Expert Systems for Environmental Applications - ACS Publications

Chapter 10

The Activated Sludge Advisor Prototype 1

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Joseph Schmuller and Michael R. Morlino 1

Expert Systems Team, CDM Federal Programs Corporation, 13135 LeeJackson Memorial Highway, Fairfax, VA 22033 Camp Dresser and McKee, Inc., Dallas, TX 75231

Downloaded by NATL UNIV OF SINGAPORE on May 5, 2018 | https://pubs.acs.org Publication Date: July 5, 1990 | doi: 10.1021/bk-1990-0431.ch010

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The Activated Sludge Advisor Prototype (ASAP) i s an Expert System designed to assist wastewater treatment operators at the Dallas Water U t i l i t i e s Central Wastewater Plant. As i t s name implies, ASAP's focus i s the activated sludge process, a biological method that uses microorganisms to speed up decomposition of wastes in wastewater. This process i s routinely susceptible to a large number of operational problems. Solving these problems is often d i f f i c u l t for novice operators, particularly if there are no experienced operators around to help. ASAP's knowledge base contains the heuristics and insights of an experienced operator. Written i n KnowledgePro ( r ) , ASAP resides on an IBM PC/AT (or compatible); it incorporates a window-based userinterface, graphics, and hypertext. Starting from a set of a half-dozen treatment plant symptoms, ASAP leads the user through sequences of questions resulting i n selection of one of 50 diagnoses.

Wastewater

Treatment

A wastewater t r e a t m e n t p l a n t i s a complex s t r u c t u r e w i t h a s i m p l e g o a l - - t o i n s u r e t h a t d i r t y w a t e r coming i n ( t h e i n f l u e n t ) w i l l be t u r n e d i n t o c l e a n w a t e r g o i n g out (the e f f l u e n t ) . The s t r u c t u r e encompasses a number o f i n s t r u m e n t s , p r o c e s s e s , and p e o p l e , a l l o f w h i c h a r e needed because o f t h e wide v a r i e t y o f s u b s t a n c e s w h i c h might contaminate the w a t e r . A t r e a t m e n t f a c i l i t y i s u s u a l l y c o n s i d e r e d t o have t h r e e major components -- c o l l e c t i o n , t r e a t m e n t , and d i s p o s a l . Wastewater i s c o l l e c t e d and brought t o t h e f a c i l i t y t h r o u g h a complex network o f p i p e s and pumps; t h e system w h i c h b r i n g s w a t e r from h o u s e h o l d s , commerce, and i n d u s t r y i s t y p i c a l l y s e p a r a t e from the system w h i c h b r i n g s storm r u n o f f water from s t r e e t s , l a n d , and r o o f s o f b u i l d i n g s . 0O97-6156790/0431-O139$06.00A) © 1990 American Chemical Society

Hushon; Expert Systems for Environmental Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

E X P E R T SYSTEMS FOR

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ENVIRONMENTAL APPLICATIONS

When the wastewater a r r i v e s a t the t r e a t m e n t p l a n t , i t moves through a s e r i e s o f p r o c e s s e s w h i c h remove the waste from the w a t e r and reduce i t s t h r e a t t o p u b l i c h e a l t h . Treatment a t the p l a n t c o n s i s t s o f p r e - t r e a t m e n t , p r i m a r y t r e a t m e n t , and secondary t r e a t m e n t . P r e - t r e a t m e n t p h y s i c a l l y s c r e e n s out l a r g e d e b r i s , and removes sand, g r a v e l , and o i l . P r i m a r y t r e a t m e n t removes s e t t l e d and f l o a t i n g materials. Secondary t r e a t m e n t i n v o l v e s b i o l o g i c a l , c h e m i c a l , and p h y s i c a l p r o c e s s e s w h i c h remove suspended and d i s s o l v e d s o l i d s ; secondary t r e a t m e n t a l s o k i l l s p a t h o g e n i c organisms.


