Determination of SVI, MLSS, and MLVSS

1. Title

Determination of Sludge Volume Index (SVI), Mixed Liquor Suspended Solids (MLSS), and Mixed Liquor Volatile Suspended Solids (MLVSS)

2. Objective

• To determine the Mixed Liquor Suspended Solids (MLSS) concentration in the aeration tank sample.
• To determine the settleability of sludge using the 30-minute settling test.
• To calculate the Sludge Volume Index (SVI) to assess the settling characteristics and quality of the activated sludge.

3. Apparatus Required

• Imhoff cone (1-liter capacity)
• Vacuum filtration assembly
• Glass fiber filter discs (Grade GF/C) with a pore size of 1.2 µm
• Analytical balance (up to 0.1 mg precision)
• Drying oven
• Volumetric pipettes
• Porcelain crucibles
• Forceps and tongs
• Stopwatch
• Wash bottles

4. Theory

The Activated Sludge Process (ASP) is a biological wastewater treatment method where microorganisms (biomass) consume organic pollutants. These microorganisms are suspended in the wastewater, forming a mixture called “Mixed Liquor.”

• Mixed Liquor Suspended Solids (MLSS): The concentration of suspended solids in the mixed liquor, usually expressed in mg/L. It represents the total biomass (living bacteria, dead cells, and inert solids) available to treat the waste.
• Sludge Volume Index (SVI): Defined as the volume in milliliters occupied by one gram of activated sludge after settling for 30 minutes. It is a critical parameter used to evaluate the sludge’s ability to settle and compact in the secondary clarifier.

5. Principle

• MLSS Determination: It is based on gravimetric analysis. A known volume of the sample is filtered, dried at 103–105°C, and weighed.
• SVI Calculation: It combines the physical settling test (SV₃₀) and the biological concentration (MLSS). The formula is:

6. Procedure

A. Determination of Settled Sludge Volume (SV₃₀)

1. A 1-liter Imhoff cone was filled up to the 1-liter mark with the mixed liquor sample.
2. The sample was allowed to settle undisturbed for 30 minutes.
3. The volume of the settled sludge layer (SV₃₀) in mL/L was recorded.

B. Determination of MLSS

1. A clean, dry glass fiber filter paper was weighed (W₁).
2. A mixed sample (10 mL) was taken from the aeration tank.
3. The sample was filtered through the filter paper using a vacuum filtration assembly.
4. The filter paper with the residue was dried in an oven at 103–105°C for 1 hour.
5. The filter paper was cooled in a desiccator, weighed, and marked as W₂.

7. Observation and Calculation

Data from Sample I (Aeration Tank)

Calculation for MLSS (Sample I):

Calculation for SVI (Sample I):

Data from Sample II (Secondary Settling Tank — SST)

Calculation for MLSS (Sample II):

Calculation for SVI (Sample II):

8. Result

The obtained data are as follows:

Sample	MLSS	SV30	SVI
Sample I (Aeration Tank)	11,300 mg/L	520 mL/L	46.01 mL/g
Sample II (Secondary Settling Tank — SST)	8,400 mg/L	440 mL/L	52.38 mL/g

9. Discussion and Conclusion

The activated sludge from the Guheshwori Wastewater Treatment Plant showed an SVI value of 46 mL/g. This value falls into the rapid settling category (< 80 mL/g), which indicates that the sludge is very dense and settles quickly.

An ideal SVI range for a healthy activated sludge system typically operates between 80 and 150 mL/g. MLSS and SVI are process control parameters and do not have direct limits in WHO and NDWQS guidelines. However, these parameters strongly influence effluent Total Suspended Solids (TSS). In Nepal, the discharge standard for industrial effluents into inland surface waters generally limits TSS to about 30–50 mg/L.

In conclusion, the system is carrying a very high solids concentration (MLSS is extremely high), and the sludge has excellent settling properties (rapid compaction).

10. Engineering Significance

1. F/M Ratio Control: MLSS helps control the Food-to-Microorganism (F/M) ratio; too high increases aeration cost and causes poor settling, while too low reduces treatment efficiency.
2. Sludge Health Indicator: SVI indicates sludge health — values below 80 mL/g show old dense sludge, 80–150 mL/g represent good settling, and above 150 mL/g indicate bulking and poor settling.
3. Pin Floc Risk: An SVI of 46 mL/g shows very fast settling sludge, but it may create pin flocs that make the effluent slightly cloudy.
4. Overloaded Clarifiers: An MLSS of 11,300 mg/L is above the normal limit (~6,000 mg/L), putting heavy load on settling tanks and increasing the risk of pollution.
5. Oxygen Transfer: High sludge concentration increases viscosity, reduces oxygen transfer efficiency, and may create anaerobic zones in the aeration tank.

11. Precautions

1. Immediate Testing: The SVI test must be performed immediately (within 15 minutes) of sampling. If the sludge sits for too long, biological activity changes its settling characteristics.
2. Stable Surface: The Imhoff cone must be placed on a vibration-free surface and kept out of direct sunlight.
3. Desiccator Cooling: The filter paper must be thoroughly cooled in a desiccator while determining the MLSS to avoid weighing errors.
4. No Air Bubbles: Do not allow air bubbles to get trapped under the filter paper while performing vacuum filtration.