2.3 Clarification using plants and plant material

Native plants have traditionally been used to improve the quality of the water in a number of countries in Africa and Latin America. For example, the seeds of the Moringa oleifera are commonly used in Guatemala, and peach and bean seeds are used in Bolivia, as coagulant aids to clarify water. Dried beans (vicia fava) and peach seeds (percica vulgaris) also have been used in Bolivia and other countries for this purpose. An emergent aquatic plant used for water quality treatment in Bolivia and Peru is Schoenoplectus tatora, commonly known as totora in those countries. This plant, which is similar to the cattail, is used to remove phosphorus and nitrogen from effluents before they are discharged to natural drainage systems. The plant biomass is then used for a variety of handicraft purposes, including the weaving of baskets and the production of the well-known reed boats of Lake Titicaca.

In addition to providing the basis for clarification, aquatic plants are also used in aquaculture applications, the production of aquatic organisms (both floral and faunal, but generally including fish) under controlled conditions. Aquaculture has been practiced for centuries, primarily to grow food, fiber, and fertilizer.

The use of aquaculture as a means of treating wastewater involves both natural and artificial wetlands and the production of both algae and higher plants (submersed and emersed), invertebrates, and fish, to remove contaminants such as manganese, chromium, copper, zinc, and lead from the water. The water hyacinth (Eichhornia crassipes) appears to be one of the most promising aquatic plants for the treatment of wastewater and has received the most attention in this regard. Other plants are also being studied, among them duckweed, seaweed, and alligator weed.

An experimental technology that has been tested successfully on the Bogotá savannah in Colombia is a form of hydroponic cultivation of grasses using domestic wastewater. This procedure works through three mechanisms: physical, adsorption, and absorption. It not only removed more than 70% of the organic content and suspended solids but produced a large grass crop that could be used to pasture livestock. It might also be practicable for restoring eroded lands. Because of the space requirements, it is best suited to rural areas. Since it has been tried only under controlled conditions, its real cost and possible disadvantages need further assessment.

Technical Description

· Native Plant Seeds

The seeds of many plants native to the South American continent contain essential oils and have other properties that have been exploited by traditional cultures for centuries. Among these is the ability of certain seed extracts to flocculate particulates in water. To prepare the seeds for use as a coagulant aid, the following procedure is commonly used:

· Extract the seeds from the plant or fruit.

· Dry the seeds for up to three days.

· Grind die seeds to a fine powder.

· Prepare a mixture of water and ground seed material; the volume of water depends on the type of seed material used (in the case of Moringa oleifera, add 10 cm³ of water for each seed; for peach or bean seeds, add 1 l of water to each 0.3 to 0.5 g of ground seed material).

· Mix this solution for 5 to 10 minutes; the faster it is stirred, the less time is required.

· Finally, after the sediments settle, decant the treated water. Testing it for pH, color, and turbidity is recommended.

· If the test results are acceptable, the treated water can be used for consumption and other domestic purposes.

· Aquatic Plants

Several aquatic plants have been used in water purification and wastewater treatment. Among die most widely used are cattails, totora, water hyacinth, and duckweed.

Totora and cattails grow in shallow lakes, rivers, and impoundments. The plants are rooted in the soil or bottom sediments of the body of water at depths of about 1 m and grow to between 2 m and 3 m above the water surface. These plants can absorb nitrate, phosphate, heavy metals such as manganese, and other chemical compounds. They are generally used to provide secondary treatment of effluents, in small lagoons filled with cattails or totora. Several physical and chemical processes take place in these lagoons:

· Sedimentation of suspended solids.

· Biological decomposition of organic compounds.

· Nitrogen removal through absorption by the plants and fixation by the plants and attached organisms, and denitrification by aerobic bacteria associated with the plants that convert organic forms of nitrogen into inorganic forms, including N₂ and N₂O gases that escape into the atmosphere (at high pH, ammonium is converted into ammonia gas, which also escapes into the atmosphere).

· Phosphorus removal by absorption and fixation in the plant biomass and/or its adsorption onto suspended particulates which later settle to the bottom of the lagoon (the amount of phosphorus removal is a function of the plant density in the treatment area).

· Removal of manganese, copper, zinc, and lead.

· Reduction of pathogenic microorganisms due to the grazing by protozoans, adsorption onto clay particles, and exposure to environmental extremes such as pH variations within the lagoon.

