X. WATER PURIFICATION

This is probably the most important chapter in this handbook. The subject was only briefly mentioned in the first edition. Several papers did refer to the capacity of water plants to extract plant nutrients from the water in which they grew. But the interest in this rather obvious fact was related to the ability which this property conferred to farmers in South-East Asia to establish a nutrient cycle. This double cycle, plants-pigs-humans-fish-humans-plants, enabled a limited set of nutrients to be continually reutilized in ponds in a system powered by the sun. Other papers, which described the fertilizing potential of water plants, recognized that the nutrient content of water plants had been salvaged from water and might otherwise have been lost to the sea.

The reason which has prompted the spate of papers within the last ten years on the nutrient extraction possibilities of aquatic plants is the increasing awareness of the problems of water pollution - both fresh and salt water - as a consequence of population growth and industrial development, and the disposal of human, animal and industrial wastes into inland waters and into the sea.

In the past industrial wastes scarcely existed and human and animal wastes were carefully returned to the land to conserve the fertility which it was recognized that they contained. Now it is often cheaper to buy fresh supplies of fertilizer in concentrated, easily applied forms and flush away wastes into sewers or streams. Many examples of the devastating consequences of this waste on formerly clean and useful rivers and lakes have aroused public and scientific awareness of the need not only to arrest this practice but to try and reverse it by extracting the pollutants. The remarkable ability of aquatic plants, particularly the water hyacinth, to extract compounds and elements from water efficiently has become well recognized.

It is particularly interesting that among the scientists who have devoted their energies to this subject are those of the U.S.A. National Aeronautics and Space Administration. The National Space Technology Laboratories have recently published a series of papers on the usefulness of water hyacinth (Eichhornia crassipes) and also alligator weed (Alternanthera philoxeroides) in removing pollutants from sewage and industrial waste. The U.S. National Academy of Sciences have included in their book “making aquatic weeds useful: some perspectives for developing countries” (1976), a chapter on “Wastewater treatment using aquatic weeds”, which gives a useful illustrated summary of recent work together with a valuable list of references. As the book is recent and readily available there is no need to cite the data reviewed in it on the uses of water hyacinth, duckweeds (Lemna spp.), submerged and emergent weeds, all of which are shown to have a potential and actual use for purifying water. One interesting item mentioned is research in progress to see if aquatic plants can be used to extract silver, gold and other precious metals from ore-refining waste waters.

The interest of the U.S.A. in the subject of water purification by aquatic plants is emphasized by the fact that of the 48 papers cited in this chapter only seven are from outside the U.S.A.

Boyd, C.E., 1970c Vascular aquatic plants for mineral nutrient removal from polluted waters. Econ.Bot., 24:95–103

This paper is one of the earliest to go into the question of reducing the pollution of lakes by the harvesting of aquatic plants which have withdrawn nutrients from the water. The author points out that all aquatic plants can serve this purpose but small plants, like phytoplankton, or submerged plants are more difficult and expensive to harvest than the floating and emergent vascular plants. Four species are considered suitable: Eichhornia crassipes (water hyacinth), Alternanthera philoxeroides, Justicia americana and Typha latifolia. (For mineral content and productivity see Chapter III.) Boyd points out that plants grown in nutrient-rich effluent would probably contain 1.5 to 2.0 times the mineral analyses figures given.

Boyd states that water hyacinth would be ideally suited for nutrient removal systems. As it floats on the surface and is not rooted, harvesting is facilitated. By continuous harvesting the population could be kept in a rapidly expanding phase during which uptake rates of nutrients are at their highest. Waters beneath dense stands are anaerobic so additional N would be lost by denitrification. There would be considerable microbial activity beneath the hyacinths and nutrients would be absorbed by these organisms. In addition considerable organic matter would reach the water by the loss of root fragments and probably have a fairly high biological oxygen demand (BOD) and it might prove necessary to use conventional sewage holding ponds to reduce the BOD prior to final release.

Alternanthera philoxeroides could probably be best harvested by draining the pond and then using modified forage harvesting equipment. Typha latifolia as an emergent species could be grown in ponds about 1 m deep so that the water remains anaerobic, allowing bottom soils to remove P from solution. But it would be better, if space permitted, to grow the plants in water 15–20 cm deep to maximize soil absorption of P. At a stage when P equilibrium had been reached then the ponds could be dried and used for conventional crops until the P levels are reduced.

In nutrient-rich waters phytoplankton frequently produces dense growth and prevents the development of rooted species by light limitation. This could be avoided by starting the plants in shallow water and increasing the water depth as the plants grow. Ponds could be started at different dates so that harvesting could be staggered and thus a constant supply of plants for feedstuffs would be provided. Water hyacinth would not need these arrangements, but as it cannot overwinter in temperate climates it would have to be introduced in early spring. There would be some danger in introducing these plants into new areas, but it would be minimal for J. americana and T. latifolia.

Large quantities of nutrients are taken from the water, as shown in the following table:

Quantities of elements that could be removed by continual culture of some aquatic plants (kg/ha/year)

At the rates shown 1 ha of pond would be sufficient to extract the N contribution in domestic wastes of 500 people, and the P from 225. The yield in protein from J. americana is calculated as being able to supply human requirements for one year based on a generous estimate of 70 g being needed per person per day.

Nutrient removal by plants would be most effective when the nutrient is concentrated. This is typical of the effluents from feed lots of cattle. If plants were grown on the ponds holding these effluents they could be fed to the cattle. This would be ideal because handling and transport of the plants would be kept to a minimum. Nutrient removal and plant utilization could also be handled by the same organization.

The author points out difficulties from mosquitoes and other insects breeding in the ponds, and the care needed to prevent disease transmission from the effluent to the plants.

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

The author reviews papers on the potential of aquatic plants, especially water hyacinth, for removal of nutrients from polluted water. He cites an example involving purification of water for fish culture. Farming of channel catfish (Ictalurus punctatus) involves addition to the water of large quantities of fish feed. Waste feed and the excretions of the fish lead to the development of dense blooms of phytoplankton. When the plankton die their decaying residues can result in oxygen depletion and fish kill. Water hyacinth, confined in barriers so that it only covers 10% of the pond, and regularly harvested, can remove sufficient nutrients to avoid excessive phytoplankton growth.

In shallow, polluted lakes the dominant vegetation is likely to be submerged plants. If these are regularly removed the levels of nutrients in the lake can be reduced to an acceptable level.

Boyd, C.E., 1976 Accumulation of dry matter nitrogen and phosphorus by cultivated water hyacinths. Econ.Bot., 30(1):51–6

Reviewing the literature Boyd cites three estimates of the amount of nitrogen and phosphorus that water hyacinth could be expected to extract from nutrient-rich waters under good growing conditions: (t/ha/year)

The paper discusses experiments to determine the N and P uptake by water hyacinth from earthen ponds enriched with N and P by application of artificial fertilizer. The plants were contained within enclosures anchored to the middle of the ponds. As the plants grew to fill the enclosures half of the plants were removed periodically for weighing and analysis. The pond water was also analysed and the phytoplankton content determined. Fertilizer was broadcast over the ponds at two-weekly intervals to give N and P concentrations (N, 0.05 to 0.18 ppm; P, 0.01–0.08 ppm) equivalent to concentrations reported from eutrophic lakes. The chlorophyll content ranged from 19–65 μg/1 indicating dense phytoplankton blooms (May-September).

