GB2472183A

GB2472183A - Processing aquatic plant material

Info

Publication number: GB2472183A
Application number: GB0909990A
Authority: GB
Inventors: Alister Gower; John Henry Edward Colton; Theresa Elizabeth Colton
Original assignee: HYDROHARVEST INTERNAT Ltd
Current assignee: HYDROHARVEST INTERNAT Ltd
Priority date: 2009-06-09
Filing date: 2009-06-09
Publication date: 2011-02-02
Also published as: GB0909990D0

Abstract

A riverside farm 10 is furnished with a system for processing water hyacinth or other plants infesting the river 12, which system comprises harvesting barges 14, 18 and 20 and an anaerobic digester 16. The barges 14, 18 and 20 are driven up and down and across the river 12 and harvest the water hyacinth, so keeping the river 12 clear. The harvested plant material is transported (directly or by a mother barge 22 or other means) to the anaerobic digester 16 located in a lagoon 54. This digests the harvested plant material to produce biogas, compost and water for use on the farm 10.

Description

PROCESSING AQUATIC PLANT MATERIAL

This invention concerns the processing of aquatic plants such as water hyacinth (Eichhornia crassipes) particularly but not necessarily exclusively for generating biogas therefrom.

In many parts of the world, and particularly in tropical and sub-tropical regions, certain aquatic plants grow as weeds and their rapid proliferation causes serious problems. Water hyacinth is notorious in this respect. Among other things, it restricts hydroelectric power schemes, it blocks irrigation systems, it interrupts water supplies, it hinders water transport and it impedes fishing. It is also believed to contribute to the spread of water-associated diseases such as malaria and : schistosomiasis (bilharzia). Other weeds which are less widespread than water *.S.

hyacinth but nevertheless similarly problematic in certain areas include water fern (Salvinia molesta), water lettuce (Pistia siratiotes), hydrilla (Hjidril/a verticul/ata), common *..SS. * S

coontail, (Ceratop,yllum demersum), water milfoil (Mjirioph5llum spicatum) and alligator S...

* * . weed (Alternanthera philoxeroides).

Many different techniques have been tried to control water hyacinth and other aquatic weeds. Broadly speaking, these techniques fall into three categories, each with a distinct disadvantage. First, chemical control entails spraying the plants with herbicides, which inevitably results in possibly hazardous contamination of the water in which the plants are growing, and possible long-term environmental effects that are difficult to predict but include the uncontrolled release of methane into the atmosphere as the plant material breaks down. Second, biological control agents such as arthropods and pathogens may be applied, but this technique tends to be slow to act and is subject to the risk that the control agent itself may become a pest. Third, the plants may be mechanically cut and removed from the water, but when the removed material rots down it produces environmentally damaging greenhouse gases.

On top of these disadvantages of known techniques for controlling aquatic weeds is the fact that none makes gainful use of the biomass embodied in them.

Water hyacinth has been used to make paper, furniture (as a substitute for rattan), baskets and fuel briquettes, but only on a small scale and with very limited impact on the proliferation of the plant or other benefit to the environment.

: One previous proposal for large-scale utihsation of water hyacinth is to * .S.

process it in an anaerobic digester to obtain biogas plus fertiliser as a by-product. A I...

0*s* proposal of this kind for use in Lake Victoria is outlined by Matthews E G in Water *s.. e. * *

Hjiacinth Project at www.angelfire.com/mac/egmatthews/geotherapy/hyacinth.html **** * *.* (Revised 2004). Matthews proposes that water hyacinth be collected from the lake and delivered to a processing site where the collected vegetation is converted into vendible products such as biogas and fertiliser. However Matthews admits that he does not know how to collect the weed from the lake and is unable to define an appropriate form of digester, for which he proposes further research. In short, Matthews fails to provide a solution.

It is an object of the present invention to provide a system for gainfully processing water hyacinth plant material and possibly other aquatic weeds.

Thus according to a first aspect of the invention there is provided a system for processing water hyacinth or other plants growing in water, which system comprises a vehicle drivable through the water and operative to cut the plants and collect the cut plant material, transfer means operative to transfer the collected plant material from the vehicle and an anaerobic digester operative to receive the transferred plant material and produce biogas therefrom.

