Microbial survival and heat generation during online sterilization of ...

(a) Heat output rate(b) Volumetric heat outputFig. 7. Output heat per unit volume of flowing cheese whey in conventional andcoil reactors.(a) Conventional reactor(b) Coil reactorFig. 8. Photographs showing fouling on UV lamps of photo-reactors.consumed in a photochemical reactiondecreased with increase in flow rate. Intheir study on heat generation and itsimpact on lamp fouling during cheesewhey sterilization using a conventionalUV reactor, Mahmoud and Ghaly (2004)found that the heat generated by a lowpressure mercury arc lamp decreasedwhile the effective radiation that isconsumed in the photochemical reactionincreased with decreasing flow.FoulingSuspended particles and high turbidity ofthe liquid medium have been found toreduce the penetration ability of the UVlight and thus reduce the efficiency of theUV conventional reactor (Mahmoud andGhaly 2004). Similarly, microorganismscan aggregate or clump together, formingparticles that potentially protectmicroorganisms within aggregates thatwould otherwise be inactivated (USEPA1999). Visual observation of the UVlamps revealed less fouling in the coilreactor compared to conventional reactor(Fig. 8). As a result of secondary flow inthe coil reactor, the hydraulics of the flowdiffered from that in the conventionalreactor and resulted in less fouling.CONCLUSIONThe coil reactor was found to be moreefficient than the conventional reactor asit resulted in higher microbial destructionin shorter retention time. The destructionefficiency of the conventional reactordecreased from 99.4% (at 5 mL/min) to31.58% (at 70 mL/min), whereas thedestruction efficiency in the coil reactorincreased from 60.77% (at 5 mL/min) to99.98% (at 30 mL/min) and thendecreased with further increases in flowrate reaching 46.2% (at 70 mL/min). Therate of microbial destruction was found tobe exponential in the conventional reactorand polynomial in the coil reactor. Theflow was laminar in both reactors (Re =1.4 - 20.1). Dean flow was observed inthe coil reactor (De = 1.09 - 15.41) andDean vortices resulted in higherdestruction efficiencies and increasedheat transfer as compared to theconventional reactor. The maximumeffluent temperatures in the conventionalreactor and coil reactor were 45.8 and46.1EC, respectively. The heat generationand thus effective radiation that wasconsumed in the photochemical reactionin the conventional and coil reactors was3.10LE GÉNIE DES BIOSYSTÈMES AU CANADA SINGH and GHALY