Method for coating dry pellets made of waste bio-solids with waste bio-solids

Method and apparatus are provided for reprocessing dry pellets made of waste bio-solids in which the pellets and wet sludge waste bio-solids are supplied to the inlet of a material combining conveyor whose angular position can be adjusted. The pellets are coated with the sludge as they travel along the material combining conveyor to its output which feeds a dryer or oven. By adjusting the speed, length and angular position of the material combining conveyor, the amount of sludge coated onto the pellets can be controlled. The material combining conveyor is preferably a sealed shaftless screw conveyor that extends in a generally veritcal direction so that pellets not properly coated fall by gravity to the conveyor inlet. Dry pellets and wet sludge can be supplied from separate bins to a common feed conveyor such as a shaftless screw conveyor which conveys the mixture to the input of the material combining conveyor.

FIELD OF THE INVENTION 
The invention relates to the method and apparatus for mixing dry pellets of 
bio-solid material with wet sludge in order to achieve a desired degree of 
size and density for finished pellets. 
BACKGROUND OF THE INVENTION 
The handling and disposal of bio-solids, such as human waste, presents a 
major problem. One technique utilized is to form the bio-solids into a wet 
sludge having approximately an 18% to 35% concentration of the bio-solids 
and to process the sludge into dry pellets. The pellets contain nitrogen 
in slow release form. They can be sold to fertilizer manufacturers for 
mixing with standard fertilizers or for use directly as a general 
fertilizer applied to agricultural areas. 
In a standard plant for producing the pellets, the finished pellets must 
have a certain specific density (solid content as compared to liquid) and 
size. If the final product in the pelletizing plant does not meet the 
density and size specifications, then the pellets must be reprocessed. 
This is usually accomplished by returning the reject pellets to mix with 
incoming wet sludge to be recoated with additional sludge and recooked in 
a dryer to correct the proportion of density and size. 
Many times, the reject pellets to be reprocessed amount to 60%-65% of the 
output of the pellet forming operation. The pellets to be reprocessed, 
which are dry, are transported vertically to a raised storage bin. This is 
usually accomplished by a bucket elevator. Such transport produces a 
substantial amount of dust and spill of the pellets to be reprocessed. 
From the bin, the pellets are distributed by gravity feed to a mixer, 
typically of the paddle or pin type. The wet sludge is also transported to 
the mixer. The mixer must insure that the dry pellets being reprocessed 
are evenly coated by the wet sludge and have achieved a size of to 3 to 4 
mm. From the mixer the reprocessed material, now containing the added wet 
sludge, is transported to a high heat dryer. 
This process of the prior art has a drawback in that the dust and spills 
produced possibly can give rise to a flashback explosion. Also, the 
process is lacking in providing different ways of adjusting the moisture 
and recoated sludge content of the reprocessed pellets. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention relates to a method and apparatus for reprocessing 
dry pellets. In accordance with the invention, the dry pellets being 
reprocessed and wet sludge are provided in respective supply containers 
and are fed therefrom and are brought together (combined) on a feed 
conveyor. In a preferred embodiment of the invention, the conveyors from 
the respective dry pellet and wet sludge supplies are of the shaftless 
screw type as in the feed conveyor. 
The mixture from the feed conveyor is then fed from a pressurized 
connection to a material combining conveyor to deliver the mixture to a 
dryer. Further mixing of the dry pellets and sludge and recoating of the 
original dry pellets occurs in the material combining conveyor as the 
product travels its length from inlet to outlet for delivery to the dryer. 
The angle of the material combining conveyor is also preferably adjustable 
from the horizontal through any angle up to a vertical position. It is 
preferred that a shaftless screw conveyor be used for the material 
combining conveyor since its open center allows some of the higher density 
pellet material, as compared to the sludge, to fall back through the open 
center and remix with the incoming wet sludge. 
Normally, delivery of the mixture would be with the material combining 
conveyor near vertical or completely vertical because the mixture is 
generally fed into the dryer by gravity, i.e., the mixture is dropped into 
a dryer. 
As an alternative to the material combining conveyor, there can be a simple 
low speed pin mixer which accepts delivery of the mixture of sludge and 
dry pellets from their respective conveyors that supply these items to the 
feed conveyor. This is to be contrasted with presently available mixers 
which are either of the paddle or pin type which operate at a very high 
speed and thereby have high maintenance costs. 
OBJECTS OF THE INVENTION 
An object of the invention is to provide a novel method and apparatus for 
mixing wet sludge with dry pellets of bio-solids which are to be 
reprocessed. 
An additional object is to provide a method and apparatus in which dry 
pellets to be reprocessed and wet sludge are each fed by a respective 
shaftless screw conveyor onto a feed conveyor to a mixer for recoating of 
the dry pellets and delivery to an oven. 
A further object is to provide a method and apparatus in which dry pellets 
of bio-solids to be reprocessed are mixed with wet sludge of bio-solids in 
a shaftless screw material combining conveyor for delivery to an oven. 
Yet another object is to provide a method and apparatus in which dry 
pellets are mixed with sludge in a shaftless screw conveyor extending in a 
generally vertical direction for delivery to an oven and the angle of the 
conveyor is adjustable relative to the vertical.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1-3, there is a horizontal feed conveyor 12 of any 
suitable length driven by a motor 14 and including other necessary 
conveyor elements. The feed conveyor 12 is preferably of the shaftless 
screw type. Along its length in the direction of feed conveyor travel, as 
shown by arrow A in FIG. 1, are one or more bins 20 into which dry pellets 
to be reprocessed are placed. This can be done by feeding the dry pellets 
into the top of a bin 20 from another conveyor or elevator lift (not 
shown) or being dumped in by any other means. The pellets being 
reprocessed typically have a size in the range other than the ideal, for 
example, 3 to 4 mm, and a dryness of 94% to 98% dry solids. The pellets 
are to be reprocessed, in a typical application, to a size in the range of 
3 to 4 mm and with a dryness of about 95%. 
