Title: Sludge Volume Index (SV): The Most Intuitive and Practical Test for Wastewater Sedimentation
June 23, 2026
Sludge Volume (SV) is a core testing indicator in the activated sludge process of wastewater treatment. It refers to the percentage of settled sludge volume relative to the total volume of mixed liquor after the mixed liquor from the aeration tank stands and settles in a graduated cylinder for 30 minutes. It can directly reflect the concentration, settling performance and flocculation condition of activated sludge. The key points for on-site observation are summarized as follows:
Standard Testing Procedures
Collect mixed liquor from the outlet end of the aeration tank and fill a graduated cylinder ranging from 100 mL to 1000 mL. Avoid violent shaking, which will damage sludge flocs. Let the sample settle for exactly 30 minutes without moving the cylinder to maintain a stable settling environment.
Key Observation of the Entire Settling Process
Monitor sludge flocculation speed within the first 5 minutes. Healthy sludge rapidly agglomerates into large flocs and settles down, leaving a clear supernatant. Fine flocs and slow settling indicate an excessively low sludge loading rate or dispersed sludge condition.
Observe layering performance throughout settling. Well-conditioned activated sludge presents distinct layers with a transparent supernatant free of suspended solids and turbidity. Turbid supernatant covered with abundant tiny sludge particles is a typical sign of sludge bulking and deflocculation.
Inspection of Sludge Characteristics
After 30 minutes of settling, record the volume of the sludge blanket to calculate the SV value. The optimal SV range for aeration tanks is 20%–30%.
Check sludge color and odor: normal activated sludge is yellowish-brown with an earthy odor. Black sludge with a foul smell points to insufficient aeration; pale white and loose flocs are caused by over-aeration. Pay attention to abnormal phenomena including floating sludge, attached foam and needle-shaped flocs.
System Operation Diagnosis Combined with Test Data
An excessively high SV value usually results from high sludge concentration or poor settleability, which easily triggers sludge bulking. An excessively low SV means insufficient sludge concentration and inadequate microorganisms, leading to reduced treatment efficiency.
Comprehensive analysis combining mixed liquor suspended solids (MLSS) and sludge volume index (SVI) is required. Adjust sludge wasting volume and aeration rate in a timely manner to stabilize the wastewater treatment system and guarantee qualified effluent quality.
专业术语注释（行业通用）
沉降比 SV = Sludge Volume（工程现场简称 SV，完整指标概念常搭配 SVI 污泥体积指数）
