Nicholas P. Cheremisinoff. Handbook of Solid Waste Management and Waste Minimization Technologies
Подождите немного. Документ загружается.
Most electric incinerators and many fluidized bed incinerators use venturi
scrubbers only. In a typical combination venturi/impingement tray scrubber, hot
gas exits the incinerator and enters the precooling or quench section of the
scrubber. Spray nozzles in the quench section cool the incoming gas and the
quenched gas then enters the venturi section of the control device. Venturi water
is usually pumped into an inlet weir above the quencher. The venturi water
enters the scrubber above the throat and floods the throat completely. This
eliminates build-up of solids and reduces abrasion. Turbulence created by
high gas velocity in the converging throat section deflects some of the water
traveling down the throat into the gas stream. Particulate matter carried along
with the gas stream impacts on these water particles and on the water wall.
As the scrubber water and flue gas leave the venturi section, they pass into a
flooded elbow where the stream velocity decreases, allowing the water and
gas to separate.
Most venturi sections come equipped with variable throats. By restricting the
throat area within the venturi, the linear gas velocity is increased and the
pressure drop is subsequently increased. Up to a certain point, increasing the
venturi pressure drop increases the removal efficiency. Venturi scrubbers
typically maintain 60 to 99% removal efficiency for particulate matter,
depending on pressure drop and particle size distribution. At the base of the
flooded elbow, the gas stream passes through a connecting duct to the base of
the impingement tray tower. Gas velocity is further reduced upon entry to the
tower as the gas stream passes upward through the perforated impingement
trays.
Water usually enters the trays from inlet ports on opposite sides and
flows across the tray. As gas passes through each perforation in the tray, it
creates a jet which bubbles up the water and further entrains solid particles.
At the top of the tower is a mist eliminator to reduce the carryover of water
droplets in the stack effluent gas. The impingement section can contain from
one to four trays, but most systems for which data are available have two or
three trays.
FUEL ECONOMY
When dealing with incinerators, fuel is generally the most expensive part of the
process from an operational standpoint. A ratio should be calculated beforehand
that represents the amount of fuel used for the amount of sludge inputted. If there
is a significant change to the amount of fuel consumed, it could mean that there is
a problem in the fuel supply system, or air flow to the incinerator, or that an
extensive furnace cleaning is in order.
COST CONSIDERATIONS
Minimal cost of operation and equipment maintenance is another economic
parameter for sludge incineration. Preventive maintenance is the single most
important factor in reduction of operating costs. Semiannual or quarterly
appointments must be scheduled to allow time for complete furnace checkout and
cleaning (referred to as "turnarounds"). Table 4 is a breakdown of the costs of
each incinerator. Essentially costs can be related to one basic parameter: the
lower the moisture content is in the sludge, the less expensive the incinerator will
be to operate. Also, incinerators are bought based on what moisture level of
sludge they are going to be effective with. Some incinerators can burn out sludge
with 20% moisture levels and some cannot.
The design cost will be a function of the incinerator cost plus installation, which
is normally in the range of 4 to 7%. This cost should be doubled to include
engineering services during project construction. It should be noted that with the
electric furnace, the power needed to start up results in a large connected load. In
areas of the country where there are high demand charges for electric power, this
system can be economically impractical.
Table 4. Estimated Economics for Incineration.
Type
incinerator
Multiple Hearth
Furnace
Fluid Bed
Incinerator
Electric Furnace
Cyclonic
Incinerator
Capacity
(lbs/hr)
7,000
1,000
2,900
2,400
2,400
2,000
Sludge moisture
content (%)
0
0
20
30
0
20
Installed cost
(U.S.
$)
11,000,000
900,000
1,600,000
1,3000,000
950,000
1,000,000
INDUSTRY APPROACHES TO SLUDGE VOLUME
REDUCTION
When we think of sludge, what automatically comes to mind is sewerage. Water
carriage systems of sewerage provide a simple and economical means for
removing offensive and potentially dangerous wastes from household and
industry. The solution and suspension of solids in the transporting of water
produces sewage. Thus, the role of solids and sludge removal at sewage
treatment plants is apparent. Sludge removal is complicated by the fact that some
of the waste matters go into solution while others are colloidal or become finely
divided in their flow through the sewage system. Ordinarily, less than half of
such waste remains in suspension in a size or condition that can be separated by
being strained out, skimmed off, or settled out. The remainder must then be
precipitated out by chemical means, filtered mechanically, or be subjected to
biological treatment whereby it is either removed from the water or changed in
character so as to be rendered innocuous.
