Advanced Digestion Process Development for Methane Production

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Advanced Digestion Process Development for Methane Production from Biomass-Waste Blends SAMBHUNATH GHOSH and DONALD L. KLASS Institute of Gas Technology, 3424 South State Street, Chicago, IL 60616

Biomass and organic wastes (discarded biomass or biomass-derived material) may supply up to 15% of the U.S. energy needs by the end of this century via gasification or other conversion schemes (1). One conversion process that is expected to play a role in producing methane from biomass and wastes is anaerobic digestion. Commercial production of substitute natural gas (SNG) and medium-Btu fuel gas by anaerobic digestion of waste materials has already started in the U.S. and other countries (2). Increased usage of the anaerobic digestion process for methane production is, however, hindered because of the low reaction rate and conversion efficiency of the conventional digestion process. New digestion techniques are needed to improve conversion rates and efficiencies so that the full potential of anaerobic digestion as a methane-producing process can be realized. Starting with the crude septic tank, a number of improved process configurations, including "standard-rate" digestion, stage digestion, "high-rate" digestion, and the anaerobic contact process, have evolved during the nearly 100 years of application of the anaerobic digestion process to sewage sludge stabilization. However, the design requirements of even one of the best anaerobic sludge stabilization modes, high-rate digestion, result in large expensive plants that are difficult to justify for commercial SNG production. An additional problem with the conventional digestion process is

0097-6156/81/0144-0251 $07.00/0 © 1981 American Chemical Society

Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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t h a t 4 0 t o 7 0 % of t h e feed o r g a n i c s r e m a i n u n c o n v e r t e d a n d m u s t be d i s p o s e d of at s u b s t a n t i a l cost. Considerable research, s u c h as t h a t r e p o r t e d by Pohland a n d Ghosh (3). Gavett (4), Ort (5,6). S w i t z g a b l e (7). Klass et al. (8). G h o s h et al. (9). G h o s h a n d Klass ( 1 0 , ] \ ) H a u g (12). a n d o t h e r s (13) has t h e r e f o r e been d i r e c t e d t o t h e d e v e l o p m e n t of better d i g e s t i o n m e t h o d s . t

Considerable w o r k has been d o n e at t h e I n s t i t u t e of Gas T e c h n o l o g y since 1971 t o d e v e l o p a d v a n c e d d i g e s t i o n m e t h o d s a n d process c o n f i g u r a t i o n s for t h e c o n v e r s i o n of various o r g a n i c feeds t o h i g h - B t u fuel gas (8-11). In t h i s paper, w e w i l l d e s c r i b e a f e w selected a d v a n c e d d i g e s t i o n c o n c e p t s a n d t h e results of t h e i r a p p l i c a t i o n t o b i o m e t h a n a t i o n of a m i x e d b i o m a s s - w a s t e b l e n d . T h e t e r m " a d v a n c e d d i g e s t i o n " as used in t h i s paper includes u n c o n v e n t i o n a l f e r m e n t a t i o n m o d e s a n d process c o n f i g u r a t i o n s for i m p r o v e d m e t h a n e p r o d u c t i o n rate a n d y i e l d . T h e research r e p o r t e d here c o n s i s t e d of a laboratory e v a l u a t i o n of several a d v a n c e d d i g e s t i o n m e t h o d s a n d a selected b i o m a s s - w a s t e b l e n d . T h e o b j e c t i v e of t h i s w o r k w a s t o search for an o p t i m u m b i o c o n v e r s i o n s y s t e m c o n f i g u r a t i o n , a n d t h e u l t i m a t e goal is t o a p p l y t h i s c o n f i g u r a t i o n t o t h e p r o d u c t i o n of SNG. Specifically, t h e a d v a n c e d d i g e s t i o n t e c h n i q u e s s t u d i e d w e r e d i g e s t i o n of p r e t r e a t e d f e e d , r e c y c l i n g of d i g e s t e r e f f l u e n t a n d p r o d u c t gas. aerobic p o s t t r e a t m e n t a n d r e c y c l i n g of d i g e s t e r e f f l u e n t , a n d t w o - p h a s e digestion. T h e e x p e r i m e n t a l p l a n c o n s i s t e d of: •

C o n v e n t i o n a l h i g h - r a t e d i g e s t i o n u n d e r baseline o p e r a t i n g c o n d i t i o n s of 3 5 ° C d i g e s t i o n t e m p e r a t u r e . 0.1 lb V S / f t - d a y l o a d i n g a n d a 12-day detention time. 3

•

M e s o p h i l i c (35°C) a n d t h e r m o p h i l i c (55°C) d i g e s t i o n of feed s u b j e c t e d t o m i l d alkaline (sodium hydroxide) p r e t r e a t m e n t w i t h r e c y c l i n g of spent c a u s t i c for fresh feed t r e a t m e n t a n d neutralization of t r e a t e d feed w i t h digester gas t o m i n i m i z e a c i d neutralizer r e q u i r e m e n t .

•

M e s o p h i l i c (35°C) d i g e s t i o n w i t h p r o d u c t gas r e c y c l i n g .

•

M e s o p h i l i c (35°C) d i g e s t i o n w i t h r e c y c l i n g of aerobically

posttreated

digester effluent. •

Two-phase digestion.


13.

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Methane from Biomass-Waste

MATERIALS AND METHODS Digester Feeds Four f e e d s t o c k s , w a t e r h y a c i n t h (Eichhornia grass (8) (Cynodon

dactylon)

crassipes)

a n d Coastal B e r m u d a

i n d i g e n o u s t o t h e Gulf Coast, a n d s e w a g e

s l u d g e a n d m u n i c i p a l solid w a s t e ( M S W ) w e r e c o n s i d e r e d f o r b i o c o n v e r s i o n t o m e t h a n e . C o n v e n t i o n a l m e s o p h i l i c (35°C) h i g h - r a t e d i g e s t i o n of a b i o m a s s s l u d g e b l e n d p r e p a r e d w i t h t h e s e m a t e r i a l s s h o w e d t h a t t h e m i x e d feed w a s superior t o t h e single feeds in t e r m s o f n u t r i t i o n a l b a l a n c e a n d m e t h a n e y i e l d a n d p r o d u c t i o n rate (14-16). U t i l i z a t i o n o f a b i o m a s s - w a s t e b l e n d has several a d v a n t a g e s . Use of a b l e n d f a c i l i t a t e s f e e d s t o c k s u p p l y o n a y e a r - r o u n d basis, a n d a m i x e d feed m a y be s u p e r i o r t o b i o m a s s alone in t e r m s o f process economics.

In a d d i t i o n ,

use of a b i o m a s s - w a s t e

blend

provides t h e

o p p o r t u n i t y f o r s i m u l t a n e o u s e n e r g y recovery a n d w a s t e s t a b i l i z a t i o n in an o p t i m i z e d i n t e g r a t e d s y s t e m . T h e feed used f o r t h e w o r k d e s c r i b e d in t h i s paper is a m i x t u r e of s e w a g e l a g o o n e f f l u e n t - g r o w n w a t e r h y a c i n t h , Coastal B e r m u d a grass, t h e c o m b u s t i b l e f r a c t i o n o f m u n i c i p a l solid w a s t e , a n d m i x e d a c t i v a t e d - p r i m a r y s l u d g e b l e n d e d in t h e mass ratio of 3 2 . 3 : 3 2 . 3 : 3 2 . 3 : 3 . 1 o n a volatile solids (VS) basis. T h i s p a r t i c u l a r ratio w a s selected based o n t h e p r o j e c t e d availability of these feed c o m p o n e n t s f o r a c o m m e r c i a l p l a n t in t h e Gulf States area. T h e b i o m a s s a n d M S W c o m p o n e n t s w e r e finely g r o u n d before b l e n d i n g . The m i x e d feed had a m e d i a n p a r t i c l e size of 0.25 m m . It had m o i s t u r e , VS, c a r b o n , n i t r o g e n , p h o s p h o r u s , sulfur, h y d r o g e n , c a l c i u m , s o d i u m , p o t a s s i u m , m a g n e s i u m , cellulose, h e m i c e l l u l o s e . l i g n i n . a n d c r u d e p r o t e i n c o n t e n t s a n d a h i g h h e a t i n g v a l u e o f 8 8 . 9 7 . 8 2 . 7 8 (of t o t a l solids). 4 3 . 1 4 , 1.64, 0.43. 0 . 3 1 , 5.60, 1.23. 0.78, 1.05, 0.22. 37.5. 31.8. 4.6. a n d 10.1 w t %. a n d 7.445 B t u / l b (dry), respectively. T h e m i x e d feed h a d t h e e m p i r i c a l f o r m u l a C3595 H5545 Ο1.979

N

0.117

p

o.oi4

S0.010 A s h

1 7 2

2

at a m o l e c u l a r w e i g h t of 100, a n d a

t h e o r e t i c a l m e t h a n e y i e l d o n a n a e r o b i c d i g e s t i o n o f a b o u t 6.4 SCF/lb V S a d d e d (14).* A l s o , it w a s d e t e r m i n e d b y l o n g - t e r m b a t c h d i g e s t i o n tests t h a t this m i x e d feed had a n u l t i m a t e a n a e r o b i c b i o d e g r a d a b i l i t y or v o l a t i l e solids d e s t r u c t i o n e f f i c i e n c y of 6 6 % (14). Digester feed slurries w e r e p r e p a r e d b y d i l u t i n g a w e i g h e d mass o f t h e feed a c c o r d i n g t o t h e l o a d i n g rate w i t h d i s t i l l e d d e m i n e r a l i z e d w a t e r t o a v o l u m e d e t e r m i n e d b y t h e h y d r a u l i c d e t e n t i o n t i m e o f t h e run. •

Assumes 30% of volatile solids converted to cells on one pass through the digester.


