12 Small-Scale Source Densification of Navy Solid Waste DONALD BRUNNER Civil Engineering Laboratory, Naval Construction Battalion Center, Port Hueneme, CA 93043
In the Navy, as everywhere else, the disposal of solid waste is becoming more and more costly as environmental regulations become more restrictive. This paper will examine some of the economic and technical aspects of a new concept for reducing the overall cost for solid waste management. The problem of increasing cost of solid waste management within the Navy is not the same as in the private sector. When the Navy is directed to meet specific requirements of new regulations or executive orders, Congress often does not appropriate additional funds or allocate new manpower to accomplish the new requirements. Increased costs cannot be passed on to the consumer since the Navy is the consumer. Expenses must be absorbed into the existing Navy budget thereby diverting funds from the primary mission. The Navy generates approximately 1.75 million tons of solid waste annually. Current costs are approximately $30 per ton for collection, transportation, and disposal. This represents an annual expenditure of approximately $50 million. The Navy could staff up, operate, and maintain a squadron of aircraft at this price. With these costs expected to double or even triple in the next few years due to more restrictive environmental regulations, it becomes prudent for the Navy to develop techniques for managing their waste that are both environmentally and economically acceptable. The collection process consumes about 70% of a l l the dollars expended to manage Navy solid waste as illustrated in Figure 1. The high costs of collection derives from the characteristic fluctuation in the generation rate, the nature of the assortment of materials involved, the variable and generally low density and the requirement to remove it from the premises at frequent regular intervals. Solid waste, for the most part, is an organic material having a high surface area to mass ratio. In the presence of moisture, i t becomes biologically active fostering the development of unacceptable and noxious decomposition by-products. It can be a fire hazard as well as a breeding ground for flies and rodents. This chapter not subject to U.S. copyright. Published 1980 American Chemical Society
152
THERMAL
CONVERSION OF
SOLID W A S T E S A N D BIOMASS
COLLECTION 70% ($20.50 T O N )
RESOURCE RECOVERY (SALEABLE PRODUCTS)
Figure 1.
Cost of solid waste management unit operations
12.
ΒR U N N E R
Source Densification of Navy Solid Waste
153
I t can a t t r a c t b i r d s , animals and human scavengers and i s subject to becoming windblown and u n s i g h t l y . H o s p i t a l wastes add a f u r ther dimension to waste management i n that they are subject to becoming a s i g n i f i c a n t v e c t o r f o r p a t h o l o g i c a l b a c t e r i a and v i r u s e s and therefore o f t e n require s p e c i a l handling. H i s t o r i c a l l y the only a v a i l a b l e s o l u t i o n to most of these problems has been frequent, regular removal of the waste from the generating source. Cost e f f e c t i v e mating of equipment and man power to the h i g h l y v a r y i n g waste generation rates has, of neces s i t y , been subordinated to h e a l t h and s a f e t y c o n s i d e r a t i o n s . Hence the d i s p r o p o r t i o n a t e amount of d o l l a r s devoted to the c o l lection activity. A study of the problem i n d i c a t e s that processing s o l i d waste to a high density, s e l f encapsulated, b i o l o g i c a l l y s t a b l e s l u g a t the generating source has the p o t e n t i a l f o r reducing c o l l e c t i o n frequency up to 18:1. This w i l l allow major reductions i n c o l l e c t i o n equipment, manpower, and containers by i n c r e a s i n g the number of loads per crew day (1., 2). A d d i t i o n a l savings can a l s o accrue i n d i s p o s a l by i n c r e a s i n g land f i l l c a p a c i t y , reducing p o l l u t i o n problems and most s i g n i f i c a n t l y by i n c r e a s i n g a v a i l a b l e d i s p o s a l options. New options w i l l include d i s p o s a l as a refuse derived f u e l and environmentally sound d i s p o s a l of shipboard generated wastes a t sea. Feasibility The C i v i l Engineering Laboratory began i n v e s t i g a t i n g the f e a s i b i l i t y of d e n s i f y i n g waste i n t o s e l f - e n c a p s u l a t e d b i o l o g i c a l l y s t a b l e slugs i n 1974. At that time, two e x t r u s i o n concepts were i n v e s t i g a t e d : ram e x t r u s i o n and screw e x t r u s i o n (3). Ram e x t r u s i o n was examined using a modified commercially a v a i l a b l e M i l Pac System shown i n Figure 2. A schematic showing the operation of the system i s presented i n Figure 3. Unprocessed waste was f i r s t f e d i n t o a v e r t i c a l s h a f t hammermill where i t was reduced to a nominal 2-inch p a r t i c l e s i z e . The shredded refuse was then pneumatically conveyed to the charge hopper of the extruder where h y d r a u l i c rams forced the m a t e r i a l through 2-inch square e x t r u s i o n d i e s . Continuous processing with t h i s system was never r e a l i z e d . The e x t r u s i o n dies jammed o f t e n as the character i s t i c s of the refuse changed. The d e n s i f i e d p e l l e t s or slugs produced by t h i s process v a r i e d considerably. The experiment d i d demonstrate that high d e n s i t y slugs could be produced. D e n s i t i e s as high as 92#/ft (1.47xl0 kg/m ) were achieved. The high d e n s i t y slugs were c h a r a c t e r i z e d by a glazed surface which tended to bind them together. Also because of the batch type processing r e s u l t i n g from the r e c i p r o c a t i n g a c t i o n of the ram, the slugs contained transverse shear planes or laminations. The slugs could e a s i l y be broken across these planes i n t o square c h i p s . The 3
3
3
154
THERMAL
Figure 2.