Sewage contains mineral and organic matter in suspension (coarse and fine
suspended matter), in colloidal state (very finely dispersed matter), and in
solution. Living organisms, notably bacteria and protozoa, find sewage to be an
abundant source of food, and their lives' activities result in the decomposition of
sewage. Sewage becomes offensive as a result of its own instability together with
the objectionable concentration of suspended materials. In addition, the potential
presence of disease-producing organisms makes sewage dangerous. Removal or
stabilization of sewage matters may be accomplished in treatment works by a
number of different methods or by a suitable combination of these methods.
While sewage sludge is rich in nutrients and organic matter, offering the potential
for applications as a biosolid (next chapter), or has a heating value making it
suitable for incineration and possible heat or energy recovery, many industrial
sludges are often unsuitable for reuse.
A more common practice with industrial sludge is to try and identify a reclaim
value; i.e., if the sludge can be concentrated sufficiently there may be a portion
of this waste which is reclaimable or can enter into a recycling market. If instead
an enterprise can sell its waste, then some of the costs associated with waste
management can be offset. For example, copper is a heavy metal and the sludge
is a hazardous waste which is expensive to dispose of. Identifying an off-site
recycling market that will take responsibility for this waste, even if they do not
pay for it, can reduce or eliminate such costs as stabilizing and concentrating the
sludge, as well as transport and tipping fees at the landfill. This option may not,
however, eliminate the long-term liabilities associated with the waste if for some
reason the reclaimer has an incident in which the hazardous material impacts on a
third party. Of concern, however, with any sludge management problem, are the
costs for recovery of potentially valuable by-products. There are many situations
in industrial settings where large volumes of toxic sludges are generated on a
continual basis. As with POTWs, waste management programs tend to focus on
volume reduction techniques that reduce the costs for transport and disposal. The
mechanical volume reduction techniques described earlier for wastewater
treatment operations are equally applicable in these cases. But in some situations,
thermal drying methods must be employed in order to achieve significant volume
reductions, whether for disposal or reclaiming purposes.
Thermal techniques such as flash drying, the use of drying rooms, rotary kiln
driers,
and various other methods are costly. Ongoing energy costs as well as
OM&R costs contribute to waste management programs. In many situations less
passive drying techniques tend to be help reduce these treatment and recovery
costs,
such as the use of drying beds and lagoons.
The use of drying beds is one of two common methods of dewatering based upon
passive thermal energy. Drying beds are generally used for dewatering of well-
digested sludges. Attempts to air-dry raw sludge usually result in odor problems.
Sludge drying beds consist of perforated or open-joint drainage pipe laid within a
gravel base. The gravel is covered with a layer of sand. Partitions around and
between the drying beds are generally open to the weather but may be covered
with ventilated greenhouse-type enclosures where it is necessary to dewater
sludge in wet climates. The drying of sludge on sand beds is accomplished by
allowing water to drain from the sludge mass through the supporting sand to the
drainage piping and natural evaporation to the air. As the sludge dries, cracks
develop in the surface, allowing evaporation to occur from the lower layers
which accelerates the drying process.
There are many design variations used for sludge drying beds, including the
layout of the drainage piping, thickness and type of materials in the gravel and
sand layers, and construction materials used for the partitions. The major
variation is whether or not the beds are covered. Any covering structure must be
well ventilated. In the past, some beds were constructed with flat concrete
bottoms for drainage without pipes, but this construction has not been very
satisfactory. Asphalt concrete (blacktop) has been used in some drying beds.
The only sidestream is the drainage water. This water is normally returned to the
raw sewage flow to the plant or to the plant headworks. The drainage water is
not normally treated prior to return to the plant. Experience is the best guide in
determining the depth of sludge to be applied, however, typical application depth
is 8 to 12 inches. The condition and moisture content of the sludge, the sand bed
area available, and the need to draw sludge from digesters are factors to
consider. It is not advisable to apply fresh sludge on top of dried sludge in a bed.
The best time to remove dried sludge from drying beds depends on a number of
factors, such as subsequent treatment by grinding or shredding, the availability of
drying bed area for application of current sludge production, labor availability,
and, of course, the desired moisture content of the dried sludge. Sludge can be
removed by shovel or forks at a moisture content of 60%, but if it is allowed to
dry to 40% moisture, it will weigh only half as much and is still easy to handle.