Water hyacinth growth rates averaged (dry weight) 190 kg/ha/day. The author compares this with growth rates cited in the literature:

540 kg/ha/day - Yount and Crossman (1970). This was a maximum, most yields being between 150–200 kg/ha/day. A lower rate was: 146 kg/ha/day (Penfound and Earle (1948)).

Average rates of N and P removal over the entire period were 3.4 and 0.43 kg/ha/day, respectively. This is equivalent to an extraction, over 12 months of growth, of N, 1.20 t/ha/year; P, 0.16 t/ha/year. The hyacinths generally absorbed N 5–10 times as rapidly as P. The content of N in the plants decreased from June-September, while P content was similar at the end of June and September but lower at other dates.

The overall results for growth based on a 12-month period were between 39–142 t/ha dry matter which included: N, 0.8–1.8 t/ha; P, 0.1–0.3 t/ha.

The author concludes that these results, though lower than others cited in the literature above, are similar to those reported in his earlier paper (Boyd 1970c). The data confirm the capability of water hyacinth to remove large amounts of N and P from water.

Brink, D.N., 1969 Aquatic weeds. Proc.Swed.Weed Conf., 10:1–2

A river, into which sewage was being discharged, had a population of Sparganium ramosum, S. simplex andButomus umbellatus. The amount of N and P discharged into the river was measured, as was the amount of these nutrients taken up by the plants to a distance of 8.4 km downstream from the sewage entry. It was found that about 4% of the N and 13% of P were assimilated by the vegetation. Most of this was later released to the water again when the plants died. Most of the sewage nutrients were deposited in the stream bottom by physical, chemical and microbial action. The author advocates mowing the vegetation as the best means of controlling it, and recommends that this be done in the first half of August, because at this time the plants contain maximum amounts of N and P.

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

The author describes methods of harvesting aquatic weeds (see Chapter IV) and emphasizes the usefulness of aquatic plants in purifying water. He comments that water plants are one of the most efficient ways of extracting nutrients from water. By releaing oxygen they also contribute to water clarity. If large aquatic plants are eradicated their place is taken by algal blooms. This can be observed after weeds are killed by herbicides, when the decayed plants release the nutrients they contain back into the water. This may convert a clear lake into water with dense algal growth.

Denike, T.J. and R.W. Geiger, 1974 The utilization of Chara in water management. Hyacinth Control J., 12:18–20

The authors consider that the alga Chara spp (see Chapter VIII) is an excellent nutrient absorber. Management ofeutrophic lakes has been arranged to suppress undesirable aquatic weeds while encouraging the growth of Chara. Under this system water clarity has been improved, in one case from 1 m to about 3 m, and in another from 0.3 mto 2 m.

Dunigan, E.P., R.A. Phelan and Z.H. Shamsuddin, 1975 Use of water hyacinths to remove nitrogen and phosphorus from eutrophic waters. Hyacinth Control J., 13:59–61

Glass-house and field trials were arranged to measure the uptake of nitrogen and phosphorus by water hyacinths from water having varying concentrations of N and P. The N was added as ammonium chloride and potassium nitrate. The N and P concentrations at the beginning of the trials were arranged to be 50, 100 and 250 ppm. After 21 days all the ammonium N at the lower and medium levels, and the nitrate N at the lowest level, had been taken up. About half the P at each concentration had also been absorbed. In the field trials the hyacinths increased the rate of lossof ammonium N, but were ineffective in removing nitrate N. The removal of P was low.

Dunigan, E.P., R.A. Phelan and Z.H. Shamsuddin, 1975a Can water hyacinths “eat pollution”? Compost Sci., 16(2):11

In green-house trials water hyacinth completely absorbed from water 50–100 ppm ammonium N in 15–21 days, 50 ppm nitrate N in 23 days, and 13–35 ppm phosphate P in 35 days. The N:P uptake ratio was about 5:1. Evapo-transpiration from the hyacinths exceeded transpiration from eutrophic waters by 320% and hastened recycling of chemically pure water back into the natural hydrological cycle. (From Weed Abstracts)

Fekete, A., D. Riemer and H.L. Motto, 1972 Removal of Rhizoclonium from a pond and its relationship to dissolved nutrients. Proc.Northeast.Weed Sci.Soc., 26:193–6

A small pond (.03 ha) was infested with the filamentous alga Rhizoclonium sp. Over two months in the summer the weed was raked out, weighed, and analysed for N and P (see Chapter III). At the same time the water in the pond was similarly analysed. The amount of N removed in the tissues of the alga was equivalent to a concentration of 23 ppm in the total pond water. Yet the maximum content determined at any time in the pond water was 4.1 ppm. Similarly, the amount of P removed by the alga was equivalent to a concentration of 1 ppm of P in the pond. Yet water analysis never revealed a P concentration of more than 0.04 ppm. Thus the N levels in the pond were apparently replenished from bottom sediments, or ammonia in rain water, or N fixation, or all three. The P levels in the water were presumed to have been restored from reserves in the bottom sediments. The authors conclude:

1. Large amounts of N and P can be removed from a pond by harvesting filamentous algae.

2. The effect on dissolved N and P levels in the water may be slight in relation to the amount removed, due to a continuous resupply.

3. Continual removal of the algae may alter the condition of the pond so that only a limited amount of algal regrowth will occur.

Fekete, A., D. Riemer and H.L. Motto, 1976 A bioassay using common duckweed to evaluate the release of available phosphorus from pond sediments. J.Aquat.Plant Manage., 14:19–25

Under laboratory conditions Lemna minor was used to measure the release of P from pond sediments. It was found that the growth characters of L. minor (frond numbers, frond diameter, root length and dry weight) consistently reflected P concentrations in solution. Different sediments with high, medium and low P content were incubated under aerobic and anaerobic conditions. The greatest amount of P extracted by the test plant was from the sediments with high P content and incubated under aerobic conditions. The results obtained, using the bioassay with L. minor, proved superior to purely chemical analyses carried out at the same time.

Fitzgerald, G.P., 1969 Field and laboratory evaluations of bioassays for nitrogen and phosphorus with algae and aquatic weeds. Limnol.Oceanogr., 14(2):206–12

Techniques are described for detecting conditions of surplus, or limiting concentrations of, nitrogen and phorphorus in algae and aquatic plants. The report also discusses a simple method for measuring rates of nitrogen fixation by blue-green algae. The test plants used were Ceratophyllum sp., Cladophora sp., Hydrodictyon sp., Aphanizomenon sp., Microcystis sp., Anabaena sp., and Nostoc sp.