The A c t i v a t e d Sludge

Process

A c t i v a t e d sludge c o n s i s t s o f p a r t i c l e s produced i n wastewater by the growth o f organisms i n the presence o f d i s s o l v e d oxygen. The term " a c t i v a t e d " r e f l e c t s the h i g h d e n s i t y o f b a c t e r i a , f u n g i , and p r o t o z o a on the p a r t i c l e s . The a c t i v a t e d s l u d g e p r o c e s s i s a secondary treatment which uses these microorganisms to speed up the decomposition of wastes. When a c t i v a t e d s l u d g e i s added t o wastewater, the microorganisms f e e d and grow on waste p a r t i c l e s i n the wastewater, and groups o f them come t o g e t h e r t o form c l u s t e r s c a l l e d "floe." They use the wastes f o r food and as a source o f energy f o r t h e i r l i f e p r o c e s s e s and f o r the r e p r o d u c t i o n o f more organisms, w h i c h w i l l i n t u r n use more o f the waste f o r f o o d . A l s o , the a c t i v a t e d sludge forms a l a c y network, c a l l e d a " f l o e mass", t h a t e n t r a p s many m a t e r i a l s not used as f o o d . In a t r e a t m e n t p l a n t , the i n f l u e n t f l o w s c o n t i n u o u s l y i n t o an a e r a t i o n t a n k , where a i r i s i n j e c t e d t o mix the a c t i v a t e d sludge w i t h the wastewater and t o s u p p l y the oxygen needed f o r the microorganisms to decompose the waste. The m i x t u r e o f a c t i v a t e d sludge and wastewater i n the a e r a t i o n tank i s c a l l e d "mixed l i q u o r " . The mixed l i q u o r f l o w s from the a e r a t i o n tank t o a "secondary c l a r i f i e r " , a tank a d j a c e n t t o the a e r a t i o n t a n k , where the a c t i v a t e d s l u d g e i s s e t t l e d . Most o f the s e t t l e d sludge i s r e t u r n e d t o the a e r a t i o n tank t o m a i n t a i n a h i g h p o p u l a t i o n o f organisms t o break down the w a s t e s . As more a c t i v a t e d sludge i s produced than can be used i n the p r o c e s s , some o f the r e t u r n sludge i s d i s p o s e d o f (wasted). To keep t h i s p r o c e s s moving smoothly, a wastewater treatment o p e r a t o r has t o p e r f o r m a d e l i c a t e b a l a n c i n g a c t a c r o s s s e v e r a l f a c t o r s . One o f these f a c t o r s i s the f o o d - t o - o r g a n i s m s r a t i o , w h i c h i s m a i n t a i n e d by w a s t i n g . A n o t h e r i s the amount o f d i s s o l v e d oxygen (DO) i n the a e r a t i o n t a n k . I f DO i s too low, f i l a m e n t o u s b a c t e r i a c o u l d d e v e l o p , w h i c h would keep the sludge f l o e from s e t t l i n g i n the secondary c l a r i f i e r ; i f DO i s t o o h i g h , p i n p o i n t f l o e ( t i n y c l u s t e r s of organisms) w i l l develop and not be removed i n the secondary c l a r i f i e r . One f i n a l f a c t o r i s the need t o keep f l o w d i s t r i b u t e d e v e n l y among m u l t i p l e t r e a t m e n t u n i t s . In c o n t r o l l i n g the p r o c e s s , an e x p e r i e n c e d o p e r a t o r l o o k s f o r c e r t a i n s i g n s l i k e the c o l o r o f the sludge and the amount o f t u r b u l e n c e a t the s u r f a c e o f the mixed l i q u o r . An e x p e r i e n c e d o p e r a t o r might a l s o use h i s / h e r sense o f s m e l l . I n a d d i t i o n t o these p e r s o n a l o b s e r v a t i o n s , an o p e r a t o r must a l s o a n a l y z e samples o f wastewater t a k e n from d i f f e r e n t l o c a t i o n s throughout the p r o c e s s t o c o n f i r m h i s / h e r judgments. A f a i r amount o f e x p e r t i s e , t h e n , i s needed to m a i n t a i n smooth o p e r a t i o n o f an a c t i v a t e d s l u d g e p r o c e s s .


10.

