Prof. Jorge Loredo

University of Oviedo

ABSTRACT

The application of natural systems for wastewater treatments in small communities and rural areas includes a vast array of technologies including soil-based and aquatic systems. The common element in all of them is that the treatment is made by use of“natural” environmental components such as soils, vegetation and native microorganisms. Natural systems typically require fewer operational personnel, consume less energy, and produce less sludge than the higher-rate systems. A necessary requirement previous to the consideration of any type of natural treatment is the characterisation of the wastewater to be treated : flow rate, composition, and their variation over time. Experience with the assimilation capacity of natural wetlands has led to the construction of man-made wetland systems (constructed wetlands) which are relatively simple to design and build. They can effectively remove large quantities of pollutants including organic matter, suspended solids, metals, and excess nutrients, and the effectivity of treatment is largement dependent on substrate chemistry. Some physical, chemical and biological processes contribute to the immobilisation and/or transformation of pollutants. Furthermore, through absorption and assimilation, wetland plants remove nutrients for biomass production. Important aspects to be in consideration for design criteria are: cell depth, vegetation type and substrate, which provide physical support for plants and attachement surfaces for microbial populations. Constructed wetlands offer an effective and economical self-maintain alternative to conventional treatment systems, and in consequence they appear to have very broad applicability for domestic wastewater treatment systems in small communities and rural areas.

1. INTRODUTION

In rural areas and/or small communities, the use of conventional physico-chemical methods (small sewage treatment facilities) in centralised systems implies a high cost of construction, maintenance and transport that in many cases are difficult to be afforded. Then, the wastewater treatment is usually carried out by septic tanks / field line systems, which, occasionally, have been proven inadequate for wastewater treatment and suspected to be a major cause of water pollution. These systems are many times ineffective due to shallow soils or poor soil percolation rates, high groundwater table, karst topography or other aspects leading to inefficient treatments. Inadequate treatment of domestic wastewaters contributes to pollution of surface waters and might led to groundwater pollution.

Then, the application of natural systems for wastewater treatments in small communities and rural areas is another possible option to be in mind. They include a vast array of technologies including soil-based and aquatic systems, which have in common the major contribution made by the “natural” environmental components (soil, vegetation and micro-organisms) which provide the desired treatment. Between the soil based systems it can be considered the subsurface systems (septic tanks) and the land treatment systems (slow rate, rapid infiltration and overland flow). The aquatic systems include the stabilization ponds, the aquatic systems with floating plants, and the wetland systems. These systems are well suited for small communities and rural areas and small communities because of the low flows of wastewater to be treated and the availability of suitable land area according to the volume of wastewater flow. Natural treatment systems which can be considered land intensive methodologies, avoid, in a great part, the use of energy, materials and chemicals, and typically require very low costs for operation and maintenance.

A necessary requirement to consider previously deciding to apply natural systems for the wastewater treatment of a particular community is to find out is any of the types of natural systems currently applied can be appropriate for this community, in function of the population number, the sources of sewage, the climate and the characteristics of the land. In all cases, it is necessary the characterisation of the wastewater to be treated: characteristics such as flow rate, chemical composition, and their variation over time, are fundamental parameters to be in consideration. In general, domestic wastewater is composed of a variable array of components characterised by the presence of biodegradable organic matter, particulate and dissolved solids, and nutrients.

