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The City of Roseville, CA (City) operates the Dry Creek Wastewater Treatment Plant (DCWWTP) which is a full tertiary recycled water facility with a design treatment capacity of 18 million gallons per day (mgd). The current average dry weather flow is 16.5 mgd and the DCWWTP disinfects the flow with gaseous chlorine and subsequently dechlorinates the effluent with sulfur dioxide. Currently, the DCWWTP uses 3000 lbs per day of chlorine gas.

Chlorine gas is the most widely used disinfectant for municipal wastewater in the United States. However there are several safety concerns associated with the use, storage, and transportation of chlorine gas. Most safety concerns are due to the human health impacts that would result from an accidental release. Other concerns arise from the chlorine residual, which is toxic to aquatic life, and various disinfection byproducts. With these and other concerns, the City selected Carollo Engineers (Carollo) to evaluate disinfection alternatives.

In order to select the disinfectant that best meets the needs of the DCWWTP, a consistent method of evaluation was needed to compare the alternatives using appropriate criteria. The evaluation method used in this study is the analytic hierarchy process (AHP). The AHP is a multiple criteria decision making (MCDM) process that uses a fundamental scale for pairwise comparison. This scale has intensities which range from 1 (equal importance) to 9 (extreme importance), as well as the reciprocal values of these intensities which are used in the evaluation.

During the study, Carollo met with City staff to discuss three disinfection alternatives: Chlorine disinfection, Peracetic acid (PAA), and Ultraviolet (UV) light. The criteria used to evaluate the alternatives, and subalternatives, for the DCWWTP were:

Employee safety. What are the risks to DCWWTP employees?

Offsite Safety. What are the risks to the surrounding public due to: transportation, storage, and use?

Regulations present and proposed. What is included in the current discharge permit? What may be included in future permits?

Water quality issues. How will the receiving water be impacted by the disinfection alternatives?

System reliability. Will disinfection be consistently achieved through the intended strategy?

Operation and maintenance. How much time is required to maintain the system and comply with safety regulations?

Costs. What are the life cycle costs for the alternatives?

It appears that the most appropriate disinfection alternative for the City of Roseville DCWWTP is primary disinfection using ultraviolet light, while using launder and filter covers to control algae and other undesirable growth and onsite generated sodium hypochlorite for a residual in the reclaimed Title 22 water. The AHP method was an important part of this evaluation and provided a good opportunity for discussion of the criteria and concerns during development of the City's consensus ratings. The outcome of the criteria ranking also proved consistent with the City's goals of protecting human health and the environment.

Document Type: Research Article


Publication date: January 1, 2003

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