Process for treating a waste sludge of biological solids

A process for treating a waste sludge of biological solids including the steps of blending the sludge with an acid, mixing an oxide-chemical with the blended sludge so as to cause a reaction which elevates a temperature of the sludge, pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a. and retaining the mixed sludge at such pressure for a period of time of no less than 15 seconds, and discharging the pressurized mixed sludge. The step of pressurizing is accomplished by passing the mixed sludge as a flow into the pipe. The pipe has a length and diameter such that the flow of the mixed sludge takes longer than 15 seconds to pass through the pipe. The mixed sludge is heated as the flow passes through the pipe. An immersion heater or a magnetic field is installed so as to apply heat directly to the mixed sludge as the mixed sludge passes through the pipe.

TECHNICAL FIELD 
The present invention relates to processes for the treatment of waste 
sludges. More particularly, the present invention relates to processes 
that render biological waste sludges pathogen-free, vector-free and 
sellable. 
BACKGROUND ART 
In the past, various techniques have been developed for the purpose of 
sterilizing or decontaminating biological sludges and wastes. The most 
common process is the process of mixing lime with the sludge. The reaction 
of lime with the water in the sludge serves to elevate the temperature of 
the sludge to a maximum of 100.degree. C. 
In the past, various U.S. patents have issued relating to processes for the 
decontamination and treatment of wastewater sludges. For example, U.S. 
Pat. No. 4,038,180, issued on Jul. 26, 1977 to N. K. Talbert, describes a 
process of dewatering sewage sludge in which the sludge from a municipal 
or industrial sewage treatment facility is mixed with a mineral acid or 
anhydride thereof to release the entrapped water in the sludge. The 
resulting mixture of the sludge solids and diluted acid or anhydride is 
then mixed with a basic material, such as ammonia, such that the heat 
generated by the reaction of the base and the acid evaporates the water to 
form either a completely dry mixture of sludge solids and a salt or a 
mixture having a predetermined moisture content which may be air dried. 
U.S. Pat. No. 4,500,428, issued on Feb. 19, 1985 to Lynch et al., describes 
a method for the treatment of a wastewater sludge using a pair of reaction 
vessels, sequentially, to treat the sludge. Both of the vessels are 
pressurized. The first vessel has an aerator for aerating the sludge. This 
vessel receives sulfuric acid and chlorine therein through a port. A 
dewatering device is provided upstream of the first vessel. The outlet of 
the first vessel is coupled to an inlet of the second vessel through 
another dewatering device. The second vessel creates a final-treatment 
chamber in which the sludge is exposed to ozone, air and lime. 
U.S. Pat. Nos. 4,781,842 and 4,902,431, issued to Nicholson, teach 
processes for the decontaminating of wastewater sludges to a level which 
meets or exceeds U.S. E.P.A. process standards. The process mixes sludge 
with an alkaline material sufficient to raise the pH of the end product to 
12 or higher for at least one day. This process will raise the temperature 
to 50.degree. C., but will not sterilize the sludge nor does it eliminate 
the pathogenic organisms. 
U.S. Pat. No. 4,306,978, issued to Wurtz, relates to a process of lime 
stabilization of wastewater treatment plant sludge. This patent discloses 
the dewatering of the sludge and intimately mixing calcium oxide to raise 
the temperature so as to produce a stabilized sludge particle. 
U.S. Pat. No. 5,482,528, issued on Jan. 9, 1996 to Angell et al., teaches a 
pathogenic waste treatment process for the processing of solid waste and 
for the converting of such solid waste into useful products. This is 
accomplished by combining the waste with an acid, such as concentrated 
sulfuric acid, and a base, such as fly ash. These exothermically react and 
thermally pasteurize the waste and add mineral value to the product. 
Pozzolanic materials, such as fly ash, agglomerate the product. The 
calcium oxide in the fly ash reacts with sulfuric acid to form calcium 
sulfate dihydrate. 
None of these prior art patented processes are capable of achieving 
temperatures, when mixed with the sludge, of greater than 100.degree. C. 
None of the prior art techniques allow for the shorter drying times as 
required by 40 C.F.R. Subchapter O, Part 503.32. 
U.S. Pat. No. 5,635,069 issued on Jun. 3, 1997 to the present inventors. 
