Floating Aquatic Plant Systems

Aquatic plant systems are engineered and constructed systems that use aquatic plants in the treatment of industrial or domestic wastewater. They are designed to achieve a specific wastewater treatment goal. Aquatic plant systems can be divided into two categories:

• Systems with floating aquatic plants such as water hyacinth, duckweed, and pennywort

• Systems with submerged aquatic plants such as waterweed, water milfoil, and watercress

The use of aquaculture as a means of treating wastewater involves both natural and artificial wetlands and the production of algae and higher plants (submersed and immersed) to remove contaminants such as nitrogen compounds, BOD5, hydrocarbons, and heavy metals from the wastewater. Floating aquatic plants such as water hyacinth (Eichhornia crassipes) and duckweed (Lemna spp.) appear to be some of the most promising aquatic plants for the treatment of wastewater and have received the most attention in this regard. Other plants are also being studied, among them seaweed and alligator weed.

These systems are basically shallow ponds covered with floating plants that detain wastewater at least one week. The main purpose of the plants in these systems is to provide a suitable habitation for bacteria that remove the vast majority of dissolved nutrients. The design features of such systems are summarized in Table 6.8.

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 because they must be harvested from the treatment systems and disposed of).

Table 6.8. Performance of four different wastewater effluent treatment systems using water hyacinth (source: USEPA, 1976).

BOD COD TSS Nitrogen Phosphorus Source Reduction Reduction Reduction Reduction Reduction

Table 6.8. Performance of four different wastewater effluent treatment systems using water hyacinth (source: USEPA, 1976).

BOD COD TSS Nitrogen Phosphorus Source Reduction Reduction Reduction Reduction Reduction

Secondary effluent

35%

n/a

n/a

44%

74%

Secondary effluent

83%

61%

83%

72%

31%

Raw wastewater

97%

n/a

75%

92%

60%

Secondary effluent

60-79%

n/a

71%

47%

° The cost of plant seeding 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.

• Temperature (climate) is a major limitation because effective treatment is linked to the active growth phase of the immersed (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 windscreens 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 that can serve as mosquito breeding habitats; 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 because 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.

The water hyacinth is a perennial, free-floating freshwater aquatic macrophyte with rounded, upright, thick, waxy, and glossy green leaves and spikes of lavender flowers (see Fig. 6.13), native to South America and found naturally in waterways, bayous, and other backwaters in temperate and tropical regions. The water hyacinth is considered one of the worst weeds in the world—aquatic or terrestrial—for its fast growth, which tends to clog the waterways for boat traffic and prevent sunlight and oxygen from getting into the water. 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. Although many uses of the plant material have been investigated,

Figure 6.13. Water hyacinth in a Florida waterway (courtesy of Center for Aquatic and Invasive Plants at University of Florida).

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.

Duckweed (Lemna sp., Spirodela sp., and Woljjia sp.) are small, green freshwater plants with fronds from one to a few millimeters and a short root, usually less than 1 cm in length. Duckweed are the smallest flowering plants. They grow as small colonies of plants floating on the surfaces of quiet bodies of water. Their growth can be extremely rapid, given the proper conditions. These plants are almost all leaf, having essentially no stem tissue, and only one, or a few, very fine roots. Fig. 6.14 shows three species of duckweed. In nature, duckweed serves as food for many species of fish and aquatic birds. In the United States, crayfish are often released in irrigated rice fields in rice-growth areas of the United States to control weeds (often duckweed). Because they are high in proteins and also (3-carotene, harvested duckweed can be used as feed for grass carp, sheep, chicken, and tilapia. Duckweed can tolerate and grow under a wide range of conditions, including on water polluted with high concentrations of bacteria and some agricultural wastes. These characteristics have brought duckweed to the attention of environmental engineers and aquculturists alike.

Water hyacinth systems

Water hyacinth systems are predominantly floating aquatic plant systems for wastewater treatment. Three types of the systems exist today for BOD5 and nutrient removals: aerobic nonaerated, aerobic aerated, and facultative anaerobic (Metcalf and Eddy, Inc., 1991). These types of water hyacinth systems reflect the types of stabilization ponds used for wastewater treatment. Not surprisingly, floating aquatic plants, such as water hyacinth, for wastewater treatment thrive in natural or artificial ponds and constructed wetlands. However, use of water hyacinth has been limited, in geographic location, to warm weather regions because of the sensitivity of water hyacinth to freezing conditions (see Fig. 6.15). Water hyacinth systems have been most often used for either removing algae from oxidation pond effluents (by blocking the sunlight to reach the water) or for nutrient removal following secondary treatment.

Figure 6.14. Three species of native duckweed in a Florida waterway (courtesy of Center for Aquatic and Invasive Plants at University of Florida).

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.

Figure 6.15. A map of a water hyacinth growing region in the U.S. (USEPA, 1993).

Duckweed systems

Duckweed systems have been used successfully to improve the effluent quality from facultative stabilization ponds by reducing algae population, BOD5, and nutrients. Duckweed is sensitive to wind and may be blown in drifts to the leeward side of the pond resulting in exposure of a large surface area that is prone to algae blooming. The regions in the U.S. that are suitable for duckweed systems can be seen in Fig. 6.16. Redistribution of the plants to cover the surface requires manual labor. Piles of decomposing plants can result in the production of odors. Drift control, such as floating baffles, can be used to divide the surface area into smaller cells, thus reducing the amount of open surface area to blowing wind.

Similar to water hyacinth systems, design criteria of duckweed systems include hydraulic detention time, water depth, pond geometry, BOD5 loading, and hydraulic loading rate. Typical treatment results from several U.S. locations involving duckweed wastewater treatment systems for effluents from facultative stabilization ponds are provided in Table 6.9.

Figure 6.15. A map of a water hyacinth growing region in the U.S. (USEPA, 1993).

[\ Vj Growth is Hkety during IVS i 6 months of the year

Figure 6.16. A map of a duckweed growing region in the U.S. (USEPA, 1993).

[\ Vj Growth is Hkety during IVS i 6 months of the year

Figure 6.16. A map of a duckweed growing region in the U.S. (USEPA, 1993).

Table 6.9. Performance of duckweed system for treating facultative pond effluents in several U.S. locations (adapted from USEPA, 1988).

BOD5, mg/l

TSS, mg/l

Detention

Location

Influent Effluent

Influent

Effluent

Depth, m

Time, Days

Biloxi, MS

30 15

155

12

2.4

21

Collins, MS

33 13

36

13

0.4

7

Sleep Eye, MN

420 1B

364

34

1.5

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Reed, S.C., Crites, R.W., and Middlebrooks, E.J. 1995. Natural Systems for Waste Management and Treatment, 2nd edition. New York: McGraw-Hill, Inc.

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Food and Agricultural Waste Water Utilization and Treatment

Sean X. Liu

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