The anaerobic digester preferably operates on a continuous digestion process. In the continuous process the plant material is fed into the digester regularly and the biogas drawn off as required, not synchronised with the feedstock input. This is more convenient than compared with the alternative batch process * ., S and has other advantages including generally better digestion and less effluvia. S..

*: Preferably the anaerobic digester is located substantially in a lagoon of the S..... * .

water. This has four important benefits as follows. First, the enclosing water limits * )..

leakage of gas or liquid, thereby containing any smell, and makes any such leakage easy to detect and rectify. Second, the water both bears most of the weight of the digester and largely balances hydrostatic pressure therein, so that the digester is physically supported against buckling or collapse and can be manufactured from light and relatively inexpensive material such as synthetic plastics. Third, the water surrounding the digester reduces heat loss from the digester, thereby stimulating the digestion process and reducing energy input requirements -both of which are

S

further improved as water in the lagoon is warmed up by solar energy during the day and retains heat at night. Fourth, having the digester in the water facilitates transfer of the plant material from the collecting vehicle to the digester.

The vehicle preferably comprises a barge equipped with a propulsion unit operable in the weed-infested water to drive the barge. The barge may carry a cutter configured and arranged to be deployed under the water to cut the plants.

The barge may carry a conveyor operable to convey the cut plant material into the barge, and the conveyor may be reversible to discharge the cut plant material from the barge. The barge may have a bulkhead movable aftward as the cut plant material is conveyed into the barge and forward as the cut plant material is discharged therefrom. The barge may be equipped with an engine configured and arranged to be fuelled with biofuel derived from the biogas.

The transfer means preferably comprises a hopper whereinto in use the cut plant material is discharged from said vehicle. This hopper may be mounted on a * S * SS * *. 15 vessel drivable through the water to deliver the cut material from the hopper to the *SSS ** anaerobic digester. Otherwise the hopper may be fixed, with the transfer means **** * S.....

* including a macerator operative to macerate the cut plant material, a conduit : between the hopper and the anaerobic digester and a pump (eg a rotary *.**e * displacement pump) operative to pump the macerated material from the hopper to the anaerobic digester by way of the conduit.

The macerator may comprise a plurality of rolls configured and arranged to be driven to crush the cut plant material. The macerator may also comprise a shredder operative to shred the crushed plant material.

The hopper preferably contains a sprayer operative to spray water over the cut plant material to facilitate its passage through the hopper.

The anaerobic digester preferably comprises a fermentation tank wherein the plant material is circulated in a substantially continuous process. The anaerobic digester may comprise a heater for the tank. The fermentation tank may have a wall shaped to facilitate circulation of the plant material therein, which wall may comprise a sheet, possibly of synthetic plastics material, curved about a substantially horizontal axis. This wall may have a cross section substantially in the form of a cardioid having two lobes and a cusp therebetween, the cusp being directed generally upwards, and some of the biogas may be fed back to the fermentation tank and therein directed through said cusp so as to stimulate *::::* 15 circulation of the plant material around each of said lobes.

S. The system preferably comprises a plurality of fermentation tanks in series. S...

S

* With this arrangement locating the anaerobic digester in a lagoon has an advantage S...

* S.: additional to those previously enumerated in that the water in the lagoon may IS....

S S

provide a heat transfer medium between the tanks, thereby improving the efficiency of fermentation throughout the digester, particularly where the fermentation is exothermic, Preferably at least one said fermentation tank has an outlet for residue from the fermentation connected to a separator operative in use to separate liquid from solid components of the residue, and the system may include means for treating the solid component to produce compost and means for treating the liquid component to extract usable water therefrom.

The system may include means for mixing the plant material with an additional material such as a fermentation conditioner, possibly dung, to accelerate anaerobic fermentation of the material.

In a further aspect the invention comprises a method of operating a farm on land adjacent a body of water containing water hyacinth or other plants, which method comprises: harvesting the plants; transferring the plant material to an anaerobic digester; fermenting the plant material in the anaerobic digester to produce gas and a slurry; capturing gas from the fermentation and converting at least part of thereof into biomethane fuel; separating the slurry into solid and liquid components; composting the solid component for spreading on the land; treating S...

the liquid component to recover water for use on the farm; and using at least some of the captured gas and/or the biomethane fuel to power harvesting the *..: plants, transferring the plant material, operating the anaerobic digester, spreading *S.*S.