This patent described a process for treating a waste sludge of biological 
solids which included the steps of mixing the sludge with an 
oxide-containing chemical and sulfamic acid so as to elevate the 
temperature of the sludge, pressurizing the mixed sludge to a pressure of 
greater than 14.7 p.s.i.a., and discharging the pressurized mixed sludge. 
The oxide-containing chemical could be either calcium oxide, potassium 
oxide, or potassium hydroxide. The sludge has a water content of between 5 
and 85 percent. The oxide-containing chemical and the acid are reacted 
with the sludge so as to elevate the temperature of the sludge to between 
50.degree. C. and 450.degree. C. The pressurized mixed sludge is flashed 
across a restricting orifice or passed into a chamber having a lower 
pressure. The evaporated liquid component can be condensed and used as 
part of the process or external of the process. 
Experiments with the process of this prior art patent have disclosed that 
this process is extremely effective in the treatment of waste sludges. 
However, certain improvements were found possible with the process of U.S. 
Pat. No. 5,635,069 which renders the process more economic and more 
assuredly pathogen-free. In particular circumstances, it was found that 
the cost of the oxide-containing chemical could be replaced, in certain 
environments, by applying heat directly to the pressurized sludge. In 
other circumstances, if heat is applied directly to the pressurized mixed 
sludge, then the sulfamic acid could be replaced by less expensive 
chemicals, such as carbon dioxide. In certain circumstances, it was found 
that the use of electricity for the heating of the pressurized sludge was 
less than the cost of certain chemicals used to elevate the temperature of 
the sludge. 
It is important to note that in existing processes for the treating of 
waste sludges, it is common to use a conveyor belt onto which the 
dewatered sludges are placed. This conveyor belt will pass the dewatered 
sludges under infrared radiation so as to effectively heat the sludge. 
Unfortunately, the use of such radiant energy for the heating and pathogen 
destruction of the waste sludge is extremely unefficient. In other cases, 
the application of such infrared energy to the waste sludge will cause 
serious odor and toxicity problems. It was found that the use of such 
infrared radiation for the treating of pathogens in the waste sludge had 
an energy efficiency of only 18 percent with respect to the amount of heat 
that could be applied to the waste sludges. As such, a need developed for 
the cost-effective and efficient pathogen-destruction and heating of the 
waste sludge so as to conform with E.P.A. requirements. 
It is an object of the present invention to provide a process for rendering 
a biological waste sludge pathogen-free and vector-free. 
It is another object of the present invention to provide a process that 
converts the biological waste sludge into a sellable end product. 
It is a further object of the present invention to provide a process that 
eliminates or reduces waste incineration and landfilling of waste sludges. 
It is another object of the present invention to provide a process which 
reduces the odor emitted during the heating of the waste sludge. 
It is still a further object of the present invention to provide a process 
that efficiently utilizes energy for the heating of the waste sludge. 
It is a further object of the present invention to provide a process that 
is adaptable for optimizing the cost of chemicals used for the treatment 
of the waste sludge. 
It is still a further object of the present invention to provide a process 
for the treating of waste sludges that is cost effective, easy to use, and 
easy to install. 
These and other objects and advantages of the present invention will become 
apparent from a reading of the attached specification and appended claims. 
SUMMARY OF THE INVENTION 
The present invention is a process for treating waste sludge of biological 
solids which comprises the steps of: (1) blending the sludge with an acid; 
(2) mixing an oxide-containing chemical with the blended sludge so as to 
cause a reaction which elevates a temperature of the sludge; (3) 
pressurizing the mixed sludge to a pressure of greater than 14.7 p.s.i.a. 
for a period of time of no less than 15 seconds; and (4) discharging the 
pressurized mixed sludge. In the present invention, the sludge has a 
solids content of greater than 7 percent. The oxide-containing chemical is 
selected from the group consisting of calcium hydroxide, sodium hydroxide, 
potassium hydroxide, lithium hydroxide, calcium oxide, sodium oxide, 
potassium oxide and lithium oxide. 
In the present invention, it is possible that the steps of blending and 
mixing occur simultaneously. In the preferred embodiment of the present 
invention, the acid is sulfamic acid. However, with the proper heating of 
the waste sludge during the step of pressurizing, it is possible that 
carbonic acid could be the acid which is used. Such a carbonic acid would 
be produced by the reaction of carbon dioxide with the waste sludge. 
