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Plants growing in water require special techniques to harvest them and transport them to the bank for whatever processing is needed for their ultimate utilization or disposal. The material is wet, it may be muddy, and in some circumstances may harbour dangerous micro-organisms or even animals.

The different growth habits of water plants - submerged, rooted with floating leaves, wholly floating, or tough rank plants emerging from shallow water - can add to harvesting difficulties. Each type of plant needs different tools and a different method of attack.

The literature in the main describes various mechanical techniques which have been evolved, mostly in the U.S.A. Yet for most of the people living in developing countries, to whom aquatic weeds may be a dangerous nuisance as well as a potential crop, harvesting must be done by hand from the bank, or by using small boats, and with conventional or specially designed hand tools. The literature on this aspect is sparse and reflects the needs and opportunities for simple technical advance in manual harvesting equipment.

An alternative approach is to introduce suitable animals which do the harvesting by eating the plants and then, at suitable intervals, to harvest the animals. There is an extensive literature on the use of fish, but a variety of other animals, including livestock, offer possibilities.

Hand and mechanical harvesting methods are considered separately in this chapter, and the use of aquatic plants as food for livestock in Chapter VII.

Harvesting of weeds can be rapid and efficient. It is a form of biological control by man. Other forms of biological control - by fish, snails or other animals - all take direct management of the situation out of man's hands. Robson (1974, p.72) makes the same point: “Being wholly controlled by man, both cutting and dredging permit him to exert the maximum control over the amount of weed removed within the limitations of the machine used, in contrast to the indirect and less determinate effects of herbicides and biological control agents. Because of this, cutting especially has certain advantages over these other methods of weed control. Also cutting and removal of the plant material has the advantage of providing an opportunity for the utilization of the vegetation.”

Fox and Prentice (1975) have reviewed harvesting aquatic weeds from lakes and the culture and removal of macrophytes. They contend that large-scale methods of removal in natural lakes are inefficient and expensive. The costs would have to be justified in terms of increased recreational value, not nutrient removal. (From Weed Abstracts)

Methods of surveying aquatic plants from the air by photography have been described by Benton and Newman (1976) and Edwards and Brown (1960).

1. Hand Harvesting

Boyd, C.E., 1974 7. Utilization of aquatic plants. In Aquatic vegetation and its use and control, edited by D.S. Mitchell. Paris, Unesco, pp.107–14

Large floating plants such as Eichhornia crassipes and Pistia stratiotes can be lifted from the water by hand or with a hay fork. Smaller floating plants such as Lemna or Azolla can best be removed with small mesh seines or dip nets. Submerged plants can be harvested by pulling rakes through the underwater meadows. Emergent and floating-leaved plants can be cut at the desired height with knives or, in areas with loose bottom soil, pulled from the substrate by hand. One man can harvest 1 500 kg or more of fresh weight of plants per day from moderately dense stands of most species.

* Hora, S.L., 1951 The water hyacinth problem and pig farming. Sci.Cult., 17(6):231–2

The daily harvesting of water hyacinth from ponds to feed pigs is referred to in this article.

Kamal, I.A. and E.C.S. Little, 1970 The potential utilization of water hyacinth for horticulture in the Sudan. PANS, 16(3):488–96

The authors describe and illustrate various simple items of equipment suggested for the use of workers collecting water hyacinth floating past on the river Nile. The project was supported by the World Food Programme and its objective was to encourage collection of the hyacinth both to help clear the weed from the river and to put it to use.

Methods tested included a boom made of floating drums linked together by wire and anchored at an angle to the bank to deflect clumps moving with the current towards the bank. Grapnels on long handles were tested as a means of extending the reach of a person working from the bank, as workers were reluctant to stand in the water because occasional snakes were found hiding in the weed. Simple metal hooks, similar to those used by wharf labourers, were found effective against the hazard of cercareae of bilharzia, or liver fluke organisms, and also enabled heavier loads to be carried with less fatigue. The methods are illustrated.

The wet condition of water weeds and the high water content of the plants are difficulties of harvesting frequently referred to by writers. In the Sudan, where solar radiation is high and accompanied by low humidity, conditions are good for drying plant material. The authors carried out experiments measuring the rate of drying of weighed samples of freshly collected water hyacinth, and concluded that a reduction in weight of the freshly harvested plant to about 25% of its original weight could be obtained 2 to 3 days after being spread in the sun.

Little, E.C.S., 1969 The floating islands of Rawa Pening. PANS, 15(2):147–53

A lake in Central Java, Indonesia, clogged with floating islands of plants bound together with water hyacinth is described, with illustrations. The local people have devised an ingenious system of cutting suitable sized portions off these islands with long saws. The pieces are then assembled into rafts, about 70 m long × 6 m wide, by binding them together with thin strips of bamboo. The workers, using long poles, can push the islands several kilometres across the lake. Canoes placed on the islands give a firm stance to the polers. The mass of vegetation is usually discharged to waste over the spillway in a surge of released water after opening the sluice gates of the lake. Consideration was being given, instead, to utilizing all this material for agriculture. This would only be practicable after a proposed dyke had been built round the lake which would provide convenient points at which the material could be lifted out and deposited on dry ground.

