Porto is the second-largest city in Portugal after Lisbon and is located on the north-west coast of Portugal. Geographically, it is located between 41.1579° N, 8.6291° W, covering an area of about 2400 sq. km and is one of the most important urban areas of the Iberian Peninsula. Aveiro and Viseu districts border Porto to the South, while Brage and Vila Real district border it to the North and East respectively. Porto has an average elevation of about 104 meters, with Monte Tadeu being the highest elevation of 149 meters. With an estimated population of over 215000 in 7 civil parishes (Instituto Nacional de Estatística Portugal), it is recognized as a global city, the only one after Lisbon. Due to the ever-growing population, the demand for water has drastically increased and this is posing a problem, not only in Porto, but in Portugal as a whole.
The temperate oceanic climate with its mild and rainy winters, but pleasantly warm summers, is characteristic of coastal Portugal and more so, Porto. November and March, with an average rainfall of about 1100 mm per year are the wettest months, while June to September are the warmer months of the year. Porto, being a coastal city, is characterized by its low slope along the coast, which then generally increases towards the East, with the highest elevation being 149 m at Monte Tadeu. The Douro River that originates in Spain is the major river of Porto and is situated at the mouth of the river at the Atlantic Ocean. Water scarcity in Portugal is presently a grave issue due to the lack of precipitation and rising temperatures, associated with global warming. This has led to many droughts in Portugal, with the latest and most devasting one occurring in 2005. Hence the construction of a wastewater treatment plant will ensure that clean water can be reused time and time again, be it for domestic use or agriculture and this will help ease the burden of water scarcity.
2. Data collection
The data used for this study are as follows:
Roads Diva GIS
Rivers Diva GIS
Porto administrative boundary Diva GIS
Porto Elevation/DEM SRTM DEM
Points of Interest MapCruzin
3. Data Analysis
The potential sites for the construction of Wastewater Treatment Plants (WWTPs) are established using GIS tools that provide accurate results to help in the study. Tools such as Spatial Analyst which include slope, reclassify, and raster calculator, Conversion such as feature to raster, Analysis tools like buffer and clip and last but not the least, Data Management tools like project are applied to process the data. The processed information is then used to identify the potential sites in the study area. This involved analysis of several criteria such as digital elevation model (DEM), points of interest like schools, monuments, parks and educational institutions to name a few, water bodies such as rivers and streams and roads. The suitable sites for this study are identified using a suitable set of criteria which are as follows:
• Atleast 500 meters away from roads, as a wastewater treatment plant close to roads can affect public health.
• At least 500 meters away from water bodies as the sewage effluents should not seep into them and thus contaminate them.
• 500 meters away from thickly habituated/populated areas and areas of interest like monuments, parks, institutes etc.so that the odor does not affect the normal working in such areas.
• Slope of the surface should be less than 20°, in order to minimize pumping costs associated with the treatment plant.
The following methods are employed to produce the outcomes described above:
1. Download the data from the relevant sites mentioned in Data Collection under Methodology.
2. This data is then added to the map, as different layers. Select Porto administrative region from the administrative boundaries layer of Portugal and export its as a new layer. This is carried out as Porto is the area of interest for the study. Once this is done, add the other layers to the map, such as roads, water bodies and point of interest.
3. Create multiple ring buffers of 500m, 1000m and 1500m around the roads, water bodies and points of interest layers. This is done using the Geoprocessing>Spatial Analyst Tools>Multiple Ring Buffer. These buffers are chosen as they will either allow inclusion or exclusion into the potential sites.
4. Since the above layers contain information regarding Portugal and not specifically Porto, they need to be combined to the Porto layer. This is done using the Geoprocessing>Overlay>Union and then clipped to Porto using the Spatial Analyst Tools>Clip.
5. These newly created layers/features are then reclassified into 3 categories, “Highly suitable”, “Suitable” and “Unsuitable”, with values of 1,2, and 3 respectively. This is done to create raster datasets that will be used to calculate the potential sites.
6. The DEM for Portugal is obtained and joined to the Porto layer. Since it is a rater data, the Extract by Mask tool is used to join it to the study area. As the criteria requires the data to been in slope (degrees), the Slope tool is used to calculate the slope of Porto from the raster dataset. The Reclassify tool is then used to reclassify the slope into 2 categories, “Suitable”, with slopes less than 20° and “Unsuitable”, with slopes greater than 20°
7. The last step in the analysis involves calculating the potential sites for the Wastewater Treatment Plants (WWTPs). This is done using the Raster Calculator tool. Here, all the 4 reclassified layers (roads, water bodies, points of interest and slope) are added and the final output of potential sites is generated.