Design criteria for a treatment system using cattails or totora include the flow rate of the water to be treated; the initial nitrogen and phosphorus concentrations; the initial concentrations of other water quality parameters, such as heavy metal concentrations and pH; the desired water quality of the effluent; and the potential uses of the treated water. In Peru, a small system capable of treating 5 l/s required 900 m² of totora lagoon, with a maximum water depth of 0.9 m. These techniques are especially useful in rural areas where advanced technology for water treatment is not available and where high turbidity and color are the primary water quality problems.

The water hyacinth, a native of South America, is found naturally in waterways, bayous, and other backwaters. It thrives in nitrogen-rich environments, and consequently does extremely well in raw and partially treated wastewaters. When it is used for effluent treatment, wastewater is passed through a water-hyacinth-covered basin, where the plants remove nutrients, suspended solids, heavy metals, and other contaminants. Batch treatment and flow-through systems, using single and multiple lagoons, are used. Because of its rapid growth rate and inherent resistance to insect predation and disease, water hyacinth plants must be harvested from these systems. While many uses of the plant material have been investigated, it is generally recommended as a source of methane when anaerobically digested. Its use as a fertilizer or soil conditioner (after composting), or as an animal feed, is often not recommended owing to its propensity to accumulate heavy metals. The plant also has a low organic content (it is primarily water) and, when composted, leaves behind little material with which to enrich the soil.

Design criteria for wastewater treatment using water hyacinth include the depth of the lagoons, which should be sufficient to maximize root growth and the absorption of nutrients and heavy metals; detention time; the flow rate and volume of effluent to be treated; and the desired water quality and potential uses of the treated water. Land requirements for pond construction are approximately 1 m²/m³/day of water to be treated. Phosphorus reductions obtained in such systems range between 10% and 75%, and nitrogen reductions between 40% and 75% of the influent concentration. Table 8 presents performance data from four different wastewater treatment systems using the water hyacinth.

Table 8 Performance of Four Different Wastewater Effluent Treatment Systems Using Water Hyacinth

Wastewater treatment using natural and constructed wetland systems remains largely in the developmental stage, although several full-scale experimental demonstration systems are in operation, including one in Puno, Peru. Wetland treatment systems generally use spray or flood irrigation to distribute the wastewater into the wetland area. Alternatively, the wastewater may be passed through a system of shallow ponds, lagoons, channels, basins, or other constructed areas where emersed aquatic vegetation has been planted and is actively growing.

Extent of Use

The use of plant materials is a traditional technology for clarifying potable water that is still in widespread use in rural areas of Latin America. The use of natural products has recently been rediscovered by water-supply technologists and is being further developed along more scientific lines.

Treatment of wastewaters using artificial wetlands is still experimental, but is receiving a moderate amount of use. It has been tested and is currently being used in Guatemala and to treat water from rivers near La Paz, Bolivia. Totora technology is also being used in Bolivia and in Puno, Peru, on the shores of Lake Titicaca, to treat small wastewater flows (of 5 to 6 l/s). However, higher flow rates (30 to 50 l/s) can be treated using larger aquatic plant pools. The totora treatment systems used in Bolivia involve transplanting natural plants into the treatment lagoons. Experimental results from Bolivia indicate that heavy metals are absorbed by totora rooted in a gravel bed. The use of aquatic plants appears to be effective only during the growing season, and is subject to temperature constraints. This technology should be very useful in developing countries with hot climates and low land costs.

Treatment systems using water-hyacinth-based technology are also still in the developmental stage, with a number of full-scale demonstration systems in operation. Some small water-hyacinth systems are in use in Mexico. This technology is useful for polishing treated effluents. It has potential as a low-cost, low-energy-consuming alternative, or addition, to conventional treatment systems, especially for small flows. It has been successfully used in combination with chemical treatment and overland flow land treatment systems. Wetland systems may also be suitable for seasonal use in treating wastewaters from recreational facilities, some agricultural operations, and/or other waste-producing activities where the necessary land is available. It also has potential application as a method for the pretreatment of surface waters for domestic supply and stormwater management.

Operation and Maintenance

Operation and maintenance of plant-based water clarifiers are very simple. For plant-seed solutions a household mixer or blender is the only equipment needed. The totora treatment systems are also simple, requiring no machinery or specialized labor. Maintenance involves periodic removal of non-biodegradable materials, and the harvesting and disposal of plant material. Disposal may either be in the form of composting, methane gas generation, or use for fiber-based handicrafts. Dredging of sediments may be required every 3 to 5 years.

