water hyacinths as a resource in agriculture and energy production

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Waste Management 27 (2007) 117–129

Technical paper

Water hyacinths as a resource in agriculture and energy production:A literature review

Carina C. Gunnarsson a,*, Cecilia Mattsson Petersen b

a Department of Biometry and Engineering, Swedish University of Agricultural Sciences, P.O. Box 7032, SE-750 07 Uppsala, Swedenb Department of Mathematics, Science and Environment, Dalarna University College, SE-781 88 Borlange, Sweden

Accepted 6 December 2005Available online 31 March 2006

Abstract

Water hyacinths are becoming a problem in lakes, ponds and waterways in many parts of the world. This paper contains a literaturestudy of different ways to use water hyacinths, mainly in agricultural or alternative energy systems.

The literature review indicated that water hyacinths can be rich in nitrogen, up to 3.2% of DM and have a C/N ratio around 15. Thewater hyacinth can be used as a substrate for compost or biogas production. The sludge from the biogas process contains almost all ofthe nutrients of the substrate and can be used as a fertiliser. The use of water hyacinth compost on different crops has resulted inimproved yields. The high protein content makes the water hyacinth possible to use as fodder for cows, goats, sheep and chickens. Waterhyacinth, due to its abundant growth and high concentrations of nutrients, has a great potential as fertiliser for the nutrient deficient soilsof Africa and as feed for livestock.

Applying the water hyacinths directly without any other processing than sun drying, seems to be the best alternative in small-scale usedue to the relatively small losses of nutrients and workload required. To meet the ever-growing energy demand, biogas production couldbe one option but it requires investments and technological skills that would impose great problems in developing countries where thewater hyacinth is often found. Composting as an alternative treatment has the advantage of a product that is easy to work into the soilcompared with dried water hyacinths, because of the decomposed structure. Harvesting and transport of water hyacinths can be con-ducted manually on a small scale and does not require a new harvesting technique to be introduced. Transporting of fresh water hya-cinths means, if used as fertiliser in amounts large enough to enhance or effect crop growth, an unreasonably large labour requirement.Based on the labour need and the limited access to technology, using dried water hyacinths, as green manure is a feasible alternative inmany developing countries.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

The water hyacinth (Eichhornia crassipes) is a free float-ing aquatic weed originated in the Amazon in South Amer-ica (Bolenz et al., 1990) where it was kept under control bynatural predators (Lee, 1979). The plant has, throughintroduction by man, spread throughout the whole tropicalzone (Aweke, 1993). Due to its fast growth and the robust-ness of its seeds, the water hyacinth has since then causedmajor problems in the whole area, e.g., a reduction of fish.Other effects of the fast growth are physical interference

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doi:10.1016/j.wasman.2005.12.011

* Corresponding author. Tel.: +46 18 672578.E-mail address: [email protected] (C.C. Gunnarsson).

with fishing, obstruction of shipping routes and losses ofwater in irrigation systems due to higher evaporation andinterference with hydroelectric schemes and increased sed-imentation by trapping silt particles. It also restricts thepossibilities of fishing from the shore with baskets or lines(Aweke, 1993) and can cause hygienic problems (Moursi,1976a; Becker et al., 1987; Abdelhafiz, 1989, from Abdelh-amid and Gabr, 1991).

Attempts to control the weed have caused high costs andlabour requirements, leading to nothing but temporaryremoval of the water hyacinths. Since the most favourableconditions for the growth of the water hyacinth often arefound in developing countries, very limited resources havebeen put into curbing them. Fighting the water hyacinth

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118 C.C. Gunnarsson, C.M. Petersen / Waste Management 27 (2007) 117–129

generates neither food nor income, and the weeds are there-fore left to cover the lakes. Fast growth is a feature valuedin crops grown by man. The water hyacinth would, there-fore, have a great potential if seen as raw material forindustries or if incorporated into agricultural practice. Thispaper contains a literature review on utilisation of waterhyacinths, mainly in an agricultural or alternative energysystem.

2. The water hyacinth

2.1. Growth and harvest

Water hyacinths regenerate prolifically from fragmentsof stems and the seed can remain viable for more thansix years. These ways of regeneration make it very difficultto control the weed (Lee, 1979). The number of plants canmore than double in seven days in conditions of high tem-perature and humidity (Lareo and Bressmi, 1982, from TagEl-Din, 1992) and up to 140 ton of DM/ha and yr are pro-duced (Abdelhamid and Gabr, 1991). The plant normallyforms cohesive floating mats and can cover large areas ofthe water surface. The spreading of the water hyacinth isalso thought to be enhanced by winds (Gay, 1960, fromAweke, 1993). The plant flourishes in nutrient-rich watersand on shallow shores with mud rich in nutrients.

To estimate how much of a certain water hyacinth prod-uct (soil amendment, fertiliser, gas, fodder, etc.) can be pro-duced, it is necessary to make approximations of howmuch biomass can be harvested. Thomas and Eden(1990) estimate the possible harvesting of water hyacinthsto 320 ton of DM/ha and yr. The figure above is basedon conditions in Bangladesh, and nothing is said abouthow much water hyacinth was present before the harveststarted. In order to not over-estimate the yield, we assumethat no more than the yearly production of biomass can beharvested, i.e., 140 ton of DM/ha and yr (Abdelhamid andGabr, 1991). The water hyacinth mats are driven by thewind. The harvest possibilities will, therefore, depend onthe local conditions and winds.

2.2. Chemical analysis

The water hyacinth has an excellent ability to take upnutrients and other chemicals from its environment, andthe chemical composition of the water hyacinths dependsstrongly on its environment (Musil and Breen, 1977, fromPoddar et al., 1991). Poddar et al. (1991) reported a nitro-gen content of 1.78% (db) in water hyacinths growing inmarshy land where the nitrogen level in the water was only2.40 mg/L. Abdelhamid and Gabr (1991) state the nutrientcontent is lower in the stem and root compared with thenutrient content in the leaves.

Several studies on the chemical composition of thewater hyacinths have been reported. Abdelhamid andGabr (1991) and Bolenz et al. (1990) made studies to eval-uate the water hyacinth’s nutritional value for ruminants.

Chanakya et al. (1993) and Patel et al. (1993a) analysedwater hyacinths as a substrate for anaerobic digestion.The analysis by Poddar et al. (1991) compared the nutrientcontent of water hyacinths grown in different habitats. Thefigures in Table 1 are for water hyacinth grown in a pond.Polprasert et al. (1980) evaluated water hyacinth as a sub-strate for compost. In the analysis of water hyacinths fromLake Victoria, Kenya (Gunnarsson and Mattsson, 1997)the plants were not washed; hence the analysis alsoincluded sand etc. found in the roots. In Table 1 the chem-ical composition of the water hyacinths from differentsources is summarised.

