This invention relates to a process concerning generating energy by using a combination of different techniques. As raw material is utilised organic material discharged from water closets and sludge in suspension form from biological refining treatment.

Prior art

In municipal wastewater purifying plants particles, biological oxygen demand matters, nutrients and other impurities , for instance infectious matters are just partly removed by mechanical and biological treatment.

Normally the sewage sludge is first treated by a mechanical purification. Thus the suspension is passed through a lattice followed of a screen, or a sand trap or a sedimentation basin Thereinafter follows a biological purifiction treatment performed in a plant having activated sludge or a layer of a biological bed. In that biological treatment matters that are break down to carbon dioxide and water, partly due to forming of flocculates from small solid particles which are removed in a settlement basin. Usually the treatment is finished by a chemical treating stage. Thus divalent or trivalent salts of metals (for example Al ³⁺ ) are added and which salts are able to precipitate solved phosphor compounds and collected in form of sludge.

Before the sludge can be used or stored (deposited) the sludge has to be stabilised by rotting or by aeration. Outlet from industries are sometimes connected to the municipal wastewater purifying plants.

Often biological gas (methane) is produced from organic material such as sludge from both communities and industries. The residual sludge product contains large volumes of water and must be thickened by using different types of apparatus such as drum filters or decanter centrifuges. After thickening dry solids from about 6 % up to 25 % is obtained. The thickened sludge could be used as a fertiliser. Alternatively the residual sludge could be composted and used as soil.

Drawbacks with prior art

Since a lot of chemicals are used within the communities and that bacteria, virus and parasites of different types are common some of them are found in the sludge. It should be pointed out that within the EU-group about 110.000 commercial chemicals are registered. The National Research Council, USA, has classified about 10 millions chemicals. Many of the chemicals are not so good for the peoples health and should not be used as a fertiliser. Moreover the sludge leak odour that disturbs the surroundings.

The energy yield of the bio gas production from the sludge is <1 MWh per ton solid organic material. Due the high content of water in the sludge achieved even combustion does not seems do be an acceptable technique from an economical point of view. However, by using technique described in the Swedish patent 523 787 there is a possibility to reach solid content >50 %.

Description of the invention

This invention relates to a procedure concerning generating energy by utilise different techniques. As raw material organic material coming out from water closets and sludge in suspension form from biological refining is utilised. The invention comprises important stages such as thickening, production of the gas methane, dewatering and thermal treatment by combustion or by gasification. Apparatus and theirs function will be described in the following text and by referring to figure 1.

Outlet 1 from stables , purifying plants or a water closet is fed to a rotting tank 2 where the methane gas 3 is formed. As a residual product sludge 4 is obtained.

Since the residual sludge is diluted in large volumes of water the main part of the water must be removed before entering a thermal treatment (combustion or gasification). That thickening can be performed by mechanical filtration and/or by dewatering in several stages.

In accordance with the invention the solid content after the thickening would be from 5 % up to 25 % by using a drum filter 5 or by using a centrifuge.

A distinguishing feature of this invention is that the partially thickened sludge 6 is conveyed to a press 7 or a double wire machine where the solid content is increased to at least 48 %.

According to the Swedish patent 523787 solid matter (henceforth named agent) in particle form could be mixed together with the sludge before entering the dewatering device. That alternative is marked by a dotted arrow 8 in figure 1. Another manner in order to increase the solid content is according to the Swedish patent 528781 where the first stage is freezing the sludge and that the second stage is melting the frozen sludge before the pressing operation is to be carried out.

The dewatered sludge 9 having a solid content of at least 48 % is conveyed to a combustion chamber 10 which size might be suited after the actual demand. Of course a less combustion chamber is required when solely burning sludge compared with mixtures of sludge and another organic materials, for instance wood, bark and bio oil.

When burning the sludge a very important destruction of synthetic compounds is achieved.

After the burning a valuable residual product in form of ash 11 is obtained. The ash contains oxides of calcium, potassium and phosphor. The ash would be converted to granules or pellets.

