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Effective design for a laboratory discard sterilizer
aTournal of Hospital Infection ( I 9 8 5 ) 6, 326--332 SHORT
REPORT
Effective design for a laboratory discard sterilizer S t u a r t J. L i n e
Public Health Laboratory, Bowthorpe Road, Norwich N R 2 3 T X Accepted for publication 10 ffanuary 1985 S u m m a r y : A modification of the direct air d i s p l a c e m e n t s y s t e m of p r o c e s s •ng l a b o r a t o r y dis'card material in an a u t o c l a v e is d e s c r i b e d . T h e m e t h o d o v e r c o m e s m a n y o f t h e d r a w b a c k s e n c o u n t e r e d p r e v i o u s l y with existing laboratory sterilizers a n d y e t p r o v i d e s an effective m e a n s of replacing air with steam in difficult l a b o r a t o r y discard loads. Plastic P e t r i dishes were u s e d as the s t a n d a r d load a n d plastic b o x e s as the container. T h e r m o c o u p l e results indicate that the d e s c r i b e d m e t h o d gives g o o d s t e a m p e n e t r a t i o n o f the load and allows safe h a n d l i n g b y the operator.
Introduction M a n y designs of laboratory discard sterilizers have b e e n p u b l i s h e d (Everall, M o r r i s & Yarnell, 1978; P u b l i c H e a l t h L a b o r a t o r y Servic~ S u b c o m m i t t e e , 1978; P u b l i c H e a l t h L a b o r a t o r y Service S u b c o m m i t t e e , 1981; Oates et al., 1983). O n e of the m a j o r design p r o b l e m s has b e e n the i n a b i l i t y of the sterilizer to r e m o v e air c o m p l e t e l y f r o m d i s c a r d boxes filled w i t h infected l a b o r a t o r y waste. T h e direct air d i s p l a c e m e n t m e t h o d was f o u n d to be the m o s t efficient air r e m o v a l design p u b l i s h e d (Everall et al., 1978; P u b l i c H e a l t h L a b o r a t o r y Service S u b c o m m i t t e e , 1981). In practice this m e t h o d caused p r o b l e m s as plastic residue f r o m m o l t e n Petri dishes f o r m e d r o u n d the f u n n e l . A glass f u n n e l could not be r e t r i e v e d w i t h o u t breakage. I f the f u n n e l was m e t a l it h a d a very l i m i t e d life, as plastic a d h e r e d strongly. A f u n n e l n e e d e d to be placed into the discard box before filling with disc a r d plates, a nd took up a c o n s i d e r a b l e v o l u m e of the b o x w i t h the result tha t a dditiona l autoclave v o l u m e was r e q u i r e d for a give n work load. W e describe an air extraction s y s t e m w h i c h ove r c ome s m a n y of the practical p r o b l e m s in use a n d over the last 2 years has p r o v e d to be effective a n d reliable.
Method T o o v e r come the p r o b l e m s d e s c r i b e d above, m e t a l t u b e s were u s e d to inject s t e a m directly into the l o a d boxes, d i s p l a c i n g the air as illustrated in F i g u r e 1. T w o British S t e r i l i z e r 6001 (14 c u b i c feet) M o t o c l a v e s (British Sterilizer (~ t985 T h e Hospital Infection Society
0195-.670l[85[030326+n7 $03.00/0
326
L2 ;boratory d i s c a r d s t e r i l i z e r d e s i g n Chamber jackeJed wall
distribution pipe against the steonn inlet port ~-3
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$:~al of the steam distribution pipe to the steam inlet port
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Figure 1. Steam inlet design. A cross-section of the sterilizer chamber illustrating two boxes of waste material only to aid clarity.