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Digesters T h e d i g e s t i o n runs w e r e c o n d u c t e d in c u s t o m - m a d e , f l a t - b o t t o m e d , c y l i n d r i c a l Plexiglas digesters h a v i n g f o u r vertical baffles m o u n t e d 9 0 ° apart o n t h e inside w a l l s t o p r e v e n t v o r t e x i n g of t h e c u l t u r e d u r i n g m e c h a n i c a l a g i t a t i o n by t w o stainless steel impellers at 1 3 0 r p m . T h e digesters w e r e h e a t e d by p l a c i n g t h e m in a c o n s t a n t - t e m p e r a t u r e c h a m b e r or by t h e r m i s t o r c o n t r o l l e d h e a t i n g tapes. A l l digesters w e r e g e o m e t r i c a l l y similar in c o n s t r u c t i o n . T h e c u l t u r e v o l u m e s of t h e v a r i o u s digesters are i n d i c a t e d in t h e t a b u l a t i o n s of t h e e x p e r i m e n t a l results. Except for t h e t w o - p h a s e s y s t e m , all digesters w e r e m a n u a l l y f e d o n c e per d a y after w i t h d r a w i n g an equal v o l u m e of digester e f f l u e n t . A s d e s c r i b e d later, t h e t w o - p h a s e s y s t e m w a s e q u i p p e d w i t h an a u t o m a t e d f e e d i n g s y s t e m . A D V A N C E D S Y S T E M CONFIGURATION A N D OPERATION Digestion of Pretreated Feed T h e m i x e d h y a c i n t h - g r a s s - M S W - s l u d g e feed w a s p r e t r e a t e d w i t h c a u s t i c soda s o l u t i o n at m i l d t e m p e r a t u r e s a n d pressures in an a t t e m p t t o i m p r o v e feed b i o d e g r a d a b i l i t y a n d m e t h a n e y i e l d . T r e a t m e n t t e m p e r a t u r e s (5°. 2 5 ° , 5 5 ° . 1 0 0 ° . 121°C) a n d pressures ( a t m o s p h e r i c a n d 3 0 psig) a n d d i l u t e c a u s t i c c o n c e n t r a t i o n s w e r e selected for t h e p r e t r e a t m e n t s t u d i e s for reasons of l o w e r reaction vessel costs, r e d u c e d e n e r g y a n d c h e m i c a l i n p u t s , a n d t o keep t h e salt c o n c e n t r a t i o n in t h e d i g e s t e r l o w . A l k a l i n e t r e a t m e n t u n d e r these c o n d i t i o n s is e x p e c t e d t o be a c o s t - e f f e c t i v e m e t h o d for increasing m e t h a n e p r o d u c t i o n f r o m cellulosic feeds (14.17). D i g e s t i o n of t h e c a u s t i c - t r e a t e d feed w a s c o n d u c t e d at selected m e s o p h i l i c (35°C) a n d t h e r m o p h i l i c (55°C) t e m p e r a t u r e s . Flow d i a g r a m s d e p i c t i n g t h e e x p e r i m e n t a l s e q u e n c e are p r e s e n t e d in Figures I a n d II. T h e p r o c e s s i n g steps i n c l u d e d m i x i n g of t h e u n d i l u t e d feed w i t h c a u s t i c s o l u t i o n , p r e t r e a t m e n t at t h e c h o s e n t e m p e r a t u r e , pressure, a n d t i m e ; d i l u t i n g t h e t r e a t e d feed w i t h d i s t i l l e d d e m i n e r a l i z e d w a t e r a n d neutralization of t h e digester feed slurry w i t h h y d r o c h l o r i c a c i d or digester g a s ; a n d d i g e s t i o n of t h e p r e t r e a t e d neutralized f e e d . In s o m e runs, t h e d i g e s t e r feed slurry w a s neutralized w i t h t h e d i g e s t e r gas t o r e d u c e neutralizer a c i d r e q u i r e m e n t a n d d i g e s t e r salinity, a n d t o increase t h e b i c a r b o n a t e alkalinity (buffer c a p a c i t y ) . In still o t h e r runs, t h e c a u s t i c - t r e a t e d feed w a s v a c u u m f i l t e r e d , a n d a p o r t i o n of t h e filtrate c o n t a i n i n g t h e spent c a u s t i c s o l u t i o n w a s r e c y c l e d for fresh feed pretreatm e n t . Filtrate r e c y c l i n g r e d u c e d t h e c a u s t i c r e q u i r e m e n t for feed pretreatm e n t . t h e acid r e q u i r e m e n t t o neutralize t h e pretreated feed, a n d t h e salt c o n c e n t r a t i o n in t h e feed slurry a n d digester.


13.

255

Methane from Bio mass-Waste

GHOSH AND KLASS

II-/

DIGESTER GAS

CAUSTIC NoOH

ι

SOLUTION /

PRETREATMENT j j ^ "

MIXING

1

H 0 2

^

1

FEED S L U R R Y PREPARATION

NEUTRALIZATION EFFLUENT

Figure 1.

QAS COLLECTION

Experimental sequence for mesophilic digestion of caustic-treated feed

MIXING

PRETREATMENT IN 55"C/KX)*C INCUBATOR - ALTERNATE PATH

Figure 2 .

Experimental sequence for thermophilic digestion of caustic-treated feed


256

BIOMASS AS A NONFOSSIL F U E L SOURCE

Digestion W i t h Product Gas Recycling T h e s y s t e m used t o c o n d u c t t h e gas r e c y c l i n g studies is s h o w n in Figure III. Digester gas w a s c l e a n e d a n d d r i e d by passing it t h r o u g h a g l a s s - w o o l p a r t i c u l a t e t r a p , a w a t e r - c o o l e d c o n d e n s a t e t r a p , a n d a C a S 0 -filled gasd r y i n g c o l u m n . T h e c l e a n , d r y gas w a s r e c y c l e d at a selected recycle ratio (defined as t h e ratio of r e c y c l e d gas f l o w rate in d r y s t a n d a r d c u b i c feet at 6 0 ° F a n d 3 0 in. Hg t o t h e d i g e s t e r gas p r o d u c t i o n rate in d r y s t a n d a r d c u b i c feet) i n t o t h e d i g e s t i n g c u l t u r e t h r o u g h a glass diffuser. E x p e r i m e n t s w e r e c o n d u c t e d at gas recycle ratios r a n g i n g f r o m a b o u t 2 t o 125. T h e pressure d i f f e r e n c e across t h e gas p u m p c o r r e s p o n d i n g t o t h e s e recycle ratios r a n g e d f r o m a b o u t 1 t o 9 psi. 4

Digestion W i t h Recycling of Aerobically Posttreated Digester Effluent T h e p u r p o s e of t h i s s t u d y w a s t o e x a m i n e t h e effect of aerobic biological p o s t t r e a t m e n t of d i g e s t e r e f f l u e n t o n t h e b i o d e g r a d a b i l i t y of r e c a l c i t r a n t feed c o m p o n e n t s passing u n c o n v e r t e d or partially c o n v e r t e d t h r o u g h t h e digester. P r e s u m i n g t h a t aerobic t r e a t m e n t of t h e d i g e s t e r e f f l u e n t i m p r o v e d b i o d e g r a d a b i l i t y . r e c y c l i n g of t h e t r e a t e d solids w a s e x p e c t e d t o increase m e t h a n e p r o d u c t i o n rate a n d yield w i t h s i m u l t a n e o u s a n d e n h a n c e d r e d u c t i o n of t h e e f f l u e n t solids a n d soluble o r g a n i c s load d i s c h a r g e d f r o m t h e overall a n a e r o b i c - a e r o b i c s y s t e m . In t h e e x p e r i m e n t a l process c o n f i g u r a t i o n (Figure IV). d i g e s t e r e f f l u e n t w a s aerobically t r e a t e d in a 1 4 / c u l t u r e v o l u m e s e m i c o n t i n u o u s l y . or in a 2 / c u l t u r e v o l u m e b a t c h a c t i v a t e d s l u d g e unit. A e r a t i o n a n d m i x i n g w e r e a c c o m p l i s h e d by d i f f u s e d a e r a t i o n . Dissolved o x y g e n w a s m o n i t o r e d by a lead-silver g a l v a n i c p r o b e inserted in t h e c u l t u r e . T h e 1 4 / a c t i v a t e d s l u d g e u n i t w a s a t w o - c o m p a r t m e n t t a n k , t h e aeration c h a m b e r of w h i c h w a s separated f r o m t h e a d j a c e n t s e t t l i n g zone by a v e r t i c a l plate. This u n i t w a s used for s e m i c o n t i n u o u s o p e r a t i o n at aerator d e t e n t i o n t i m e s greater t h a n 2 days. Settled s l u d g e w i t h d r a w n f r o m t h e b o t t o m of t h e settler w a s r e c y c l e d t o t h e digester. T h e 1 4 / unit w a s o p e r a t e d at 3 5 ° C w i t h an air f l o w rate of 1 / / m i n . V a r i o u s runs w e r e c o n d u c t e d at selected aerator d e t e n t i o n t i m e s a n d s l u d g e recycle ratios (defined as t h e v o l u m e of recycle s l u d g e d i v i d e d by t h e v o l u m e of t h e d i g e s t e r feed). T h e 2 / b a t c h u n i t w a s o p e r a t e d u n d e r c o n d i t i o n s of l i m i t e d aeration (0.12 / / m i n ) at an a m b i e n t t e m p e r a t u r e of a b o u t 25°C). T h e f i l l - a n d - d r a w a c t i v a t e d s l u d g e u n i t w a s f e d d a i l y w i t h 1.667 m l of fresh digester e f f l u e n t after w i t h d r a w i n g an equal v o l u m e of m i x e d liquor aerated for 2 4 hours. Selected v o l u m e s of t h e aerator m i x e d liquor w e r e r e c y c l e d t o t h e d i g e s t e r t o


13.

