The measurement of suspended solids
Someone recently asked, “What are the procedures for measuring sludge concentrations and amounts? How is sludge quality determined?”
Sludge quality can be determined by measuring the solids concentration in various tanks, by solids inventories or by determining the sludge-return and wasting flow rates.
Solids quantities and concentration in the activated-sludge process is measured to achieve balance or a correct ratio for the food amounts -- organic solids or BOD -- entering the treatment system and the bacteria amounts available to treat (eat) it. This “F/M” balance or ratio impacts process performance, with "F” being food entering the process and "M" being the system’s bacteria mass. Bacteria must be present sufficient to handle food entering.
Another reason to analyze concentrations is to know sludge thickness and flow rate returned from the final clarifier back to the treatment system.
Concentration measuring methods include the total-suspended solids (TSS) method, which measures solids concentration in activated-sludge samples. Widely accepted, it accurately and precisely measures actual suspended-solids concentrations. Because settling is not a part of the procedure, problems caused by differences in sludge sample settle-ability are eliminated.
The volatile suspended solids (VSS) method is a rough measure of solids concentration in samples of activated sludge derived by measuring volatile solids.
Because bacteria are mostly organic, the VSS test is a better indicator of organic-solids concentrations and therefore, the amount of bacteria in a sample.
Calculate solids quantities
The TSS and VSS solids concentrations are usually expressed in milligrams per liter (mg/L). This concentration data is used to calculate – in pounds of solids or of pounds per day of solids -- amounts in a basin or flowing into or out of “something.”
Let's look at how to convert the concentration in mg/L to an amount in pounds (lbs). The formula for this conversion is:
Lbs = Volume (MG) X TSS Concentration (Mg/L) X 8.34
If we determine the concentration of solids in Mg/L of VSS, the calculation would be the same except we substitute the VSS concentration for the TSS concentration. The result would be lbs. of VSS instead of lbs. of TSS. An example of their ratio might be VSS as 75% of TSS.
To determine settled sludge concentration: If we know mixed liquor settle-ability, we can use those results while measuring solids settling and observing settling concentrations.
While performing the settle-ability test, recording settled sludge volumes (SSV) is useful.
Plotting the settle-ability values on a curve shows that as time passes, the sludge volume settling in the settlometer decreases. At the same time, TSS or VSS concentration measures and visual observation of settling sludge show sludge-blanket solids-concentration increases.
Using this information, a formula can calculate the settled sludge concentration (SSC) for a given time (T). These calculations identify the same relationships as the settle-ability curve.
Settling information is important to activated sludge process operators. However, sludge quality may be defined as the relationship between SSV and either TSS or VSS.
A “normal” settling, good-quality activated sludge from a conventional activated-sludge process may reach ultimate compaction -- that is, it stops settling -- somewhere between one and two hours.
Additional useful information
For conventional activated-sludge process, normal sludge ages range from three to six days and for extended aeration activated sludge process, the normal ages from 10 to 30 days.
Rapid-settling sludge reaches its endpoint concentration in less than one hour and is usually an and, over-oxidized sludge. Rapid-settling sludge is undesirable because by settling too fast it releases pin floc and other solids from the sludge blanket. These solids are then discharged in the plant effluent. A slower settling sludge consistently produces a higher quality effluent.
Slow-settling sludge that is old will be dark brown and settles significantly faster if diluted. Diluting is using equal amounts of sample and water. Any foam present will be brown and scummy-greasy looking. Young, slow-settling sludge will be light-brown to tan in color and settle about as fast whether diluted or not. Any foam present will be white and crispy-looking.
For process control, lower the wasting rate to increase sludge age which, in turn, will reduce sludge settling rates. Conversely, increase to wasting rates to decrease sludge age. This, in turn, will increase sludge settling rates.