Also positioned along the length of feed conveyor 12 is a bin 30 for wet 
sludge that is to be used to coat or otherwise mix with the dry pellets 
from bins 20 being reprocessed. The wet sludge is supplied to its bin 30 
from any suitable source and can be dumped or pumped into the bin. While 
bins 20 and 30 are shown as being on opposite sides of the feed conveyor 
12, they can be on the same side. 
At the bottom of each dry pellet bin 20 is a conveyor 24, preferably of the 
shaftless screw type, driven by a motor and other necessary conveyor 
elements 26. Similarly, at the bottom of the sludge bin 30 is a conveyor 
34, also preferably of the shaftless screw feed type, driven by a motor 
and other necessary conveyor elements 36. Each of the bin conveyors 24 and 
34 has an outlet to the feed conveyor 12. Thus, dry pellets being 
reprocessed empty from a bin 20 into its conveyor 24 and are carried to 
the main feed conveyor 12. Similarly, wet sludge from bin 30 empties into 
its conveyor 34 from which it is carried to the feed conveyor 12. 
Each of the bins 20 and 30 has the necessary conventional elements to 
control the material discharge rate from the feeding bin onto its 
respective conveyor 24 and 34. Similarly, each of the conveyors 24, 34 has 
the necessary control elements, such as motor speed control, screw size, 
etc., to control material feeding rate from the conveyor 24, 34 to the 
feed conveyor 12, thereby to control the mix proportions. 
The dry pellets from a bin 20 and the wet sludge from bin 30 are mixed 
somewhat as they are conveyed by the feed conveyor 12 along its length. 
The mixture is delivered to a pressurized axial connection 38 which is the 
inlet of an elongated material combining conveyor 44. The connection 38 
also permits rotational adjustment of the angle of the material combining 
conveyor 44 relative to the vertical direction. Its position is basically 
adjustable to any angle from the horizontal direction to the vertical. 
Conveyor 44 is also preferably of the shaftless screw type driven by a 
motor and other necessary elements 46. The material combining conveyor 44 
has a discharge outlet 48 at its (upper) end remote from its inlet. The 
outlet 48 of conveyor 44 feeds the reprocessed pellets to a dryer or an 
oven (not shown). 
As the mixture travels the length of conveyor 44 between inlet 38 and 
outlet 48, the originally dry pellets are recoated with sludge. That is, 
there is further mixing of the dry pellets with the sludge. As described, 
the material combining conveyor 44 is rotatable about connection 38 to be 
positioned relative to the horizontal from 0.degree. to 90.degree. . The 
ability to adjust the inclination angle of material combining conveyor 44 
has several advantages. First, it can adjust the height of the discharge 
outlet 48 to the height of the dryer or oven into which the discharge 
outlet delivers the recoated pellet product. Second, the angle of 
inclination of conveyor 44 is a control parameter for the reprocessing of 
the dry pellets. That is, depending upon the inclination angle of conveyor 
44, more or less of the wet sludge adheres to the dry pellets being 
recoated with wet sludge. The rotational speed of the conveyor screw also 
can be controlled. Further, the conveyor 44 can be made as long as needed. 
The use of a shaftless screw type conveyor for the conveyor 44 has an 
advantage in that the conveyor open center permits some of the higher 
density material, i.e., dry pellets, to fall by gravity through the open 
center of the conveyor back to the pressurized inlet 38 to mix again with 
additional wet sludge supplied from feed conveyor 12. 
Normally, the material combining conveyor 44 has a high angle of 
inclination, or is kept vertical, since delivery of the final reprocessed 
product from its outlet 48 to the dryer or oven is by gravity. In general, 
the closer the angle of conveyor 44 inclination is to the vertical, the 
better is the mixing. 
Factors which affect the reprocessing of the rejected dry pellets are: 
a. delivery rate of pellets from bin 20 to feed conveyor 12. 
b. delivery rate of sludge from bin 30 to feed conveyor 12. 
c. feed rate of feed conveyor 12. 
d. inclination of the material combining conveyor 44. 
e. feed rate of the material combining conveyor 44. 
f. length of the material combining conveyor. 
One or more of these parameters can be adjusted to achieve the desired 
density and size of the reprocessed product being fed to the dryer or oven 
from the outlet 48 of the material combining conveyor. Shaftless screw 
conveyors that can be utilized in the invention are those manufactured by 
Spinac of Malmo, Sweden. Such a Model U-355 (for the conveyors 12, 24 and 
34) and model OK-355 (for the material combining conveyor 44). Suitable 
shaftless screw type conveyors of other manufacturers can be used. 
The system of FIGS. 1-3 has several advantages. The presence of dust from 
the dry pellets at the entry to a dryer or oven is minimized. Also, as 
listed above, a number of separately adjustable control parameters are 
provided to control the characteristics of the reprocess pellets supplied 
from the discharge 48 of the material combining conveyor 44. 
A variation of the invention includes supplying the dry reject pellets from 
at or near ground level and lifting them vertically directly to the feed 
conveyor 12 by a shaftless screw conveyor. The dry reject pellets are fed 
by a horizontal conveyor to the inlet of a vertical conveyor, preferably 
of the shaftless type, such as 44. The sludge can be supplied directly to 
the input of the vertical conveyor.