2. CONSTRUCTED WETLANDS FOR DOMESTIC WASTEWATER TREATMENT

From the different types of natural systems potentially applicable for wastewater treatment in rural areas and small communities, the wetland systems occupy an outstanding place because of its advantages and capabilities in order to obtain high quality effluents. Wetlands are transitional areas between aquatic ecosystems and uplands, and they are typically considered to be part of contiguous surface waters. With this consideration, they can be defined as land where the water surface is at or above the ground surface for a long enough period each year to maintain saturated soil conditions and the growth of related vegetation (Reed, 1990). Wetlands have a high rate of biological activity and they can transform many of the common pollutants of conventional wastewaters into harmless by-products or essential nutrients that can be used for additional biological productivity. It has long been recognised that natural wetlands help protect water quality and the natural wetlands have been used as convenient wastewater discharge sites for as long as sewage has been collected (Kadlec and Knight, 1996). In coastal plain areas of U.S. natural wetlands have historically been used as convenient receiving waters for wastewater discharges. EPA documented the existence of 326 discharges to surface waters that could be defined as wetlands (U.S.EPA, 1983a, 1983b). Several natural wetlands designed for wastewater management have been extensively studied in the U.S. These include the Hoghton Lake fen in Michigan (Kadlec, 1987), the Florida cypress domes (Ewel and Odum, 1984; Fritz and Helle, 1982), southern cypress and gum swamps (Knight, 1987; CH2M Hill, 1987), Carolina bays (CH2M Hill, 1988; Knight et al., 1981) and marshes (Dolan et al., 1981; Fetter et al., 1978).

Currently, most of the natural wetlands are protected areas, so wastewater application is nor permitted. Then, in order to make use of the knowledge and the experience with the assimilative capacity of natural wetlands has been proposed the construction of manmade wetland systems (constructed wetlands) for wastewater treatment. After Hammer and Bastian (1989) is possible to define constructed wetlands as a designed and manmade complex of saturated substrates, emergent and submergent vegetation, animal life, and water that simulates natural wetlands for human use and benefits. Constructed wetlands systems mimic the treatment that occurs in natural wetlands and they apply a combination of physical, chemical and biological processes in a same manner that they work in the natural wetlands, so they are similar in macroscopic functions such as organic matter and pollutants assimilation potential.

Constructed wetlands are engineered systems of wastewater treatment that are designed and constructed to utilize the natural processes that occur in natural wetlands. A constructed wetland consists of a properly-designed basin that contains soils or other selected substrate, water column, and wetland vegetation, as main elements. Other important components that assist in the treatment of the wastewater, such as the communities of microorganisms, develop naturally. The substrate is placed over an impermeable layer constituted by combinations of clays and geotextile liners, in order to protect the subsoil of pollution from wastewater. Constructed wetlands are designed to take advantage of the same physical, chemical and biological processes that occur in natural wetlands, but do so within a more controlled environment (USEPA, 1993).

Different wetland systems types and alternative plant species can be considered in constructed wetlands. The two basic types of constructed wetland treatment systems include constructed Free Water Surface Systems (FWS) and constructed Subsurface Flow Systems (SFS). In the Free Water Surface type, the water in the system is exposed to the atmosphere and surface aeration is the major oxygen source. They are designed to simulate natural wetlands, with the water flowing over the soil surface at shallow depths. In the constructed subsurface flow type, the water level is maintained below the surface of the permeable media used in the bed, and in this case the major oxygen source is transferred by the plants to their root systems. These systems are designed to create subsurface flow through a permeable medium, keeping the water being treated below the surface. A scheme of both types of wetlands is represented in Figure 1.

Figure 1: Schematic representation of two types of constructed wetlands.

Many chemical and biological transformations take place within the substrate, which is constituted by soil, sand, gravel, rock, and organic materials such as compost. It serves to provide physical support for plants and microbial populations, and as reactive surface area for complexing ions in order to removecontaminants. The vegetation represents an important component in both types of systems. The wetland plants stabilize the substrate, slow water velocities allowing suspended materials to settle, filter, absorb and adsorb pollutants, provide support for microorganisms development, and leakage of oxygen from subsurface plant structures creates oxygenated microsites within the substrate. Typical plant species used in constructed wetlands include common reed (Phragmites australis), cattail (Typha spp.), bulrush (Scirpus spp.), and another aquatic macrophytes.