Little, E.C.S., 1970 From water weeds to milk. (Letter) PANS, 16(1):198–9

The author describes how an emergent sedge, Cyperus digitatus, could be hand-cut from the margins of a lake for chopping up and feeding to dairy cows.

Robson, T.O., 1974 6. Control of aquatic weeds. 6.1. Mechanical control. In Aquatic vegetation and its use and control, edited by D.S. Mitchell. Paris, Unesco, pp.72–84

A description is given of the traditional tools used in Europe for centuries for hand-cutting of water weeds, which have evolved from agricultural implements: scythes, sickles and grass hooks, rakes and forks, chain scythes and chain knives.

U.S. National Academy of Sciences, National Research Council, Making aquatic weeds useful: 1976 some perspectives for developing countries. Washington, D.C., Agency for International Development, 175 p.

This book illustrates (p.68) how hand harvesting may supply enough feed or soil amendment for small farms.

Sawing an island of aquatic plants in Rawa Pening lake, Central Java, Indonesia

Poling islands to the spillway in Rawa Pening lake, Central Java, Indonesia

“Necklace” boom of floating drums anchored to the bank of the Nile river at Malakal, Sudan, to collect water hyacinth

Simple hooks used to carry clumps of wet water hyacinth from the bank of the Nile river in the Sudan

2. Mechanical Harvesting

Bates, R.P. and J.F.Hentges, 1976 Aquatic weeds - eradicate or cultivate? Econ.Bot., 30(1): 39–50

The authors review the literature from the U.S.A. on harvesting and note the wide range of output and costs which have been recorded. They point out that these costs tend to make crops of aquatic weeds more expensive than ordinary low value crops. Harvesting may also be much more expensive than chemical weed control. It is the transport and disposal of the large quantities of weed that imposes a serious economic limitation on the method. Harvesting becomes worth while when aquatic weeds, for example water hyacinth, are cultivated as a means of removing nutrients from polluted water.

Blanchard, J., 1968 Winter Parks' lake weed management program. Hyacinth Control J., 7:30–1

Mechanical harvesting of lake weeds, including Eichhornia crassipes (water hyacinth) Vallisneria americana (valisneria) and Hydrilla verticillata (Florida elodea), has been carried out with machines known as “Aquatic Scavengers”. These boats combine both cutting and carrying the weeds. They can cut normally to a depth of 1.5 m if necessary. To deliver the collected weed on shore the conveyor is run in the opposite direction. Each unit can remove 100 t of Hydrilla, or 140 t of Vallisneria, each week. One unit was programmed to work in shallow water near the shore and the other out in deeper water.

Mechanical removal has been criticized, but the author believes that it is justified because of its psychological and practical effects. The expenditure of public money in its operation removes a public nuisance. Also it is no more harmful, in spreading cut weeds through the water, than the action of many boat propellors and the activities of birds. In removing the weeds from the lake they are not left to repropagate by seeding or, in the case of Hydrilla, by turions. Best results in controlling the weeds are obtained by a suitable combination of mechanical harvesting and the use of herbicides.

Boyd, C.E., 1968 Fresh water plants: a potential source of protein. Econ.Bot., 22:359–68

The author comments that aquatic plants present unique problems of harvesting and processing. Quite different approaches should be taken by developing tropical countries compared with technologically advanced countries. Most tropical areas have cheap manual labour and the plants could be harvested by hand, but in the U.S.A. labour is so expensive that the plants must be harvested mechanically. The machines needed for this require considerable capital outlay and are costly to operate. In most areas weather conditions are too unstable to permit air drying, so mechanical driers would have to be developed. Most aquatic plants contain 2–4 times less dry matter than forages, so drying also would be very expensive. For example, to obtain 1 ton of dry Elodea canadensis or Eichhornia crassipes 14 and 20 t of wet material, respectively, would have to be processed. This excessive moisture content would prohibit transportation to a central processing plant so drying would have to be done on a barge or by a portable unit located on the shore. An operation as outlined above would have to be in almost continuous use and only very large plant stands could be exploited. So, based on this simple assessment of the problems of harvesting and drying, it appears very doubtful that aquatic plants have direct commercial possibility.

N.B. Boyd (1974, p.107) reinforced this view, commenting on Bailey (1965): “…the cost of harvesting and processing the plants by mechanical techniques has prohibited commercial exploitation.”

Bruhn, H.D., R.G. Koegel and D.F. Livermore, 1975 Utilization of aquatic vegetation. Paper presented to the Annual Meeting of the Northern Atlantic Region of the American Society of Agricultural Engineers, New York, 13 p.

Discussing the merits of harvesting aquatic weeds the authors consider that the value of the weeds, in terms of the various uses to which they may be put, could only recover about 10% of the costs of harvesting them. Therefore they point out the need for harvesting to be made cheaper if there is any prospect of aquatic weeds being profitable as a crop. (From Weed Abstracts)

Bryant, C.B., 1970 You need weeds. Proc.Annu.Meet.South.Weed Sci.Soc., 23:301–5

The author claims that the traditional means of removing lake weeds by labourers using long-handled rakes is a thing of the past. He describes the modern mechanical approach to harvesting as the means by which excessive weed growth can be efficiently removed, ensuring that the ecology of the lake is not unduly disturbed.

