method for processing sewage sludge

A method for drying sludge which may contain combustible vapors in a manner to positively prevent fires and explosions within the sludge dryer and the associated sludge-drying piping. The method provides for a substantial reduction of the oxygen content of the sludge-drying vapors by deviating a part of the sludge-drying vapor stream from the principal vapor circuit and passing it as secondary combustion air into the combustion chamber which generates hot gases for drying the sludge. Furthermore, a portion of the hot effluents from the combustion chamber is deviated from the sludge-drying circuit and is used to heat the sludge-drying vapors whereafter it is vented to the atmosphere. In a variant of the invention, an acid scrubber is provided to remove hydrochloric acid. A number of embodiments is presented.

FIELD OF THE INVENTION 
The invention relates to the processing, especially the drying, of sewage 
sludge in which the sludge is subjected to heated gases and is 
subsequently treated in a cyclonic solid precipitator. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
In a known method for sewage sludge treatment, the digested sludge is 
subjected to treatment in an apparatus which includes a combustion 
chamber, a dryer, a cyclonic dust precipitator and a suction blower, all 
connected in series. It is necessary to label the inlet temperatures of 
material admitted to the dryer below approximately 550.degree. C. This 
requires that a substantial amount of cold air is added to the hot gases 
generated in the combustion chamber prior to admission to the dryer. As a 
result, these gases acquire an excess of air leading to an air factor 
.lambda. between 2 and 3, i.e., a relatively high value. Even if the 
outlet temperature of the dried vapors is held at the relatively low 
temperature of 100.degree. C., the thermal efficiency of the drying 
process is relatively low. Furthermore, the high air factor .lambda. means 
that the dryer is operated with gases which have an oxygen content of more 
than 13%. Such gases cause numerous dryer fires and even explosion-like 
combustion within the dryer. Furthermore, the known dryer apparatus 
connected in the manner described above causes emissions of dust and 
foul-smelling substances into the atmosphere which is contrary to the 
goals of environmental protection. The dust contained in the gas could be 
eliminated by placing an air scrubber between the cyclonic precipitator 
and the suction blower but the unpleasant odors are generally not removed 
from the gas by scrubbing. 
Another known method for sludge drying and treatment has a provision for 
returning sludge vapors due to drying to the inlet of the dryer to be 
admixed there with the hot gases coming from the combustion chamber. This 
method results in improved heat economies and a relatively lower air 
excess factor without increasing the temperature of the dryer beyond 
permissible limits. However, the aforementioned emissions of dust and 
odor-causing vapors are not eliminated. Accordingly, the known sludge 
drying apparatus and method can be used only for drying pure mineral 
sludges in which the drying vapors are odor-free. 
OBJECT OF THE INVENTION 
It is thus a principal object of the present invention to provide a method 
for drying sludge in which relatively high heat efficiencies are achieved 
and in which the danger of fires or explosions within the dryer is largely 
eliminated. It is an associated object of the invention to provide a 
method for drying sludge which results in gaseous emissions that are 
substantially free of dust and odiferous components. 
Briefly, the sludge is dried and the solids contained in the drying vapors 
are separated by solid precipitators. Subsequently, a vapor blower causes 
a continuous movement or circulation of the drying vapors in a closed 
circuit which is so constructed that the vapor stream ahead of the dryer 
is subdivided into a first and second vapor stream. The first vapor stream 
is split off from the vapor circuit ahead of the dryer and is applied as 
secondary air into the combustion chamber while the second vapor stream 
remaining in the circuit is admixed with a portion of the hot gases 
generated in the combustion chamber. The latter step heats the vapors to 
the required drying temperature after which they are admitted to the 
dryer. A further feature of the invention is that the remainder of the hot 
gases generated in the combustion chamber is passed through a heat 
exchanger and is cooled by thermal contact with a medium serving to dry 
the sludge whereafter it is exhausted into the atmosphere. 
The method according to the invention provides that the air excess factor 
.lambda. within the main circuit of the drying vapors is only 
approximately 1.3-1.5 and the exhaust temperature only approximately 
150.degree. C., leading to substantially improved thermal economy. 
