What is MLSS?
MLSS stands for mixed liquor suspended solids, which is commonly given in grams per litre.In the activated sludge process, mixed liquor is a mixture of raw or settled wastewater and activated sludge held in an aeration basin. In the suspended growth process, MLSS are employed to control the wastewater treatment plant. Thus, the concentration of suspended particles in an aeration tank during the activated sludge process, which occurs during waste water treatment, is known as mixed liquid suspended solids.
Measurement of MLSS
The MLSS units are usually measured in milligrams per litre (mg/L), however for activated sludge, they are usually measured in grams per litre [g/L], which is equal to kilogrammes per cubic metre [kg/m3]. In an aeration tank, it is a mixture of raw or unsettled wastewater, pre-settled wastewater, and activated sludge. Microorganisms and non-biodegradable suspended materials make up the majority of MLSS. It is an important aspect of the activated sludge process because it ensures that there is always enough active biomass to absorb the applied amount of organic pollutant. It is described as food to microbe ratio or F/M.
The biomass will consume a large percentage of the food if this ratio is maintained at an adequate level. This reduces the amount of food residue in the treated effluent. In basic terms, the lower the biochemical oxygen demand (BOD) in the discharge, the more biomass is consumed. It's critical that MLSS removes COD and BOD from water in order to cleanse it for clean surface waters, as well as clean drinking water and hygiene.
When is MLSS considered healthy in wastewater treatment plants?
When we meet the following criteria, we term MLSS healthy:
1: Obtain effluent treatment objectives;
2: The SV30 and SVI are both within simple operating range;
3: Low growth rates F/M in goal range D.O. is above 2 mg/L throughout the system;
4: There are indicator protozoa present and active.
There is a reversion to k-rate organisms like Pseudomonas, Bacillus, and a number of lesser-known fast-growing organisms that flourish with high COD/BOD and associated low D.O. and possibly quasi-toxic components early in the system starting or following a big upset. These organisms have a high respiration rate and grow quickly in order to reduce COD/BOD. The cells agglomerate as the MLSS lowers, decreasing the amount of free bacteria in solution. If an upset occurs, they colonise the existing floc mass, which contains extracellular polymers, dead biomass, particles, and a decreased percentage of normal live biomass.
As the system grows, the bacteria mix begins to prefer r-rate strategy microorganisms, which thrive in low F/M environments such as those seen in activated sludge or when water quality meets effluent standards. This helps to maintain and grow floc.
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