Harvesting also prevents the inevitable algal blooms which follow destroying weeds by herbicide or cutting weeds and then leaving the plants in the water to rot. Harvesting equipment designed by Aquamarine Corporation is described in detail, and Bryant contends that an aquatic weed harvester should be propelled by paddle wheels so that it can operate in shallow water (down to 30 cm depth). Paddles also enable the craft to turn on its axis and thus manoeuvre in confined places.

The author suggests that harvesting cannot be looked at as the last and most expensive resort. It is the most promising method of aquatic weed control and deserves research into costs and effectiveness. He claims that control of weeds by mechanical harvesting may be as low as 20% of the cost of chemical methods.

Bryant, C.B., 1970a Aquatic weed harvesting - effect and costs. Hyacinth Control J., 8(2):37–9

Illustrations are given of the H-650 AQUA-TRIO Harvester, the T-650 AQUA-TRIO transport and the S-650 AQUA-TRIO Shore Conveyor. An analysis is given of the results of trials with the equipment harvesting 20 acres (8 ha) of Hydrilla verticillata in Lake Maitland, Florida, U.S.A. The amount of weed harvested was about 300 t, averaging 2.5 t per load. The job took 170 man hours. Costs were US.S$620.00 for a 40-hour week, yielding 216 tons. This figure was arrived at as follows:

ItemWeekly cost (US$)
AQUA-TRIO set, initial cost $44 0001 
Dump truck, initial cost $3 5001100.00
Interest at 10% on $47 500100.00
Labour - 3 men at $8/hr for 40 hours320.00
Maintenance and running costs100.00

1 Depreciation calculated on 10-yr lifetime

The rate of harvesting day by day throughout the period of the operation is also tabulated. The figures show a fairly regular progress of efficiency up to a maximum of 9.3 t per 3-man crew per hour.

A later operation, using 2 sets of AQUA-TRIOs on a river, resulted in 12 t of weed being harvested by each crew per hour. Bryant concludes that yield is related to the efficiency of the crew, which improves with experience. He also points out that before extrapolating these costs onto any other waters or weed infestations adjustments must be made to allow for changes in labour rates, weight of harvested weed, average weight of unharvested weeds per acre of lake, distance of weeds to shore conveyor site, dump truck haul distance and design of harvesting equipment.

Bryant, C.B., 1973 Control of aquatic weeds by mechanical harvesting. PANS, 19(4):601–6

Continuing his case for harvesting of aquatic weeds instead of destroying them with chemicals, the author states: “Any method of control which does not remove nutrients from the water does not solve the problem. When weeds are killed they fall to the bottom and decay. During the decaying process they use up a portion of the oxygen in the water and deny that oxygen to the many forms of animal life in the water, thus endangering them or even suffocating them. … Since mechanical harvesting on a large scale is relatively new, little work has been done to document its long-term benefits. But there seems to be little or nothing to indicate that it is harmful.”

Bryant lists the criteria required for efficient aquatic harvesting machinery, then goes on to show how the AQUA-TRIO equipment designed by his organization meets them. The equipment consists of a harvester 11.99 m long with a cutter bar at the forward end, the transporter, 9.14 m long, which takes the weeds from the harvester, and a two-part shore conveyor which can take the weeds from either harvester or the transporter and then lift them into a truck. The criteria given are:

  1. shallow water operation;

  2. to cut weeds from 0–1.5 m deep and extract all cuttings from the water into the hold;

  3. capable of hitting unseen immovable objects at high speed without serious damage;

  4. maximum load capacity with minimum empty harvester weight;

  5. fast off-loading capability;

  6. all controls powered;

  7. capability to transport large quantities of weed to shore after harvesting in minimum time and with minimum manpower;

  8. loading into truck capability;

  9. able to be installed in many varied access points at low cost and still operate at top efficiency;

  10. towable by road at a maximum width of 2.44 m;

  11. launchable via a ramp into the water or, alternatively, by crane.

Much work has yet to be done to make the costs of harvesting equipment decrease to the the level of, say, farm equipment. High production can easily result in these lower costs. Toward this end much research is being financed by the U.S. Department of Interior, Office of Water Resources Research. Among the areas under research are higher harvesting speeds and weed chopping, pressing and de-watering schemes. If end products from the harvested weeds can be developed, a great step toward financing weed harvesting can be made.

Bryant adds that the equipment described has been used in nine states of the U.S.A., four provinces of Canada, and in Scotland, Iceland and Braxil. The paper has four illustrations.

Bryant, C.B., 1974 Aquatic weed harvesting, costs and equipment - 1972. Hyacinth Control J., 12:53–5

The author tabulates harvesting operation costs for various locations and states that the cost of harvesting aquatic weeds seems to be diminishing as equipment design improves. The selection of equipment is broadening and new developments in the near future will further reduce costs. Thus harvesting should be increasingly used as a method of controlling growth of aquatic vegetation. Costs can be further drastically reduced if work can be continued for longer hours, including into the night, instead of just for the usual working day. Harvesters can be, and have been, fitted with lights to make night operation possible.