Due to the low air excess .lambda. the vapors within the dryer circuit have 
an oxygen content of only between 5-6% and are thus incapable of 
sustaining combustion which would require an oxygen content of at least 
8%. Accordingly, fires and explosion-like combustion within the dryer are 
positively eliminated. The volume of vapor circulating within the closed 
circuit remains independent of the total load. Furthermore, the entire air 
flow may be adjusted by two halves controlling the first and/or second 
partial vapor streams in the low temperature domain. The ratio of 
circulating vapor to exhaust gas is approximately 2.5:1 so that the 
maximum temperature attained by the metallic parts of the heat exchanger 
is only approximately 500.degree.-550.degree. C. and does not result in 
excessive thermal stresses and attendant high temperature corrosion. 
Furthermore, all drying vapors which are vented to the atmosphere are 
thermally deodorized so that malodorous emissions no longer occur. 
Still further characteristics and advantages of the sludge drying method 
according to the invention will become evident from a detailed description 
of a number of preferred exemplary embodiments which are to be considered 
in conjunction with the drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a first embodiment of the invention illustrated in FIG. 1, sewage sludge 
to be dried flows in the direction of the arrow A1 into a dryer embodied 
preferably as a grinding or pulverizing type dryer 1. The drying vapors 
generated in the dryer 1 are carried in a closed circuit through a 
cyclonic solid separator 2, a filter 3, a vapor blower 4 and a heat 
exchanger 5, all of which are connected in a serial stream and in which 
the material is transported continuously as illustrated by the heavy line 
C of FIG. 1. The solid materials separated in the cyclonic separator 2 are 
carried away for subsequent processing as indicated by the arrow A2. A 
combustion chamber 6 which is supplied with fuel and combustion air, in a 
manner not shown but indicated schematically by two arrows AIR & FUEL, 
generates a hot effluent gas which is used for sludge drying and is 
admitted for that purpose to the dryer 1 at point 1a. The drying vapor 
stream circulating in the circuit C is subdivided at a point 5a, i.e., 
downstream of the heat exchanger 5 but ahead of the grinding dryer 1 so as 
to constitute two partial streams P1 and P2. The first partial vapor 
stream P1 is taken from the main circuit C and is used to supply secondary 
air into the combustion chamber 6. The remaining, i.e., second, partial 
vapor stream P2 is retained within the circuit C but is admixed at the 
point 1a ahead of the dryer 1 with a portion of the hot gases generated in 
the combustion chamber 6 and is thereby heated to the temperature required 
for drying the sludge, whereafter it is admitted to the dryer 1. The 
remaining hot gases generated in the combustion chamber 6 are fed to the 
heat exchanger 5 where they flow counter to and reheat the drying vapors 
of the main circuit C, whereafter they are pulled by the suction blower 7 
to be vented to the atmosphere. 
The adjustment of the flow in the circuit and its branches takes place by 
means of two air flaps D1 and D2, associated, respectively, with the first 
and second partial vapor streams P1 and P2 and disposed within the 
low-temperature region of the apparatus. The filter 3, indicated in FIG. 1 
by dashed lines and serving to separate very fine dust from the drying 
vapors, may be placed upstream of the suction blower 7 if the dried sewage 
sludge is to be used as fuel in the combustion chamber 6, for, in that 
case, the exhaust gas retains flying ash due to the powdered fueling of 
the combustion chamber 6 (compare FIG. 3). Similarly, the heat exchanger 5 
indicated in dashed lines in FIG. 1, which is generally used for 
recovering exhaust gas heat, can also be placed in a different part of the 
circuit if it is intended to combust the dried sludge. The maximum 
temperature of the sludge at the inlet side of the dryer 1 is 
approximately 550.degree. C. 
A second embodiment of the invention is illustrated by the flow diagram of 
FIG. 2 for drying sludge produced by a communal sewage sludge system with 
sludge digestion. In this sewage system, the fermentation gas produced 
during the digestion of the raw sewage in the system, is used for drying 
the previously dehydrated digested sludge. The fermentation gas is 
combusted in the combustion chamber 6 to produce hot gases while the 
exhaust gas heat is recovered by using the drying exhaust gases from the 
combustion chamber to generate hot water which is used in turn to heat the 
digestion chambers of the sewage treatment plant, i.e., it is used for 
heating the raw sewage in the sewage plant. 