Figure 2: Two macrophytes that are used in constructed wetlands: A) Typha latifolia; B) Iris pseudacorus

A fundamental characteristic of wetlands as wastewater treatment systems is that their functions are almost solely regulated by microbiota and their metabolism (Wetxel, 1993). Microorganisms are major sink for organic carbon and many nutrients, transform a great number of organic and inorganic substances into innocuous or insoluble substances and involve in the recycling of nutrients. They alter contaminant substances to obtain nutrients or energy to carry out their life cycles. The final effectiveness of constructed wetlands managed for wastewater treatment is dependent on developing and maintaining optimal environments for desirable microbial populations.

In comparison to the physico-chemical conventional treatment systems, the use of constructed wetlands municipal wastewater presents quite advantages but also some disadvantages. The main advantages to consider are the low cost of construction and low operation/maintenance expenses (both energy and supply), the simplicity of operation that can be run by relatively untrained personnel (included homeowners), the flexibility of operations and the low susceptibility to variations in loading rate, the possibility of water reuse and recycling, the possibility too to be used for wildlife habitat and the aesthetic enhancement of open spaces. On the other site, between the main disadvantages to consider are: the requirement of large land areas, the possible decreased performance during cold seasons in temperate regions, the possible existence of bad odours and mosquitoes. The land area needed for treatment facilities can be used for wetlands habitat, attracting wildlife and giving a more ecological component to the installation.

The use of constructed wetlands for domestic wastewater treatment has become more prevalent in recent years as a result of a wider understanding of the working and advantages of these natural systems. Available bibliographic information indicates that constructed wetlands can effectively remove or convert large quantities of pollutants including organic matter, suspended solids, metals, and excess nutrients, and the effectivity of treatment is largement dependent on substrate chemistry.

3. MECHANISMS OF TREATMENT IN CONSTRUCTED WETLANDS

In constructed wetlands, some physical, chemical and biological processes occurring in the substrate-water matrix and in the plant rhizosphere contribute to the immobilisation and/or transformation of pollutants. The most important mechanisms to remove aquatic pollutants are listed in Table 1.

Table 1: Removal mechanisms in wetlands (after Brix, 1993)

Removal rates represent the change in mass per area between waters entering and exiting the wetland. Types of removals included in the rates are net changes due to adsorption, absorption, plant up-take, gaseous release to the atmosphere, infiltration losses, and burial as new sediments. Available bibliographic information indicates that constructed wetlands can effectively remove from waste waters: suspended solids, organic matter, and excess nutrients, as well as natural die-off of pathogens.

microorganisms (Tchobanoglous, 1987). Wetlands transform nitrogen through the actions of microorganisms, and plant uptake and tissue synthesis. Microorganisms are largely responsible for transforming ammonia to nitrate through nitrification and from nitrate to nitrogen gas through denitrification. Wetlands assimilate phosphorus through soil adsorption or plant uptake; total phosphorus removal efficiency increases with higher input concentrations and with high hydraulic residence time. Wetland treatment systems have significant potential to reduce the concentrations of metals and other toxics in wastewater through physical, chemical, and biological processes (Richardson and Nichols, 1985). Generally these chemical are oxidized and precipitated in the sediments and biota in the wetland.

As with other natural system alternatives, wetlands can be designed in a variety of shapes and sizes to provide treatment and disposal of wastewater. Constructed wetlands offer an effective and economical self-maintaining alternative to conventional treatment systems, and in consequence they appear to have very broad applicability as wastewater treatment systems for an array of water pollution problems, including domestic wastewater treatment in small communities. Wetlands have been found to be effective in treating biochemical oxygen demand, suspended solids, nitrogen, and phosphorus, as well as for reducing metals, organics, and pathogens (Kadlec and Knight, 1996).

