Apparatus and method for processing sewage scum

A method and apparatus for reclaiming fats and oils from sewage scum includes a scum cooker, a solids separation system, a vacuum dryer, a condenser and product storage tanks. The scum cooker liquifies and renders sewage scum obtained from a sewage treatment plant by maintaining the raw sewage scum at an elevated temperature and an elevated pressure to sterilize and separate the scum into the solid, aqueous, and oily components. The solid separation system removes the solid and particulate matter from the liquified scum for disposal, preferably after washing the solids with treated water from the sewage treatment plant. The solid-free liquified scum is then transferred to the vacuum dryer which is maintained at an elevated temperature and preferably at least a moderate vacuum to remove the water and other volatiles from the liquified scum. These volatile components are condensed in the condenser for retreatment in the vacuum dryer or return to the sewage treatment plant to be processed with the remainder of the sewage. The oily components of the liquified sewage scum collected in the vacuum dryer are stored in a storage tank which is heated to make the oily substances flowable.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus and method for processing sewage 
scum; in particular, the invention relates to an apparatus and method for 
processing the collected primary skimmings, for example, from wastewater 
treatment plants. The purpose of this process is to separate and treat the 
solids portion of these skimmings so that they are acceptable for ordinary 
landfill disposal, to separate and treat the fats and oils portion of the 
skimmings for sale as a raw product for distillation, and to return the 
oil and solid-free water back into the treatment plant influent. 
One of the first steps in the treatment of municipal sewage or raw sewage 
from other sources is skimming an upper layer from the raw sewage for 
disposal prior to treating the sewage for discharge, by flocculation, 
aerobic digestion, or other techniques. Typically, a layer of scum rises 
to the surface of the raw sewage when it is, for example, held in a 
settling tank or basin. This scum is typically skimmed from the surface of 
the raw sewage for separate treatment to prevent interference by the 
materials in the scum with treatment processes applied to the remainder of 
the sewage. Removal of this scum layer is typically accomplished with a 
primary skimmer. The scum thus removed from the raw sewage typically 
contains about 70% water, about 15% plastics and other solids, and about 
15% fats, oils, and other oily substances. Until recently, the scum 
removed from the raw sewage in this way was either disposed of by dumping 
or landfilling the scum, possibly after the removal of the aqueous 
component of the scum, or by drying and incinerating or burning the scum. 
With increased interest in the environment and increased regulatory 
requirements for pollution abatement equipment, such methods of scum 
disposal are no longer practicable. Dumping or landfilling of scum is 
undesirable due to the potentially harmful environmental consequences, the 
continually increasing high cost of landfilling, and the increasingly 
limited locations in which such disposal is permitted. Burning of 
dewatered scum is possible, but, because of the very high incinerator 
temperatures that are required to completely combust the plastics 
typically found in sewage scum, burning of large quantities of scum in a 
way that will meet regulatory requirements is generally infeasible. 
SUMMARY OF THE INVENTION 
A method and apparatus for processing sewage scum are disclosed. The sewage 
scum processor apparatus preferably includes a scum cooker, a solids 
separation system, a vacuum dryer, a condenser, and product storage tanks. 
The scum cooker, or cooking tank, heats the sewage scum obtained, for 
example, from a primary skimmer of a sewage treatment plant, by 
maintaining the raw sewage scum at an elevated temperature and elevated 
pressure to liquify and sterilize the raw scum and render the raw scum 
separable into its solid, aqueous, and oily components. The solid 
separation system removes the solid and particulate matter from the thus 
liquified scum for disposal, preferably after washing the solids with 
treated water from the sewage treatment plant and a germicide. The 
solid-free liquified scum is then transferred to the vacuum dryer. The 
vacuum dryer is maintained at an elevated temperature and preferably at 
least a moderate vacuum to remove the water and other volatiles from the 
fluid components of the liquified scum. The volatile components of the 
liquified scum (which include water vapor) removed with the vacuum dryer 
are condensed in the condenser and returned to the sewage treatment plant 
to be processed with the remainder of the sewage. Preferably, in the 
condenser, the water and volatiles are washed and cooled with treatment 
plant effluent and deodorized by injection of a deodorizing chemical. The 
non-condensable components of the liquified sewage scum are then released 
to the atmosphere, preferably following treatment with a deodorizing 
chemical. The oily components of the processed sewage scum collected in 
the vacuum dryer are preferably stored for ultimate disposition in a 
heated storage tank; the storage tank is heated to prevent solidification 
of any of the oily substances and to maintain a relatively low viscosity. 