Gravity flows are generally used in wastewater treatment systems using the water hyacinth. Energy to operate the water-hyacinth-based systems is provided by sunlight. However, the plants must be harvested regularly. Fifteen to 20 percent of the plants should be removed at each harvest. While the water hyacinth system can successfully cope with a variety of stresses, the health of the plants must be maintained for most effective treatment. Several precautionary steps have been identified. Studies have shown that the presence of high chlorine residuals inhibits plant growth. Therefore, chlorination of the effluent is best done after water hyacinth treatment. However, if local conditions dictate that pre-treatment chlorination is necessary, care should be taken to maintain chlorine residuals in the influent at less than 1 mg/l. The system should also be monitored for the presence of weevils and other insects that damage the plants. Diseased or damaged plants should also be removed.

In wetland treatment systems, a knowledge of the mosquito life cycle and habitat needs helps managers avoid mosquito breeding problems. Open water areas, which are subject to wind action and provide easy access to predators (such as fishes), will limit mosquito production. Maintaining good water circulation in vegetated areas also gives access to predators and lessens mosquito production. The vegetation resulting from wetland systems can be utilized as compost or as animal feed supplements, or digested to produce methane. Depending on the plant species involved and their fiber content, plant material can also be used for handicrafts and the manufacture of specialty papers. Skill requirements for the operation and maintenance of wetland treatment systems are low.

Level of Involvement

These forms of treatment have been practiced primarily by the private sector in rural areas, and by universities and government institutions for research and development purposes. The Government of Peru has contributed financial and technical resources to the construction of two experimental treatment facilities using totora in Puno, Perú. In Bolivia, experiments have been performed at the University of San Andrés (UMSA).

Costs

Very little information is available concerning the cost of plant-based technologies. This is especially true in the case of water clarification using Moringa oleifera and other seeds. The main cost appears to be the labor in acquiring the plant seeds and producing the flocculent solution.

Cost estimates of wetland-based wastewater treatment systems are equally scarce. The cost of the totora treatment system in Peru is estimated at $65 000. Generalized construction, operation, and maintenance costs for wetland systems are shown in Figure 23. The costs shown in this figure were derived from wetland treatment systems at Vermontville and Houghton Lake in Michigan, U.S.A.

Effectiveness of the Technology

In using ground seeds for water clarification, the size of the particles is an important factor: generally speaking, the smaller the particles, the more efficient the clarification process. This is particularly important in the removal of color using peach and bean seeds (Figure 20). The concentration of the resulting coagulant solution has also an effect on the reduction of turbidity in the product water (Figure 21). For most plant seeds, the lower the pH of the water, the more effective the treatment. Suspended materials coagulate better at lower pH values. Peach seeds are an exception to this rule of thumb. Moringa oleifera was found to be more efficient at reducing turbidity than aluminum sulfate (alum). In general, also, the higher the initial turbidity, the higher the removal rate.

Figure 20: Percent Color Removal as a Function of Seed Particle Size.

Source: Freddy Camacho Villegas, Institute of Hydraulics and Hydrology, UMSA, La Paz.

Figure 21: Turbidity Reduction as a Function of Coagulant Concentration.

Source: Freddy Camacho Villegas, Institute of Hydraulics and Hydrology, UMSA, La Paz.

Wetland treatment systems using totora are quite efficient at removing nutrients and oxygen-demanding substances from effluents. Table 9 shows the percentage of removal of chemical compounds from wastewater by the system in Puno. Parasites were also removed from the inflow waters, and total and fecal coliforms were reduced in concentration by 80% and 99%, respectively. The experiments performed in Bolivia on the removal of heavy metals by totora show that lead, silver and copper can be removed from effluents in less than 2 days. Figure 22 shows the decline in concentration of several heavy metals in a typical effluent.

Table 9 Removal of Chemicals by Totora

Figure 22: Absorption of Heavy Metals by Totora.

Source: Freddy Camacho Villegas, Institute of Hydraulics and Hydrology, UMSA, La Paz.

Figure 23: Generalized Construction and Operation and Maintenance Costs for Aquaculture and Wetland Systems.

Source: Edward J. Martin. Handbook for Appropriate Water and Wastewater Technology/or Latin America and the Caribbean Washington, D.C., PAHO and IDB, 1988.

Suitability

These technologies are useful in areas where suitable plants are readily available. In areas where they are not, any introduction of plants species must be undertaken with caution to minimize the possibility of creating nuisance growth conditions. Even introducing them into constructed enclosures should be done carefully, and with the foreknowledge that there is a strong likelihood that they will enter natural water systems (especially as they must be harvested from the treatment systems and disposed of).

