Mixed Liquor Suspended Solids (MLSS) - The content of the aeration tank in an activated sludge system is called mixed liquor. The MLSS is the total amount of organic and mineral suspended solids, including microorganisms, in the mixed liquor. The normal concentration of mixed liquor expressed as suspended solids (MLSS) is between 2000–5000 mg/L
Monitoring the MLSS and MLVSS concentration is an effective means of controlling the microbial activity in an aeration basin because of the direct correlation between the suspended solids concentration and the size of the microbe population. The longer the process has been in the tank, the greater the microbe population.
MLSS: Measured in the same way as suspended solids in wastewater: by filtering a known volume of the mixed liquor sample through Whatman GF/C filter paper and weighing it after drying in an oven at 105 C.
A = weight of filter + dried residue, mg,
B = weight of filter, mg.
Range Of Values Of Mixed Liquor Suspended Solid Particles In Activated Sludge:
Conventional = 1,500 - 3,000 MLSS (mg/l)
Step Aeration = 2,000 - 3,500 MLSS (mg/l)
Extended Aeration = 3,000 - 6,000 MLSS (mg/l)
Pure Oxygen System = 6,000 - 8,000 MLSS (mg/l)
High Rate Aeration = 4,000 - 10,000 MLSS (mg/l)
Mixed Liquor Volatile Suspended Solids (MLVSS) - The organic portion of MLSS is represented by MLVSS, comprises nonmicrobial organic matter, live microorganisms && cellular debris and represents approximately 65–75 % of MLSS.
Due to settling && supernatant pump-out, MLSS concentration in the effluent sludge is typically double the influent MLSS concentration. Part of the mixed liquor is returned to the secondary clarifier. The rest is "wasted”- diverted from the return loop to the thickening/digesting stage for further processing. Removing the waste activated sludge maintains the MLSS loading level in the aeration basin.
MLVSS: The MLSS residue dried as described as above is ignited at 550°C in a muffle furnace. An ignition time of 15 to 20 minutes is usually sufficient for 200 mg residue. The remaining solids represent the fixed total, dissolved, or suspended solids, while the weight lost on ignition is the volatile solids. The determination is useful in control of wastewater treatment plant operation because it offers a rough approximation of the amount of organic matter present in the solid fraction of wastewater, activated sludge, and industrial wastes.
The ratio of MLVSS to MLSS for activated sludge is in the range of 0.65 to 0.90
MLSS and MLVSS: These provide a crude estimate of biomass concentrations when coupled with influent BOD5 values to determine F/M ratios. The MLVSS represents the organic proportion of the biomass, but not necessarily its active portions.