Are pipelines the answer t o waste Even with present technology, piping solid waste to disposal points appears attractive
T 1. here are a number of disadvantages inherent in the collection of solid w'ast'e and its removal to the point of disposal by truck. Obvious ones associated with storage at the source include unsightliness, fire h'azard, and the harboring of flies, rats, and other vermin: those disadvantages associated with collection are the need for frequent collections, labor problems, effect on the health of the labor force, and the tremendous waste of human resources on an unproductive job. Associated with the removal of solid wastes to the pohts of disposal are such disadvantages as the effects on the transportation arteries of the cities, and the effect of snow on the collection procedure. Finally, truck transportation becomes more expensive as the distance of haulage to the point of disposal increases beyond a certain limit. This last considera6ion has forced cities to dispose of their waste close to the center of population, resulting in poor utilization of valuable land and severely restricting the disposal processes. Obviously, the steady growth of ci'ties, combined with the increased waste produced by an affluent society, will only intensify these unfavo,rable characteristics. No management miracles or mere refinement of present soli'd waste technology is capable of ehiminating all these objectionable aspects. If their elimination is desired, ;I new technology must be developed. We have examined the applicability of one such new technology-bulk solid transport in pipelines-to the collection and removal of solid waste. While the application of pipeline technology to the field of solid waste is new, the pumping of bulk solids in pipeline is not. Such a system, if feasible, would eliminate virtually all the difficulties associated with the present truck collection operation. and create 812
Environmental Science & Technology
r
Shredder. First .step in pipelirie transport would be shredding waste to small size capabilities beyond those presently available. This investigation was supported by a Public Health Service research grant from the Bureau of Solid Waste Management. Whether the pipeline transport of solid waste may be a seri'ous dternative to truck collection depends upon answers to four fundamental questions: Is pipeline transport of solid waste technologically feasible? How does it compare economically with truck collection? What method of treatment and disposal is best suited to this type of collection? What governmental instrument is needed to bring about the change to pipeline transport and what will be the sociological impact of such a change? Here, we aim to answer only the first two questions. The thi'rd question
has been discussed elsewhere; no attempt has yet been made to investigate the fourth one, We do not intend to claim that the solid waste pipeline will provide a universal solution to the very complex problem of handling solid wastes. Rather, we hope to provide another alternative which, alone or in combination with other methods of transport, may ease our difficulties. Technology
It is a well known fact of fluid mechanics that liquids or gases travelling at high velocity are capable of entrapping solid matter and carrying it along. This characteristic of flowing fluids has been utilized for transport of bulk solids. A fully automatic system of refuse collection and removal should begin at each service point, terminate at the point or points of disposal, and
feature collection dilemma? lraj Zandi and John A. Hayden Towne School of Civil and Mechanical Engineering University of Pennsylvania Philadelphia, Pa. 19104.
Available presizing units are too massive and too expensive to be installed in each building. consequently, a combination plan would have to be worked out. There are two alternatives : A semi-automatic system, utilizing trucks to collect the solid waste of different neighborhoods and remove it to various small, centrally located points where the wastes would be crushed and mixed with a small percentage of sewage to produce slurry to be pumped over long distances for disposal. Of course, this system has many of the disadvantages associated with truck collection. A pneumo-slurry system, where pneumatic pipelines would replace the truck collection of the previous alternative. This system produces a flexible scheme which is fully automatic, capable of long distance transport, and does not require a presizing unit at each building. Experimental work
Tests. One shredder used in authors’ tests was a Gorator, marketed b y Dorr-Oliver should be designed so as to admit almost any type of waste. If the distance between points of generation and point of disposal is short (one or two miles), a pneumatic system could well serve the purpose. In Sundeberg, a suburb of Stockholm, Sweden, a vacuum-sealed pipeline system collects the solid waste of nearly 5000 apartments. This solid waste is swept along in an air stream of about 90 feet per second mean velocity. The destination of the waste is the incinerator-boiler of a space heating plant, 1.7 miles away, from which heat is distributed to the residential units. Underground pipes in this system range in diameter from 20-24 inches; sufficient vacuum is produced by five turbo-extractors, each driven by a 100 kw electric motor. This summer, upon completion of the 5000 apartments for which the system is designed, about
20 hours of operation will be necessary. No presizing is required when dimensions of the waste are less than 20 inches-this includes almost 98 % of all household wastes. In the eight years that the Swedish system has been in operation, no clogging has occurred, although it has been reported that even a car battery has been carried through it. When the haulage is more than a few miles, and/or the total volume of waste in the pipe at a single moment is excessive, the pneumatic system becomes impractical or uneconomical. By contrast, a water slurry pipeline becomes less expensive per ton-mile as the haulage and volume increase, and it is natural that its application should be examined. Yet, a slurry system requires presizing of materials, such as by a crusher or shredder at each point of generation.
