Energy from Fresh and Brackish Water Aquatic ... - ACS Publications

5 Energy from Fresh and Brackish Water Aquatic Plants JOHN R. BENEMANN

Downloaded by UNIV LAVAL on May 10, 2016 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch005

Ecoenergetics, Incorporated, 5691 Van Fleet Avenue, Richmond, CA 94804

The large-scale cultivation of aquatic plants and their conversion to fuels has often been suggested in recent years as a potential energy source. Large-scale systems for cultivation of microalgae (1,2), cattails (3,4), and water hyacinths (5,6) have been proposed without, however, sufficient supporting analysis. Historically, the concept of cultivating aquatic biomass for energy dates back twenty-five years when microalgae were suggested as a renewable source of methane (7). This concept was demonstrated experimentally a few years later (8) and subjected to a general analysis which, based on very favorable assumptions, concluded that the concept could be a low-cost future energy source (9). Recently, a more detailed analysis, also based on very favorable assumptions, again concluded that microalgae could be economically cultivated in large-scale systems and converted to fuels (10). A related study (11) using a similar design concept and analysis, concluded that emergent aquatic plants (e.g.. water hyacinths) would be favored over microalgae because they would not be limited by the availability of an enriched carbon dioxide source. All of these analyses and proposals were based on relatively superficial considerations of the requirements for cultivation, harvesting, and conversion of these aquatic plants. This review attempts to advance the concepts of aquatic biomass energy farming based on a more detailed review of the biological data base and the technical limitations and potentials for cultivating aquatic plants. This review is based, in part, on recent reports and publications by the author and colleagues (12,13).

0097-6156/81/0144-0099$05.75/0 © 1981 American Chemical Society

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

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A r e v i e w of t h e a q u a t i c plant literature (13) reveals t h a t s u b m e r g e d plants, brackish w a t e r m a r s h plants (Spartina). small f l o a t i n g plants ( d u c k w e e d ) , a n d b l u e - g r e e n algae are not as p r o d u c t i v e as e m e r g e n t f r e s h w a t e r m a r s h plants (cattails, bull rushes), w a t e r h y a c i n t h s , a n d p l a n k t o n i c g r e e n algae. T h u s , t h i s paper w i l l c o n s i d e r o n l y t h e latter plant t y p e s . Particular a p p l i c a t i o n s of these p l a n t s in c h e m i c a l p r o d u c t i o n a n d utilization of m a r g i n a l lands a n d w a t e r


resources are c o n s i d e r e d . Specific c o n c e p t u a l s y s t e m s are p r e s e n t e d for each t y p e of a q u a t i c plant. T h e p o t e n t i a l of h a r v e s t i n g n a t u r a l , u n m a n a g e d s t a n d s of m a r s h plants or a q u a t i c w e e d s (e.g.. w a t e r hyacinths) a n d c o n v e r t i n g t h e biomass t o fuels is c o n s i d e r e d small by this a u t h o r (12!). H o w e v e r , m a n a g e m e n t a n d h a r v e s t i n g of natural s t a n d s of a q u a t i c p l a n t s is t a k i n g place for a q u a t i c w e e d c o n t r o l (e.g.. w a t e r hyacinths) a n d for w i l d l i f e m a n a g e m e n t (marsh plants). T h u s , t h i s o p t i o n s h o u l d be c o n s i d e r e d t o a greater e x t e n t in t h e f u t u r e . It is not possible, at present, t o p r o v i d e either a d e t a i l e d resource base assessment (e.g.. p o t e n t i a l l y available w a t e r , l a n d , or n u t r i e n t resources), or a d e t a i l e d cost analysis of a q u a t i c p l a n t p r o d u c t i o n . T h u s , t h i s r e v i e w presents general c o n c e p t s of a q u a t i c biomass f a r m i n g e x e m p l i f i e d by t h r e e s y s t e m s — m i c r o a l g a e f a r m i n g for lipid fuel a n d c h e m i c a l s p r o d u c t i o n , cattail c u l t i v a t i o n for c o n v e r s i o n t o alcohol fuels, a n d g r o w i n g w a t e r h y a c i n t h s for m e t h a n e gas g e n e r a t i o n . W a s t e w a t e r a q u a c u l t u r e a p p l i c a t i o n s are not c o v e r e d in this r e v i e w nor are t h e a c t u a l c o n v e r s i o n processes by w h i c h a q u a t i c biomass w o u l d be c o n v e r t e d t o fuels. M I C R O A L G A E F A R M I N G FOR L I P I D S D u r i n g a n d after W o r l d W a r II. b o t h in G e r m a n y a n d t h e U.S.. t h e h i g h lipid c o n t e n t of m i c r o a l g a e (up t o 8 6 % for Chlorella) a t t r a c t e d a t t e n t i o n as a possible source of fats a n d oils (14-16). This led t o a c o n c e r t e d effort in t h e U.S. in t h e late 1940's a n d early 1950's t o d e v e l o p m i c r o a l g a e p r o d u c t i o n t e c h n o l o g y as a p o t e n t i a l source of f o o d . This w o r k , w h i c h c u l m i n a t e d in a pilot-scale p r o j e c t by t h e A r t h u r D. Little Co.. s u p p o r t e d by t h e Carnegie I n s t i t u t e , is r e p o r t e d in t h e book e d i t e d by B u r l e w e n t i t l e d Algae Cultivation from Laboratory to Pilot Plant (V7). A l t h o u g h n o t d i r e c t l y a c k n o w l e d g e d , t h e results of t h i s early w o r k w e r e not e n c o u r a g i n g ; t h e large plastic t u b e used for t h e pilot-scale algal c u l t u r e w a s s u s c e p t i b l e t o leaks a n d o v e r h e a t i n g . Harvesting p r o v e d q u i t e d i f f i c u l t , r e q u i r i n g expensive c e n t r i f u g e s . Recycling of t h e m e d i a appeared t o give s o m e p r o b l e m s . In general, costs far o u t w e i g h e d benefits in p r o t e i n or lipids p r o d u c t i o n .