If the sludge gets too dry (10 to 20% moisture) it will be dusty and will be
difficult to remove because it will crumble as it is removed. Many operators of
smaller treatment plants use wheelbarrows to haul sludge from drying beds.
Planks are often laid on the bed for a runway so that the wheelbarrow tire does
not sink into the sand. Wheelbarrows can be kept close to the worker so that the
shoveling distance is not great. Most plants use pick-up trucks or dump trucks to
transport the sludge from the drying bed. Dump trucks have the advantage of
quick unloading.
Where trucks are used, it is best to install concrete treadways in the sludge
drying bed wide enough to carry the dual wheels since the drying bed can be
damaged if the trucks are driven directly on the sand. The treadways should be
installed so that good access is provided to all parts of the beds. If permanent
treadways have not been installed, heavy planks may be placed on the sand.
Large plants will normally utilize mechanical equipment for handling the dried
sludge. Some communities have encouraged public usage of the dried sludge. In
some cases users are allowed to remove the sludge from the beds, but this may
not be satisfactory in many cases. Local regulations should be reviewed before
attempting to establish a public utilization program.
The use of sludge lagoons is a technique that relies both on the settling
characteristics of sludge and solar evaporation. The considerable labor involved
in sludge drying bed operations may be avoided by the use of sludge lagoons.
These lagoons are nothing but excavated areas in which digested sludge is
allowed to drain and dry over a period of months or even a year or more. They
are usually dug out by bulldozers, or other dirt-moving equipment, with the
excavated material used for building up the sides to confine the sludge. Depths
may range from 2 to 6 feet. Areas vary, and although drainage is desirable, it is
not usually provided. Digested sludge is drawn as frequently as needed, with
successive drawings on top of the previous ones until the lagoon is filled. A
second lagoon may then be operated while the filled one is drying.
After the sludge has dried enough to be moved, a bulldozer, or a tractor with an
end-loader, may be used to scoop out the sludge. In some locations it may be
pushed from the lagoon by dozers into low ground for fill. Lagoons may be used
for regular drying of sludge, reused after emptying, or allowed to fill and dry,
then leveled and developed into lawn. They can also be used as emergency
storage when the sludge beds are full or when the digester must be emptied for
repair. In the latter case it should be treated with some odor control chemicals,
such as hydrated or chlorinated lime.
The size of the lagoon depends upon the use to which it will be put. Lagoons may
take the place of sludge beds or provide a place for emergency drawings of
sludge, but they may be unsightly and even unwanted on a small plant site.
However, they are becoming more popular because they are inexpensive to build
and operate.
Although lagoons are simple to construct and operate, there can be problems
associated with sizing them. These problems largely arise from uncertainty in
estimating the solar evaporative capacity. In semiarid regions evaporation ponds
are a conventional means of disposing of wastewater without contamination of
ground or surface waters. Evaporation ponds as defined herein will refer to lined
retention facilities. Successful use of evaporation for wastewater disposal requires
that evaporation equal or exceed the total water input to the system, including
precipitation. The net evaporation may be defined as the difference between the
evaporation and precipitation during any time period. Evaporation rates are to a
great extent dependent upon the characteristics of the water body. Evaporation
from small shallow ponds is usually considered to be quite different from that of
large lakes, mainly because of differences in the rates of heating and cooling of
the water bodies because of size and depth differences. Additionally, in semi-arid
regions, hot dry air moving from a land surface over a water body will result in
higher evaporation rates for smaller water bodies. The evaporation rate of a
solution will decrease as the solids and chemical composition increase.
Depending upon its origin, evaporation pond influent may contain contaminates
of various amounts and composition. Decreases in evaporation rates compared to
fresh water rates can seriously increase the failure potential of ponds designed on
fresh water evaporation criteria. Designers of settling ponds and lagoons that rely
on evaporation need to know the probability level of their designs being
exceeded. Confidence limits for published evaporation normals have not been
given, nor have analyses been made of the effects of uncertainty in the estimated
normals or of the temporal variation of net evaporation. Definition of the spatial
and temporal distribution of parameters such as evaporation and precipitation is
difficult in mountainous regions. A concern is that the application of many of the
empirical equations, based on climatological data, for estimating evaporation has
not been thoroughly tested for high-altitude conditions. In particular, the ability
of these equations to define the variability of evaporation is basically unknown.
Historically, pan data are the most common means for defining free water
evaporation. However, the density of evaporation pan stations is much less than
that of weather stations.