The sludge is dewatered prior to the step of blending. This dewatering 
causes the sludge to have a water content of less than 93 percent. 
Ideally, the mixed sludge is pressurized to a pressure of greater than 
20.7 p.s.i.a. 
The present invention pressurizes the mixed sludge by passing the mixed 
sludge as a flow into a pipe. The pipe maintains the mixed sludge at the 
pressure of greater than 14.7 p.s.i.a.. The pipe has a length and diameter 
such that the flow of the mixed sludge takes longer than 15 seconds to 
pass through the pipe. The mixed sludge is heated as the flow passes 
through the pipe. The heating of the mixed sludge can be accomplished in 
two different ways. Preferably, an immersion heater is installed into the 
interior of the pipe. The immersion heater applies heat directly to the 
mixed sludge. Alternatively, a magnetic field can be applied around the 
pipe as the mixed sludge flows through the pipe. The magnetic field can be 
tuned to a frequency which causes a desired elevation of the temperature 
of the mixed sludge. 
In the present invention, the step of discharging includes the steps of: 
(1) flashing the pressurized mixed sludge across a restricting orifice; 
and (2) evaporating a liquid component of the sludge.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is shown at 1 the process for the treatment of a 
waste sludge of biological solids in accordance with the preferred 
embodiment of the present invention. In the process 1 of the present 
invention, the sludge 1a and the acid 2 are delivered together into a feed 
hopper 4. Within the concept of the present invention, the sludge 1a is a 
waste sludge, including sewage sludges and animal feces. The sludge 1a 
will have a solids content of greater than 7 percent or a water content of 
less than 93 percent. It is important for the sludge 1a to have a water 
content such that the remaining chemicals introduced to the process can 
properly react with the sludge. If the sludge 1a has a water content of 
greater than 93 percent, then it should be suitably dewatered prior to 
introduction into the process 1 of the present invention. Such dewatering 
can occur in a conventional manner by using belt presses or centrifuges. 
It is only necessary for the dewatering process to reduce the water 
content to below 93 percent. 
Within the present invention, the preferred acid 2 is sulfamic acid. 
Sulfamic acid is otherwise known as amidosulfonic acid (H.sub.3 NO.sub.3 
S). Sulfamic acid is obtained from chlorosulfonic acid and ammonia or by 
treating urea with H.sub.2 SO.sub.4. Typically, sulfamic acid is otherwise 
used in acid cleaning, in nitrite removal, and in chlorine stabilization 
for use in swimming pools, colling towers, and paper mills. 
Importantly, within the concept of the present invention, the acid 2 which 
is used is not limited to sulfamic acid. Various other acids could 
possibly be used provided a suitable amount of heat could be imparted to 
the sludge as it passes a later point in the process of the present 
invention. For example, carbon dioxide could be substituted for the 
sulfamic acid. The carbon dioxide would form carbonic acid when reacted 
with the waste sludge. Although experiments have shown that such carbonic 
acid would not optimally work in the process of the present invention, it 
would be possible to use such carbonic acid, or other acids, so as to 
accomplish the purposes of the present invention. 
After the sludge 1a and the acid 2 are added together into the feed hopper 
4, the mixture is auger fed into the feed section 7 of a screw conveyor 6. 
The screw conveyor 6 will rotate so as to transport the mixture of the 
sludge 1a and the acid 2 to a feed section. During the transport of the 
mixture of the sludge 1a and the acid 2, these materials are mixed 
together by the screw conveyor. 
At the feed section 8, an oxide-containing chemical 3 is added to the feed 
section. In particular, the oxide-containing chemical 3 is delivered into 
the feed hopper 5 and auger delivered to the feed section 8. As used in 
the present invention, the oxide-containing chemical 3 can be either 
calcium hydroxide, sodium hydroxide, potassium hydroxide, lithium 
hydroxide, calcium oxide, sodium oxide, potassium oxide and lithium oxide. 
