A Small Community's Goal of Improving Effluent Quality, Reducing Energy Costs and Managing Their Water Resources
Abstract: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), as more stringent effluent requirements are being promulgated. By and large, it can be said that the greater the required level of treatment – the greater the energy demand. In addition, both influent concentrations and the type of biological treatment processes used to meet the regulatory requirements play a considerable role of the factors that must be considered. In most cases, many facilities over aerate, with no regard to how much air is required for the process in order to obtain adequate margin of safety against permit exceedances. The result is that the actual discharge concentrations of these constituents are well below the permitted discharge concentration, while a significant amount of energy is wasted in accomplishing this.
Another concern facing utilities are diminishing freshwater supplies, impacts from climate change, population growth, and more stringent effluent disposal and water quality limitations, all of which have all placed greater demands on the development of reclaimed water facilities to supplement the use of this resource in lieu of potable water. Not only can the use of reclaimed water help conserve potable water by replacing potable water for certain non-potable water uses, it can also help recharge groundwater supplies. As a result, utilities are finding synergistic solutions to water supply, wastewater treatment and water resources management issues. Therefore, the adequacy and protection of our water supplies will be one of the more challenging issues that utilities will face in the 21st century.
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 desired to minimize the impact that their facility had on the environment, and thus incorporated these three principal goals:
Improve effluent quality well below permit requirements.
Provide for an energy efficient treatment process.
Reuse 100-percent of the effluent generated.
The City's first step was to define the characteristics of the wastewater entering the facility and throughout their existing treatment process. Using this information the facility was not only upgraded, but rerated to maximize the process tankage in-place using BioWin™ modeling. The resultant of this modeling was that the City added nearly and additional 42-percent of treatment capacity with no additional capital expenditures. During the design phase, the City recognized the importance of incorporating more efficient equipment to provide the necessary aeration for the treatment process, and incorporation of technologies to provide a higher level of treatment, beyond what is required for their operations permit. In addition, the City incorporated process controls to monitor and maintain adequate aeration for the process to function, while not wasting energy.
Construction of the improvements began in February 2008, and in August 2009 the new treatment process was put on-line. The upgrades have reduced energy costs ranging from 12% to greater than 40%, and averaged slightly greater than 26% from the previous year. Based on the energy savings, the payback period for the facilities installed as part of the aeration process has been estimated to be approximately 7-years. In addition to the energy savings experienced at the facility, the process has reduced the total nitrogen (TN) concentration to consistently less than 6 mg/L, or an average TN reduction of nearly 63%. More importantly, 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, approximately 80-percent of the effluent has been reused for irrigation of residential lawns, golf courses, parks, schools and urban greenspace areas; thereby minimizing the impacts of non-potable water uses on the regions potable water supply.
This paper provides utility managers with:
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 rerating and design.
Process control as a means of energy management.
The importance of an integrated water resources management plan.
Operations data illustrating resulting energy savings, effluent quality data, reclaimed
water usage, etc.).
Document Type: Research Article
Publication date: January 1, 2010
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