Twenty years ago, the first wastewater facility implemented a real-time solids retention time (SRT) control system. Since then, such systems have provided the following benefits: an increase of plant capacity by as much as 30% due to the improved sludge settleability; foam reduction; decreased disinfectant usage due to lower secondary effluent TSS. Furthermore, SRT real time control automation led to up to 30 % reduction of aeration energy usage; reduction of polymer consumption by as much as

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... -quarter; and up to 90% decrease in process control related laboratory TSS tests. These improvements were realized by optimizing SRT targets to satisfy conflicting objectives and then reliably maintain these targets with ±2.5% accuracy. Automatic SRT control software utilizes an activated sludge computer model, machine learning algorithms, and an advanced output data filtration module. A combination of these tools has been providing continuous improvements in both precision and reliability of the SRT control.