SCHMULLERANDMORLINO

The Activated Sludge Advisor Prototype


ASAP The A c t i v a t e d Sludge A d v i s o r P r o t o t y p e (ASAP) was d e s i g n e d t o h e l p t h e D a l l a s C e n t r a l Wastewater P l a n t ' s n o v i c e o p e r a t o r s c o n t r o l t h e a c t i v a t e d sludge p r o c e s s . Our u l t i m a t e g o a l i s t o develop an e x p e r t system w h i c h d e a l s w i t h a l l a s p e c t s o f p l a n t f u n c t i o n i n g . The a c t i v a t e d sludge p r o c e s s was chosen as t h e f o c u s o f o u r i n i t i a l e f f o r t because (a) i t i s c r u c i a l t o t r e a t m e n t p l a n t f u n c t i o n i n g , (b) i t e n t a i l s a c o n s i d e r a b l e amount o f e x p e r t i s e , and (c) i t s c i r c u m s c r i b e d n a t u r e would p e r m i t development o f an e x p e r t system w i t h i n a r e a s o n a b l e time frame. ASAP was developed by CDM FPC's E x p e r t Systems team, through a c o n t r a c t h e l d by Camp D r e s s e r & McKee-Dallas w i t h t h e D a l l a s Water U t i l i t i e s . The domain e x p e r t , an employee o f Camp D r e s s e r & McKeeD a l l a s , has over 20 y e a r s * e x p e r i e n c e i n t h e wastewater f i e l d , and works c l o s e l y w i t h D a l l a s Water U t i l i t i e s . Development proceeded a c c o r d i n g t o well-known e x p e r t system development g u i d e l i n e s , f a c i l i t a t e d by t h e wastewater t r e a t m e n t e x p e r i e n c e o f one o f t h e knowledge e n g i n e e r s on t h e team. We e s t i m a t e t h a t h e r e x p e r i e n c e saved about 2 person-months i n the 12 person-month development e f f o r t . P r e l i m i n a r y knowledge was a c q u i r e d by r e a d i n g Water P o l l u t i o n C o n t r o l F e d e r a t i o n p u b l i c a t i o n s , and wastewater o p e r a t o r i n s t r u c t i o n manuals ( 1 ) , e n a b l i n g t h e knowledge e n g i n e e r s t o use wastewater t e r m i n o l o g y when i n t e r v i e w i n g the domain e x p e r t . B e f o r e i n t e r v i e w i n g t h i s e x p e r t , t h e knowledge e n g i n e e r s i n t e r v i e w e d a Camp D r e s s e r & McKee e n g i n e e r who has e x t e n s i v e e x p e r i e n c e i n t h e d e s i g n o f wastewater treatment p l a n t s . A l s o , p r i o r t o knowledge a c q u i s i t i o n , t h e development team t o u r e d a t r e a t m e n t f a c i l i t y i n V i r g i n i a , and t h e C e n t r a l Wastewater P l a n t i n D a l l a s . Two 3-day knowledge a c q u i s i t i o n s e s s i o n s were h e l d w i t h t h e domain e x p e r t . Three knowledge e n g i n e e r s were p r e s e n t a t t h e f i r s t s e s s i o n , two a t t h e second. A f t e r t h e f i r s t s e s s i o n , t h e knowledge e n g i n e e r s ' notes were c o m p i l e d and used as t h e b a s i s f o r c o n s t r u c t i n g a " d e c i s i o n diagram" ( a l s o c a l l e d a "knowledge t r e e " ) -- a p i c t u r e w h i c h i l l u s t r a t e s t h e sequence o f an e x p e r t ' s d e c i s i o n s i n g i v e n s i t u a t i o n s , and c a n be c o n v e r t e d i n t o a s e t o f p r o d u c t i o n r u l e s . I n the s i x weeks between s e s s i o n s , t h e domain e x p e r t was sent t h e d e c i s i o n diagram f o r comments and a d d i t i o n s , and c o d i n g was i n i t i a t e d . A t t h e second s e s s i o n , t h e d e c i s i o n diagram was used as t h e b a s i s f o r i n t e r v i e w i n g the expert. The d e c i s i o n diagram grew b o t h i n depth and i n b r e a d t h as a r e s u l t o f t h e second s e s s i o n . I n i t s f i n a l form, t h e diagram c a n take one o f s i x p a t h s , each o f w h i c h has s e v e r a l p o s s i b l e branches. Each p a t h s t a r t s w i t h a r e a d i l y o b s e r v a b l e symptom i n t h e a c t i v a t e d sludge p r o c e s s and u l t i m a t e l y d i v e r g e s i n t o s e v e r a l d i a g n o s e s , so t h a t ASAP can conclude w i t h any o f 50 d i a g n o s e s . Thus, a l t h o u g h ASAP wears t h e " p r o t o t y p e " l a b e l , t h e depth and b r e a d t h o f i t s knowledge base take i t beyond t h e l e v e l o f a p r o t o t y p e . The s p e c i f i c i t y o f some o f t h e parameter v a l u e s i n i t s knowledge base c o n f i n e s i t t o d e c i s i o n s f o r the C e n t r a l Wastewater P l a n t , b u t i t c o u l d be c u s t o m i z e d f o r o t h e r facilities. The a l m o s t - f i n i s h e d system was shown t o C e n t r a l Wastewater P l a n t o p e r a t o r s , so t h a t t h e knowledge e n g i n e e r s c o u l d i n c o r p o r a t e t a r g e t u s e r s * comments on t h e u s e r i n t e r f a c e , and t h e i r i m p r e s s i o n s about i n t e r a c t i n g w i t h t h e system. The f i n i s h e d system and t h e f i n a l