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Figure 4.


257

Recycling system for aerobically-posttreated digester effluent


258


o b t a i n t h e desired recycle ratio (defined as t h e ratio of t h e v o l u m e of m i x e d liquor t o t h e v o l u m e of fresh feed). In s o m e e x p e r i m e n t s , t h e aerator feed (fresh digester effluent) w a s pretreated w i t h d i l u t e s o d i u m h y d r o x i d e s o l u t i o n for 2 4 hours at 100°C. T h e recycle s l u d g e w a s d e o x y g e n a t e d w i t h a h e l i u m p u r g e in s o m e runs before c h a r g i n g it t o t h e digester. Two-Phase Digester T w o - p h a s e d i g e s t i o n has c o n s i d e r a b l e p o t e n t i a l f o r increasing m e t h a n e p r o d u c t i o n rate a n d yield (3.10,18,^9). It is a m u l t i - s t a g e , h i g h - r a t e d i g e s t i o n process in w h i c h a c i d o g e n i c a n d m e t h a n o g e n i c f e r m e n t a t i o n s are o p t i m i z e d in separate digesters. T h e t w o - p h a s e s y s t e m used in o u r w o r k c o n s i s t e d of a c o m p l e t e l y m i x e d a c i d - p h a s e digester a n d a c o m p l e t e l y m i x e d m e t h a n e phase digester or a m e t h a n e - p h a s e a n a e r o b i c filter p a c k e d w i t h Raschig rings (Figure V). T h e a c i d digester w a s g r a v i t y fed f r o m a sealed o v e r h e a d feed reservoir h a v i n g a h e l i u m or a r g o n b l a n k e t a b o v e t h e feed slurry. Caustict r e a t e d feed w a s delivered t o t h i s d i g e s t e r in small slugs u p t o 7 0 t i m e s per d a y by a t i m e r - o p e r a t e d v a l v e t o o b t a i n o p e r a t i n g c o n d i t i o n s closely a p p r o a c h i n g t h o s e of c o n t i n u o u s f e e d i n g . Effluent f r o m t h e acid digester o v e r f l o w e d d i r e c t l y t o t h e c o m p l e t e l y m i x e d m e t h a n e - p h a s e digester. A l t e r n a t i v e l y , part of t h i s e f f l u e n t w a s v a c u u m f i l t e r e d , a n d t h e f i l t r a t e w a s f e d c o n t i n u o u s l y t o t h e a n a e r o b i c filter f r o m a c o n s t a n t - h e a d M a r i o t t e b o t t l e s u p p l i e d w i t h h e l i u m t o fill t h e reservoir gas phase. T h e p a c k e d - b e d anaerobic filter had a gross v o l u m e of 1 8 . 5 / a n d a v o i d ratio of 0.63. Filter e f f l u e n t w a s r e c i r c u l a t e d c o n t i n u o u s l y t o t h e inlet e n d at a r e c i r c u l a t i o n ratio of 5.1 (defined as t h e ratio fo t h e recycle f l o w rate t o t h e daily feed f l o w rate) t o d i l u t e t h e i n c o m i n g feed a n d accelerate t h e t r a n s p o r t of d i g e s t i o n p r o d u c t s o u t of t h e c u l t u r e . T h e filter h a d a h y d r a u l i c d e t e n t i o n t i m e of a b o u t 2 . 3 3 . d a y s (defined as t h e gross v o l u m e d i v i d e d by t h e daily feed f l o w rates). The t w o - p h a s e s y s t e m feed w a s s u p p l e m e n t e d w i t h external n i t r o g e n , p h o s p h o r u s , a n d m a g n e s i u m t o ensure t h a t feed hydrolysis, a c i d i f i c a t i o n , a n d gasification were not nutrient limited. RESULTS A N D DISCUSSION Conventional High-Rate Digestion Steady-state p e r f o r m a n c e of c o n v e n t i o n a l h i g h - r a t e m e s o p h i l i c d i g e s t i o n of t h e b i o m a s s - w a s t e b l e n d at t h e baseline loading a n d d e t e n t i o n t i m e is presented in Table I. The m e t h a n e yields f r o m replicate baseline runs ranged


to vo

Figure 5. Two-phase system for biomass-waste blend

Τ CAKES EFFLUENT



3

* ΐ

20 0.18 9 0.868 58.7 2.88 6 29.9 32.4

0.570 60.7 3.45 27 34.5 38.8

0.550 63.9 3.53 35 33.6 39.7

0.550 62.0 3.40 42 33.3 38.3

Run 3MA/7

20 0.1 12

3MA/2

5 0.1 12

5

Baseline Runs* 6M

0.1 12

Runs 5M and 6M were replicates. Calculated by formula suggested by Klass and Ghosh (21).

Operating Conditions Culture Volume, t Loading. l b V S / f t - d a y Detention Time, days Gas Production Rate, std vol/vol culture-day Methane Content, m o l % Methane Yield. SCF/lb VS added Effluent Quality Volatile Acids, m g / f as acetic Efficiency VS Reduction. %* Energy Recovery in Collected Methane. %

6M

Table I. C O N V E N T I O N A L MESOPHILIC (3B°C) DIGESTION OF HYACINTH-GRASS-MSW-8LUDGE BLEND UNDER BASELINE A N D N O N - B A S E L I N E CONDITIONS

to

H

ι

I

>

δ

ο

OS

13.

GHOSH AND KLASS


261

b e t w e e n 3.4 a n d 3.5 SCF/lb V S a d d e d . These yields w e r e b e t w e e n 53.1 a n d 5 4 . 7 % o f t h e t h e o r e t i c a l m e t h a n e y i e l d . A s e x p e c t e d f o r digesters o f similar g e o m e t r y a n d m i x i n g c o n f i g u r a t i o n , c u l t u r e v o l u m e had n o d i s c e r n i b l e effect o n m e t h a n e p r o d u c t i o n (compare Runs 5 M , 6 M , a n d 3 M A ) . N u t r i t i o n a l studies, t h e details o f w h i c h w e r e p r e s e n t e d in a n earlier paper (14), s h o w e d t h a t c o n v e n t i o n a l d i g e s t i o n o f t h e b i o m a s s - w a s t e feed w a s n o t n u t r i e n t or growth-factor limited. The performance of subsequent advanced digestion runs w a s e v a l u a t e d w i t h reference t o t h e baseline p e r f o r m a n c e d a t a reported in Table I. M e t h a n e yield decreased t o 2.4 SCF/lb V S a d d e d (43.8% o f t h e o r e t i c a l yield) w h e n t h e l o a d i n g rate w a s increased t o 0.18 lb V S / f t - d a y a n d t h e d e t e n t i o n t i m e decreased t o 9 days (Table I). These d a t a i n d i c a t e t h a t c o n v e n t i o n a l d i g e s t i o n process e f f i c i e n c y w o u l d decrease s u b s t a n t i a l l y as t h e l o a d i n g rate is increased a n d t h e d e t e n t i o n t i m e is decreased b e y o n d t h e baseline values of these parameters. U n c o n v e n t i o n a l or a d v a n c e d d i g e s t i o n m e t h o d s are t h u s needed t o o v e r c o m e t h e l i m i t a t i o n s o f t h e c o n v e n t i o n a l h i g h - r a t e process. 3

Digestion o f Pretreated Feed A s p o i n t e d o u t previously, o n l y a b o u t 6 6 % of t h e h y a c i n t h - g r a s s - M S W s l u d g e feed V S w a s d e t e r m i n e d t o be b i o d e g r a d a b l e u n d e r l o n g - t e r m b a t c h d i g e s t i o n c o n d i t i o n s . A b o u t 5 2 % o f t h e b i o d e g r a d a b l e o r g a n i c s a n d 3 2 % of t h e cellulose c o m p o n e n t o f t h e feed w e r e gasified at t h e baseline o p e r a t i n g c o n d i t i o n s of 0.1 lb V S / f t -day l o a d i n g a n d a 12-day d e t e n t i o n t i m e (14). These data s u g g e s t t h a t 3 4 % o f t h e feed o r g a n i c s resisted anaerobic d e g r a d a t i o n , a n d t h a t o n l y a b o u t one-half or less o f t h e b i o d é g r a d a b l e s c o u l d be gasified by c o n v e n t i o n a l h i g h - r a t e d i g e s t i o n . One p r o b a b l e e x p l a n a t i o n for t h e l o w b i o c o n v e r s i o n e f f i c i e n c y w a s t h a t hydrolysis a n d a c i d i f i c a t i o n l i m i t e d t h e d i g e s t i o n of t h e h i g h l y f i b r o u s l i g n o c e l l u l o s i c feed (14). If this is t h e case, t h e n considerable i m p r o v e m e n t w o u l d be e x p e c t e d w h e n t h e feed is p r e t r e a t e d c h e m i c a l l y t o hydrolyze t h e c o m p l e x p o l y m e r i c s u b s t a n c e s or t o c o n v e r t t h e m t o a f o r m s u i t a b l e f o r s u b s e q u e n t e n z y m a t i c hydrolysis. 3