Preferably, the scum cooker, vacuum dryer, and storage tank are heated with 
a heated exchange fluid, such as a high temperature oil, cycled through a 
heat exchanger. In the heat exchanger, the exchange fluid is heated to the 
desired elevated temperature with a burner utilizing, for example, diesel 
fuel. In alternative embodiments of the invention, other heat sources 
could be used, including arrangements in which separate heat sources are 
used to heat the scum cooker, vacuum dryer, and storage tanks. 
The cleaned and purified fat and oil component of the sewage scum can, 
following the process of the present invention, be recycled for use as 
fuel oil or otherwise sold. The recovered fats and oils can also be 
distilled for disposition of specific fractions of the mixture according 
to their intended use. 
The solids that are extracted from the sewage scum according to the process 
of the present invention may be landfilled safely and inexpensively due to 
the treatment according to the present process and the relatively low 
volume of the extracted solids. The water and aqueous fractions removed by 
the process of the present invention can be returned to the sewage 
treatment plant for processing with the raw sewage that remains after 
removal of the scum with the primary skimmer. 
In addition, the apparatus disclosed is relatively inexpensive to operate; 
the primary expense is for fuel for heating the cooker, vacuum dryer, and 
storage tanks. The process is also non-polluting. 
Following treatment of the sewage scum using the present invention, it is 
necessary to dump or landfill only about 10%-15% of the amount of scum 
which would have to have been so disposed of without the use of the 
invention. This 10%-15% will be inert water wet solids which are 
compatible with any municipal landfill operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the invention, shown schematically in FIG. 1, 
has a cooking tank 10 for liquifying and sterilizing the raw sewage scum 
and rendering the raw scum separable into solid, aqueous, and oily 
components, a solids separation system 12 for separating the plastics and 
other solids from the aqueous and oily phases of the liquified scum, a 
vacuum dryer 14 for removing water and other volatiles from the combined 
aqueous and oily phases, a condenser 16 for liquefying water and other 
volatiles removed in the vacuum dryer, and storage tanks 18 for storing 
the dried and treated oily phase pending final disposition of the 
recovered fats and oils. The preferred embodiment also includes a heat 
exchanger 20 for providing heat to cooking tank 10, vacuum dryer 14, and 
storage tanks 18. In alternative embodiments of the invention, however, 
other heat sources could be used, such as arrangements for individually 
heating cooking tank 10, vacuum dryer 14, and storage tanks 18. 
Cooking tank 10 is provided with an inlet 22 for introduction of raw scum 
from the primary skimmer of a sewage treatment system. Typically, the raw 
scum includes about 70% aqueous components, about 15% fats and oils, and 
about 15% plastics and other solids; however, without further treatment, 
the scum mixture is not readily separable into its component parts. 
Treatment of the raw scum from the primary skimmer in cooking tank 10 
renders the scum to enhance the separability of the solids, the oily 
phase, and the aqueous phase of the scum; this treatment also sterilizes 
the scum. 
Cooking tank 10 also includes a liquified scum outlet 24, a gas relief 
outlet 26 equipped with a high pressure relief valve 27, and exchange 
fluid circulating jackets 28 and 30. Preferably, cooking tank 10 is a 
cylindrical tank having a conical lower end for efficient drainage of tank 
10. Other designs and shapes suitable for cooking tank 10 are well known; 
alterations in tank design will require appropriate changes in the design 
of circulating jackets 28 and 30, which are formed to adapt to the 
particular shape of cooking tank 10. Drainage of tank 10 into solid 
separator 12 through liquified scum outlet 24 is controlled by outlet 
valve 29. 
During the heat and pressure treatment of the raw scum from the primary 
skimmer of the sewage treatment system, substantial quantities of gas are 
evolved. Excess gas and pressure are released through gas relief outlet 
26; the elevated pressure is maintained in the cooking tank 10 by high 
pressure relief valve 27, which is adjustable to maintain the desired 
pressure in tank 10. Typically, tank 10 is maintained at an elevated 
pressure of about 25 psi (pounds per square inch) during this stage of the 
scum treatment process. 