2.3. Health aspects

The increased growth rate of the water hyacinths has ledto worsened health conditions for the people living in theaffected areas. The floating water hyacinth mats can serveas a breeding ground for vector organisms carryingmalaria, bilharziosis and river blindness (Moursi, 1976a;Becker et al., 1987; Abdelhafiz, 1989, from Abdelhamidand Gabr, 1991). At some places precautions against watersnakes, hippos and crocodiles need to be taken. The waterhyacinths consume so much oxygen when decaying that itleads to less oxygen remaining in these waters. Thedecreased oxygen content in the water leads to less oxygenin the fish. This, combined with fewer algae and other foodsources for the fish, cause the meat of the fish to go bad fas-ter than before. Decreased possibility to store fish leads tolower income and food security (Sunday Standard, Kenya,12/1-1997). This means that decreasing the amount ofwater hyacinths could hopefully improve the healthsituation.

When working in water hyacinth infested areas, one ofthe problems is the risk of catching waterborne diseases,in the case of Lake Victoria especially Schistosomiasis.The problem is that the snails that serve as a host forthe bilharzia parasites are very likely to be found in waterhyacinths. If drying the water hyacinths is to be a success-ful way of eliminating the risk of catching bilharziadepends on if the water hyacinth can be harvested withoutsnails.

2.4. Harvesting

Harvesting the water hyacinths means a mechanicalcontrol of growth. Presently, the water hyacinths are onlyharvested to control their propagation where chemical orbiological (e.g., introduction of water hyacinth eatinginsects) methods are prohibited or unsuccessful. This is,according to Petrell and Bagnall (1991), because mechani-cal harvesting is too expensive and time-consuming. Theadvantages of mechanical control of water hyacinths are,according to Verbandt (1990):

� The removal of superfluous nutrients.� The immediate result without damage to the ecosystem.

Table 1Chemical analysis of water hyacinths according to different sources

Parameter(% on DM basis)

Abdelhamid andGabr (1991)a

Bolenz et al.(1990)

Chanakya et al.(1993)

Patel et al.(1993a)

Poddar et al.(1991)b

Polprasert et al.(1980)

Gunnarssonand Mattsson(1997)

Fresh Driedc

Dry matter (% on wb) 9.5 6.2 9.4 – – –Organic matter (VS) 74.3 – 83.65 – 83.61 –Crude protein 20 – – 11.9 16.25 –Ether extract 3.47 – – – 1.61 –Crude fibre 18.9 – – – 16.34 –Nitrogen free extract 31.9 – – – 49.41 –Ash 25.7 15 – 20.2 16.39 – 35.6 52.07C/N ratio – – – – – 15.8 23.5 25.1Neutral detergent fibre 62.3 – – – 56.14 –Acid detergent fibre 29.0 – – – 37.72 –Hemicellulose 33.4 22 33.97 43.4 18.42 –Cellulose 19.5 31 18 17.8 25.61 –Lignin 9.27 7 26.36 7.8 9.93 –Water soluble – – 21.68 – – –Phosphorus 0.53 – – – 0.53 0.5 0.26 0.32Carbon 27.6 18.54Nitrogen – – – – 2.76 2.9 1.18 0.74Magnesium 0.17 – – – – –Calcium 0.58 – – – 2.29 –Potassium – – – – 2.44 – 4.53 2.27Metabolizable energy

for ruminants (MJ/kg)6.35 – – – – –

a Abdelhamid and Gabr (1991) made the chemical analysis on water hyacinths collected from a canal and a ditch at Mansoura, Egypt.b The cell wall composition was fractioned according to Van Soest (1963) and Van Soest and Wine (1967) from Poddar et al. (1991).c Dried 13 days with full natural aeration.

C.C. Gunnarsson, C.M. Petersen / Waste Management 27 (2007) 117–129 119

� Waterbodies can be used more widely (e.g., for irriga-tion of agriculture areas and drinking water supply).� Mechanical methods are possible in open flowing as well

as in closed water systems.

Another advantage of harvest is that it makes it possibleto use the water hyacinths in, for example, agriculturalpractice.

A weed screen cleaner with continuously moving rakes isbeing developed in Belgium. The plan is to construct aback-raked screen that is mobile by being mounted on aconverted military amphibian (Verbandt, 1990). Anotherpossibility might be to use agricultural harvesting machin-ery such as conveyor belts. In the Philippines they arereported to use barges with some kind of hand poweredwinch when harvesting open-water water hyacinths (Tho-ren, 1997). When the hyacinths are close to the shoreline,one person can, in an easy and sustainable tempo, harvestapproximately 200 kg of fresh water hyacinths per hour(Gunnarsson and Mattsson, 1997).

If the water hyacinths are not growing directly on theshoreline, they must be transported there. Petrell andBagnall (1991) conducted a study to determine drag prop-erties of water hyacinths. The maximum towing velocity,before the mat became unstable and the leading edgestarted to roll forward under the water, was found to be0.40 m/s. The maximal size of the mats in this experimentwas 2.44 by 1.22 m.

2.5. Transport

When using water hyacinths in agricultural practise,transport of the fresh or treated plant is necessary. The needand design of the transport vary depending on the treatment.The design also depends on the scale of the operation. Inanaerobic digestion, the water hyacinths will have to betransported to the biogas digester that also requires a lowdry matter concentration. The sludge from the process, alsohaving a high water content, must be transported to the fieldfor spreading. In alternatives with compost or green manure,it is advantageous if the treatments are taking place close towhere the water hyacinths are harvested. That means lesstransport of the water hyacinths while they still have a highwater content, and thus less transport of water that mightneed to be added to the compost. The compost product couldbe transported to the field in baskets carried on the head(Eklund, 1996). The dried water hyacinths used as greenmanure could be carried in nets or pieces of cloth.

In Table 2 is a summary of the transport requirementsfor providing 1 kg of plant available nitrogen.

3. Treatment

3.1. Carbonisation

The main product from this three-stage process (gasifi-cation, pyrolysis and carbonisation) is charcoal, as a

Table 2Transport requirements for providing 1 kg plant available N throughwater hyacinth products (Gunnarsson and Mattsson, 1997)

Parameter Anaerobicdigestion

Compost Greenmanure

Unit

Fresh waterhyacinth to harvest

3676 8351 2973 kg wh (wb)

Dry matterto harvest

349 795 284 kg wh (DM)

Dry matterafter process

243 459 270 kg wh (DM)

Dry matterof product

10 70 85 %

Amount to spread 2378 649 324 kg whproduct (wb)

wh = water hyacinth.


by-product gas is obtained that can be used for internalprocesses (Thomas and Eden, 1990).