Because there has been no previous experience with solid waste slurry pipelines, an experimental facility was built in our laboratory to establish the feasibility of pumping a solid waste slurry and to find the influence of different system variables. These variables include material properties (size, shape, size distribution, composition, and density) ; transporting medium properties (viscosity, density, and temperature) ; pipeline properties (diameter, slope, and material) ; flow properties (mean velocity); and slurry properties (concmtration and rheological behavior). These studies would establish the design criteria for a solid waste pipeline network. At present, only a portion of the goals have been realized and active experimentation is in progress. The experimental setup consists of three 80 foot test sections of 2, 4, and 6 inch diameter galvanized iron pipes. The flow rates are measured by a magnetic flow meter, and, in the tests completed at the present time, differential manometers were used to measure pressure losses. Our first series of tests was conducted o n solid waste originating in the center city of Philadelphia, Pa., as delivered by city operated collection trucks. Prior to pumping, these materiVolume 3, Number 9, September 1969 813
Slurry. Shredded waste materials (shown before and after shredding) are pumped easily when slurried with writer als were crushed in a hammer mill, after which objects up to one inch in size still were present. These materials then were added to a mixing tank, pumped through pipes, and the pressure drops were measured for different concentrations, mean velocities, and pipe diameters. These studies showed that, at a low concentration of solid waste, the head losses are similar to those for water alone. When the concentration (by weight) of solid waste was 6% or more, the measurement of head losses by the manometer became quite erratic and the readings lost their reliability. The fact that the head losses seem to be independent of velocity for 11% concentration suggests the possibility that the manometer's lines were clogged during these runs. For this reason. the data for concentrations above 6% have been disregarded, even though actual slurries of up to 12% were pumped with little difficulty. It is interesting to note that untreated solid waste slurries, with as low as 2% concentration by weight, tend to form a stable suspension. The solid waste slurry forms a matrix which holds heavier materials in suspension. Because of this phenomenon, it seems that minimum velocity, a major design criterion for pipeline transport of discrete solids, loses its importance for solid waste. The practical implication of this is that settling of solids due to a reduction in mean velocity (usually the primary cause of pipe clogging) is less significant for a solid waste pipeline than for pipeline transport of discrete materials. This first series of tests shows that slurries formed from untreated. 814 Environmental Science & Technolog)
crushed solid waste, with concentrations up to 12% by weight, can be conveyed through pipes. For concentrations below 4 % , the pipeline can be designed as though the medium is water alone, as long as the characteristics of untreated solid waste are similar to those tested. Furthermore, minimum velocities required to sustain stable suspensions of solid waste slurries are much lower than those ordinarily found in, for example, coal or sand suspensions. However, i n order to ascertain, with satisfactcry reliability, the design criteria for an optimized solid waste pipeline, more data are needed. Different types and compositions of solid waste need to be tested, and data for slurries with higher concentrations should be obtained. Tests are underway to obtain these data. Waste shredders
Obviously, the success of a solid waste pipeline depends largely upon the ability to shred all the components of solid waste to a reasonable size. A number of different crushers may be used, directly or with some modification. One such unit is the Goratormarketed by Dorr Oliver, 1nc.-which features a notched, flat-disk impeller mounted at an oblique angle on the end of the shaft. The notches mesh with serrations in the removable liner sections to create a chopping and pumping action. Rotation of the disk results in rapid oscillation, creating ;I continuous cavity into which the slim! flows axially through a fully open inlet. Then, the slurry contacts the rotating disk and is discharged A positive feed is required.
