Method for producing a compostable mixture of sludge from sewage purification plants

The invention relates to a method for producing a product from which compost can be suitably produced and which contains sludge obtained from a sewage purification process, in which method the sludge is mixed in a fluid state with a finely-divided carbon-carrier selected from the group sawdust, peat and bark, and with polyelectrolyte, and is then mechanically dewatered to a dry substance content of at least about 28 percent by weight.

The present invention relates to a method for producing a compostable 
mixture of sludge obtained from sewage purification plants. 
It is well known that decomposition of such sludge to produce compost is 
encumbered with difficulties. There are a number of reasons to account for 
this. Firstly, the amount of decomposable carbon present in the material 
is not sufficient to enable the material to react satisfactorily with the 
enzymes of microorganisms responsible for the decomposition of the 
material to form compost. Secondly, it is extremely difficult to dewater 
the material to a solids content at which the material can be effectively 
composted. Although it may be possible to dewater the material to a 
relatively high solids content with the aid of highly effective technical 
auxiliary means, the properties of the resultant material are not such as 
to enable the material to be brought into effective and uniform contact 
with the air, or oxygen-containing medium, required to effect a composting 
process. 
It has now been found that the aforementioned disadvantages can be avoided, 
completely or substantially completely, by means of a method in which 
sludge taken from a sewage purification plant and maintained in the same 
form as that which it had when leaving the plant is mixed with a 
carbon-carrier taken from the group sawdust finely-divided bark or 
finely-divided peat, and with polyelectrolyte, and the resultant mixture 
is then mechanically dewatered to a solids content of at least about 28 
percent by weight. 
As a result of mixing the sludge with the finely-divided carbon-carrier in 
an unthickened state, while the sludge is highly fluid, there is readily 
obtained an intimate mixture in which the sludge particles adhere to the 
larger particles of carbon-carrier, to form an agglomerate from which 
water can be readily removed with the aid of the polyelectrolyte when 
pressing the mixture. Subsequent to pressing the mixture there remains a 
homogeneous mass or pulp, which when introduced into a container packs to 
a porous layer or a pure porous mass through which the oxygen-bearing gas 
used to decompose the mass is able to pass without channeling. This is 
extremely important with view to the fact that the composting process also 
includes a heat-treatment step which aims at killing pathogenic 
microorganisms and in which all parts of the material are required to be 
brought into contact with the oxygen-containing medium, such that no area 
remains at a temperature which is not sufficiently high to ensure that the 
harmful microorganisms are killed. 
One important advantage afforded by the method according to the invention 
is that the amount of carbon-carrier introduced can be reduced within the 
framework of that amount required to ensure that the mixture can be 
decomposed to compost form. This effect can be ascribed to the fact that 
the sludge particles and carbon-carrier are so intimately mixed in the 
mixture that the carbon-carrier can be utilised more effectively. Whereas 
it is normal practice when preparing compostable material to use 
carbon-carrier in amounts of roughly 25 percent by weight, this amount can 
be reduced to additions in the range of from 10 to 20 per cent by weight 
when proceeding in accordance with the invention. 
An extremely important advantage afforded by the method according to the 
invention is that it can be carried out with smaller 
apparatus-contributions; this applies in particular to the step of mixing 
the main ingredients, sludge and carbon-carrier, which step does not 
require the power-consuming assistance of mechanical auxiliary devices. 
It has been found that by means of the combination of procedures according 
to the invention, it is possible, in simple fashion, to prepare 
compostable mixtures of sludge having high solids contents of at least 28 
percent by weight, and particularly 28-42 percent by weight. 
For example, when the material as mixed with the sludge is sawdust, the 
important advantage is afforded that the material can be mixed directly 
with the wet sludge, without needing to be finely-divided in a separate 
stage. The present invention enables sawdust products of high water 
content to be put to useful use while solving, at the same time, the 
problem of disposing waste sawdust produced in the saw mill; hitherto 
sawdust products of high water content have been rejected as a fuel due to 
the necessity of drying the products. 
In accordance with one embodiment of the invention, the ingredients can be 
mixed together while injecting thereinto an oxygen-bearing medium, 
preferably air. It has been found that the mixture desired can be quickly 
obtained by injecting an oxygenous medium into the premix. At the same 
time there is also afforded the advantage that the aerobic microorganisms 
present are made active, so that the subsequent composting process can be 
commenced with microorganisms which are more active than would otherwise 
be the case and thus resulting in a more rapid process of decomposition. 
According to another embodiment of the invention, the carbon-bearing 
substance used may be one which has a relatively high water content. The 
aforementioned materials, sawdust, peat and bark, have a low value when 
highly saturated with water, since they are then less attractive as fuel. 
The invention provides the advantage of using such water-saturated 
materials in a beneficial manner. The high water contents do not impose 
any economic or technical load on the process, since it is necessary, 
under all circumstances, to subject the material to a subsequent pressing 
stage in order to separate the water from said material. 
The invention is particularly advantageous when the water content of the 
material is so high that the material can be said to be in suspension. 