* the compost and distributing the water; characterised in that the farm is thereby substantially self-sustaining and carbon-neutral or better.

The plant material may be mixed with dung with to facilitate anaerobic fermentation thereof, and some of the captured gas and/or the biomethane fuel may be used to power collection and mixing of the dung.

Gas from the digester may also provide fuel for cooking in the local community, thereby reducing the demand for locally sourced wood and deforestation, with further significant benefits for global warming.

Other aspects of the invention will be apparent from the following description which is made by way of example only with reference to the accompanying drawings in which -Figure 1 is a schematic plan view of a farm, adjacent a body of water, embodying the invention; Figure 2 shows in side elevation a vehicle for harvesting aquatic plant material as part of the invention; *:::: Figure 3 illustrates in vertical cross section a macerating and pumping *. 15 subsystem of the invention; and S...

SS* Figure 4 is a block diagram of an anaerobic digester of the mventlon.

* The description which follows is based on use of the invention in a rural * area of a tropical or subtropical country. Many such areas are relatively *.**.* * S underdeveloped and therefore likely to gain particular benefit from the invention.

However it should be understood that the invention is not limited to use in areas which are either rural or underdeveloped. Further, whilst the invention is described in relation to its use on a farm, it may equally be operated by an extended community such as a village, possibly on a shared basis. The only essential requirement is the proximity of a body of water such as a river or lake containing a substantial amount of water plants.

Referring first to Figure 1, this illustrates a farm 10 the nominal perimeter of which is shown in broken lines. The farm 10 is typical of small mixed farms in tropical countries, with some cattle and poultry as well as crops. It is situated on land adjacent a river 12, where fishing provides an important supplement to the local community.

The river 12 is infested with water hyacinth, and before the introduction of the present invention this blocked the river and severely restricted fishing so that the community found it difficult even to achieve a subsistence level. From time to time, attempts were made to remove the water hyacinth manually. But it proved impossible to keep pace with the proliferation of the plant, which can double in :::: biomass in less than a week. And workers involved in manual clearance were e'. 15 threatened by crocodile or hippopotamus attack and exposed to increased risk . from water-associated disease including malaria and schistosomiasis.

I

I.....

* As will now be explained, the invention has transformed the situation. The S...

*..: * invention comprises three key elements -means to harvest the water hyacinth, * means to ferment the harvested water hyacinth and means to transfer the water hyacinth from the harvesting means to the fermenting means. The harvesting means comprises a barge 14 (to be described in more detail hereinafter with reference to Figure 2) which is driven up and down and across the river 12 and harvests the water hyacinth, so keeping the river 12 clear. The fermenting means comprises an anaerobic digester 16 (to be described in more detail hereinafter with reference to Figure 4) which digests the harvested plant material to produce biogas, compost and water. The transfer means comprises a mother barge 18 (to be described in more detail hereinafter with reference to Figure 3) where the harvested plant material is macerated and pumped to the anaerobic digester 16 by way of a floating pipeline.

As well as the benefits of a clear river -allowing fishing as well as the passage of water-borne transport -the invention creates a self-sustaining virtuous circle for the farm 12, and the broader community, as follows: -The water hyacinth is perceived to be a valuable commodity, and managed accordingly, and so becomes a resource rather than a nuisance; :::: -With a substantially unlimited amount of water hyacinth as feedstock, s. 15 the anaerobic digestion process is continuous, which is more efficient * : : and effective than batch processing; ** S. .S * -Anaerobic digestion of the water hyacinth generates biogas, some of *..* * which is burned to input heat to the digestion process; * S -The remainder of the biogas is converted (by means well known, including the extraction of carbon dioxide) to biomethane; -The biomethane fuels the harvesting, maccrating and pumping operations; -Solid residue from the digestion process can be turned into compost for use on the farm (with possibly a surplus for sale); -Liquid residue from the digestion process can be treated to extract water for irrigation (and possibly potable use); -The biomethane also fuels a tractor which among other things carries the compost about the farm; -The biomethane also fuels a pump to deliver the extracted water to points of use; -The compost and the irrigation stimulates growth of crops on the farm; -The compost and the irrigation also stimulates growth of fodder for * : : : cattle on the farm; and ***.