Co. Ltd., H a i n a u l t , Essex) were u.sed to carry out this work. T h e steam injection tubes were in the f or m of 10 m m d i a m e t e r 250 m m long stainless steel tubes covered with silicone r u b b e r with a silicone r u b b e r connection to the steam d i s t r i b u t i o n pipe ( F i g u r e 1). T h e discard loading s y s t e m f o r m e d part of the sterilizer f u r n i t u r e and allowed c o n v e n i e n t loading and unlo a d ing. T h e loading cage was m o u n t e d on wheels a n d could be easily transported on a trolley to the sterilizer a nd loaded into the m a c h i n e w h e n r e q u i r e d ( F i g u r e 2). W e adopted as our s t a n d a r d load a plastic box 2 9 4 m m x 2 6 0 r a m x 2 3 0 m m deep, code W C B . W l 1 6 ( W . C . B . C o n t a i n e r s Ltd., S t a m f o r d Works, Bayley Street, S t a l y b r i d g e , C h e s h i r e , S K 1 5 1 Q Q ) filled with used plastic Petri d i s h e s 104 m m d i a m e t e r c o n t a i n ' i n g 15 m l agar. E i g h t V¢CB boxes could be loaded oi1 to the trolley and p r o c e s s e d in one cycle. In the laboratory, plastic Petri dishes were d i s c a r d e d into W . C . B . boxes lined w i t h autoclavable plastic bags, 640 m m wide b y 820 m m long. W h e n a box was full to w i t h i n 20 m m of the top, the plastic bag w a s twisted up and sealed with adhesive tape. T h i s m e t h o d allowed safe h a n d l i n g d u r i n g t r a n s i t f r o m the se c t ion of the laboratory g e n e r a t i n g the d i s c a r d to the sterilizer. T h e filled boxes were m o u n t e d on a l o a d i n g f r a m e ( F i g u r e 2) a n d a s t e a m injection t u b e w a s p l u n g e d into the plastic b a g w i t h i n each box (Everall & Mo r r i s, 1976). It h a s n o t be e n f o u n d necessary to force the injection t u b e t h r o u g h the plastic of the waste material, b u t m o v i n g the t u b e w h i l e p u s h i n g into t h e load m a d e insertion easier. A n y u n u s e d i n j e c t i o n t u b e s w e r e inserted into available boxes, a s m o r e t h a n one t u b e p e r box c o u l d be used.
328
S.J. L i n e
Steam distribution tubes
Plaslic petri dish discord material inside on outoclavoble plastic bag
inlet port
Figure 2. Tile load and loading frame. T h e r e w e r e e i g h t i n j e c t i o n t u b e s a v a i l a b l e in t h e d e s i g n d e s c r i b e d , b u t this n u m b e r c o u l d be c h a n g e d , d e p e n d i n g on t h e size o f the s t e r i l i z e r c h a m b e r a n d t h e e x p e c t e d m a x i m u m n u m b e r o f b o x e s to be p r o c e s s e d in one load. A s t h e s t e a m s u p p l y to t h e injection t u b e s is t a k e n off t h e s t e a m d i s t r i b u t i o n p i p e at t h e t o p o f t h e l o a d i n g f r a m e the n u m b e r o f o u t l e t s a t t a c h e d to this p i p e c o u l d b e a l t e r e d in m a n u f a c t u r e to suit r e q u i r e d n e e d s . T h e l o a d e d f r a m e w a s w h e e l e d into t h e s t e r i l i z e r c h a m b e r . T h e s t e a m inlet to t h e c h a m b e r w a s on t h e b a c k wall a n d sited to c o i n c i d e w i t h a r i n g at t h e e n t r a n c e to t h e s t e a m d i s t r i b u t i o n pilSe o n t h e h e a d of t h e l o a d i n g f r a m e . W h e n t h e m a c h i n e d o o r w a s closed t h e r i n g on t h e f r a m e w a s p r e s s e d a g a i n s t t h e s t e a m inlet f o r m i n g a seal, so all t h e i n p u t s t e a m f l o w e d into t h e l o a d i n g f r a m e d i s t r i b u t i o n p i p e v i a an a u t o m a t i c c o n n e c t i o n m o u n t e d w i t h i n t h e load c a r r i e r f r a m e w o r k ( F i g u r e 1).