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d e c i s i o n diagram were g i v e n t o another Camp D r e s s e r & McKee employee who has e x t e n s i v e wastewater e x p e r i e n c e . T h i s e x p e r t worked w i t h t h e system and checked i t s r u l e s ; he was n o t i n v o l v e d w i t h t h e development of ASAP i n any o t h e r way.


ASAP

Implementation

ASAP was implemented i n KnowledgeGarden's PC-based e x p e r t system s h e l l c a l l e d KnowledgePro ( r ) , and i t works on a PC/AT o r e q u i v a l e n t machine. We s e l e c t e d t h i s s h e l l because o f i t s f l e x i b l e knowledge r e p r e s e n t a t i o n , i t s i n t e r f a c e t o g r a p h i c s packages, i t s use o f h y p e r t e x t , and i t s f a c i l i t i e s f o r e a s i l y d e v e l o p i n g a u s e r - f r i e n d l y i n t e r f a c e . Our v e r s i o n o f KnowledgePro was w r i t t e n i n Turbo P a s c a l , w h i c h can be used as an e x t e n s i o n language. O r i g i n a l l y , we thought we might have t o do some P a s c a l programming t o implement a r e c o r d k e e p i n g c a p a b i l i t y f o r ASAP ( i . e . , t o keep t r a c k o f t h e u s e r ' s o b s e r v a t i o n s , u s e r - s y s t e m i n t e r a c t i o n s , system recommendations, u s e r ' s a c t i o n s t a k e n , and consequences o f those a c t i o n s ) , b u t i t t u r n e d out t h a t KnowledgePro i t s e l f was e q u a l t o t h e t a s k . One drawback o f KnowledgePro (which d i d n o t a f f e c t t h i s p r o j e c t ) , i s t h a t i t does n o t c o n t a i n a b u i l t - i n f a c i l i t y f o r implementing certainty factors ( q u a n t i t a t i v e r e p r e s e n t a t i o n s o f how sure an e x p e r t i s o f h i s / h e r c o n c l u s i o n s ) ; CDM-FPC's E x p e r t Systems Team i s c u r r e n t l y d e v e l o p i n g a way t o do t h i s i n KnowledgePro f o r another p r o j e c t . Knowledge r e p r e s e n t a t i o n i n KnowledgePro i s b u i l t around t h e t o p i c , a s o f t w a r e s t r u c t u r e w h i c h has been c a l l e d t h e " u l t i m a t e l y f l e x i b l e knowledge r e p r e s e n t a t i o n scheme" ( 2 ) . KnowledgePro uses t o p i c s f o r s t o r i n g i n f o r m a t i o n , f o r o r g a n i z i n g s t r u c t u r e , and as t h e b a s i s f o r h y p e r t e x t ; t o p i c s can a l s o be used t o s e t up f r a m e - l i k e s t r u c t u r e s w h i c h can be used i n an o b j e c t - o r i e n t e d f a s h i o n . D e c i s i o n p o i n t s on the ASAP d e c i s i o n diagram were e a s i l y and i n t u i t i v e l y r e p r e s e n t e d as KnowledgePro t o p i c s , and o p t i o n s a t each d e c i s i o n p o i n t were e a s i l y r e p r e s e n t e d w i t h i n t h e t o p i c s . P r o d u c t i o n r u l e s ( t h e e x p e r t ' s " h e u r i s t i c s " ) were a l s o r e p r e s e n t e d w i t h i n t o p i c s , p r o v i d i n g o r g a n i z a t i o n t o t h e knowledge base. U n l i k e o t h e r e x p e r t system development t o o l s (such as 0PS5), a p r o d u c t i o n r u l e ' s l o c a t i o n ( w i t h r e s p e c t t o t o p i c s ) i s o f extreme importance i n KnowledgePro. ASAP i n c o r p o r a t e s h y p e r t e x t , a t e c h n i q u e w h i c h connects p i e c e s of t e x t i n a n o n - l i n e a r f a s h i o n . H y p e r t e x t a l l o w s t h e d e v e l o p e r t o l i n k a term o r a phrase t o i t s d e f i n i t i o n o r t o o t h e r c l a r i f y i n g i n f o r m a t i o n , so t h a t the a d d i t i o n a l i n f o r m a t i o n i s h i d d e n from v i e w when t h e term i s on the s c r e e n ; t h e u s e r , a t h i s / h e r o p t i o n , can b r i n g the a d d i t i o n a l i n f o r m a t i o n onto t h e s c r e e n . A term can be l i n k e d t o a d e s c r i p t i v e g r a p h i c , as w e l l as t o t e x t . The a d d i t i o n a l i n f o r m a t i o n can be s i m i l a r l y l i n k e d t o more i n f o r m a t i o n , and so on. ASAP i n c o r p o r a t e s a l l o f these t e c h n i q u e s , r e s u l t i n g i n a u s e r i n t e r f a c e w h i c h each u s e r , i n e f f e c t , t a i l o r s t o h i s / h e r own c a p a b i l i t i e s d u r i n g each s e s s i o n . That i s , each u s e r has a p o t e n t i a l l y different background and l e v e l o f e x p e r i e n c e , and w i l l know t h e d e f i n i t i o n s o f some terms and n o t o t h e r s . H y p e r t e x t p r o v i d e s an o p p o r t u n i t y f o r a u s e r t o l o o k up a d e f i n i t i o n w i t h o u t b r e a k i n g away from the system. The g e n e r a l development o f t h e u s e r i n t e r f a c e proceeded a l o n g w i t h t h e development o f t h e system. O r d i n a r i l y , w i t h any o f a number of o t h e r e x p e r t system s h e l l s , we w o u l d have f i r s t developed t h e knowledge base, t e s t e d i t , and then developed t h e u s e r i n t e r f a c e .



