Wetlands for wastewater treatment: Opportunities and limitations

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Abstract

This paper gives some introductory information on the use of wetlands for wastewater treatment. It focuses mainly on the functioning of constructed wetlands, in particular surface-flow and infiltration wetlands. The various processes which lead to water purification are briefly explained, in relation to the factors which influence their efficiency. The possibilities for optimization of the design and management of such systems are illustrated with data on the functioning of a wastewater infiltration wetland in The Netherlands. In general, constructed wetlands can be designed to remove more than 90% of BOD, COD, suspended solids and bacteriological pollution from the through-flowing wastewater. Removal of N and P remains, however closer to 50% in most cases.

Introduction

Wetlands have been used for water purification in different parts of the world since the 1950s. Environmental concerns over insufficiently performing individual septic systems, as well as high costs involved in the construction of sewer systems with centralized water purification have spurred investigations into the suitability of wetland ecosystems for this purpose. Generally, natural as well as constructed wetlands have been loaded with several types of wastewater. The natural systems include lake marginal wetlands, extensive fen systems and floodplain marshes, in which large helophytes such as Phragmites australis, Typha spp., Scirpus spp., often dominate the vegetation. Constructed wetlands also have helophyte beds and are of two main types: (i) surface-flow wetlands, in which the wastewater is flowing horizontally over the wetland sediment; and (ii) infiltration wetlands, in which the wastewater flows vertically through a highly permeable sediment and is collected in drains. The wastewater applied usually has received only a primary filtration of coarse material; there are also quite a few cases in which a wetland is being used for polishing the effluent of a conventional purification plant. Wetland ecosystems have special characteristics which make them particularly suitable for wastewater purification:

  • 1.

    They are semi-aquatic systems which normally contain large quantities of water. The flooding caused by wastewater addition is a normal feature of the system.

  • 2.

    They have partly oxic, partly anoxic soils in which organic matter breakdown takes place through special pathways involving electron acceptors other than oxygen, e.g. nitrate, sulphate and iron. As a result, N as well as P dynamics are very different from those in upland ecosystems.

  • 3.

    They support a highly productive, tall emergent vegetation capable of taking up large amounts of nutrients and responding to enrichment with nutrients with enhanced growth. The helophytes also aerate the soil rhizophere through aerenchyma in the roots.

Research over the last two decades has accumulated much quantitative information on the performance of these systems. A useful recent review of the current state of the art is the book by Kadlec and Knight (1996), which not only gives detailed documentation on the wastewater purification function of natural as well as constructed wetlands, but also gives elaborate engineering guidelines for the construction and management of wastewater wetlands. An important data base increasingly used in the literature is the North American Data Base (NADB) containing data from over 100 wastewater wetlands compiled for US-EPA by Knight et al. (1992). This data base shows that the performance of wetlands used for water purification depends strongly on the loading rate (wastewater per area per time) and on the wetland's specific hydrological and ecological characteristics. Removal percentages were generally high for COD, BOD and bacterial pollution (80–99% removal in most cases). Removal rates for P and N were lower and more variable. N removal was higher than 50% in most cases, P removal was mostly lower and subject to saturation after prolonged loading. The nutrient removal rates can be optimized by chosing the right loading rate (i.e. prevention of overloading the system), by installing a regime of fluctuating water levels and by the addition of absorbing materials to the wetland sediment.

From a practical standpoint, constructed wetlands offer better opportunities for wastewater treatment than natural wetlands. They can be designed for optimal performance of the BOD, COD and nutrient removal processes and for maximum control over the hydraulic and vegetation management of the wetland. The use of natural wetlands should also be discouraged because of the great conservational value of many of these systems. The characteristics of two types of wastewater wetlands and the processes responsible for the purification of the through-flowing water will be further outlined below. The functioning of these systems and the possibilities for optimization the purification will be illustrated by an example of an infiltration wetland in The Netherlands which has been studied in great detail by Meuleman (1994).

Section snippets

Surface-flow wetland

Surface-flow wetlands often include a presettling basin and a number of compartments with a shallow water layer (0.2–0.4 m) planted with helophytes such as Phragmites, Typha or Scirpus spp. The wastewater is often mixed with surface water or purified effluent and generally flows through the system with a minimum residence time of 10 days. The purification processes include:

  • 1.

    Settlement of suspended solids.

  • 2.

    Diffusion of dissolved nutrients into the sediment.

  • 3.

    Mineralization of organic material.

  • 4.

Infiltration wetland

Infiltration wetlands typically have a relatively coarse sediment type (sand) so that the wastewater can easily penetrate the soil. By surrounding the wetland (compartments) with a drainage ditch with a lower water table, the wastewater is forced vertically into the sediment by gravity. The infiltration process can be enhanced by burying drainage tubes at a depth of 60–100 cm. It is essential that the wetland is sealed off from the lower sediment layers by an impervious clay layer or by plastic

Nutrient removal processes in wastewater wetlands

As is clear from the previous section, wastewater wetlands generally perform well for COD, BOD and bacterial pollution, but show limited capacity for nutrient removal. The high removal rates for COD and BOD are caused by sedimentation of suspended solids and by rapid decomposition processes in the water and upper soil layers. As nutrient removal is often also an important objective, optimization of the nutrient removal processes should always be attempted. Knowledge of the various nutrient

Performance of an infiltration wetland in The Netherlands

To illustrate the wastewater purification capacity of wetlands, we will give an overview of the performance of a wastewater infiltration wetland which has been in operation for 10 years as a treatment facility for a recreational housing project. The 1.3 ha constructed wetland was located near Lauwersoog in the northern part of The Netherlands and was studied intensively with respect to water and nutrient budgets, COD and BOD removal, bacterial pollution and the contribution of the plant uptake,

Concluding remarks

Wetlands may be the better alternative for wastewater purification in the following examples:

  • 1.

    Remote location of small residential areas which excludes the connection to conventional sewer systems. Examples of such small-scale wastewater wetlands include cases from cold climates where they proved to work under winter conditions if a snow cover is present (Brix, 1994).

  • 2.

    Recreational facilities which are mostly used during the warmer season. A constructed infiltration wetland in Holland receiving

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