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Mixed Liquor Suspended Solids (MLSS)

Monitoring Suspended Solids in the Activated Sludge Process

The activated sludge process deals with the treatment of sewage (and industrial wastewater) and was developed around 1912-1914. Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to biologically oxidise organic pollutants, producing a waste sludge (or floc) containing the oxidized material. Atmospheric air or pure oxygen is bubbled through primary treated sewage (or industrial wastewater) and combined with organisms to develop a biological floc which reduces the organic content of the sewage. The combination of raw sewage and biological mass is commonly known as Mixed Liquor. In all activated sludge plants, once the sewage (or industrial wastewater) has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge (RAS), is returned to the head of the aeration system to re-seed the new sewage entering the tank. Mixed Liquor is a mixture of raw or settled wastewater and activated sludge within an aeration tank in the activated sludge process. Mixed Liquor Suspended Solids (MLSS) is the concentration of suspended solids in the mixed liquor, usually expressed in milligrams per litre (mg/l).

Why Measure MLSS

If MLSS content is too high the process is prone to bulking and the treatment system becomes overloaded, this can cause the dissolved oxygen content to drop with the effect that organic matters are not fully degraded and biological ‘die off’. Conversely, if the MLSS content is too low the process is not working efficiently, and is likely to be wasting energy whilst not treating the effluent effectively.The typical control band is 2,000 to 4,000 mg/l.

Fixed installation Monitor

With a permanently installed monitor 24/7 trending and alarming is available. This brings in the possibility of automated control, which improves plant efficiency by providing stability and continuity to the treatment process.Continuous On-Line Monitoring reduces the need for time-consuming laboratory analysis, removing operator dependency from the measurement.

Portable SS Monitor

Measurement using a properly calibrated portable monitor is now very reliable and repeatable, the data is also available immediately and can be used to make site adjustments to improve the performance of the site.

Laboratory Tests

The sample is sent away to a remote laboratory and the results are typically received 24 hours+ after the event, this makes remedial action on site during the initial visit impossible. Additionally the sample degrades in transit.

Site Settling Jars

The test takes 30 minutes or more and is extremely operator dependent, this means that a site can be kept in control when the same operator is looking after the site and has time to make the measurement carefully. If the operator changes or if time is limited the validity of the settling jar test becomes very questionable.


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