Many methods exist for either measuring or estimating evaporative losses from
free water surfaces. Evaporation pans provide one of the simplest, least
expensive, and most widely used methods of estimating evaporative losses. Long-
term pan records are available, providing a potential source of data for
developing probabilities of net evaporation. The use of pan data involves the
application of a coefficient to measured pan readings to estimate evaporation
from a larger water body. Among the most useful methods for estimating
evaporation from free water surfaces are the methods which use climatological
data. Many of these equations exist, most being based directly upon the a method
which was originally intended for open water surfaces, but is now commonly
applied to estimates of vegetative, water use.
Monthly evaporation estimates can be made using the Kohler-Nordenson-Fox
equation with a pan coefficient of 0.7. The Kohler-Nordenson-Fox equation
describes evaporation as the combination of water loss due to radiation heat
energy and the aerodynamic removal of water vapor from a saturated surface.
The general form for the combination equation is:
E = (d/(d+y)) R
n
+ (Y/(d+Y)) E
a
where E is the evaporation in inches per day, d is the slope of the saturation
vapor pressure curve at air temperature in inches of mercury per degree
Fahrenheit, Y is the psychrometric constant in inches of mercury per degree
Fahrenheit, R
n
is the net radiation exchange expressed in equivalent inches of
water evaporated, and E
a
is an empirically derived bulk transfer term of the
form:
E
a
= f(u) (e
s
- e
d
)
Where f(u) is a wind function and (e
s
- e
d
) is the vapor pressure deficit. Kohler-
Nordenson-Fox evaluated the aerodynamic term using pan data resulting in the
form:
E
a
= (0.37 + 0.0041U
p
)(e
s
- e
a
)
where e
a
is in units of inches of water per day, U
p
is the wind speed 2 feet above
the ground expressed in miles per day, and e
s
and e
a
are the saturation vapor
pressures at mean air and mean dew-point temperatures, respectively (expressed
in inches of mercury). For development of the wind function, an adjustment in
the psychrometric constant is generally made to account for the sensible heat
conducted through the sides and bottom of the evaporation pan. One may also
apply as an approximation the following expression for the psychrometric
constant:
Y = 0.000367P
where P is the atmospheric pressure in inches of mercury. My own experience in
designing surface lagoons and evaporation ponds over the years, and
substantiated in the literature, has been to apply a pan coefficient of 0.7.
Of concern is that very little information often is available concerning the effects
of common waste waters on evaporation rates. As noted, the evaporation rate of a
solution will decrease as the solids and chemical concentrations increase.
However, the overall effects on evaporation rates of dissolved constituents as
well as color changes and other factors of wastewater are largely unknown.
Evaporation from surface ponds is usually based upon estimates of annual net
evaporation. Calculation of annual evaporation rates requires estimates during
periods when the surface may be frozen. Most studies related to cold weather
evaporation have been concerned with snow rather than ice. In general, the
evaporation from a snowpack is usually much less than the amount of melting
that occurs. Considering the large percentage of the annual evaporation which
occurs during the warmer months and the overall uncertainties involved in
estimates of evaporation from water surfaces, the amount of evaporation from
frozen ponds during winter can reasonably be neglected in calculating annual
evaporation. A more important consideration is the evaporation which occurs
during winter from ponds which may remain unfrozen because of the
introduction of warm wastewater. In these cases, water temperature will
influence the evaporation rates. However, the low value of the saturation vapor
pressure of the air above any water body will limit evaporation. Annual estimates
of evaporation herein can be made by applying the Kohler-Nordenson-Fox
equation throughout the year. Such estimates should provide near maximum
possible evaporation estimates. For lined ponds, evaporation will be confined
mainly to the water surface area. Evaporation from the soil and vegetation on the
banks surrounding the pond should be minimal. However, for ponds which have
appreciable seepage to the surrounding area, evaporation from this area will be
dependent upon the type and amount of vegetation, as well as the moisture
content of the upper soil layers. Methods foe estimating evaporation and/or
evapotranspiration in these instances are readily available, and you can find some
of these studies and estimating procedures by doing a Web search.