10.

SCHMULLERANDMORUNO


KnowledgePro, however, e n a b l e s the d e v e l o p e r t o c o n s t r u c t a windowbased i n t e r f a c e v e r y e a s i l y and q u i c k l y . We b e l i e v e t h a t d e v e l o p i n g the u s e r i n t e r f a c e i n t h i s way ( i . e . , w h i l e i m p l e m e n t i n g the system as a whole) r e s u l t e d i n a time s a v i n g o f about 10Z. We a l s o f e e l t h a t i t r e p r e s e n t s a more n a t u r a l way t o d e v e l o p a system. A s e s s i o n w i t h ASAP b e g i n s w i t h the u s e r e n t e r i n g i d e n t i f i c a t i o n i n f o r m a t i o n and i n i t i a l comments about observed c o n d i t i o n s . The s e s s i o n proceeds w i t h ASAP q u e s t i o n i n g t h e u s e r , and t h e u s e r answering by c h o o s i n g one of s e v e r a l o p t i o n s , e x c e p t when ASAP d i r e c t s the u s e r t o e n t e r q u a n t i t a t i v e i n f o r m a t i o n . As shown i n F i g u r e 1, each q u e s t i o n appears i n a window, and a n s w e r i n g o p t i o n s appear on a menu w i t h i n the window. Terms l i n k e d t o t h e i r d e f i n i t i o n s v i a h y p e r t e x t are h i g h l i g h t e d , so t h a t t h e u s e r always knows w h i c h terms have f u r t h e r i n f o r m a t i o n a v a i l a b l e ; f u n c t i o n keys a l l o w t h e u s e r t o n a v i g a t e among these terms and d i s p l a y t h e i r d e f i n i t i o n s . Some o f the terms are l i n k e d t o t e x t , some t o g r a p h i c s . As F i g u r e 2 i l l u s t r a t e s , a t e x t - b a s e d d e f i n i t i o n or c l a r i f i c a t i o n , when brought t o the s c r e e n , appears i n a window w h i c h i s s m a l l e r t h a n a question-window, and p a r t i a l l y o v e r l a p s w i t h t h e window w h i c h l i n k s t o i t . G r a p h i c h y p e r t e x t dominates most o f the s c r e e n . ASAP's c o n c l u s i o n s a l s o appear i n windows. A s e s s i o n concludes w i t h the u s e r e n t e r i n g comments, a c t i o n s t a k e n , and consequences o f those a c t i o n s . The e n t i r e s e t o f q u e s t i o n s , responses, and comments i n a s e s s i o n i s t h e n w r i t t e n t o a file. Summing Up At p r e s e n t , wastewater o p e r a t o r t e s t s o f ASAP are p e n d i n g . Management s t a f f a t D a l l a s Water U t i l i t i e s have run t h r o u g h s e s s i o n s w i t h ASAP, and t h e i r r e a c t i o n has been v e r y p o s i t i v e . We e n v i s i o n s e v e r a l f u t u r e d i r e c t i o n s f o r our work w i t h ASAP. F i r s t , we t h i n k t h a t ASAP c o u l d be used as a t r a i n i n g t o o l as w e l l as a t r o u b l e s h o o t i n g t o o l . I n t h e s h o r t term, t h i s w i l l o c c u r as o p e r a