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On-Site Sludge Volume Index (SVI) Analysis: A Practical Guide for Wastewater Treatment Plant Operators

The Sludge Volume Index (SVI) is a vital, quick, and cost-effective test used in Activated Sludge Process (ASP) wastewater treatment plants to assess the settling characteristics and overall health of the activated sludge. Regular SVI monitoring is crucial for effective process control and to prevent issues like sludge bulking and solids carryover in the final effluent. This guide outlines the step-by-step procedure for conducting an SVI analysis directly at your treatment plant site.

I. Purpose of SVI Analysis

The primary purpose of the SVI test is to:

  • Quantify Settling Ability: Provide a numerical value indicating how well the activated sludge settles and compacts.

  • Quantify Settling Ability: Provide a numerical value indicating how well the activated sludge settles and compacts.

  • Aid Process Control: Help operators make informed decisions regarding sludge wasting rates (sludge age), aeration levels, and other process adjustments.

II. Equipment Required

To perform an SVI analysis, you will need the following standard laboratory and sampling equipment:

  • 1-Litre Graduated Cylinder: A clean, clear glass or plastic cylinder with 1000 mL graduation marks.

  • Mixed Liquor Sampling Container: A clean bucket or wide-mouth jug (e.g., 2-5 litre capacity) to collect the representative mixed liquor sample from the aeration tank.

  • Stopwatch or Timer: For accurate timing of the 30-minute settling period.

  • Smaller Measuring Cylinder/Beaker: A clean and accurate measuring cylinder (e.g., 50 mL or 100 mL) to collect a precise subsample for MLSS analysis.

  • MLSS Testing Equipment: To determine the Mixed Liquor Suspended Solids (MLSS) concentration. This typically includes:


    • Glass Fiber Filter Papers: Pre-weighed (or weighable on-site) and suitable for suspended solids analysis (e.g., Whatman GF/C or equivalent).


    • Filtration Apparatus: A filter funnel, vacuum flask, and vacuum pump (or hand pump) for rapid filtration.


    • Drying Oven: Capable of maintaining a temperature of 103-105°C (usually located in the plant's lab).


    • Desiccator: For cooling filter papers and samples to prevent moisture absorption before weighing.


    • Analytical Balance: Accurate to 0.1 mg (typically in the lab).


    • Forceps or Tongs: For handling filter papers.

III. Step-by-Step Procedure for SVI Analysis

The SVI analysis involves two main parts: determining the settled sludge volume (SV30) and determining the Mixed Liquor Suspended Solids (MLSS).

Part A: Determining Settled Sludge Volume (SV30)
1. Sample Collection (Aeration Tank Outlet):

  • Go to the outlet of your aeration tank, just before the mixed liquor flows into the secondary clarifier.

  • Using your clean sampling container, collect a representative sample of mixed liquor. Ensure the sample is well-mixed and homogeneous, reflecting the typical conditions entering the clarifier.

  • Crucial: Collect enough volume (e.g., 2-3 litres) to perform both the SV30 and the MLSS subsample immediately.

2. Fill Graduated Cylinder:

  • Immediately after collection and ensuring the sample is still homogeneous, gently and carefully pour exactly 1.0 litre (1000 mL) of the mixed liquor sample into the 1-litre graduated cylinder.

  • Avoid vigorous pouring or agitation, which can break up flocs and lead to inaccurate results.

3. Initiate Settling & Timing:

  • Place the graduated cylinder on a stable, level surface where it will remain undisturbed.

  • Start your stopwatch or timer immediately.

4. Observe and Record SV30:

  • Allow the sample to settle undisturbed for precisely 30 minutes.

  • After exactly 30 minutes, read the volume (in millilitres, ML) of the settled sludge layer at the bottom of the cylinder. This is your SV30 value.

  • Note the appearance of the supernatant (clear, cloudy, turbid) and observe if any foam or floating solids are present.

Part B: Determining Mixed Liquor Suspended Solids (MLSS)

This part usually requires basic laboratory facilities available at the treatment plant.