* 15 -The cattle produce dung to be mixed with the harvested water * hyacinth to accelerate anaerobic digestion (and produce more biogas).

****** * At this point a very important additional benefit of the invention should be *.** * S...

* noted. When a river or other body of water is choked by water hyacinth, the water S.....

S

tends to be stagnant and this gives rise to much anaerobic decomposition of dead vegetative matter, releasing large quantities of methane as a particularly damaging greenhouse gas. If the weed is removed to land and allowed to rot, the decomposition is essentially aerobic and releases gas mostly in the form of carbon dioxide, and whilst carbon dioxide is much less damaging to the environment than methane, the volume released is likely to be much greater because the mass of water hyacinth is so great. Either way, water hyacinth and similar aquatic weed growth exacerbates the problem of global warming through release of carbon (as carbon dioxide or methane) into the atmosphere. By means of the invention, this carbon is captured. That is to say, the invention is better than carbon neutral in environmental terms.

Returning to Figure 1, it will be understood that the invention may be operated at a variety of different levels of sophistication. At a first, basic, level the barge 14 may deliver the harvested water hyacinth directly to the anaerobic digester 16, operated on and for the specific benefit of the individual farm 12. At a second level the invention may be shared among a number of farms, to spread the cost, *: :: : and thus waterside farms adjoining the farm 10 may each have a barge (indicated at 15 20 and 22 in Figure 1) harvesting aquatic weed from the river 12 and delivering it to the anaerobic digester 16 on the farm 10. At a third level the mother barge 18 S...

I

* may range up and down the river 12, collecting harvested water hyacinth from the S...

: barges 14, 18 and 20 and transporting it to the anaerobic digester 16 (where there S.....

S

may be a jetty 24 for mooring the mother barge 22 and to serve as a landing stage).

And at a fourth level the mother barge 18 may be moored at a location convenient to receive plant material collected by the plurality of harvesting barges 14, 20 and 22 and be provided with shredding/maccrating and pumping equipment for transferring the plant material, macerated, to the anaerobic digester 16.

Referring now to Figure 2, this shows in more detail the barge 14 of Figure 1 being used to harvest a mass of water hyacinth 30 infesting the river 12. The water hyacinth floats in the water and/or grows up from the bed 32 of the river 12 to well above the surface 34 of the river 12, which at this point has a depth of about 1.5m. The barge 14 is driven through the water by means of a stern-mounted helix drive propulsion unit 36 powered by an inboard engine (not detailed).

The barge 14 carries a cutter-conveyor assembly indicated at 38 having a cutter at its forward end and a belt conveyor extending rearwardly therefrom. The cutter-conveyor assembly is powered by a hydraulic power take-off (not detailed) from the engine of the barge 14 and has three different positions shown in Figure 2 as 38a, 38b and 38c. In position 38a the cutter-conveyor assembly 38 is retracted "S * * * and inoperative. In position 38b the cutter-conveyor assembly 38 is deployed so * .**, that the cutter extends below the water surface 34, towards the bed 32 of the river S...

12. In this deployed position the cutter is operated to cut the water hyacinth 30 as * the barge 14 is driven forwards and the conveyor is operated to carry the cut plant *....

* material upwards and rearwards into the hold of the barge 14. Position 38c of the cutter-conveyor 38 is a discharge position in which the cutter is inoperative and the conveyor is operated in reverse (relative to position 38b) to carry cut the plant material out of the hold of the barge 14 and forwards over the bow of the barge 14. It will be understood from the foregoing description with reference to Figure 1 that position 38c of the cutter-conveyor assembly 38 enables harvested plant material to be discharged from the barge 14 into the mother barge 18 for shedding/maceration and pumped transfer to the anaerobic digester 16. (At a more basic level of the invention a harvesting barge may discharge the collected plant material direct to an anaerobic digester).

The hold of the barge 14, which has a load capacity of about 3m3, contains a constrictor 40 which can be driven (from the power take-off by means not detailed) fore and aft. When cutting commences, the constrictor 40 is in a forward position and it is progressively moved aft as the Cut plant material is carried into the hold by the conveyor. By this means the hold is filled progressively and substantially evenly, to make best use of its capacity. When the hold is full, the constrictor 40 is at its aftmost position. \X7hen the load is to be discharged (the *: : : cutter-conveyor 38 being then in its discharge position 3 8c and the conveyor being *::: :* 15 operated to carry the cut plant material out of the hold of the barge 14) the constrictor 40 is moved progressively forward to tip the load on to the conveyor, **** * S....