It is estimated that since 1945, 50–75 aquatic weed harvesters have been produced commercially. As most of the work is against submerged weeds the method has developed into a standard system. A powered barge has underwater cutters mounted in front of an inclined porous conveyor, with a holding area for the harvested weeds; there is a handling or transporting system to shore.

The bulk of harvesting design and innovations appear to have been centred in Wisconsin, U.S.A. Bryant refers to Grinwald (1968) as being the originator of mechanical harvesting machinery (see p.58). A succession of enterprises has resulted in the Aquamarine Corporation, with 25 harvesters operating in 1972.

Cifuentes, J. and L.O. Bagnall, 1976 Pressing characteristics of water hyacinth. J.Aquat. Plant Manage., 14:71–5

In order to facilitate harvesting of aquatic weeds reduction in the high water content is desirable. Using water hyacinth the authors investigated the effectiveness of a screw press in expressing water from the fresh plant. The plant had to be chopped for worth-while results. The smaller the size of the particle the faster the process. Increasing pressure up to 600 kPa increased juice extraction but higher pressure did not remove much more juice. If the cake was agitated between additional pressings, expression was increased by 12–20%. About 70% of the water was expressed from water hyacinth with an energy input of less than 100 J/kg of water expressed, but the energy required increased rapidly as more water was removed. The optimum expression time was about 40 seconds.

Deutsch, A. (Ed.), 1974 Some equipment for mechanical control of aquatic weeds. Rep.Int.Plant Prot.Cent.Ore.State Univ., (74–2):18 p.

A number of firms throughout the world construct and market specialized equipment for cutting, harvesting or otherwise mechanically controlling aquatic weeds. Some of these units are briefly described and addresses of the firms involved are given in this paper. The information represents most of those firms suggested by experts in aquatic weed control from a number of countries. The equipment is designed, with a few exceptions, to operate on the water surface. The text is divided into sections on aquatic weed cutters and rakes, other equipment (including prototypes), and a list of companies.

Many of the illustrations show operators working without safety equipment (life jackets, etc.) and some belt and drive shafts are without shielding. The author points out the need for adequate safety gear to protect machinery operators.

The following machines and equipment are illustrated and described:

  1. Airlec Industries Inc. (Madison, Wisconsin, U.S.A.)

    WC10 Aquatic weed cutter and rake. The cutter is U-shaped, 107 cm wide and can cut to 107 cm deep. It is powered by a 2-cycle, 3 h.p. engine. The equipment weighs 99 kg and may be mounted on the bows of most boats of 4.5 m or longer. The same frame can carry the rake which is 2.8 m wide. Operation may be at 5–10 km/h depending on weed density. The untio requires two operators, one to steer the boat by the outboard motor.

  2. American Waterweed Harvesting Co. (shreveport, Louisiana, U.S.A.)

    Waterbug. The unit, 6.1 m long and 1.5 m wide, 13 cm draft, has two polyethylene pontoons supporting a metal platform. It is propelled by a 1.2-m diameter steerable (360°) air fan powered by an 8 h.p. engine, giving a speed of 8 km/hr. The cutter consists of two vertical blades and a 2.4-m horizontal blade. A 3.7-m wide blade is also available. Cutting can be to a depth of 1.2 m. The unit is operated by one man.

    Waterweed harvester. This is a large unit 21 m long, 5.5 m wide and draws 30.5 cm. It is propelled by a propane-burning engine (6.4-litre) powering the rear mounted propulsion fan, and an electric generator or a hydraulic pump. There is a powered conveyor at the bows, which has, on its leading edge, a 4.9-m wide sickle bar cutter capable of cutting to a depth of 1.8 m. The conveyor may be raised for unloading operations. The cut plant material is conveyed into central storage area in the craft with a capacity of 60 m3. There is a second conveyor in the bottom of the storage area. The direction of movement of both conveyors is reversed for unloading.

  3. Aquamarine Corporation (Waukesha, Wisconsin, U.S.A.)

    Sawfish. The craft has paddle wheels, side mounted at the rear. In the bows are one horizontal and two vertical sickle blades in a U-configuration. The units cuts a swathe 2.4 m wide and down to a depth of 1.5 m. The craft (one operator) can work in water as shallow as 25 cm. A petrol engine powers three hydraulic motors on the cutting assembly and one motor for each of the paddle wheels. (See also Bryant, 1973, onp.53)

  4. G. Bouwmeester (Sappermeer, Netherlands)

  5. Boat sweeper. The boat can be either 3.5 m or 4.0 m long. They each draw 25 cm and can operate in waterways approximately 1.4 m wide. They are fitted with special sweeping knives. Also a side cutter bar is available for longer craft. Power comes from a 10 h.p. diesel engine with reverse gear which drives an anti-fouling propellor. Sweeping can be done at 4–5 km/hr or slower when using side cutting. One-man operation. A trailer is also provided which is fitted with roller track winch and cable for retrieving, launching and transporting the boat.