This enables the material to be readily introduced into the mixing zone, 
e.g. by pumping or like procedures, thereby also enabling the material to 
be dispensed more positively in metered quantities. 
The product produced by the method according to the invention is preferably 
intended for producing compost. The nature of the product enables it to be 
composted by various kinds of composting processes. Preferably, the 
material is composted in relatively small containers having connected to 
the bottoms thereof supply lines for air or optionally for some other 
oxygenous medium, the material in said containers forming a static bed or 
a static layer. In order to nake the composting process as advantageous as 
possible with respect to apparatus contributions, the process is suitably 
divided into two stages, namely a pre-composting stage which is effected 
in a container and the main purpose of which is to kill pathogenic 
microorganisms, to which end there is required treatment at a high 
temperature of at least 50.degree. C., preferably 60.degree. C., over a 
long period of time, a seven day period at 50.degree.-60.degree. C. being 
considered to safely kill such organisms, and a subsequent composting 
process which can be carried out with the least possible 
apparatus-contribution and in which the pre-composted and thus hygienized 
material can be placed on a gas-permeable plate and treated with air until 
the desired degree of decomposition has been reached. 
By composting the product of the method in a container, the important 
advantage is afforded that bio-activated exhaust air from an adjacent 
container in which an earlier composting process has been carried out can 
be used to initiate the composting process in the first container and, 
once the process has achieved a certain rate of decomposition, to maintain 
this rate. By bio-activated air is meant the carbon-dioxide containing hot 
exhaust gas which departs from the upper surface of the mass being 
composted when, as is most normally the case, air is fed to the bottom of 
the mass, this exhaust air, due to its saturation with water vapour, 
containing a mist of fine water droplets saturated with microorganisms of 
the kind taking part in a composting process. 
The invention also relates to compost-producing apparatus comprising a 
first station which includes a plurality of transportable containers each 
of which having a space for the material undergoing decomposition and each 
being intended to serve as transport means for transporting partially 
decomposed or composting material to a second compost-producing station, 
characterised in that said apparatus at said first station is provided 
with means for connecting the containers to an air-supply source and means 
for connecting said containers to an outlet conduit for exhaust air.

Wet sludge obtained from a sewage purification plant and having a solids 
content of about 3 percent by weight is introduced into a mixing container 
1 through a first infeed line or conduit 2 with the aid of a suitable 
pump. Sawdust taken from a storage vessel 3 is introduced into the 
container 1 through a second infeed line 5 with the aid of a screw feeder 
4. The sawdust has a dry solids content of 40 percent by weight and a 
density of 350 kg/m.sup.3. Normally, such sawdust is not suitable for use 
as fuel, because of its high water content, which amounts to 60 percent. 
The mixing container 1 has a volumetric capacity of 30 m.sup.3 
.multidot.25 m.sup.3 of sludge is fed through the line or conduit 2, while 
1.3 m.sup.3 of sawdust is fed through the line or conduit 5, i.e. 455 kg. 
The mixing container 1 has arranged at the bottom thereof an 
air-distributing means 34, through which air is introduced into the 
contents of the container 1, in a manner to ensure that the said contents 
are stirred or turned to the extent required, this stirring or turning of 
the contents thus being effected without the aid of mechanical auxiliary 
devices. Because the sludge is fluid it will be rapidly and effectively 
mixed with the sawdust, to form a homogeneous mass, with the aid of the 
aforementioned air-stirring of said ingredients. Although there is nothing 
to prevent mechanical auxiliary devices from being used in order to obtain 
a more thorough mixing of the ingredients, the mixing effect obtained with 
the aid of air is normally sufficient to render the use of such auxiliary 
devices unnecessary. The injection of air into the mixture of sludge and 
sawdust also affords the advantage of initiating or accelerating an 
aerobic decomposition process, and results in a more rapid increase in 
temperature in the subsequent composting stage. The resultant suspension 
of sludge and sawdust is then withdrawn by suction from the mixing 
container 1 and passed through an exhaust conduit 6 to a screening 
belt-filter 8. At a location upstream of the point at which the suspension 
is passed to the filter 8, the suspension is supplied with polyelectrolyte 
solution in an amount corresponding to 2 kg of polyelectrolyte, calculated 
on the whole of the suspension, this polyelectrolyte solution being pumped 
through a conduit 9 by means of a pump 10. The belt filter 8 comprises two 
endless belts which are brought together at a pressing zone. The mixture 
of wet sludge sawdust and polyelectrolyte are introduced into the pressing 
zone and there pressed to a solids content of 30 percent by weight, 
whereupon the water pressed out from the suspension runs down through a 
drain 11. The high dry solids content of the thus pressed mixture imparts 
surprising properties thereto, which render the mixture extremely suited 
for a subsequent composting process. Thus, there is obtained a homogeneous 
and porous mass without requiring the use of mechanical auxiliary devices 
in the preparation of the mixture. The main purpose of the sawdust 
supplied is to serve as a carbon carrier in the resultant, pressed, 
compostable mixture. The sawdust, together with the polyelectrolyte, also 
serves to facilitate dewatering of the sludge. It has been surprisingly 
found that within the framework of what is required with respect to the 
function of a carbon-bearing substance, less sawdust than would be 
expected is required to achieve a given dewatering effect. 
