Use of wetlands for water quality improvement. Artificial purification swamps and wastewater gardens.

Abstract

People were trying to adapt the natural purification of different types of water systems, using both plants and animals (microorganisms) to purify municipal and industrial sewage. These microbes, in addition to the wetland plants, are very important in the usage of excess nutrients in the water, particularly nitrates and phosphates. Initially, artificial swamps were only used for the purification of overflow water from sewers and the effluent of sewer purification installations. Nowadays, forming artificial swamps as the last stage of water purification process is not only cheap but also a modern way of improving quality of water delivered by purification plants. A natural water purification process are used also in wastewater gardens.
Authors: Mateusz JAKUBIAK, Małgorzata ŚLIWKA

1. Wetlands as natural purification plant

The significance of wetlands in the improvement and maintenance of water quality is now widely accepted all over the world. Both natural and constructed wetlands are increasingly being used to purify water from such sources as storm water drains, waste water treatment plants, agriculture and industry. This quality improvement is achieved through two major processes – the trapping of suspended and dissolved contaminants, and the usage of excess nutrients.

There are a number of advantages to using wetlands to improve water quality. One of the most important is simply that it’s relatively cheap. There is no new or complex technology required and wetlands do not require constant attention as in the case of a conventional treatment plant. The system relies on renewable energy sources (solar and kinetic) as well as plants and microorganisms. It can also tolerate varying influxes of water and contaminant levels. Wetlands, particularly those in or near cities, are also highly valued for their aesthetic value.

The action of wetlands in improving water quality can be summarized as follows. Wetlands effectively act as settling basins, slowing the water flow and increasing the settling of suspended particles. Contaminants attached to these suspended particles are thus removed from the water as well. They are then stored in the wetland (in the soil or plants) or converted into usable form by microbes. These microbes, in addition to the wetland plants, are very important in the usage of excess nutrients in the water, particularly nitrates and phosphates [2].

The settling of suspended particles from the water is caused primarily by the wetland plants. The simple physical presence of these plants, with their proportionally large and extensive subsurface biomass, slows water flow to very low levels and turns the wetland into an efficient settling basin. The slowing of the water flow causes silt, carried in the water from erosion and runoff, to settle out. Pollutants, toxins, metals and other harmful substances are often attached to the silt, and so they are removed from the water as well. As more sediment is deposited on the wetland bed, the contaminants are covered and stored in the wetland soils. These soils are often deficient in oxygen, which slows the decomposition of dead plant material. This results in a soil type that is generally dark, quite heavy and relatively high in organic material, (15% to 20%). This organic material provides many charged particles that are capable of attracting and binding organic molecules such as pesticides, thus removing them from the water as well. Another consequence of the high organic matter levels is that the soil is capable of supporting huge populations of microbes. Some of these microbes are able to use pesticides and other organic molecules as food. The second major process of water quality improvement is the usage of excess nutrients, both by plants and microbes. Wetland plants are among the most productive of ecosystems, often producing from 10 to 1000 times the biomass of nearby ‘dry’ systems. As a result, relatively large amounts of nutrients are removed from water passing through a wetland, particularly nitrate. Wetland plants also use up some excess phosphate. Nitrogen is also removed by microbes living on the surface of plant roots, in a process known as nitrogen assimilation. Nitrate is converted to ammonia by the microbes, which is then released into the atmosphere. Some researchers have estimated that the microbes remove as much as ten times as much nitrate as the actual plants.

Wetland plants are also capable of accumulating heavy metals and various organic pollutants. However, it must be remembered that there are limits to the amount of pollutants that a plant can take up without being damaged or killed.