A c i d or alkaline t r e a t m e n t s o f p a r t i c u l a t e feeds have been s h o w n t o i m p r o v e digester gas yields (17,22-25). A c i d hydrolysis w a s n o t used in o u r w o r k because severe reaction c o n d i t i o n s are r e q u i r e d , a n d there is considerable d e c o m p o s i t i o n o f t h e h y d r o l y t i c p r o d u c t s u n d e r these c o n d i t i o n s (26). Dilute alkaline p r e t r e a t m e n t w a s e v a l u a t e d because alkali w a s s h o w n t o be more e f f e c t i v e in p r o m o t i n g hydrolysis of cellulosic biomass t h a n acid (j_7,26). It is p o s t u l a t e d , for e x a m p l e , t h a t s o d i u m h y d r o x i d e breaks d o w n t h e cross-linked l i g n i n m a c r o - m o l e c u l e s s u r r o u n d i n g t h e cellulose fibers into alkali-soluble l o w e r - m o l e c u l a r - w e i g h t units. In t h i s w a y . t h e cellulose fibers are e x p o s e d f o r


262


e n z y m a t i c hydrolysis d u r i n g anaerobic d i g e s t i o n . A l s o , it has been s u g g e s t e d t h a t alkali t r e a t m e n t hydrolyzes ester b o n d s b e t w e e n t h e uronic acids of h e m i c e l l u l o s e a n d l i g n i n (27), t h e r e b y e n h a n c i n g t h e b i o d e g r a d a b i l i t y of hemicellulose. A s already

mentioned, digestion

runs w i t h

caustic

t r e a t e d feed

were

c o n d u c t e d u n d e r a v a r i e t y of o p e r a t i n g c o n d i t i o n s . T h e results of a f e w selected runs w i t h p r e t r e a t e d feed are p r e s e n t e d in Table II. T h e d a t a in t h i s table s h o w t h a t t h e h i g h e s t m e s o p h i l i c (35°C) m e t h a n e y i e l d . 4.10 SCF/lb VS a d d e d , f r o m d i g e s t i o n of t h e p r e t r e a t e d feed at a l o a d i n g of 0.1 lb V S / f t - d a y a n d a 12-day d e t e n t i o n t i m e w a s o b t a i n e d w i t h w e t feed p r e t r e a t e d w i t h 3 w t % c a u s t i c s o l u t i o n . H o w e v e r , t h e m e s o p h i l i c m e t h a n e y i e l d (3.92 SCF/lb VS added) f r o m feed t r e a t e d w i t h 1 w t % NaOH s o l u t i o n w a s n o t s i g n i f i c a n t l y different. A l s o , neutralization of t h e p r e t r e a t e d feed or a d d i t i o n of e x t e r n a l n i t r o g e n t o t h e d i g e s t e r d i d n o t affect t h e m e s o p h i l i c m e t h a n e yield. These o b s e r v a t i o n s i n d i c a t e t h a t an increase in m e s o p h i l i c m e t h a n e y i e l d u p t o 2 0 % m a y be e x p e c t e d at a l o a d i n g of 0.1 lb V S / f t - d a y a n d a d e t e n t i o n t i m e of 12 days w i t h alkaline p r e t r e a t m e n t . 3

3

T h e r m o p h i l i c (55°C) d i g e s t i o n of t h e c a u s t i c - t r e a t e d feed at 0.4 lb V S / f t - d a y loading a n d a 6-day d e t e n t i o n t i m e s h o w e d t h e s a m e m e t h a n e yield of a b o u t 4 SCF/lb V S a d d e d as o b s e r v e d d u r i n g m e s o p h i l i c d i g e s t i o n of t h e p r e t r e a t e d feed at a 0.1 lb V S / f t - d a y l o a d i n g a n d a 1 2 - d a y d e t e n t i o n t i m e (Run 5 T / 2 4 . Table II). H o w e v e r , t h e t h e r m o p h i l i c gas p r o d u c t i o n rate w a s 4 - 5 t i m e s t h e m e s o p h i l i c rate. A l s o , t h e c a u s t i c r e q u i r e m e n t for feed p r e t r e a t m e n t w a s l o w e r for t h e t h e r m o p h i l i c r u n o w i n g t o t h e r e c y c l i n g of t h e spent c a u s t i c solution. 3

3

A t h e r m o p h i l i c m e t h a n e yield of a b o u t 3.7 SCF/lb V S a d d e d , w h i c h w a s still larger t h a n t h e baseline y i e l d , w a s o b s e r v e d at a l o a d i n g rate of 0.43 lb V S / f t -day a n d a d e t e n t i o n t i m e of 5.5 days w h e n t h e c a u s t i c - t r e a t e d feed w a s neutralized t o p H 10 w i t h digester gas instead of t o p H 9 w i t h h y d r o c h l o r i c acid(Run 5 T / 2 6 . Table III). This r e d u c e d yield resulted f r o m increased l o a d i n g a n d decreased d e t e n t i o n t i m e . A s e x p e c t e d , a h i g h e r b i c a r b o n a t e alkalinity c o u l d be m a i n t a i n e d w h e n t h e d i g e s t e r w a s c h a r g e d w i t h p r e t r e a t e d feed neutralized w i t h d i g e s t e r gas. In a d d i t i o n , neutralization of t h e alkaline feed w i t h p r o d u c t gas e l i m i n a t e d t h e need for neutralizing acid a n d p r o v i d e d a m e t h o d of c a r b o n d i o x i d e r e m o v a l f r o m t h e digester gas. This t e c h n i q u e s h o u l d p r o v i d e a r e d u c t i o n in t h e cost of feed t r e a t m e n t a n d digester gas cleanup. 3


13.

263


GHOSH AND KLASS

Table II. MESOPHILIC (3B C) AND THERMOPHILIC (55°C) DIGESTION OF HYACINTH-QRAS8-M8W-SLUDGE BLEND PRETREATED WITH CAUSTIC SODA 80LUTION a

Neutralization Method Operating Conditions Culture Volume. I Loading, lb VS/ft -day Detention Time, days Qas Production Rate, std vol/vol culture day Methane Content, mol % Methane Yield. SCF/lb VS added Effluent Quality PH Volatile Acids. mg/f as acetic Total Alkalinity. mg/f asCaC03 Bicarbonate Alkalinity. mg/f asCaC03 3

VS Reduction. % Energy Recovery in Collected Methane. %

Mesophilic Run 6M A/13* No neutralization of caustictreated feed

Run •MA/22 Whole feed to pH 8-8.4 with acid. b

Thermophilic Digester Feed Run 5T/24 Run 5T/26 Mixed vacuum filter Mixed vacuum filter cakes plus filtrate to cakes plus filtrate pH 9 with acid. to pH 10 with acid. C

d

10 0.1 12

10 0.1 12

5 0.4 6.0

5 0.43 5.5

0.606 59.5

0.670 58 1

2.851 56.4

2.911 56.0

4.10

392

4.00

3.68

684

6.71

7.35

743

20

21

99

140

2618

2433

6075

6298

2592

2412

5993

6240

41 9

41.0

43.1

40.0

46 1

44.1

45.0

41.4

The fresh feed was treated with 140 ml of 3 wt % NaOH solution at 55°C for 24 hr The treated feed was not neutralized, and no N H 4 C I was added Feed pretreatment same as Run 6MA/13 except that 140 ml of 1 wt % NaOH solution was used at 25°C for 24 hr. The treated feed was neutralized with HCI to the indicated pH and supplemented with 10 ml of 120-g/l NH CI solution. Fresh feed was treated with 160 ml of 3 wt NaOH solution at 100°C for 24 hr. The treated feed was vacuum filtered, and about 158 g of filter cake and 200 ml of filtrate were fed to the digester after dilution to the proper volume and neutralization with HCI to pH 9. Fresh feed was treated in the same way as in Run 5T/24. except that the alkaline feed slurry was neutralized by bubbling a portion of the digester gas (Ave 9.2 f . range 7.19-11.55 f ) through it. The average pH of the digester feed after gas neutralization was 9.98 (range 9.31 -11.55). 4


Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981. d e o x y g e n a t i o n ; 2 5 7 vol % recycle.

VS added

recycled gas/std f

digester gas produced-day.

43.7

55.0

41.6

4645

4667

20

6.80

culture volume, batch, activated sludge unit operated

43.1

—

37.9

2819

2826

9

6.69

3.88

56.7

3.83

0.687

61.5

t h r o u g h it.

S a m e o p e r a t i n g c o n d i t i o n s as i n R u n 1 M A / 1 3 . e x c e p t t h a t t h e r e c y c l e a e r a t o r s l u d g e w a s d e o x y g e n a t e d b y b u b b l i n g h e l i u m

at a m b i e n t t e m p e r a t u r e . A v o l u m e o f 1 2 0 0 m l o f a e r a t e r c o n t e n t w a s r e c y c l e d w i t h 4 6 7 m l o f f r e s h f e e d d a i l y .

D a i l y d i g e s t e r e f f l u e n t w a s a e r a t e d (air f l o w r a t e 0 . 1 2 f / m i n ) i n a 2.0- f

R e c y c l e r a t i o is s t d f

Methane. %

44.4

39.7

Energy Recovery in Collected

39.7

Overall V S Reduction. %

3

VS Reduction. %

Efficiency

Bicarbonate Alkalinity, as C a C 0 3997

3997

mg/f

Total Alkalinity, as C a C 0

3

8

Volatile Acids, m g / f as a c e t i c

mg/f

7.06

PH

Effluent Quality

3.95

60.5

Methane Content, m o l %

Methane Yield. SCF/lb

0.596

Rate, s t d v o l / v o l c u l t u r e - d a y

0.630

12

12

12

Production

Detention Time, days

Gas

20 0.1

20 0.1

20

3

0.1

6

56 v o l % r e c y c l e .

8

recycle ratio.

Culture volume, f Loading, lb V S / f t - d a y

Operating Conditions

Run I M A / 1 8 A e r a t e d digester effluent after

Run 1 M A / 1 3 Aerated effluent;

MESOPHILIC

Run 2 M A / 1 1

BLEND

Digester Gas: 3 9 gas

(3B°C) DIGESTION OF H Y A C I N T H - G R A S S - M S W - 8 L U D G E

EFFECT OF PRODUCT QA8 A N D P08TTREATED EFFLUENT RECYCLING ON

Recycled Material and Amount

T a b l e III.

