By Steve Hides and Alberto Burgoa
Mexico’s 120 million people are facing increasing shortages of good quality water at the same time as the country is emerging as one of the world’s most dynamic economies. The water table and supply potential of good quality water continues to drop alarmingly, with efforts to recharge the aquifers proving frustratingly difficult and expensive. The recent introduction, however, of measures to conserve, clean up and recycle wet-season rainwater in the major provincial city of Monterrey (the state capital of Nuevo Leon in the arid North East) are providing a cost-effective and simple solution. In addition to demonstrating longer-term advantages for the water cycle, they have also shown immediate social and local environmental benefits made possible through private funding.
Surface water storm events
Monterrey, a rocky basin between 540 and 720 meters (1,771 and 2,362 feet) above sea level with surrounding mountains rising to 1,800 and 3,710 meters (5,905 and 12,171 feet) above sea level, is ranked as the third largest metropolitan area in Mexico. It has an overall dry climate, but rainfall events can be extreme when they occur. Here, the normal wet season is May through October. The metropolitan streets effectively act as stormwater drainage into the River Santa Catarina, which rises in the local mountains, and floods extensively every five to 10 years. The result is that heavy downpours in Monterrey create great volumes of runoff from the associated hard surfaces, including roofs, roads and sidewalks. The high volume of frequent first-flush events after storms with a heavy load of erosion sediment, trash and floatable debris blocks and floods the sewerage systems, which affects the city’s sanitation services. Recent data shows extreme rainfall of 200 mm (7.8 inches) in 24 hours, as occurred on September 20, 2012. Events of such intensity can happen every five years and have been influenced by recent hurricanes. (There were other similar events in 1988, 2005, 2007 and 2010.)
Water and life quality improvements
In one area, a recent project has shown that simple stormwater filtration and retention measures can markedly improve the water quality and control flash flooding. The investment has also initiated welcome social rehabilitation in a run-down quarter. Until the project started, however, certain areas were the focus of many undesirable urban social problems, and the difficulties in this area were increased by frequent flooding, which undermined any efforts to create a more healthy local community.
The locality consists of four adjacent districts adjoining a section of the Tomas Alba Edison Avenue, known as the Prolongación. With around 24,000 inhabitants, this area is at the low point of a catchment basin of 540,000 m2 (133 acres), draining almost 300,000 m3 (10,500,000 cubic feet) of rainwater a year and can flood to around a meter (three feet) deep after high-intensity rainfall. This can happen four or five times in the wet season. Here a major urban avenue, 1,000 meters (0.8 mile), straight four-lane road, runs through a mixed residential and small retail area. Eight islands up to 80 meters (240 feet) long are sited between the roads, with a total area of 1.5 hectares (3.7 acres). These islands are planted with 170 trees, the drought-resistant Monterrey Oak (Mexican White Oak), which should provide an amenity asset with shade and greenery if cared for.
Construction of modular tanks
Planning for improvements for the islands and the surrounding community was initiated in 2012, with the support of a combination of private foundations, local community, municipal and state organizations. The urban and social regeneration of the Edison district, being delivered through grass-roots development and citizen participation, was led by a well-known national convenience-store chain headquartered in the area. The chain gathered together a group of investors and sponsors to form The Trust Poligono Edison. The stormwater solution was supported by FEMSA Foundation as part of its program for Sustainable Development of Water Resources. The ambitious project aimed to provide for recreational facilities for all ages, from children to adults, as well as more general leisure and community amenities. It was hoped that these would encourage family-orientated social interaction, which would improve the community and discourage anti-social elements. It was also important, however, to prevent damage to and destruction of the installations by the regular surface water inundations.
A rainwater management company proposed an integrated water recycling solution with two parts. At its heart was a large water retention facility, large enough to accommodate the frequent volumes of flood water and keep it off the streets. This could then be used for recycling, such as irrigation water for the trees along the islands to encourage their health. By treating the rainwater, the whole project would mitigate the common problems of stormwater runoff, such as eroded stones and sediments, trash and floating objects carried in flash-flood events, as well as hydrocarbons (oil, petrol etc.), which are typical of urban street environments. Sediments have also been shown to carry toxic metals and organic materials with them. Otherwise, the volumes of these undesirable materials could rapidly fill up pipework and tanks, thus nullifying any benefit. The irrigation water would also remain of poor quality and even damage the vegetation.
Robust filtration a key element
To improve water quality, innovative filtration solutions were proposed to ensure recycled water entering the storage would be silt- and pollution-free. An advanced hydrodynamic vortex separator removed gross and fine sediments, floating trash and hydrocarbons before the main retention tank, and an upward flowing media filter unit provided final pollutant removal before storage in the irrigation tank. With no moving parts or power requirements, each of these low-maintenance devices was installed into standard manholes where they were easily accessed. The advanced hydrodynamic vortex separator removed greater than 80 percent of fine particles with a mean particle size of 106 microns, gross pollutants, liquid- and sediment-bound hydrocarbons and over 90 percent of floatable trash. The upward flowing media filter typically removed 80 percent of fine particles as small as 20 microns, as well as high pollutant loads, liquid- and sediment-bound hydrocarbons, sediment-bound heavy metals and organic materials and nutrients.