The author writes that aquatic plants whichhave been recently exposed to a relatively high concentration of available P could absorb enough to indicate surplus P conditions by their total P content. This difference could be used to advantage in ecological studies of the effects of high dosage of nutrients or accidental spills in river systems since the relative rate of growth of plants would be a function of the time required for their enzymatic activity to decrease to its lowest level.

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

A detailed description is given of trials, both in the laboratory and in the field, to overcome the problems of harvesting algae from sewage effluent. The principal species considered were Scenedesmus and Chlorella. Flocculation, flotation, straining, filtration, centrifugation, and gravity separation were among the methods tested. Drying the slurry was also considered, especially by means which would not impair the quality of the resultant product as an animal food. The following is a summary of findings from the trials.

Harvesting involves three steps: initial concentration which brings algal solids to 1–2%of the slurry, dewatering (secondary concentration), and final drying.

Chemical precipitation and centrifugation were the best for the initial process, though the latter is expensive in equipment and power. The best flocculating agents were alum, lime and synthetic organic polyvalent cationic polymers. Alum is needed at arate of from 40–120 mg/litre, the higherthe dose the cleaner the effluent. Freshly hydratedlimerequires 100–200 mg/litre. Complete removalof algae was achievedusing a polymer (Sondellite) at 2.4 mg/litre.

Dewatering the slurry was done successfully with a modified industrial gravity filter with two hatch type centrifuges and a continuous solid bowl centrifuge. Dewatering and drying can bedone at the same time on specially prepared sand beds. On a typical summer day 1 470 m²of bed per hectareof pond is required. Otherwise drying as a thin film on a heated drum is the best method. This process improves the digestibility of the algal product and at the same time sterilizes it.

The authors conclude that the question as to whether harvesting and processing of algae grown in sewage is worth while depends on whether there is a profitable market for the product and/or for the reclaimed water. If thereis, then communities forced by rigid water pollution control requirements to undertake tertiary treatment should consider algae production as an early and ultimate step intheir waste disposal systems.

Haller, W.T., E.B. Knipling and S.H. West, 1971 Phosphorus absorption by, and distribution in, water hyacinths. Proc.Soil Crop Sci.Soc.Fla., 30(1970):64–8

Water hyacinth was grown in the laboratory in culture solutions containing phosphorus at varying concentrations. The P concentration critical for maximum growth was 0.1 ppm. Below this level growth was limited, above it thehyacinths took up P inluxury amounts without any increase in yield. (From Weed Abstracts)

Haller, W.T. and D.L. Sutton, 1973 Effect of pH and high phosphorus concentrations on growth of water hyacinth. Hyacinth Control J., 11:59–61

The growth of water hyacinth in nutrient solutions of different pH and P content was measured. It was found that pH between thewide limits of 4.0 and 8.0 did not appear to have any obvious influence on growth. The P contenthada great effect on growth rate, morphology of the plant, and the distribution of P within the plant tissues. When grown in solution without P the plant roots developed until their total proportion to plant size was twice that of hyacinths growinginsolutions containing P. The roots also became iridescent blue in contrast to the normal grey-black colour. Growth increased with P content of the solution up to a P concentration of 20 ppm. At 40 ppm growth declinedand the effect of increasing P was increased toxicity to the plants. the P content of the plants with rising P concentration is shown, together with P distribution in the plant, in the following table:

As the P concentration in solution becomes greater it can be seen that the distribution of P within the plant becomes more uniform. At low concentrations more P accumulates in the leaves compared to the stem and roots. It was noted that the P uptake increased in the hyacinth even after some toxic effect of the high concentration in the nutrient solution (40 ppm P) had become evident in reduced growth. This “luxury consumption” appeared to be approaching its limit for plants growing in 40 ppm P.

The authors comment: “High P absorption together with the high productivity of water hyacinth indicates that these plants could be beneficial in removing large amounts of nutrients from eutrophic waters by mechanical harvesting. The cost of this is prohibitive but it would be worth while to know the value of removing nutrients from a body of water and then including this value in determining the economic feasibility of the operation.”

Kohl, W., 1974 The bacterial colonization of aquatic plants. Proc.Eur.Weed Res.Counc.Int. Symp.Aquat.Weeds, 4:31–6

The author reviews the extent and species distribution of bacteria on aquatic vegetation and their importance in water purification in Austria, and also in the breakdown of organic matter from dead vegetation and as indicators in the assessment of water quality. (From Weed Abstracts)

Lawrence, J.M. and W.W. Mixon, 1970 Comparative nutrient content of aquatic plants from different habitats. Proc.Annu.Meet.South.Weed Sci.Soc., 23:306–10

Three aquatic plants were grown in water with different degrees of pollution, from raw sewage to unpolluted. The uptake of nutrients was analysed (see Chapter III). “Luxury consumption” of nutrients was observed from water containing large amounts of N, P and K. The authors conclude: “As the availability of the nutrient changes so does the concentration of nutrient in the plant. This occurs to a far greater extent thanthe plant can physiologically utilize. Yet such plants retain a similar healthy external appearance to those growing with normal levels of nutrient content.”

Mackenthun,K.M., 1962 A review of algae, lakeweeds and nutrients. J.Water Pollut.Control Fed., 34:1077–85

This comparatively early paper discusses the relationship of nutrients in water to algae and aquatic plants. Commenting on the amounts of nutrients released into water the author states: “The annual contribution of nitrogen and phosphorus from domestic sewage per head of population is about 1.9 kg of Nand 0.5 kg of P. This is sufficient N to fertilize 0.4 ha of lake water to a depth of 1.6 min respect of N, and 2.8 ha of lake to the same depth in respect of P to such an extent that nuisance algal blooms might occur during summer months.” He points out thathigh nutrient concentrations in water can be substantially reduced by the passage of water through a lake, nutrient removal being influenced by temperature, biological activity andflow. The slower the flow and the longer the retention time, the greater the removal. No direct reference to the part played by aquatic plants in the removal is made except: “The temporary control of excessive aquatic growths is often a necessity for the full recreational utilization of the water. Certainly the most adequate form of control would be complete removal from the lake basin, thus removing thosenutrients which are combined with biological growth. Unfortunately this is now impractical in most cases and, in other instances, such mechanical control may be laborious, time consuming and expensive. Thus attention has been focussed on the useof chemical methods for temporary control of aquatic nuisances.”

That the author considers this attitude unsatisfactory is clear from his final paragraph: “The growth and development of established aquatic plants will depend on the available nutrients in the water, suitable climatic conditions and competition with other species. The void created by the destruction of one type of biological growth may be filled by another and different type of biological activity.”

Mackenthun, K.M., 1971 Nutrients and their relationship to weed and algal growths. Hyacinth Control J., 9(1):58–61

The author reviews the relationship and importance of aquatic plants to their habitat. When the growth of these plants is stimulated in some manner so that they interfere with water use then a public outcry develops. The various nuisances which excessive water plants can cause include unpleasant smells and taste, toxicity to man and animals, and other negative effects on the environment, as well as physical impediments to water use.

