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You Don't Need to be Large to Make a Difference – A Small Community's Goal of Improving Effluent Quality While Reducing Energy Costs

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Recent societal pressures to reduce the costs associated with energy consumption and the related greenhouse gas emissions have created a driver that is an inconsistent with the traditional goals of water quality and environmental protection. The conflict between these goals is particularly compelling for wastewater treatment facilities (WWTFs) throughout the United States, as more stringent effluent requirements are being promulgated. In addition, to the stricter effluent requirements, influent loads and the type of biological treatment processes both play a considerable role of energy used at a facility. In general terms, it can be said that the greater the required level of treatment – the greater the energy demand. In most cases, we have found that many facilities over aerate, with no regard to how much air is actually required for the process in order to obtain adequate margin of safety against permit exceedences. The result is that the actual discharge concentrations from these facilities are well below the permitted discharge requirements, while a significant amount of energy is wasted in accomplishing this.

While energy issues energy issues are primarily a concern with large treatment facilities, there are much more utilities that treat less than 38,016 m3/day (10 MGD), and face this issue. For example, in Florida there are 2,059 domestic WWTFs statewide, and of these facilities there are only 57 (2.77%) WWTFs with a treatment rated capacity of greater than 38,016 m3/day (10 MGD). In most cases, the same can be said for other regions of the United States.

In 2007, the City of North Port, Florida embarked on an ambitious capital program to expand and upgrade their existing WWTF to meet the growing needs of this small community. But, rather than expand the facility solely to meet growth needs, the City wanted to make a difference and minimize the impact that their facility had on the environment, and thus incorporated these three principal goals into their program:

Provide for an energy efficient treatment process to minimize their carbon footprint.

Remove nitrogen from their wastewater, although not required by their permit.

Increasing the quantity of effluent beneficially reused within their service area.

The City's first step of this program was to define the characteristics of the wastewater entering the facility and throughout their existing treatment process. The data that was gathered during this phase was used to calibrate a BioWin™ model. Using this model, the facility was not only upgraded, but rerated to maximize the process tankage in-place and predict the performance and process aeration requirements of the expanded and upgraded facilities.

The facility improvements included converting a conventional activated sludge facility that was required to meet secondary standards to a Modified Ludzack-Ettinger (MLE) facility that met advanced secondary standards (5 mg/L BOD, 5 mg/L TSS and <10 mg/L TN), and high level disinfection. This required the addition of an internal mixed liquor recycle pumping system, deep bed filtration and high level disinfection. Other than the conversion of the treatment process and the addition of new equipment, the primary improvements that were associated with the biological treatment process included:

High-speed single stage centrifugal blowers with dual vane control;

Replacement of coarse bubble diffusers with a tapered fine-bubble diffuser arrangement.

Incorporation of DO control to manage the airflow to the individuals zones of the aeration basins.

Construction of the improvements began in February 2008, and in September 2009, the new treatment process was put on-line. The power consumption data before construction, during construction and after successful start-up was evaluated. The operating data that has been collected indicates that the average power usage has decreased by slightly greater than 40%, and has ranged from 24% to nearly 50% over this period, even though additional equipment was added that increased the power draw at this facility. What is more telling is that the influent flows have increased by approximately 15%. Based on the energy savings and the current cost of energy during off-peak and peak demand periods, the City can expect its investment for the facilities installed as part of the aeration process to pay for itself in approximately 6-years.

As noted earlier, the City had two other goals – reducing the nitrogen in their effluent and increasing the volume of effluent beneficially reused within their service area. Both of these goals were achieved. The upgrades to the treatment process has reduced the total nitrogen (TN) concentration to consistently less than 7 mg/L, or an average TN reduction of nearly 63%. Additionally, the facility is now capable of reusing 100% of the wastewater treated at public access reclaimed water sites throughout the City, rather than disposing of it using their deep injection well. Since the upgrades have been completed, the annual average use of public access reclaimed water has increased from approximately 35% to nearly 76%; thereby minimizing the impacts of non-potable water uses on the regions potable water supply.

The intended take-home message for utilities managers and operators is that combining multiple project elements creates a relationship could result in a project performance that exceeds expectations. The importance of process modeling during planning and design should not be overlooked, and the advancing of aeration and control technologies now offers utilities relevantand cost-effective solutions to today's energy conservation challenges while improving other aspects of a treatment process cost effectively. This paper presents to the audience:

A case study that demonstrates that in not all cases does improving effluent quality increase energy costs.

A discussion of the importance of process modeling for predicting aeration demands.

Process control as a means of aeration control.

Operations data illustrating resulting energy savings.
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Keywords: Process aeration control; aeration; energy management; nutrient removal; reclaimed water reuse; water resources management

Document Type: Research Article

Publication date: 2011-01-01

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