* so that the discharge of the cut plant material is both efficient and effective. **..

: * The construction and operation of the maceration and pumping equipment *.*..* on the mother barge 18 and its associated equipment will now be described with reference to Figure 3.

Referring then to Figure 3 this shows a hopper 80 arranged to receive cut plant material 82. Within the hopper 80 is a first row 84 and a second row of rolls 86 arranged generally in parallel for rotation as indicated by the arrows in Figure 3 and driven by some means not shown such as a hydraulic drive. The rolls 84 and 86 are formed and arranged to function generally like the rolls of sugar mill, crushing the cut plant material as it passes down through the hopper 80. To facilitate the passage of the cut plant material down through the hopper 80, a pump 88 connected to a hose 90 draws water up from the river in (or by) which the hopper 80 is situated and sprays it over the cut plant material as indicated at 92.

After the cut plant material has passed down through the hopper 80, a rotating blade 94 shreds it. As a result, what reaches the bottom 80a of the hopper 80 is a wet pulp which can be readily pumped by means of a rotary displacement pump 96, also driven by means such as a hydraulic drive. The pump 96 draws in the pulp and pumps it out through a duct 98 connected by a Bauer hose coupling or the like to the conduit 26. Thus the pulp is delivered to the anaerobic digester 16 (Figure 1), by way of intermediate pumps (not shown) if required.

*::* 15 The construction and operation of the anaerobic digester 16 will now be *. described with reference to Figure 4. The digester 16 comprises a pair of * S fermentation tanks 50 and 52 connected in series in a lagoon 54. (As indicated in ***.

Figure 1, the lagoon 54 may be cut out of a bank of a river or another body of * S water. It may be lined with concrete to provide a substantially permanent structure). Each of the tanks 50 and 52 is heated (by means not detailed) and the water in the lagoon provides insulation and may also be heated by the sun, to improve efficiency.

Macerated plant material pumped through the floating conduit 26 is delivered at A into a holding tank 56, where it is mixed with dung to accelerate facilitate fermentation. A slurry pump 58 delivers the mixture from the holding tank 56 to the first fermentation tank 50, by way of a slurry valve 60, and from there to the second fermentation tank 52.

The fermentation tanks 50 and 52 operate on a continuous process. The mixture of the macerated plant material and dung, which is essentially a solid-liquid suspension, is continuously circulated as it ferments. The fermentation causes gas (principally methane with a proportion of carbon dioxide) to be produced and this is captured in a pressure vessel 62, pressurised by a gas pump 64, and can be drawn off for use as required. Some of the pressurised gas is fed back from the pressure vessel 62 to the fermentation tanks 50 and 52, where it is bubbled through the mixture therein to stimulate its circulation and maintain its suspended state.

:::: Sensors 66 are arranged on or in each of the fermentation tanks 50 and 52 to I...

** 15 measure a parameter of the fermentation process (such as temperature or pressure or sound) and are connected to gas valves 68 controlling the feedback of the

I

I...,.

* pressurised gas. I..,

Pressuriseci is drawn off at B. Some of this may be burned to heat the * I. III * I fermentation tanks 50 and 52. The rest may be used for other purposes (including use as fuel within the local community, thereby reducing deforestation in the area) and it may be upgraded to biomethane by removing carbon dioxide, water vapour and other impurities in known manner.

Residue from the fermentation is delivered to a separator 70 which separates solid from liquid components. The solid component is delivered to a composter 72 wherein it is formed into compost. The compost is then packaged in a bagging machine 74 from which it is delivered at C for use on the farm 10 (Figure 1) and/or for sale. The liquid component is delivered into a settling tank 76 and then passes to water treatment equipment 78 from which water is delivered at D. The quality of the water delivered at D depends upon the nature of the water treatment equipment 78: it may be potable (in which case the water treatment equipment must provide chelation as well as filtering) or it may be adequate for irrigation on the farm 10 (which can be provided, unlike irrigation direct from the river 12 or other body of water, without material risk of agrestal escape of the water hyacinth).

