In 2003 this Minnesota city began the process of upgrading its lagoon-based wastewater treatment facility (WWTF) into a new mechanical treatment facility. Nine years and two construction phases later, the city now has a fully operational facility and has upgraded its aeration system to lower energy and maintenance costs.
The wastewater influent flow of the WWTF is primarily from residential sources. Commercial and industrial users contribute less than five percent of the current wastewater flow. The average influent Biochemical Oxygen Demand (BOD5) is 185 mg/L with Total Suspended Solids (TSS) of 167 mg/L. Mixed Liquor Suspended Solids (MLSS) are kept around 4,000 mg/L. The activated sludge, extended aeration system was constructed for a capacity of over 8,000 people and a design flow of 687,000 gpd. Ferric chloride (FeCl3) is added for phosphorus control. The effluent is processed through clarifiers for solids removal. Biosolids generated from the plant are aerobically digested and stored in a covered tank onsite. The two, 19-foot deep earthen aeration basins were also designed to provide flow equalization. By releasing a constant flow, the Intermediate Pump Station causes the water levels to rise and fall inside the basins. UV light is used for disinfection.
At the onset of the plant upgrade several surface aerator types and brands were considered. Based on information available at the time, some high-speed aerators with stainless steel float systems were selected. The eight 15Hp aspirator type aerators were installed and put to work. Unfortunately, the float systems began suffering corrosion at the water line. In addition to float problems the units suffered from constant mechanical problems with shafts, bearings and seal failures. The sales/service agreement called for replacing the above and below water seal packs every 6 months. As the mechanical problems worsened the operators were instructed to change parts every 3 months and then monthly. The shaft alone was costing $2,500 to replace plus the bearing and seal costs. The straw that broke the camel’s back was the manufacturer’s recommendation for a new service contract to have all parts replaced at the factory every 6 months for “only” $20,000 per year. After several costly and emergency aeration situations the city decided it was time to search for an upgrade.
A short search turned up the highly recommended Aire-O2 Triton® Process Aerator/Mixer. With what the city was spending on parts and energy per year they could afford to buy a replacement Triton. As budget permitted they were able to eventually switch out a whole basin worth of aerators. The Water &Wastewater Treatment Plant Operator shared many details about the upgrade to the Tritons. “We are saving a great amount of maintenance costs as a result of the switch. Gone are the days of constantly replacing shafts, bearings and seals. The simple shaft and bearing design of the Triton allows for smooth and trouble-free operation.” The Triton has very few moving parts; a design which inherently allows for failure-free operation. The water-lubricated bearings allow for smooth operation without need for artificial and manually applied lubrication. The float system is constructed of molded low-density polyethylene with ultraviolet inhibitor — there are no more problems with corrosion.
Before the aeration system upgrade the total plant power consumption was between 36–37 kW hours/day, a figure that Dale and his staff meticulously records on a daily basis. After the upgrade the plant is now consuming 17–19 kW hours/day which represents a 50% energy savings. The operator directly links the energy savings to the four new 10Hp Tritons. The units were designed to perform at a speed of 900 RPM allowing for much larger mixing propellers and ultimately producing more efficient oxygen transfer. The energy savings have been so noticeable that the city accountants who pay the electrical bill called Dale to confirm that the energy consumption was correct; they were concerned about what was going on at the plant! Thanks to the major energy impact the city is now working on an energy rebate with the local power company.
The moral of the story – don’t let your aeration system clean out your maintenance and energy checkbook. The Aire-O2Triton Process Aerator/Mixer and the process engineering professionals of Aeration Industries can provide you with much needed relief. A simple analysis can be provided to help you discover how easy it is to upgrade your equipment and process to help meet ever tightening effluent standards.