Fundamental factors affecting the selection of a constructed wetland option for wastewater treatment in small communities or rural areas are the cost of the system and the level of treatment that is required. Constructed wetland may be used as component processes in a wastewater treatment or may provide overall treatment, depending of the characteristics of the residual water to be treated and the requested quality for effluents. The required effluents quality varies widely depending of the characteristics of the project; if wetland effluents will be reused they will met the corresponding regulation established for that field.

4. REUSE OF THE TREATED WATER

The reuse of treated wastewater establishes the quality standards to be met, according to regulations. In this case the objective will be to obtain treated water with: BOD₅ 10 mg/l (considering the regulation for water reuse from USEPA).

The agricultural reuse of water constitutes the more extend exploitation of treated wastewaters. This is one of their best possible uses, as in this case water does not need to have a high quality, so the necessary treatment will be minor. Other advantage is the presence of nutrients (mainly N, P, K) in the effluent due to their fertilizer power; and the trace elements (Cu, Fe, Mb, Zn, etc), in the small quantities they are usually present at this water, are micronutrients instead of contaminants (Rosemblum and Sheikh, 1998; Colmenarejo Morcillo, 1995). After the correct treatment in a constructed wetland wastewaters can be reused for irrigation. The water quality after the pass through the wetland can be high enough for this use, which save the use of water of higher quality at the same time.

The main disadvantage of reusing wastewaters for irrigation is the hazard caused by the possible presence of pathogens, so this is the main issue to control. The treatment must be enough to accomplish the established quality criteria and norms of use to avoid sanitary risk.

5. CONCLUSIONS

Constructed wetlands have proved to effectively remove suspended solids, organic matter, metals and excess nutrients from waste waters. In rural areas or small communities, the constructed wetlands represent a good alternative as opposed to the use of septic tanks or centralised small sewage treatment conventional physico-chemical plants, in order to reduction of costs and integration in the environment. The utilization of constructed wetlands for the treatment of domestic wastewater in small communities and rural areas has become more prevalent in recent years as a result of their contrasted advantages as opposed to the conventional treatment methods.

Ability of artificial wetlands to perform integrated wastewater treatment by removing bacterial indicators as well as BOD5, suspended solids, and nitrogen makes them an attractive alternative for the wastewater treatment needs of small communities and rural areas. Where land is available, constructed wetlands offer a simple process design, low operation and maintenance expenses, and wildlife enhancement value, and they represent an innovative solution to cost-effective wastewater treatment.

Having the concept of sustainability in mind, the reuse of waste waters for irrigation of agricultural or recreation lands, after being correctly treated to reach an appropriated quality, grows in importance day by day, as it saves the use of higher quality water. Then, constructed wetlands appear to have very broad applicability as wastewater treatment systems for an array of water pollution problems.

For small communities, constructed wetlands treatments can be a cost-effective option with ability to remove pollutants from residual wastewaters. Since expensive wastewater collection systems can be minimised and central treatment and discharge completely avoided, they represent effective management tools for the treatment of domestic wastewater in small communities. The decentralised systems that promote the natural systems are appropriate and present abundant benefits for low density communities or rural nucleus.

REFERENCES

Bastian R.K., Hammer D.A. (1993): The Use of Constructed Wetlands for Wastewater Treatment and Recycling. In Constructed Wetlands for Water Quality Improvement,ed. G.A. Moshiri. CRC/Lewis.

Brix H. (1993): Wastewater Treatment in Constructed Wetlands: System Design, Removal Processes, and Treatment Performance. In Constructed Wetlands for Water Quality Improvement,ed. G.A. Moshiri. CRC/Lewis.

Colmenarejo Morcillo J.M. (1995): Reutilización de agua regenerada (aplicaciones). Published in Los residuos como fuente de recursos. Comité de Energía y Recursos.

CH2M Hill (1987). Central wastewater treatment plant wetlands discharge – First Annual report, June 1986 – June 1987. Paper prepared for Grand Strand Water and Sewer Auth.