ON

13.

GHOSH AND KLASS


265

T h e e x p e r i m e n t a l d a t a s u g g e s t t h e f o l l o w i n g s e q u e n c e of o p e r a t i o n s f o r improved digestion of the hyacinth-grass-MSW-sludge feed: 1.

Caustic t r e a t m e n t of u n d i l u t e d feed for 2 4 hr at 100°C w i t h 3 w t % NaOH a n d r e c y c l i n g of spent c a u s t i c s o l u t i o n . Fresh caustic is a d d e d at t h e rate of a b o u t 3 m e q per g r a m of VS. Spent c a u s t i c recycle f l o w rate is 7 5 v o l % o f t h e feed slurry f l o w rate.

2.

D e w a t e r i n g of t h e pretreated feed. D e w a t e r e d cakes a n d t h e balance of t h e filtrate after r e c y c l i n g are fed t o t h e digester.

3.

Neutralization o f alkaline feed slurry w i t h digester gases.

4.

T h e r m o p h i l i c d i g e s t i o n o f t h e feed at a l o a d i n g rate of 0.4 lb V S / f t - d a y 3

a n d a d e t e n t i o n t i m e of 6 days. Digestion W i t h Product Gas and P o s t t r e a t e d Effluent Recycling T h e s e c o n d major a d v a n c e d s y s t e m w a s c o n c e r n e d w i t h t h e r e c y c l i n g of p r o d u c t gas a n d l i q u i d e f f l u e n t t o t h e digester. T h e effluent w a s p o s t t r e a t e d before r e c y c l i n g . The p r o d u c t gas w a s d e h y d r a t e d before r e c i r c u l a t i o n t o t h e d i g e s t i n g c u l t u r e . T h e a q u e o u s e f f l u e n t w a s s u b j e c t e d t o various f o r m s o f p o s t t r e a t m e n t i n c l u d i n g s o n i c a t i o n , s i m p l e heat t r e a t m e n t f o r 3 0 m i n at 2 7 0 ° F a n d 2 5 psig. a n d a c t i v a t e d s l u d g e t r e a t m e n t t o i m p r o v e t h e b i o d e g r a d a b i l i t y o f t h e u n d i g e s t e d solids. Recycling of s o n i c a t e d a n d heatt r e a t e d s l u d g e t o t h e digester d i d n o t increase m e t h a n e yield a b o v e t h e baseline yield. Gas r e c y c l i n g a n d a c t i v a t e d s l u d g e p o s t t r e a t m e n t w h i c h e f f e c t e d a higher s y s t e m V S r e d u c t i o n , w i l l be discussed here. Gas Recycling Recycling of p r o d u c t gases t h r o u g h t h e d i g e s t i n g c u l t u r e w a s e x p e c t e d t o increase m e t h a n e p r o d u c t i o n because of a d d i t i o n a l m e t h a n e f e r m e n t a t i o n f r o m increased r e d u c t i o n of c a r b o n d i o x i d e a n d because t h e s w e e p i n g a c t i o n of t h e recycled gas m i g h t be e x p e c t e d t o accelerate r e m o v a l o f t h e gaseous products surrounding the microorganisms thereby minimizing end-product repression. Digester gases w e r e r e c y c l e d at various recycle ratios f r o m 2 t o 125. T h e best m e t h a n e yield o f 3.95 SCF/lb V S a d d e d , w h i c h w a s a b o u t 1 5 % h i g h e r t h a n t h e baseline y i e l d , w a s o b s e r v e d a t a g a s recycle ratio o f 3 9 (Table III). T h e gas-phase d i l u t i o n rate at t h i s recycle ratio w a s a b o u t 0.2 hr"'. The e x i s t e n c e of an o p t i m u m gas recycle ratio is rationalized as f o l l o w s : A s t h e gas recycle


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ratio is increased, t h e o p p o r t u n i t y for c a r b o n d i o x i d e r e d u c t i o n a n d s w e e p i n g of t h e gaseous d i g e s t i o n p r o d u c t s o u t of t h e d i g e s t e r increases t h e r e b y s t i m u l a t i n g m e t h a n e p r o d u c t i o n . H o w e v e r as t h e gas r e c y c l i n g ratio is increased f u r t h e r , t h e c u l t u r e is increasingly s a t u r a t e d w i t h gaseous e n d p r o d u c t s ; t h i s w o u l d t e n d t o repress or i n h i b i t a d d i t i o n a l m e t h a n e p r o d u c t i o n . T h e o p p o s i n g effects of increasing gas recycle o n m e t h a n e p r o d u c t i o n e q u a l each o t h e r at t h e o p t i m u m gas recycle ratio w h i c h maximizes methane yield. It s h o u l d be n o t e d t h a t d u r i n g gas r e c y c l i n g , t h e test d i g e s t e r w a s m e c h a n i c a l l y m i x e d as in t h e baseline c o n t r o l runs so t h a t t h e effect of t h e r e c y c l e d gas o n test digester m e t h a n e p r o d u c t i o n c o u l d be e v a l u a t e d . Since m e c h a n i c a l m i x i n g alone w a s d e s i g n e d t o p r o v i d e c o m p l e t e m i x i n g of t h e d i g e s t e r c o n t e n t s , t h e beneficial effect of gas r e c y c l i n g m a y n o t be a t t r i b u t e d t o effects s u c h as s u b s t r a t e t r a n s p o r t a n d s u b s t r a t e - m i c r o o r g a n i s m c o n t a c t . M i l d m e c h a n i c a l m i x i n g also p r o v e d t o be beneficial d u r i n g gas r e c y c l i n g because s c u m f o r m a t i o n , w h i c h arises d u e t o gas f l o t a t i o n of d i g e s t e r solids, w a s s u r p r i s i n g l y n o t a p r o b l e m even at very h i g h gas recycle ratios. The reason for t h i s w a s t h a t m e c h a n i c a l a g i t a t i o n at a m i x i n g Reynolds n u m b e r of a b o u t 9,000 dispersed t h e surface solids a n d m o v e d t h e m d o w n i n t o t h e c u l t u r e by t h e f o l d i n g a c t i o n of t h e t w o propeller m i x e r s p l a c e d at h e i g h t s of one a n d t w o i m p e l l e r d i a m e t e r s a b o v e t h e digester b o t t o m . The results of t h e gas r e c y c l i n g e x p e r i m e n t s s h o w e d t h a t m o d e s t increases in m e t h a n e yield c a n be o b t a i n e d by r e c y c l i n g p r o d u c t gas at an o p t i m u m gas recycle ratio. Dual m e c h a n i c a l a n d gas m i x i n g e l i m i n a t e d t h e s c u m p r o b l e m associated w i t h gas m i x i n g alone. Aerobic Sludge Posttreatment of Digester Effluent T h e o b j e c t i v e of aerobic p o s t t r e a t m e n t is t o treat t h e " r e f r a c t o r y " e f f l u e n t o r g a n i c s t o render t h e m b i o d e g r a d a b l e , a n d t o increase m e t h a n e p r o d u c t i o n by r e c y c l i n g t h e p o s t t r e a t e d m a t e r i a l for f u r t h e r d i g e s t i o n . P o s t t r e a t m e n t m a y be preferred t o p r e t r e a t m e n t because it o n l y treats t h e recalcitrant residue r e m a i n i n g after t h e b i o d e g r a d a b l e material is gasified a n d not t h e t o t a l solids in t h e feed. T h u s , t h e o r g a n i c loading rate on t h e p o s t t r e a t m e n t process is s u b s t a n t i a l l y l o w e r t h a n t h a t for a similar p r e t r e a t m e n t process. A l s o , w h i l e i m p r o v i n g t h e b i o d e g r a d a b i l i t y of t h e r e c a l c i t r a n t f e e d f r a c t i o n , p r e t r e a t m e n t m a y adversely affect t h e d i g e s t i b i l i t y of feed c o m p o n e n t s t h a t are easily gasified in t h e i r o r i g i n a l f o r m s . P o s t t r e a t m e n t o b v i a t e s t h i s p r o b l e m . A e r o b i c b i o c h e m i c a l a n d c h e m i c a l - b i o c h e m i c a l p o s t t r e a t m e n t s w e r e invest i g a t e d because several species of f u n g i a n d aerobic o r g a n i s m s are k n o w n t o hydrolyze a n d d e g r a d e c o m p l e x lignocellulosic s u b s t a n c e s t o s i m p l e r