This unit can reduce the size of the so-called hard components of solid waste, thus making them suitable for a pipeline system. Actually, from the standpoint of slurry pipe transport, this unit is wasteful, because it crushes the solid waste to a much finer size than is required for piping. The main problem with this shredder is that the size of the inlet to the pump limits the size of waste which can be handled. This difficulty, however, can be surmounted by using a modified version of a Wascon pulper in series with the Gorator. Wascon pulping equipment can admit very large objects and can easily shred all components of solid waste. except metallic ones. But the design is such that the metallic objects entering the pulper do not interfere with the process of pulping and remain there until eventually bent, deformed, or sheared. Those which canlnot be deformed will be rejected. If a Wascon pulper is installed ahead of the Gorator, the hard components of soli'd waste can remain in the pulper until reduced to a size which can be admitted and processed by the Gorator. In this manner, aImost all components of soli'd waste can be suitably processed for pumping. To examhe the feasibility of this idea, arrangements were made with both Wascon and Dorr-Oliver to build a setup incorporating a Gorator and a pulper. This setup was constructed by Wascon. while Dorr-Oliver donated a Gorator. Solid waste shredded by this type of equipment in our laboratory could be pumped readily. Obviously, the hope is that a new design can combine these two operations
and produce a compact and efficient crusher-pump device which can handle any type and size of solid waste. It is important to note that many steps must be taken to develop an efficient and reliable system of crusher-pumps and piping; but even with the present technology. almost all household solid wastes can be collected and removed through a pneumo-slurry system. Economics
Whether solid waste pipelines will be considered as a future means of collecting domestic. commercial, and some industrial solid waste depends largely on cost as compared with the truck collection system. To make a realistic comparison, a center section of the city of Philadelphia was chosen as a study area. We made a compilation of the current cost of collection and removal of waste generated in the study area, and estimated the external costs of the present system. Having then looked at the relevant costs of the system for 1968, we made a projection of these costs for the future. Second, we designed and estimated the cost of a pi'peline transport system for the study area. Finally. a cost-benefit analysis was made. using the costs of the present systems as a measure of the benefits of the proposed pipeline system. We selected an appropriate discount rate (6%) in order to convert the benefits and costs to a given present date (1 968). The particular study covers 3.2 square miles and about 2.5% of the total city area-and was selected primarily because its boundary closely coincides with census tracts and city solid waste collection routes. This portion of Philadelphia contains the central business district, with much commercial property. There is also a great deal of rental housing, especially in the form of small apartment units. Population of the study area is 48,000, about 2.3% of the city's two million residents. The collection and removal system in this area includes both city and private operations (large bulky items, especially automobiles, are disposed of by private junk dealers). The city collects all wastes generated by residences (up to a limit) and city institutions with its own trucks or through contract with private collectors. Both quantity of waste produced and expenses incurred by the city are well documented: in 1968, the city spent $900.0800 to collect 55,300 tons of solid waste in the
Quantities and costs of truck collection
Waste quantity in 1968 (thousands of tons) Collection costs in 1968 (millions of dollars) Quantity of waste in 50th year of design period (thousands of tons) Total costs for 50 year design period at 1968 values (mi I I ions of d 01I a rs)
City collection
collection Private
60.14
193.0
253.0
$ .90
$1.30
$2.20
161.9
519.9
681.0
$27.60
$39.92
$67.50
Total
Survey. Data for private collection i n t h e Philadelphia study area were obtained from a m a i l questionnaire. City data included neither disposal costs, nor t h e collection and disposal of bulky items, t h e quantity of which amounted t o 4150 t o n s in 1968, a n d is estimated t o rise t o 11,200t o n s i n t h e 50th year of the design period. Costs were projected a t 4% a n n u a l increase basis.
