Resumen de Integrating enhance biological phosphorus removal (ebpr) in a resource recovery scenario
Exhaustion of many natural resources that are vital to our species constitutes one of the main challenges that our planet faces, which has lead the scientific community in the pursuit of immediate solutions for this problem. Currently, to counteract this situation, sources that long last were inconceivable, are being looked about, such as wastewater. Wastewater is potentially rich in resources as energy, biopolymers and nutrients; however, these are not leveraged and are lost during treatment, which in a summarized way, focus on the elimination and degradation of pollutants.
Phosphorus (P) is a resource that could be recovered from wastewater. This element has an important role for life, since it is structurally implied in both the cell membrane and the bones. Furthermore, it is essential for agriculture because it is one of the main components of fertilizers. Nowadays, exploitation of this resource has lead to a decrease in the quality of the reserves and according to estimations, it is anticipated that they could run out in the following 50 to 100 years.
Even if it is true that phosphorus is important for the preservation of life, it is also potentially a pollutant agent. Drainage of waters rich in phosphorus in rivers and lakes is the main cause of the phenomenon known as Eutrofization, which produces damage on the quality of these superficial bodies of water. This situation has lead to the creation of treatment processes to remove phosphorus from wastewater; being the chemical precipitation through addition of inorganic salts the most used one. However, this method has many disadvantages as its high cost and the excessive production of sludge. On the other side, the process of biological elimination of phosphorus (EBPR) is gaining popularity and it is being implemented in wastewater treatment plants worldwide (WTP). The EBPR process is based upon the metabolic activity of a group of bacteria called phosphorus accumulating organisms (PAO) to remove this nutrient from wastewater.
Although EBPR process has been implemented successfully at real scale, it does not fit in the perspective of modern WTP, since they are being designed to work as bio-refineries nowadays. This is, systems where components and/or energy are recovered counting on an efficient utilization of resources. That is why research in this thesis has focused mainly in the achievement of more sustainable systems SBR-EBPR, characterized by minimum energy consumption, methane production and phosphorus recovery. With this approach, a SBR-EBPR system was studied in the long term by modifying its configuration in order to obtain an anaerobic supernatant enriched in phosphorus, which can be used in a recovery process as, for example, struvite (chapter 4). The behavior of the EBPR activity using short cell retention times (SRT) was also studied. This operational parameter is discussed in the literature as one of the factors that obstruct the integration of process BEP to systems more energetically efficient (chapters 5 and 6). Lastly, PHA influence was studied in methane production from biomasses obtained in different operational periods of system SBR-EBPR at different SRT (chapter 7).
In chapter 4, the configuration of system SBR-EBPR included one stage for the extraction of supernatant at the end of anaerobic stage (after a period of sedimentation). The extracted liquid has the largest concentration of P in the SBR cycle, which implies that the availability of this nutrient would be limited for the metabolic requirements of the PAO, compromising the efficiency of the process EBPR. For this reason, different extraction volumes were assessed. Results obtained showed that extracting up to 10% of the total volume the system’s phosphorus removal capacity was not affected, achieving removal efficiency even higher than 90 %. Conversely, by extracting 15 % of total volume, the EBPR activity decreased. On the other side, liquids with P concentration between 50-100 mg/L were obtained, which are adequate to be used in processes of P recovery.
Stability of process EBPR was studied in chapters 5 and 6 in order to assess the possibility of its integration to energetically efficient systems. With this purpose, different SBR were operated with a conventional configuration. In chapter 5, three different SBR were operated at 25 °C and at 3-14 days SRT. It was observed that with the 3.6 days SRT the system sustained its EBPR activity. At a three-day SRT, EBPR activity was lost. A four-day SRT is recommended to achieve a good performance of the EBPR process. However, this value is much higher than SRT used in systems energetically efficient (for example, A/B process). In chapter 6, the influence of temperature and SRT on the EBPR process was assessed (in both the short and long terms) using three EBRP systems. Results obtained from these systems demonstrated that the EBPR process was stable at 20 °C and at a five-day SRT. Under these conditions, up to 90% removal efficiency was achieved. Nonetheless, at 10 °C and at a 5-day SRT, EBPR activity was lost progressively. A similar behavior was observed with a 10-day SRT at 20 °C, with an 86 % P removal, but when temperature was reduced to 15 °C the efficiency of removal decreased to 71 % and it was lost at 10 °C. The temperature coefficients obtained from the experiments in the short and the long terms showed that speeds of reaction involved in the EBPR process are highly dependable on temperature at 3.5-day SRT.
In chapter 7, biomasses obtained from systems SBR-EBPR were tested through anaerobic digestion. Since every biomass was obtained at different phases of the EBPR cycle (aerobic and anaerobic), these contained different concentrations of PHA. Results obtained showed that to a lesser SRT, PHA concentration in biomass increased. For example, the content of PHA in the biomass obtained at a 5-days SRT was 18 %, whilst, the PHA content in biomass obtained at a 15-days SRT was 8%. It was also observed that biomasses with a larger content of PHA registered a larger methane production. For example, with an 18 % PHA content biomass, a 401 mLCH4/g VSS methane production was obtained, whilst with an 8% PHA content biomass, a 329 mLCH4/g VSS was obtained.
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