There are two problems in using the water hyacinth formaking charcoal. First, there is a need to reduce the watercontent, and second the ash content (40% according toThomas and Eden, 1990) of air-dried water hyacinth istoo high to get a good fuel as an end product. The highinvestments and technological level necessary also makecarbonisation an unfavourable alternative in developingcountries (Thomas and Eden, 1990).

3.2. Hydrolysis and fermentation

Hydrolysis together with fermentation will give a liquidfuel, for example, ethanol. The process is well suited formaterial with a high moisture content, which would makeaquatic weeds a good substrate. Hydrolysis and fermenta-tion also require yeast fermentable sugars that are availableonly to a very low extent in water hyacinth and other aqua-tic weeds. Some kind of pre-treatment is, therefore, neededto make the sugar more easily available for chemicalhydrolysis. The pre-treatment requires a relatively hightemperature, strong acids and pressurised reactors. Enzy-matic hydrolysis is an option, but difficult because of thehigh lignin content. Slesser and Lewis (1979, from Thomasand Eden, 1990) reported a negative energy balance of sucha process. Thomas and Eden conclude that hydrolysis ofwater hyacinths to produce fuel is, because of the negativeenergy balance, only feasible in situations where there is ahigh need for ethanol as a liquid fuel.

3.3. Fodder and silage

Water hyacinth as roughage is an interesting alternativeto reduce the shortage of animal feed. Tag El-Din (1992)conducted a study to see the effect of substituting beanstraw with water hyacinth hay when feeding sheep. Norton(1982, from Tag El-Din, 1992) considered 9% crude pro-tein, on a DM basis, to be the minimum in the fodderfor ruminants. Abdelhamid and Gabr (1991) give a crudeprotein content of 20% on DM basis showing that water

hyacinth in that aspect is a good roughage for ruminants.Tag El-Din (1992) found that using water hyacinth hayas a sole roughage greatly reduced the average daily weightgain. They concluded that for growing sheep, up to 30% ofthe roughage (bean straw) can be substituted with haymade from dried water hyacinths, without a loss in growthrate.

According to Bolenz et al. (1990), the stalk tissues of thewater hyacinths contain intercellular spaces filled with air,which soak up water while the animals are digesting. Thatleads to excessive water consumption and the animals feelreplete, although having little material of nutritional valuein their rumens. Bolenz et al. (1990) also found, whenexamining the water hyacinth tissue in microscope, sharpneedles formed of calcium oxalate. Bolenz et al. (1990)assumed that these needles could damage the digestive tractof animals fed with water hyacinths, if they were not dis-solved by digestive acid. To avoid these problems, Bolenzet al. (1990) suggested the following preparation of thewater hyacinths used for feeding animals: Chop the tissuesto eliminate the air included and to negate its ability toabsorb water. After chopping, the solid material shouldbe separated from the soluble components in the juice bypressing and centrifuging. The solid phase could be washedwith acid to eliminate the acid-soluble calcium-oxalate andthen processed to a ruminant fodder. The juice could beconcentrated, dried and used as a protein enriched foddercomponent (Bolenz et al., 1990).

It is possible to produce silage from water hyacinths, butthe water hyacinths need to be chopped into fine pieces toremove the air in the tissues; otherwise these could lead togrowth of aerobic moulds during the fermentation. Alsosorbic acid can be added to suppress the moulds. For suc-cessful fermentation the pH needs to be lowered (below 4),which is achieved by adding sugar. The water hyacinthswere calculated to contain 0.52% fermentable sugar in rela-tion to fresh weight so an addition of 0.4% sugar wasenough to reach the desired pH value (Bolenz et al., 1990).

3.4. Drying

Drying is a pre-treatment step done to get the rightmoisture content for the subsequent treatment, but alsoto decrease the weight and thereby make transport easier.It can also be a treatment in itself. Drying the water hya-cinths makes it possible to store them for later use andmake them easier to decompose once put in the soil. Thismight be achieved by sun drying, i.e., simply spreadingthe water hyacinths directly on the ground. Experiencesin Egypt of sun-drying of water hyacinths reported noobserved mould or contamination during the drying period(Tag El-Din, 1992). Polprasert et al. (1980) report that sun-drying water hyacinths for a few days reduce the weight byabout half. Apori (1994) found the when plantain and cas-sava peels were sun dried, the material had to be dried forfour days to attain a dry matter content of 87.0% (limit forstorability) or above. There were no significant losses of


nutrients, the maximum loss in crude protein was 0.3%(Apori, 1994).

In Kenya water hyacinth roots dried with full naturalaeration (hung over a fence) reached a dry matter contentof 87% after 13 days. The leaves of the same plants did notreach more than 66% DM. It was also found that the nutri-ent level in the water hyacinths decreased during the dryingprocess. It should be pointed out that these trials were car-ried out during the rainy season, and it is likely that thewhole plants would reach a dry matter content sufficientfor storage and the nutrient losses can be decreased if driedduring the dry season (Gunnarsson and Mattsson, 1997).

3.4.1. IncinerationSun drying and direct burning is used on a small scale in

certain parts of the world. Fresh water hyacinth has amoisture content of about 90% (Abdelhamid and Gabr,1991). Even when the moisture level is reduced to 10%the energy density is not more than 1.3 GJ/m3 (Thomasand Eden, 1990). This may be compared with 9.8 GJ/m3

in firewood and does not make the water hyacinth veryattractive for direct incineration. The major part of thefresh water hyacinth is water, and any processing or trans-port is therefore worthwhile only if the water content canbe reduced to 10–15% with ‘‘relative ease’’ (Thomas andEden, 1990).

3.4.2. Briquetting

Thomas and Eden (1990) mention briquetting as a pos-sible treatment. The briquettes are made by sun-drying thewater hyacinth for a few days, disintegrating, screening andchopping the dried water hyacinths to pieces about 6 mmlong. The shredded water hyacinth can then be compressedinto briquettes or pellets. The material resulting afterbriquetting water hyacinth has an energy density of8.3 GJ/m3, which is comparable to charcoal that has9.6 GJ/m3. This process requires initial investments formachinery and a rather large area for the drying, whichmight be expensive.

3.5. Anaerobic digestion

Anaerobic digestion is the biological process by whichorganic matter is degraded in the absence of oxygen andbiogas is produced. The three-step process results in a gasthat can be used directly for cooking, heating or produc-tion of electricity and a nutrient-rich slurry.

