PARTICLE ACUMMULATION WITHIN A UV DISINFECTION SYSTEM: CAUSES AND EFFECTS ON PERFORMANCE

It has been demonstrated that the presence of particles on the order of 10 μm can effectively shield coliform bacteria from UV light and result in residual effluent coliform concentration. To ensure the effective performance of a UV disinfection facility, the presence of these >10 μm particles must be minimized by pre-disinfection filtration. A study was conducted at a wastewater treatment plant that was found to be a suitable candidate for UV disinfection. Collimated beam and particle size distribution testing that were conducted at this site demonstrated that the pre-disinfection filtration technology effectively minimized the presence of particles such that the UV system would effectively disinfect the effluent and meet the effluent coliform limits. A study at this site was conducted to determine the cause and effect that solids accumulation within a UV disinfection reactor will have on the performance of the UV system. It was determined that solids accumulation within a UV reactor consistently occurred during normal operation of a UV disinfection facility. The solids that accumulated with in the UV reactor contained high concentrations of coliform bacteria. Sloughing of a small portion of these accumulated solids within the UV reactor will result in violations of the effluent coliform standards. To ensure the consistent performance of a UV disinfection facility, the accumulation of solids within the UV channel should be minimized. Steps that can be taken to prevent and or mitigate solids accumulation within a UV reactor include: 1) the addition of a small concentration of a chemical disinfectant prior to the UV system, 2) operating the system at the higher end of the validated flow range resulting in scouring of the channel, 3) frequently cleaning the UV channel reactors, 4) designing the UV system such that during normal operation of the system the channel can be easily flushed to prevent solids accumulation within the UV reactor, and 5) optimizing the pre-filtration coagulation step to prevent and or minimize the occurrence of post filtration flocculation.