Cycle details Pre-Vacuum andFlush
A n initial v a c u u m o f 0.2 b a r A was d r a w n b y a w a t e r r i n g p u m p o v e r a p e r i o d o f 6 rain. A s m a l l b l e e d o f s t e a m was a l l o w e d to e n t e r t h e c h a m b e r d u r i n g this stage. T h i s s t e a m flow was c o n t r o l l e d b y a
Laboratory discard sterilizer d e s i g n
329
flow l i m i t i n g disc in the s t e a m i n p u t line . T h i s c o m b i n a t i o n of v a c u u m a n d st e a m flush assisted air r e m o v a l a n d alIowed t h e t e m p e r a t u r e o f ' t h e plastic Petri d ishes to rise slowly. M o s t of the air a p p e a r e d to be r e m o v e d f r o m the load before the plastic m e l t e d , w h i c h p r e v e n t e d the f o r m a t i o n o f pockets of p o t e n t i ally infective material. Plastic pockets, once formed, w e r e v e r y difficult to b u r s t and p r o v i d e d a p r o b l e m w h e n w a s t e was s u b s e q u e n t l y discarded, as sterility could not be g u a r a n t e e d . T o p r e v e n t c o n t a m i n a t e d air r e a c h i ng the w o r k i n g area in frollt of the m a c h i n e or service space b e h i n d the sterilizer, the d r a i n outlet f r o m the u n i t was d e s i g n e d as illustr a te d in F i g u r e 3. C o n t a m i n a t e d gases were ve nte d to a t m o s p h e r e via a stack pipe o p e n i n g well away f r o m w i n d o w s or ventilator inlets. L i q u i d flowed into the drain. T h i s s y s t e m p r e v e n t e d a bu ild up of p r e s s u r e at the d r a i n h e a d the r e f o r e c o m p l y i n g w i t h local water a u t h o r i t y regulations.
Sterilizing
T h e c h a m b e r was s h u t off f r o m the v a c u u m p u m p a n d s t e a m
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330
S.J. Line
allowed to enter the c h a m b e r t h r o u g h a disc bypass pipe. T h e load t e m p e r a t u r e r a p i d l y rose to a steady s t a t e : t h i s was set at 126°C for 38 rain of w h i c h 2 0 m i n was the sterilizing t i m e u n d e r the wor st case c o n d i t i o n s . T h e w a r m u p p e r i o d to allow the c o l d e s t p a r t of the load o f 8 W C B boxe s fiZled with u s e d p l a s t i c Petri dishes to reach 126°C was 18 m i n . T h i s p e r i o d was a d d e d to the sterilizing t i m e required.
Cooling O n c o m p l e t i o n of the sterilizing t i m e steam was s h u t off a nd the c h a m b e r allowed to cool n a t u r a l l y to 100°C, w h i c h n o r m a l l y took 15 m i n . O n r e a c hing this t e m p e r a t u r e c old wa te r was p u m p e d r o u n d the c h a m b e r jacket for 20 m i n , and the air vent valve ope ne d, allowing filtered air into the c h a m b e r to replace c¢;ndensed steam. C o o l i n g of the load was rapid, for e x a m p l e m i x e d plastic materials cooled to 80°C w i t h i n 3 4 m i n of t h e e n d of the sterilizing stage at 1'26°C. T h e cooling t i m e r was set for the p e r i o d taken for 5 0 0 m l water in a 1 1 bottle to reach 80°C in the hottest p a r t of the c h a m b e r (Gillespie & G i b b o n s , 1975), as this was the m a x i m u m size of bottle likely to be i n c l u d e d in the disc a r d load. Cycle End O n c o m p l e t i o n of the cooling pe r iod both the cooling t i m e r a n d the load s i m u l a t o r required to show safe c o n d i t i o n s before the door c ould be o p e n e d (Gillespie & G i b b o n s , 1975). W h e n the load was r e m o v e d f r o m the sterilizer, the tubes were easily w i t h d r a w n f r o m the pack of plastic, w h i c h does not stick to the silicone r u b b e r sheaths. I f unc ove r e d me ta l lances were used the plastic a d h e r e d strongly and was difficult to peel off. Drain design It is been f o u n d i m p o r t a n t not to allow too m u c h agar to enter the d r a i n line. I f this oc c ur r e d blockage with solid agar e nsue d. T o p r e v e n t this p r o b l e m the boxes were not filled above 20 m m f r o m the top. D r a i n blockage with agar has b e e n f o u n d difficult to avoid w i t h o u t resorting to e x p e n sive jacketed tubes a nd large v o l u m e s of d i l u t i n g water. T h e i n c l u s i o n of a trap in the d r a i n line n e a r the c h a m b e r outlet was useful, b u t r e q u i r e d regular cleaning at least every 4 m o n t h s .