The fermentation gas is fed to the burner 9 of the combustion chamber 6 by 
a pump, not shown, where it is admixed with combustion air delivered by a 
blower 8. At a point 5a of the circuit C, a first vapor stream P1 is 
diverted and applied as secondary combustion air to the combustion chamber 
6. Some parts E1 of the hot gases generated in the combustion chamber 6 
are carried to a mixing chamber 1a within the circuit C where they are 
mixed with the remaining partial vapor stream P2 which is thereby heated 
to the required drying temperature of approximately 550.degree. C. 
maximum. The remainder E2 of the hot gases, may if required, be mixed with 
cool air supplied by a cool air blower in a low temperature mixing chamber 
6m, the cooled effluent of which is passed to an exhaust conduit 11 that 
is ultimately vented to the atmosphere by a chimney 12 in a manner still 
to be described in detail. 
The principal drying circuit C contains the previously described grinding 
dryer 1, followed by the solid separator 2, the filter 3 for removing fine 
dust, the vapor transport blower 4, the heat exchanger 5 for reheating the 
vapor and the mixing chamber 1a upstream of the dryer 1 which serves to 
mix a part of the hot gases generated with the drying vapors within the 
circuit C. All of the aforementioned apparatus is connected in process 
series. The prehydrated digested sludge is admitted as indicated by arrow 
A1 by a sludge pump 13 or some other suitable metering device to the 
grinding dryer 1, the rate of sludge transport being regulated on the 
basis of a temperature sensor t disposed downstream of the dryer 1. Within 
the dryer 1, the sludge travels in the same direction as the vapors heated 
by the hot burner gases to a temperature of approximately 550.degree. 
maximum and is thereby dried. The solid material removed from the drying 
vapors by the cyclonic separator 2 and the filter 3 is removed from the 
circuit C in the sense of the arrows A2 and A3 of FIG. 2 and is 
transported to a dry goods silo 14 from which it may be removed by 
railroad cars or trucks to serve as a top soil component for various 
purposes of agriculture. The flow rates of the partial vapor streams are 
adjusted by means of valves D1 and D2 disposed respectively in the main 
circuit C and the subsidiary circuit which carries the stream P1. The 
aforementioned exhaust gas conduit 11 will now be explained in further 
detail. 
Due to the fact that the exhaust gases from the combustion chamber 6 have a 
relatively high steam content of between 50-60% (corresponding to a 
saturation temperature of approximately 80.degree. C.) and because they 
are practically dust-free due to the employment of the gas burner 9, the 
exhaust gas heat may be readily recovered in a condenser 16 which serves 
as a hot water heater which supplies hot water for heating the raw sewage 
sludge in the communal sewage plant, for example by being used to heat the 
digestion chamber. The flow of hot water from the condenser 16 to the 
digestion chamber of the sewage plant is indicated by the arrow 16a and 
the cold water return is indicated by the arrow 16b while the arrow 16c 
indicates the flow from an overflow outlet. 
The exhaust gases from the combustion chamber 6 are carried through the 
heat exchanger 5 where they reheat the circulating drying vapors within 
the circuit C in the sense of the arrow Q pointing to the left of FIG. 2 
as well as for reheating the exhaust gases cooled in the condenser 16 as 
indicated by the arrow Q pointing to the right of FIG. 2. The flow of both 
of the heat-absorbing streams is in the opposite direction to the flow of 
the hot exhaust gases. 
The reheating of the gases prior to entry in the chimney 12 prevents the 
generation of a customary cloud of steam emerging from the chimney 12 
which results in an undesirable condensation sometimes called "chimney 
rain." The exhaust gases emerging from the heat exchanger 5 are passed 
through the aforementioned condenser 16 where the steam contents of these 
gases are condensed. Subsequently, they are used to reheat the effluent 
from the condenser 16 in the manner mentioned previously whereafter they 
are exhausted through the chimney 12 by the blower 7. 
FIG. 3 is a flow diagram of a sludge-drying apparatus for drying and 
combusting communal sewage sludge. Elements of this system identical with 
those of the embodiment of FIG. 2 have the same reference numerals. The 
powered solids separated by the dust separator 2 are used to generate the 
hot gases employed for sludge drying. In this way, the combustion chamber 
6 of FIG. 2 which was fixed with fermentation gases is here replaced by a 
combustion chamber 60 fired with dried sludge powder. However, an external 
fuel, for example fuel oil or gas, may also be used in the start-up of the 
sludge drying plant. The overall configuration of components in the main 
sludge drying circuit C is similar or identical to that previously 
described. However, when sludge solids are used for combustion, the dust 
filter 3 is placed in the exhaust stream 11 as indicated in FIG. 3 to 
remove flying ash from the exhaust gas. The separated solids are 
transported by a conveyor screw 17 to the powder burner 90 of the 
combustion chamber 60 which also receives combustion air from a blower 8. 