Projected costs for pipeline network Distance f r o m study area t o disposal p o i n t 0 20 m i l e s 50 m i l e s (millions of dollars)
Pipe costs
$25.20
$30.00
$37.10
Air seals and house fittings
68.02
68.02
68.02
Crusher a n d pump Vacuum blowers a n d filters Slurry pumps and p u m p houses Operation Maintenance Supplementary collection of bulky materials ($20/ton)
2.08
2.08
2.08
0.97
0.97
0.97
1.00
1.39
2.03
10.17
12.10
14.20
18.48
18.84
19.41
3.0
3.0
3.0
$128.9
$136.4
$146.8
Total cost
area. Both these figures are about 6% of the citywide total, and exclude nonmunicipal collection. In the study area, some manufacturing establishments, large wholesalers, commercial companies, and apartment houses utilize private collectors. For central Philadelphia, this part of the collection is considerably larger than city collection. Data regarding the quantity and cost of this portion of collection and removal were lacking, since private collectors do not have to report their activities to the city. This presented a major data problem. To obtain information as to the quantity and cost of private collection, we surveyed the area, utilizing a stratified random sample of real properties in the study area. Our technique employed both printed, mailed questionnaires and indepth telephone interviews, and the results were analyzed statistically. This survey has been described in detail elsewhere; one table accompanying this article shows a summary of the information obtained from the survey.
In spring 1967, we undertook a test survey, using the results in a preliminary report on the pipeline system for Philadelphia. However, information presented here supersedes that of the preliminary report and is the most reliable data now available. Cost of present system
Since the aggregate benefits of the new pipeline system represent the projected costs of the present system, an estimate of the projected costs of the present system over the next 50 years was made. This includes the direct collection cost and the spillover or indirect cost of the present arrangements. We estimated direct benefits (cost of present system) from data supplied by the city and those resulting from the survey, Total city expenses for the study area, including private collectors who work under contract, were estimated at $900,000 per year. Based on a per capita solid waste increase of no more than 2% per year plus some change in relative labor costs and popVolume 3, Number 9, September 1969 815
site Mode of collection
50 Year study period using:
Mean of present costs
1
I
Truck Pipelinee Collection Collection
67.5
128.9
miles
miles
Truck Pipelinee Collection Collection
Truck Piuelinee Collection Collection
~
I
_
100.6
_
_
136.4
miles
,-
miles
~
miles
.
153.2
146.8
'
I
67.50
105.6
i
~
100.6
112.7
'
153.2
122.6
I
(a) The indirect benefits of pipeline systems are not included i n this comparison. (b) A 25% decrease in the costs of blowers, crusher-pumps a n d discharge values (vacuum seals) is assumed. (c) Cost of truck transport assumed t o be 5b/ton/mile. (d) An annual increase of 2.0% i n quantity of solid waste is assumed. (e) These figures include t h e collection of bulky material which is $3,000,000 over the next 50 years at the 1968 cost (estimated t h a t it will cost $20/ton to remove the bulky materials).
dation level, we assumed an expected growth rate of 4.0%. According to city estimates, the volume of city collections has been increasing at the rate of 2-396 per year in recent years. These yearly values were discounted at a rate of 6% for the 50 year design period. The summation of yearly discounted values provided the total city expenditure of $27,600,000 over the next 50 years at the first year basis (1968). The private collection costs were obtained from a stratified random sample of real properties in the study area, selected from city records. Within the basic categories of commercial and residential buildings, 45 different but significant sub-categories (such as commercial buildings used for light manufacturing, or residential buildings with garage. etc.) alre distinguished. Each sub-category was further divided into two groups according to the number of stories in the structure. A pretested questionnaire was sent to a statistically siignificant number of buildings in each category. The survey produced information on cost, waste quantity, and a number of sodal indices. We found the mean expenditure for yearly private collection in 1968 to be $1,220.000 for commercial firms and $80,000 for residential units, making private collection cost $1,3OO,OOO per year. This yearly expenditure was also assumed to increase annually at a rate of 4 % , and was again discounted at the rate of 6% per year to convert it to the present day value for the next 50 years. This figure ($39,920,000), when added to the cost of city collection, results in a total collection cost in the area of $67.5 million, adjusted to present cost, for the next 50 years, 816
Environmental Science & Technology
It should be noted that the survey predicted the amount of solid waste collected by the city of Philadelphia (mean value) within 9 % of the actual value based on Sanitation Department figures. This result was interpreted as an independent check on the accuracy in the survey. Pipeline system costs