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s u b s t a n c e s (26). Trichoderma viride. Cellulomonas, and Cytophaga hutchinsonii. Cytophaga vulgaris are e x a m p l e s o f aerobic m i c r o o r g a n i s m s t h a t m e d i a t e these reactions (28.29). Because s o m e aerobes c a n n o t e f f i c i e n t l y a t t a c k lignocellulosic cellulosic c o m p l e x e s (30,31). o n e r u n w a s also c o n d u c t e d in w h i c h t h e aerator feed (digester effluent) w a s p r e d i g e s t e d w i t h hot c a u s t i c t o m a k e t h e fibers available f o r aerobic d e c o m p o s i t i o n (28). Finally, a c t i v a t e d s l u d g e p o s t t r e a t m e n t o f t h e d i g e s t e d residue w a s also c o n d u c t e d u n d e r c o n d i t i o n s o f " l i m i t e d " aeration t o arrest cellulose b r e a k d o w n before t h e f o r m a t i o n of m o n o m e r i c p r o d u c t s (hexose, pentose, etc.) w h i c h are readily oxidized aerobically a n d , t h u s , b e c o m e unavailable f o r gasification), so t h a t t h e p r o d u c t s o f partial aerobic d i g e s t i o n c o u l d be gasified anaerobically u p o n r e c y c l i n g t o t h e digester. Evidence o f partial cellulose d e g r a d a t i o n b y l i m i t e d aeration w a s presented b y Kalnins (32), w h o s h o w e d t h a t Bacterium protoziodes d e c o m p o s e d cellulose t o dextrose w i t h an u n l i m i t e d s u p p l y of o x y g e n . H o w e v e r , w h e n t h e s u p p l y o f o x y g e n w a s l i m i t e d , dextrose p r o d u c t i o n w a s s t o p p e d , b u t t h e o r g a n i s m d e c o m p o s e d an increased q u a n t i t y of cellulose t o derive a n a m o u n t of e n e r g y e q u i v a l e n t t o t h a t o b t a i n e d d u r i n g d e g r a d a t i o n of cellulose t o dextrose. T h u s , t h e a d v a n t a g e of l i m i t e d aeration p o s t t r e a t m e n t is t h a t it s h o u l d l i m i t d e g r a d a t i o n of t h e residual solids t o f o r m s t h a t are lost b y o x i d a t i o n before r e c y c l i n g t o t h e digester. P o s t t r e a t m e n t studies o f digester e f f l u e n t c o n d u c t e d in t h e 1 4 - / m e s o p h i l i c (33°-34°C) aerobic s l u d g e u n i t at d e t e n t i o n t i m e s o f 2.4 a n d 5.3 days, a n d an air f l o w rate of 1 / / m i n p r o d u c e d settleable s l u d g e w h i c h , w h e n r e c y c l e d t o t h e d i g e s t e r at recycle ratios o f 5.3 a n d 33.7%, e f f e c t e d digester m e t h a n e yields based o n fresh-feed volatile solids r a n g i n g b e t w e e n 2.81 a n d 3.51 SCF/lb V S a d d e d . These yields w e r e l o w e r t h a n or equal t o t h e baseline m e t h a n e yield o b s e r v e d u n d e r t h e same d i g e s t e r o p e r a t i n g c o n d i t i o n s , w h i c h i n d i c a t e d t h a t aerobic biological p o s t t r e a t m e n t at long d e t e n t i o n t i m e s a n d a h i g h air f l o w rate ( p r o d u c i n g residual aerator dissolved o x y g e n c o n c e n t r a t i o n of 0.7 t o 2.8 m g / / ) d i d n o t e n h a n c e m e t h a n e p r o d u c t i o n . H o w e v e r , residue p o s t t r e a t m e n t in t h e 2 - / b a t c h unit o p e r a t e d u n d e r l i m i t e d aeration c o n d i t i o n s (no residual dissolved o x y g e n in aerator) w a s superior in t h a t r e c y c l i n g o f t h i s s l u d g e at a 5 6 % recycle ratio led t o a m o d e s t increase in t h e d i g e s t e r m e t h a n e yield (Run 1 M A / 1 3 . Table III). T h e m e t h a n e yield d i d n o t increase f u r t h e r w h e n t h e recycle s l u d g e w a s d e o x y g e n a t e d . t h e recycle ratio w a s increased f r o m 5 6 t o 2 5 7 % (Run 1 M A / 1 8 ) , or t h e feed t o t h e aerator w a s pretreated w i t h h o t caustic. It is i n t e r e s t i n g t o note t h a t t h e volatile solids r e d u c t i o n increased w h e n t h e aerobic p o s t t r e a t m e n t m e t h o d w a s used as s h o w n b y t h e overall V S r e d u c t i o n o f Run 1 M A / 1 8 .


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Two-Phase Digestion of Slurry Systems T w o - p h a s e studies w e r e u n d e r t a k e n t o d e v e l o p a m u l t i s t a g e h i g h - r a t e process superior t o c o n v e n t i o n a l d i g e s t i o n . Results of selected t w o - p h a s e runs are presented in Tables IV a n d V. T w o - p h a s e d i g e s t i o n of t h e u n t r e a t e d feed at an overall d e t e n t i o n t i m e of 7.6 days a n d a l o a d i n g rate of 0.26 lb V S / f t - d a y e x h i b i t e d a t o t a l gas p r o d u c t i o n rate of 1.35 s t d v o l / v o l of c u l t u r e d a y a n d an e f f l u e n t volatile a c i d c o n c e n t r a t i o n of a b o u t 19 m g / / (Run A 3 0 / M 1 6 . Table IV). T h u s , t h e t w o - p h a s e s y s t e m h a d a gas p r o d u c t i o n rate t h a t w a s a b o u t 2.5 t i m e s t h a t of c o n v e n t i o n a l baseline d i g e s t i o n , a n d y e t had a b o u t t h e s a m e e f f l u e n t v o l a t i l e acid c o n c e n t r a t i o n as t h a t o b s e r v e d d u r i n g c o n v e n t i o n a l d i g e s t i o n at a 12-day d e t e n t i o n t i m e . T h e m e t h a n e y i e l d for t h i s r u n . h o w e v e r , w a s a b o u t 2 SCF/lb V S a d d e d , a n d t h e volatile a c i d (VA) yield f r o m t h i s s y s t e m w a s e s t i m a t e d t o be 0.31 (mass of V A as a c e t i c d i v i d e d by mass of V S added), w h i c h i n d i c a t e d t h a t h y d r o l y s i s a n d a c i d i f i c a t i o n of t h e feed o r g a n i c s w e r e inefficient. T o i m p r o v e t h i s c o n d i t i o n . Run A 3 C / M 3 C w a s c o n d u c t e d w i t h c a u s t i c - t r e a t e d feed. Gas p r o d u c t i o n rate, m e t h a n e y i e l d , a n d a c i d yield f r o m t h i s run w e r e 1.5 s t d v o l / v o l of c u l t u r e - d a y . 3 SCF/lb VS a d d e d , a n d 0.49, respectively, all of w h i c h w e r e s u b s t a n t i a l l y h i g h e r t h a n t h o s e o b s e r v e d w i t h t h e u n t r e a t e d f e e d . T h e v o l a t i l e acid yield c o e f f i c i e n t of 0.49 for t h e solid b i o m a s s - w a s t e feed w a s l o w e r t h a n t h e acid yield of 0.73 r e p o r t e d by Ghosh a n d Pohland (33) for t w o - p h a s e d i g e s t i o n of t h e s i m p l e soluble sugar, g l u c o s e , s u g g e s t i n g t h a t it m a y still be possible t o i m p r o v e t h e V S - t o - a c i d c o n v e r s i o n e f f i c i e n c y b e y o n d t h a t realized b y c a u s t i c t r e a t m e n t . Further acid y i e l d or b i o d e g r a d a b i l i t y increase is e x p e c t e d t o be d i f f i c u l t t o a c h i e v e a n d m a y require m o r e severe feed p r e t r e a t m e n t . 3

Run A 7 C / M 7 C in Table IV had t h e s a m e o p e r a t i n g c o n d i t i o n s as t h e t w o o t h e r runs d i s c u s s e d above, b u t received o n l y external n i t r o g e n instead of e x t e r n a l n i t r o g e n , p h o s p h o r u s a n d m a g n e s i u m . I n s p e c t i o n of t h e d a t a in Table IV s h o w s t h a t e l i m i n a t i o n of e x t e r n a l p h o s p h o r u s a n d m a g n e s i u m f r o m t h e feed d i d n o t affect t w o - p h a s e process p e r f o r m a n c e . This o b s e r v a t i o n correlated w i t h t h e results of t h e c o n v e n t i o n a l d i g e s t i o n runs w h i c h s h o w e d t h a t t h e b i o m a s s - w a s t e b l e n d used in t h i s w o r k w a s not n u t r i t i o n a l l y deficient. A l l t w o - p h a s e process feeds, h o w e v e r , w e r e f o r t i f i e d w i t h external n i t r o g e n t o g u a r d against a n y d e f i c i e n c y of t h i s e l e m e n t d u e t o loss t h a t c o u l d o c c u r d u r i n g c a u s t i c t r e a t m e n t of t h e f e e d . A d d i t i o n of e x t e r n a l n i t r o g e n w o u l d not be r e q u i r e d in a c o m m e r c i a l process utilizing a p r o p e r l y d e s i g n e d c o n t i n u o u s p r e t r e a t m e n t reactor. To test kinetic p o t e n t i a l of t h e t w o - p h a s e s y s t e m , a d d i t i o n a l runs w e r e c o n d u c t e d at decreased d e t e n t i o n t i m e s of 5 a n d 4 days, a n d increased loading rates (Table V). A t e m p e r a t u r e of 5 5 ° C w a s selected for t h e a c i d -


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Table IV. EFFECT OF FEED PRETREATMENT ON TWO-PHASE MESOPHILIC (35°C) DIGESTION OF HYACINTH-QRA88-MSW-8LUDGE BLEND Untreated Feed Run AM/MI β Operating Condition*' Acid-Phase Culture Volume, t Methane-Phase Culture Volume. / Acid-Phase Loading. lbVS/ft -day Methane-Phase Loading. lbVS/ft -day Acid-Phase Detention Time, days Methane Phase Detention Time. days Overall Loading, lb VS/ft -day Overall Detention Time, days External Nutrient Additions to Acid-Phase Feed Gas Production Rate, std vol/vol culture-day Acid Phase Methane Phase Total Methane Content, mol % Acid Phase Methane Phase Total Gas Yield. SCF/lb VS added Acid Phase Methane Phase Total Total Methane Yield. SCF/lb VS added Effluant Quality PH Acid Phase Methane Phase Volatile Acids, mg/y as acetic Acid Phase Methane Phase Efficiency VS Reduction. % Energy Recovery in Collected Methane. % 3

3

3

16 45

Caustic Traatad Feed Run A3C/M3C Run A7C/M7C 16 45

16 45

1.0

10

10

0.32 2.0

0.32 2.0

0.32 2.0

5.6 0.26 76

5.6 0.26 76

5.6 0.26 76

N.P.Mg

N.P.Mg

Ν

1.346 0.924 1.035

1484

— 1.443

49 2 59.0 497

528

559

1.34 2.91 3.98

563

5.50

1 98

2.87

3.07

5.86 6.44

6 72 7.03

6 82 7.15

1047 19

2440 287

I860 148

24 2

34 2

33 4

22 3

33 4

34.5

" No alkali was used for digester pH control. The acid-phase feeds for Runs A3C/M3C and A7C/M7C were pretreated for 24 hr with 2.561 of 1 wt % NaOH solution in a total volume of 4 8 f under ambient conditions. The product gases from each phase were collected together for these two runs.