An anaerobic digester which may be usable in the invention is that described in European Patent EP12301 73. This digester has a fermentation tank of cardioid cross-section extending along a generally horizontal axis with a wall formed by I..' * * * curving a thin sheet of synthetic plastics material around the axis. This is inexpensive. And because the digester 54 is located in a lagoon its lightweight structure is adequately supported and resistant to buckling as well as being well S...

* .. * insulated. Further, the water contained in the lagoon 54 acts as both a heat S.. Sal insulator for the fermentation tanks 50 and 52 and provides a medium for heat transfer between the tanks, thereby improving the efficiency of fermentation. It should be understood, however, that the invention is not limited to use of the digester the subject of EP1230173.

Further, various modifications and additions to the system as described may be made without departing from the scope of the invention. For instance, particularly in shallow water, a frxed hopper may be located (in place of the mother barge 18) at a position along the river 12 where a plurality of barges 14, 20, 22 etc may each conveniently discharge its harvest of water hyacinth, this being there shredded/macerated and pumped to the anaerobic digester 16 by way of a floating conduit. Other alterations will be apparent to those skilled in the art. s.. * I* I. I * ,** * I * )SI ***. * I Is.

I s,,*a * *

I I...

S

*.***S * I

Claims

CLAIMS1 A system for processing water hyacinth or other plants growing in water, which system comprises a vehicle drivable through the water and operative to cut the plants and collect the cut plant material, transfer means operative to transfer the collected plant material from the vehicle and an anaerobic digester operative to receive the transferred plant material and produce biogas therefrom.
2 A system for processing plants growing in water as claimed in Claim I characterised in that the anaerobic digester operates on a continuous digestion process.
3 A system for processing plants growing in water as claimed in Claim 1 or Claim 2 characterised in that the anaerobic digester is located substantially in a lagoon of the water.
4 A system for processing plants growing in water as claimed in any * : ::. preceding claim characterised in that the vehicle comprises a barge equipped with a *. 15 propulsion unit operable in the water to drive the barge.*
5 A system for processing plants growing in water as claimed in Claim 4 S.. **5*S** . * charactensed in that the barge carries a cutter configured and arranged to be I...

*..: deployed under the water to cut the plants.

*S***S *
6 A system for processing plants growing in water as claimed in Claim 5 characterised in that the barge carries a conveyor operable to convey the cut plant material into the barge.
7 A system for processing plants growing in water as claimed in Claim 6 characterised in that the conveyor is reversible to discharge the cut plant material from the barge.
8 A system for processing plants growing in water as claimed in Claim 6 or Claim 7 characterised in that the barge carries a bullthead movable aftward as the cut plant material is conveyed into the barge and forward as the cut plant material is discharged therefrom.
9 A system for processing plants growing in water as claimed in any of Claims 4 to 8 characterised in that the barge is equipped with an engine configured and arranged to be fuelled with bio fuel derived from the biogas.
A system for processing plants growing in water as claimed in any preceding claim characterised in that the transfer means comprises a hopper whereinto in use the cut plant material is discharged from said vehicle.
11 A system for processing plants growing in water as claimed in Claim *::: 15 10 characterised in that the hopper is mounted on a vessel drivable through the * water to deliver the cut material from the hopper to the anaerobic digester.