Reviewing the literature on eutrophication (the enrichment of water by various nutrients) he points out the importance of phosphorus, which can vary greatly in concentration in water depending on the degree of contamination from agricultural or urban areas. On the question of algal populations in relatior to P concentration he writes: “Growth of algae in sewage has been reported in the laboratory at 1–2 g/1 (dry weight), and in sewage ponds at 0.5 g/litre. Thus, assuming optimal growth conditions, and maximum phosphate utilization, the maximum algal crop that could be grown from 1 kg of P would be 1 tonne of wet algae under laboratory conditions or 250 kg in the field. If a cellular content of P in algae is 0.7% then 1 kg of P could be distributed among 1 450 kg of algae (wet weight). This suggests that to prevent biological nuisances total P in flowing water should not exceed 100 μg/1, and in standing water not more than 50 μg/litre.”

Nichols, D.S. and D.R. Keeney, 1976 Nitrogen nutrition of Myriophyllum spicatum: variation of plant tissue nitrogen concentration with season and site in Lake Wingra. Freshwat.Biol., 6(2):137–44

The nitrogen content in Myriophyllum spicatum from different sites in a hypereutrophic lake in Wisconsin, U.S.A. was determined. From all sites the N concentration in the water was similar, but it differedin the sediments. And the N content of the plant varied in relationship to the N in the sediments, indicating a dependence on the sediment for this nutrient (which is understandable because it is arooted plant). The concentration of N in the plants was highest in spring, decreased steadily during the summer, and increased again in the autumn. The plant grows very little during the winter but continues to accumulate N. The death and decay of a considerable portion of the plant in autumn contributes only insignificant amounts of N to the lake water. (From Weed Abstracts)

Ornes, W.H. and D.L. Sutton, 1975 Removal of phosphorus from static sewage effluent by water hyacinth. Hyacinth Control J., 13:56–8

Water hyacinth was grown outdoors in concrete tanks containing sewage effluent. Over a period of five weeks the uptake of P was measured as 5.5 mg/g of the dry weight of the plant. The P concentration in the effluent was 1.4 mg/litre at the start of the experiment and was reduced to 0.2 mg/litre at the end. Of this decrease 70% took place in the first two weeks and 80% by the end of three weeks. The hyacinth increase in (dry) weight was at a maximum during the first week and totalled 97 g/m² of water surface, which represented a 45% increase in the dry weight of the plants at the start of the experiment. The authors conclude that this study indicated that water hyacinths could be used to reduce P in sewage effluent to low levels. However the length of time involved may not make this a practical method because of the space required to hold the sewage effluent under static conditions. For example, to hold the sewage effluent from one day's production of a treatment plant processing 3.8 million litres/day would require a surface area of 8 190 m² with a depth of 0.5 m. A more practical approach would be to grow the plants in a sewage lagoon and then harvest when growth and nutrient content of the water hyacinth is maximum. This study suggests that weekly harvests of water hyacinths of 5 kg dry weight/m². Based on this amount of dry weight these plants would contain 0.9 kg of crude protein and 22 g of P. The plants could then be used for mulch, soil amendment or other purposes.

Rao, K.V., A.K. Khandekar an D. Vaidyanadham, 1973 Uptake of fluoride by water hyacinth, Eichhornia crassipes. Ind.J.Exp.Biol., 11(1):68–9

The authors were interested in the possibility of using water hyacinth for extracting fluorine from water. They analysed plants growing in a reservoir which had a fluorine content of 1.0 ppm. The plants were found to have 23 ppm F in the leaf and 60 ppm F in the petiole.

Hyacinths were then grown in nutrient solution to which a range of concentrations of sodium fluoride had been added. Over four weeks the uptake of fluoride by hyacinth plants was measured. Over a range of concentrations of 6 ppm to 26 ppm fluoride, the plants (each about 150 g) took up from 11 to 75 mg of fluoride. However the authors conclude that the efficiency of uptake was too low for hyacinth to be considered of practical use in extracting fluorine from the typically low concentration present in natural waters.

Reay, P.F., 1972 The accumulation of arsenic from arsenic-rich natural waters by aquatic weeds. J.Appl.Ecol., 9(2):557–65

Several aquatic plants have been found to accumulate arsenic from the Waikato river in New Zealand to levels well above those associated with toxicity of this element. The species examined differed in the concentration to which they accumulated arsenic, and the average values ranged from 30–650 mg/kg dry weight. Accumulation was influenced by the amount of arsenic in the water but not by the amount inthe river and lake bottoms (for details see Chapter III).

Rogers, H.H. and D.E. Davis, 1971 Nutrient absorption from sewage effluent by aquatic weeds. Proc.Annu.Meet.South.Weed Sci.Soc., 24:352 (Abstr.)

Eichhornia crassipes (water hyacinth), Alternanthera philoxeroides and Myriophyllum spicatum were grown for two months in plastic pools containing water with or without added sewage effluent. Similar control pools contained no plants. Nitrogen and phosporus concentrations were measured at intervals. The sewage-enriched water initially contained five times as much N and three times as much P as unenriched water. Six days after planting the N content in sewage-enriched water decreased to less than 10% of the original value. During the same period the N content in unplanted pools with sewage-enriched water decreased to about 20%. P concentrations did not decrease as rapidly as the N concentration, and the effect of plants on depletion was not as great. The experiment was impaired by an unknown disease which killed many of the water hyacinths. A. philoxeroides tolerated the low fertility of unenriched water better than the other two species. (From authors' abstract)

Rogers, H.H. and D.E. Davis, 1972 Nutrient removal by water hyacinth. Weed Sci., 20(5):423–8

In laboratory experiments the uptake of N and P by water hyacinth was measured in static and in flowing water. The nutrients were either added as sewage effluent or as a standard solution (Hoagland's) which at 10% strength contains 3 mg/1 P and 22 mg/1 N, equivalent to the sewage effluent which had 3.6 mg/1 P and 22 mg/1 N. Measurements of N and P uptake by the hyacinth per day at various concentrations of Hoagland's solution and sewage effluent, in flowing and in static water, are tabulated below.

Uptake by individual hyacinth plants (mg/day)

On the basis of an estimate by Lee (1970) that the annual domestic waste per person is 0.9 kg P and 3.2 kg N, and another estimate (Penfound and Earle, 1948) that 1 ha of water hyacinth contains about 1.62 x 10⁶ plants it is assumed that 1 ha of water hyacinth could absorb the N and P wastes of 800 people. This assumes a maximum uptake and growth by the plants throughout the year. The authors suggest that consideration be given to growing hyacinths under a plastic roof both to extendits growing season and also to enable transpiredwater to be condensed for reuse, or to dilute the effluent. A further means of promoting hyacinth growth would be to make use of the waste hot water from power stations.

Transpiration by individual hyacinth plants, per day, averaged 175 ml from still water and 225 ml from flowing water.

Scarsbrook, E. and D.E. Davis, 1970 The effect of sewage effluent on growth of five aquatic species. Proc.Annu.Meet.Weed Sci.Soc., 23:305 (Abstr.)