5.

BENEMANN

Energy from Aquatic Plants

101

S u b s e q u e n t w o r k w a s c o n c e n t r a t e d m a i n l y in J a p a n , leading t o t h e d e v e l o p m e n t of very " h i g h t e c h n o l o g y " algal c u l t i v a t i o n systems, s o m e of w h i c h even g r e w t h e algae h e t e r o t r o p h i c a l l y (on acetic acid) u n d e r sterile f e r m e n t a t i o n c o n d i t i o n s (18). P r o d u c t i o n costs o f t h e algal biomass p r o d u c e d by s u c h s y s t e m s are very h i g h , e x c e e d i n g $ 1 0 , 0 0 0 / t o n (dry) d u e t o t h e use o f c e n t r i f u g e s , d r y i n g plants, a n d elaborate p o n d i n g systems. T h e algae are


used as a health f o o d a n d specialty feed (e.g.. f o r t r o p i c a l fish). In t h e 1960's. a n u m b e r of projects w e r e i n i t i a t e d for t h e use of m i c r o a l g a e in a q u a c u l t u r e f o o d c h a i n s (see l â f o r a review), f o r f o o d p r o d u c t i o n (particularly by t h e G e r m a n a n d Czechoslovakia g r o u p s , see (20), a n d f o r w a s t e w a t e r t r e a t m e n t a n d feed p r o d u c t i o n . H o w e v e r , at present, o n l y o n e c o m m e r c i a l p r o d u c t i o n s y s t e m is o p e r a t i n g t o date o u t s i d e o f t h e Far East — t h e Spirulina p r o d u c t i o n plant o f t h e Sosa T e x c o c o C o m p a n y near M e x i c o City (2JJ. T a k i n g a d v a n t a g e of t h e n a t u r a l l y favorable c o n d i t i o n s in s o m e areas o f t h e i r b i c a r b o n a t e e v a p o r a t i o n p o n d s , this c o m p a n y operates a 10 hectare Spirulina p r o d u c t i o n p o n d f o r t h i s f i l a m e n t o u s b l u e - g r e e n alga. H a r v e s t i n g is n o p r o b l e m , as t h e long f i l a m e n t s a l l o w easy r e m o v a l by relatively w i d e - m e s h screens. T h e s p r a y - d r i e d p r o d u c t sells f o r a b o u t $ 5 , 0 0 0 / t o n , m a i n l y t o t h e Japanese market. P r o d u c t i o n costs are u n k n o w n . The o t h e r major practical use o f m i c r o a l g a e w a s in w a s t e w a t e r t r e a t m e n t a p p l i c a t i o n s (22). M i c r o a l g a e are capable of p r o v i d i n g t h e dissolved o x y g e n required in m e e t i n g t h e biological o x y g e n d e m a n d of m u n i c i p a l a n d other w a s t e w a t e r s . S e w a g e o x i d a t i o n p o n d s have been used in t h e U.S. f o r m a n y d e c a d e s ; t h e y are s i m p l e e a r t h e n lagoons, one t o t w o meters deep, a n d u p t o f i f t y acres or m o r e in size. Several lagoons are usually o p e r a t e d in series t o effect w a s t e w a t e r t r e a t m e n t . T h e m i c r o a l g a e c u l t u r e is neither c o n t r o l l e d n o r h a r v e s t e d ; t h u s , n o t r u e c u l t i v a t i o n process is i n v o l v e d . O s w a l d in t h e early 1 9 5 0 ' s applied m o r e c o n t r o l l e d " h i g h r a t e " p o n d s t o w a s t e w a t e r t r e a t m e n t (23). These w e r e essentially s h a l l o w (20-50 c m ) , m e c h a n i c a l l y m i x e d , a n d baffled p o n d s w h i c h a l l o w e d m a i n t e n a n c e of a dense c u l t u r e of m i c r o a l g a e w h i c h c o u l d m o r e e f f i c i e n t l y p r o v i d e t h e o x y g e n required in w a s t e w a t e r t r e a t m e n t . A l t h o u g h these s y s t e m s w e r e s t u d i e d in detail b o t h in t h e U.S. (24) a n d m o r e recently in Israel (25), o n l y f e w p o n d s y s t e m s of this t y p e have been built. This is because t h e h i g h algae c o n c e n t r a t i o n makes h a r v e s t i n g i m p e r a t i v e , and m i c r o a l g a e h a r v e s t i n g w a s expensive. The a u t h o r , in association w i t h W . J . O s w a l d , over t h e past four years has s t u d i e d l o w e r cost algal p r o d u c t i o n a n d h a r v e s t i n g systems f o r a p p l i c a t i o n t o b o t h w a s t e w a t e r t r e a t m e n t a n d energy p r o d u c t i o n (26-28). The research has c o n c e n t r a t e d o n t h e p r o b l e m s o f m i c r o a l g a l harvesting a n d species c o n t r o l in e x p e r i m e n t a l a n d pilot-scale s e w a g e h i g h - r a t e p o n d systems. T h e first



