In 2003, the city of Lonsdale, Minn., 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.
Treatment Process
The wastewater influent flow of the WWTF is primarily from residential sources. Commercial and industrial users contribute less than 5% of the current wastewater flow. The average influent biochemical oxygen demand is 185 mg/L with total suspended solids of 167 mg/L. Mixed liquor suspended solids are kept around 4,000 mg/L. An activated sludge, extended aeration system was constructed to handle a capacity of more than 8,000 people and a design flow of 687,000 gpd. Ferric chloride 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 on site. Two 19-ft deep earthen aeration basins also were 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. Ultraviolet (UV) light is used for disinfection.
Aeration System
At the start of the plant upgrade, several surface aerator types and brands were considered. Based on information available at the time, high-speed aerators with stainless steel float systems were selected. The eight 15-hp 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 six months. As the mechanical problems worsened, the operators were instructed to change parts every three months and then monthly. The shaft replacement alone cost $2,500, plus the bearing and seal costs. The manufacturer also recommended a new service contract to have all parts replaced at the factory every six months for $20,000 per year. After several costly emergency aeration situations, the city decided it was time for an upgrade.
A short search turned up the Aire-O2 Triton process aerator/mixer manufactured by Aeration Industries Intl. of Chaska, Minn. With what the city was spending on parts and energy per year, it could afford to buy a replacement Triton. As budget permitted, the city was able to eventually switch out a whole basin worth of aerators.
“We are saving a great amount of maintenance costs as a result of the switch,” said Dale Furrer, Lonsdale Water & Wastewater Treatment Plant operator. “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 aerator/mixer has few moving parts. The water-lubricated bearings allow for smooth operation without the need for artificial and manually applied lubrication. The float system is constructed of molded low-density polyethylene with a UV inhibitor, so there are no more problems with corrosion.
Energy Savings
Before the aeration system upgrade, the total plant power consumption—a figure that Furrer and his staff meticulously records on a daily basis—was between 36 and 37 kWh per day. After the upgrade, the plant now consumes 17 to 19 kWh per day, which represents a 50% energy savings. Furrer linked the energy savings to the four new 10-hp Tritons. The units were designed to perform at a speed of 900 rpm, allowing for larger mixing propellers and ultimately producing more efficient oxygen transfer. The energy savings have been so noticeable that the city accountants who pay the electric bill called Furrer to confirm that the energy consumption was correct; they were concerned about what was going on at the plant. Thanks to the energy impact, the city is now working with Furrer on an energy rebate with the local power company.
To sum it all up, the city of Lonsdale’s experience is a good example of how a new aeration system can give facilities much-needed relief in terms of maintenance and energy consumption.
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