CH2M Hill (1988). 201 Facilities Plan Amendment for the Grand Strand Region (Plant“A” Service Area). Volume 3. Carolina Bay Effluent Discharge Pilot Program Final Project Report. Rep. Prepared for Grand Strand Water and Sewer Auth., Conway, S.C.

Dolan, T.J. et al. (1981). Phosphorus dynamics of a Florida freshwater marsh receiving treated wastewater. J. Appl. Ecol., 18, 205.

Ewell, K.C. and Odum, H.T. (1984). Cypress swamps. Univ. Presses of Fla., Gainesville

Fetter, C.W. et al. (1978). Use of a natural marsh for wastewater polishing. J. Water Pollut. Control Fed., 50, 290.

Fritz, W.R. and Helle, S.C. (1982). Cypress wetlands for tertiary treatment. In: Progress in wetlands utilization and management. P. McCaffrey et al., (Eds.), 349pp.

Hammer, D.A. and Bastian, R.K. (1989). Wetlands ecosystems: natural water purifiers´?. In: Constructed wetlands for wastewater treatment, Hammer, D.A. (Ed.). pp. 5-21. Lewis Publ. USA.

Kadlec, R.H. (1987). Northern natural wetland water treatment systems. In: Aquatic plants for water treatment and resource recovery. K.R. Reddy and W.H. Smith (Eds.), Magnolia Pub., Inc., Orlando, Fla, 913pp.

Kadlec, R.H. and Knight, R.L. (1996). Treatment wetlands. Lewis Publishers.

Knight, R.L. (1987). Effluent distribution and basin design for enhanced pollutant assimilation by freshwater wetlands. In: Aquatic plants for water treatment and resource recovery. K.R. Reddy and W.H. Smith (Eds.), Magnolia Pub., Inc., Orlando, Fla, 913pp.

Knight, R.L. et al. (1981). Carolina Bays – Feasibility for effluent advanced treatment and disposal. Wetlands, 4, 177pp.

Lorion R. (2001): Constructed Wetlands:Passive Systems for Wastewater Treatment. Technology Status Report prepared for the US EPA Technology Innovation Office.

Reed, S. (1990).Natural systems for wastewater treatment. Water Environment Federation. 270pp. Alexandria. VA.

Richardson, C.J. and Nichols, D.S. (1985). Ecological analysis of wastewater management criteria in wetland ecosystems. In: Ecological considerations in wetlands treatment of municipal wastewaters. P.J. Godfrey ey al., (Eds.), Van Nostrand Reinhold Co., New York, 137.

Rosemblum E., Sheikh B. (1998): Choosing to reuse. Published in Water Environment and Technology, May 1998.

Steiner G.R., Combs D.W. (1993): Small Constructed Wetlands System for Domestic Wastewaters Treatment and Their Performance. In Constructed Wetlands for Water Quality Improvement,ed. G.A. Moshiri. CRC/Lewis.

Tchobanoglous, G. (1987). Aquatic plant systems for wastewater treatment engineering considerations. In: Aquatic plants for water treatment and resource recovery. K.R. Reddy and W.H. Smith (Eds.), Magnolia Pub. Inc., Orlando.

U.S. EPA (1983a). Phase 1 report. Freshwater wetlands for wastewater management. U.S. EPA, Region IV, Atlanta, Ga.

U.S. EPA (1983b). The effects of wastewater treatment facilities on wetlands in the Midwest. EPA-905/3-83/002, U.S. EPA Tech. Rep. Region V, Chicago, III.

U.S. EPA. Office of Water (1993): Constructed Wetlands for Wastewater Treatment and Wildlife Habitat. 17 Case Studies. EPA832-R-93-005.

U.S.EPA (2001). Constructed wetlands treatment of municipal wastewaters. EPA 625/R-99/010.

Wetzel R.G. (1993): Constructed Wetlands: Scientific Foundations Are Critical. In Constructed Wetlands for Water Quality Improvement,ed. G.A. Moshiri. CRC/Lewis.