Results
F i g u r e 4 gives the t h e r m o c o u p l e results o b t a i n e d f r om an average of two complete cycles using the above syste m. T h e 'worst case' h e a t - u p t i m e f r o m start to t h e sterilizing t e m p e r a t u r e of 12 6 °C( + 4 °C, " 0 ° C ) was 18 m i n for plastic m a t e r i a l and 23 m i n for 5 0 0 m l Water in a 1 1 bottle. T h e cooling tim e f r o m t h e e n d o f the ster!hzmg . . . t e m!p e r a~t u r e ~t0 80 oC in the load was 34 m i n for p l a s t i c m a t e r i a l and 4 7 m i n f o r the lwater bottles. T h e t o t a l cycle times are therefore 77 m i n f o r a plastic only load a n d 95 min~ for a m i x e d l o a d i n c l u d i n g 500 m l l v o l u m e s of liquid. T h e longer cycle has : b e e n :adopted in, this la bor a tor y as the sterilization o f l i q u i d is
Laboratory discard sterilizer design
331
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F i g u r e 4. Discard cycle t e m p e r a t u r e a n d pressure chart. , Drain temperature; ---, 500 ml water in 1 1 bottle; ..... ' w o r s t case' bin of petri dishes c o n t a i n i n g 15 ml agar.
occasionally required. These times compare favourably with those produced by the original direct air displacement method described by Everall et al., (1978). Discussion
T h e cycle described provided a reliable, fast, effective m e t h o d of processing laboratory discard material safely within a total period of 95 rain. T h i s time could be reduced if volumes of liquid in sealed bottles are not processed or if the sterilizing time and temperature of 20 rain at 126°C is considered to be excessive. Over 1500 cycles have now been completed over the last 2½ years. Results obtained on retest and recommissioning are similar to those aescribed. Over t h i s period the cycle parameters have been stable and engineering failures few. T h e special requirements o f the machine were a water ring p u m p and a load carrier, T h e u n i t described, therefore, s h o u l d n o t b e m u c h itnore expensive t h a n a s t a n d a r d s t e r i l i z e r : T h e greater efficiency, safety, :;:a n d ease of h a n d l i n g more than ~paid for itself. Sealing discard material i n s i d e a n
332
S.J. Line
autoclave plastic bag which is then not subsequently opened has proved a very safe handling method. The sterilizer operator needs only to plunge the steam injection tube into the discard material before processing to give good steam penetration and efficient air removal. The use of two separate sterilizers, one for discard material and one for media production has proved to-be an advantage. It has been possible to incorporate the best characteristics for the cycle required without compromise. I am grateful to Mr D. Cutts (East Anglia Regional Health Authority) for his advice and assistance, and to British Sterilizer Co. Ltd. for assistance and drawings.
References Everall, P. H. & Morris, C. A. (1975). Some observations on cooling in laboratory autoclaves. Journal of Clinical Pathology 28, 664-669. Everall, P. H. & Morris, C. A. (1976). Failure to sterilize in plastic bags. Journal of Clinical Pathology 29, 1132. Everall, P. H., Morris, C. A. & Yarnell, R. (1978). Sterilization in the laboratory autoclave using direct air displacement by steam. Journal of Clinical Pathology 31, 144-147. Gillespie, E. H. & Gibbons, S. A. (1975). Autoc!aves and their dangers and safety in laboratories. Journal of Hygiene (Cambridge) 75, 475-487. Oates, K., Deverill, C. E. A., Phelps, M. & Collins, B. J. (1983). Development of a laboratory autoclave system. Journal of Hospital Infection 4, 181-190. Public Health Laboratory Service Subcommittee on Laboratory Autoclaves. (1978). Autoclaving practice in microbiology laboratories: report of a survey. Journal of Clinical Pathology 31, 418-422. Public Health Laboratory Service Subcommittee on Laboratory Autoclaves. (1981). Specifications for laboratory autoclaves. Journal of tIospital Infection 2, 377-384.