If water losses from the surrounding area are a major component of the total
evaporative losses of the pond, then soil moisture conditions will be expected to
be high. Under nonlimiting soil moisture conditions vegetative moisture losses
are often defined as "potential" losses. Evaporative losses in this case would not
be expected to differ greatly from free water evaporation. The literature
recommends in fact that lake evaporation be used as a measure of potential
evapotranspiration. Thus, for high soil moisture conditions, evaporation rates
calculated for the water surface should be applicable to the surrounding area. The
influence upon evaporation of vegetative growth within a pond is uncertain. The
literature is inconclusive as to whether vegetation will increase or decrease
evaporation compared to an open surface. It appears that the effect may be
somewhat dependent upon the size of the water body. Literature studies indicate
vegetation will decrease evaporation for extensive surfaces with the effect being
less for smaller surface areas. It is very possible, however, that the introduction
of vegetation upon the surface of a water body of more limited extent may
increase its evaporative water loss, but only while the vegetation remains in a
healthy, robust condition. Thus, the effect of the presence of vegetation appears
to range from being a water conservation mechanism to that of increasing
evaporation. In either case, the potential effects appear to be quite large with
reported ratios of vegetative covered to open water evaporation under extreme
conditions ranging from 0.38 to 4.5. In most instances, this ratio would be
expected to be much closer to unity.
Drying techniques based on passive thermal energy are still costly waste
management programs because ultimately there are operating costs associated
with the collection, transportation, and disposal of the dried waste. And in the
end the long-term liabilities still persist, since there are waste materials that
continue forever.
A SHORT REVIEW
Volume reduction techniques help to reduce the costs of disposal, but they are
essentially treatment technologies. Among the range of technologies that are
available, some are less costly in terms of operating costs and capital investments
than others, but all of them contribute to the overall waste management dilemma
of long-term liabilities since there is a final waste form that must be disposed of.
Incineration as a technology is very expensive and carries many hidden costs that
are associated with the required air pollution controls. When used in industrial
applications, the ash is generally hazardous and costly to dispose of. Even in
situations where resource recovery may be possible through a volume reduction
technique, the investment needs to be examined carefully to assess whether there
are long-term financial benefits and reduction of future and long-term liabilities.
RECOMMENDED RESOURCES
1.
Foisy, B. F., L. I. Ramon, "Sewage Sludge Incineration: Meeting Air
Emissions in the Nineties and Beyond," Proceedings of the National Waste
Processing Conference, ASME, 1994.
2.
Modern Pollution Control Technology, Volume II, Research and Education
Association, New York, 1980.
3.
Environment Canada, Utilization of Ash from Incineration of Municipal
Sewage Sludge - Draft Literature Review, Waste water Technology Centre,
November, 1992.
4.
Lue-Hing, C, "Sludge Management Costs Going Up," Resource Recovery,
1989.
5.
Metropolitan Council of Twin Cities Area. Analysis of Sludge Ash for Use in
Asphalt, Concrete, Fertilizer and Other Products. Publication No. 12-82-
103,
October, 1982.
6. Metropolitan Waste Control Commission. Sewage Sludge Ash Use in
Bituminous Paving. Minnesota Department of Transportation, Minnesota
Pollution Control Agency, October, 1990.
7.
California Integrated Waste Management Board. Summary Report:
Appropriate Level of Regulatory Control for Sludge Ash and Contaminated
Soil, April, 1995.
8. Tay, Joo-Hwa. "Bricks Manufactured from Sludge," Journal of
Environmental Engineering, Vol. 113, No. 2, 1987.
9. Micale, F. J., A Mechanism for Ash Assisted Sludge Dewatering. USEPA,
EPA-600/2-76-297, 1976.
10.
Wegman, D. E. and D. S. Young, "Testing and Evaluating Sewage Sludge
Ash in Asphalt Paving Mixtures," Presented at the 67th Annual
Transportation Research Board Meeting, Washington, DC, January, 1988.
11.
Khanbiluardi, R. M. Ash Use from Suffolk County Wastewater Treatment
Plant, Sewer District No. 3. City University of New York, Draft Report,
August, 1994.
12.
Chesner, W., "Waste Glass and Sewage Sludge Frit Use in Asphalt
Pavements," Utilization of Waste Materials in Civil Engineering
Construction. American Society of Civil Engineers, September, 1992.
13.
Braun Intertec Environmental.Sewage Sludge Ash Use in Bituminous Paving,
Report on Additional Testing. Prepared for Metropolitan Waste Control
Commission, 1991.
14.
Braun Intertec Environmental. Sewage Sludge Ash Use in Bituminous
Paving, Report on Additional Testing, Prepared for Metropolitan Waste
Control Commission, 1992.
15.
Gray, D. H. and C. Penessis, "Engineering Properties of Sludge Ash,"
Journal of Water Pollution Control Federation, Vol. 44, No. 5, May, 1972.