The energy gained, as a result of the burning, should be used for recovering heat 12 (warming houses) and/or for generating electricity 13.

Instead of burning the residual product of sludge gasification could be used which technique is able to produce hydrogen and carbon monoxide. Two gases having high energy values.

Moreover the gases could be converted to, for instance, to synthetic diesel.

Example in accordance with the invention

At the purify plant Knorthem the annual solid weight (equal to the volume) of sludge is about 300 tons (or m ³ ) while the total weight with the water included is about 7500 tons/ m ³ since the average solid content is about 4 %. After generating of bio gas in the rotting tank (2) the residuals (4) is thickened in a decanter centrifuge (5) to solid contents of 23 % (± 2 %). Even after the thickening in the centrifuge the solid content of sludge is too low from a economical point of view if the sludge has to be burned. Therefore the partially thickened sludge was dewatered in a piston press (7) equipped with outlet pipes coupled to a main pipe (14) which one fed the excess of water back to the rotting chamber (2). By bringing back water the yield of bio gas could be increased due to the rotting reactions seldom is totally completed. Before pressing the sludge received from the centrifuge modified saw dust (8). was added and mixed together with the sludge. The content of saw dust calculated by the total weight was 40 %, while the solid content obtained of the mixture was 32,8 %. After the pressing a solid content of 57 % was achieved.

The theoretical energy value of the mixture having a solid content of 57 % is estimated to be about 4 MWh per ton dried organic material. The theoretical energy contribution from the sludge was so high as 2,4 MWh (0,6x4) per ton.

According to the invention it is possible to recover about 1 MWh by the rotting process and 2,4 MWh by burning or by gasification, which makes a total energy production of 3,4 MWh. That means, the theoretical energy yield is, indeed, surprisingly high or about 85 %. The mixture of sludge and saw dust having the solid content of 57 % was tested as a fuel by burning in a bio fuel furnace owned by OviksEnergi AB. There was no problems at all detected regarding the emptying the mixture from the container or feeding the furnace. The combustion of the mixture was carried out whiteout any problems. Surprisingly, indeed, was that the coatings of insoluble substances on the combustion walls was eliminated by presence of the mixture containing sludge. Moreover, it should be stated, that the ash received was easy to handling in form of fast micro particles. Of the ash granules was made having high quality with respects to hardness (means high density) and the even sizes (spread 3-8 mm).

The ash contained a surprisingly high content of CaO (6,2 %), K ₂ O (3,5 %) and phosphor as P ₂ O ₅ (phosphorus penta-oxide) The economical value was estimated to be in the range of about 600 Swedish crowns per ton bone dried ash.

There are a lot of unknown chemicals in sludge produced at purifying plants. Therefore it is from a environment point of view very important to get a destruction of the chemicals. Since no organic substances could be detected in the ash it was confirmed that the combustion of the mixture had been effective. Due to the ash reacts as a strong alkalinity chemical the foreign matters as bacteria, virus and parasites was effectively destroyed in combination with the thermal process.

Advantages

The invention is of importance with respects to create a better environment and a decreased consumption of energy. Furthermore all forms of energy reduction results in a better environment due to the consumption of fossil fuels can be reduced Thus, bio gas is produced by the rotting process and after dewatering of the residuals from the rotting a product with high energy value is achieved. By bringing back the excess water from the dewatering stage to the rotting chamber a higher yield of bio gas is obtained compared to known technique. The discharge of carbon dioxide among other things can be reduced significantly. Theoretically speaking a reduction in water content by 1 m ³ per ton oven dried sludge results in a reduction of the level of carbon dioxide (CO ₂ ) emitted by 200 kg. Moreover the inventions results in an effective destruction of unknown chemicals, bacteria, virus and parasites in the sludge received in the municipal wastewater purifying plants. Another positive result when using the invention is that after the combustion a valuable ash is achieved which can be utilised as a fertiliser. In case gasification is utilised both heat and valuable gases is produced.