t o r s become i n c r e a s i n g l y c o m f o r t a b l e w i t h ASAP, and e n t e r h y p o t h e t i c a l wastewater p l a n t s i t u a t i o n s , as w e l l as s i t u a t i o n s w h i c h they have observed. The o p e r a t o r s ' n o t i n g and remembering ASAP's responses w i l l e f f e c t a t r a n s f e r o f e x p e r t i s e from ASAP's knowledge base (and, u l t i m a t e l y , from our domain e x p e r t ) t o the o p e r a t o r s . I n the l o n g term, ASAP c o u l d serve as one component o f an I n t e l l i g e n t C o m p u t e r - A s s i s t e d I n s t r u c t i o n (ICAI) system. ICAI systems combine the knowledge o f a domain e x p e r t and the knowledge o f an e x p e r i e n c e d teacher, r e s u l t i n g i n a computerized t u t o r i n a p a r t i c u l a r f i e l d . A u s e r i n t e r a c t s w i t h an ICAI system i n much the same way t h a t he/she would i n t e r a c t w i t h a human t u t o r -- by a s k i n g q u e s t i o n s , s o l v i n g problems posed by the t u t o r (which are geared t o the u s e r ' s mastery of the f i e l d and l e a r n i n g r a t e ) , and h a v i n g h i s / h e r problem s o l u t i o n s e v a l u a t e d by the t u t o r . We a l s o t h i n k t h a t a l o g i c a l n e x t s t e p f o r ASAP i s t o expand the knowledge base t o d e a l w i t h t r e n d s i n the mass o f d a t a w h i c h i s c o n t i n u o u s l y b e i n g gathered a t the C e n t r a l Wastewater P l a n t . As KnowledgePro has database i n t e r f a c e f a c i l i t i e s , we f e e l t h a t t h i s c a p a b i l i t y c o u l d be implemented v e r y e a s i l y . F i n a l l y , we b e l i e v e t h a t ASAP w i l l serve as the c o r n e r s t o n e f o r r e a c h i n g our a f o r e m e n t i o n e d g o a l of a system w h i c h h e l p s o p e r a t o r s t o c o n t r o l and t r o u b l e s h o o t a l l phases o f wastewater p l a n t o p e r a t i o n ,


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F i g u r e 1. An ASAP Q u e s t i o n - S c r e e n

F i g u r e 2.

ASAP H y p e r t e x t


10.

S C H M U L L E R AND MORLINO


i n c l u d i n g p r e - t r e a t m e n t , p r i m a r y t r e a t m e n t , secondary t r e a t m e n t , and d i s p o s a l o f removed w a s t e s . Such a system might someday be i n t e r f a c e d t o d e v i c e s f o r a u t o m a t i c p r o c e s s c o n t r o l , thus automating many o f the o p e r a t i o n s o f a wastewater p l a n t .


Literature Cited 1.

Operation of Wastewater Treatment Plants: A Field Study Training Program, Volumes 1 and 2, U. S. Environmental Protection Agency, Office of Water Program Operations, 1988.

2.

Shafer, D. PC AI 1988, 2(2), 37-39.

RECEIVED

April 13, 1990


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