TO UV OR NOT TO UV, IS THAT THE QUESTION? OR IS IT: WHEN IS CLEAR WATER NOT TRANSPARENT?

It is becoming common for communities to switch from chlorine to ultraviolet light (UV) for wastewater treatment effluent disinfection. The concern for releases of chlorine to the environment due to accidents, the need to dechlorinate prior to discharge, as well as the introduction and refinement of medium pressure UV lamps has accelerated this transition. Additionally, other communities that never disinfected in the past are considering UV as the only viable means to comply with tighter bacteriological regulations.In many cases there has been a complacent attitude in this transition because of the pervasive belief that the UV disinfection process will work without risk of failure. This complacency extends to the point that many communities are not even bothering to measure the actual transmissivity of their wastewater prior to installing the new equipment. Rather, designs are based on assumed worse case conditions.This paper presents a case study of one community that did not bother to study their wastewater, but instead decided to base a design and a major construction project on an assumed wastewater transmissivity. What they found after everything was it place is that a perfectly clear effluent can have transmissivities well below 40 percent depending on the mix of domestic, industrial, and commercial wastes influent to the treatment plant.This investigation was conducted in two phases. Phase 1 included an in depth review of the design process and the interactions between the community staff, its consultants, and the UV equipment manufacturer. Wastewater treatment plant records, engineer reports and designs, correspondence, shop drawings, industrial waste discharge records, and interviews with personnel were all reviewed in piecing together what went wrong with what should have been a successful project.In Phase 2 a more detailed study of the transmissivities of the wastewater entering the plant and what happens in the wastewater treatment plant to UV blocking compounds was completed.The intent of this paper is to examine the decisions that were made for this community's project so that others do not make the same series of errors that can result in failure of a disinfection system, additional work for community staff, potential litigation for consultants, embarrassment for public officials, and poor relations with businesses in the community. Included in this paper is a listing of the simple steps that can be taken to prevent these problems in your community should you decide to consider making the transition to UV.