Eichhornia crassipes, Alternanthera philoxeroides, Egeria densa, Najas flexilis and Potamogeton crispus were grown in plastic pools in well water, with or without the addition of 25% of sewage effluent. Of the five test plants, E. crassipes showed the maximum growth response to the sewage effluent, with A. philoxeroides second. In well water alone A. philoxeroides was the only plant to survive, indicating its ability to tolerate very low nutrient levels. (From Weed Abstracts)

Scarsbrook, E. and D.E. Davis, 1971 Effect of sewage effluent on growth of five vascular aquatic species. Hyacinth Control J., 9(1):26–30

The growth of five aquatic plants in well water to whichsewage effluent had been added was recorded together with their uptake of nitrogen, phosphorus and potassium. The plants were Eichhornia crassipes (water hyacinth), Alternanthera philoxeroides, Potamogeton crispus, Egeria densa and Najas flexilis. The tests were in plastic pools 66 cm deep and 2.7 m in diameter containing no soil. A mixture of all the plants was put in each pool. Half the pools had 25% sewage effluent, the others well water only. In the latter pools all the test plants died exceptA. philoxeroides, but its growth was stunted and unhealthy.

In the sewage-treated pools, by the endof 11 weeks water hyacinth had dominated all the other species and had covered 71% of the water surface. The next in vigour was A. philoxeroides, though its growth was much less than water hyacinth. The other testplants did not grow sufficiently in face of the competition to justify harvesting. The results are shown in the following table:

Growth over 23 weeks (April-October - g dry weight

^* Includes weight of plantsharvested at 11 weeks.

The water hyacinth had removed 6.9 g of N, 2.9 g of P and 8.7 g of K from the sewage pools. A. philoxeroides did not gain N and K, instead the plants released these elements to the water; but they took up some P (0.15 mg). The authors conclude that water hyacinth could be usefully employed to extract nutrients from sewage.

Sheffield, C.W., 1967 Water hyacinth for nutrient removal. Hyacinth Control J., 6:27–30

In a literature review the author cites findings that the P content was 0.22 mg/1 in the effluent from a biological treatment sewage system. Trickling filters cannot remove P but they can remove about 10% of the total N content. By the activated sludge process 10–90% of N can be removed, depending on the temperature and loading rates employed. One plant reported up to 95% P removal by a modified activated sludge system, but this result was considered exceptional.

Considering the alternative approach of using water hyacinth as a means of nutrient removal from effluent the author cites the claim of Van Vuran (1948) that water hyacinths could remove annually, under ideal conditions, 3 400 kg of N per hectare of water surface.

In the laboratory, treatmentof sewage effluent by an algal pond system was compared with water hyacinth having an air stripping and coagulant system, in terms of efficiency in removing nitrate N, ammonia N, and P. The results were tabulated:

The author points out that the water hyacinth system was far superior to the algal pond when air stripping and coagulation techniques were included. He states that without them the hyacinth initially removed 50% of the P or better. Then as more and more bottom settlements occurred within the pond the phosphates seemed to recycle back into the water resulting in only 10–15% P removal.

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

This paper reviews the effectiveness of different aquatic plants in extracting nutrients from polluted water (for details see Chapter III). Steward writes: “Nitrogen and phosphorous are present on average in plant tissues at a ratio of 10:1. Thisis the ratio at which these elements often occur in natural waters.” Hecites evidence that N and P are entering waters at a ratio of nearly 5:1. In domestic sewage the ratio is 3:1. Thus plants can only remove one third to one half of the P, since N supplies would be limiting. He calculates that water hyacinth,with a theoretical productivity of 165 t/ha, could effectively remove the yearly contribution of N from 1 430 people and thus P from only 450 people. He quotes unpublished reports of the sucessful control (in the U.S.A.) of eutrophication in a small lake by growing water hyacinth in a fenced area in the centre of the lake. Afterone year the lake was clear and supporting fish.

Another report (from Germany) states that Juncus lacustris has been used to purify industrial effluents on a massive scale. The effluentsflowed at a rate of 5 million m³/day through 20 basins each 400 m long x 50 mwide.

Sutton, D.L. and W.H. Ornes, 1975 Phosphorus removal from static sewage effluent using duck-weed. J.Environ.Qual., 4(3):367–70

The duckweeds Lemna gibba and L. minor were grown (200 g fresh weight/m²) in secondary treated sewage effluent diluted to 6, 12 and 25% concentration. These concentrations gave increases of growth at 16, 23 and 31% higher than plants grown in pond water. Increasing the sewage concentration did not further increase the growth rate. Over eight weeks of the experiment the P content in the sewage decreased by 97%. The P content in the plants continued to increase with higher concentrations of P in the sewage until the sewage concentration of P reached 2.1 mg/litre; then the P content of the plants began to decline.

Protein content of the plants grown in 50% and 100% effluent was almost three times that of plants grown in pond water. The protein content of the plants rose rapidly to about 40% during the first week and then declined to about 12% at the fourth week, and then remained at about the same level for the remaining four weeks of the experiment.

The authors conclude: “This study indicated that duckweed could be used to reduce P to low levels insewage effluent held under static conditions. The rate of P removal would depend on the amount of duckweed, concentration of P inthe effluent, and length of contact time.”

Telitchenko, M.M., G.V. Tsttsarin and Ye.L. Shirokova, 1970 Trace elements and algal “bloom”. Hydrobiol.J., 6(6):1–6

The authors point out that trace elements in water are responsible for massive increases in numbers of different species of algae - the so-called “bloom”. It was establishedthat Aphanizomenon flos-aquae and Microcystis aeruginosa (Cyanophyceae) concentrate 18 trace elements from water, oneof whichis copper. When copper stocks are exhausted the “bloom”of Cyanophyceae in the water ceases. Repeated blooms are noted only when the water is enriched with copper by the death of the previous generation of algae. A conclusion, therefore, is that algal bloom should not be controlled by treatment with copper sulphate, though some algae (e.g. Spirogyra) are extremely susceptible.

Timmer, C.E. and L.W. Weldon, 1967 Evapotranspiration and pollution of water by water hyacinth. Hyacinth Control J., 6:34–7

The growth of water hyacinth will pollute water by adding to it particles of decaying tissue. With evaporation tests the author measured 3.7 times greater evaporation of water under hyacinth than from clean water, thus confirming earlier similar findings (Penfound and Earle, 1948). These factors need to be taken into account when considering the use of hyacinths to remove pollutants from water.

Note: These findings were similar to those of Little (1967) who recorded, from glass-house tests in the United Kingdom, that the rate of evapotranspiration of water hyacinth with leaves 30–40 cm long was 4–5 times that from the surface of open water undersimulated tropical conditions. He commented: “It seemslikely that with the much larger and more vigorous plants, commonly found in the tropics, rates of evapo-transpiration may even exceed the up to six times normal reported by Penfound and Earle (1948)”. Little also found that Pistia stratiotes (leaves 10 cm long) had an increased evapotranspiration rate of 2–3 times that of open water. Salvinia auriculata gave only a slightly increased rate (but its speed of growth was well below that normal in the tropics).