102

BIOMASS AS A NONFOSSIL F U E L

SOURCE

c o n c e p t s t u d i e d w a s t o harvest t h e algae by m i c r o - s c r e e n s — r o t a t i n g b a c k w a s h e d fine m e s h screening devices. T h e critical p a r a m e t e r is t h e screen o p e n i n g — a 2 6 m i c r o n screen size w a s c h o s e n as t h e m o s t cost effective. This required m a i n t a i n i n g in t h e p o n d s colonial t y p e s of algae as average single cell algae sizes range b e t w e e n 2 a n d 2 0 m i c r o n s . H o w e v e r it w a s e x p e r i m e n t a l l y d e t e r m i n e d t h a t relatively l o n g d e t e n t i o n t i m e s w e r e required t o a l l o w m a i n t e n a n c e of c o l o n i a l g r e e n algal c u l t u r e s w h i c h resulted in a s i g n i f i c a n t loss of b i o m a s s p r o d u c t i v i t y (about haJf of t h e total) (28). T h e e m p h a s i s s h i f t e d t o an even l o w e r c o s t m e t h o d of algal h a r v e s t i n g — s p o n t a n e o u s f l o c c u l a t i o n of t h e algae, f o l l o w e d by s e d i m e n t a t i o n of t h e m i c r o a l g a e c u l t u r e . T h e results of over t w o years of s t u d y , w i t h t h e last year b e i n g d e v o t e d t o pilot-scale (0.1 hectare) d e m o n s t r a t i o n of t h i s process, have s h o w n t h a t it is possible t o c u l t i v a t e y e a r - r o u n d a m i c r o a l g a l c u l t u r e t h a t exhibits both high productivity and good harvestability (flocculations e d i m e n t a t i o n ) (29). Data f r o m over o n e year of o p e r a t i o n is s h o w n in Table I. A l t h o u g h t h i s process remains t o be d e m o n s t r a t e d in p r a c t i c e , it appears t h a t l o w - c o s t algal h a r v e s t i n g is feasible. A n e c o n o m i c analysis of m i c r o a l g a l b i o m a s s p r o d u c t i o n m u s t be based o n a n u m b e r of a s s u m p t i o n s , o n l y one of w h i c h is t h e availability (and practicality) of a l o w - c o s t h a r v e s t i n g process. O t h e r a s s u m p t i o n s m u s t be m a d e a b o u t t h e specific d e s i g n a n d t h e c a p i t a l cost of t h e p o n d s y s t e m (e.g.. lined vs. u n l i n e d ) . t h e a v a i l a b i l i t y a n d q u a l i t y of w a t e r , t h e feasibility of r e c y c l i n g w a t e r , t h e n u t r i e n t utilization e f f i c i e n c y , t h e source a n d transfer e f f i c i e n c y of c a r b o n (carbon d i o x i d e ) , a n d t h e p r o c e s s i n g costs after t h e initial h a r v e s t i n g (defined as r o u g h l y t h e first 1 0 0 - f o l d c o n c e n t r a t i o n ) . M o r e i m p o r t a n t l y , a s s u m p t i o n s m u s t be m a d e a b o u t t h e a b i l i t y t o g r o w c e r t a i n algal species or t y p e s , p r e v e n t i n g c u l t u r e instabilities (e.g., z o o p l a n k t o n prédation), m a n a g e m e n t r e q u i r e m e n t s , a n d p r o d u c t i v i t y . T h e reason so m a n y d i f f e r e n t a s s u m p t i o n s are r e q u i r e d is t h e lack of d e t a i l e d a n d / o r available i n f o r m a t i o n , i n c l u d i n g p r o d u c t i v i t y d a t a . T h e Japanese a n d Far East s y s t e m s m e n t i o n e d a b o v e are not a g o o d g u i d e because no f i r s t - h a n d t e c h n i c a l d a t a are available; these are c o m m e r c i a l p r o p r i e t a r y projects. Similarly, t h e M e x i c a n Spirulina p r o d u c t i o n project is not d e s i g n e d for o p t i m a l p r o d u c t i o n , as it o n l y i m p r o v e s o n t h e natural s i t u a t i o n . H o w e v e r , an overall p r o d u c t i o n f i g u r e of 1 0 - 1 2 g / m - d a y averaged over 10 m o n t h s has been r e p o r t e d (21). T h e best p r o d u c t i v i t y d a t a for large-scale systems (above 100 m ) are available f r o m h i g h - r a t e s e w a g e o x i d a t i o n p o n d s as s u m m a r i z e d in Table II. Even in this case, serious short c o m i n g s of t h e data are a p p a r e n t w h e n r e v i e w i n g t h e original literature; s e w a g e solids, for e x a m p l e , are i n c l u d e d in these p r o d u c t i o n figures. H o w e v e r , based o n available d a t a , a biomass p r o d u c t i o n rate of 4 0 - 5 0 t / h a - y r appears feasible a n d p r a c t i c a l . 2

