How to Determine the Ideal Return Sludge Flow Rate (RAS)

return activated sludgeIn order to determine and come up with the ideal sludge return flow rate or RAS, you will need an indicator that can tell you what is needed to be done. Basically the main consideration on what everyone is using is by referring to the Solids Level in the wastewater system. In order to control and adjust the solid level, there are 3 different approaches which you can rely on to provide you the necessary information and to serve as a guideline. Although most operators will argue that based on experience coupled with observations, all that should be sufficient to determine the return flow rate, but nevertheless you will still need something which you can quantify and measure.

The first and easiest method is by using sludge depth which is basically very much straightforward to apply. Sludge Depth is determined by measuring the level of sludge blanket that accumulates in the clarifier. Generally, a rule of thumb is to maintain the bottom sludge blanket not more than a quarter of the clarifier. Adjustment should be carried out at least once a day and then monitored, if further needed, follow-up adjustments should be done based on the changing wastewater loading rate going to the system. Ideally, in order to avoid frequent adjustments carried out several times a day, one should strive to maintain a constant loading rate to the clarifier or else, if this is not possible, then the measurement should be determined based on maximum loading rate for the day which is during peak flow period.

The second approach to determine the best return activated sludge rate is by using the solids mass balance equation. Although it might sound complicated to some, in general, this is better than the earlier approach which is basically relying on crude observation. To determine the correct return flow rate, the equation below should apply.

Return Flow Rate = (Q x MLVSS) / (VSS – MLVSS)

Whereby, Q is loading rate to the system and VSS is the volatile suspended solids for the returned sludge

The calculation can be performed easily as long as the all required data are available. To do that, a duty roster can be created so that the operators can collect the flow meter reading based on a set timeline. Also, to ensure that there is no mistake in the calculation, you can consider creating a simple spreadsheet table for easy and fast computation. Not to forget, one must also ensure that the flow meter is working well and most important the right type of flow meter is installed based on the wastewater characteristics. Refer to the guideline on selection of wastewater flow meters so that problem arising from inaccurate data can be avoided. For MLVSS testing, make sure that the proper facility is available to carry out testing and provide fast results when is needed.

The third method of determining the return sludge flow rate is by using the settleability criteria. Basically settleability is defined as the volume occupied or taken up by the sludge after it is left settling for 30 minutes. Similar like the second method mentioned earlier, the data can also be obtained and calculated based on simple equation. Here, settleability is expressed in mL/L. If you need further clarification on what it all means, you can refer to measurement of SVI and MLSS guide. Simply put,

Return Flow Rate = (Q x SV30) / (1000 – SV30)

Whereby SV30 is your sludge settling volume after 30 minutes while Q is your loading rate.

Although this method is basically much easier as you do not need to carry out MLVSS testing which will take longer time to get the results, SV30 test has its own weakness whereby settleability test carried out in the field using test cylinder will not give an accurate representation on the actual settling rate happening in the clarifier. Finally as a crude guideline, the typical value for solids level is should be somewhere within a certain range but take note that different effluent treatment systems have different working parameters mainly due to varying operating conditions such as feed rate, wastewater loading characteristics, accompanying facilities and other factors. Refer to the table below for a general guideline on the amount of solids in a conventional activated sludge process.

Measuring ParameterTypical value (mg/L)
MLSS2000 – 3500
MLVSS1500 – 2500
SS (Suspended solid)6000 - 8000
VSS (Volatile Suspended Solid)4500 – 6000

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