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Table V. THERMOPHILIC IBB'C) ACID-PHASE AND MESOPHILIC (3B"C) METHANE-PHASE DIGESTION OF PRETREATED HYACINTH-GRAS8-M8W-8LUDQE BLEND Run A3B/M21 Operating Conditions' Acid-Phase .Culture Volume, f Methane-Phase Culture Volume. I Acid-Phase Loading, lb VS/ft -day Methane-Phase Loading, lb VS/ft -day Acid-Phase Detention Time, days Methane-Phase Detention Time, days Overall Loading, lb VS/ft -day Overall Detention Time, days External Nutrient Additions to Acid-Phase Feed Gas Production Rate, std vol/vol culture-day Acid Phase Methane Phase Total Methane Content, mol % Acid Phase Methane Phase Total Gas Yield. SCF/lb VS added Acid Phase Methane Phase Total Total Methane Yield. SCF/lb VS added Effluent Quality PH Acid Phase Methane Phase Volatile Acids, mg/f as acetic Acid Phase Methane Phase Efficiency VS Reduction. % Energy Recovery m Collected Methane. % 3

3

Run A37/M23

5 20 2.0 044 1.0 4.0 0.40 50

5 20 2.5 0.56 0.80 32 0.50 4.0

Ν

Ν

3.189 1 553 1.880

3.472 1.740 2.086

60.3 59.6 598

58.5 59.1 57.9

1.53 335 4.70 2.81

1.40 3.11 425 2.46

699 7.05

7.05 7.05

1734 347

2255 591

28.6 31.6

25.8 27.7

No alkali was used for digester pH control. The acid-phase digester feed was pretreated for 24 hr with NaOH solution at ambient conditions. The caustic concentration in the slurry was 142 meg/f


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phase digester because of t h e earlier o b s e r v a t i o n t h a t this t h e r m o p h i l i c t e m p e r a t u r e i m p r o v e d g a s i f i c a t i o n rates at higher loadings a n d shorter d e t e n t i o n t i m e s w i t h o u t s i g n i f i c a n t l y a f f e c t i n g m e t h a n e yield. A s s h o w n in Table V, an overall s y s t e m gas p r o d u c t i o n rate of 1.9 std v o l / v o l c u l t u r e - d a y , a m e t h a n e yield o f 2.8 SCF/lb V S a d d e d , a n d a n e f f l u e n t volatile acid c o n c e n t r a t i o n o f a b o u t 3 5 0 m g / / w e r e observed at a s y s t e m loading a n d d e t e n t i o n t i m e o f 0.4 lb V S / f t - d a y a n d 5 days, respectively. A s e x p e c t e d , gas p r o d u c t i o n rate a n d e f f l u e n t volatile acids c o n c e n t r a t i o n increased t o a b o u t 2.1 s t d v o l / v o l c u l t u r e - d a y a n d 6 0 0 m g / / , a n d m e t h a n e yield decreased t o a b o u t 2.5 SCF/lb V S a d d e d w h e n t h e s y s t e m d e t e n t i o n t i m e w a s decreased t o 4 days a n d t h e s y s t e m l o a d i n g w a s increased t o 0.5 lb V S / f t - d a y . Volatile acid yields at t h e 5- a n d 4 - d a y d e t e n t i o n t i m e s w e r e , h o w e v e r , a b o u t t h e same. 0.46 a n d 0.50, respectively. C o m p a r i s o n of acid p r o d u c t i o n yield c o e f f i c i e n t s f o r t h e runs in Tables IV a n d V i n d i c a t e d t h a t volatile solids c o n v e r s i o n t o volatile acids is s i g n i f i c a n t l y increased b y alkaline feed p r e t r e a t m e n t . F u r t h e r m o r e , it w a s o b s e r v e d t h a t , w h i l e t h e V S - t o - a c i d c o n v e r s i o n e f f i c i e n c y of t h e p r e t r e a t e d feed r e m a i n e d t h e same as t h e s y s t e m d e t e n t i o n t i m e w a s decreased f r o m 7.6 t o 4 days, t h e gas p r o d u c t i o n rate increased b y 5 0 % c o m p a r e d t o a 1 7 % decrease in m e t h a n e yield. These o b s e r v a t i o n s indicate t h a t it is desirable t o o p e r a t e t h e t w o - p h a s e s y s t e m at a d e t e n t i o n t i m e o f 4 days or less, a n d t o c o u p l e this s y s t e m t o a cell mass r e c y c l i n g d e v i c e (e.g.. anaerobic settler) t o p r e v e n t m e t h a n e yield r e d u c t i o n s a n d volatile acids a c c u m u l a t i o n associated w i t h short d e t e n t i o n t i m e s . It s h o u l d be n o t e d , h o w e v e r , t h a t s e t t l i n g of relatively c o n c e n t r a t e d d i g e s t e d b i o m a s s - w a s t e slurry a n d anaerobic settler o p e r a t i o n w e r e p r o b l e m a t i c a n d appeared i m p r a c t i c a l in light o f o u r e x p e r i e n c e w i t h c u s t o m - d e s i g n e d laboratory settlers. A n alternate a p p r o a c h , w h i c h has t h e same effect o f m a i n t a i n i n g higher cell d e n s i t y a n d increasing solids r e t e n t i o n t i m e (SRT) as in a settler, is a p a c k e d bed anaerobic filter. W i t h this reactor, it is be possible t o c o n d u c t d i g e s t i o n at short h y d r a u l i c r e t e n t i o n t i m e (HRT) a n d still o b t a i n h i g h m e t h a n e yield a n d l o w e f f l u e n t volatile acid c o n c e n t r a t i o n because o f t h e h i g h SRT. 3

3

Packed-Bed Anaerobic Digester T h e p a c k e d - b e d anaerobic digester, c o m m o n l y referred t o as an anaerobic " f i l t e r " , w a s o p e r a t e d w i t h filtrates f r o m t h e v a c u u m f i l t r a t i o n of t h e a c i d digester effluent. Filtrate w a s used because u n f i l t e r e d a c i d - d i g e s t e r effluents t e n d e d t o c l o g t h e p a c k e d bed. T h e feed t o t h e p a c k e d - b e d digester h a d volatile acids c o n c e n t r a t i o n s b e t w e e n 1 5 0 0 a n d 2 0 0 0 m g / / as acetic a n d a solids c o n t e n t o f a b o u t 0.5 w t %. T h e d i g e s t e r w a s o p e r a t e d at a n HRT of a b o u t 2.3 days a n d a l o a d i n g rate o f a b o u t 0.15 lb V S / f t - d a y based o n t h e gross filter v o l u m e (1.5 days a n d 0.24 lb V S / f t -day w h e n based o n t h e v o i d 3

