Constructed Wetland – Phytoremediation
Category / type: civil wastewater treatment (physical, biological and chemical);
Application: tertiary treatment;
Population equivalent (PE): from 2 up to 100;
Flow: from 1 up to 20 m3 per day;
Normative references and guarantees: local regulations;
Material: gravel, pebbles, plastic sheeting, LDPE, GRP and concrete;
Simple and natural method with good landscaping.
The phytoremediation uses wetlands to clean up contaminated environments. Plants can clean up many types of contaminants including metals, pesticides, and oil. However, they work at best, where contaminant levels are low, because high concentrations may limit plant growth and may need too long time to clean up. Plants also help in preventing wind, rain, and groundwater flow from carrying contaminants away from the site to surrounding areas or deeper underground.
Anthropic treatment wetlands are able to remove various types of pollutants present in wastewater. They are constructed to recreate, as much as possible, features and functions of natural wetlands. They possess a rich microbial community grown in the sediment able to effect the biochemical transformation of pollutants. They are biologically productive and they are self-sustaining. These factors make constructed treatment wetlands a very attractive option for water treatment compared to conventional systems, especially when lifetime operating costs are compared. Constructed wetlands (CW) are also used as treatment facilities for household (blackwater or grey water) and biodegradable municipal or industrial wastewater. Constructed wetlands can be used as a treatment step for Decentralized Wastewater Treatment Systems and they can even be used as a tertiary treatment system after an activated sludge plant or a trickling filter plant to increase effluent quality.
There is horizontal flow (HF), vertical flow (VF), hybrid and free water surface (FWS) constructed wetland. The systems differ in surface and subsurface type.
How It Works
Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilize, and/or destroy contaminants in wastewater and groundwater. There are several different types of phytoremediation mechanisms and several ways in which plants are used to clean up or remediate contaminated sites. To remove pollutants from soil, sediment and water, plants can break down, or degrade organic pollutants or contain and stabilize metal contaminants by acting as filters or traps.
Sizing depends on flow and pollutant concentrations.
There are various types of constructed wetland systems for treating wastewater, based on the type of plants, type of media and flow dynamics.
In horizontal flow system, efficiency depends on horizontal surface; maximum flow depends on vertical surface.
It normally requires 5 up to 10 m2 per PT (*).
The two main types of constructed wetlands are surface flow (SF) and subsurface flow (SSF). In general, SF wetlands require a wider area than SSF wetlands for the same pollution reduction while they are easier and cheaper to design and build. SSF systems are often more efficient but cost significantly more than equivalent SF wetlands; however, recent data may show efficiency is more equitable. Many small treatment wetlands have been implemented for residential wastewater treatment, for which SSF wetlands have several advantages, particularly in regard to the limited exposure pathway of pathogenic microbes and other contaminants. SSF wetlands are suitable for treatment of industrial effluents or other wastewater containing hazardous contaminants due to SSF wetlands’ inherent exposure-limiting characteristic.
The water level in the subsurface horizontal filtration is maintained below the ground surface to ensure a submerged flow and avoid odours problems or accidental contact with wastewater: this level is variable (from 5-15 cm up to even 30 cm). Even the total depth varies depending on the vegetation type and on the radical apparatus length: it can typically be around 60 cm (*).
An important role of treatment efficiency is the oxygen supply. Horizontal filter beds have a very small external oxygen transfer and a smaller inlet compared to a vertical flow constructed wetland. Therefore they require a wider area.
In vertical flow plants (VF), the filter goes through stages of being saturated and unsaturated, by wastewater inlet intermittently dosing.
Vertical flow phytoremediation is much more effective in nutrients removal, in particular nitrogen.
Size parameter referred to the population equivalent can change from 1 up to 4 m2 per PE (*) depending on climatic conditions and water provision.
The system needs a waterproof bottom (clay or geotextile, plastic, CA) to avoid dispersion in soil and groundwater.
Inlet and outlet areas are important for inflow aeration and water distribution in all operating conditions.
Some devices are used as output level control as manhole and adjustable standpipe.
Filling materials are gravel (river gravel to ensure a good porosity), sand and soil. The fine fraction can cause problems of permeability and angular sedimentary particles (such as crushed material) may damage the waterproofing mainly made of plastic.
The system needs pretreatment (and primary treatments) to stop oils, fats, foams and sedimentable solids.
The main parameter for civil wastewater plant (or equivalent) projects is the population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE).
A proper wastewater characterization for incoming organic loads a better estimation of nutrients are possible considering the possibility of dosing additives for biological reactions.
Sizing is normally based on population equivalent and standard organic load (BOD5); it is very important to understand exactly what is the main project parameter for each product and plant. For example, in European countries there are different standard value for inhabitants water consumption.
Operation and maintenance
In general the operation requirements for constructed wetlands are relatively simple. However, they will always require some maintenance for the duration of their life. This aspect is important for economics considerations.
During the startup season, it is important to remove weeds that can compete with the planted wetland vegetation. During time, the gravel will become clogged due to the accumulation of solids and bacterial film. The filter material at the inlet zone will require replacement every 10-15 years. Maintenance activities should focus on ensuring that primary treatment is effective in reducing the solid concentration in the wastewater, before it enters the wetland. Maintenance should also ensure that trees do not grow in the area, preventing the roots can harm the liner.
Odour problems may occur only if the water level will increase more than ground level. This is an indicator for anaerobic conditions. In this case the filter should be rested and the loads must be readjusted. Pre-treatment facilities need to be checked regularly if they work properly and they have to be emptied frequently and sludge must be discharged correctly.
Multiple line plants is always a good design choice, first for maintenance and second for climate changing conditions.
Many national or local regulation define constructed wetland system design and sizing. Often a minimum surface for equivalent inhabitant is required by law.
As international reference can be taken a EPA manual called Manual – Constructed Wetlands Treatment of Municipal Wastewaters – 1999. It recommends the evaluation of permeability and the technical feature. The Tennessee Valley Authority (TVA, 1993) defines some useful parameters for design, too.
(*) – sample parameters, compare according to local regulation and design consideration.
Features and benefits
Research studies have shown that wetland systems have great potential in controlling water pollution from domestic, industrial and non-point source contaminants. It is a widely recognised simple, effective, reliable, economical and natural technology compared to several other conventional systems.
Constructed wetland is a good solution for small civil settlements for domestic wastewater treatment, where there is no sewer service and as a tertiary treatment in medium and large wastewater plants for nutrient removal.
Phytoremediation is a natural system and it does not require power consumption except for small electric pump for vertical flow (VF) type. No chemical reagents are needed. Wetland maintenance is needed for entire plant useful life.
The success of phytoremediation may be seasonal, depending on location. Other climatic factors will also influence its effectiveness.
It requires considerable surface areas. It is much more common in rural areas than in urban areas because of building areas cost.
Branches, cuttings and material from maintenance can be used for a further integrated solution in composting and biogas production.
- low operating costs (wetland);
- common materials (sand, gravel, soil);
- natural process;
- good BOD and SS reduction;
- no odour problems (groundwater flow);
- low or no energy consumption;
- nitrification (vertical);
- minimal environmental impact;
- minimum impact on the landscape.
- occupied areas (footprint);
- pre-treatment required;
- frequent maintenance;
- long startup time;
- careful laying;
- not suitable for cold climatic conditions.