Aerobic Digestion

One of the major jobs at a wastewater treatment plant is the wasting of sludge produced during the process of treating wastewater. The sludge wasted from the secondary clarifier is normally sent to an aerobic digester, a thickener or both.

The sludge from the secondary clarifier is normally concentrated to less than one percent solids (10,000 mg/l). The digester and/or thickener further concentrate the sludge and reduce the volume of sludge to be dewatered by further digesting the volatile solids left in the sludge. The normal detention time in an aerobic digester is 10 to 15 days. If properly operated, the volume of sludge should be reduced by 35 to 50 percent to a concentration of 2.5 to 5 percent solids.

This is accomplished by filling the aerobic digester with sludge from the secondary clarifier and aerating it for several hours, depending on the digester volume and the amount of sludge wasted; then you must allow it to settle. The aeration (1-2 mg/l DO) allows the bacteria to continue to degrade the pollutants in the water and forces some of the bacteria into the endogenous respiration phase.

In this phase, the bacteria are feeding on the volatile material left in the sludge, dead bacteria and the stored energy in their cells. This process results in a lower volume of sludge that has to be dewatered at the end of the process.

After settling, the clear water (which is not clean) from the top section of the digester is decanted back to the headworks of the treatment plant. This flow has to be retreated due to the amount of soluble BOD, suspended solids, nitrogen and phosphorus in it. At this point, the flow, BOD, TSS, nitrogen and phosphorus should also be sampled from the decant, as these have an effect on the sludge age and F/M of the plant, which in turn has an effect on the total treatment.

The digester is then filled with more wasted sludge, and the process repeats until the solids reach the desired level for proper and economical dewatering and final disposal.

The final product of wastewater treatment is sludge that is ready for final disposal. The aerobic digestion process is only one of a few ways to prepare sludge for final disposal. In future posts, I will discuss other methods to prepare sludge for dewatering and disposal.


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