Native Plants of North America

The Use of Aquatic Plants to Treat Waste Water

by Cristian Frers, Senior Environmental Manager and Consultant; Translated by Veronica Wisniewski

We are publishing this article in this edition as an exploration of the alternative treatment methods being employed in other countries around the world. Cristian Frers is a Senior Environmental Manager and Consultant in Argentina where the techniques discussed below are being employed to treat industrial waste water. Thanks to Ecoportal (www.ecoportal.net) and Mr. Frers for allowing us to translate and reprint this article.

Because they require little or no energy to function, constructed wetlands offer distinct advantages over alternative treatment systems. If there is adequate inexpensive land suitable for aquatic conditions available for the installation of artificial wetlands, such an approach to wastewater treatment can be a cost effective alternative. As an added advantage, such wetlands provide habitat for wildlife.

INTRODUCTION

The importance of wetlands has changed over time. Wetlands serve as transition zones between aquatic and terrestrial environments and provide a dynamic link between the two. Moving along a gradient, water picks up chemicals and sediments which, as they move through the wetland, are transformed and transported to the surrounding landscape.

Wetlands are effective nutrient sinks and absorbers of organic and inorganic pollutants. This absorptive capacity of a natural wetland is the mechanism employed in a constructed wetland for the purpose of treating wastewater from businesses and municipalities.

The biotechnological solution to wastewater involves installing artificial wetlands to act as natural filters. By locating constructed wetlands between the wastewater source and aquatic resources (rivers, lakes, lagoons), these systems require no maintenance, consume no electricity, and cost less than one quarter that of a traditional waste treatment system. Such constructed wetlands employ different species of plants that commonly abound in natural wetlands: cattails, waterlilies and rushes.

ARTIFICIAL WETLANDS FOR SEWAGE AND INDUSTRIAL WASTE

1. Sewage flows into the constructed wetland, which is an excavated cell filled with sand that serves to filter out odors.

2. The filter consists of a large vegetative planting, in this case rushes, whose roots in the sand are fed by the wastewater.

3. The nutrients in the water are absorbed by rushes (Juncus), which sequester them in their tissues as they grow.

4. The nutrients absorbed are eliminated with vegatative dieback of the rushes, whose remnants form an insulating layer.

5. The purified water filters from the wetland into the lagoon.

6. Proportioning a wastewater treatment wetland: The area required is proportional to the size of the residential population and is calculated as follows: 1 person = around 5 m2.

WASTEWATER TREATMENT WITH AQUATIC PLANTS

Treatment systems using aquatic plants consist of shallow reservoirs containing floating or submerged aquatic plants. The best studied wastewater systems are those utilizing duckweed (Lemna minor). Generally, treatment systems break into two types based on the dominant plant types. The first type uses floating plants which are distinguished by their ability to meet their need for carbon dioxide and oxygen directly from the atmosphere. Such plants derive their mineral needs from the water.

The second type of treatment system consists of submerged plants, which are distinguished by their ability to absorb oxygen, carbon dioxide, and minerals directly from the water column. Submerged plants are easily inhibited by high turbidity because their photosynthetic parts are under water.

SAMPLE CASE

Maria Alejandra Maine brought together a team from UNL to investigate methods for treating the effluents of the Bahco Company. They constructed a wetland on the grounds of the plant in Sao Tome – Province of Santa Fe Argentina, to perform the final treatment of the company’s effluent of both fluids and industrial waste.

The idea of constructing an open air wastewater treatment system utilizing aquatic plants to absorb and purify liquid waste arose in 2001 when Bahco Argentina decided to improve its environmental management strategy. To minimize the impact generated by manufacturing processes, the company decided to add a final polishing phase to its wastewater treatment system.

ARTIFICIAL WETLANDS

An artificial wetland is a system of treating wastewater in a shallow constructed pond or channel no more than 0.60 meters deep, in which aquatic plants have been planted, and natural processes are utilized to treat wastewater. Artificial or constructed wetlands have the advantage over alternative treatment systems in that they require little or no energy to operate. If there is adequate cheap land available near the effluent source, installation of the water treatment wetlands can be a cost effective alternative. In addition, constructed wetlands provide habitat for wildlife, and are aesthetically pleasing to the eye.

Advantages:

1. Plants can be used as low-cost extraction devices to purify polluted water.

2. In some cases, plants decompose waste faster than microorganisms.

3. The method can be applied to large areas or to complete the decontamination of restricted areas in lengthy periods.

Limitations:

1. The process is limited to shallow water or the depth to which roots can penetrate.

2. The process can be time consuming.

3. There is a biological limit to the metals and compounds that can be captured.

The various processes through with plants can incorporate pollutants are illustrated below and described in the following table:

FUNCTIONS OF WETLANDS

Human activities have given and continue to give rise to a variety of wetlands valuable for a variety of plant species.

Physical remediation processes:

Constructed wetlands are capable of physically removing contaminants bound to particulate matter efficiently.

Biological remediation processes:

Biological remediation is perhaps the most important mechanism removing pollutants in constructed wetlands. Wetland plants are widely recognized for their ability to capture and remove contaminants, particularly since some of the pollutants are essential nutrients, such as nitrate, ammonium and phosphate which are easily taken up in such wetlands.

Chemical remediation processes:

Wetland soils perform the important process of chemical absorption, which leads to short-term retention or long-term immobilization of various kinds of pollutants.