AWWA WQTC58920

Anticipating implementation of ultraviolet (UV) for compliance with the upcoming Stage 2Disinfection/Disinfection Byproducts Rule and the Long Term 2 Enhanced SurfaceWater Treatment Rule (USEPA, 2003a), the US Environmental Protection Agency (USEPA) is in the process of draftingguidance for implementation of UV disinfection [UV Guidance Manual, UVGM](USEPA, 2003b). This guidance will include recommendations for reactor design andoperation, dose tables, and a bioassay validation protocol. Because of the demonstratedeffectiveness of UV disinfection for controlling pathogens such as Cryptosporidium,facilities with capacities ranging from less than 1 MGD to well over 1000 MGD haveexpressed interest in UV as a pathogen control technology. In response, manufacturershave developed reactors capable of treating in excess of 40 MGD. However, full-scalereactor validation will be required before utilities will be awarded disinfection creditsbecause of the inaccuracies associated with scaling up validation data from smallerreactor designs. Large scale validation presents numerous logistical challengesincluding: an absence of test sites offering suitable water quality, sufficient volumeand discharge capacity; the need to produce exceptionally high levels of testmicroorganisms for seeded validation studies; and, the difficulties of arranging forNPDES permits for receiving waters. These needs were recently met by a UVvalidation facility site developed at Portland, Oregon.Bioassay validation of large reactors involves comprehensive characterization ofmicrobial inactivation through reactors under a range of operating conditions.Depending on the doses examined and the indicator organism(s) used, microbialreductions may reach 6 logs or more, requiring substantial concentrations ofmicroorganisms in the feed stream (possibly 106 or more viable microorganisms perunit volume, with volumes dictated by the enumerative assay used to quantify microbialreductions). UV reactor validation has traditionally relied upon the use of coliphagesand endospore-forming bacteria; however, validation of reactors with capacitiesexceeding 20 MGD may require microbial seed concentrations in excess of 1014 perminute. Achieving steady state feed conditions may require several minutes per test,numerous conditions may be evaluated for each day of testing, and sufficient replicationmust be practiced to characterize experimental variability; hence, total microberequirements may exceed 1016 per day, with enumerative assays consuming 3,000 ormore Petri dishes per validation. Moreover, the dose response of the seed organismsmust be determined at the bench scale to establish the microbial inactivation kinetics fora given water. This paper discusses these considerations and examines the logisticalconsiderations for providing test microorganisms and conducting bioassay studies forUV reactor validation at scales in excess of 20 MGD. Includes 3 references.

Product Details

Edition:

Vol. - No.

Published:

11/02/2003

Number of Pages:

6

File Size:

1 file , 240 KB