changes that cuts energy without touching effluent quality.
Industrial Wastewater Applications
Food and Beverage Processing
Food processing wastewater carries high suspended solids from organic matter:
Typical Characteristics:
– TSS: 500-5000 mg/L raw waste
– High BOD:COD ratio (0.4-0.6)
– Variable pH and temperature
Monitoring Benefits:
– Equalization basin control based on TSS loading
– DAF (Dissolved Air Flotation) optimization
– Biological treatment load balancing
– Effluent compliance verification
Pulp and Paper Industry
Pulp mill effluent presents unique monitoring challenges:
Interferences Addressed:
– High color (lignin compounds)
– Variable fiber content
– Wood pitch and extractives
Mill-wide TSS monitoring network typically includes:
– Screen effluent (0-2000 mg/L)
– Primary clarifier overflow (50-200 mg/L)
– Bioreactor influent/effluent
– Final effluent (compliance point)
Mills that instrument these points run their fiber recovery and effluent treatment from actual measurements — which matters in an industry where both fiber lost to effluent and effluent charges hit the same bottom line.
Installation and Maintenance Guidelines
Sensor Siting
Critical Considerations:
– Representative sampling location (avoid dead zones)
– Minimum velocity: 0.3 m/s to prevent settling
– Avoid air entrainment (bottom of drop structures)
– Protection from damage (turbulent flows, floating debris)
For primary clarifier installations, flow-through cells hold the sensor in a consistent orientation and keep air bubbles from fouling the optics — worth the extra plumbing on almost every installation.
Calibration Verification
Calibration Schedule:
– Weekly: single-point verification against grab sample
– Monthly: two-point calibration check
– Quarterly: full calibration with laboratory comparison
– Annual: third-party certification
Grab Sample Correlation:
Regular grab sample comparison (minimum weekly) establishes correlation between online sensor readings and gravimetric reference values. Correlation drift beyond ±10% triggers recalibration — the lab is always right, and the sensor works for it.
Article #858 | ChiMay SS Sensor | ChiMay Suspended Solids Sensor for wastewater monitoring