REPORT
Effective design for a laboratory discard sterilizer S t u a r t J. L i n e
Public Health Laboratory, Bowthorpe Road, Norwich N R 2 3 T X Accepted for publication 10 ffanuary 1985 S u m m a r y : A modification of the direct air d i s p l a c e m e n t s y s t e m of p r o c e s s •ng l a b o r a t o r y dis'card material in an a u t o c l a v e is d e s c r i b e d . T h e m e t h o d o v e r c o m e s m a n y o f t h e d r a w b a c k s e n c o u n t e r e d p r e v i o u s l y with existing laboratory sterilizers a n d y e t p r o v i d e s an effective m e a n s of replacing air with steam in difficult l a b o r a t o r y discard loads. Plastic P e t r i dishes were u s e d as the s t a n d a r d load a n d plastic b o x e s as the container. T h e r m o c o u p l e results indicate that the d e s c r i b e d m e t h o d gives g o o d s t e a m p e n e t r a t i o n o f the load and allows safe h a n d l i n g b y the operator.
Introduction M a n y designs of laboratory discard sterilizers have b e e n p u b l i s h e d (Everall, M o r r i s & Yarnell, 1978; P u b l i c H e a l t h L a b o r a t o r y Servic~ S u b c o m m i t t e e , 1978; P u b l i c H e a l t h L a b o r a t o r y Service S u b c o m m i t t e e , 1981; Oates et al., 1983). O n e of the m a j o r design p r o b l e m s has b e e n the i n a b i l i t y of the sterilizer to r e m o v e air c o m p l e t e l y f r o m d i s c a r d boxes filled w i t h infected l a b o r a t o r y waste. T h e direct air d i s p l a c e m e n t m e t h o d was f o u n d to be the m o s t efficient air r e m o v a l design p u b l i s h e d (Everall et al., 1978; P u b l i c H e a l t h L a b o r a t o r y Service S u b c o m m i t t e e , 1981). In practice this m e t h o d caused p r o b l e m s as plastic residue f r o m m o l t e n Petri dishes f o r m e d r o u n d the f u n n e l . A glass f u n n e l could not be r e t r i e v e d w i t h o u t breakage. I f the f u n n e l was m e t a l it h a d a very l i m i t e d life, as plastic a d h e r e d strongly. A f u n n e l n e e d e d to be placed into the discard box before filling with disc a r d plates, a nd took up a c o n s i d e r a b l e v o l u m e of the b o x w i t h the result tha t a dditiona l autoclave v o l u m e was r e q u i r e d for a give n work load. W e describe an air extraction s y s t e m w h i c h ove r c ome s m a n y of the practical p r o b l e m s in use a n d over the last 2 years has p r o v e d to be effective a n d reliable.
Method T o o v e r come the p r o b l e m s d e s c r i b e d above, m e t a l t u b e s were u s e d to inject s t e a m directly into the l o a d boxes, d i s p l a c i n g the air as illustrated in F i g u r e 1. T w o British S t e r i l i z e r 6001 (14 c u b i c feet) M o t o c l a v e s (British Sterilizer (~ t985 T h e Hospital Infection Society
0195-.670l[85[030326+n7 $03.00/0
326
L2 ;boratory d i s c a r d s t e r i l i z e r d e s i g n Chamber jackeJed wall
distribution pipe against the steonn inlet port ~-3
'~
Chamber door /
~
rubber connection
,tl
Stainless s t e e l / injection tube
.......
--
~:
327
$:~al of the steam distribution pipe to the steam inlet port
-
/'~'[li]~.([
"
---,--
"
"-
- inJel
.
Steam distribution
~-._-'.~
pipe
,. ~. 8
11 f tl
' ~ ~ ~,..~'~
.
.
.
.
.
.
.
.
.
.
.
.
-'-'-" -£-! [-i:~-Discord box ful, of Wasfe goods
.
Step
flow into
discord material
Figure 1. Steam inlet design. A cross-section of the sterilizer chamber illustrating two boxes of waste material only to aid clarity.
