Mixed liquor suspended solids (MLSS) and Sludge Volume Index SVI

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Please can you help explain the difference between MLSS and Sludge Volume Index (SVI). How to ​calculate or ​determine these.​

The context is for Aeration ​tank in the dairy ​industry

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4 Answers

  1. =mL/g  

     

     

    Hope this explains the difference.  SV is volume of settled solids in one-liter graduated transparent measuring cylinder after 30 minutes settling period.

    mL/L – MLSS – Mixed Liquor Suspended Solids

    ppm SVI is a key factor when it comes to the clarifier design so that clear water discharge can be obtained without significant carryover of sludge

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  2. The MLSS is a measurement of the concentration of the suspended solids in the aeration tank.  The SVI is a measure of how well the suspended solids settle and compact when left undisturbed for 30 minutes, except that the lower the number the better it has settled since it is expressed in units of ml per gram.

    I looked in an old textbook of mine to refresh my memory of these terms ("Unit Operations and Processes in Environmental Engineering" by Tom D. Reynolds, published by PWS-KENT in 1982).  The glossary provided these two definitions.

    • Mixed Liquor Suspended Solids (MLSS): The suspended solids in the mixed liquor- that is, the mixture of wastewater and activated sludge undergoing aeration at an activated sludge wastewater treatment plant.
    • Sludge Volume Index (SVI): The volume in millilitres occupied by one gram of mixed liquor after settling for 30 minutes in a litre graduated cylinder.

    The book also gave this overview of how the two measurements are used to regulate the operation of the plant's recycle ratio.  See attached PDF.

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  3. I would suggest to have a look at https://www.tpomag.com/editorial/2010/03/what-the-heck-is-svi. It gives the complete answer.

  4. The SSLM is the measurement of the amount of solids found in the aeration pond (or bioreactor). SSVLM are the volatile suspended solids, which represent the amount of biomass (microorganisms) found in the reactor. On the other hand, the SVI is an indicator of the sedimentability of the mud. The calculation relates the solids that settle in 30 min, with the concentration of solids. For the calculation, the sedimentable solids (SedS) are divided by mL / L by the concentration of total solids (SSLM) in g / L and the result indicates how sedimentable the mud is. According to Von Sperling, an SSVI of 0 - 50 mL / g is "very good", 50 - 80 "good", 80 - 140 "medium", 140 - 200 "poor" and above 200 is "very poor". .. all this will indicate the probability of having problems in the secondary clarifier.

    2 Comments

    1. MLSS is the mg/L of suspended solids in the aeration tank and is simply a measure of the solids themselves, having been filtered, and the water evaporated in a drying oven, then weighed. The SVI is a measure of how well the solids settle, or compact, in the quiescent environment of the clarifier and mimicked in a settleometer, in 30 minutes. A high SVI is a bulking or "fluffy" sludge that does not compact well. Since SVI is an expression of "how many mls of the settled sludge it takes to make a gram", a low SVI means it is compacting well and consequently takes less mls to make that gram. If you have a high SVI you can do a 50:50 test, using 1/2 plant effluent and 1/2 activated sludge and try that in the settleometer. If it settles much better, you are carrying too many solids and need to waste more sludge from the system. If it settles only slightly better, then you probably have an issue with long filament dominence and need to find the problem. It is often related to areas in the system with very low D.O. such as a recycle wastestream from an aerobic digester with only intermittent aeration or even scum build up on the basins, but there are various types of filaments, and even a long line feeding the plant can select for them as it loses oxygen.

    2. 0-50 ml/g is possible only in the start of process, later it is something like 80-130 ml/g, even cross this.

      1 Comment reply

      1. En realidad también es posible cuando el lodo se encuentra demasiado mineralizado y sedimenta de forma abrupta, rápida y con poco volumen... eso es señal de una buena sedimentabilidad, pero de una mala "salud" del lodo...