*
12 A system for processing plants growing in water as claimed in Claim **S *..: * 10 characterised in that the hopper is fixed and the transport mechanism includes a * S macerator operative to macerate the cut plant material, a conduit between the hopper and the anaerobic digester and a pump operative to pump the macerated material from the hopper to the anaerobic digester by way of the conduit.
13 A system for processing plants growing in water as claimed in Claim 12 characterised in that the hopper is carried by a floating platform moored in the water.
14 A system for processing plants growing in water as claimed in Claim 12 or Claim 13 characterised in that the macerator comprises a plurality of rolls configured and arranged to be driven to crush the cut plant material.
A system for processing plants growing in water as claimed in Claim 14 characterised in that the macerator comprises a shredder operative to shred the crushed plant material.
16 A system for processing plants growing in water as claimed in any of Claims 10 to 15 characterised in that the hopper contains a sprayer operative to spray water over the cut plant material to facilitate its passage through the hopper.
17 A system for processing plants growing in water as claimed in any of Claims 10 to 16 characterised in that the pump comprises a rotary positive * S * *::SS 15 displacement pump. S...
18 A system for processing plants growing in water as claimed in anyS*...SI. . . . . * * precedmg claim charactensed m that the anaerobic digester compnses a *SS.* ** fermentation tank wherein the plant material is circulated in a substantiallySSS S *continuous process.
19 A system for processing plants growing in water as claimed in Claim 18 characterised in that the anaerobic digester comprises a heater for the tank. -21 -
A system for processing plants growing in water as claimed in Claim 18 or Claim 19 characterised in that the fermentation tank has a wall shaped to facilitate circulation of the plant material therein.
21 A system for processing plants growing in water as claimed in Claim 20 characterised in that the wall of the fermentation tank comprises a sheet curved about a substantially horizontal axis.
22 A system for processing plants growing in water as claimed in Claim 21 characterised in that the sheet is formed of synthetic plastics material.
23 A system for processing plants growing in water as claimed in Claim 21 or Claim 22 characterised in that the wall has a cross section substantially in the form of a cardioid having two lobes and a cusp therebetween, the cusp being directed generally upwards.
24 A system for processing plants growing in water as claimed in any of Claims 18 to 23 characterised in that some of the biogas is fed back to the * 15 fermentation tank and therein directed through said cusp so as to stimulate circulation of the plant material around each of said lobes. *S***

*
25 A system for processing plants growing in water as claimed in any of **** Claims 18 to 24 characterised in that the system comprises a plurality of said tanks *..*S* * * in series.
26 A system for processing plants growing in water as claimed in Claim characterised in that the water in the lagoon provides a heat transfer medium between the tanks.
27 A system for processmg plants growing in water as claimed in any of Claims 18 to 26 characterised in that the fermentation tank has an outlet for residue from the fermentation connected to a separator operative in use to separate liquid from solid components of the residue.
28 A system for processing plants growing in water as claimed in Claim 27 characterised in that the system includes means for treating the solid component to produce compost therefrom.
29 A system for processing plants growing in water as claimed in Claim 27 or Claim 28 characterised in that the system includes means for treating the liquid component to extract usable water therefrom.A system for processing plants growing in water as claimed in any of Claims 18 to 29 characterised in that the system includes means for mixing the plant material with an additional material to accelerate anaerobic fermentation of the material. 15 31 A system for processing plants growing in water as claimed in Claim characterised in that the additional material comprises a fermentation S...S *** S conditioner. ***.* 32 A system for processing plants growing in water as claimed in Claim S.....S Ior Claim 31 characterised in that fermentation conditioner comprises an organic material.33 A system for processing plants growing in water as claimed in Claim 32 characterised in that fermentation conditioner comprises dung.34 A system for processing water hyacinth or other plants growing in water substantially as hereinbefore described with reference to and as shown in the accompanying drawings.A method of operating a farm on land adjacent a body of water containing water hyacinth or other plants, which method comprises: harvesting the plants; transferring the plant material to an anaerobic digester; fermenting the plant material in the anaerobic digester to produce gas and a slurry; capturing gas from the fermentation and converting at least part of thereof into biomethane fuell; separating the slurry into solid and liquid components; composting the solid component for spreading on the land; treating the liquid component to recover water for use on the farm; and using at least some of the captured gas and/or the biomethane fuel to power harvesting the plants, transferring the plant material, operating the anaerobic digester, spreading the compost and distributing the water; *:::: characterised in that the farm is thereby substantially self-sustaining and carbon-* .** 15 neutral or better.** 36 A method of operating a farm as claimed in Claim 35 characterised in S...S0 05 S* * that said method comprises mixing the plant material with the dung with to *.*5 facilitate anaerobic fermentation thereof.*.**S.S S37 A method of operating a farm as claimed in Claim 36 characterised in that said method comprises using at least some of the captured gas and/or the biomethane fuel to power collection and mixing of the dung.38 A method of operating a farm as claimed in Claim 30 characterised in that some of the gas is used for cooking on the farm.39 A method of operating a farm substantially as hereinbefore described with reference to and as shown in the accompanying drawings. * .** * * * S. S. * .** * S S... *SSS * . S...S..... * S S.SS *.*.S*....SS