The combustion air is preheated in the heat exchanger 5 by receiving heat 
from the exhaust gases of the burner 16 by heat transported in the sense 
of the arrow Q pointing to the right of FIG. 3. The combustion chamber 60 
is illustrated as being disposed vertically and the flame of the powdered 
fuel burner 90 enters the combustion chamber 60 from the top. Ash is 
removed at the base of the combustion chamber 60. The drying vapor 
circulating in the main circuit C is subdivided at the point 5a behind the 
heat exchanger 5 into two partial vapor streams P1 and P2, the vapor 
stream P1 being diverted from the main circuit C and used as secondary 
combustion air which is applied to the powdered fuel burner 90. 
Some of the hot gases generated in the combustion chamber 60 are fed to the 
mixing chamber 1a where they are mixed with the remaining vapor stream P2 
to heat the latter prior to admission to the grinding dryer 1. The 
remaining hot gases are fed to a mixing chamber 60m where they are mixed 
with air supplied by the cold air blower 10 causing them to be cooled off, 
whereafter they are exhausted by the suction blower 7 through the chimney 
12 after passing through the heat exchanger 5 and the ash filter 3. The 
sewage sludge to be dried is introduced into the grinding dryer 1 by the 
sludge pump 13 in the sense of the arrow A1 and the flow rate of the 
admitted sludge is controllable on the basis of signals from a temperature 
sensor t which gauges the prevailing vapor temperature. The powdered 
solids separated from the stream by the dry particle separator 2 are 
transported to the silo 14 in the sense of the arrow A2. The flow rate in 
the conduits is adjustable by means of two valves D1 and D2 which control 
the magnitude of the two partial streams P1 and P2 in the low-temperature 
region of the sludge drying system. The settings of these valves may be 
continuously controlled on the basis of the temperature of the system. 
The exhaust gases from the drying plant are thermally deodorized in the 
combustion chamber 60, thereby positively preventing any malodorous 
emissions. Furthermore, the entire plant is operated at sub-atmospheric 
pressure so that none of the gaseous contents and any possible attendant 
odors could emerge from the plant due to leakage. The temperature sensor t 
and the associated temperature control system for the combustion chamber 
60 insures that the minimum temperature of 800.degree. C. for deodorizing 
the gases is maintained at all times. In spite of the relatively low 
excess air factor .lambda. of between 1.3 and 1.5, the temperature of 
800.degree.-900.degree. C. required for thermal deodorizing may be 
attained due to the recycling of drying vapors into the combustion chamber 
60. At that temperature level, any odor-causing components are oxidized so 
that the gases emerging from the combustion chamber are completely 
odor-free. 
The sludge drying and combustion plant illustrated schematically in FIG. 3 
is very flexible and can be adapted to a number of uses. Depending on the 
requirements, the separated solid material, instead of being used as fuel 
for the burner 90, may also be removed for various agricultural purposes 
and replaced in the required amounts by some external fuel, for example 
fuel oil or gas, which is then admitted to the burner 90 by a suitable 
fuel pump 18. 
The construction of the powdered fuel burner 90 is substantially equivalent 
to the well-known and proven design of the powdered coal burners known in 
the art and consisting substantially of two telescoped coaxial tubes. The 
inner tube of the burner carries the primary combustion air and the 
powdered dried sludge which serves as fuel while the external tube carries 
the secondary air which comes from the first partial vapor stream that is 
deviated from the main circuit C. The detailed construction of the 
powdered fuel burner 90 has been omitted from FIG. 3 to increase the 
overall clarity of the figure. It may be suitable to impart a vortex flow 
to either or both of the primary and secondary air streams by means of 
stationary air guide vanes. Due to a very high contact of volatile fuel 
components of approximately 50% at 500.degree. C., the powdered sludge 
burns substantially more readily than ordinary coal dust. A flame monitor 
device tf is provided for emergencies to control both of the vapor stream 
valves D1 and D2. The system according to FIG. 3 also has the advantageous 
thermal conditions already described for the system of FIG. 2. In 
particular, the system has a low exhaust gas temperature of approximately 
200.degree. C. and is operated with a relatively low air excess factor 
.lambda. of between 1.3-1.5. Furthermore, the plant is well-insulated 
thermally and the powdered sludge is combusted fully, the glow losses 
being below 3-5%. Condensation and the attendant corrosion may be 
prevented by careful thermal insulation of the principal vapor circuit C. 