The resultant compostable mixture of high solids content can then be 
subJected to a composting process. To this end, there is used a plurality 
of transportable composting containers 12, 13 and 14 each of which has a 
volumetric capacity of 15-20 m.sup.3 and to which compostable, pressed 
material from the filter 8 can be selectively transported with the aid of 
a conveyor belt 15. Each container 12, 13 and 14 has arranged at the 
bottom thereof a device 33 for supplying air to respective containers and 
distributing said air therein, in a manner to cause the air to pass 
through the mass of compostable material present in the container. 
Connected to the top of each container is an exhaust-air conduit, the 
conduit extending from the container 12 being referenced 16 and the 
conduit from the container 13 being referenced 21. Exhaust air is passed 
to a three-way valve, the valve allotted the container 12 being referenced 
17 and the valve allotted to the container 13 being referenced 18. The 
valve 17 is set so that exhaust air deriving from the container 12 can be 
passed through the exhaust conduit 16 and conduit 19 directly to the 
surrounding atmosphere (optionally after being cleansed) through the 
outlet 20. The exhaust air obtained in the container 13 passes through the 
exhaust conduit 21 to the three-way valve 18, which is set so that said 
exhaust air is partly recycled through recycling conduits 22 and 23 and 
partly discharged to the surrounding atmosphere, through conduits 23, 32 
and 19. The recycling conduit 23 is connected to an inlet conduit 24, 
which incorporates a fan 25, and a control valve 26. The inlet conduit 24 
is connected to a distributing conduit 27, which in turn is connected to a 
branch conduit 28 leading to the container 12, and to a branch conduit 29 
leading to the container 13. The branch conduit 28 incorporates a control 
valve 30 which controls the supply of composting air to the compostable 
mass present in the container 12, and the branch conduit 29 incorporates a 
control valve 31 having the same purpose as the control valve 30. Surplus 
gases in the system are passed to the conduit 19 and the outlet 20, 
through a conduit 32, when the control valve 17 is set to the position 
illustrated in the drawing. As will be understood, the valves 17 and 18 
can be reset in each particular case, to provide the same effect, 
wherewith surplus gas will then pass out through the conduit 22. The 
container 14 illustrated in broken lines symbolizes the possibility of 
providing transportable composting containers additional to the containers 
12 and 13 in the station in which the composting process is carried out. 
Slightly above the device 33 for supplying air to the container 12, 13 or 
14 there is arranged a grid-like bottom 35 which serves as a support for 
the mass undergoing composting in the respective container. Below the 
grid, there is formed a space for the device 33 thereby to attain that the 
air streams leaving orifices in the device are united into one stream 
which is uniformly contacted with the underside of the grid and caused to 
enter uniformly distributed into the mass of composting undergoing 
material after having passed the grid bottom. 
Immediately a container 12 or 13 has been filled with compostable material 
from the dewatering filter 8, air is supplied through the conduit 28 in an 
amount controlled by means of the control valve 30. The material contains 
sufficient microorganisms to initiate the formation of enzymes and 
therewith the decomposition of the material while generating heat. After 
24 hours it was found that the temperature had risen to 50.degree. C. The 
material can be considered free from harmful pathogenic microorganisms (to 
be hygienized) after 7 days at a temperature within the composting 
material of 50.degree.-60.degree. C. The container, together with its 
contents, can now be collected by a loader and moved to a further site, 
where the material is finally composted and then matured. The material can 
be finally composted in the open air, in which case the material is placed 
on an airpermeable plate and treated with air until a product having the 
desired properties is obtained. The container used to transport the 
material is then moved back to the composting site. Subsequent to filling 
the container with further compostable material of high solids content 
from the dewatering filter 8, a further treatment period is initiated and 
driven to a temperature of 50.degree. C., whereafter the material is 
partially composted and hygienized at a temperature of 
50.degree.-60.degree. C. 
The exhaust air departing from respective containers through the exhaust 
conduit 16 or 21 is warm and saturated with moisture and consequently 
condenses very readily to form a mist of fine water droplets. These water 
droplets are able to form seats for microorganisms from the mass of 
material being composted in said container. By recycling exhaust air 
containing microorganisms to a further container, it is possible to 
initiate the formation of enzymes in the compostable mass present in the 
container more rapidly than if the decomposition process was commenced 
with the sole use of microorganisms obtained from the sewage sludge. The 
valve 18 is set so that warm and moist air containing mioroorganisms and 
obtained from the container 13 is recycled to the container 12 via the 
inlet conduit 24, the distributing conduit 27 and the conduit 28. If the 
container 12 has been newly filled with compostable material, the 
decomposition process will be initiated very rapidly as a result of 
recycling said exhaust air.