Figure 3.
CONVERSION
OF
SOLID
WASTES
Ram extrusion modified Mil Pac system
Schematic of ram extrusion process
AND
BIOMASS
12.
BRUNNER
Source Densification of Navy Solid Waste
155
thickness of these chips depended on the q u a n t i t y o f m a t e r i a l processed during each c y c l e o f the ram. F o r these experiments, the nominal chip thickness was 0.25 inch (6.35xl0~ m). The e f f e c t s of the second process, screw e x t r u s i o n , was i n v e s t i g a t e d using a 3-inch (7.62xl0~ -m) diameter s i n g l e screw extruder s i m i l a r to those used by the p l a s t i c and rubber industries. In t h i s concept, shown i n F i g u r e 4, the r o t a t i o n o f the screw provided the pressure necessary to force the shredded s o l i d waste through the c y l i n d r i c a l e x t r u s i o n d i e . The waste slugs produced by t h i s process were more continuous and not c h a r a c t e r i z e d by transverse shear planes e x h i b i t e d by the ram extruded slugs. A l s o , because the a c t i o n of the screw tended to "work" the m a t e r i a l and compact i t before e n t e r i n g the d i e , high temperatures were generated i n the b a r r e l of the screw and the r e s u l t i n g slugs tended to have a harder and more cohesive surface f i n i s h . F i g u r e 5 shows comparative samples of ram and screw e x t r u s i o n products. A d d i t i o n a l f i n d i n g s of these e a r l y i n v e s t i g a t i o n s showed that: 3
2
(a) The d e n s i t y , s t r u c t u r a l cohesiveness, and surface f i n i s h of the product from e i t h e r process can be c o n t r o l l e d by r e g u l a t i n g the p r o c e s s i n g temperatures and c o n t r o l l i n g the r e s i s t a n c e o f f e r e d by the e x t r u s i o n d i e . (b) Both d e n s i f i c a t i o n concepts e f f e c t i v e l y reduced the b i o l o g i c a l l y a c t i v e surface area and impart a degree of b i o l o g i c a l s t a b i l i t y to the refuse. (c) The moisture content of the refuse i s a s i g n i f i c a n t processing parameter. I t a f f e c t s the r e s i s t a n c e developed w i t h i n the e x t r u s i o n d i e , and t h e r e f o r e , the d e n s i t y and cohesiveness of the r e s u l t i n g product. V a r i a t i o n s i n moisture content can cause the die to jam and thus i n h i b i t continuous p r o c e s s i n g . These p r e l i m i n a r y t e s t s were designed to i d e n t i f y the p r o c e s sing parameters and product c h a r a c t e r i s t i c s , but were not o f s u f f i c i e n t depth to q u a n t i f y the r e l a t i o n s h i p between p r o c e s s i n g c h a r a c t e r i s t i c s and product c h a r a c t e r i s t i c s . They d i d show the need and d i r e c t i o n f o r f u r t h e r research. At the time CEL was performing i t s f e a s i b i l i t y i n v e s t i g a t i o n s other experiments were also i n progress (4, 5). F o r the most p a r t , these experiments were d i r e c t e d a t c e n t r a l i z e d p r o c e s s i n g of s o l i d waste i n t o small p e l l e t s f o r c o - f i r i n g with c o a l . Commerc i a l u t i l i z a t i o n of p e l l e t i z e r s f o r s o l i d waste p r o c e s s i n g was being explored by a number o f companies i n c l u d i n g Papakube Corpora t i o n i n San Diego, C a l i f o r n i a . In the summer of 1978, CEL processed Navy wastes from the San Diego area a t Papakube. The cubes were made using a modified John Deere extruder (shown i n F i g u r e 6) and measured approximately 1 inch square (2.54xl0~ m) and 1.5 2
THERMAL
Figure 4.
