Rebates Encourage Installation of Energy Saving Aeration Components

By Guy Mace

Aeration equipment and instrumentation technology available today can provide power savings up to 50 percent over the performance of outdated aeration systems, and equipment manufacturers often will guarantee these power savings with today's more efficient equipment. Rebates for capital equipment purchases by power generating utilities also can act as a catalyst to move sometimes stalled upgrades forward.

The decade of the 1970's saw most wastewater treatment plants in the United States installing or upgrading to secondary biological treatment because of newly mandated regulations for wastewater treatment. This period experienced the largest installation of wastewater capital equipment in U.S. history. It's now 20-30 years later and much of this equipment is nearing the end of its useful life and must be replaced. The industry thus has an unprecedented opportunity to upgrade equipment with greater efficiencies.

The wastewater aeration process can consume up to 60 percent of the power needs of a wastewater treatment plant. To fully appreciate the impact of energy savings aeration equipment can have, a review of the advancements in aeration systems is in order.

An aeration system is comprised of the following equipment:

1. Aeration Diffusers
2. Aeration Blowers
3. Piping & Valves
4. Primary Instruments
5. Control Panels
6. Software

All of these items affect efficiency, and all have seen an evolution of improvements that have had a positive impact on lowering power costs, albeit by employing equipment choices with higher first cost.

Aeration Diffusers - Ceramic and elastomeric fine bubble diffusers were developed in the 1970s and 1980s, respectively. Advancements in elastomeric membrane diffusers are still occurring as the quest for ever finer air bubbles and the optimal distribution/detention of these air bubbles continues. Conversion to fine bubble diffusers from coarse bubble diffusers, mechanical surface aeration, or rotating types of bio-reactors can improve efficiency by 25 percent to 30 percent in certain applications.

Aeration Blowers - Evolution has progressed from positive displacement to multi-stage to integrally geared, high efficiency, dual vane, single-stage air blowers. Each blower type is more efficient than the other, albeit each has a higher first cost. However, the most efficient single-stage has a smaller footprint (less floor-space means smaller blower building).

The multi-stage offers variable discharge air capacity of 100 percent to about 70 percent, while the single-stage has a variable capacity of 100 percent to 45 percent. This variable capacity capability is vital in advanced aeration systems that require precise control of air flow. Matching the air demand with the metabolic needs of the biomass allows operators to provide only the compressed air volume needed for biological treatment. Upgrading to high efficiency single-stage blowers can improve efficiency by 25 percent to 30 percent.

Piping and Valves - Sizing pipe too small results in excessive pressure drop (higher operating cost) and excessive noise. Oversizing piping results in excessive first cost. Air flow control valves metering air to the aeration tanks must be properly sized to work properly. Generally 1-2 sizes smaller than the air manifold piping, these valves are variable position and have feed forward, feed back controls and limit switches for automated process control. Another element of energy savings design are air blow-off electric operated butterfly valves. They are now supplied for virtually all blower equipment over 100 hp to allow blowers to come on-line in a no-load condition. This minimizes peak power demand.

Instrumentation - The quest for ever superior dissolved oxygen (DO) probes continues. Recent developments include improved cleaning devices, calibration, and control probe status feedback for the purpose of status and replacement.

Empirical experience in the late 1990's evidenced the necessity of an air flow meter to control each air flow control valve. Earlier control schemes used the DO probe to directly control the air flow control valve. This results in an unstable control system. These and several other primary instruments are still undergoing an advancement of the art.

Controls - This is the area of fastest development today. Modern local blower control and master system control panels use man-machine operator interfaces with touch screen controls. The operator interface and their operating screens are undergoing a startling evolution, making it ever easier for operators to control the aeration process. And with more control comes better efficiency.

Software - Modern local and master control panels contain a programmable logic controller (PLC) with software. It is the software that ties the aeration system equipment together to optimize operation and efficiency. Proper operating software:

• Provides the start-stop and safety control of the aeration blowers.
• Provides the process control to make today's sophisticated aeration systems work. Step aeration processes offer more efficient systems control and superior treatment. Biological Nutrient Removal (BNR) removes phosphorus and nitrogen now required in many metropolitan areas. This process requires close control of air flow to each of several aerated cells.
• Monitors and controls the entire aeration process system at the lowest overall air header pressure by keeping the modulating air flow control valves in their most open position. Instrumentation and controls thus improve efficiency by 10 percent - 15 percent and lower power costs two ways:
• By controlling the volume of air flow to the aeration cells, thereby compressing only the air required by the biomass.
• Operating the aeration system at the lowest possible air header pressure, likewise saving power.

Ideal Aeration System

A properly designed aeration system combines the most energy efficient equipment with an instrumentation and controls system to optimize the process control and realize energy savings. In large part, "you get what you pay for". Considerable power savings can be realized by upgrading with the advanced equipment and instrumentation and controls available today.

The following case studies are only a few examples where power utility rebates had an influence on the selection of more energy efficient equipment in lieu of lower first cost equipment.

Case Studies

Woodstock Ontario, Canada - This 5 mgd wastewater treatment plant went through an expansion in 1978 to handle a predicted 15 year future wastewater flow. The plant upgraded with fine bubble diffusers and multi-stage blowers. By the early 1990's it was apparent the aeration blowers were oversized because the population didn't grow as predicted, infiltration was reduced and the fine bubble diffusers were more efficient than predicted.

As a result, excessive air was compressed because the existing multi-stage units were oversized and could only turn down to about 75 percent capacity.

Energy savings at this plant were calculated at $77,000 (Canadian dollars), by upgrading blowers to the most efficient variable capacity, single-stage type. A rebate of $86,671 from Ontario Hydro served to move this project forward. Using a simple payback of first cost investment versus operating cost savings, payback was calculated at 2.5 years. With the Ontario Hydro subsidy for first cost, payback was reduced to 1.4 years.

Irvine Ranch, CA, Michelson WRP - The Irvine Ranch WRP was extensively renovated in 1994-1997. The aeration process was upgraded from conventional activated sludge to nitrification, a more complex and energy consumptive process. The upgrade incorporated all the advancements of fine bubble diffusers, single-stage aeration blowers, and advanced instrumentation. During the period of upgrade, plant flow increased from 9.5 to 10.6 mgd. Today, maximum plant flow has grown to over 14 mgd.

Documented power savings of 40-45 percent were realized with the upgrade, which equated to $110,000 per year at $0.07/kWh. An energy rebate from Southern California Edison was instrumental in providing the final affirmative decision to install energy efficient equipment in lieu of less expensive, less efficient new equipment.

St. Cloud, MN WWTP - This 10 mgd plant upgraded to high efficiency, fine bubble diffusers and energy efficient blowers in 1997. The upgrade was initiated by a Northern States Power (NSP) "Bounty" on energy savings of $0.10 for each kWh of energy savings during the first year after retrofit, and the equipment manufacturer's guarantee of a certain level of power savings.

The NSP bounty resulted in a rebate of about $200,000 the first year. The utility also provided a 10-year interest free loan limited to two times the first year's power bill and free engineering services up to 15 percent of the loan value or retrofit costs.

About the Author

Guy Mace, a chemical engineer and President of Turblex, has published more than 50 technical papers on rotating equipment. He is a recognized expert on low pressure air compressors and energy efficient wastewater aeration system design.