Biogas is a form of energy that has very useful by-prod-ucts and positive impacts on public health and pollution.This, together with the growing shortage of firewood andrising cost of fossil fuels, has made anaerobic digestionincreasingly interesting. These advantages of the processmight make it well suited for use in developing countries.According to Gunnersson and Stuckey (1986) ‘‘Plants suchas water hyacinths. . .can be degraded easily, and give quitea high gas yield. In these cases, digestion of these weeds cansolve the problems caused by excess growth in canals and

provide energy as well.’’ Day et al. (1990) see biogas as areliable energy source that can improve the environmentboth on a large and on a small scale, e.g., deforestationand smoke reduction in kitchens.

However, technical requirements might limit the possi-bility to use anaerobic digestion as a treatment for waterhyacinth in rural areas. Lack of water and cow manureas a substrate has been mentioned as other limitations.When water hyacinths are intended as the main substrate,large amounts of animal manure are not needed and, dueto their growth place, water is generally available.

3.5.1. Pre-treatment

Chopping the water hyacinths increases the specific sur-face of the substrate and thereby enhances the access ofmicrobes to the plant material, which is important for awell-working biogas process (Haug, 1993). Moorheadand Nordstedt (1993) conducted experiments with differentparticle sizes, nitrogen contents and inoculum volumes, in amesophilic process (35 �C). The total biogas and methaneproduction was largest for water hyacinth when the plantmaterial was chopped into 6.04 mm pieces (compared with1.6 and 12.7 mm).

Water hyacinths have a high content of hemicelluloseand cellulose, but the existing hemicellulose has a ratherstrong association with the lignin in the plant, which makesit unavailable for the microorganisms (Patel et al., 1993a).To optimise biogas production, the plants must undergosome kind of pre-treatment. Patel et al. (1993a) used ther-mochemical pre-treatment to solve these problems andthereby increased the gas production.

Patel et al. (1993b) found that the addition of metal ions:Fe3+, Zn2+, Ni2+, Co2+, and Cu2+, will enhance gas pro-duction and increase the methane content in the producedgas and also result in better process stability. The waterhyacinths used in these experiments were, however, takenfrom a pond that did not receive municipal or industrialeffluent. Water hyacinth from polluted water might alreadycontain sufficient amounts of heavy metals. Geeta et al.(1990) reported increased biogas production when nickelwas added to water hyacinth or a mixture of water hya-cinth and cow dung.

El-Shinnawi et al. (1989) produced biogas from waterhyacinth mixed with cow dung, and found the cow dungto provide enough microorganisms to serve as inoculum.The conclusion from these reports is that in developingcountries it is probably better to not use costly pre-treat-ment and instead use a longer residence time.

3.5.2. Digestion product

3.5.2.1. Sludge. Gunnersson and Stuckey (1986) write thatthe sludge from the anaerobic process is rich in nutrientsand organic matter and provides a good way to recyclethese nutrients. A wet biogas process has a dry mattercontent of 2–10% (Thomas and Eden, 1990; El-Shinnawiet al., 1989; Madamwar et al., 1991). Essentially all ofthe nutrients contained in biomass used for anaerobic


methane generation remain in the digester sludge (Honset al., 1993; Stout, 1983) as long as it is not de-wateredand stored airtight. Nutrient concentration will increaseslightly during digestion because of the loss of volatile sol-ids, associated with methane generation. The high concen-tration of nutrients gives the sludge a high potential asfertiliser (Hons et al., 1993). Due to the anaerobic condi-tions, most of the nitrogen in the sludge will be found inorganic form, followed by ammonium (NHþ4 ) (20–50% ormore) and a very small part as nitrate (NO�3 ) (Honset al., 1993). Anaerobic sludge is easily de-watered andthere have been thoughts on marketing the dried sludgeas a fertiliser. This practice is only feasible for very largefeedlots (Stout, 1983). Using the sludge as a fertiliser with-out de-watering will mean more effort in transport of thematerial.

Much of the nitrogen in anaerobic sludge is in ammo-nium (NHþ4 ) form and therefore is less likely to leach fromthe soil than nitrite (NO�2 ) and nitrate (NO�3 ). The decisionwhether ammonium (anaerobic sludge) or nitrite andnitrate (aerobic sludge or chemical fertilisers) are to be pre-ferred will be based on the soil type (Stout, 1983). FromChina an increase in agricultural productivity by 30% overfarmyard manure is reported when using anaerobic sludgeas a fertiliser (van Buren, 1979, from Gunnersson and Stuc-key, 1986). This is probably due to the nitrogen in thesludge being more accessible than, in for example, farm-yard manure (Gunnersson and Stuckey, 1986). There is arisk of contaminating nearby watercourses (Stark andClapp, 1980) and decreasing seed germination by applyingtoo much sludge (Hons et al., 1993).

Ammonia volatilisation can lead to significant losses ifthe material is spread on the surface of the soil or storedin containers that are not airtight. Some authors reportonly negligible volatilisation from sludge. How much is lostwill depend on the characteristics of the sludge, method ofapplication, and soil properties (Hons et al., 1993). The riskof ammonia volatilisation increases with high ammoniumconcentration and pH (Moorhead and Nordstedt, 1993).If the sludge from a biogas process is to be applied as a soilfertiliser in areas with high temperatures, it must be workedinto the soil or in some other way covered to minimise thenutrient losses due to ammonia volatilisation. The nitrogen

Table 3Summary of reported gas yields with water hyacinth as a main substrate

Source Biogas pr

(l/g DM)

Chanakya et al. (1993) 0.291Chanakya et al. (1993) 0.245Chynoweth et al. (1983, from Moorhead and Nordstedt, 1993) –Ellegard et al. (1983) –Hanisak (1980, from Moorhead and Nordstedt, 1993) –Moorhead and Nordstedt (1993) –Patel et al. (1993a) 0.190Patel et al. (1993b) 0.143Madamwar et al. (1991) 0.19

W = water hyacinth as only substrate. WC = a mix of water hyacinth and cat

losses might otherwise be as high as 70–80% (Thyselius,1997). Parker and Sommers (1983) report that 15% of theorganic nitrogen remaining in the sludge will be availableto the plants during the first growing season. They alsostate that the risk of nitrogen immobilisation in the soil ishighest when the sludge has a C/N ratio above 20.

3.5.2.2. Biogas. Chanakya et al. (1993) found that waterhyacinth has a high content of fermentable matter andtherefore a high potential for biogas production, but thehigh lignin content can reduce the actual production.The low bulk density could result in large voids with poorcompaction and low feed rates (Chanakya et al., 1993) asa result. El-Shinnawi et al. (1989) conducted trials withanaerobic digestion of agricultural waste. Rice straw,maize stalks, cotton stalks and water hyacinths mixedwith cow dung were digested in different containers. Themixture of water hyacinth and cow dung was found toproduce more biogas per kilogram VS added than maizeand cotton stalks, but the total biogas production perkilogram DM added was lower for the water hyacinths.The low values for total gas production was probablymostly due to the high lignin content and low percentageof volatile solids in the water hyacinths (El-Shinnawiet al., 1989). Table 3 is a summary of reported gas yieldswhere water hyacinth made up all or the major part of thesubstrate. This shows that water hyacinths can competewell with any kind of animal manure as a substrate forbiogas production.