2



8.5 15.8 20.1 22.6 22.0 21.7 19.9 16.3

25.5 25.5 11.6 4.7 4.9 6.4

2

85 71 83 64 56 74 74 53 74 91 89 94 87 69 8.0 11.3 9.8 6.6 4.7 9.3 16.5 16.2 21.3 20.2 35.5 35.6 35.5 27.8 23.5 22.7 3.1 3.3 4.2 5.2 6.9 12.0 17.7 20.6 20.2 19.1 18.7 13.7

2

92 89 27 70 85 82 81 76 88 91 92 88 94 84

2

East Pond 24-hr Imhoff Cone* % removal

Total Production g/m -da y

Harvestable Production g/m -da y

W e s t Pond 24-hr Imhoff Cone* % removal

• Imhoff cone removals indicate t h e percentage of algal biomass that will spontaneously flocculate and settle

Sept 7 8 Oct Nov Dec Jan 7 9 Feb Mar Apr May Jun Jul Aug Sep Oct

Date

Total Production g/m - d a y

Table I. S U M M A R Y OF 0.1 H E C T A R E HIGH-RATE P O N D OPERATIONS A T RICHMOND, CALIFORNIA 1 9 7 8 - 7 9 . The two ponds were operated at variable detention times, depths, and mixing speeds, accounting for differences in productivity and harvestability (29).


Klass; Biomass as a Nonfossil Fuel Source ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 12 15

30 days summer 6 0 days Sep-Oct

5 days 2.6 days

variable

variable (semi-batch)

6 days 9.3 days

3 days fixed 4.5 days variable

3 days 3 days

Duration of Time

29

34

.32.

32

ai

.22

Ref.

*The productivities were generally not corrected for non-algal sewage solids (except for Shelef et al. 1977) and sometimes calculated indirectly from t h e data presented.

2500 1000

Richmond. Calif.

average of yearround experiments

-15-25

100

Manila. Philippines

1-2 week ave. summer

— 14-18

646

Southern California

30 days Mar 30 days A p r

14.3 17.4

2800

365 days 365 days

30.2 20.2

10 days A u g 2 mos. Nov-Dec

Productivity* Experiment

120 150

2

Scale g/m -day

25.3 12.2

2

70

m

Melbourne, Australia

Haifa, Israel

Richmond, Calif.

Location

Table II. PRODUCTIVITIES OF M I C R O A L G A L C U L T U R E S IN HIGH-RATE S E W A G E P O N D S



5.

BENEMANN

105


D e p e n d i n g o n a s s u m p t i o n s , c a l c u l a t e d p r o d u c t i o n costs of m i c r o a l g a l biomass can a m o u n t t o as little as $ 5 0 / t o n or u p t o several t h o u s a n d dollars per t o n . One analysis, carried o u t b y this a u t h o r , w a s d e s i g n e d t o explore t h e l o w e r cost limits o f algal biomass p r o d u c t i o n at very large scales a n d arrived at a p r o d u c t i o n cost of $ 5 0 / d r y t o n . This cost analysis w a s based o n an u n l i n e d p o n d s y s t e m c o n s i s t i n g of very large i n d i v i d u a l p o n d s (100 acres), d i v i d e d into long s e r p e n t i n e c h a n n e l s b y baffles, m i x e d b y p a d d l e - w h e e l s , a n d harvested b y a 4 8 - h r c y c l e b a t c h s e t t l i n g p o n d . H o w e v e r , this analysis w a s n o t realistic nor detailed in a n u m b e r of specifics. It is d o u b t f u l t h a t a n y m i c r o a l g a l s y s t e m c o u l d p r o d u c e a c o n c e n t r a t e d slurry of 2 - 5 % algae at a cost of less t h a n $ 2 0 0 / t o n (dry w e i g h t basis), even if w a t e r a n d n u t r i e n t s w e r e s u p p l i e d free of c h a r g e or e f f i c i e n t l y r e c y c l e d . T h u s , c o n t r a r y t o m a n y assertions, m i c r o a l g a e d o n o t appear t o be a suitable c h o i c e f o r e n e r g y f a r m i n g , as a p r o d u c t i o n cost of at least $ 1 0 / 1 0 Btu f o r " r a w " biomass is foreseen. This does n o t d e t r a c t f r o m t h e p o t e n t i a l of m i c r o a l g a l w a s t e w a t e r t r e a t m e n t - e n e r g y p r o d u c t i o n systems, w h e r e w a t e r a n d n u t r i e n t s are p r o v i d e d free of c h a r g e a n d c r e d i t is t a k e n f o r w a s t e w a t e r t r e a t m e n t . T w o i n d e p e n d e n t analyses o f a m i c r o a l g a l w a s t e w a t e r t r e a t m e n t - e n e r g y p r o d u c t i o n s y s t e m (based o n a s s u m p t i o n s o f c a r b o n or n i t r o g e n as l i m i t i n g nutrients) c o n f i r m e d t h a t m u n i c i p a l w a s t e t r e a t m e n t s y s t e m s c o u l d c o m p e t i t i v e l y p r o d u c e fuel f r o m m i c r o a l g a l b i o m a s s if h a r v e s t i n g c o u l d be carried o u t b y a l o w - c o s t process s u c h as m i c r o s t r a i n i n g (35). However, m u n i c i p a l w a s t e w a t e r s are a l i m i t e d resource base; even c o n s i d e r i n g energy c o n s e r v a t i o n , a m a x i m u m e n e r g y c o n t r i b u t i o n of a b o u t 0 . 1 % of national energy needs can be foreseen f r o m all w a s t e w a t e r a q u a c u l t u r e s y s t e m s , m i c r o a l g a e b e i n g o n l y o n e o f these t e c h n o l o g i e s (12). 6

