Modeling and Efficiency of Ultraviolet Disinfection Systems

D i s i n f e c t i o n of wastewater e f f l u e n t s w i th u l t r a v i o l e t ( W ) l i g h t h a s r e c e n t l y become a r a p i d l y growing and c o m p e t i t i v e a l t e r n a t i v e o c h l o r i n a t i o n . Over 20 l a r g e scale W systems t h e past 2 y e a r s i n North America. U l t r a v i o l e t d i s i n f e c t i o n can avo id t h e environmental problems a s s o c i a t e d w i t h c h l o r i n a t i o n . Chlor ine residuals can be t o x i c t o a q u a t i c l i f e (Ward and DeGrave, 1978) and c h l o r i n a t i o n may resu l t i n c a r c i n o g e n i c byp roduc t s (Na t iona l Research Counci l , 1980) . U l t r a v i o l e t d i s i n f e c t i o n may be r e l a t i v e l y more e f f e c t i v e than c h l o r i n a t i o n i n k i l l i n g v i r u s e s compared t o t h e doses needed t o k i l l i n d i c a t o r bacteria (Yip and Konasewich, 1 9 7 2 ) . . ( o v e r 1 0 0 0 m f /day of e f f l u e n t ) , have been b u i l t or planned i n Recent c o s t estimates from a f u l l scale wastewater p l a n t showed W d i s i n f e c t i o n t o be t h e least expens ive d i s i n f e c t i o n method f o r small systems. Even i n l a r g e systems t r e a t m e n t c o s t s were only 38% higher than c h l o r i n a t i o n ( S c h e i b l e and Bassel, 1981) . Very l i t t l e r e s e a r c h has been done on t h e e f f i c i e n c y of W d i s i n f e c t i o n systems. With f u r t h e r development, W may prove less expens ive t h a n c h l o r i n a t i o n , even f o r l a r g e systems. a r e s i d u a l d i s i n f e c t a n t which may be d e s i r a b l e i n d r i n k i n g water t r ea tmen t . Another d i sadvantage has been t h e d i f f i c u l t y of measuring W dose and t h e consequent d i f f i c u l t y i n p rocess c o n t r o l . Because of d i f f i c u l t i e s i n measuring u l t r a v i o l e t ( W ) dose, and i n measuring t h e immediate resul ts of d i s i n f e c t i o n , modeling can p lay a p a r t i c u l a r l y u s e f u l r o l e i n W d i s i n f e c t i o n . While there have been a number of p i l o t and f u l l scale i n v e s t i g a t i o n s of W d i s i n f e c t i o n of wastewater (Schei b l e and Bassel, 1981; Petrasek, d, 1980; Roeber and Hoot, 1975; One of t h e d i sadvan tages of W d i s i n f e c t i o n i s t h e l a c k of Johnson and Q u a l l s , 1981) most of t h e s e s t u d i e s focused on demonst ra t ing t h e r e l i a b i l i t y of a given system. There have been s e v e r a l problems w i t h t h e measurements made on W systems which have prevented comparisons of e f f i c i e n c y : ( a ) u n t i l r e c e n t l y t h e r e has been no d i r e c t method of measuring W in tens i t i e s i n t h e i n t e r i o r of a W u n i t , ( b ) c a l c u l a t i o n s of in tens i t y have o f t e n relied on i n a p p r o p r i a t e e q u a t i o n s (Qua l l s and Johnson, 1983) and (c) t h e r e have been qo s y s t e m a t i c methods developed f o r comparing W d i s i n f e c t i o n system performance as a f u n c t i o n of des ign and o p e r a t i o n a l v a r i a b l e s . Most c u r r e n t W d i s i n f e c t i o n systems employ s e v e r a l t u b u l a r ge rmic ida l lamps each enc losed i n a second q u a r t z tube submerged i n a chamber through which t h e f l u i d f lows. Flow p a t t e r n s a r e complex and may e i t h e r be parallel or pe rpend icu la r t o t h e lamp axes. Contact time or t h e t i m e any p o r t i o n of t h e flow is i r r a d i a t e d depends on t h e s e flow p a t t e r n s . I n t e n s i t y p a t t e r n s a r e a l s o complex i n t h e abso rb ing f l u i d i n t h e chambers and cannot be d i r e c t l y measured wi th a meter. The o b j e c t i v e s of t h i s s tudy were: (1) t o develop a model of W d i s i n f e c t i o n which accoun t s f o r t h e complex i n t e n s i t y p a t t e r n s , non-ideal flow pat terns , and non-l inear s u r v i v a l curves found i n flow through W d i s i n f e c t i o n systems, ( 2 ) t o compare s i m u l a t i o n s t o p i l o t p l a n t d a t a , ( 3 ) t o u s e t h e model t o e v a l u a t e f a c t o r s i n f l u e n c i n g e f f i c i e n c y and ( 4 ) t o develop comp a r i s o n methods f o r W systems t o e v a l u a t e b i o c i d a l e f f e c t i v e n e s s i n cont inuous flow systems. A model h a s been developed of s u r v i v a l i n flow-through W d i s i n f e c t i o n systems. The model c o n s i d e r s t h e complex i n t e n s i t y p a t t e r n i n a c y l i n d r i c a l r e a c t o r c o n t a i n i n g s e v e r a l t u b u l a r lamps, t h e d i s t r i b u t i o n of r e s idence times from non-ideal flow p a t t e r n s , and usua l non-Chick's Law or non-l inear log s u r v i v a l vs. W dose organism response. I t calculates t h e s u r v i v a l i n each p o p u l a t i o n f r a c t i o n of t h e r e s i d e n c e time d i s t r i b u t i o n s e p a r a t e l y and sums t h e s u r v i v o r s t o calculate t h e average surv i v a l . The model uses as i n p u t d a t a t h e i n j e c t i o n b ioassay or calculated average W i n t e n s i t y w i t h i n t h e system, t h e actual r e s idence time d i s t r i b u t i o n , and a n expe r imen ta l ly determined dose-surv iva l cu rve i n a batch , columnated beam system where dose i s a c c u r a t e l y measured. The p r e d i c t i o n s of t h e model corresponded w e l l w i t h t h e resu l t s found i n cont inuous f low u n i t s used i n a W p i l o t p l a n t s tudy.