WQCF Aeration Improvements
City of Manteca WQCF staff, HERWIT Engineering, and APG-Neuros Inc. implemented a state-of-the-art aeration system for nitrogen control that does an excellent job of biological treatment to meet ammonia levels, stable process treatment at minimum energy usage.
Project Background
City of Manteca, located 15 miles north of Modesto California, provides sewer service and provides treatment at the City’s Wastewater Quality Control Facility (WQCF). The WQCF treats an average dry weather flow (ADWF) of about 7.6 mgd and has an original average dry weather design capacity of 9.9 mgd. However, historic water use reductions in the community combined with population growth have drastically increased the concentration of biological oxygen demand (BOD) and total Kjeldahl nitrogen (TKN) in the influent wastewater. Incoming wastewater became of higher strength overall and the biological and nitrogen loading on the plant increased, thereby requiring better nitrogen control in the form of nitrification and denitrification to meet ammonia limits of 1.5.
Project Development
The WQCF is separated into North and South WWT Plants. The WQCF operates a Ludzak-Ettinger (MLE) process. Aeration efficiency and control improvements were subsequently made to the WQCF for the North Plant in 2014/2015, and the South Plant in 2017/2018. Both facilities were design with complete aeration control systems provided by APG-Neuros that included initially dissolved oxygen (DO) control with most open valve (MOV) control of 20 aeration zones at each plant. The aeration improvements produced audited energy savings of 963,000 kW-hr annually and received energy rebates of $93,000.
After these improvements the team took further steps to optimize the aeration control system by implementing a nitrogen removal system for both facilities. This proved to be effective in response to the increased nitrogen loadings at the WQCF. The City of Manteca has an Ammonia limit less than 1.5 mg/l and a total nitrogen limit nitrite plus nitrate less than 10 mg/l. All improvements were made in a systematic step by step manner in a team effort between the City of Manteca WQCF staff, HERWIT Engineering, and APG-Neuros Inc. To improve nitrogen removal efficiency WQCF staff began the process by adding on-line nitrate and ammonia sensors in selected Zones of one process train at each plant. The team added cycling of DO in selected Zones between low and high DO values that showed immediate noticeable improvements in nitrogen removal levels in these zones.
The treatment process was further improved upon by testing and auto adjustment of the oxidation reduction potential (ORP) using dedicated ORP probes in selected Zones. Upon completion of testing, the team implement ORP cycling control throughout the process and some zones no longer ran on DO control. This proved that in specific zones, the nitrogen control improved under ORP control over DO control.
Results
The nitrogen control has been successfully accomplished by deploying a step-by-step implementation approach of advanced aeration control that combined Most Open Valve (MOV) control, DO Control and ORP control in selected zones. WQCF has been able to accomplish ammonia levels well below the 1.5 and nitrate far below 10, while respecting all sludge concentration and other process requirements.