O t h e r p o t e n t i a l c o n t r i b u t i o n s of m i c r o a l g a e f o r energy p r o d u c t i o n are of interest. One possibility is t h e p r o d u c t i o n o f speciality c h e m i c a l s , w h e r e h i g h u n i t prices c o u l d defray h i g h p r o d u c t i o n costs. S u c h c h e m i c a l s i n c l u d e fats, h y d r o c a r b o n s , p i g m e n t s , proteins, a n d p o l y s a c c h a r i d e s . Glycerol is a g o o d e x a m p l e of s u c h a p r o d u c t . It w a s d i s c o v e r e d a n u m b e r o f years ago t h a t t h e Dead Sea m i c r o a l g a e Dunaliella w i l l p r o d u c e a h i g h f r a c t i o n of its t o t a l d r y w e i g h t as g l y c e r o l , as m u c h as 5 0 % , in response t o h i g h salt c o n c e n t r a t i o n s in its m e d i u m , as a m e t h o d o f m a i n t a i n i n g a n o s m o t i c e q u i l i b r i u m (36.37). T h e p r o d u c t i o n of Dunaliella has been p r o p o s e d (38,39), a n d t h e d e v e l o p m e n t o f a n a p p r o p r i a t e t e c h n o l o g y is u n d e r w a y in Israel (38). The c o n c e p t is t o c o - p r o d u c e p r o t e i n a n d c a r o t e n e p i g m e n t s w i t h t h e glycerol. T e c h n i c a l l i m i t a t i o n s a p p a r e n t l y i n c l u d e h a r v e s t i n g a n d c u l t u r e stability. E c o n o m i c a l l y , t h e process is c o m p e t i n g w i t h g l y c e r o l p r o d u c e d d u r i n g f a t r e n d e r i n g , w h i c h sells f o r a b o u t $ 1 / k g , b u t w h i c h c o u l d be s u b j e c t t o s i g n i f i c a n t d o w n w a r d price shifts. A l s o , t h e p r o d u c t i o n of this algae requires very h i g h - s t r e n g t h brines a n d t h e p r o d u c t has a l i m i t e d market.Thus, a l t h o u g h it is t h e process


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nearest t o c o m m e r c i a l i z a t i o n , g l y c e r o l p r o d u c t i o n f r o m m i c r o a l g a e s h o u l d be c o n s i d e r e d as o n l y o n e of m a n y t y p e s of m i c r o a l g a l c h e m i c a l s y s t e m s . U n d e r fairly o p t i m i s t i c a s s u m p t i o n s a n d c o n d i t i o n s , it m a y be possible t o p r o d u c e h i g h - v a l u e l i q u i d h y d r o c a r b o n fuels f r o m m i c r o a l g a e . either by c o n v e r s i o n of t h e lipids or by d i r e c t h y d r o c a r b o n p r o d u c t i o n (40). In s o m e cases, very h i g h lipid c o n t e n t s have been reported in m i c r o a l g a e u p t o 8 6 % (of d r y w e i g h t ) (mostly C - C o f a t t y acids) for t h e u n i c e l l u l a r algae Chlorella (16) a n d a similar a m o u n t of l o n g - c h a i n ( C 2 6 ) h y d r o c a r b o n s in t h e colonial Botryococous (41). H o w e v e r , t h e s e h i g h c o n c e n t r a t i o n s are o n l y a c h i e v e d at t h e e n d of a l o n g period of l i g h t or n i t r o g e n l i m i t a t i o n , w h i c h result in e x t r e m e l y l o w rates of lipid p r o d u c t i o n . W h e t h e r it is possible t o o p t i m i z e lipid c o n t e n t a n d p r o d u c t i v i t y is u n c e r t a i n . A b o u t 2 0 % lipids are present in s e w a g e - g r o w n m i c r o a l g a e ( A a r o n s o n . personal c o m m u n i c a t i o n ) . H o w e v e r , t h a t appears s o m e w h a t l o w for p r o c e s s i n g purposes. It m a y be possible t o d o u b l e t h i s a m o u n t by s t r a t e g i c s c h e d u l e s of p o n d o p e r a t i o n s a n d n u t r i e n t additions w i t h o u t significantly lowering total productivity. A high product i v i t y of Phaeodactylum tricornutum w i t h a b o u t 4 0 % lipids, u s i n g very s h a l l o w c u l t u r e s , has been r e p o r t e d (42).