U.S. National Aeronautics and Space Administration, 1976 Aquatic blooms as problem solvers. NASA Activ., 7(8):22–4

This NASA news publication reports onexperiments with waterhyacinth to purify sewage. An extract is given below.

Water hyacinths were enclosed on a 16-ha lagoon which received sewage from 6 000 households. The plants soon grew at a rate of 20–40 t/ha/day fresh weight. It was found that an area of about 5 ha of water hyacinth was more thanenough to purify the lagoon.

The cost of a standard chemical filtration plant to remove waste products from a sewage lagoon of a small community, to meet current pollution standards, would be about US$500 000. Experiments using water hyacinths have shown how adequate water purification standards could be achieved at a cost of only the few thousand dollars needed for stocking and repeatedly harvesting the plants.

Effluent containing industrial and chemical waste flowing at a rate of about60 000 litres/day has been purified by water hyacinths growing in a zig-zag canal type lagoon, 12 m wide x 0.8 m deep and 250 m long, to a level exceeding effluent standards.

A system is being proposed by which an assessment of heavymetals present in various water bodies may bemade by analysis of the plantsgrowingin the water rather than thewater itself.

It has been found that duckweed can be used as a supplemental filtration system to water hyacinth. When surface quantities of hyacinth diminished in cold weather the amounts of duckweed increased and continued to absorb nutrientsfrom the water, though at a reduced rate. When temperatures rose and hyacinths increased again the duckweed reduced in number.

The results of the various aspects of this research programme at NASA have stimulated a number of similar experiments in other regions in the U.S.A. andin other countries.

U.S. National Space Institute, 1976 Will hyacinth become first moon flower? Natl. Space Inst. Newsl., U.S., 1(9):6

This Newsletter refers to a proposal that water hyacinths should beincludedin space experiments to purify wastes in space ships. Growing in a tank containing about 10 m³ of water, the plants should purify the wastes of 10 people.

Wahlquist, H., 1972 Production of water hyacinths and resulting water quality in earthen ponds. Hyacinth Control J., 10:9–11

The response of water hyacinth to added fertilizer in ponds was measured. One observation was that ponds treated with fertilizer, but without hyacinth, had a rich, dark green bloom of plankton. Similarly treated ponds with hyacinth had clear water,or only a light plankton population. It was concluded that hyacinths were more successful at extracting nutrients than the algae. The author cites Penfound and Earle (1948) as having measured a yield of water hyacinth of 414 t/ha. In the trials reported here the yield without fertilizer would have been 175 t/ha; with P only added, 550 t/ha; with N and P both added, 590 t/ha. Wahlquist suggests that these results indicate the potential value of hyacinth as a means of alleviating the pollution of eutrophic waters.

Widyanto, L.S., 1975 The effect of industrial pollutants on the growth of water hyacinth (Eichhornia crassipes). Tropical Pest Biology Program, BIOTROP, Bogor. Proc.Indonesian Weed Sci.Conf., 3:328–39

A survey of the waste water of four textile factories revealed that in two factories the total dissolved solids was higher (1 185 and 1 204 mg/l) than the acceptable concentration of total dissolved solids (1 000 mg/l) in water for domestic and irrigation purposes. The sulphate content of the water in one case was so high (719 mg/l) that water hyacinth died after five days when grown in it, or even when the impure water was diluted by 50%.

Dilution of the waste from another factory (concentration not stated) had the effect of stimulating hyacinth growth. This suggests that, with appropriate dilution of factory waste waters, hyacinth could be used to purify such waters for reuse or for crop irrigation.

Wolverton, B.C., 1975 Water hyacinths for removal of cadmium and nickel from polluted waters. NASA Tech.Memo., (TM-X-72721):11 p. Issued also in Sci.Tech.Aerospace Rep., 13(7):795–6

The author points out the hazards to health from the release of heavy metals like cadmium and nickel to waterways in industrial effluents. He notes the efficiency of water hyacinth in extracting a range of elements from water, and describes laboratory experiments to measure its extraction capacity for cadmium and nickel. It was found that, within 24 hours, hyacinths were able to absorb and concentrate in their roots: cadmium up to 0.67 mg/g dry matter, and nickel up to 0.5 mg/g dry matter from water polluted with 0.6–2.0 ppm of these metals.

The roots represent about 18% of the total dry weight of the hyacinths, and in them was found 97% of the heavy metals absorbed by the whole plant. No toxic effects from the heavy metal concentration in the water was observed in the plants. It was concluded that, as 1 ha of hyacinth has an approximate growth rate of 600 kg/dry matter/ day, the plants could potentially remove about 300 g of heavy metals from 1 ha of polluted water per day.

Wolverton, B.C., 1975a Water hyacinth for removal of phenols from polluted waters. NASA Tech. Memo., (TM-X-72722): 18 p. Issued also in Sci.Tech.Aerospace Rep, 13(7):795

Phenol and phenolic derivatives are common organic pollutants in domestic and industrial wastes, and in drinking water. In addition, chlorophenols, which have an extremely objectionable odour and taste, are produced by chlorination of drinking water when it is contaminated with phenolic compounds. Laboratory trials are described which measured the ability of water hyacinth to extract phenol from water to which known amounts of phenol (25, 50 and 100 ppm) had been added. Tests were carried out in both distilled and river water. It was found that the hyacinths were able to extract phenol from water at a rate of 36 mg/g dry matter in 72 hours. On a basis of 1 ha of water surface carrying 1.62 × 10⁶ plants, with an average dry weight each of 2.75 g, it was calculated that 1 ha water hyacinth would extract 160 kg of phenol in 72 hours.

No phenol could be detected in any part of the hyacinths which achieved this extraction. Also no evapotranspiration of phenol could be measured. Therefore it was concluded that the phenol, as it is removed from the water by the hyacinths, is rapidly metabolized to other components. None of the concentrations of phenol used appeared to adversely affect the hyacinths.

Wolverton, B.C. and D.D. Harrison, 1975 Aquatic plants for removal of mevinphos from the aquatic environment. J.Miss.Acad.Sci, 19:84–8

In laboratory trials Nymphaea odorata, Paspalum distichum and Juncus repens were used to evaluate the effectiveness of aquatic plants in removing the insecticide mevinphos from waters contaminated with this chemical (at 43–58 ppm). The emergent plants N. odorata and P. distichum removed 93 and 87 ppm of mevinphos, respectively, from the water in less than two weeks without apparent damage to the plants. J. repens, a sunmerged plant, removed less insecticide than the controls which contained contaminated water and soil only. The controls still contained toxic levels of the insecticide after 35 days, as detected by fish bioassay studies.

The authors speculate that the filtering capability of aquatic plants depends on absorption, translocation, concentration and detoxification by metabolic breakdown. Emergent aquatic plants may be more effective in removing mevinphos than submerged species because of the quantity of water they transpire.

