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Mixed liquor suspended solids

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Title: Mixed liquor suspended solids  
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Subject: Membrane bioreactor
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Mixed liquor suspended solids

Mixed liquor suspended solids (MLSS) is the concentration of suspended solids, in an biomass will consume high percentages of the food. This minimizes the loss of residual food in the treated effluent. In simple terms, the more the biomass consumes the lower the biochemical oxygen demand (BOD) will be in the discharge. It is important that MLSS removes BOD in order to purify water for clean drinking water and hygiene. Raw sewage enters in the water treatment process with a concentration of sometimes several hundred mg/L of BOD. Upon being treated with MLSS and other methods of treatment, the concentration of BOD in water is lowered to less than 2 mg/L, which is considered to be clean, safe to use water.

The total weight of MLSS within an aeration tank can be calculated by multiplying the concentration of MLSS (mg/L) in the aeration tank by the tank volume (L).


MLSS is responsible for removing the biochemical oxygen demand make-up of a large portion of the solids that are retained in the activated sludge process within the water treatment process. They are the "active" part of activated sludge process. Mixed liquor suspended solids are the solids under aeration. MLSS is measured by filtering a known volume of the mixed liquor sample, which is the same way that suspended solids are measured in wastewater. Some of the MLSS may be inorganic material. Sometimes this may represent a large percentage of the solids present in the wastewater.

Environmental engineering focuses on the particles suspended in water and the suitable operation of water treatment plants. Therefore, it is important to measure the total mass of suspended solids, which is the MLSS, as well as the mass of organic matter suspended in the activated sludge unit. These measurements allow engineers to adjust the flow rate of return sludge from the secondary clarifier into the secondary treatment reactor. This ensures that influent organic matter will be treated with a correct concentration of microorganisms.

Mixed liquor volatile suspended solids

The portion of the MLSS that is actually eating the incoming food is referred to as the Mixed Liquor Volatile Suspended Solids (MLVSS). The volatile solids concentration in a sample of mixed liquor will consist mostly of microorganisms and organic matter. As a result, the volatile solids concentration of mixed liquor is approximately equal to the amount of microorganisms in the water and can be used to determine whether there are enough microorganisms present to digest the sludge.

Removal process

MLSS is removed by secondary treatment in a settling tank in the activated sludge process.

Effects of MLSS in water treatment

If MLSS content is too high

  • The process is prone to bulking of solids and the treatment system can become overloaded.
  • This can cause the dissolved oxygen content to drop; this may reduce the efficiency of nitrification and the settleability of the sludge.
  • Excessive aeration will be required which wastes electricity.

it will create thick foam on upper layer.

If MLSS content is too low

  • The process may not remove sufficient organic matter from the wastewater.
  • The sludge age may be too low to enable nitrification.

The typical control band for the concentration of MLSS in wastewater is 2,000 to 4,000 mg/L for conventional activated sludge, or up to 15,000 mg/l for membrane bioreactors.

One of the easiest control procedures for activated sludge systems is the Constant Mixed Liquor Suspended Solids method. In this method, the operator selects a certain MLSS concentration or range of mix liquor concentrations that produces the best effluent and the highest removal efficiencies. This specific value or range must be determined experimentally. When the operator finds the optimum MLSS concentration for each plant, he attempts to maintain this value by adjusting the sludge wasting rate. One rule of thumb for activated sludge systems is that for every pound of BOD removed in the secondary system a half a pound of new solids is generated through reproduction of the organisms and addition of new organisms from the influent wastes. So, the operator tries to waste the proper amount of solids to keep his selected optimum mix liquor concentration constant. If the MLSS concentration is above the desired concentration, the wasting of the excess solids will have to be started or increased. If the MLSS concentration is below the desired concentration level, wasting should be decreased or stopped.

Calculations of MLSS

MLSS (mg/L) = [SV(1000 mg/g)]/SVI

SVI= sludge volume index (mL/g)
SV= Volume of settled solids per 1 litre


Q = wastewater flow rate (m3/d)
Qr = return sludge flow rate (m3/d)
X' = MLSS (g/m3)
X'r = maximum return sludge concentration (g/m3)
Qw = sludge wasting flow rate (m3/d)

Estimating MLSS

Materials: Obtain a sample of mixed liquor. Transfer the sample into a laboratory in order to analyze it. Measure the sample volume weight of the sample. Remove two filter papers from a desiccator and record the weight of each. Place the filter holder on a vacuum flask and then place another filter on top of the filter holder by using a pair of tweezers. Stir the sample of mixed liquor in order to get a good mixture for the experiment. After sample is mixed pour 5 mL into a graduated cylinder. Turn on the vacuum pump and pour the sample into the filter holder. After all the mixed liquor has gone through, run three portions of 10 mL distilled water through the filter holder to rinse any particles that may have stuck to the glass. Allow the vacuum pump to run an additional three minutes. This will help remove any extra water from the filter before drying. Switch the vacuum pump off and remove the filter from the filter holder and place in the

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