FIG. 3a illustrates a variant of the flow diagram of FIG. 3. The elements 
of the apparatus which perform identical functions as they do in FIG. 3 
retain the same reference numerals. FIG. 3a also illustrates an apparatus 
and a process for drying and combusting communal sewage sludge for the 
situation in which the raw sludge has a high percentage of dry materials 
so that a considerably greater amount of dried solids becomes available 
during the drying process than is required to generate the hot gases in 
the combustion chamber. The excess heat developed during the combustion of 
the dried sludge may thus be used to generate steam or hot water in a 
suitable boiler. For this purpose, the combustion chamber, whose principal 
purpose is the generation of hot gases for sludge drying, can be 
integrated with the steam or hot-water system. 
As was in the case in FIG. 3, the drying vapor circuit C in FIG. 3a 
includes a grinding dryer 1, a solid separator 2, the principal 
transporting blower 4 and a mixture chamber 1a for mixing the hot gas 
components used for sludge drying. The heat exchanger 5 of FIG. 3 which 
was used for recovering exhaust heat is replaced in the variant of FIG. 3a 
by the above-mentioned boiler. 
In the sludge drying method illustrated by the flow diagram of FIG. 3a, the 
powdered dried material separated from the sludge vapors by the separator 
2 is used for generating hot gases for sludge drying. The portions of the 
hot gases used for drying are fed to the mixing chamber 1a where they are 
combined with the second vapor stream P2 in the circuit C which is thus 
heated to the required drying temperature. The combustion chamber 60a is 
now part of a steam or hot water boiler 19 which is also provided with a 
heat exchanger 5a which serves as an air preheater for preheating the 
combustion air supplied by the combustion air blower 8. The dried 
combustible sludge is transported from the silo 14 by an air blower 20 
which blows the air carrying the powdered fuel through a pipe 21 to the 
powdered fuel burner 90a. The boiler water is supplied to the boiler 19 in 
accordance with the arrow 22 and the steam or hot water generated in the 
boiler is removed therefrom in accordance with the arrow 23. The 
combustion exhaust gases are removed from the combustion chamber 60a by 
means of a suction fan 7 which vents them into the open air. 
FIG. 4 is a partial flow diagram of a sludge drying system associated with 
the sewage system of a chemical plant. Portions of this flow diagram which 
are identical to that of FIG. 2 have been omitted for clarity. The 
principal difference of the variant of FIG. 4 with respect to that of FIG. 
2 is the inclusion of an acid scrubber downstream of the heat exchanger 
within the exhaust gas line. The acid scrubber has the task of scrubbing 
the exhaust gas free of hydrochloric acid which is generated when chemical 
solvents originating in the sewage system of the plant are combusted. 
Such solvents, which may contain up to 10% chlorine, are transported to the 
burner 900 by means of a burner fuel pump 19 and are combusted at an 
excess air factor .lambda. of 1.3-1.5. The combustion air which is 
admitted by the combustion air blower 8 into the combustion chamber 600 
consists of oxygen-enriched, odiferous exhaust air originating from 
biological oxygenating processes of the plant sewage system as well as the 
exhaust air of local septic tanks. They may include any other air 
components which have a variety of odors and originate in various parts of 
the sewage system of the plant. These air streams may contain a percentage 
of solvent vapors. The sludge enters the dryer 1 and is dried therein by 
traveling together with the drying vapors which are heated to a 
temperature of approximately 550.degree. C. maximum. The principal 
transport blower 4 which maintains the vapor flow within the circuit C 
forces the drying vapors into the heat exchanger 5 at a temperature of 
approximately 130.degree.-140.degree. C. and they are heated in the heat 
exchanger 5 to approximately 330.degree. C. by heat derived from the 
exhaust gas of the drying system. 