Cooking tank 10 is maintained at an elevated temperature of about 
200.degree. F. (95.degree. C.), preferably with a heated exchange fluid 
circulated through circulating jackets 28 and 30. Other temperatures may 
be used as long as the temperature is sufficiently high to sterilize and 
render the scum. These jackets 28 and 30 are in thermal contact with, are 
conformed to the exterior of, and cover most of the external surface area 
of, cooking tank 10. The exchange fluid, which will be described more 
fully below, is received from heat exchanger 20 into jackets 28 and 30 via 
inlets 32 and 34. The cooled exchange fluid is returned to heat exchanger 
20 via outlets 36 and 38. 
Following rendering and sterilization of the scum in cooking tank 10, the 
liquified scum is forwarded to solids separation system 12 using pump 40. 
Suitable pumps 40 for forwarding the heated slurry output of tank 10 to 
solids separation system 12 are well known. 
Solids separation system 12 includes a solids separator 41 which 
substantially separates the scum solids from the aqueous and oily 
components of the liquified scum. The separated solids, which may be a 
sludge-like material, are produced at an outlet 42 for disposal or further 
processing, while the combined aqueous and oily phases of the liquified 
scum are removed from solids separator 41 through a fluids outlet 44. 
Solid separator 41 is of a conventional design, such as that described in 
U.S. Pat. Nos. 3,773,661 or 3,707,235. Suitable substitutes for solid 
separator 41 are well known. 
Since the rate of output of fluids from solids separator 41 through fluid 
output 44 may be uneven, a surge tank 46 is provided between fluid output 
44 of solids separator 41 and vacuum dryer 14. Surge tank 46 is equipped 
with an overflow outlet 48 and a drain 50; overflow outlet 48 provides a 
relatively even flow of fluids to vacuum dryer 14. Excess aqueous phase 
may be drained from surge tank 46 through drain 50 and returned to the 
main sewage treatment plant controllably with valve 52. All free bottom 
water can be drained through 50 back into treatment plant influent 
following solids removal. When aqueous phase becomes oily, the flow is 
diverted through outlet 48 into the vacuum dryer. 
Using the remaining components of solids separation system 12 shown in FIG. 
1, the solids produced by solids separator 41 are preferably washed with 
sewage treatment plant discharge water prior to disposal of the solids, 
although this is not necessary to the practice of the invention. Water, 
preferably together with a germicide such as chlorine for further 
sterilization, is mixed with the solids in a mixing screw conveyor 54 
connected to outlet 42 of solids separator 41. The resulting slurry is 
introduced to a second separator 56, which conveniently is of the same 
type as solids separator 41. The solids produced by separator 56 through 
outlet 58 are collected in a disposal container 60 in a form suitable for 
ultimate disposal. Liquids obtained from separator 56 are returned to the 
sewage treatment plant. 
Fluid communication between surge tank 46, vacuum dryer 14, condenser 16, 
and storage tanks 18 is controlled by flow network 70. Flow network 70 
includes, in the preferred embodiment of the invention, four valves 72, 
74, 76, and 78, and a pump 80. With valves 72, 74, and 80 open and valve 
74 closed, pump 80 forwards fluid from surge tank 46 to vacuum dryer 14; 
for this operation, fluid is prevented from entering storage tanks 18 by 
valves 82, discussed below. When valves 74, 76, and 78 open and valve 72 
is closed, partially dried fluid from vacuum dryer 14 or condensate from 
condenser 16 may be recycled through vacuum dryer 14 for further drying. 
Satisfactorily dried fluid may be forwarded to storage tanks 18 by opening 
valves 74 and 76 and closing valves 72 and 78. 
As flow network 70 is shown in FIG. 1, flow network 70 is particularly 
adapted to processing of batches of raw sewage scum. That is, for example, 
the non-volatile phase collected in vacuum dryer 14 cannot be transferred 
to storage tanks 18 simultaneously with the transfer of condensate from 
condenser 16 to the sewage treatment plant headworks via valve 138. 
However, as will be apparent to those skilled in the art, a more complex 
flow network 70 could be substituted for that shown in FIG. 1 for 
substantially continuous operation of the sewage scum processor according 
to the invention. 