Wolverton, B.C. and R.C. McDonald, 1975 Water hyacinths and alligator weeds for removal of lead and mercury from polluted waters. NASA Tech.Memo., (TM-X-72723)

Following successful trials on the uptake of nickel and cadmium by water hyacinths (Wolverton, 1975) further glass-house tests with this plant to measure uptake of lead and mercury were carried out. The work was expanded to include also Alternanthera philoxeroides (alligator weed) which is known to tolerate higher levels of salinity than hyacinths.

It was found that, during a 24-hour period, the following maximum amounts of metals were extracted by the plants from water containing initially 1.09 ppm lead and 0.875 ppm mercury:

It was concluded that, at these rates and under a continuous harvesting system, it is possible for water hyacinths to remove 105.6 g of lead and 90.0 g of mercury per day from 1 ha of polluted water.

Wolverton, B.C. and R.C. McDonald, 1975a Water hyacinths and alligator weeds for removal of silver, cobalt, and strontium from polluted waters. NASA Tech.Memo., (TM-X-72727)

Eichhornia crassipes (water hyacinth) and Alternanthera philoxeroides (alligator weed) were grown in the laboratory in distilled water and in river water. To the water was added (in separate containers) known amounts of silver nitrate, cobalt chloride and strontium nitrate in order to measure the ability of the test plants to take up these metals, which are often pollutants in waste water. The concentrations of these elements in the test solutions were: Ag 0.6–2.4 ppm; Co 0.5–2.0 ppm; Sr 0.4–2.0 ppm

It was found that water hyacinths were able to remove about 0.44 mg Ag, 0.57 mg Co and 0.54 mg Sr per g of plant dry matter in 24 hours.

Extrapolating to field conditions the authors conclude that the test plants could potentially extract 263 g Ag, 341 g Co, and 326 g Sr per hectare per day “provided that the metal-saturated plants are harvested at regular intervals.”

Alligator weeds removed a maximum of 0.44 mg Ag, 0.13 mg Co, and 0.16 mg Sr per gram of plant dry matter per day.

Wolverton, B.C. and R.C. McDonald, 1975b Water hyacinths for upgrading sewage lagoons to meet advanced wastewater treatment standards. Part 1. NASA Tech.Memo., (TM-X-72729)

A field trial to measure the ability of water hyacinth to purify the water of a secondary sewage lagoon is reported.

In their introduction the authors state that water hyacinth is an easily harvested plant which possesses adequate levels of mineral and protein for it to be grown as a crop in warm climates for use in many ways. In temperate conditions it could be used in conjunction with hydroelectric generating stations. Thermal discharges from the condenser cooling water could be mixed with sewage pumped into large lagoons nearby and water hyacinths grown there throughout the year in the warm, nutrient-enriched water. The method could also utilize the waste hot water from nuclear power plants. In this role hyacinths could also provide a safeguard to absorb radioactive isotopes should they be accidentally released into the waste water.

The trial ran for three months in the summer. It was found that effluent entering the lagoon had a N content of about 3 ppm. After passing through the hyacinths the water leaving the lagoon had about 1.2 ppm N, a recuction of about 60%. The P inflow concentration was 5.5 ppm, which reduced to 4.1 ppm for the first five weeks (26% removal). The authors state that phosphorus reduction rates after this period of time suggest that plants should be harvested at 5-weekly intervals for maximum P removal.

It was concluded that based on data presented in this preliminary report, cities located in the tropical and subtropical regions of the world should be able to utilize water hyacinths as a final filtration system for reducing the levels of polluting substances in domestic sewage to levels which comply with advanced waste water treatment standards. For nearly complete removal of P an area of about 4 ha would be needed per 5 000 people. But complete removal of P is not usually required, or even desirable. Therefore much smaller systems could be used to meet present and future waste water effluent standards.

Wolverton, B.C. and R.C. McDonald, 1976 Don't waste waterweeds. New Sci., 71(1013):318–20

This popular review article reports on the work being done at the National Space Technology Laboratories on the usefulness of water hyacinth for purifying water.

The authors state that when grown in warm enriched domestic sewage water hyacinths produce over 17.8 tonnes of wet biomass per hectare per day. Such plants contain 17.2% crude protein, 15–18% fibre and 16–20% ash, with the following analyses:

Growth rates suggest annual production rates of 212 tonnes of dried plant material per hectare.

Raw sewage from small Mississippi communities contains an average of 35 ppm of N and 10 ppm P. On this basis a 0.5-ha lagoon covered with water hyacinths, with a minimum sewage retention time of two weeks, should be able to purify to acceptable levels the daily waste of 1 000 people. An experimental lagoon did reduce pollutant levels by 75–80%.

Water hyacinths could also prove useful in treating effluents polluted with toxic heavy metals. In static laboratory experiments the plants rapidly absorbed gold, silver, cobalt, strontium, cadmium, nickel, lead and mercury. The roots concentrated 97% of the Cd and N within 24 hours, though having only 18% of the plants' total dry weight. Water hyacinths can also absorb, or metabolize, phenols and other trace organic compounds of the type commonly found in the drinking water supplies of many large cities.

Duckweeds are being tested as a means of sewage filtration during cold months when water hyacinth is temporarily inactive.

Wolverton, B.C., R.C. McDonald and J. Gordon, 1975 Water hyacinths and alligator weeds for final filtration of sewage. NASA Tech.Memo., (TM-X-72724)

Eichhornia crassipes (water hyacinth) and Alternanthera philoxeroides (alligator weed) were grown for two weeks in laboratory conditions, in sewage water taken from the inflow and outflow of a wastewater lagoon. Analyses of the plants and water were made for nitrogen, phosphorus and a series of metals before, during and after the growth period. The results are summarized in the following tables:

Reduction in original levels of plant nutrients from sewage (%) after 14 days

Heavy algal growth occurred in the control vessels but no visible algae were present in vessels containing plants.

Concentrations of heavy metals (ppm) in water initially, and in plant roots after 14 days

The authors conclude that the test plants demonstrated their potential as secondary and tertiary filtration systems capable of producing clean water from sewage lagoon effluent. They were also free of toxic levels of trace heavy metals after a 2-week growth period. Therefore the harvested material, relatively high in protein and mineral content, is an excellent candidate for feed and/or food products.

Wolverton, B.C., R.M. Barlow and R.C. McDonald, 1976 Application of vascular aquatic plants for pollution removal, energy and food production in a biological system. In Biological control of water pollution, edited by J. Tourbier and R.W. Pierson, Jr. Philadelphia, University of Pennsylvania Press, pp.141–9

This article is an illustrated review of the work carried out at the U.S.National Space Technology Laboratories on the use of Eichhornia crassipes (water hyacinth) and Alternanthera philoxeroides (alligator weed) in the purification of water (the relevant papers are cited in this chapter). The authors state that the vast potential of vascular aquatic plants for removing chemical pollutants from water systems is just beginning to be fully appreciated by environmental scientists.