  6. Hockney Company (Silver Lake, Wisconsin, U.S.A.)

    HC10 underwater weed cutter. A metal pontoon, which can be plastic foam filled, is 3.7 m long, 1.2 m wide, 30 cm deep and draws 13 cm. An H7 version also offered is slightly narrower. The boat has a rear mounted paddle driven by a 4 h.p. engine. A 4 or 5 h.p. engine drives a horizontal sickle bar 3 m wide. There are two vertical cutter bars on each side. Cutting depth is from 13 cm to 1.5 m. A smaller version, HP7, can be bought as a unit to fit to any suitable flat-bottomed boat.

  7. Krinke and Kruger GmbH (Hanover, Federal Republic of Germany)

    A range of cutter bars of different sizes and power are offered which may be fitted to different sized boats. These are propelled by non-clogging Archimedean-type screws.

  8. John Wilder (Engineering Ltd.) (Wallingford, Berks, England)

    Wilder water weed cutter. A double-skinned fibre glass hull, with flip-up ends, has side mounted, independently hydraulically-driven paddles. It is fitted with a U-shaped cutter with flexible, reciprocating, hydraulically-powered blades working against a fixed blade. Cutting can be made down to a depth of 1.7 m. Cutting width is 3.5 m. Cutting can be changed to work ahead of, or behind, the boat. Power is from a 15 h.p. diesel engine. The boat can turn easily in narrow waterways. A trailer is available to launch and load the cutter. A rake foreloader is available to deposit debris well back from the water's edge.

  9. AOA Research and Development (Orlando, Florida, U.S.A.)

    Underwater weed cutter. This is a manually operated device which has two double-edged serrated blades, each 15 cm long and fitted to a handle, with handle extensions 1.5 m long.

Frye, O.E., 1972 Weed control as it relates to the aquatic environment. Hyacinth Control J., 10:12–3

This author contends that the one big advantage of mechanical harvesting is the removal of nutrients from the water. But unfortunately the relatively high expense has not permitted full exploitation of this advantage. It is therefore important to find a use for the end product so as to obtain a return to offset the cost. Possible uses suggested include paper-making, food supplements for livestock, mulch, and a source of protein. To date no commercial use (in the U.S.A.) has been found for either water hyacinth or submerged plants.

Golueke, C.G. and W.J. Oswald, 1965 Harvesting and processing sewage grown algae. J.Water Pollut.Control Fed., 37(4):471–98

The authors extensively studied methods of harvesting planktonic algae. There were three stages in the process:

  1. Initial concentration. This involved bringing the algal solids concentration from that in the pond to 1–2% of the wet weight of the slurry. Chemical precipitation or centrifugation were the best of many methods tried. Centrifugation required ranged from 3 300 to 6 200 kW of power per ton of dried algae when the pond concentration was 200 mg/litre with outputs of 100–300 g/min.

  2. Suitable flocculating agents included alum, lime and polyvalent cationic polymers. Figures are given for the amounts of chemicals needed to achieve different degrees of clarification of the water.

  3. The algal slurry was dehydrated as a thin film on a heated drum, a method which sterilizes the product and improves digestibility. Alternatively, the algae may be dewatered and air-dried in one operation on specially prepared sand beds. About 1 470 m2/ha is required. The costs of these operations are discussed.

Grinwald, M.E., 1968 Harvesting aquatic vegetation. Hyacinth Control J., 7:31–2

The author points out that nutrient removal from lakes is often necessary to maintain natural resources in the best possible condition. Mechanical weed removal is one way to accomplish this end both economically and effectively. It has the following advantages:

  1. it can provide immediate relief from local nuisance conditions;

  2. it probably does not tend to alter plant to animal life balance as drastically as may be the case with chemical treatments;

  3. it does not introduce foreign substances into the water;

  4. by removing nutrients it should tend to reduce the rate of lake filling by plant residues.

Cutting should be followed by extraction and disposal of the cut material. Otherwise it may block channels and cause a nuisance by decomposition.

A detailed description, with photograph, is given of the unit built by the Grinwald-Thomas Corporation in Wisconsin, U.S.A. The capacity of the equipment is 0.4 ha/hour in medium to average weed growth, and it can remove 6 t of weed per hour under average conditions.

Mechanical harvesting has proved effective against Myriophyllum spicatum which is difficult to control by chemicals.

A channel in Lake Pewaukee, Wisconsin, kept clear of heavy weed over a period of four years by mechanical harvesting, did not require harvesting during the fifth year. Weeds on either side of the channel were as dense as ever. At another site (Rib Lake, Wisconsin) after two years of harvesting practically none was required the third year. Water clarity and fishing conditions have been reported to have improved considerably.

Grinwald suggests that good records of harvesting work are needed together with studies of the consequences of harvesting on plant/animal relationships.

Hughes, H.R., 1976 Research into aquatic weeds in New Zealand waterways: a review. Inf.ser. Dep.Sci.Ind.Res.N.Z., (116)

Reviewing work done in new Zealand the author refers to experiments on weed cutting and mechanical harvesting (which are unpublished).

