St. Cloud Wastewater Treatment Facility Case Study
City of St. Cloud, MN—Waste Water Treatment Facility Renewable Today for a Greater Tomorrow
The City of St. Cloud officials had made the decision in 2015 to become more aggressive in their quest for sustainability. This started by looking at their largest energy consumer, the City’s Waste Water Treatment Facility. The City in partnership with Apex Efficiency Solutions, SBC has since completed the implementation of both energy saving measures and renewable energy projects. These projects are saving the City over $300,000 per year and are reducing energy consumption by over 5.8 million kWh per year. In addition, this partnership has kicked off a series of additional energy savings initiatives and renewable energy projects throughout the City, resulting in greater energy savings and cost savings to the City. All work was completed such that the cost savings more than paid for the energy savings and renewable upgrades, resulting in a positive cash flow to the City of more than $100,000 per year and totaling over $8.3 million dollars over 20 years.
240 kW AC Solar PV System
633kW AC Biogas Generator Energy
LED Lighting Retrofits
Hot Water System
VFD’s Building Envelope Improvements
New Compressed Air System
Total Energy Savings: 655,903 kWh
Annually Total Renewable Energy Produced: 5,313,015 kWh
Annually Total Cash Savings to the City: $302, 208 Annually
Energy Savings Upgrades:
A facility wide audit was done to identify cost effective energy savings opportunities. The audit found opportunities for the following upgrades; new high efficient compressed air system, variable flow hot water modifications, LED lighting replacements, Controls upgrades including energy efficient strategies for HVAC systems, and building envelope improvements. This energy saving work also allowed for several capital repairs to be made as part of the project. The first phase of the effort resulted in savings of over 650,000 kWh of electricity and 3,300 therms of gas per year.
After energy saving upgrades were complete, we turned to renewables. Photo Voltaic solar modules were installed at the site which, at peak performance, provide 240 kW AC of power. This is divided into two separate arrays. The first is a 220kW AC ground mounted system which includes a special wild flower mix of pollinator-friendly native plants that does not interfere with power generation. The second is a 20 kW AC roof mounted system. Combined the two systems generate an average of 342,000 kWh of power each year. On a sunny day the solar garden can produce enough energy to power all plant operations for the day. The facility also generates a large amount of biogas because of the treatment process. This biogas was looked at by the City and Apex as another source of renewable energy. As a result, a biogas generator was installed which converts the organic waste to electricity and heat. The waste heat from the generator is rejected back to the plants hot water heating system. This biogas power generator produces 4.9 million kWh annually resulting in savings of over $226,000 per year. Eventually the biofuel will be stored so it can be used to keep the plant running all the time, even when the solar panels aren’t producing. The facility could become the first in MN to be completely energy independent.
With the renewable energy produced annually, greenhouse gas emissions are expected to be reduced by 3,734 Metric Tons of CO2 per year, which is equivalent to:
789 Passenger vehicles driven for 1 yr.
3,984,383 Pounds of coal burned, or
551 Homes electricity use for 1 yr.