3


272


or c u l t u r e v o l u m e ) . Effluent r e c i r c u l a t i o n a n d c u l t u r e t e m p e r a t u r e had s i g n i f i c a n t effects on gas p r o d u c t i o n (Table VI). Recirculation of t h e e f f l u e n t f r o m t h e t o p t o t h e b o t t o m of t h e bed increased gas p r o d u c t i o n rate by 7 6 % , w h i l e also increasing t h e m e t h a n e yield by 4 5 % . M e t h a n e c o n t e n t w a s a b o v e 7 0 m o l % w i t h or w i t h o u t r e c i r c u l a t i o n . Increase in m e a n d i g e s t e r t e m p e r a t u r e f r o m 3 3 ° t o 3 8 ° C decreased gas p r o d u c t i o n rate by a b o u t 2 0 % a n d m e t h a n e yield by a b o u t 25%. D e p e n d i n g on t h e o p e r a t i n g c o n d i t i o n , volatile acids c o n c e n t r a t i o n s in t h e filter e f f l u e n t varied b e t w e e n 9 0 a n d 1 9 0 m g / / (Table VI), w h i c h w e r e still l o w e r t h a n t h o s e in t h e slurry m e t h a n e d i g e s t e r o p e r a t e d at a h i g h e r HRT of 5.6 days (Table V). It s h o u l d be p o i n t e d o u t t h a t t h e p e r f o r m a n c e of t h e p a c k e d - b e d m e t h a n e d i g e s t e r is m o r e d e p e n d e n t perhaps o n t h e i n f l u e n t v o l a t i l e acids c o n c e n t r a t i o n t h a n t h e overall VS l o a d i n g rate. T h u s , filter p e r f o r m a n c e w i t h a c i d - d i g e s t e r filtrate m i g h t be e x p e c t e d t o be better t h a n t h a t o b t a i n e d w i t h m e t h a n e - d i g e s t e r e f f l u e n t s h a v i n g c o m p a r a b l e volatile acids c o n t e n t . Hypothetical Multi-Stage Digestion Process T h e u l t i m a t e o b j e c t i v e of t h i s research w a s t o synthesize a h y p o t h e t i c a l b i o m a s s - w a s t e pilot process d e s i g n based o n t h e results of o u r i n v e s t i g a t i o n of p r o m i s i n g a d v a n c e d d i g e s t i o n m o d e s . T h e w o r k p r e s e n t e d here p r o v i d e d i n f o r m a t i o n o n t h e effects of alkaline p r e t r e a t m e n t alkali r e c y c l i n g , d i g e s t i o n t e m p e r a t u r e , a n d d i g e s t e r gas r e c y c l i n g ; h e a t s o n i c a t i o n , a n d aerobic p o s t t r e a t m e n t a n d r e c y c l i n g of d i g e s t e d s l u d g e u n d e r different a e r a t i o n , s l u d g e recycle, a n d aerator d e t e n t i o n t i m e c o n d i t i o n s ; slurry a n d p a c k e d - b e d d i g e s t i o n ; a n d t w o - p h a s e d i g e s t i o n . T h e results of t h i s w o r k are s u g g e s t i v e of an a d v a n c e d b i o m a s s - w a s t e d i g e s t i o n s y s t e m as d e p i c t e d in Figure VI. A d d i t i o n a l w o r k t o refine t h e pre- a n d p o s t t r e a t m e n t t e c h n i q u e s a n d t w o phase process o p t i m i z a t i o n is better c o n d u c t e d in a pilot s y s t e m similar t o t h a t of Figure V I . T h e h y p o t h e t i c a l s y s t e m is necessarily a m u l t i - s t a g e s y s t e m t o a c c o m o d a t e p r e t r e a t m e n t m u l t i - s t a g e phasic d i g e s t i o n , a n d d i g e s t e d residue p o s t t r e a t m e n t . T h e h y d r a u l i c residence t i m e in t h e t o t a l s y s t e m is a b o u t 6 days a l l o w i n g for 12 hr of d i l u t e c a u s t i c p r e t r e a t m e n t 1 2 - 2 4 hr of t h e r m o p h i l i c acid d i g e s t i o n . 2-5 days of slurry-phase m e s o p h i l i c m e t h a n e d i g e s t i o n . 2 days of m e s o p h i l i c p a c k e d - b e d m e t h a n e d i g e s t i o n , a n d 12 hr of l i m i t e d - a e r a t i o n b i o l o g i c a l t r e a t m e n t of t h e d i g e s t e d s l u d g e . Based o n t h e d a t a c o m p i l e d in t h i s w o r k , m e t h a n e yields u p t o 5.5 SCF/lb V S a d d e d are e x p e c t e d for t h i s t y p e of c o n f i g u r a t i o n .


Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 27.0 34.3 45.9

31.7

6.86 68

6.86 131

6.73 186 34.3

0.698 68.7 3.05

0.872 72.3 4.08

0.495 73.5 2.82

23.3

38 0.16 2.34 510

Run 8MA/6

33 0.15 2.33 510

Run 8 M A / 5

35 0.13 2.30 0

Run 8MA/4

FED MESOPHILIC

The packed-bed digester had a gross v o l u m e of 18.5 f The bed had a void ratio of 0.63. Filtrate from a slurry-phase acid digester effluent having a volatile acid content of 1550-2000 m g / J as acetic acid and a solids content of about 0.5 w t % was used as feed. No pH control w a s used.

Efficiency VS Reduction, % Energy Recovery in Collected Methane, %

3

Operating Conditions* Temperature. °C Loading, lb V S / f t - d a y Detention Time, days Effluent Recycle Ratio, % Qas Production Rate, std v o l / v o l culture-day Methane Content, mol % Methane Yield. SCF/lb VS added Effluent Quality PH Volatile Acids, m g / j f as acetic

Table VI. S T E A D Y - S T A T E P E R F O R M A N C E OF C O N T I N U O U S L Y P A C K E D - B E D M E T H A N E DIGESTER

274



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SUMMARY A series o f e x p l o r a t o r y anaerobic d i g e s t i o n e x p e r i m e n t s w a s p e r f o r m e d w i t h a m i x e d b i o m a s s - w a s t e feed t o search f o r d i g e s t i o n c o n f i g u r a t i o n s t h a t p r o v i d e i m p r o v e d p e r f o r m a n c e over t h a t o f c o n v e n t i o n a l h i g h - r a t e d i g e s t i o n . T h e t e c h n i q u e s s t u d i e d w e r e p r e t r e a t m e n t o f t h e feed w i t h caustic soda, p r o d u c t gas r e c y c l i n g t o t h e digester, r e c y c l i n g of aerobically t r e a t e d digester e f f l u e n t t o t h e digester, t w o - p h a s e d i g e s t i o n w i t h c o m p l e t e m i x a c i d - a n d m e t h a n e - p h a s e reactors, a n d p a c k e d - b e d . m e t h a n e - p h a s e d i g e s t i o n o f t h e e f f l u e n t f r o m an a c i d - p h a s e reactor. A m b i e n t - t e m p e r a t u r e p r e t r e a t m e n t of t h e feed b l e n d w i t h d i l u t e caustic a n d r e c y c l i n g of t h e p r o d u c t gas each a f f o r d e d higher m e t h a n e yields a n d volatile solids r e d u c t i o n efficiencies t h a n h i g h - r a t e d i g e s t i o n alone. It w a s f o u n d t h a t spent c a u s t i c c o u l d be r e c y c l e d for fresh feed p r e t r e a t m e n t a n d t h a t neutralization w a s n o t necessary before f e e d i n g t o t h e digester. T w o - p h a s e d i g e s t i o n in t h e c o m p l e t e - m i x reactors gave m e t h a n e yields a n d r e d u c t i o n efficiencies a b o u t t h e same as t h o s e of h i g h - r a t e d i g e s t i o n b u t at m u c h h i g h e r loadings a n d r e d u c e d d e t e n t i o n t i m e s t h e r e b y o f f e r i n g s i g n i f i c a n t r e d u c t i o n s in e q u i p m e n t size f o r t h e same t h r o u g h p u t s . The use o f a p a c k e d - b e d a n a e r o b i c filter as a m e t h a n e - p h a s e reactor also s h o w e d c o n s i d e r a b l e p r o m i s e f o r o p e r a t i o n at r e d u c e d d e t e n t i o n t i m e s w h e n t h e filter e f f f u e n t w a s r e c y c l e d t o t h e filter inlet. A n a l y s i s of t h e data f r o m t h e e x p e r i m e n t s c o n d u c t e d t o s t u d y each a d v a n c e d d i g e s t i o n t e c h n i q u e indicates t h a t a n i n t e g r a t e d series o f unit processes c o n s i s t i n g of dilute caustic pretreatment, thermophilic acid-phase digestion, mesophilic complete-mix and packed-bed methane-phase digestion, and limitedaeration aerobic t r e a t m e n t of t h e m e t h a n e - p h a s e effluents c o u p l e d w i t h r e c y c l i n g s h o u l d e x h i b i t d i g e s t i o n efficiencies a n d m e t h a n e yields near t h e u p p e r practical limits. ACKNOWLEDGEMENT This research w a s s u p p o r t e d b y U n i t e d Gas Pipe Line C o m p a n y (UGPL). H o u s t o n , Texas. T h e project w a s d o n e u n d e r t h e m a n a g e m e n t o f UGPL a n d is c u r r e n t l y m a n a g e d b y t h e Gas Research Institute. The g u i d a n c e a n d help of Mr. Robert C h r i s t o p h e r a n d Dr. V i c t o r E d w a r d s , b o t h o f UGPL, w e r e invaluable. The assistance of Dr. B. C. W o l v e r t o n of N A S A , M r . D a w s o n M. J o h n s of LSU. Mr. Robert Power o f W a s t e M a n a g e m e n t , Inc., a n d t h e staff of t h e M e t r o p o l i t a n Sanitary District o f Greater Chicago in p r o v i d i n g t h e feed samples is a p p r e c i a t e d . T h e a u t h o r s also a c k n o w l e d g e t h e efforts of Janet Vorres, M i c h a e l Henry, A l v i n Iverson, M o n a S i n g h , Frank Sedzielarz, Phek H w e e Y e n , a n d R a m a n u r t i Ravichandran in c o l l e c t i n g t h e e x p e r i m e n t a l data.


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Klass, D.L. "Abstracts", Biomass for Energy, sponsored by the U.K. Sec. Int. Solar Energy Soc., London, England, July 3, 1979.

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Ghosh, S.; Conrad, J.R.; Klass, D.L. Research Symposium of the 47th Annual Conference, Water Pollution Control Federation, Denver, Colo., Oct. 6-11, 1974. Ghosh, S.; Klass, D.L. U.S. Patent 4 022 665, 1977.

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Klass, D.L. "Proceedings", Bio-Energy World Congress and Exposition, Atlanta, Ga., April 21-24, 1980; Bio-Energy Council: Washington, D.C., 1980.

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Ghosh, S.; Henry, M.P.; Klass, D.L. Second Symposium on Biotechnology in Energy Production and Conservation, Gatlinburg, Tenn., Oct. 3-5, 1979.


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Klass, D.L.; Ghosh, S.; Conrad, J.R. "Symposium Papers", Clean Fuels From Biomass, Sewage, Urban Refuse and Agricultural Wastes, Symposium sponsored by the Institute of Gas Technology, Orlando, Fla., Jan. 27-30, 1976; Institute of Gas Technology: Chicago, 1976.

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McCarty, P.L. et al."Proceedings", Microbial Energy Conversion, H.G. Schlegel; J. Barnea, Eds.; Erich Goltze KG: Gottingen, FRG, 1976.

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Ghose, T.K.; Bisaria, U.S. Biotechnol. Bioeng. 1979, 21, 131-46.

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RECEIVED JUNE 24, 1980.


Advanced Digestion Process Development for Methane Production

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