Vacuum dryer 14 includes a vacuum tank 90, which is conveniently 
approximately equivalent to cooking tank 10. Vacuum tank 90 is equipped 
with an inlet 92 and a sprayer 94, a vapor outlet 96 communicating with 
the interior of vacuum tank 90 through a demister 98, and a liquid outlet 
100. Vacuum dryer 14 also includes heating jackets 102 and 104 having 
inlets 106 and 108 and outlets 110 and 112 coupled to heat exchanger 20 
with suitable valves and conduits. Heating jackets 102 and 104 are 
substantially equivalent in both structure and function to circulating 
jackets 28 and 30, discussed above. 
Sprayer 94 disperses the fluid received into vacuum dryer 14 from surge 
tank 46 through inlet 92 for enhanced evaporation of the volatile 
components of the combined aqueous and oily phases of the liquified scum. 
The volatile components of the liquid received into vacuum dryer 14 are 
removed via vapor outlet 96. Since the vapors withdrawn from vacuum dryer 
14 typically include small droplets of liquid or mist from the sprayer, 
the vapor is preferably directed through demister 98; the volatile 
fractions and some liquid mist enter demister 98 through demister inlets 
97. The mist or droplets are there collected for return to vacuum dryer 14 
via return pipe 101. The volatilized fractions of the liquid are removed 
from vacuum dryer 14 through a vacuum outlet valve 96 to condenser 16. The 
dried non-volatile fractions may be withdrawn through liquid outlet 100 
and forwarded to storage tanks 18 or recycled to vacuum dryer 14 via flow 
network 70, as discussed above. 
Vacuum dryer 14 is preferably maintained at at least about 200.degree. F. 
(95.degree. C.) by the heated exchange fluid circulating through jackets 
102 and 104; at such a temperature, water has a sufficiently high vapor 
pressure to be readily removed to a high degree of effectiveness. As water 
and other volatiles are removed from the fluid introduced to vacuum dryer 
14, the heat supplied through jackets 102 and 104 causes the temperature 
in vacuum dryer 14 to rise to as much as about 250.degree. F. (120.degree. 
C.), enhancing the removal of the remaining water and volatile components 
from the fluid. Vacuum dryer 14 is preferably maintained at a pressure of 
between about 15 and 25 inches of mercury, and most preferably at about 22 
inches of mercury (0.65 atmosphere). 
The water and other volatile substances, removed from the combined aqueous 
and oily phases of the sewage scum, are transferred to condenser 16 
through vacuum line 114. Vacuum line 114 preferably also receives gases 
released through high pressure relief valve 27 from cooking tank 10. A 
chemical deodorant, such as caustic soda, may be added to vacuum line 114 
through valve 116 prior to reception into condenser 16. 
Condenser 16 is maintained at an ambient temperature so that the 
condensable volatiles received into condenser 16 through manifold 120 are 
collected in condenser 16. Condenser 16 is also provided with inlets 122 
and 124 and sprayers 126 and 128 for spraying the vapors and condensate 
with a chemical deodorant and a water rinse, respectively. Preferably, the 
chemical deodorant is caustic soda; suitable alternatives are well known. 
Water for washing the condensate and increasing the efficiency of 
condensation is conveniently derived from the treated water discharge from 
the sewage treatment plant. 
Non-condensable gases and some mist are withdrawn from condenser 16 through 
outlet 128; passage of these vapors through a trap 130 isolates the 
non-condensable gases for removal to the atmosphere via vacuum pump 132. 
Condensable materials and mists or droplets are collected in trap 130 and 
returned to condenser 16 via return inlet 132. Trap 130 is preferably a 
cold trap, such as a tube type heat exchanger cooled with plant water; 
suitable substitutes will be apparent to those skilled in the art. Vacuum 
pump 132 is a liquid ring pump in the preferred embodiment. 
Condensate collected in condenser 16 is removed through pump 134 
controllably with a valve 136. The condensate can return through flow 
network 70, as discussed above, to the sewage treatment plant via a valve 
138. 
