GB1599760A - Conversion of organic matter to methane fertiliser or soil conditioners - Google Patents

Description

(54) CONVERSION OF ORGANIC MATTER TO METHANE, FERTILISER, OR SOIL CONDITIONERS (71) We, UNIVERSITY OF STRATH- CLYDE, a Body Corporate organised and existing by Royal Charter, of 204 George Street, Glasgow, Great Britain, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the anaerobic microbial conversion of organic matter, and is primarily concerned with the production of methane gas and with products which are of value to the growth of plants.

Known processes for the production of methane gas by means of the anaerobic microbial conversion of organic matter have been carried out in apparatus requiring mechanical stirring devices in order to maintain in suspension the microbial matter, and additionally the known processes have produced a relatively low yield of methane gas accompanied by by-products in solid form.

It is an object of the present invention to provide an improved process for the anaerobic microbial conversion of organic matter which can be performed without the need for mechanical stirring devices and which provides a maximum yield of methane gas accompanied by a minimum yield of byproducts in solid form.

According to the present invention there is provided an anaerobic process for the microbial conversion of organic matter to produce methane and a liquiform product, comprising confining a suspension and/or solution of organic matter in the presence of a mixedspecies culture of nutrient-supported microorganisms including methane-producing micro organisms to a column having a heightto-width ratio within the range 7:1 to 20:1, intermittently feeding the column with fresh organic matter to be converted, and maintaining the column at temperatures within the range 5--50"C.

Preferably the process is intermittently fed with organic matter in slurry form. The organic matter may be fed to the column at any location, but conveniently is fed to the base of the column.

The organic matter may be from a wide variety of sources of which examples are: pig slurry, cow slurry, yeast slurry, ground cereal slurry, alcoholic washes, yeast extracts, fish by-products, slaughterhouse effluent, seaweed residues, cellulose, organic materials protected from deterioration by content of organic acids.

The methane-producing mixed culture of micro organisms may be taken from a wide variety of sources of which examples are: cattle or other animal manure, grass clippings, mud from beneath stagnant waters, the digestive tract of animals, industrial effluents, municipal anaerobic digestor.

We have found that by confining the process to a columnar form whose height to width ratio lies within the range 7:1 to 20:1 the process continues without the need for any impressed form of mixing, such as by mechanical means and we believe that this is due to a continuous recirculation process occurring within the reactant mass by virtue of gravitational movement of solid matter accompanied by upward movement of gases produced within the reactant mass. The gases produced within the reactant mass are methane and carbon dioxide and these gases appear to cause partial flotation of the downwardly drifting solid matter. Because of this process the organic matter which is of solid form is retained within the reactant mass until such time as it is digested by the micro organisms and converted into methane gas and liquid products so that the process can provide for substantially complete conversion of solid organic matter. The retention time of the organic matter within the column may lie within the range 5 to 300 days.

Conveniently the column of reactant mass is defined by a container or alternatively the column may be one of several such columns arranged side by side in a single container having internal partitions defining the boundaries of the respective columns.

The produced methane gas collects within the container at the top of the column and liquid and solid products of the process are conveniently extracted through a trap (for example a water trap). The liquid products contain inorganic chemicals which are of value to growing plants. Also produced are organic chemicals some of which represent non-biodegradable solids which may be used in agriculture, some of which contain microbial solids which also are of value as fertilisers, and some of which are present as solutes in the liquid products mentioned and have no known value. All the organic solids have a value as a soil conditioner and humus agent.

Because the process of the present invention is regenerative the functioning of the column can last indefinitely provided that the column is intermittently fed with fresh organic matter to be converted and, since the microbial matter is not removed to any extent from the column, the process can be initiated with a heterogeneous mixture of microbes a substantial number of which become automatically adjusted to those which form flocks comprising microbes only or microbes attached to or otherwise associated in near proximity to solid organic particles some of which are undergoing digestion, i.e. a clustering together, the non clustering or flocing bacteria being continuously carried out of the process with the liquid products. This retention of floc material contributes towards a process of great inherent stability which can survive shortterm process upsets such as failure of temperature control or failure to feed fresh organic matter.

Experiments have shown that if the temperature is maintained within the range 5-50 C the process is self perpetuating and there is no requirement to confine the temperature to any further extent. By way of example the temperature may be ambient or a heater may be used to raise the process temperature above ambient.

Experiments have also shown that control of pH is non-critical and good results have been obtained from processes operating at a pH within the range 3.5 to 9.0. It has been found that the pH is dependent upon the particular feed to the column and for any given feed the pH of the process remains substantially constant.

It has also been shown that in the vicinity of the flocing material the Redox potential of the reactant mass reaches values as low as - 2OOmV during the production of high yields of methane gas.

Various further advantages derive from the process, including (i) there is an absence of scum build-up in the container; (ii) in comparison to known processes the yield of methane is substantially greater.

Known processes usually yield about 1 unit of methane per day whereas with the present process the yield is in the range 2-14 units of methane per day (the column volume being taken as the unit of measurement); (iii) the production of methane gas occurs at a substantially constant level which is minimally disturbed by process failures (such as described above); (iv) the residence time of organic matter within the reactant mass is substantially constant and is controllable in part by the selection of the dimensions of the container and in part by the feed rate of fresh organic matter.

The fresh organic matter may be prepared in a variety of forms from the previously mentioned sources. One typical form is a slurry containing solid and liquid matter, the latter providing the initial nutrient for the micro organisms. Another typical form is after preliminary digestion of say grass cuttings. In this case the cuttings are subjected to silage-like storage resulting in a partial transformation into materials which are readily consumed (digested) by the micro-organisms in the environment of the present invention.