1 6

2

If fuel is t o be p r o d u c e d f r o m t h e algae, s o m e t y p e of s u b s i d y is required even w h e n c o m p e t i n g against spot m a r k e t prices for oil. O n e specific e x a m p l e involves t h e use of m i c r o a l g a l p o n d s in t h e e v a p o r a t i v e disposal of brackish a g r i c u l t u r a l d r a i n a g e w a t e r s w h o s e m a n a g e m e n t a n d disposal is a serious p r o b l e m in m a n y areas. T h u s , in t h e Central Valley of California, elaborate d i s c h a r g e s y s t e m s t h r o u g h artificial marshes are b e i n g p r o p o s e d . T h e use for m i c r o a l g a l p r o d u c t i o n of brackish-saline w a t e r s u n s u i t a b l e for c o n v e n t i o n a l a g r i c u l t u r e is o n e of t h e m o s t i m p o r t a n t p o t e n t i a l a p p l i c a t i o n s of m i c r o a l g a l p r o d u c t i o n systems. In c o n c l u s i o n t h e r e are several, n e a r - t e r m a p p l i c a t i o n s of m i c r o a l g a e biomass s y s t e m s in e n e r g y p r o d u c t i o n . M u n i c i p a l w a s t e w a t e r t r e a t m e n t s y s t e m s are t h e m o s t i m m e d i a t e ones f o l l o w e d by s y s t e m s d e s i g n e d t o p r o d u c e speciality c h e m i c a l s s u c h as polyols (e.g., glycerol), lipids, p o l y s a c c h a r i d e s a n d p i g m e n t s . The w a s t e w a t e r t r e a t m e n t credits, a n d lack of alternative uses for a q u a t i c biomass g r o w n o n s e w a g e , or t h e h i g h u n i t prices for s o m e c h e m i c a l s a l l o w t h e relatively h i g h p r o d u c t i o n costs forecast for m i c r o a l g a l biomass. S u c h a p p l i c a t i o n s have, h o w e v e r , an a g g r e g a t e p o t e n t i a l i m p a c t o n U.S. e n e r g y supplies t h a t m u s t be characterized as. at best, rather minor. Larger i m p a c t s i n v o l v i n g l i q u i d fuels p r o d u c t i o n m a y be possible if m i c r o a l g a l b i o m a s s p r o d u c t i o n c o u l d be c o m b i n e d w i t h t h e m a n a g e m e n t or disposal of brackish-saline a g r i c u l t u r a l w a s t e w a t e r s or by s i g n i f i c a n t t e c h n o l o g i c a l b r e a k t h r o u g h s s u c h as a c o n t i n u o u s a n d s p o n t a n e o u s s e t t l i n g or f l o t a t i o n process for algal h a r v e s t i n g .


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A L C O H O L FUELS F R O M M A R S H P L A N T S


M a r s h plants have been relatively little e x p l o i t e d by m a n . T h e i m m e n s e stands of Phragmites (bullrush) c o v e r i n g a b o u t 6 0 % of t h e D a n u b e Delta, over 3 m i l l i o n hectares, are, perhaps, t h e best e x a m p l e of large-scale m a n a g e m e n t a n d h a r v e s t i n g o f a n e m e r g e n t marsh plant s y s t e m (43). T h e plants are harvested o n a sustainable yield basis a n d are used as fiber f o r paper m a n u f a c t u r e , as w e l l as s o m e t r a d i t i o n a l uses (construction) a n d c h e m i c a l s . In t h e U n i t e d States, large areas o f fresh brackish w a t e r marshes are c u t on a m o r e or less regular basis b o t h in t h e N o r t h e r n Lakes area and o n t h e East Coast t o i m p r o v e t h e o p e n w a t e r s u r f a c e - t o - m a r s h plants ratio o p t i m a l f o r m i g r a t o r y birds (about half a n d half). T h e c o n c e p t of using this t y p e of biomass s y s t e m f o r e n e r g y p r o d u c t i o n has been p r o p o s e d , particularly in M i n n e s o t a (3,4). M a r s h l a n d s border t h e areas o f m o s t o f t h e inland and coastal w a t e r s o f t h e w o r l d a n d t h e U n i t e d States. Detailed statistics o n m a r s h l a n d areas w e r e n o t r e v i e w e d b y t h i s a u t h o r ; h o w e v e r , a g o o d e s t i m a t e is t h a t in t h e U.S. a b o u t 2 0 m i l l i o n hectares of marshes exist w i t h an equal a m o u n t already d r a i n e d or filled since t h e e s t a b l i s h m e n t o f t h e U.S. M a j o r areas w i t h marsh lands are in t h e Great Lakes area, s u c h as M i n n e s o t a w i t h 4 m i l l i o n hectares, t h e s o u t h e r n states (Louisiana h a v i n g 3 m i l l i o n hectares), t h e East Coast s u c h as t h e Carolinas a n d t o a lesser e x t e n t , t h e S a c r a m e n t o Delta region o n t h e W e s t Coast. H o w e v e r , e x i s t i n g natural m a r s h lands are n o t likely t o be used f o r e n e r g y p r o d u c t i o n purposes t o a great e x t e n t unless t h e y c a n be d e m o n s t r a t e d t o be c o m p a t i b l e w i t h preservation of e n d a n g e r e d plant a n d a n i m a l species, are c o n d u c i v e t o w i l d l i f e m a n a g e m e n t , a n d e n h a n c e e n v i r o n m e n t a l a n d c o m m u n i t y benefits. S u c h a c c o m m o d a t i o n s m a y be possible. For e x a m p l e , m a n y m a r s h s y s t e m s are essentially m o n o c u l t u r e s o f specific species s u c h as Phragmites communis (bullrush) or Typha augustifolia (cattail). Thus, o n e e c o l o g i c a l o b j e c t i o n o f e n e r g y f a r m i n g is o v e r c o m e . A n o t h e r f a c t o r t h a t m u s t be c o n s i d e r e d is t h a t t h e c u l t i v a t i o n of these a n n u a l plants m a y n o t a l l o w c o m p l e t e harvest because t h a t w o u l d p r e v e n t rapid regeneration w i t h o u t e x p e n s i v e r e p l a n t i n g . This w o u l d allay t h e o b j e c t i o n against c l e a r - c u t t i n g as in tree e n e r g y f a r m i n g . For m a x i m a l w i l d l i f e m a n a g e m e n t , partial c u t t i n g is already u n d e r t a k e n as m e n t i o n e d above. The U. S. E n v i r o n m e n t a l P r o t e c t i o n A g e n c y p o l i c y is t o m i n i m i z e " a l t e r a t i o n s " of q u a l i t y or q u a n t i t y of t h e natural w a t e r s t h a t affect w e t l a n d s (44). These w e t l a n d s are recognized as sensitive e c o l o g i c a l areas a n d . t h u s , a n y neart e r m use o f s i g n i f i c a n t areas m u s t be c o n s i d e r e d unlikely. This a u t h o r envisions t h a t in t h e n e a r - t e r m , m a r s h p l a n t - e n e r g y systems c a n be established o n a l r e a d y - d i s t u r b e d or m a r g i n a l w e t l a n d areas, or w h e r e