Advantages

· Moringa oleifera trees are hardy and drought-resistant, fast-growing, and a source of large numbers of seeds. They are nontoxic and effective coagulants useful for removing turbidity and bacteria from water.

· The cost of both seed treatment and wetlands is very low, in most cases negligible.

· These technologies are traditional, rudimentary, and easy to implement, ideal for rural areas.

· Wetland systems are easy to build, simple to operate, and require little or no maintenance.

· Most small-scale wetland treatment systems require relatively small land areas.

· Wetland technologies reduce nutrient contamination of natural systems.

· Heavy metals absorbed by the plants in wetland treatment systems are not returned to the water.

· Water-hyacinth-based and other wetland systems produce plant biomass that can be used as a fertilizer, animal feed supplement, or source of methane.

Disadvantages

· In some places plant seeds may not be readily available.

· Totora treatment systems require an initial capital investment that may not always be easily accessible to potential users.

· The lifespan of totora as an efficient water quality treatment technology is still undetermined.

· Temperature (climate) is a major limitation, since effective treatment is linked to the active growth phase of the emersed (surface and above) vegetation.

· Herbicides and other materials toxic to the plants can affect their health and lead to a reduced level of treatment.

· Duckweed is prized as food by waterfowl and fish, and can be seriously depleted by these species.

· Winds may blow duckweed to the windward shore unless wind screens or deep trenches are employed.

· Plants die rapidly when the water temperature approaches the freezing point; therefore, greenhouse structures may be necessary in cooler climates.

· Water hyacinth is sensitive to high salinity, which restricts the removal of potassium and phosphorus to the active growth period of the plants.

· Metals such as arsenic, chromium, copper, mercury, lead, nickel and zinc can accumulate in water hyacinth plants and limit their suitability as fertilizer or feed materials.

· Water hyacinth plants may create small pools of stagnant surface water which can serve as mosquito breeding habitat; this problem can generally be avoided by maintaining mosquitofish or similar fishes in the system.

· The spread of water hyacinth must be closely controlled by barriers, since the plant can spread rapidly and clog previously unaffected waterways.

· Water hyacinth treatment may prove impractical for large-scale treatment plants because of the land area required.

· Evapotranspiration in wetland treatment systems can be 2 to 7 times greater than evaporation alone.

· Harvesting the water hyacinth or duckweed plants is essential to maintain high levels of system performance.

Cultural Acceptability

Seed treatment is not widely known in Latin America and the Caribbean, and its acceptability cannot be conjectured.

Use of aquatic plants as a wastewater treatment medium is well accepted in areas where it is a traditional technology. It is especially well accepted in the Andean areas, where the plants used in the treatment process have value for handicraft production, cattle feed, and other economic uses.

Further Development of the Technology

Other native plants and plant materials should be investigated as coagulants for use in the removal of color and turbidity, and the control of pH. Additional studies will be needed to establish the appropriate dosages of flocculent solutions to be used in water quality treatment.

The use of totora or other aquatic plants can help to clean nutrient- and metal-laden water from agricultural and mining operations, both for water reuse and to eliminate downstream contamination. Future development should be focused on determining appropriate aquatic plant densities required to clean certain types of wastewaters and improving the efficiency of plant uptake after several water treatment cycles. Other uses of the harvested plants should be investigated to make this technology economically attractive.

Sources of Information

Contacts

Freddy Camacho Villegas, Instituto de Hidráulica e Hidrología (IHH), Universidad Mayor de San Andrés (UMSA), Casilla Postal 699, La Paz, Bolivia. Tel. (591-2)79-5724 - 25. Fax (591-2)79-2622.

Erika Gehler A, Carlos Arce L, Hans Salm and Alfredo Alvarez C., Instituto de Ciencias Químicas, UMSA, Calle 27 s/n, Cota Cota, Casilla de Correo 303, La Paz, Bolivia. Fax (591-2)79-2622

Luis A. Ochoa Marroquín, Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrología (INSIVUMEH), 7 Avenida 14-57, Zona 13, Guatemala, Guatemala. Tel. (502-2)31-4967/31-9163. Fax (502-2)31-5005.

Juan Ocola Salazar, Instituto Nacional de Desarrollo (INADE), Proyecto Especial Binacional Lago Titicaca (PELT), Ave. El Son 839, Puno, Perú. Tel. (51-54)35-2305/35-2392.

Guillermo Sarmiento, Dirección de Agua Potable y Saneamiento Básico, Ministerio de Desarrollo Económico, Bogotá, Colombia. Tel. (57-1)287-9743. Fax (57-1) 245-7256/212-6520.

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