City of Ouray, Wastewater Treatment Plant: Surface Aerators vs. Air Diffusion Systems Bottom Laid Diffuser Aeration

Executive Summary

Aqua Sierra, Inc. performed a cost benefit comparison of the current surface agitator aerators employed at the City of Ouray WWTP versus a proposed Air Diffusion Systems (ADS) bottom-laid fine bubble diffuser system. This comparison evaluates current operational costs associated with the surface aerators versus operational costs associated with a complete ADS system. The cost-benefit analysis identifies differences in energy consumption and treatment efficiencies between the two systems and weighs associated "non-fixed" costs such as sludge removal and disposal. Additionally, the study examines the differences in energy consumption and treatment efficiencies between several municipal and industrial wastewater treatment systems throughout the nation, which have already switched from surface aerator systems to a new ADS system.

Energy consumption comparison results indicate substantial cost savings associated with the ADS system. This comparison includes the initial purchase cost of the new system. Additionally, treatment performance comparison results indicate improved treatment through the bottom laid diffuser method.

The study data demonstrates that ADS Bottom-laid Fine Bubble Diffusion Aeration Systems achieve better treatment while substantially reducing operating costs. Outstanding advantages include complete treatment, primary settling, biological oxidation, final settling, internal sludge digestion, and odor and mosquito control.

Introduction

Aerated lagoons are commonly used for treatment of municipal and industrial wastewater. Aerated lagoons treat sewage economically with far less capital expense than larger more intensive treatment plants. Aerated lagoons treat wastewater through the introduction of air (oxygen) into the wastewater to overcome Biological Oxygen Demand (BOD) and Suspended Solids (SS). Introduction of air into wastewater can be accomplished through various means.

Most aerated lagoon wastewater treatment systems today incorporate surface aerators to introduce air into the lagoon systems. Unfortunately, there are several problems inherent in surface aerators including incomplete treatment efficiency and high energy consumption.

Operational Cost Savings with the ADS Bottom-laid System
A comparison of annual operational costs show an annual savings of approximately $34,000 per year. While the majority of the cost savings are due to a substantial reduction in energy consumption, approximately $15,000 are saved through increased treatment (promoting internal digestion of sludge by beneficial bacteria) resulting in virtual elimination of sludge handling. Blower and surface aerator energy data is listed in Appendix A.

The cost of the total system including, all equipment and labor to install is $75,000. With annual operational savings of approximately $34,000 per year, the proposed ADS system can be completely paid off within 2.3 years. After five years the annual cost savings would result in an additional savings of over $70,400 (includes paying for the system in the above stated 2.3 years). After ten years, the annual cost savings would result in a total savings to the City of Ouray of over $246,000. The cost savings can be used to purchase additional land to expand the treatment facility lagoon system in order to meet the demands of future population growth.

Highest Oxygen Transfer Rates in the Industry
High oxygen transfer rates result in a direct savings in energy costs. In most cases, ADS aeration systems will use approximately one-quarter to one-half the total horsepower of surface aerator systems.

Low Power Costs
In most cases the ADS wastewater aeration system can be paid for under a lease option agreement or other available programs with the savings in energy consumption alone. When other operational costs are included in the equation, such as reduced sludge handling, the savings are even more substantial.

Seasonal variation in waste loading will not upset the ADS System
Excess volumes or intermittent high BOD inputs do not upset ADS treatment systems. The water volume, continual circulation and high oxygen transfer efficiencies combine to virtually guarantee continual effective treatment.

Reduction of septic "off smells"
ADS aeration systems work to constantly keep all areas of the water column aerobic (in presence of oxygen), therefore there are no septic odors caused by anaerobic conditions. Complaints from neighbors will be eliminated as "off smells" are eliminated.

Minimal Sludge Handling
Surface aerators treat and aerate only the top layer of the water column. This type of treatment results in rapid sludge accumulation due to anaerobic conditions on and near the bottom of the lagoon. The ADS bottom-laid fine bubble diffuser system provides high dissolved oxygen in all areas of the lagoon including on and near the bottom of the lagoon. High dissolved oxygen concentrations throughout the entire water column result in natural digestion of sludge by communities of beneficial bacteria. This process termed "bio-augmentation" reduces most organic sludge into CO2, H2O, and inert ash.

Features and Benefits of the Air Diffusion Systems Bottom-Laid Fine Bubble Aeration Wastewater Treatment Systems

• System designs and engineering free of charge
• Designs based on a 10 and 20-year life expectancy
• Proven low initial costs, with 2 year unconditional guarantee
• Simple design, installation, and operation for both new lagoon systems and retrofits
• Air delivered at the anaerobic, sludge level, converting solid waste (sludge) to food for aerobic bacteria
• Complete fine-bubble aeration provides uniformly high oxygen transfer throughout the entire water column
• Slow rising fine bubbles greatly reduce power requirements, due to long contact time
• No chemicals necessary for complete treatment
• No sludge handling necessary
• No dewatering required for installation or maintenance
• No moving parts located in the water, therefore reducing the possibility for failure
• Required maintenance is easy to perform with "out of water" header pipe

Problems Inherent in Surface Aerator Wastewater Treatment Systems

• Extremely energy consumptive, approximately 30 horsepower per one million gallons of wastewater
• In-water moving parts, thereby increasing the chance for failure
• No oxygen transfer at the bottom of water column where sludge is present
• Recurrent sludge removal and disposal
• Pump maintenance requires removal from lagoon
• Potential "freeze-up" of surface aerators during winter months

Conclusion

By keeping abreast of the latest technologies, Aqua Sierra, Inc. and Air Diffusion Systems A John Hinde Company assure an efficient, effective, cost-conscious approach to solving most wastewater treatment problems our clients may encounter.

Cost benefit comparison results clearly illustrate that ADS technology is far more energy efficient than the current surface aerator system employed at the City of Ouray Wastewater Treatment Plant. Additionally, the ADS system provides improved treatment through superior oxygen transfer efficiencies. Oxygen transfer and/or circulation efficiencies attain complete and uniform oxygen dispersion throughout the aerobic liquid phase resulting in internal solids separation and sludge digestion.

Outstanding advantages include complete treatment, primary settling, biological oxidation, final settling, internal sludge digestion, and odor and mosquito control. In Summary, ADS Bottom-laid Fine Bubble Diffusion Aeration Systems achieve better treatment while substantially reducing operating costs. Additionally, since off smells are eliminated, the benefits of the ADS System reach far beyond the treatment facility itself.

Appendix A. – Blower and Surface Aerator Operation Energy Data Calculations

ADS Bottom-laid Fine Bubble Diffuser System

Actual Blower = 12 BHP @ 7 PSI

Estimated Annual Power Costs Based on the Following:

Rate: $0.08 KWH

Motor and Drive Efficiency: 90%

Number of Blowers Required for Operation: 1

12.0 BHP (0.746 KW/HP) ($0.08 KWH) (24 HR/DAY) (365 DAY/YEAR)

0.90 Motor Efficiency = $6,970.62 per year

Current Surface Aerator System

Surface Aerator Pumps = 5 HP (9)

Estimated Annual Power Costs Based on the Following:

Rate: $0.08 KWH

Motor and Drive Efficiency: 90%

Number of Pumps for Operation: 9

45.0 HP (0.746 KW/HP) ($0.08 KWH) (24 HR/DAY) (365 DAY/YEAR)

0.90 Motor Efficiency = $26,139.84 per year