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p l e n t i f u l w a t e r resources a l l o w s u c h h i g h l y c o n s u m p t i v e use. In t h e longer t e r m (e.g., t w e n t y years), s u c h systems c o u l d e x p a n d i n t o n a t u r a l , n o n sensitive marsh areas. T h u s , e v e n t u a l l y , a s i g n i f i c a n t f r a c t i o n of t h e large w e t l a n d resources in s o m e states c o u l d b e c o m e available for biomass p r o d u c t i o n a n d be i n t e g r a t e d into t h e h i g h e r uses of w i l d l i f e m a n a g e m e n t , fisheries p r o d u c t i o n , e n v i r o n m e n t a l p r o t e c t i o n , a n d recreation. Even o n e t e n t h of all present m a r s h l a n d s (e.g.. 2 m i l l i o n hectares), a s s u m i n g a s u s t a i n e d yield of 3 0 t / h a - y r . w h i c h is relatively m o d e s t , c o u l d p r o v i d e a s i g n i f i c a n t a m o u n t of fuels, a b o u t o n e q u a d ( 1 0 Btu) of r a w biomass (higher h e a t i n g value basis).


1 5

Before s u c h p r o g n o s i s c a n be m a d e , h o w e v e r , t h e c u l t i v a t i o n a n d h a r v e s t i n g t e c h n o l o g i e s for s u c h plants m u s t be d e v e l o p e d . This requires c o n s i d e r a t i o n of t h e biological c h a r a c t e r i s t i c s of these plants. T h e first aspect t o c o n s i d e r is t h e seasonality of these plants. T h e y g r o w very rapidly in t h e s p r i n g t i m e , d r a w i n g o n t h e c a r b o h y d r a t e stored t h e p r e v i o u s fall in t h e i r root t u b e r s (rhizomes). T h e s h o o t s very rapidly a t t a i n a v e r y h i g h leaf area index, e x c e e d i n g 10 in several reports, w h i c h is h i g h e r t h a n a n y c r o p p l a n t , even sugarcane. T h e v e r t i c a l leaf a r r a n g e m e n t a l l o w s g r a d u a l light a t t e n u a t i o n a n d . t h u s , efficient light utilization. Relatively h i g h t r a n s p i r a t i o n rates a l l o w for m a x i m a l p h o t o s y n t h e s i s rates, similar t o t h o s e of t r o p i c a l grasses, a l t h o u g h t h e C p a t h w a y of c a r b o n d i o x i d e f i x a t i o n is usually absent. M o s t reports o n p r o d u c t i v i t y of these plants o n l y m e a s u r e d t h e areal parts of t h e p l a n t s , w h e r e a s a s i g n i f i c a n t f r a c t i o n of t h e p h o t o s y n t h a t e is t r a n s l o c a t e d t o t h e roots w h i c h m a y c o n t a i n u p w a r d s of 4 0 % of t h e t o t a l biomass. T h u s , t h e d a t a o n a c h i e v a b l e p r o d u c t i v i t y by these plants is a f f e c t e d by t w o critical p r o b l e m s — t h e seasonality of their g r o w t h a n d t h e t r a n s l o c a t i o n t o their e x t e n s i v e root s y s t e m s . 4

T h e root s y s t e m of m a r s h p l a n t s e v o l v e d t o tolerate t h e anaerobic c o n d i t i o n s in t h e b o t t o m layers of w e t l a n d areas. T w o basic a d a p t a t i o n s are f o u n d — internal air passages e x t e n d i n g f r o m t h e leaf bases t o t h e rhizomes. Rhizomes are enlarged roots w h i c h a l l o w for storage of c a r b o h y d r a t e s a n d t h e e x t e n s i o n of lateral roots a n d n e w shoots. A n a e r o b i c roots are c a p a b l e of anaerobic m e t a b o l i s m w i t h e t h a n o l (instead of t h e lactic a c i d f o u n d in a n i m a l tissues) as e n d p r o d u c t (45). It is u n c e r t a i n h o w h i g h a rate of e t h a n o l p r o d u c t i o n c a n be s u s t a i n e d , b u t it is not t o o f a r - f e t c h e d t o p o s t u l a t e t h e possibility of a p p l y i n g g e n e t i c selections t o this s y s t e m t o a level w h i c h w o u l d a l l o w d i r e c t e t h a n o l p r o d u c t i o n f r o m harvested rhizomes. S o m e a d v a n t a g e s of s u c h a s y s t e m are t h e h i g h solids (substrate) c o n c e n t r a t i o n feasible a n d t h e s i m p l i f i c a t i o n of t h e process. In t h i s c o n t e x t t h e areal part of m a r s h plants are l o w e r in l i g n i n c o n t e n t t h a n terrestrial land plants, m a k i n g t h e m m o r e suitable for e n z y m a t i c or c h e m i c a l hydrolysis and s u b s e q u e n t use