Co. Ltd., H a i n a u l t , Essex) were u.sed to carry out this work. T h e steam injection tubes were in the f or m of 10 m m d i a m e t e r 250 m m long stainless steel tubes covered with silicone r u b b e r with a silicone r u b b e r connection to the steam d i s t r i b u t i o n pipe ( F i g u r e 1). T h e discard loading s y s t e m f o r m e d part of the sterilizer f u r n i t u r e and allowed c o n v e n i e n t loading and unlo a d ing. T h e loading cage was m o u n t e d on wheels a n d could be easily transported on a trolley to the sterilizer a nd loaded into the m a c h i n e w h e n r e q u i r e d ( F i g u r e 2). W e adopted as our s t a n d a r d load a plastic box 2 9 4 m m x 2 6 0 r a m x 2 3 0 m m deep, code W C B . W l 1 6 ( W . C . B . C o n t a i n e r s Ltd., S t a m f o r d Works, Bayley Street, S t a l y b r i d g e , C h e s h i r e , S K 1 5 1 Q Q ) filled with used plastic Petri d i s h e s 104 m m d i a m e t e r c o n t a i n ' i n g 15 m l agar. E i g h t V¢CB boxes could be loaded oi1 to the trolley and p r o c e s s e d in one cycle. In the laboratory, plastic Petri dishes were d i s c a r d e d into W . C . B . boxes lined w i t h autoclavable plastic bags, 640 m m wide b y 820 m m long. W h e n a box was full to w i t h i n 20 m m of the top, the plastic bag w a s twisted up and sealed with adhesive tape. T h i s m e t h o d allowed safe h a n d l i n g d u r i n g t r a n s i t f r o m the se c t ion of the laboratory g e n e r a t i n g the d i s c a r d to the sterilizer. T h e filled boxes were m o u n t e d on a l o a d i n g f r a m e ( F i g u r e 2) a n d a s t e a m injection t u b e w a s p l u n g e d into the plastic b a g w i t h i n each box (Everall & Mo r r i s, 1976). It h a s n o t be e n f o u n d necessary to force the injection t u b e t h r o u g h the plastic of the waste material, b u t m o v i n g the t u b e w h i l e p u s h i n g into t h e load m a d e insertion easier. A n y u n u s e d i n j e c t i o n t u b e s w e r e inserted into available boxes, a s m o r e t h a n one t u b e p e r box c o u l d be used.
328
S.J. L i n e
Steam distribution tubes
Plaslic petri dish discord material inside on outoclavoble plastic bag
inlet port
Figure 2. Tile load and loading frame. T h e r e w e r e e i g h t i n j e c t i o n t u b e s a v a i l a b l e in t h e d e s i g n d e s c r i b e d , b u t this n u m b e r c o u l d be c h a n g e d , d e p e n d i n g on t h e size o f the s t e r i l i z e r c h a m b e r a n d t h e e x p e c t e d m a x i m u m n u m b e r o f b o x e s to be p r o c e s s e d in one load. A s t h e s t e a m s u p p l y to t h e injection t u b e s is t a k e n off t h e s t e a m d i s t r i b u t i o n p i p e at t h e t o p o f t h e l o a d i n g f r a m e the n u m b e r o f o u t l e t s a t t a c h e d to this p i p e c o u l d b e a l t e r e d in m a n u f a c t u r e to suit r e q u i r e d n e e d s . T h e l o a d e d f r a m e w a s w h e e l e d into t h e s t e r i l i z e r c h a m b e r . T h e s t e a m inlet to t h e c h a m b e r w a s on t h e b a c k wall a n d sited to c o i n c i d e w i t h a r i n g at t h e e n t r a n c e to t h e s t e a m d i s t r i b u t i o n pilSe o n t h e h e a d of t h e l o a d i n g f r a m e . W h e n t h e m a c h i n e d o o r w a s closed t h e r i n g on t h e f r a m e w a s p r e s s e d a g a i n s t t h e s t e a m inlet f o r m i n g a seal, so all t h e i n p u t s t e a m f l o w e d into t h e l o a d i n g f r a m e d i s t r i b u t i o n p i p e v i a an a u t o m a t i c c o n n e c t i o n m o u n t e d w i t h i n t h e load c a r r i e r f r a m e w o r k ( F i g u r e 1).