1. Collect MLSS Subsample (From Original Mixed Liquor):

  • Crucial Point: Immediately after taking the 1-liter sample for SV30, and while the original mixed liquor sample (from step 1, Part A) is still well-mixed, collect precise subsample for MLSS analysis.

  • The volume of this subsample depends on your expected MLSS concentration. A typical volume might be 50ml or 100ml. Use your smaller, accurate measuring cylinder.

  • Important: DO NOT take the MLSS sample from the settled sludge in the graduated cylinder after the SV30 test, as this will not be representative of the MLSS in the aeration tank.

2. Prepare Filter Paper:

  • obtain a clean, pre-weighed (W1, in grams) glass fiber filter paper.

  • Record W1 accurately to the nearest 0.1 mg.

3. Filter the Subsample:

  • Set up your filtration apparatus (filter funnel, vacuum flask, vacuum pump.

  • Carefully place the pre-weighted filter paper into the filter funnel.

  • Pour the precisely measured MLSS subsample onto the filter paper. Apply vacuum to filter the water through, retaining the suspended solids on the filter.

  • Rinse the measuring cylinder and the inner walls of the filter funnel with a small amount of distilled water to ensure all solids are transferred to the filter paper.

4. Dry the Filter Paper with Solids:

  • Carefully transfer the filter paper with the retained solids to a pre-weighed drying dish or directly into a drying oven.

  • Dry the sample in the oven at 103-105'C for at least 1 hour (or unitll a constant weight is achieved).

5. Cool and Weigh:

  • Remove the filter paper from the oven and immediately transfer it to a desiccator to cool to room temperature. This prevents moisture absorption.

  • Once cooled, accurately weigh the filter paper with the dried solids (W2, in grams) on the analytical balance.

6. Calculate MLSS:

  • Weight of Solids (mg) = (W2 - W1) * 1000 (if W1 and W2 are in grams)

  • MLSS (mg/L) = Weight of Solids (mg) / Volume of Subsample (L)


    • Example: If w1 = 1.000 g. w2 = 1.0050 g, and subsample volume = 0.050 L (50 mL):


    • Weight of Solids = (1.0050 - 1.0000) * 1000 = 5.0 mg


    • MLSS = 5.0 mg / 0.050 L = 100 mg/L (This is a low example for illustration; typical MLSS is 2000 - 4000 mg/L)

IV. SVI Calculation

Once you have both your SV30 and MLSS values, calculate the SVI using the formula:

SVI (mL/g) = [SV30 (mL/L) / MLSS (mg/L)] * 1000

  • Example Calculation:


    • SV30 = 250 mL/L (from your 1-liter cylinder reading)


    • MLSS = 3000 mg/L (from your MLSS analysis)


    • SVI = (250 / 3000) * 1000 = 83.3 mL/g

V. Interpretation of SVI Values (General Guideline)

  • 50 - 150 mL/g: Indicates good settling sludge. The plant is likely operating efficiently.

  • 150 - 200 mL/g: Fair settling. This range suggests a potential operational shift or an early indicator of mild bulking. Close monitoring and minor adjustments might be needed.

  • > 200 mL/g: Poor settling, indicating sludge bulking. Immediate investigation into the cause (e.g., filamentous growth, low F/M, nutrient deficiency, low DO) and corrective actions are required to prevent effluent quality deterioration.

  • > 50 mL/g: Very dense sludge, sometimes referred to as "pin floc." While it settles rapidly, it can sometimes be associated with hazy effluent due to very fine dispersed particles that do not flocculate well.

Note: The ideal SVI range can vary slightly depending on the specific plant design, wastewater characteristics, and desired effluent quality. Consistently tracking your plant's SVI and understanding its normal operating range is more important than comparing it to absolute universal values.

By regularly performing this SVI analysis, operators can gain valuable insights into their activated sludge process, troubleshoot issues proactively, and maintain optimal plant performance and effluent quality.