The N.Z. Electricity Department devised a weed pulveriser, a machine capable of harvesting and discharging to shore wet pulverised weed. Netting booms, 3.7 m deep, were made to collect weed drifting into the penstocks of hydroelectric generators. The booms were fitted with alarm systems to give warning when critical masses had accumulated.

Koegel, R.G., 1973 et al., Increasing the efficiency of aquatic plant management through processing. Hyacinth Control J., 11:24–30

The authors have investigated the main problem which limits the rate of mechanical harvesting. That is the high water content of the plants. They give a diagram showing the total matter of the plants divided into: surface water, 44.5%; cellular water, 45.5%; solids, 10%. Thus for every 10 kg of aquatic plants collected, 9 kg of water have to be transported in order to harvest 1 kg of dry matter.

They therefore carried out a series of experiments to measure the amount of water that could be mechanically expelled from the plants by cutting and pressing without an unacceptable loss of useful nutrients in the expressed liquid. Treatment also included heating the material with a view to causing cell collapse and precipitation of protein to allow easier extraction of liquid and, at the same time, better retention of the protein. The plant used in the studies was Myriophyllum spicatum (Eurasian water milfoil). The following table summarizes their results after pressing for 1 hour at 7 kg/cm2:

TreatmentInitial moisture content %Final moisture content %Dry matter in expressed liquid %
2 min at 100°c92.573.00.64
3 min at 100°c93.671.40.92

Little, E.C.S., 1975 Harvesting aquatic weed - a neglected opportunity. N.Z.Farmer, 96(8): 46–8

This paper suggests that the problem of aquatic weeds in New Zealand lakes should be treated as an opportunity to carry out experiments on harvesting and utilization. From this work advice could be given to developing countries with similar problems. Thus if the costs were considered as a form of foreign aid the economics of the use of harvesting machinery would be subordinated to the research and development needed. The author states that harvesting has the supreme advantage that it is directly under control, whereas herbicide spraying can be affected by unpredictable gusts or updrafts of wind.

Harvesting from boats in calm waters enables man to take the weeds where and when he wants, and in the right amounts. The work can then be done with the minimum of disturbance to people using the amenities. No pollution is involved by this method, and the status of nutrients in the lakes, weed obstruction, algal blooms and general pollution are under control. If the weeds are put to use they can then help to pay for the costs involved.

Livermore, D.F., H.D. Bruhn and B.W. Pollock, 1971 Processing characteristics of subsurface macrophytes of Madison, Wisconsin, lakes in relation to mechanical harvesting systems. Hidrobiologia, Bucharest, 12:341–50

The authors have investigated various methods by which the large quantities of water typically present in aquatic weeds may be reduced in order to facilitate and cheapen disposal costs. The weeds in their trials were mainly Myriophyllum spp. and filamentous algae. Harvesting of such weeds involves transporting about 9 tons of water for every ton of dry matter.

  1. Fluidising. The process involved macerating the plants by hammer milling through fine screens, or by an agricultural field forage harvester equipped with a fine recutter screen. In some cases the fluidised material was pumpable through self-priming pumps without the addition of water. It was concluded that a substantial reduction in labour might be accomplished by transporting aquatic vegetation to disposal areas as liquid rather than a fibrous bulk material.

  2. Dewatering. Three methods were tested:

    1. Centrifuging of unprocessed plants successfully removed surface water, but within practical radial acceleration limits did not remove cell moisture.

    2. Crushing between rollers resulted in moisture reduction from 87% (wet basis) to 78% on the third pass. By improving the removal of expressed moisture, reduction to 76% on the first pass was accomplished. By the addition of rubber matting to the roller surfaces, to increase the time of pressing at a given roll speed, the degree of dewatering can be further increased with additional passes. Reductions from 86 to 65% have been attained. Higher roll pressures result in an increasing degree of dewatering on each pass and also a higher percentage of the dry matter being discharged with the liquid fraction.

    3. Screw pressing, using a conventional industrial processing machine, commonly used to separate liquid and fibrous fractions of various types of material. Such machines are expensive and have high power consumption because of internal friction on the material being dewatered. But the capacity and consolidation on the fibrous fraction makes them worth study.

      Detailed results are given in the paper on the amount of liquid expressed at a range of pressures (from 1.4–5.2 kg/cm2). This removed from 45–63% of the original weight. Two passes through the machine raised these figures to 70–78%. Losses of dry matter in the expressed liquid were measured. At 2.1 kg/ cm2 the content was 5% of the dry matter, of which less than 25% was protein. The power needed was measured. It was concluded that 2–4 horse-power hours per 450 kg of material processed at normal harvesting rates is about as much as can be employed economically.

  3. Combustion. Results showed that material dewatered to 65% wet basis should support combustion. But agitation of the material, during combustion, would be necessary. Using a fuel oil burner and retort, higher moisture material has been incinerated. It was pointed out that if the harvested material were reduced to ash then transportation and disposal would be reduced to a minimum. Also it was found that while some livestock would not eat the dewatered fibrous fraction they readily consumed the same material in a partially dehydrated and charred condition.

The conclusion of the studies was that harvesting and transporting of aquatic vegetation can be made 4–6 times more productive by providing processing equipment in the harvester barge. However, harvesting rates may be limited if discarding the expressed liquid from the harvester barge unduly upsets water conditions. Research is needed into this aspect.

