Industrial Utility Efficiency

Considerations for Temporary Blower Air at a Wastewater Treatment Facility


Executive Summary

There are many reasons why a wastewater operator may have a need to leverage temporary blowers in their facility. Emergency needs, supplemental seasonal needs, or an upgrade project to name a few.  Historically, operators have relied on the local general rental industry to provide diesel driven compressors that are readily available locally and easily installed.  What is often overlooked is how inefficient these compressors are when supplying air to a wastewater treatment plant and how environmentally insensitive this approach is.  When selecting a solution to solve a temporary need, the operator should consider the “Total Cost of Rental” which includes recurring and one-time project costs, energy costs, as well as the environmental impact.

 
Introduction

The wastewater treatment process typically utilizes volumes of air in the 5-14 PSIG range for a variety of aeration and mixing functions. The volume of air required per plant will vary with the capacity of the plant and the wastewater treatment process employed. The blower technology chosen for permanent installation in a plant may vary by treatment process requirements, age of installation, and even space available for the blower installation. The various blower technologies utilized at a wastewater facility include multistage centrifugal, rotary lobe, screw blower, geared centrifugal, and high speed turbo to name a few. When the circumstance presents itself for a project that requires temporary low-pressure air, understanding the efficiency of these technologies and installation / operational costs that accompany them are key to determining the “Total Cost of Rental” for a project. This article aims to discuss the various technologies of equipment that could be presented to a wastewater operator faced with a temporary need for blower air and to help the operator understand the impact (both monetarily through a “Total Cost of Rental” approach and environmentally) of their decisions.

 
Plant Air Compressors and Their Efficiency

These compressors are designed to support manufacturing operations that utilize compressed air at 90-150 PSIG and are typically 1 or 2 stage rotary screw machines that employ either oil flooded or oil free technologies within the stages of compression. These units are readily available in the rental markets with diesel driven or electric driven options. When running these rental compressors in the 5-14 PSIG range required for most wastewater treatment plant applications, there is considerable waste in that the machine utilizes energy to compress the air up to the level of their minimum allowable discharge pressure before expanding via a pressure reducing regulator to the lower pressure required. The electrical energy consumption of electric drive plant air compressors varies from ~18 to 21 kW per 100 cfm. This level of energy consumption does not vary across the demand side pressure range of 5-14 PSIG. Electric driven plant air compressors are rarely utilized in temporary blower projects in wastewater treatment plants due to the considerably higher electrical power consumption as compared to a blower of any technology. Most wastewater plants don’t have the necessary electrical infrastructure to supply power to an electric drive plant air compressor and can only consider a diesel driven option. For diesel driven compressor units, the most readily available units in the rental markets deliver from 1500 to 1600 cfm and consume 17-19 gallons of diesel fuel per hour to deliver their rated flow at their minimum operating discharge pressure point which can then be regulated down to the range of 5-14 PSIG. Note additional equipment such as pressure regulators and storage tanks are often required to utilize these compressors in wastewater applications.

 
Blowers and Their Efficiency

Blowers are designed to operate at peak efficiency at the pressure levels required for the wastewater treatment processes. The energy consumption of these blowers varies as a function of the demand side pressure range and the efficiency of the design / operations. Diesel drive blowers do not exist in the rental markets therefore this overview exclusively looks at energy consumption from electric motor driven blowers. If electrical power is not available, these electric driven blowers can be powered by diesel driven generators that are available locally in most markets. At a delivered pressure of 5 PSIG, blowers can consume from ~1.8 to 2.2 kW per 100 cfm. At a delivered pressure of 14 PSIG, blower energy consumption can increase to ~4.5 to 6 kW per 100 cfm.

Temporary aeration blowers installed at a wastewater treatment plant.

 

Total Cost of Rental

When evaluating the appropriate solution to a temporary blower need, a plant operator needs to consider the “Total Cost of Rental”. The total cost of a rental project includes all of the following project costs:

  • Rental rates for the duration of the project inclusive of shift charges for diesel driven equipment.  This shift charge is typically a multiplier of the “standard published rate” which typically allows for up to 40 hours of run time.  A “double shift rate” allows for up to 80 run hours per week and a “triple shift rate” allows for unlimited usage. 
  • Environmental fees associated with diesel driven equipment
  • Round trip freight 
  • Preventative Maintenance during the project to keep the equipment operating reliably (diesel engines may require service every 250 to 500 run hours)
  • Installation, commissioning, and decommissioning costs
  • Energy consumption whether it is from plant supplied electricity or diesel fuel (including diesel fuel drop charges and tank pump out at the completion of a project if applicable)
  • Ancillary equipment rental such as pressure regulators, storage tanks, transformers, compressed air hose, pipe, fittings, external fuel tank, etc.