A diagram is shown of a zig-zag secondary sewage treatment lagoon growing hyacinths for conversion to methane production. Another illustration suggests a layout of hyacinth culture based on the effluent from livestock feed-lots and poultry farming. Harvesters extract the hyacinths from the secondary treatment lagoon for conversion to animal foods, fertilizer and methane.

Wooten, J.W. and J.D. Dodd, 1976 Growth of water hyacinth in treated sewage effluent. Econ. Bot., 30:29–37

The literature on the uses and potentials of water hyacinth, especially as a means of removing nutrients from sewage effluent is reviewed. Discussing the alternative of using algae the authors contend that there is no feasible mechanism for harvesting algae (in contrast to the simple harvesting techniques for hyacinths). Thus algae can only be utilized by harvesting higher organisms in the food web with which they are associated, i.e. fish.

In their experiments Wooten and Dodd used five ponds, each 465 m² in area and 0.8 m deep through which sewage effluent flowed in sequence at a rate of 480 l/min. In mid-April 2 000 water hyacinth plants were placed in the ponds. By the end of July the plants had increased to 500 000 covering the ponds completely, the weight estimated at 645 t/ha (dry weight 29.7 t/ha). This represents a growth rate (dry weight) of 29 g/m²/day, and approximates to a production of 30 metric tons of organic matter per hectare in 105 days. Growth rates of water hyacinth seem to reach optimum levels when nutrients are maintained at high levels and the plants are periodically harvested to alleviate overcrowding. Thus in a climate permitting continuous growth throughout the year more than 100 t/ha of organic matter could be produced annually, starting with a small inoculum every 100 days and with continuous harvesting under ideal conditions. Up to 150 t/ha could possibly be easily achieved, which is much higher than can be obtained with crop plants.

The rate of disappearance of N and P from each of the five ponds, as shown by analyses of the outflow waters, together with the analysis of the effluent as it came in, is shown in the following table which covers a 2-week period when the ponds were fully covered by hyacinths.

Analyses (ppm)

The authors comment that these tables reflect the wide variation in nutrient content that can occur in effluents. But they have found that water hyacinth will grow vigorously in solutions containing 100 ppm N, contrary to other authors who have claimed that 25 ppm N is the concentration which gives maximum growth. Therefore the usual levels of N in sewage-enriched waters will not be high enough to inhibit growth.

Regarding P uptake, it is pointed out that the above results should be considered in the light that evaporation and transpiration water losses have occurred and have the effect of concentrating N and P in the remaining solution. Also P use by plants is less than for N and, as N is depleted, less P is absorbed. The table also shows that the hyacinths can deplete P from solutions with concentrations of up to 28.1 ppm, contrary to other workers who have found 7.5 ppm limiting. When the flow of effluent into the ponds was later stopped for two weeks the average drop in P content was found to be 40%.

The authors concluded that though their experiments suffered from various difficulties (e.g. a leak developed in pond 4), “the potential value of using water hyacinths as a type of tertiary treatment of sewage effluent is clear, and warrants a larger and more sophisticated experimental programme to define the nutrient budget of this plant. This would include experimental determination of the total growth needed to reduce phosphates to acceptable levels. It is conceivable that N or other nutrients might become limiting before the desired reduction is achieved and the addition of fertilizer might be required to maintain a balanced growth.”

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

The authors point out that the pollution of lakes can be reversed by the harvesting of fish, algae and other organisms because in this way nutrients would be removed from the lakes. Floating plants were chosen for their trials because they are the most convenient organisms to harvest, and because they can be anchored in any part of the lake. Floating plants have an advantage over submerged and planktonic plants because, being in the atmosphere, supplies of carbon dioxide are not limiting. They can also compete with algae by effectively shading them from light. The plants used were water hyacinth and Salvinia rotundifolia, and the trials were carried out in ponds fertilized with various amounts of waste water sludge containing at least 5.5% N and 3% P. To control biting mosquitoes, males and females of the fish Gambusia affinis were put into the ponds.

Growth rates of hyacinth of 21 g/m²/day were measured. The yield of various insects was also recorded. It was found that production of insects was greatest in ponds with added nutrients. Yount and Crossman suggest that when nutrients are added in small amounts algae may be more efficient than hyacinths in absorbing them and thus increase insect productivity. They suggest that by concentrating nutrients into streams, and using hyacinths to extract the nutrients there before the waters reach the lakes, the numbers of insects produced could be reduced - an important matter when biting insects are present in serious numbers.

The trials showed that the regular removal of hyacinth from test ponds effectively reduced the productivity of the water (and in consequence the growth of the test plants). The authors conclude: “We feel that large-scale harvesting from natural waters can be expected to reduce the productivity of these waters and probably reverse the trend towards hypertrophy, especially in polluted waters. Furthermore it is evident that the present method of controlling the ‘pest’ water hyacinth and other plants by chemical sprays is returning their contained nutrients to the lakes and exacerbating hypertrophy of these lakes.”

Zafar, A.R., 1976 Economic significance of certain species of Scirpus sp. In Aquatic weeds in South East Asia, edited by C.K. Varshney and J. Rzeska. Proceedings of a Regional Seminar on noxious aquatic vegetation in tropics and sub-tropics, New Delhi, 12–17 December, 1973. The Hague, Dr. W. Junk, B.V. pp.387–91

The characteristics of those aquatic plants which may be expected to synthesize ethanol or phenol under anaerobic conditions in underground stems are listed. Such characteristics would also, it is suggested, confer on the plant the ability to absorb phenol which is often an important contaminant of industrial effluents. Some species of Scirpus, especially S. lacustris, are considered to meet the necessary criteria. These are:

1. having rhizomes which grow buried in oxygen-deficient soil;

2. the plant should be emergent, tall, with long stems (this minimizes oxygen transport to the rhizomes because of the distance of travel);

3. leaves should be on the upper part of the stem only and sheathing it - again limiting oxygen transport;

4. the stem should be without large air chambers.

S. lacustris is reported also to exude phenol from roots and rhizomes which can inhibit the growth of bacteria, a characteristic which could make the plant useful to grow for water purification in water works.

Anon., 1971a Water hyacinths help in treatment of waste water. Compost Sci., 12(5):27

Work at Iowa State University, U.S.A., with water hyacinths growing in sewage is reported. Nitrogen removal by the plants was estimated at 560 kg/ha/year. It was found that water hyacinths do well in water containing N at a concentration of up to 100 ppm. Above this concentration the hyacinths are adversely affected. A heavily used sewage lagoon might have 200–300 ppm N. A holding pond for effluent drawn from the lagoon may be down to 100–200 ppm N. Thus if hyacinths were to be grown on the pond some adjustment of effluent dilution may be desirable, and then it may be possible to grow the plants efficiently on both types of ponds.

Advantages of water hyacinths for sewage purification are stated to include the increased evapotranspiration of water and shading of the water to reduce algal growth. A disadvantage, in colder regions, is that the population of plants may be killed off in winter by frosts thus requiring replanting of the ponds each year.

The article emphasizes the need to harvest and remove the plants from the lagoons, otherwise they will die, decay and return their contained nutrients to the water.