Routine Wastewater Tests: What SV30, SVI, MLSS, DO & More Reveal About Plant Health

Routine wastewater tests like SV30, SVI, MLSS/MLVSS, DO, OUR, and clarifier bed depth give operators a real-time snapshot of biological health, settling performance, and system stability. Together, they guide process control decisions and help prevent effluent violations.

Understanding the Core Tests in Wastewater Treatment
Routine monitoring in activated sludge systems isn’t just about compliance—it’s about staying ahead of potential upsets. Here’s how each test contributes to a holistic view of plant performance:

SV30 & SVI: Settling Behavior and Sludge Quality

• SV30 (Sludge Volume after 30 minutes) measures how well mixed liquor settles in a graduated cylinder. A high SV30 may indicate poor floc formation or bulking sludge.
• SVI (Sludge Volume Index) standardizes settling performance by dividing SV30 by MLSS or MLVSS (in g/L).
  • SVI < 120 mL/g suggests good settling.
  • SVI > 150 mL/g may indicate filamentous bulking or poor compaction.

Operators use SV30/SVI to assess floc structure, predict clarifier performance, and adjust wasting or aeration strategies.

 MLSS & MLVSS: Biomass Concentration

• MLSS (Mixed Liquor Suspended Solids) includes all suspended solids in the aeration tank—biomass plus inert material.
• MLVSS (Mixed Liquor Volatile Suspended Solids) isolates the organic (biological) fraction.

Tracking these helps operators:

• Maintain target biomass levels.
• Calculate food-to-microorganism (F/M) ratios.
• Diagnose shifts in microbial health or influent composition.

DO (Dissolved Oxygen): Aerobic Efficiency

• DO levels in aeration tanks often have target residuals of 2 - 3 mg/L
• Low DO can lead to filamentous growth, loss of nitrification, poor BOD removal, and odor issues.
• High DO may waste energy and indicate over-aeration.

DO is a frontline indicator of microbial respiration and aeration system performance. 

OUR (Oxygen Uptake Rate) & SOUR (Specific Oxygen Uptake Rate): Microbial Activity

• OUR measures how fast microbes consume oxygen—essentially their metabolic rate.
• A sudden drop in OUR may signal toxicity or nutrient deficiency.
• A spike could indicate a shock load or increased organic loading.

Operators use OUR to fine-tune aeration and assess microbial vitality in real time. SOUR standardizes OUR by accounting for MLSS/MLVSS concentration. (SOUR = OUR/(MLVSS in grams).

Secondary Clarifier Bed Depth: Settling Capacity

• Monitoring sludge blanket depth helps prevent solids washout.
• A rising blanket may indicate poor settling, hydraulic overload, or excessive return sludge.
• Ideal bed depth varies by design but should remain well below the weir to avoid effluent TSS issues

Putting It All TogetherThese tests aren’t isolated—they’re interlinked.
For example:

• A high SVI with normal MLSS might prompt a closer look at DO and OUR.
• Poor SV30 results could lead to wasting adjustments or filament control strategies.

By routinely tracking these metrics, operators can:

• Optimize sludge age and wasting rates.
• Prevent bulking, pin floc, or clarifier overload.
• Respond proactively to influent variability or process upsets.