 

Environmental Impact

In addition to the monetary costs associated with various rental options, there are also environmental aspects to be considered. The ideal approach to a temporary blower project is to utilize electrically driven rental blowers that have energy consumption levels comparable to the permanently installed machines and that can be powered using the plant’s existing electrical infrastructure. Any solution that fits this “ideal” criteria, will have a neutral environmental impact as compared to the plant’s normal operations. Any solution that utilizes diesel driven equipment is putting an incremental amount of emissions into the environment. Consider a temporary blower need of 3000 cfm at 10 PSIG for 3 months.  A solution utilizing diesel driven compressors will consume ~78,000 gallons of diesel fuel.  A solution utilizing electric driven rental blowers powered by the site’s existing electrical infrastructure will consume ~282,000 kWh of grid supplied power.  When these two solutions are compared from a “Carbon Dioxide Equivalent” approach (https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator), it can be demonstrated that the diesel driven compressor solution delivers ~4 times the greenhouse gas emissions that the grid powered electric driven blower solution delivers.  While there is no direct monetary cost that can be assigned to a project around greenhouse gas emissions, it is just one additional and quite significant benefit of utilizing the correct technology for a temporary air project.

 
Case Study

A wastewater treatment plant in the Midwest had a collapsed underground aeration supply header that was used to transport air from the blower building to the basins. See Figure 1 for a high-level overview of the facility. The site could not cease water treatment operations for the time required to compete the repair and had to continuously supply the basins with a total of 7000 cfm of air at 6.8 PSIG. To carry out the repair, the plant decided to bring in rental equipment to deliver the air needed to sustain operations and to introduce that air at the opposite end of the distribution header through an existing 20” blanked off flanged connection and to isolate the failed section of pipe by taking a short planned outage to install an isolation valve. An alternate approach evaluated was to install a temporary pipe bypass from the blower building to any point on the distribution header that is downstream of the repair area thus allowing the permanently installed blowers to be used and avoiding any rental equipment but this option was not viable due to the site layout and the time and expense associated with designing and installing a temporary piping system.


 
Figure 1.  Facility Image

Because of the physical location where the temporary blowers needed to be installed, utilizing in house power via long cable runs was deemed not viable.   This meant any solution chosen had to be diesel driven.  The plant considered proposals from 2 different rental companies.  

Company A is a general rental company with a local presence that offered 125 psi rated diesel driven plant air compressors to complete the project.   To deliver the required volume of air, their proposal included 5 of these compressors, pressure regulated down to 6.8 PSIG. 

Company B is a specialty rental company that recommended electric driven tri lobe blowers powered by diesel driven generators that could be supplied from Company A.  Company B is almost 600 miles away from the plant.  Table 1 shows the Estimated Relative Costs on each “Total Cost of Rental” project line item and is sorted from the largest cost category at the top to the smallest cost category at the bottom.

Total Cost of Rental Category

Sorted by highest project cost to lowest project cost

Company A (diesel driven plant air compressors) vs Company B (blowers powered by diesel generators)

 

Diesel Fuel Costs

Company A option is ~5-6 times more expensive

Rental Charges including shift adjustments for driven Diesel Units

Company A option is ~2 times more expensive

Round Trip Freight

Comparable

Installation, commissioning, and decommissioning costs

Company B option is ~ 2 times more expensive

Preventive Maintenance Costs

Company A option is ~2 times more expensive

Total

Company A option is ~2.5 times more expensive

Table 1.  Estimated Relative Total Cost of Rental for the Sample Project

Based on a comprehensive estimate of the Total Cost of Rental, the plant went with the solution presented by Company B for an electric drive blower + diesel powered generator.

 

Project Outcome

  • The “Total Cost of Rental” for choosing the solution presented by Company B was estimated to be less than half of solution proposed by Company A.  
  • Company B’s solution consumed almost 66,000 fewer gallons of diesel fuel as compared to Company A’s proposed solution. The environmental impact was ~1.5 million lbs of CO2 emissions avoided. That’s the green house gas emission equivalent of ~1.7 million miles driven by an average passenger vehicle.

 

Conclusion

Over the lifecycle of a wastewater treatment plant, temporary needs for blower air are likely to arise. When these needs arise, it is critical that the plant take a “Total Cost of Rental” approach to select the solution that is the most economical and least impactful to the environment. Although equipment may be available from local suppliers with faster delivery timelines, this equipment is typically misapplied to the needs of the wastewater industry and very inefficient. Researching available suppliers of low-pressure rentals and establishing contingency plans proactively will help plant operators be prepared to make the most cost-effective and environmentally responsible decision when the need presents itself.

 

About Aerzen Rental
Specializing in temporary oil-free blower and compressor solutions under 50 psig, Aerzen Rental supplies complete solutions to customers in a wide range of industries. From rental units for immediate deployment in the event of a production failure or shortfall, to planned projects, Aerzen Rental is your expert partner. For more information about Aerzen Rental, contact sales@aerzenrental.com, tel: 1-844-400-AERZ (2379), or visit www.aerzenrentalusa.com.
All photos courtesy of Aerzen Rental.
 
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