CONVERSION
OF
SOLID
WASTES
Screw extrusion process
AND
BIOMASS
BRUNNER
Source Densification of Navy Solid Waste
Figure 5. Samples of ram- and screw-extruded solid waste
HELICAL FEED S C R E W
Figure 6. Schematic of John Deere/Papakube cuber
158
THERMAL
CONVERSION OF SOLID WASTES AND BIOMASS
2
inches ( 3 . 8 x l 0 ~ m) i n length. Some minor measurements were made during the processing of the wastes but the primary o b j e c t i v e of the t e s t s was to o b t a i n d e n s i f i e d m a t e r i a l f o r performing p h y s i c a l and chemical t e s t s and analyses of the product. Preliminary r e s u l t s i n d i c a t e the moisture of the cubes v a r i e d between 20% and 36% and the energy content between 4,400 (10.2xl0 J/kg) and 6,000 ( I 4 x l 0 J/kg) Btu/lb. A report of the p r o c e s s i n g and subsequent p h y s i c a l and chemical t e s t s of the cubes i s n e a r l y completed (6). A p o r t i o n of the cubes were brought to CEL and stored a d j a cent to shredded s o l i d waste to o b t a i n an i n d i c a t i o n of the r e l a t i v e b i o l o g i c a l s t a b i l i t y of the d e n s i f i e d m a t e r i a l . The cubes were stored i n 40-yard (30.6-m~ ) drop boxes and covered with black p l a s t i c . Probes were i n s e r t e d at various p o i n t s i n the containers to allow e x t r a c t i o n of gas samples and to measure f l u c t u a t i o n s i n temperature. Results of temperature f l u c t u a t i o n over a 75 day storage p e r i o d are shown i n F i g u r e 7. Note that the temperatures of the cubes t r a c k very c l o s e l y with those of the ambient a i r temperatures i n d i c a t i n g very l i t t l e i n the way of composting of the refuse was t a k i n g p l a c e . On the other hand, the temperature of the shredded refuse was c o n s i s t e n t l y higher than the ambient, a t t a i n i n g an average temperature c l o s e to 50°C, i n d i c a t i n g a much higher l e v e l of b i o l o g i c a l a c t i v i t y . These t e s t s demonstrated the b i o l o g i c a l l y s t a b i l i z i n g e f f e c t of d e n s i f i c a t i o n and the p o t e n t i a l f o r long term storage without adverse environmental e f f e c t s . A report on these storage t e s t s i s curr e n t l y being prepared and should a l s o be a v a i l a b l e i n the near future (7). More in-depth a n a l y s i s of the b i o l o g i c a l aspects of d e n s i f i e d waste storage are planned at our S o l i d Waste R&D t e s t s i t e c u r r e n t l y being constructed at NAS J a c k s o n v i l l e , F l o r i d a . We w i l l a l s o be using the t e s t s i t e to e s t a b l i s h p r o c e s s i n g c r i t e r i a and c r i t e r i a f o r s o l i d forms of waste-derived f u e l f o r use by the Navy. 6
6
3
Application/Benefits D e n s i f i c a t i o n p r o c e s s i n g can be a p p l i e d b a s i c a l l y e i t h e r at the generating source or at a c e n t r a l i z e d l o c a t i o n . We have emphasized the generation source l o c a t i o n i n our s t u d i e s . The o p e r a t i o n b e n e f i t s of source d e n s i f i c a t i o n are derived from the t e c h n i c a l o b j e c t i v e s of high d e n s i t y s e l f - e n c a p s u l a t i o n and b i o l o g i c a l s t a b i l i t y . High d e n s i t y has the obvious e f f e c t of decreasing the volume of the waste. This i n t u r n allows a decrease i n the s i z e of the storage area whether i t i s a dumpster, a truck or a c e l l i n a landfill. D e n s i f i c a t i o n to 6 5 # / f t (104 kg/m ) represents an approximate volume r e d u c t i o n of 18:1 over that of unprocessed r e f u s e . The higher d e n s i t y a l s o implies a reduced surface area and t h e r e f o r e a reduced f i r e hazard and lower b i o l o g i c a l a c t i v i t y . 3
3
12.
BRUNNER
Source Densification of Navy Solid Waste
159
"to •S
I -Ci
s
-à
S a S Ci,
I
I ·&s .1 Σ! Q Ci,
ί Κ