Cycle details Pre-Vacuum andFlush
A n initial v a c u u m o f 0.2 b a r A was d r a w n b y a w a t e r r i n g p u m p o v e r a p e r i o d o f 6 rain. A s m a l l b l e e d o f s t e a m was a l l o w e d to e n t e r t h e c h a m b e r d u r i n g this stage. T h i s s t e a m flow was c o n t r o l l e d b y a
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flow l i m i t i n g disc in the s t e a m i n p u t line . T h i s c o m b i n a t i o n of v a c u u m a n d st e a m flush assisted air r e m o v a l a n d alIowed t h e t e m p e r a t u r e o f ' t h e plastic Petri d ishes to rise slowly. M o s t of the air a p p e a r e d to be r e m o v e d f r o m the load before the plastic m e l t e d , w h i c h p r e v e n t e d the f o r m a t i o n o f pockets of p o t e n t i ally infective material. Plastic pockets, once formed, w e r e v e r y difficult to b u r s t and p r o v i d e d a p r o b l e m w h e n w a s t e was s u b s e q u e n t l y discarded, as sterility could not be g u a r a n t e e d . T o p r e v e n t c o n t a m i n a t e d air r e a c h i ng the w o r k i n g area in frollt of the m a c h i n e or service space b e h i n d the sterilizer, the d r a i n outlet f r o m the u n i t was d e s i g n e d as illustr a te d in F i g u r e 3. C o n t a m i n a t e d gases were ve nte d to a t m o s p h e r e via a stack pipe o p e n i n g well away f r o m w i n d o w s or ventilator inlets. L i q u i d flowed into the drain. T h i s s y s t e m p r e v e n t e d a bu ild up of p r e s s u r e at the d r a i n h e a d the r e f o r e c o m p l y i n g w i t h local water a u t h o r i t y regulations.
Sterilizing
T h e c h a m b e r was s h u t off f r o m the v a c u u m p u m p a n d s t e a m
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allowed to enter the c h a m b e r t h r o u g h a disc bypass pipe. T h e load t e m p e r a t u r e r a p i d l y rose to a steady s t a t e : t h i s was set at 126°C for 38 rain of w h i c h 2 0 m i n was the sterilizing t i m e u n d e r the wor st case c o n d i t i o n s . T h e w a r m u p p e r i o d to allow the c o l d e s t p a r t of the load o f 8 W C B boxe s fiZled with u s e d p l a s t i c Petri dishes to reach 126°C was 18 m i n . T h i s p e r i o d was a d d e d to the sterilizing t i m e required.
Cooling O n c o m p l e t i o n of the sterilizing t i m e steam was s h u t off a nd the c h a m b e r allowed to cool n a t u r a l l y to 100°C, w h i c h n o r m a l l y took 15 m i n . O n r e a c hing this t e m p e r a t u r e c old wa te r was p u m p e d r o u n d the c h a m b e r jacket for 20 m i n , and the air vent valve ope ne d, allowing filtered air into the c h a m b e r to replace c¢;ndensed steam. C o o l i n g of the load was rapid, for e x a m p l e m i x e d plastic materials cooled to 80°C w i t h i n 3 4 m i n of t h e e n d of the sterilizing stage at 1'26°C. T h e cooling t i m e r was set for the p e r i o d taken for 5 0 0 m l water in a 1 1 bottle to reach 80°C in the hottest p a r t of the c h a m b e r (Gillespie & G i b b o n s , 1975), as this was the m a x i m u m size of bottle likely to be i n c l u d e d in the disc a r d load. Cycle End O n c o m p l e t i o n of the cooling pe r iod both the cooling t i m e r a n d the load s i m u l a t o r required to show safe c o n d i t i o n s before the door c ould be o p e n e d (Gillespie & G i b b o n s , 1975). W h e n the load was r e m o v e d f r o m the sterilizer, the tubes were easily w i t h d r a w n f r o m the pack of plastic, w h i c h does not stick to the silicone r u b b e r sheaths. I f unc ove r e d me ta l lances were used the plastic a d h e r e d strongly and was difficult to peel off. Drain design It is been f o u n d i m p o r t a n t not to allow too m u c h agar to enter the d r a i n line. I f this oc c ur r e d blockage with solid agar e nsue d. T o p r e v e n t this p r o b l e m the boxes were not filled above 20 m m f r o m the top. D r a i n blockage with agar has b e e n f o u n d difficult to avoid w i t h o u t resorting to e x p e n sive jacketed tubes a nd large v o l u m e s of d i l u t i n g water. T h e i n c l u s i o n of a trap in the d r a i n line n e a r the c h a m b e r outlet was useful, b u t r e q u i r e d regular cleaning at least every 4 m o n t h s .