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for e t h a n o l i c f e r m e n t a t i o n s . T h u s , o n b o t h a c c o u n t s , these plants c a n be c o n s i d e r e d p r o s p e c t i v e sources o f e t h a n o l . W h e t h e r their fruits, w h i c h also c o n s u m e a large a m o u n t o f p h o t o s y n t h a t e , c o u l d be f e r m e n t e d is n o t k n o w n . Of course, c o m b u s t i o n is a s t r a i g h t - f o r w a r d a n d m o r e efficient use o f t h e p l a n t f o r energy. H o w e v e r , t h e s i g n i f i c a n c e of e t h y l alcohol as a liquid fuel e x t e n d e r makes it t h e preferred c o n v e r s i o n route, even at a m u c h h i g h e r cost or l o w e r efficiency. The data o n p r o d u c t i v i t y o f cattails are s u m m a r i z e d in Table III a n d are l i m i t e d by t h e absence of t o t a l p r o d u c t i v i t y d a t a , b o t h a b o v e a n d b e l o w g r o u n d . S o m e o f t h e best data w e r e c o l l e c t e d in M i n n e s o t a as s u m m a r i z e d in Table IV, w h i c h s h o w s t o t a l , above, a n d b e l o w g r o u n d p r o d u c t i o n . It s h o u l d be n o t e d t h a t natural stands c a n have as h i g h , or higher, t o t a l p r o d u c t i v i t i e s t h a n m a n a g e d (fertilized) plots. Peat soils had s o m e w h a t l o w e r p r o d u c t i v i t i e s . In late s u m m e r , s h o o t d r y w e i g h t reaches a m a x i m u m , a n d roots start a c c u m u l a t i n g p h o t o s y n t h a t e . P r o d u c t i v i t i e s of 4 0 t / h a - y r have been e s t i m a t e d f o r cattails in M i n n e s o t a (51). A c h i e v a b l e p r o d u c t i v i t y w i l l d e p e n d o n d e v e l o p m e n t of a p p r o p r i a t e c u l t i v a t i o n a n d h a r v e s t i n g t e c h n o l o g i e s . T h e p r o d u c t i o n s y s t e m itself w i l l likely be relatively s i m p l e , c o n s i s t i n g o f large ( 1 0 - 1 0 0 hectares) level g r o w t h areas s u r r o u n d e d b y a l o w soil e m b a n k m e n t a n d p r o v i d e d w i t h i n l e t / o u t l e t s t r u c t u r e s . The actual o p e r a t i o n s w o u l d need t o be w o r k e d o u t : H o w m u c h a n d w h e n t o harvest; w h e t h e r o n l y a b o v e or also b e l o w g r o u n d biomass w o u l d be h a r v e s t e d ; h o w m u c h n u t r i e n t a n d h o w t o a p p l y it; h o w t o c o n t r o l possible pests; etc. A l t h o u g h t o t a l biomass p r o d u c t i v i t y in a w e l l - m a n a g e d s y s t e m w o u l d likely exceed 5 0 t / h a - y r , based o n t h e data in Table II. t h e a c t u a l harvestable p r o d u c t i v i t y is likely t o be s i g n i f i c a n t l y less, possibly in t h e range of t h e 3 0 t / h a - y r . T h e cost o f p r o d u c t i o n , h o w e v e r , s h o u l d be l o w if h a r v e s t i n g does n o t present t o o great a p r o b l e m a n d if n u t r i e n t s are available t o sustain h i g h p r o d u c t i v i t i e s . Typha a n d o t h e r similar a q u a t i c m a r s h plants have n u t r i e n t c o n c e n t r a t i o n s as % o f dry m a t t e r o f a b o u t 0.5-3% N. 0.1-0.3% P, a n d 1.6-3.5% Κ (13). T h e actual n u t r i e n t c o n c e n t r a t i o n d e p e n d s o n t h e part of plant analyzed, t h e season (or age of plant), a n d , m o s t i m p o r t a n t l y , o n t h e n u t r i e n t s u p p l y t o t h e plant. N u t r i e n t l i m i t a t i o n reduces light c o n v e r s i o n e f f i c i e n c y a n d p r o d u c t i v i t y . H o w e v e r , t h e m i n i m a l c o n c e n t r a t i o n s required t o m a i n t a i n h e a l t h y g r o w t h are n o t w e l l characterized. Critical n u t r i e n t tissue levels (at w h i c h n u t r i e n t d e f i c i e n c y sets in) are 0.09% f o r Ρ a n d 2.5% f o r Κ in a Typha h y b r i d (52); for n i t r o g e n it is likely b e t w e e n 0.5-1.0%. For s u p p l y of s u c h n u t r i e n t levels o n a large-scale a n u m b e r of sources can be c o n s i d e r e d — a g r i c u l t u r a l fertilizers, s e w a g e a n d a n i m a l w a s t e s , a n d recycled n u t r i e n t s f r o m a processing plant.


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