The gas produced during the process consists mainly ofmethane, carbon dioxide and ammonia, but small amountsof hydrogen sulphide may occur. The proportion of meth-ane in the produced gas is usually up to 60%, but dependson the substrate (Gunnersson and Stuckey, 1986). Temper-ature, pH and pressure may alter the gas compositionslightly. The specific gravity decreases when the methanecontent increases (Constant et al., 1989).

3.6. Composting

Another possible treatment method for water hyacinthsis aerobic decomposition, i.e., composting. Due to the lackof infrastructure and capital, small-scale composting is the

oduction Residence time (days) Methane (%) Substrate

(l/g VS)

0.348 300 60 W (fresh)0.292 300 60 W (dry)0.19 – – W0.4 – – W0.24 – – W0.20–0.28 15–60 63–67 W0.293 8 62–66 W0.286 8 – W0.4 8 65 WC

tle dung was digested.


main interest in developing countries. Composting can bedefined as the biological decomposition and stabilisationof organic substrates, under conditions that allow develop-ment of thermophilic temperatures as a result of biologi-cally produced heat. The final product should be stable,free of pathogens and plant seeds, and beneficial whenapplied to land (Haug, 1993).

To decrease evaporation and losses of nitrogen, asammonia, the compost can be covered with a layer of,for example, straw, grass or plastic. The advantage ofusing straw is that the microorganisms use straw as asource of energy and then catch the escaping ammoniain order to fill their nitrogen demand (Claesson and Stei-neck, 1991). Covering the compost with straw is alsoadvantageous since it decreases the losses of nutrientfrom rainwater leaching through the compost pile (Ulen,1991).

3.6.1. Pre-treatment

Enhancement of the access of the microbes to the plantmaterial by chopping the fresh water hyacinths is impor-tant for a well-functioning compost (Dalzell et al., 1979).To enhance bacterial decomposition, Polprasert et al.(1980) mention that the water hyacinths were shredded intoabout 5-cm long pieces before they were composted. In astudy by Elserafy et al. (1980), the fresh water hyacinthswere also chopped before being composted. Compostingthe water hyacinth without size reducing them might, onthe other hand, decrease the need for additional struc-ture-supporting material and decrease the labourrequirement.

3.6.2. Amendments

For a well-working compost process, conditioning of thefeed substrate is sometimes needed. Amendments can beadded to prevent lack of energy, nutrients or other chemi-cal substances (Haug, 1993) or to establish a suitablemicro-fauna and increase degradation of, for example, cel-lulose and lignin (Adhikary et al., 1992).

In a study of composting water hyacinths conducted inEgypt by Elserafy et al. (1980), lignin and cellulose werereported to remain undegraded after 185 days. The waterhyacinths were spread alternating with a microbial activa-tor consisting of ammonium sulphate, superphosphate andlime, in order to keep the process slightly alkaline (Elserafyet al., 1980).

Haug (1993) mentions that a cellulose-rich substratemay lack the nutrients necessary to sustain rapid microbialgrowth rates. Adhikary et al. (1992) investigated the micro-flora associated with different plant wastes, e.g., water hya-cinth, during composting. It was shown that a mixture offungi, actinomycetes and bacteria added to the compostincreased both the cellulose and lignin degradation com-pared with the untreated control (Adhikary et al., 1992).Since water hyacinths have a relatively high lignin content,preparation of the compost with microorganisms is ofinterest.

3.6.3. C/N ratio

The optimal C/N ratio for the microbes is 15 to 30according to Haug (1993), who claims that a decreasedratio is no problem for the composting process but leadsto losses of excess nitrogen via ammonia volatilisation.Others claim the optimal range for bacterial decompositionto be a C/N ratio of 20–40 (Achraya, 1950, from Polprasertet al., 1980). A balanced nutrient availability for the micro-organisms is important for a high decomposition rate (Pol-prasert et al., 1980). The same authors also noticed aslower composting rate for the piles prepared with waterhyacinths compared with a control consisting of cow dungand leaves. The reason, they concluded, might be thatleaves consist mainly of hemicellulose and cellulose, com-pounds reported to be more biodegradable than lignin,the major component of water hyacinths (Karim, 1968,from Polprasert et al., 1980). The water hyacinths have aC/N ratio of about 16 according to Abdelhamid and Gabr(1991), whereas Dalzell et al. (1979) report a C/N ratio of20. This means that water hyacinths need an addition ofcellulose material, such as leaves (C/N 60.8, Polprasertet al., 1980) to keep the ammonia losses low for the micro-bial decomposition.

3.6.4. Moisture content

Elserafy et al. (1980) report an optimal moisture contentof about 60% for the composting process, and Dalzell et al.(1979) give an optimal range of moisture content of 50–60%.

Elserafy et al. (1980) claim that the moisture content ofthe fresh water hyacinth is too high and hence little addi-tional water is needed during the composting. Also theevaporation rate is high in hot climates, i.e., where thewater hyacinths grow. Therefore the high moisture contentis probably not a big problem when composting water hya-cinths. To avoid too low of a moisture content in pileswhen composting in tropical areas with hot climates, thecorrect location of the compost operation is important.The preparation of 1 ton of compost product may requireup to 2700 L of water. Composting can be carried outeither in a pit or in a pile. Dalzell et al. (1979) suggest thatcompost be produced in pits during dry seasons and, toavoid water logging, in piles during rainy seasons. Thecompost should be protected from the wind to decreasemoisture losses (Dalzell et al., 1979; Njoroge, 1994). Plac-ing the compost pile out of direct sunlight can also reducethe water requirements according to Dalzell et al. (1979).

3.6.5. Pathogen reduction

As a measure of bacterial destruction, Polprasert et al.(1980) mention an initial coliform concentration between5 · 103 and 180 · 103 MPN (most probable number)/gcompost mixture and a die-off of 70–90% after 10 weekwhen composting water hyacinths together with nightsoil.As comparison, the US Environmental Protection Agencyrequires that the density of faecal coliforms must be lessthan 1000 MPN/g dry matter for sludge-based compost


intended for public distribution and marketing (Haug,1993). Such low levels of pathogens were not reached inthe trial by Polprasert et al. (1980) where the highestreported temperature was 60 �C.