In the preferred embodiment of the present invention, the oxide-containing 
chemical could be calcium oxide. Other ingredients can be added to the 
feed section 8, if desired. These other ingredients could be passed along 
with the oxide-containing chemical 3 or otherwise delivered into the feed 
section. These materials are then transported to the compression zone 9 of 
the screw conveyor 6. This compression zone 9 serves to increase the 
pressure of the mixed sludge to the desired value. Specifically, the 
compression zone 9 should increase the pressure of the mixed sludge to a 
pressure of greater than 14.7 p.s.i.a.. Experimentation has found that the 
desired effects of the present invention are achieved by pressurizing the 
mixed sludge to a pressure of between 14.7 p.s.i.a. and 120 p.s.i.a.. 
Importantly, the preferred pressure is greater than 20.7 p.s.i.a.. At such 
pressures, water is retained in the mixture and is not flashed from the 
system. When the water is flashed by pressures of less than 20.7 p.s.i.a., 
there is a loss of heat of approximately 1,000 BTU per pound of water. As 
such, to preserve the optimal heating effects in the process of the 
present invention, it would be desirable to maintain the pressure on the 
mixture to a level which would prevent the flashing of the water. 
Furthermore, the higher pressure keeps any ammonia (NH.sub.3) from 
flashing and retains the ammonia for intimate mixing with the pathogens of 
the waste sludge. The ammonia byproduct produced from the process of the 
present invention is an effective chemical for the killing of pathogens in 
the sludge. 
The adding of the oxide-containing chemical to the mixture and the 
increasing of pressure through the motive force of the screw conveyor 6 
causes an exothermic reaction along the reaction section 11. The 
combination of calcium oxide and the water within the waste sludge 
produces calcium hydroxide and liberates 235 kcal/mole of heat. This 
raises the temperature from ambient to 100.degree. C. in 0.5 seconds. The 
sulfamic acid 2 then reacts with the calcium hydroxide to form calcium 
salts. This raises the temperature from 100.degree. C. to 140.degree. C. 
in less than 1 second. 
In the present invention, the oxide-containing chemical can be produced 
from any source, such as kiln dust or lime dust. The oxide-containing 
chemical will make up between 5 percent and 50 percent of the waste sludge 
1a by weight. The acid that is added, in any form, whereby the weight 
ratio of acid to the oxide-containing chemical is between 0.33:1 and 1:1. 
In general, the temperature of the reaction chamber 11 will be between 
50.degree. C. and 450.degree. C. 
The material which exits the screw conveyor 6 enters the reaction chamber 
11 having insulation 10 extending therearound. This reaction chamber 11 
can contain static mixing elements. The material is continuously mixed as 
it progresses through the predetermined length of the pipe. The material 
is continuously under pressure within the pipe 11 so as to prevent a 
premature flashing of the water within the mixed sludge. The mixed sludge 
will pass as a flow through the length of the pipe 11. The pipe 11 should 
be sized so as to have a length and diameter such that the flow of the 
mixed sludge will continue through the pipe 11 for a period of no less 
than 15 seconds. The intimate mixing of the ammonia with the pathogens of 
the mixed sludge at such an elevated temperature and under such an 
elevated pressure will effectively destroy any pathogens or vectors which 
would occur within the mixed sludge. The intimate contact of the sludge 
with the ammonia provides great disinfecting action to the waste sludge. 
The pressure within the pipe 11 will prevent the ammonia from flashing. 
Experiments with the present invention have shown that it will reduce 
pathogens from 2.2 million colonies per gram to less than 10 colonies per 
gram. 
Importantly, in the present invention, an immersion heater 20 is installed 
into the interior of the pipe 11. The immersion heater 20 is a heating 
element which is in the form of a tube having a sealed end. The immersion 
heater 20 will extend through a wall in the pipe 11 and through a 
substantial length of the pipe 11. A power supply 21 is electrically 
connected by line 22 to the immersion heater 20 so as to provide suitable 
power to the immersion heater 20. Preferably, the immersion heater 20 can 
produce heat of up to 500.degree. F. The application of such heat in the 
area of the interior of pipe 11 will cause the mixed sludge to also 
increase in temperature. This further enhances the pathogen-killing 
behavior of the process of the present invention in the interior of the 
pipe 11. The immersion heater 20 can be used to produce heat where the 
cost of the heat from the electricity is less than the cost of heat from 
lime. If the immersion heater 20 provides 14 degrees of additional 
temperature, then it reduces the lime requirement by approximately 20 
percent. Since the pipe 11 has insulation 10 extending therearound, any 
heat produced by the immersion heater 20 is imparted directly into the 
mixed sludge. Experiments have shown that this system is at least 95 
percent efficient in passing heat into the sludge. This is in contrast to 
the approximately 18 percent efficiency associated with the aforedescribed 
infrared heaters used with conveyor belts for the treatment of sludges. As 
such, it can be seen that the immersion heater 20 as used within the pipe 
11 enhances the efficiency and optimizes chemical consumption associated 
with the process 1 of the present invention. 