Mara, M.J., 1976 Estimated values for selected water hyacinth by-products. Econ.Bot., 30(4): 383–7

Reviewing the possible uses of water hyacinth the author states that mechanical harvesting seems to be the best means of control for environmental reasons: it does not harm fish, it leaves no environmental residues and, since water hyacinths remove nutrients from the water, mechanical harvesting improves water quality. Harvesting also makes water hyacinths available for various uses which, if a market for them can be developed, would help defray the cost of mechanical control.

Nichols, S. and G. Cottam, 1972 Harvesting as a control for aquatic plants. Water Resourc. Bull., 8(6):1205–10

This paper describes the effects of a series of harvests on the subsequent growth of Myriophyllum spicatum. The experiments were on a plot scale (30 m × 30 m) so harvesting was by hand as close to the bottom of the lake as possible. The objective was to determine the effectiveness of harvesting on the control of the weed rather than primarily for its utilization. The results showed that one harvest reduced growth by at least 50%; two harvests reduced it by 75%, and three virtually eliminated plant material for the year. Harvesting also reduced biomass the following year, especially in deep water. Three harvests had the greatest effect but two harvests were not far behind. The work thus showed that harvesting could have a carry-over effect into subsequent years. Moreover, if harvesting was done just before the two growth peaks of M. spicatum (in early June and early August, but progressively later at greater depth), the best effect would be achieved because this would remove the maximum biomass.

Thus by properly timing a harvest it may be possible preferentially to select against the weed and release more desirable species. Or a single release cutting could open up dense stands, making the areas available to fish and wildlife.

Robson, T.O., 1974 6. Control of aquatic weeds. 6.1. Mechanical control.In Aquatic vegetation and its use and control, edited by D.S. Mitchell. Paris, Unesco, pp.72–84

The author reviews water weed harvesting in regard to control of the weeds rather than particularly for utilization. He points out that the development of new machines and the modification and improvement of existing equipment depends on further investment of capital which will only be made if adequate returns can be expected. Much will depend on finding profitable uses for the harvested material and economic methods for handling and processing it. Impetus will also come from the need to avoid excessive eutrophication and pollution and, in some countries, the need for animal food.

The paper describes in detail, with illustrations, various types of mechanized weed cutting and harvesting machines, including weed cutting boats, reciprocating cutters, methods of propulsion of boats, and conveyors for collecting floating cut material.

The author also reviews papers on mechanical harvesting as developed in the U.S.A.

Robson, T.O. and L.F. Fillenham, 1976 Weed control in land drainage channels in Britain. Paper presented to the Sixth Session of the Working Party on Water Resources and Irrigation, Seville, Spain, 15 p.

In a paper devoted to water weed control the authors describe equipment which could be employed for weed harvesting. The equipment includes hand-operated power tools, tractor-based equipment, hydraulic and drag-line excavators, weed cutting attachments, and weed boats. A table shows the approximate stretch of machines working from one side of drainage channels. (From Weed Abstracts)

Steward, K.K., 1970 Nutrient removal potentials of various aquatic plants. Hyacinth Control J., 8(2):34–5

The author comments on the comparative ease of harvesting floating plants compared with the difficulties presented by the need to cut rooted and emergent types.

Sy, Se Hiong, 1974 Mechanical management of aquatic vegetation. Ph.D. Thesis, Wisconsin Univ., U.S.A., 254 p. (Cited in Weed Abstr., 25(10):3023 (1976))

A study was made of methods by which water could be extracted from water weeds in order to assist the harvesting of aquatic plants. When water was expressed under constant pressure it was shown that there was an “initial rate” of water released followed by a “falling rate” period characterized by a slow decay in the rate of moisture expression.

Thompson, T.W. and H. Hartwig, 1973 Control of watermilfoil in large Wisconsin lakes. Hyacinth Control J., 11:20–3

The authors point out that mechanical harvesting leaves root systems intact and so does not significantly increase water turbidity. Since vegetable matter is removed from the lake the problems of oxygen stress, phytoplankton blooms, and the accumulation of unsightly masses of decaying vegetation are avoided. Because no toxic materials are employed dangers of fish kills or damage to sport fishing are not present. Fishing may even be improved by opening channels through the weed beds.

An interesting additional point is that mechanical operations may promote good public relations by frequent demonstrations that positive action is being taken to improve the environment. To test public reaction a survey was taken of the attitude to mechanical harvesting of weed. Of 1 370 replies, 1 139 felt that the service should be extended, 179 suggested that the present levels of spending should be continued, and only 52 felt that the programme should be cut back.

The problem weeds in this instance were Myriophyllum spp. Analysis of the costs of mechanical harvesting showed a figure of U.S.$21.38/acre ($53.00/ha). This figure was less than anticipated and was attributed to the experience gained and the use of improved equipment. These units were a Grinwald-Thomas GT 501 harvester, a GT 471 Craneveyor, 2 transport barges, and a 1970 Aquamarine H 650 harvester with T 650 transporter.

