Aprovechamiento Energético de los Lodos Obtenidos a Partir del Tratamiento de Aguas Urbanas

Abstract

Human activities signify environmental impact, regardless of its size, location o typology; every resource usage carries a high probability of an alteration of an ecosystem’s equilibrium. This behavior is not an exception to water exploitation and even more significant regarding the results of the water treatment process that are normally different types of sludge depending on the activity it is used. But there is a possibility to change this situation and come up with an energy source instead of having an environmental threat. Water treatment sludge is usually composed of organic matter with an important energetic potential, which is available through a process of anaerobic digestion, where a group of bacteria transform the matter in biogas in an acid environment. This fuel is composed about 60% of methane, better known in nowadays fuel industry as natural gas, applied for domestic or industrial use, or even electric energy generation. In this Masters Final Work, a specific quantitative method was used to estimate the biogas production potential of three different water treatment plants located in Girona, Cataluña, where the fuel production is directly proportional to the water/sludge flow going through it and of course the sludge chemical oxygen demand (oxygen required to oxidize soluble and particle organic matter in water). This potential was measured in volume (m3 of biogas/day) and expressed afterwards as power (kW/day) and energy (kWh/day), which was compared to the complete system’s energy demand to determine if this potential was enough to design a self-sustainable treatment plant using biogas as fuel only. After the mentioned calculations, it was noticed that in none of the three cases the energy produced was enough to cover the system’s demand, forcing it to come up with a viable solution; on this occasion a cogeneration power plant was proposed to use the additional heat produced to change from a mesophilic to a thermophilic bio digestion, which increased the energy production rate about 20%. As the energy was still not enough to reach a self-sustainable system, natural gas fossil fuel was chosen to cover the missing demand because of its low environmental print and low cost. Financially, only two of the three options were viable, with a payback period of 8,5 and 4,5 years depending on the energy costs savings each alternative generated, but it is important to stand out that these two options resulted economically attractive because of the usage of natural gas as a complementary energy source, since in Spain its price is about half the electricity’s, which caused higher energy saving costs and therefore viable solutions