The non-volatile fractions of the combined aqueous and oily phases of the 
sewage scum, trapped in the lower portion of vacuum dryer 14, constitute 
the recovered, processed fats and oils; these are removed through flow 
network 70 to storage tanks 18. This recovered, processed oil is received 
into tanks 18 through valves 82 and inlets 140. The recovered oil can be 
held in storage tanks 18 until it can be conveniently removed for its 
ultimate disposition. Removal is accomplished through storage outlets 142 
via valves 144 and a pump 146. To avoid partial solidification of the oils 
and fats recovered, storage tanks 18 are provided with jackets 148 and 150 
through which a heated exchange fluid from heat exchanger 20 is circulated 
in much the same manner as in circulating jackets 28, 30, 102, and 104. 
Preferably, the heated exchange fluid is received into jackets 148 and 150 
through jacket inlets 152 and 154, and removed for reheating through 
jacket outlets 156 and 158. 
The heat exchanger system for maintaining cooking tank 10, vacuum dryer 14, 
and storage tanks 18 at elevated temperatures includes, in addition to 
circulating jackets 28, 30, 102, 104, 148, and 150 discussed above, heat 
exchanger 20 and a pump 160 for circulating the exchange fluid through 
heat exchanger 20. 
Heat exchanger 20 preferably utilizes a flame jet to heat the exchange 
fluid to the desired temperature of about 350.degree. F. In the apparatus 
according to this invention, a heat exchanger with a capacity of about 2.5 
million BTU has been found to be satisfactory. The exchange fluid is 
circulated in thermal contact with the flame through a jacket 162 and 
thence to cooking tank 10, vacuum dryer 14, and storage tank 18. The fuel 
burned in heat exchanger 20 is preferably diesel fuel or natural gas, but 
other suitable substitutes are well known. Efficient and appropriate 
designs for jacket 162 within heat exchanger 20 will also be well known to 
those skilled in the art. 
Air and fuel for burning within heat exchanger 20 are received into and 
burned using burner 164. Preferably, the flame is directed along the 
length of the interior of jacket 162 with a stream of compressed air 
received in burner 164 via compressed air inlet 166. The exhausted gases 
from vacuum pump 132 can conveniently provide a source of compressed air 
for introduction into compressed air inlet 166. The combustion products of 
the flame are removed to the atmosphere via a flue 168. 
Preferably, the heated exchange fluid may be stored in a reservoir 170 for 
circulation through the various jackets of cooking tank 10, vacuum dryer 
14, and storage tanks 18. Reservoir 170 also acts as an expansion chamber 
for the heat exchanger and associated circulating jackets and tubing. 
The exchange fluid circulated through heat exchanger 20 and the associated 
conduits and circulating jackets is preferably a high temperature oil, 
such as Dow Therm.RTM., manufactured by Dow Chemical Company. 
The temperatures maintainable in cooking tank 10, vacuum dryer 14, and 
storage tank 18 will depend on a variety of factors, including the length 
and insulation of the conduits in the heat exchanger system, the geometry 
and efficiency of heat transfer in heat exchanger 20, the quantity and 
rate of fuel combustion in burner 164, the rate of circulation of the 
exchange fluid through the system by pump 160, and other factors. As will 
be apparent to those skilled in the art, adjustment of some of these 
factors, especially the rate of fuel combustion, will allow the adjustment 
of the temperatures within cooking tank 10, vacuum dryer 14, and storage 
tanks 18 to the desired temperatures. 
As used in this specification and the appended claims, the term "fats and 
oils" includes non-particulate portions of the rendered scum which are 
substantially fat- or oil-soluble and are non-volatile under the 
conditions obtaining in the above-described vacuum dryer 14. The term 
"oily phase" denotes that portion of the liquified scum remaining after 
removal of solids and free water from the liquified scum which is 
substantially hydrophobic and those components of the rendered scum which 
are substantially soluble in fats or oils. 
While the above is a complete description of the preferred embodiment of 
the invention, other arrangements and equivalents are possible and may be 
employed without departing from the true spirit and scope of the 
invention. For example, the sizes and configurations of most of the 
components of the preferred embodiment, in particular of the cooking tank 
10, vacuum dryer 14, and storage tanks 18 may be varied considerably. 
Similarly, the number and positioning of the tanks of each type is not 
critical. Therefore, the above description and illustration should not be 
construed as limiting the scope of the invention which is delineated by 
the appended claims.