The most serious problem is the transport of the heavy wet material to a disposal area. But after tests with chopped and dewatered Myriophyllum it has been found possible to sell the product as a soil conditioner and thus meet some of the transport costs.

The authors also point out the need for faster operating equipment (their rate was too slow at 1.6 km/hour). Cutter bars need to be made more able to withstand damage from underwater obstacles. It is also important for the cutters to be able to operate in all weathers, especially in strong winds.

U.S. National Academy of Sciences, 1976 National Research Council, Making aquatic weeds useful: some perspectives for developing countries. Washington, D.C., Agency for International Development, 175 p.

Mechanical harvesting methods are briefly reviewed. Illustrations are given of experimental and commercial harvesters. These include the large machines built by the U.S. Army Corps of Engineers, which used “clam shell” buckets to clear massive infestations of water hyacinth before the use of the herbicide 2,4-D provided a simpler and cheaper method of destroying the weed. Various sizes of weed cutters and collectors are shown mounted on large and small boats, Equipment has been made to enable floating weeds to be dragged on board with protection against damage to the gear from unseen underwater obstructions. Also shown are the “Hyballer” machine which picks up water hyacinth and flings it to shore, and flotation shoes which enable observers to walk over water and floating weeds.

Wunderlich, W.E., 1967 The use of machinery in the control of aquatic vegetation. Hyacinth Control J., 6:22–4

The author describes the design in 1937 by the U.S. Army Corps of Engineers of a diesel electric barge, “Kenny”, needed for the removal of water hyacinth and alligator weed from extensive infestations in waterways.

The vegetation was lifted by a 5-m wide chain and slat conveyor at a maximum speed of 30 m/minute. The lower end could submerge to a depth of 1 m. Usually it was operated just below the surface because this gave clean and sharp action and prevented vegetation from piling up and then being picked up in large heaps. Speed could be controlled, or reversed, if logs or other debris too large to pass through the machine had to be rejected. Circular saws, 1 m in diameter, were fitted as outriggers to the conveyor. These cut the weed mat into a ribbon which facilitated pick-up by the conveyor. From the conveyor the vegetation passed into a hopper and thence between two corrugated rollers 30 in. (75 cm) in diameter and operating under 18 000 kg pressure. The crushed plants were rejected over the side by conveyor. The barge was propelled by two full-weedless propellors mounted in tunnels. All equipment was powered by electric motors supplied by two main generators giving 220 V DC.

The barge was capable of clearing 1 million m2 of surface vegetation per month. It worked until 1951 when the use of herbicides replaced mechanical clearing.

For bankside clearance in shallow water, smaller units were constructed with 5-m side conveyors also fitted with saws which delivered the weed vegetation to 1-m wide side conveyors for depositing the cleared material onto the waterway banks. One unit had a high speed side conveyor to catapult the material ashore. To supplement the collectors small boats with rakes mounted over the bows were used to sweep weed toward the conveyors.

Another method of destroying the vegetation, rather than collecting it, was to use saw boats. These were fitted with banks of closely spaced circular saws revolving at 800–1 000 revs/min, and were satisfactory against both hyacinth and alligator weed. A modification, to combat water chestnut, had in addition a horizontal under-water cutter bar and a collecting trough.

Fish will leave the area when cutting is in progress but will return in large numbers when it is over. Fishermen have not made any objections to mechanical clearing since the operations do not appear to harm fish to any great extent.

Mechanical clearance tends to be more expensive than chemical because of the initially higher cost of the equipment. However, it can usefully complement chemical control methods.

Yount, J.L. and R.A. Crossman, 1970 Eutrophication control by plant harvesting. Part 2. J.Water Pollut. Control Fed., 42(5):R173–83

Experiments with water hyacinth showed that harvesting these plants from lakes reduced the productivity of the water. Therefore large-scale harvesting from such waters would reverse the trend towards hypertrophy, especially in polluted waters. The authors state that it is evident that the usual method of treating water hyacinth as a pest and destroying it with chemical sprayers returns nutrients contained in the plants to the water and thus exacerbates hypertrophy of the lakes.

Recognizing the difficulty of lifting and transporting masses of plant material containing up to 95% water, the suggestion is made that air-drying before transportation may be a solution. It is proposed that floating platforms be anchored in the lake on to which the freshly harvested weeds would be dumped. When dry to 20–30% of their wet weight the weeds could then be collected, say, for stock feed.

Anon., 1968 United Kingdom. Paddle boat mower for weedy waters. New Sci., 37(584):362

A short note, with illustrations of a cutter mounted on a 5-m glass fibre boat and propelled by paddles. It has a draught of only 25 cm and can easily be transported by road. The cutting mechanism, suspended behind the boat, consists of a large U-shaped knife with a movable serrated blade moving over a fixed one. The bottom, horizontal part of the knife cuts at a standard maximum depth of 1.4 m., and the two arms of the blade are inclined outwards so that a wedge-shaped channel, 2.6 m wide at the bottom, is cut through the weeds. The position of the arms is controlled by hydraulic rams. (See Deutsch (g), p.57)

Dugong weed cutter on Kirkgözlü lake, Antalys, Tukey

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