3.6.6. Use of compost as a soil amendmentAs the composting process proceeds, the readily degrad-

able organics in the substrate are oxidised and graduallyturned into increasingly less degradable humic material(Haug, 1993). If the compost product is not mature enoughit has been observed to contain metabolites that are toxicto plants. If the compost applied to soils still has a highC/N ratio, i.e., decomposes rapidly, it may rob the soil ofnitrogen needed by the plants. If, on the other hand, theC/N ratio of the organic matter is low, the excess nitrogenreleased as ammonia may become phytotoxic to plants(Haug, 1993).

The major nutrients important for the fertilising qualitiesof the compost are nitrogen (N), phosphorus (P) and potas-sium (K). The oxidation of nitrogen in the compost processultimately yields nitrate, which is not normally lost from thecompost pile (Polprasert et al., 1980). Since phosphorus andpotassium are physico-chemically less mobile than nitrogen,these compounds remain in the compost unless lost throughleaching. They concluded that composting of the water hya-cinths in developing countries is a feasible method becauseof its ability to retain most of the nitrogen, phosphorus andpotassium in the compost and attain a satisfactory degree ofcomposting within a relatively short period of time, i.e., 30days (Polprasert et al., 1980).

In a longer perspective, adding compost regularlyenhances the quality of the soil by improving the soil struc-ture. Advantages with compost application are (Eklund,1996):

� Increased water-holding capacity of the soil,� Improved soil structure by binding sand particles, mak-

ing soils less prone to erosion,� Adding nutrients to the soil and thus giving higher crop

yields, and� Contributing to re-circulation of organic material.

In Africa, many resource-poor farmers cannot afford topurchase fertilisers. They seldom use organic waste prod-ucts for compost (Abdel-Sabour and Abo El-Seoud,1996). In a study in India (Sharma and Mittra, 1990), par-tially decomposed water hyacinth compost was applied to asandy clay loam in a pot experiment with rice. The grainyield increased with increasing rate of application up to15 ton/ha. The decomposition of water hyacinths resultedin active mineralisation of nutrients (Kumada, 1977, fromSharma and Mittra, 1990), which enhanced tillering andgrain yield. Application of all organic materials increasedthe organic carbon and available nitrogen, phosphorusand potassium contents of the soil after harvest of the firstcrop. The higher grain yield of the second crop was associ-ated with increasing soil fertility, owing mainly to increases

in organic carbon and available nitrogen concentrations. Itwas concluded that yield may be enhanced by incorporat-ing water hyacinth compost up to 10 days before trans-planting the rice to the field (Sharma and Mittra, 1990).

Water hyacinth compost also had positive effects on ses-ame growth in Egypt (Abdel-Sabour and Abo El-Seoud,1996). Primary analysis indicated that compost additionincreased the levels of extractable nitrogen, phosphorusand potassium in the top 25 cm of the soil (Abdel-Sabourand Abo El-Seoud, 1996).

Composted water hyacinths were also tried as anorganic source of nitrogen on fibre yield of white jute.The water hyacinth compost was found to contain 0.55%nitrogen (on DM) and 45% moisture. Application of40 kg N/ha, through water hyacinth compost alone,increased the fibre yield significantly compared with thecontrol (Thakuria et al., 1991).

3.7. Green manure

One option is to use the water hyacinths as green man-ure. Green manuring involves spreading plant material(with a high nitrogen content) on the fields and sometimesalso working it into the soil (van der Werff et al., 1995).Wivstad (1997) reports that the most important featuresof a green manure are a large dry matter production anda high ability to fix nitrogen. The chemical analyses foundin literature indicate a high nutrient content of the waterhyacinth, 20% crude protein (Abdelhamid and Gabr,1991), but values as low as 7.26% have been reported (Else-rafy et al., 1980). They also have a very high dry matterproduction (140 ton of DM/ha and yr, Abdelhamid andGabr, 1991). This should make them suited for use as greenmanure.

Drying the water hyacinths before spreading them on thefields ought to be done primarily to minimise the risk of bil-harzia (Thors, 1997) and secondly to decrease the labourrequired for transportation. Drying might lead to losses inbiomass and thereby nutrients. For many plants their leaveshave a higher nutrient content than the rest of the plant.This is also true for the water hyacinths. The nitrogen con-tent of the leaves and stem is 3.7% (DM) and 2.7% (DM),respectively (Abdelhamid and Gabr, 1991). The leavesand other fragile parts will usually make up the largest partof the mechanical losses (Grant, 1990). Leaf-losses may,therefore, lead to a decrease in the average nutrient contentcalculated on the total dry matter. According to Gupta et al.(1996), leaves do not make up more than 25% of the totalbiomass of water hyacinths.

3.7.1. Application of water hyacinth

To minimise the losses of nitrogen through volatilisationonce the plant material is spread in the field, it should becovered by soil (Dalzell et al., 1979). One of the easiestways to do this would be to, while ploughing, put the waterhyacinths in the plough furrow and allow the next furrowslice to cover them. Gunnarsson and Mattsson (1997)

Table 4Nitrogen losses and amounts needed to provide 1 kg plant availablenitrogen

Parameter Anaerobicdigestion

Compost Greenmanure

Unit

Mechanical N-losses 10 0 15.8 % ofinitial N

Process losses 0 39.6 0.3 % ofinitial N

Losses duringspreading

15 0.6 0 % ofinitial N

Total N loss 24 40 16 % ofinitial N

DM loss due toprocessing

30 44 13 % ofinitial DM

N mineralised atspreading

14 4 0 % ofinitial Nas NO�3 orNHþ4

N mineralised firstseason

15 10 30 % ofremainingorg, N

Plant available nitrogen 23 9 25 % ofinitial N

Amount to harvest 135 338 124 kg wh (DM)Fresh water hyacinth

to harvest1432 3568 1305 kg wh (wb)

Amount dry matter tospread

95 192 108 kg whproduct(DM)

Dry matter in product 10 70 85 % of wetweight

Amount to spread 957 273 127 kg whproduct (wb)

Calculations are based on a dry matter content of 9.5% and a nitrogencontent of 3.2% (db) in the fresh water hyacinths (Gunnarsson andMattsson, 1997). wh = water hyacinth.


found that partially dried water hyacinths used as greenmanure continued to grow in soil, which is why it wouldbe desirable to somehow disintegrate the plants beforeusage. These methods require a certain level of mechanisa-tion. If all of the agricultural work is done by hand, thewater hyacinth could be hoed into the soil while preparingit for sowing. A method must be developed so that theworking hours spent on the fields do not drasticallyincrease, but exactly how this could be done depends onthe level of mechanisation.

