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System reduces operating costs & limits capital expenses
Faced with rising operating costs due to increasing energy and chemical prices as well as stricter effluent permit limits, many operators and engineers are turning to sensors and automation to enhance treatment performance and reduce operating costs while limiting capital expenses.
In order to overcome these challenges, an advanced process control solution was implemented in an aerobic digester in Green Lake, Wisc. By controlling the process in a smarter way, the solution significantly reduced phosphorus and nitrogen in the returns to the main treatment process and reduced energy consumption and wear on the blower.
The study was conducted at the city of Green Lake wastewater treatment plant, which uses a continuous feed ICEAS advanced sequencing batch reactor (SBR) system with aerobic digestion that has an average flow of 0.2 million gal per day (mgd). The digester includes fine bubble diffusers, positive displacement blowers and a manual telescoping valve for decanting. Digester solids are sent to a belt filter press for dewatering and ultimately land application.
The digester has four zones operating in series—two in the east digester and two in the west digester. Waste activated sludge is automatically pumped from the ICEAS system into the east digester approximately every two hours. The supernatant from the digester is returned to the SBR through the telescoping valve and is manually initiated four days per week. A sludge pump is used to remove sludge from the west digester once per week.
During the study, YSI sensors measuring ammonium, nitrate (NO3), potassium, dissolved oxygen and oxidation reduction potential were installed in the east and west digesters. Additionally, a YSI orthophosphate (PO4) analyzer (P700 model) was installed in the east digester.
The aerobic digester was upgraded with an Oscar process performance optimizer control system with Dino controller, which automatically adjusts the required aeration time in the digester based on potassium readings. The Dino controller alternates aerobic, anoxic and anaerobic conditions in the digester to optimize total nitrogen and phosphorus treatment while minimizing energy consumption.
The Oscar system with Dino controller enabled biological phosphorus removal in the digester, reducing PO4 returned to the main plant more than 90% on average. This reduction helped the plant reduce chemical consumption in the main treatment process by more than 50% while still meeting the effluent phosphorus permit.
The Dino controller also achieved 100% reduction in NO3 returned to the main treatment process when compared to previous operation, which was continuously aerated. Digester blower energy consumption was reduced at Green Lake by 95% compared to previous operation. Blower energy was shifted from 36% on-peak to 0% on-peak, reducing demand charges and the unit cost of energy.
The aerobic digester, equipped with the Oscar system with Dino controller, enhanced digester performance by reducing nutrients that were returned to the main treatment process by more than 90% and lowering energy consumption by more than 95%.