After reacting within the pipe 11, the mixed sludge is flashed across a 
restricting orifice 13. This restricting orifice can be an opening, a die, 
or a valve. The orifice 13 is positioned generally adjacent to the end of 
the pipe 11. The orifice 13 will communicate with a flash chamber 14. As 
such, the material is delivered under pressure to the orifice 13 and then 
released into the flash chamber 14. A vapor, including water vapor, 
NH.sub.3, SO.sub.2, and SO.sub.3, will exit the flash chamber 14 through 
the vent 15. This vapor can then pass through a condensor, or compressor, 
16. The products of the process can then be sold as valuable byproducts 
external of the system. 
In order to properly remove the water from the sludge, it is important that 
the flash chamber 14 has an interior pressure of between 0 and 14.7 
p.s.i.a.. As such, when the mixed sludge passes through the orifice 13, 
the sludge will be exposed to a lesser pressure. This causes the water and 
other volatile components of the sludge to be evaporated. As a result, the 
water content and the temperature of the sludge are appropriately reduced. 
The heat of vaporization of the flashed material can be passed directly 
back to the sludge by using heat exchangers, pumps or vapor compressors. 
After the sludge passes into the flash chamber 14, the resulting sludge 
will be a sterile decontaminated product which is pathogen-free and 
vector-free. This product will meet or exceed U.S. E.P.A. standards. 
The sterilized sludge then exists the flash chamber 14 through the 
discharge opening 18. 
The geometric configuration of the reaction chamber 11 is dependent upon 
the layout configuration of the facility in which it is used. The reaction 
chamber 11 should have a sufficient diameter and length so as to provide a 
residence time of the sludge within the chamber of greater than 15 
seconds. The insulation 10 is provided so as to eliminate heat loss and to 
produce an adiabatic reaction. 
Tests have been conducted with the configuration of the present invention. 
The experimental data associated with the process of the present invention 
is identified in Table I hereinbelow. During these experiments, oxalic 
acid was included in the experiments. However, it was later determined 
that the oxalic acid is a temperature depressor and can be a poison. As 
such, oxalic acid should not be included as part of the process of the 
present invention. Other test results have shown that acids such as 
HNO.sub.3 acid, acetic acid, and vinegar acid do not achieve the necessary 
reaction so as to significantly increase the temperature of the waste 
sludge. 
TABLE I 
______________________________________ 
TIME TO 
OXALIC SULFAMIC REACH 
EXP CaO ACID ACID WATER TEMP TEMP 
# gr. gr. gr. cc. F. mins. 
______________________________________ 
1 169 75 56 24 300 8 
2 189 75 112 24 607 8 
3 337 153 224 24 618 8 
4 337 306 112 24 580 4 
5 189 75 168 24 400 1 
6 169 75 112 24 667 5 
7 50 40 87 24 250 1 
8 169 0 130 24 840 1 
9 169 130 0 24 370 1 
10 169 0 0 12 213 0.2 
11 0 75 0 12 0 1 
12 0 0 56 12 0 1 
13 169 130 0 24 500 3 
14 189 0 130 24 820 1 
15 85 0 85 24 700 1 
16 189 0 130 24 750 1 
17 169 0 130 72 750 1 
18 169 0 188 24 860 1 
______________________________________ 
FIG. 2 shows an alternative form of the present invention. In FIG. 2, there 
is shown at 30 an alternative process for the treatment of a waste sludge 
of biological solids. In the process 30, the sludge 32, the acid 34 and 
the oxide-containing chemical 36 are introduced simultaneously into feed 
hopper 38. As such, these components can be added simultaneously into the 
process 30. The sludge 32, the acid 34 and the oxide-containing chemical 
36 will have a configuration similar to that described herein previously. 