Especially important is the presence of lowmolecular-weight organic acids resulting from the transformation, because such acids lead to good methane production.

The feed rate to the column depends upon the nature and quality of the organic matter but typically is of the order of 20% or less of the column volume per day.

Although the process of the present invention may be performed without impressed stirring it may be desirable to use such stirring when the form of the feed to the column hinders mixing efficiency within the reactant mass. This may occur, for example, when the feed is in slurry form and has too high a solids content or has an adequate solids content but is too viscous. By use of impressed stirring mixing within the reactant mass may be enhanced in the circumstances described, leading to higher yields of methane gas than would otherwise be the case.

The container in which the present invention is carried out may take the form depicted in Fig. 1 wherein the liquid at the top of the column of reactant mass enters a reducer the upper end of which is connected to an outlet pipe incorporating a water-trap.

The feed to the container is diagrammatically illustrated but may take any form capable of handling material in slurry form and preventing drainage of reactant liquid from the column. Operation of the feed device causes a positive displacement of liquid to occur in the column resulting in flow of liquid products along the outlet pipe.

The methane gas is removed through a vent pipe and may be stored in a conventional container.

An alternative form of container is depicted in Fig. 2 where a relatively squat outer envelope is internally partitioned into a plurality of side-by-side compartments each of which is appropriately dimensioned to fulfil the requirements of the present invention. The compartments have a common header space leading to a common outlet similar to that of Fig. 1. At their lower ends the compartments may be individually fed from separate feeding devices or they may be fed in common from a single feeding device.

The apparatus described above need not be utilised with the container disposed precisely vertically because satisfactory results can be achieved from the column even when arranged at a 45 degree inclination to the vertical.

It will be noted that the apparatus depicted in Figs. 1 and 2 do not incorporate any form of impressed stirring means and the only energy-consuming component is the feeding device. It is thus an important advantage that, because the conversion efficiency of the present process can be so great, the process operates with a nett energy gain. The nett energy gain may be enhanced by utilising a feed which is at a temperature above ambient, such an an industrial effluent.

WHAT WE CLAIM IS: 1. An anaerobic process for the microbial conversion of organic matter to produce methane and a liquiform product, comprising confining a suspension and/or solution of organic matter in the presence of a mixedspecies culture of nutrient-supported micro organisms including methane-producing micro organisms to a column having a heightto-width ratio within the range 7:1 to 20:1, intermittently feeding the column with fresh organic matter to be converted, and maintaining the column at temperatures within the range 5-50'C.

2. A process as claimed in claim I, wherein the process is intermittently fed with organic matter in slurry form.

3. A process as claimed in either preceding claim wherein the organic matter is fed to the base of the column.

4. A process as claimed in any preceding claim wherein the column of reactant mass is defined by a container.

5. A process as claimed in any one of claims 1 to 3, wherein the column of reactant mass is one of several such columns arranged side by side in a single container having internal partitions defining the boundaries of the respective columns.

6. A process as claimed in any preceding claim, wherein the organic matter is derived from any one of the sources herein recited and the methane-producing mixed culture of micro organisms is taken from any one of the sources herein recited.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. top of the column of reactant mass enters a reducer the upper end of which is connected to an outlet pipe incorporating a water-trap. The feed to the container is diagrammatically illustrated but may take any form capable of handling material in slurry form and preventing drainage of reactant liquid from the column. Operation of the feed device causes a positive displacement of liquid to occur in the column resulting in flow of liquid products along the outlet pipe. The methane gas is removed through a vent pipe and may be stored in a conventional container. An alternative form of container is depicted in Fig. 2 where a relatively squat outer envelope is internally partitioned into a plurality of side-by-side compartments each of which is appropriately dimensioned to fulfil the requirements of the present invention. The compartments have a common header space leading to a common outlet similar to that of Fig. 1. At their lower ends the compartments may be individually fed from separate feeding devices or they may be fed in common from a single feeding device. The apparatus described above need not be utilised with the container disposed precisely vertically because satisfactory results can be achieved from the column even when arranged at a 45 degree inclination to the vertical. It will be noted that the apparatus depicted in Figs. 1 and 2 do not incorporate any form of impressed stirring means and the only energy-consuming component is the feeding device. It is thus an important advantage that, because the conversion efficiency of the present process can be so great, the process operates with a nett energy gain. The nett energy gain may be enhanced by utilising a feed which is at a temperature above ambient, such an an industrial effluent. WHAT WE CLAIM IS:

1. An anaerobic process for the microbial conversion of organic matter to produce methane and a liquiform product, comprising confining a suspension and/or solution of organic matter in the presence of a mixedspecies culture of nutrient-supported micro organisms including methane-producing micro organisms to a column having a heightto-width ratio within the range 7:1 to 20:1, intermittently feeding the column with fresh organic matter to be converted, and maintaining the column at temperatures within the range 5-50'C.

2. A process as claimed in claim I, wherein the process is intermittently fed with organic matter in slurry form.

3. A process as claimed in either preceding claim wherein the organic matter is fed to the base of the column.

4. A process as claimed in any preceding claim wherein the column of reactant mass is defined by a container.

5. A process as claimed in any one of claims 1 to 3, wherein the column of reactant mass is one of several such columns arranged side by side in a single container having internal partitions defining the boundaries of the respective columns.

6. A process as claimed in any preceding claim, wherein the organic matter is derived from any one of the sources herein recited and the methane-producing mixed culture of micro organisms is taken from any one of the sources herein recited.