In order to estimate the cost of the pipeline system, we examined a number of alternative networks. The main criterion for the selection of any design was that it should accomplish the task with the present technology, In other words, the designs examined could be built today. They would be inefficient as compared with a refined system, and certainly would have problems, but they would be operative. The final scheme adopted is a combination pneumatic and slurry pipeline system. The pneumatic system would collect at four points all solid waste generated from individual buildings in the study area. Then, at each point. waste water from the sewerage system (only a few percent of the total produced) is mixed with the solid waste to produce a 6% slurry. The mixture then is fed into, respectively, a Wascon pulper and a Gorator. The Gorator produces enough head to convey the slurry to the boundary of the system, from whence the slurry can be pumped as far as desired for disposal. The pneumatic part of the system basicnlly is the same as the unit already in operation in Sundeberg, Sweden. The slurry pipeline was designed from information obtained in the present study. A 6% concentration was selected because reliable data for a 6 inch pipeline is available; pressure gra-
dients for larger pipes were estimated proportionally. We have already mentioned that slurries with concentrations up to 12% can be pumped, but that the pressure loss data for concentrations higher than 6% are unreliable. When reliable data for higher concentrations become available, a more efficient system can be designed. Other workable schemes, such as all-pneumatic systems, are possible. and should be explored for each individual situation, but the design adopted here reveals the pertinent points of comparison with truck collection. In the summary table of the basic calculations involved in the design of the new system, the unit prices selected are all conservative. They were checked with a number of consulting firms and are about the same as those used by Philadelphia's Water Department. The tables are self-explanatory and clearly indicate that all costs, including installations, operation, and maintenance, are considered. For comparison of the costs-benefits of the pipeline system, direct costs of the present system were assumed to be the benefits of the pipeline system. Three alternate points of disposal were yelected: zero, 20, and 50 miles from the boundary of the study area. Comparison was made for the design period of 50 years. According to these estimates. the city of Philadelphia will have to spend, in direct costs, some $153.2 million using mean values, to collect and remove to a distance of 50 miles the solid waste which will be generated in the study area for the next 50 years. For this same period, the cost of a 50 mile solid waste pipeline is estimated
to be $146.8 million. These figures show that pipeline transport of solid waste, economically speaking, would be competitive with the present truck collection, even disregarding the secondary benefits of the pipeline system. Additional factors
There are several other points which we must discuss. The basic premise of the economic comparison has been that no new devices will be available. However, it seems only natural to look into the future with some expectation and hope. First, it is clearly reasonable to assume that a more efficient crusher-pump unit will be developed. In this study, for instance, we assumed that 15 Wascon pulpers were needed at each collection point. It is obvious that without much effort the capacity of these pulpers can be increased. In addition, the cost estimates for different units are based on present prices, and these units are now manufactured separately because of a limited market. Once a demand is developed, a considerable reduction in price can be expected Wascon’s engineers predict a 30-40% reduction in the cost of their unit. Second, it is clear that the major portion of the cost of the pneumo-slurry system is the installation of the discharge valve assembly at each building. The cost of this item has been reduced by 40% in one year (1968)-even without the cost-reducing incentive of mass production. The summary table also indicates a cost for the pipeline system that assumes some reasonable future development. The economy of the proposed system is obvious. There are large shredding units available on the market which may be used in a slurry pipeline scheme. These large shredding units would reduce the size of the material collected in the pneumatic pipeline and also crush and shred the large, bulky items not transportable in the pipeline. Thus, large items would only have to be taken short distances to the local crushing station. This would require only a small, inexpensive supplementary truck collection by the city. This supplementary cost is included in the figures in the summary table. Study is underway to collect information as to the capability and cost of these units. The indirect benefits of the pipeline system, which make it even more attractive. are not included in the comparison In addition. no mentbon is made of the mode of disposal. Obbiously, the interrelationship between
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