On-Site Mixed Liquor Suspended Solids (MLSS) Analysis: A Practical Guide for Wastewater Treatment Plant Operators
I. Purpose of the MLSS Test:
The MLSS (Mixed Liquor Suspended Solids) test determines the total concentration of all suspended solids (both biological and inert) present in the aeration tank of an activated sludge wastewater treatment plant. This measurement is crucial for:
Biomass Quantification: Knowing the amount of active microorganisms available for pollutant degradation.
Process Control: It's a key parameter for calculating the Food-to-Microorganism (F/M) ratio and sludge age (Mean Cell Residence Time - MCRT), both vital for process stability.
Aeration Management: Higher MLSS often means higher oxygen demand.
Sludge Wasting Decisions: Directly impacts the amount of waste activated sludge (WAS) generated.
SVI Calculation: MLSS is a required input for calculating the Sludge Volume Index (SVI).
II. Principle of the Test:
The MLSS test is a gravimetric (weight-based) method. A precisely measured volume of well-mixed liquor is filtered through a pre-weighed glass fibre filter paper. The solids retained on the filter are then dried in an oven at 103-105°C until all water evaporates. The increase in the filter's weight after drying represents the mass of suspended solids in the original sample.
III. Equipment and Materials Required:
Ensure all equipment is clean and calibrated.
1. Sampling Container:
A clean, wide-mouth bucket or jug (2-5 liter capacity) for collecting the mixed liquor sample.
2. Volumetric Pipette or Graduated Cylinder:Volumetric Pipette or Graduated Cylinder:
A precise measuring device (e.g., 25 mL, 50 mL, or 100 mL) to accurately measure the mixed liquor subsample for filtration. Choose a volume that, based on your expected MLSS, will yield between 2.5 mg and 200 mg of dry solids on the filter for optimal accuracy. For typical plant MLSS (2,000-4,000 mg/L), a 50 mL sample is generally suitable.
3. Glass Fiber Filter Discs:
Standard filters, often 47 mm or 90 mm diameter (e.g., Whatman GF/C or equivalent, with a nominal pore size of 1.5 µm). These should be oven-dried and pre-weighed.
4. Filtration Apparatus:
Filter Funnel: A Büchner funnel or similar vacuum filter funnel.
Filter Flask (Vacuum Flask): A heavy-walled flask designed to withstand vacuum.
Vacuum Pump: A hand-operated vacuum pump, water aspirator, or electric vacuum pump.
Rubber Stopper/Adapter: To create a seal between the funnel and flask.
5. Drying Oven:
Thermostatically controlled, capable of maintaining a temperature of 103°C to 105°C.
IV. Step-by-Step Procedure for MLSS Analysis:
Part A: Preparing the Glass Fiber Filter Paper
This step must be performed meticulously to ensure the filter's initial weight is stable and free of moisture.
Preparation: Using clean forceps, carefully place a new glass fiber filter disc into the filtration apparatus.
Rinse: Apply vacuum to the flask. Pour a small amount (e.g., 20-30 mL) of distilled or deionized water over the filter paper to rinse it and draw out any loose fibers or impurities. Continue applying vacuum until all water has passed through.
Initial Drying: Carefully remove the filter paper with forceps and place it in a designated, clean drying dish or on a piece of aluminum foil. Transfer it to the drying oven at 103-105°C for at least 1 hour (or longer, until a constant weight is achieved for precision, meaning successive weighings do not differ by more than 0.5 mg).
Cooling: Immediately after drying, transfer the filter paper (using forceps) to a desiccator. Allow it to cool to room temperature (typically 15-20 minutes). Cooling in a desiccator is vital to prevent the filter from absorbing moisture from the air, which would lead to an inaccurate weight.
Weigh (W1): Once cooled, carefully transfer the filter paper to the analytical balance and record its initial weight (W1) in grams to four decimal places (e.g., 1.2345 g). If not used immediately, return the filter to the desiccator.
Part B: Filtering the Mixed Liquor Sample
Sample Collection:
Go to the aeration tank at your plant in Muthukadu. Collect a representative sample of mixed liquor from the outlet of the aeration tank (just before it flows into the secondary clarifier) using your clean sampling container.
Ensure the sample is well-mixed and homogeneous, reflecting the actual conditions the sludge is experiencing.
Measure Sample Volume:
Immediately and thoroughly re-mix the collected mixed liquor sample (gently swirling or inverting to ensure all solids are evenly suspended).
Using your accurate volumetric pipette or graduated cylinder, precisely measure the chosen volume of mixed liquor (e.g., 50 mL or 100 mL).
Record this exact volume (V) in milliliters (mL). This is critical for your calculation.
Filter the Sample:
Place your pre-weighed (W1), cooled filter paper back into the filtration apparatus.
Turn on the vacuum pump.
Carefully pour the precisely measured mixed liquor sample onto the center of the filter paper.
Rinse the inside of the measuring cylinder and the walls of the filter funnel with a small amount (e.g., 10-20 mL) of distilled/deionized water to ensure all solids are transferred from the glassware to the filter. Allow all water to pass through.
Part C: Final Drying and Weighing of Solids
Dry Solids:
Carefully remove the filter paper with the retained solids using forceps.
Place the filter paper in a clean drying dish or on a fresh piece of aluminum foil.
Transfer it to the drying oven at 103-105°C for at least 1 hour (or until constant weight is achieved).
Cool Solids:
Immediately remove the filter paper from the oven using forceps and transfer it to the desiccator.
Allow it to cool completely to room temperature (typically 15-20 minutes). This step is crucial for accurate weighing, as hot items can create air currents around the balance, leading to errors.
Weigh (W2):
Carefully transfer the cooled filter paper with the dried solids to the analytical balance.
Record the final weight (W2) in grams to four decimal places.
V. Calculation of MLSS:
Once you have recorded W1, W2, and the sample volume (V):
MLSS (mg/L) = [ (W2 - W1) * 1,000,000 ] / V (in mL)
Alternatively, if you prefer to use volume in Liters:
MLSS (mg/L) = [ (W2 - W1) * 1000 ] / V (in Liters)
Where:
W1 = Initial weight of clean, dried filter (grams)
W2 = Final weight of filter + dried solids (grams)
V = Volume of mixed liquor sample filtered (milliliters or liters, depending on the formula used)
1,000,000 or 1000 = Conversion factors to get the final result in mg/L (parts per million).
VI. Important Considerations for On-Site MLSS Testing:
Consistency: Perform the test at the same time each day, from the same location in the aeration tank, to ensure comparable results for trend analysis.
Accuracy of Balance: The analytical balance is the most critical piece of equipment. Ensure it is level, calibrated regularly, and protected from drafts or vibrations.
Proper Sample Volume: Select a sample volume that results in a reasonable amount of solids on the filter (2.5 mg to 200 mg). If too little, weighing errors are magnified. If too much, filtration is very slow.
Handling: Always use forceps or tongs to handle filter papers to prevent contamination from skin oils or moisture.
Record Keeping: Maintain meticulous records of all data, calculations, and observations. This data is invaluable for process optimization and troubleshooting.
By diligently following this procedure, operators of wastewater treatment plant can accurately monitor MLSS, a cornerstone of effective activated sludge process control.