Constructed wetlands have been used in the last 20-30 years in the treatment of domestic and urban waste, referred to as (CW). CW are engineered systems designed and constructed to emulate the self-purification processes of the natural wetlands, but in comparison with natural systems, The constructed wetlands are characterized with a greater degree of control, leading to a more precise and accurate measure of system effectiveness, that depends on the bed nature , type of vegetation and the hydraulic routes.
The free flow systems illustrate the best match to the natural process; but the most used water purification systems are the submerged flow plants (horizontal or vertical), that resemble bio-digesters where complex chemical, physical and biological purification processes occur, that transform and degrade the pollutants contained in the waste water, and thus lowering its negative environmental impact.
The CWs, together with many other types of filtration, are classified as purification systems called “attached growth systems” to distinguish them from traditional purification systems known as suspended growth in which the bacterial biomass is kept in suspension by mixing and aeration systems. On the other hand, in artificial wetland systems, bacteria is responsible for the degradation processes of pollutants populate the surfaces around the fills used to filter the water, forming what is called “bacterial biofilm”.
Contrary to what the Italian name “phytodepuration” may suggest, the systems do not directly contribute to water purification as they only assimilate a small percentage of the pollutants contained in the wastewater; however, they are fundamental in maintaining the hydraulic conductivity of the system (that is, ensuring not clogging of the filter over the years), as “catalysts” of many purification processes and in transferring oxygen to the filtering layers without any electrical consumption but naturally. In the last 15-20 years new techniques have been introduced (referred to as Constructed wetlands 2.0, that are an upgrade from the first techniques in terms of yields and spaces): for example French systems, eliminate the need for a primary treatment system that transform the sludge into an excellent organic soil conditioner; or aerated constructed wetland systems (Forced Bed Aeration ™), introduced and patented by S.D. Wallace and for which IRIDRA is the sole and exclusive provider in Italy. In this system, air (and therefore the fundamental oxygen for oxidative processes) is forced to circulate inside the filtering medium, increasing the purification yields and significantly reducing the necessary area used for the wetland, while keeping electrical consumption maintained compared to traditional systems.
Constructed wetlands VS Centralized technological purification plants – General
Constructed wetlands are one of the most widely applied techniques globally for domestic wastewater purification, in comparison to centralized wastewater system (that is, a number of sewers that collect and convey wastewater generated from an urban area (service area) to a single treatment plant, distant from the area of wastewater generation) that lack a reliable assessment of its environmental impacts, has resulted in serious disparities and imbalances in the water cycle: the relocation of water from a certain aquifer or water body to another basin.
The Centralized wastewater treatment plants if, in large urban areas are an almost obligatory solution, even though they impose a significant impact on the territory and the environment, due to the high energy consumption, the high production of sludge and the use of chemicals. the effluent water quality may not be safe (according to the standards and despite the high degree of purification achieved), as the huge volumes of purified wastewater (as a consequence of the centralization of purification) always contribute a considerable amount of pollutants to the disposal water streams, and are more impactful in the summer when it is a low water season .
The Decentralized constructed wetlands approach , that is, implemented for small isolated plants, that are distant from the sewage networks, in many cases can be convenient both in economic terms (As the big plants have an added cost per cubic meter of purified water reduced due to the economies of scale, the sewers cost and management) and environmental. In many cases then (for isolated fractions, scattered houses, mountain shelters, etc.) it is almost impossible to make sewage connections to centralized purifiers. The traditional plants (activated sludge) are not suitable for small-medium centers because they are sensitive to changes in hydraulic loading and daily and seasonal pollutants, additionally they require constant and continuous management by specialized personnel; While, CW represents a valid alternative to low-cost investment and management, load fluctuation tolerance, and management by non-specialized personnel in a non-continuous manner. Principles that are also required by the current water law (Legislative Decree 152/06) which indicates that constructed wetland systems are the most appropriate solution for discharges coming from groups below 2000 people, for discharges characterized by a fluctuation of the influent load, as well as tertiary treatment systems for larger users.
Constructed Wetlands VS Centralized technological purification plants – Cost of construction and management
The initial investment costs of CWs are generally comparable with conventional techniques (activated sludge plants) up to about 2000 P.E and, after which the economies of scale and the impact of the land cost can have an effect on activated sludge systems. On average, It costs around € 100 / m2 of floor space to be considered; costs may vary depending on the type, the purification objective, and on the size of the plant (for very small plants it is more reasonable to weigh at 150-200 € / m2, and depending on the cost of supply and transport of the filling material.