Located downstream of the heat exchanger 5 in the exhaust gas stream 11 is 
an acid scrubber 160 which serves to remove from the exhaust gas any 
hydrochloric acid (HCl) which is generated during the combustion of 
chlorine-containing solvents. When the exhaust gases are scrubbed in the 
acid scrubber 160, they are cooled and are then reheated to approximately 
150.degree. C. by heat derived from the unscrubbed exhaust gas within the 
heat exchanger 5. Due to the possibility of severe corrosion in this part 
of the apparatus, the heat exchanger 5 is preferably constructed of glass 
pipes. The reheating of the exhaust gases prior to passage into the 
chimney prevents the occurrence of a cloud of steam and the associated 
chimney precipitation or "chimney rain". 
In order to prevent high-temperature corrosion which occurs at metal 
temperatures above 450.degree.-500.degree. C. in the presence of 
hydrochloric acid, cool air is pumped by the cold air blower 10 to be 
mixed with the combustion chamber exhaust gases, thereby lowering their 
temperature from approximately 800.degree. C. to approximately 600.degree. 
C. Low-temperature corrosion cannot take place unless the hydrochloric 
acid has condensed. Such condensation may be prevented by adjusting the 
vapor temperature ahead of the heat exchanger 5 to approximately 
140.degree. C. and by installing efficient thermal insulation. 
The overall construction of the apparatus illustrated in FIG. 4 thus 
includes a drying circuit C which receives the heat required for sludge 
drying from the combustion chamber 600 via the heat exchanger 5. 
Conversely, the circuit C supplies excess drying vapors to the combustion 
chamber 600 at the point 5a in the form of the partial vapor stream P1. 
The apparatus of FIG. 4 further includes the hot exhaust gas conduit 11 
which includes the heat exchanger 5, the acid scrubber 160, the suction 
blower 7 and the chimney 12. 
Because the air excess factor .lambda. lies between 1.3-1.5, the gas in the 
dryer circuit C has an oxygen content of only 5-7%. Under such conditions, 
the dryer gas may be considered inert from the standpoint of combustion 
because gas having an oxygen content below 8-10% is incapable of being 
ignited. These conditions make it possible to dry sludges that still 
contain volatile solvents without causing fires or explosions. The use of 
a practically inert dryer gas within the principal vapor circuit C 
completely prevents the possibility of fires and explosions, especially 
within the dryer 1, and this property is of decisive significance in 
sewage treatment systems that serve the chemical industry and which often 
contain substantial amounts of volatile and flammable solvents. 
The maximum operating temperature of the dryer 1 is approximately 
500.degree.-550.degree. C. The maximum temperature of the walls of the 
heat exchanger 5 is below 500.degree. C. Accordingly, the operational 
safety of the plant is insured at all times even if the gases contain 
hydrochloric acid vapors. 
The exhaust gases of the sludge drying plant are thermally deodorized 
within the combustion chamber 600, thereby positively eliminating any 
odors from gases vented to the atmosphere Furthermore, the entire system 
is operated as subatmospheric pressure, thereby preventing the efflux of 
gases and thus the escape of vapors or odors at any part of the system. 
The plant according to the invention also satisifes the most stringent 
rules regarding powdered or dust effluents. The tubular filter 3 (compare 
FIG. 2) cleans the circulating vapors from any dust down to a 
concentration of 50 mg/Nm.sup.3. Any dust which is carried by the 
secondary air stream into the combustion chamber 600 is burned up so that 
the ash content of the exhaust gases is substantially below a value of 25 
mg/Nm.sup.3. 
The gas transport characteristics of the vapor blower 4 are such as to 
maintain a stable gas flow rate within the principal vapor circuit C. 
The load adjustment takes place by means of suitable control of the valves 
D1 and D2 which control the flow of the partial vapor streams P1, P2, 
respectively, in the low-temperature domain while a vapor temperature 
control maintains any previously selected load condition in a reliable 
manner. A temperature monitor in the combustion chamber 600 serves to 
insure that a minimum temperature of 800.degree. C. is maintained within 
the combustion chamber to guarantee complete deodorization of combustion 
chamber exhaust gases. 
The foregoing description relates to preferred exemplary embodiments of the 
invention. Features of one embodiment may be used with those of any other 
and various modifications are possible within the compentence of the 
person skilled in the art without departing from the scope of the 
invention.