Successful installation results
From design conception to installation, the project took a little over a year to complete and was located at the lowest local catchment point under two adjacent islands located between the cross streets, Melchor Muzquiz and Calle Miguel Barragan. Here the hydrodynamic vortex separator discharged into a plastic retention tank of 186 m3 (6,569 cubic feet) to store the water and regulate the surface water flow. From the first tank, the water was discharged through a filter into the 62 m3 (2,190 cubic feet) tank and was pumped at a rate of 4 m3/day to the 170 oak trees on the other islands via a drip irrigation system. Once the flood alleviation measures were in place, investment in a children’s playground, table tennis tables, a basketball court and fitness and exercise equipment areas would benefit the residents. To enhance the sustainability features, photovoltaic panels were used to provide power where required. Successful flood protection, in addition to public amenity and leisure activity structures, have encouraged community participation from the 24,000 local residents. Other islands have had seating and tables installed and, in 2013, a weekly Tuesday market began to flourish. The entire project will be handed over to the community Paseo Edison.
Wider application implications
Interest in the success of these water quality and volume control solutions is broad, stimulating new approaches to existing flooding and water problems at municipal, state, central government and international levels. Here, instead of irrigation that is not as critical, the proposal is to use recycled water for street cleaning and building washing, instead of expensive and precious treated mains water. In this way, groundwater stocks would not suffer depletion to the same extent, as the washing water would effectively be recycled many times. The most important opportunity for major cities is to use the captured runoff water for infiltration. In this way, rainwater will recover the natural hydrological cycle as it did prior to urbanization.
Recharging the aquifer
Insufficient expertise and lack of budgets for maintenance are among the reasons cited for the challenges faced by Mexico in replenishing its aquifers. However, municipalities could hire external companies with the right equipment and trained people once the BMP is established and necessary. A further and high potential long-term benefit, which may solve a major infiltration and groundwater regeneration problem, has already been trialled at a new UN- and World Bank-funded eco-development residential project at Tecamac, in the state of Mexico near Mexico City. In this area, housing and infrastructure are being constructed with a low environmental-impact objective.
There is no urban stormwater drainage and, after a cost-feasibility analysis, it was decided to infiltrate the rainwater runoff, generating a positive impact by replenishing the local aquifer.
This requires drilling a 100 meter- (300 feet-) deep bore hole and supplying it from a rainwater-fed concrete tank at 30-40 liters/sec (7-9 gal/sec). The problem is that sediment and trash carried with the surface water rapidly slows the infiltration flow within a few months and blocks the boreholes. On average, a well needs to be repaired or even replaced every other year, depending on site pollution. Additionally, undesirable pollution is also being carried into the groundwater.
Here, the rainwater management company installed two advanced hydrodynamic vortex separators on one borehole, to remove most of the sediment, hydrocarbons and sediment-bound pollutants before they reach the feeder tank. The result has been spectacular. The local drainage team has reported that the tank has continued to empty overnight and the borehole has not yet become clogged after the first rainy season. As well as minimizing pollution entering the groundwater, this solution meant that the cost of the drilling program could be reduced significantly. If the infiltration well lasts five years instead of one, it will produce an 80-percent saving, which easily covers the cost of the vortex separation unit; the borehole could remain viable far longer than that. In this way, willingness by the Mexican authorities to consider new solutions to old problems could greatly improve the ability to combat Mexico’s chronic water shortage.
About the authors
Steve Hides is President of Hydro Americas, based in Portland, ME, USA. After gaining a B.Sc. in geographical sciences, he joined Hydro International in October 1980. Hides transferred to the US from Hydro’s UK affiliate in February 1988 to establish the company’s North America Office to promote its stormwater, wastewater and CSO products.
Alberto Burgoa is General Director of Soluciones Hidropluviales, based in Mexico City. He worked for more than 30 years in the pipe industry and this experience made him realize Mexico requires a different way of managing rainwater. He started Soluciones Hidropluviales in 2011 to develop the rainwater management market.
About the companies
Hydro International provides cost‑effective solutions for controlling the quantity and improving the quality of water with minimal maintenance. After more than 30 years of research and development, many of Hydro’s award-winning solutions in the water, wastewater and stormwater sectors are based on sustainable vortex technologies. Hydro Americas’ Stormwater Division is headquartered in Portland, ME. For further information, email [email protected], visit www.hydro-int.com or call (207) 756-6200.
Soluciones Hidropluviales SA distributes specialized equipment solutions that ensure integrated management of rainwater. The technology ensures the best primary treatment of stormwater runoff for large volumes and catchments, such as projects for industry, shopping centers, residential and municipal developments. To date, the company has implemented rainwater harvesting and management solutions in various states of Mexico.
About the products
The Hydro Downstream Defender® is an advanced hydrodynamic separator that has been proven in temperate and tropical climates to effectively and reliably remove silts, oils and floatables from surface water runoff. Its innovative design delivers high efficiency across a wide range of flows in a small footprint. The Hydro UpFlo™ Filter is an upward flowing filter that provides high-performance, multi-stage treatment combining sedimentation and screening with fluidized bed filtration technology in one compact unit. Easy to maintain and with low installation costs, the unit delivers a high quality of surface water discharge in a much smaller footprint than alternative systems.