Results
F i g u r e 4 gives the t h e r m o c o u p l e results o b t a i n e d f r om an average of two complete cycles using the above syste m. T h e 'worst case' h e a t - u p t i m e f r o m start to t h e sterilizing t e m p e r a t u r e of 12 6 °C( + 4 °C, " 0 ° C ) was 18 m i n for plastic m a t e r i a l and 23 m i n for 5 0 0 m l Water in a 1 1 bottle. T h e cooling tim e f r o m t h e e n d o f the ster!hzmg . . . t e m!p e r a~t u r e ~t0 80 oC in the load was 34 m i n for p l a s t i c m a t e r i a l and 4 7 m i n f o r the lwater bottles. T h e t o t a l cycle times are therefore 77 m i n f o r a plastic only load a n d 95 min~ for a m i x e d l o a d i n c l u d i n g 500 m l l v o l u m e s of liquid. T h e longer cycle has : b e e n :adopted in, this la bor a tor y as the sterilization o f l i q u i d is
Laboratory discard sterilizer design
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150 120 I10 10(3
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F i g u r e 4. Discard cycle t e m p e r a t u r e a n d pressure chart. , Drain temperature; ---, 500 ml water in 1 1 bottle; ..... ' w o r s t case' bin of petri dishes c o n t a i n i n g 15 ml agar.
occasionally required. These times compare favourably with those produced by the original direct air displacement method described by Everall et al., (1978). Discussion
T h e cycle described provided a reliable, fast, effective m e t h o d of processing laboratory discard material safely within a total period of 95 rain. T h i s time could be reduced if volumes of liquid in sealed bottles are not processed or if the sterilizing time and temperature of 20 rain at 126°C is considered to be excessive. Over 1500 cycles have now been completed over the last 2½ years. Results obtained on retest and recommissioning are similar to those aescribed. Over t h i s period the cycle parameters have been stable and engineering failures few. T h e special requirements o f the machine were a water ring p u m p and a load carrier, T h e u n i t described, therefore, s h o u l d n o t b e m u c h itnore expensive t h a n a s t a n d a r d s t e r i l i z e r : T h e greater efficiency, safety, :;:a n d ease of h a n d l i n g more than ~paid for itself. Sealing discard material i n s i d e a n
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autoclave plastic bag which is then not subsequently opened has proved a very safe handling method. The sterilizer operator needs only to plunge the steam injection tube into the discard material before processing to give good steam penetration and efficient air removal. The use of two separate sterilizers, one for discard material and one for media production has proved to-be an advantage. It has been possible to incorporate the best characteristics for the cycle required without compromise. I am grateful to Mr D. Cutts (East Anglia Regional Health Authority) for his advice and assistance, and to British Sterilizer Co. Ltd. for assistance and drawings.
References Everall, P. H. & Morris, C. A. (1975). Some observations on cooling in laboratory autoclaves. Journal of Clinical Pathology 28, 664-669. Everall, P. H. & Morris, C. A. (1976). Failure to sterilize in plastic bags. Journal of Clinical Pathology 29, 1132. Everall, P. H., Morris, C. A. & Yarnell, R. (1978). Sterilization in the laboratory autoclave using direct air displacement by steam. Journal of Clinical Pathology 31, 144-147. Gillespie, E. H. & Gibbons, S. A. (1975). Autoc!aves and their dangers and safety in laboratories. Journal of Hygiene (Cambridge) 75, 475-487. Oates, K., Deverill, C. E. A., Phelps, M. & Collins, B. J. (1983). Development of a laboratory autoclave system. Journal of Hospital Infection 4, 181-190. Public Health Laboratory Service Subcommittee on Laboratory Autoclaves. (1978). Autoclaving practice in microbiology laboratories: report of a survey. Journal of Clinical Pathology 31, 418-422. Public Health Laboratory Service Subcommittee on Laboratory Autoclaves. (1981). Specifications for laboratory autoclaves. Journal of tIospital Infection 2, 377-384.