Constructed Wetlands for municipal and domestic wastewater
Constructed wetland is an optimal nature-based solution for the decentralised treatment of wastewater generated from small settlements (small town below 2000 PE, scattered houses), with numerous national and international applications in the last 30 years. However, the constructed wetland technology is not affected by an upper limit in terms of treatable PE, if enough area is available. Nowadays, much bigger towns can be served by constructed wetlands thanks to the advancement driven by research activities; for instance, IRIDRA has designed one of the biggest constructed wetland secondary treatment plant in the World, i.e. the plant of Ohrei, which treats up to 20,000 PE.
Advantages 
- high removal efficiencies for organic and nutrient removal as well as pathogens abatement
- simple construction and functioning
- simple maintenance
- CAPEX comparable with compact technological solutions
- OPEX almost negligible in comparison to technological solutions (no or almost negligible energy consumption)
- landscaping
- neither odour issues, mosquito proliferation nor aerosol
- no use of chemical products
- no need of specialised personnel
More information
In Italy, the law n. 152 of 1999 fixed that every Municipality and urban settlement has to be equipped with a wastewater treatment system. Utilization of traditional depuration systems (total oxidation, bio-disks, etc) also for little settlements has never produced real benefits in term of pollutant removal; this because the hydraulic and organic load variations typical of little and medium settlements and touristic centres don't fit well with traditional technologies, but also because the technologic management of those plants is not sustainable for a little municipality, both economically and professionally.
The solution chosen for several years has been the construction of big treatment plants where convoy and treat wastewater of many settlement: but, considered the characteristics of our land, this solution brings to the necessity of realizing long and not cheap pipes. Moreover, the logic of centralization of the depuration treatment strongly alters the balances of water resource, damaging superficial and underground water bodies: in fact, it can happen that the great quantities of water captured for human necessities are given back to watersheds completely different from the withdrawal one, or to the same watershed but strongly downstream, creating the impoverishment of long fluvial drafts.
Constructed wetlands are a modern technology that uses the depurative capacity of natural ecosystems for human activities needs. Its application to wastewater of municipalities and settlements with 500 to 5000 P.E. has given very good results, with management costs 5-6 times lower than those of traditional plants, allowing an on-site wastewater treatment in a decentralized logic of the depuration system. More, it has to be considered the better environmental impact and the greater feasibility of constructed wetlands compared to traditional plants, quality that permits to consider the depuration plant not as something to confine away from urban areas, but as an instrument or environmental restoration.
IRIDRA's experience on CW for small size towns (below 2,000 PE)
CW WWTP of Castelluccio di Norcia (1000 PE), sited in an area of high naturalistic value (Natural Park of Monti Sibillini). Designed by IRIDRA in 2011.
IRIDRA's experience on CW for medium size towns (above 2,000 PE)
Recent IRIDRA's experience suggest that thers isn't any above threshold in terms of maximum PE treatable with constructed wetlands if land is not an issue. Indeed, monitored and stable good perfomances where shown for Dicomanto WWPT (3500 PE - 1 hectar) and Orhei (20,000 PE - 5 hectars). Less footprint can be obtained with aerated CW systems.
CW WWTP of Orhei municipality (20,000 PE - Moldavia), designed by IRIDRA in collaboration with P&P, SWS, Hydea.
Constructed Wetlands vS conventional technological solutions - Investment and O&M costs
Investment costs of constructed wetland (CW) systems are usually comparable with those of conventional technological solutions (e.g. activated sludge) up to the threshold of 2000 PE; above this value, scale effect leads to less investment costs for compact technological solutions. On average, and on the basis of our Italian Experience, investment costs for CW is about 100 €/mq of net surface; this is only an indicative value, since they can vary according to with different water quality standards to be met, type of used CW system, WWTP size, and local material costs. For instance, the investment costs for small settlement can reach up to 150-200 €/mq.
Comparison among investment costs of CW and activated sludge WWPTs on the basis of Italian experience (IRIDRA's analysis published in Masotti, 2009)
Operational and Maintenance (O&M) costs are significantly lower for nature-based solutions in comparison with activated sludge systems, due to the almost negligible costs of consumed energy. Usually, O&M costs of CW for wastewater treatment is on average equal to 14 €/PE per year (even lower if french systems for raw wastewater treatment are adopted, and equal to 7-8 €/PE per year), therefore about 5-10 less than O&M costs for activated sludge systems (50-100 €/PE per year). Moreover, CW WWTPs do not require specialized and permanent personnel to be maintained, reducing, even more, the O&M costs.
Comparison of energy consumption for different conventional and nature-based treatment technologies (Kadlec&Wallace, 2009)
O&M cost comparison between CW and activated sludge WWTPs on the basis of Italian experience (IRIDRA's analysis)
References
IRIDRA's authors are highlighted in bold.
Kadlec R.H., Wallace S.D. (2009), “Treatment wetlands – Second Edition”, Lewis, Boca Raton.
Masotti L. (2011), Depurazione delle acque, Calderini.
Masi, F., Bresciani, R., Martinuzzi, N., Cigarini, G. and Rizzo, A., 2017. Large scale application of French reed beds: municipal wastewater treatment for a 20,000 inhabitants town in Moldova. Water Science and Technology, 76, 1, 68-78, 2017.
Rizzo, A., Bresciani, R., Martinuzzi, N. and Masi, F., 2018. French Reed Bed as a Solution to Minimize the Operational and Maintenance Costs of Wastewater Treatment from a Small Settlement: An Italian Example. Water, 10(2), p.156.
