In order to improve the water quality of the lake located at Bosque San Juan de Aragon (BSJA), Mexico City, and considering the site suitability, a system was designed consisting of a conveyance channel, a settler, a fan-shape constructed wetland system (FCWS) on a constructed surface wetland, and a gabion wall. This arrangement prevented the depletion of the existent water mirror, while providing environmental benefits associated with the increase in plant population, the arrival of migratory birds, and the improvement of the landscape environment. In addition, said project will serve as an environmental education site for the forest visitors. The executive project, including soil mechanics studies, basic, conceptual, and detailed engineering, as well as the environmental impact study were developed between September 2010 and March 2011 by the Interdisciplinary Environmental Academic Group (Grupo Académico Interdisciplinario Ambiental, GAIA) of the Faculty of Chemistry of the UNAM. The construction was subjected to tenders for bid between April and June 2011, and the actual construction work took place from September 2011 to June 2012. It was inaugurated in November 2012.
Figure 1. Construction of the FCWS’s external infrastructure: A) Preparation to set the SSFCW foundation B) Foundations for the SSFCW modules C) Building perimeter borders for the SSFCW modules D) Construction of the gabion wall E) Construction of the settlement tank F) Finishing of the SSFCW modules (Photo: Victor Luna)
Figure 2. Final stage of FCWS (Photo: Victor Luna)
The wetland system has a total area 8,379 m2: phosphate removal area 250 m2; subsurface wetland area 1,612 m2; surface wetland area 5,734 m2; stabilization area 168 m2; settler 44 m2, spillway and walkways 739 m2. The water flow is 62.5 m3. Average treatment capacity is 250 m3 per day. Regarding the structure, the cells of the system separately contain different packing media in successive stages, to preferably remove carbon, phosphorus, and nitrogenated compounds. Operation is batch-wise, with vertical and horizontal flow paths.
Untreated water passes through a channel packed with limestone rock (acting as a phosphate removal filter), and then into a settler. Water is then pumped to the vertex of the subsurface wetland, promoting homogenous gravity water flow into all 6 cells of the wetland. At the endpoint, treated water is collected and sent to the farthest point of the surface wetland. This arrangement facilitates the degradation process of carbon compounds as well as nitrification, resulting in treated water that purifies the lake by mixing and diluting with the exiting lake water. For that purpose, a gabion wall that separates the subsurface water from the lake water serves as a separation-purification barrier.
Figure 3. Aerial view of the components of the FCSW (Photo: Victor Luna)
Water quality is expected to provide an effluent with low N and P content, which will gradually reduce the eutrophication level of the lake water. Environmental benefits associated with the increase in plant coverage are expected, as well as a habitat for local and migratory birds; an improved landscape environment, and a site for environmental education, thereby integrating a marginal area of Mexico City.
Figure 4. Migratory birds within of the FCWS (Photo: Victor Luna)
Figure 5. Water flow diagram
Vascular plants of several species both purifying plants, like reeds and Typha, and ornamental plants, like papyrus and field horsetail, as well as aquatic plants, like Typha, jonquil, perymenium and aquatic grass, provide aesthetics according to the site suitability and raw materials exploitable by the handicraft sector or biodegradable paper manufacturing sector.
Figure 6. Cultivation of plants in the FCWS: A, B and C) Growing plants in the SFCW. D) Growing plants in the SSFCW (Photo: Victor Luna)