After the sludge 32, the acid 34 and the oxide-containing chemical 36 are 
introduced into the feed hopper 38, the mixture is auger fed into the 
compression zone 44. In the transport of the mixture of the sludge 32, the 
acid 34 and the oxide-containing chemical 36, these materials are mixed 
together by the screw conveyor. The compression zone 44 serves to increase 
the pressure of the mixed sludge to the desired value. Specifically, the 
compression zone 44 should increase the pressure of the mixed sludge to a 
pressure of greater than 14.7 p.s.i.a.. 
The material which exits the screw conveyor 42 enters the pipe 46. Pipe 46 
can be suitably insulated. The pipe 46 can contain static mixing elements. 
Material is continuously mixed as it progresses through the predetermined 
length and diameter of pipe 46. The mixed sludge is continuously under 
pressure so as to prevent a premature flashing of the ammonia or of the 
water. A temperature monitor TI and a pressure monitor PI provided along 
the pipe 46 so as to provide suitable monitoring of the reaction process 
and can provide an input for suitable reaction control systems. 
In the process 30, as shown in FIG. 2, a magnetic field generating 
apparatus 48 is provided. This magnetic field generating apparatus 48 
includes a coil 50 which is wrapped around the exterior of the pipe 46. A 
440 volt power source 52 is provided so as to generate a suitable magnetic 
field. A variable frequency controller 54 allows the magnetic field 
generated by the magnetic field generating apparatus 48 to be suitably 
tuned for the optimal heating of the mixed sludge passing through the 
interior of pipe 46. 
In the process of the present invention, the water in the mixed sludge 
passing through the pipe 46 is slightly polar. The process of the present 
invention is a magnetohydrodynamic process. There is a frequency at which 
the water molecules flip. At the optimal frequency, heat is generated by 
the flipping of the water molecules. Typically, the frequency should be 
between 100 and 3,000 cycles per second. The winding 50 extends around the 
pipe 46. The variable frequency controller 54 is adaptable so as to vary 
the cycle/second acting on the mixed sludge on the interior of the pipe. 
The variable frequency controller 54 allows the magnetic field to be 
"tuned" to the composition of the mixed sludge. The frequency can be tuned 
until it reaches an optimal frequency. At such an optimal frequency, a 
maximum amount of heat will be imparted by the water molecules to the 
mixed sludge on the interior of pipe 46. The temperature monitor TI and 
the pressure monitor PI can be examined so as to facilitate the proper 
"tuning" of the magnetic field generating apparatus 48. 
The magnetic field generating apparatus 48 achieves certain advantages over 
the immersion heater described in association with FIG. 1. For example, 
since the winding 50 extends around the exterior of pipe 46, there is no 
flow restriction on the interior of pipe 46. Additionally, the winding 50 
will not be subject to the corrosion effects of the mixed sludge. On the 
other hand, it is uncertain whether the economics for the magnetic field 
generating apparatus 48 equal the economics of the prior embodiment. 
Additionally, it is possible that the magnetic field generating apparatus 
48 could add installation and safety concerns which would not be created 
by the immersion heater of FIG. 1. 
After being reacted and heated within the interior of pipe 46, the mixed 
sludge is flashed across restricting orifice 56 at the end of pipe 46. The 
orifice 56 will communicate with the interior of flash chamber 58. As 
such, the mixed sludge is delivered under pressure to the orifice 56 and 
then released into the flash chamber 58. The vapor can then be released 
through vent 60. The byproducts released through vent 60 can then be 
accumulated in container 62 for reuse or resale. The sterilized sludge 
exits the flash chamber 58 through the discharge opening 64. Experiments 
with the present invention have shown that the byproducts which exit the 
flash chamber 58 through vent 60 can be easily resold as industrial or 
commercial gases. 
The foregoing and description of the invention is illustrative and 
explanatory thereof. Various changes in the details of the illustrated 
construction or of the steps of the described method can be made within 
the scope of the appended claims without departing from the true spirit of 
the invention. The present invention should only be limited by the 
following claims and their legal equivalents.