Nitrogen mineralization is the transformation from theorganic state into the inorganic forms of ammonium ornitrate, thereby making it available to the plants. No fig-ures on mineralization of dried water hyacinths were foundin literature, but the nitrogen and lignin content of waterhyacinths are similar to that of subterranean cloverdescribed in an incubation experiment to examine carbonand nitrogen mineralization from green manure legumesdecomposing in the soil (Marstorp and Kirchmann,1991). Also the nitrogen uptake from decomposing legumematerial by subsequent crops was determined. During 115days of incubation, approximately 30–35% of total nitro-gen in subterranean clover was mineralised. The net nitro-gen mineralization correlated well with the C/N ratio of thelegumes; the species with high C/N ratio had lower miner-alization. Calculations of nitrogen mineralised from thesame green manure legumes under field conditions indi-cated that the potentially mineralisable amount of nitrogen(N0) decreased with plant age (Kirchmann and Marstorp,1991).

4. Discussion

To establish a well-operating chain for use of the waterhyacinth, all the steps involved in the process – harvest,transport, pre-processing and processing – must be consid-ered. A participatory approach is important for adjustingthe systems to be introduced to the needs and possibilities(technological and economic) of the people. If the system isto be successful, it has to be accepted in the area and peoplemust benefit from it. In Table 4 an evaluation of differentmethods for utilisation of water hyacinths in agricultureis presented. The evaluation is based on nitrogen lossesand how much water hyacinth product must be spread toprovide the available nitrogen needed.

A method for harvesting the plant material must bedeveloped to get the weeds out of the water. In many areas,the supply of water hyacinth is almost unlimited. A goodestimation of the amount of water hyacinths available forharvest is 140 ton of dry matter per hectare, which is equiv-alent to the annual production per hectare (Abdelhamidand Gabr, 1991). The nitrogen content of the water hya-cinths is between 1.2% and 3.2% on a dry matter basis.

Harvest of the water hyacinths on a large scale can bedone with specially designed machines. If the water hya-cinths have to be transported to the place of harvest, Petrelland Bagnall (1991) found the maximum towing velocity to

be 0.40 m/s. Towing of the water hyacinths might, how-ever, not be necessary as they often are transported directlyto the shore by winds. On a smaller scale, harvesting byhand is preferable since it is a simple and inexpensivemethod.

Harvesting of the water hyacinths would improve thehealth situation for people living in water hyacinth infestedareas. Direct negative effects of water hyacinths are theincreased occurrence of diseases like malaria and bilharzia.The quality of the fish caught in areas with lots of waterhyacinths has also deteriorated (Sunday Standard, Kenya,12/1-1997). The harvesting process in itself might imposehealth problems. Harvesting will lead to contact with thewater and thereby the risk of being infected by bilharzia.The total time the plants are kept dry, or at least out ofthe lake, must exceed 48 h (Thors, 1997) to ensure destruc-tion of the bilharzia parasites if they are in the form ofcercariae. However, if the hosting snails are present, the bil-harzia parasite can survive in the snail for a long time.

Cutting the water hyacinths before processing is neces-sary for anaerobic digestion. For composting, cuttingmight not be necessary because whole plants will enhanceaeration. It is favourable if the preparation of the freshwater hyacinths can be done during the dry season. During


that season people are not engaged in fieldwork, and con-sequently have more time for working with the waterhyacinths.

Since not all water hyacinths can be expected to be useddirectly on the lakeshore, it is also necessary to find a wayto transport the harvested material. The high water contentof the fresh water hyacinths makes drying them directly onthe beach after harvesting of interest in order to decreasethe labour needed for transport. Fresh water hyacinthscan be transported in buckets or baskets. When it comesto transport of the product, the nitrogen concentration inthe sludge from an anaerobic process is, due to its highwater content, very low and the sludge is difficult to trans-port. Buckets could be used but the large amount that mustbe transported to provide nutrients, in the form of sludge,for the crop would make it necessary to develop a newtransport system. The heavy workloads otherwise mightlead to a risk of the sludge being dumped close to the pro-cessing plant. If so this would probably lead to even greatereutrophication of the lake and an increase in the amount ofwater hyacinths. The compost product can be transportedin baskets. The amount of compost product to be trans-ported to the field is twice as much as when transportingdried water hyacinths. Once in the field the compost prod-uct has a texture very similar to the soil and is thereforeeasy to work into the soil.

Use of dried water hyacinths as green manure is thealternative with the lowest transport requirement. Theproduct is also easy to handle. Drying gives the option tostore the plants so they can be harvested when there is time,during the dry season. They can then be used when neededby the farmer, during the rainy season. Sun drying for afew days decreases the weight of the water hyacinths byabout half. A dry matter content of 87% is needed to stopmicrobial activity decomposing the product (Apori, 1994).

The advantage of the anaerobic process is that it resultsin two useful products: a nutrient-rich sludge that can beused for soil improvement and a gas that can be used forcooking, heating, lighting and electricity production, thusdecreasing the need for firewood. One restriction withanaerobic digestion is the large need for water, but as waterhyacinths grow in water that should not be a problem.Implementation of a biogas program requires, accordingto experiences from China and India, strong governmentalsupport to be successful. It also requires quite large initialinvestments and technological skills. Small-scale digesterdesigns exist that, with proper implementation, could workvery well in these areas (Gunnersson and Stuckey, 1986).

Composting of water hyacinths is a possible treatmentmethod. Compared with drying, the nitrogen loss is largerbut the compost product obtained is stable, and if a tem-perature of above 55 �C is reached for a day or two essen-tially all pathogens are destroyed (Haug, 1993). To ensurekill-off, the high temperature must be reached throughoutthe compost pile. The high water content of the fresh waterhyacinths probably does not cause any problems whencomposting in hot, evaporative climates. On the other

hand, regardless of the high water content of the substrate,the compost might need watering, especially during the dryseason. If possible, the compost should be placed close to asource of water that does not dry out during the dry seasonsince that is when the water demand is highest. If the waterhyacinths are cut into smaller pieces before composting,addition of a material providing structure might be needed(Dalzell et al., 1979). As water hyacinths have a relativelyhigh ash content, 25.7% of DM (Abdelhamid and Gabr,1991), adding extra ash will probably not be needed. Thewater hyacinths are valuable in the compost due to theirhigh content of nitrogen. Compost of water hyacinths willoff-set the cost of cleaning the irrigation system of this weedand will prevent the health hazard arising from leaving theplant material on the beach (Elserafy et al., 1980). Waterhyacinth compost has been shown to have positive effectson crop growth (Sharma and Mittra, 1991; Abdel-Sabourand Abo El-Seoud, 1996; Thakuria et al., 1991). Whenusing the water hyacinth compost as organic fertiliser, itmay be advisable not to add the compost too near in timeto sowing or transplanting, since it has been concluded thatyield may be enhanced by incorporating water hyacinthcompost up to 10 days before transplanting rice to the field(Sharma and Mittra, 1990).

Due to its positive charge, ammonium can bind to thenegatively charged soil particles and the risk is low that itleaches away. The negatively charged nitrate does not bindto the soil particles but remains in the soil fluids and is thusavailable for leaching (Claesson and Steineck, 1991).Whether ammonium or nitrate is to be preferred as a fertil-iser on a certain soil depends on the type of crop grown, theclimate, soil conditions and the agricultural practices in thearea (Stout, 1983). A large proportion of the nitrogen inanaerobic sludge is in ammonium (NHþ4 ) form (Stout,1983). In the compost product only a small amount ofthe nitrogen is in the form of nitrate, and thereby directlyavailable to the plants. Most of the nitrogen is bound inhumus and must be mineralised to be plant available. Inthe dried water hyacinths, all nitrogen is in organic formand the release of nutrients might therefore be delayed withlittle becoming available to the crop during the first grow-ing season. In the long run, adding organic material will, asmentioned before, increase both the nutrient content andthe water-holding capacity of the soil.

When comparing the contents of phosphorus and potas-sium in the fresh water hyacinths with the water hyacinthsdried for 3 and 13 days (Gunnarsson and Mattsson, 1997),it can be seen that the two nutrients behave differently.While the phosphorus content stays relatively constantduring the drying process, the potassium content hasdecreased to a value that is half of the original. Phosphorusand potassium can only be lost through leaching (van derWerff et al., 1995). The fact that the dried samples lostpotassium indicates that the losses of nitrogen also weremostly due to leaching. The literature also confirms thatpotassium is lost to a greater extent than phosphorus(Ulen, 1991). The amount of nutrients lost through leach-


ing is higher if the plant material is dry at the time of pre-cipitation (Ulen, 1984).

5. Concluding remarks

Applying the water hyacinths directly without anyother processing than sun drying, seems to be the bestalternative in small-scale use, due to the relatively smalllosses of nutrients and workload required (Gunnarssonand Mattsson, 1997). This option also does not requireany large investments or new technology. If the freshwater hyacinths could be applied as mulch on the fields,the labour need for weeding could be used for handlingthe water hyacinths instead. But transport of fresh waterhyacinths means transporting a lot of water. Dryingseems to be a reasonable treatment since it will bothdecrease the labour required for transport and the riskof water hyacinths emerging in the field, as well asimproving their hygienic status. It can be assumed thatas long as drying is carried out during the dry season,reaching a high enough dry matter content is possible(Gunnarsson and Mattsson, 1997). The limitation mightinstead be finding time for harvesting and the availabilityof land to dry the water hyacinths on.

To meet the ever-growing energy demand, biogas pro-duction could be one option but it requires investmentsand technological skills that would impose great problemsin developing countries, where the water hyacinth is oftenfound. The sludge, if used as fertiliser, would also be diffi-cult to transport. It might be possible to use the containersthat are used for carrying water, but the low dry mattercontent makes this a heavy task. According to calculationsby Gunnarsson and Mattsson (1997) 88 ton of sludge mustbe transported to equal 37 kg of plant available N/ha. Theadvantage with anaerobic digestion is that the gas pro-duced has multiple uses, such as cooking and lighting,but this does not outweigh the investments and technologyneeded. For optimal operation, the anaerobic digestershould be fed continuously. Anaerobic digestion wouldhave the same labour requirements during the dry andrainy season, and might therefore interfere with the agricul-tural work.

Composting as an alternative treatment has the advan-tage of producing a product that is easy to work into thesoil compared with dried water hyacinths, because of thedecomposed structure. But the structure of the compostwill also make it more difficult to transport and will,because of the relatively high nitrogen losses, require moretransport work compared to dried water hyacinths. Alsoconsiderable work is required for taking care of the com-post. To provide 37 kg plant available nitrogen, at LakeVictoria, 132 ton of fresh water hyacinths must be com-posted (Gunnarsson and Mattsson, 1997). That the com-post product already contains a well-working flora ofmicroorganisms will be favourable for the soil.

The relatively slow mineralisation rate of organic nitro-gen prevents leakage of nutrients from the soil. The nutri-

ents will probably be stored in the soil for the next growingseason. In the green manure all nutrients are in organicform; in compost, only small amounts of the nutrientsare found in the mineralised form. It will therefore takesome time before the nutrients are available to the crop.In sludge much of the nitrogen is in the form of ammoniumand can be directly available to plants. In warm climatesthere is a big risk of gaseous losses of nitrogen as ammonia.Therefore green manure or compost might be preferable.

For other uses of the water hyacinth, such as incinera-tion on a large scale, it must be possible to reduce the mois-ture content, ‘‘with relative ease’’, to 15% or less (Thomasand Eden, 1990). The highest dry matter content reached intrials by Gunnarsson and Mattsson (1997) was 87%, inroots after one week of drying on a fence. Storing or incin-eration might be possible if drying can be accomplishedduring the dry season. Burning the dried water hyacinthsdirectly or as briquettes might therefore be a feasible solu-tion that would not only decrease the water hyacinth prob-lem but also provide energy and thereby decrease thedeforestation. Briquetting would produce a product thatis more similar to the charcoal used today, but the process-ing requires energy input and investments in machinery.The ashes could then be spread in the fields to provide min-erals, mainly phosphorus and potassium (and larvae pro-tection). The ash spreading would require a relatively lowlabour input, but the effects and application rate must beinvestigated.

An interesting feature of the water hyacinths is theirability to accumulate nutrients and metal ions from sur-rounding water. If water hyacinths are introduced to theponds for biological cleaning of the water, together withother adjustments, it might improve the degree of cleaning.This would, in a longer perspective, improve the waterquality in the recipient water by decreasing the input ofnutrients and pollutants. Reduced nutrient input might inturn decrease the growth rate of the water hyacinths inthe recipient water and thereby further improve the qualityof the water and help to control the further spread of thehyacinths.

Water hyacinths pose a big, and increasing, problem inmany places; mechanical control alone might not be suffi-cient. A more productive way to finally control the growthcan be to make use of the plant by using one or several ofthe techniques described in this paper.

Acknowledgements

The authors would like to thank the Swedish Develop-ment Agency (SIDA) for financing this study. We wouldalso like to thank Assoc Prof. Hakan Jonsson, Prof. GirmaGebresenbet, the District Commissioner of Homa Bay,Kenya, Dr. K.V. Seshu Reddy, Scientist-in-Charge at Mbi-ta Point Field Station, Britta Widen, Stephan Noll, Dr.Lennart Bengtsson and everybody at the SNFIOH WomenCentre in Homa Bay, Kenya for helping in the preparationof the manuscript.


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