Novel technologies for wwtp optimization in footprint, nutrients valorization, and energy consumption

Zusammenfassung

The continuous development of the human society causes stress to natural systems: pollution increase, deforestation, global resource depletion, etc. Water resource contamination and water scarcity are among the major challenges to be confronted by humanity in the 21st century. Present wastewater treatment technologies were developed between the end of the 19th century and during 20th century. Technology evolved from simple aqueduct sewerages that dumped in waterways without treatment to a complete treatment; including physical, chemical and biological treatments, aimed to remove solids, organic matter and main nutrients. Nowadays, novel concepts are taken into account in order to face up to new environmental, economic and social limitations. At the same time, these new concepts have to deal with aspects like the population growth, the global climate change, the water scarcity, etc. Up to now, wastewater treatment plants (WWTPs) can be defined as systems where organic matter, nitrogen and phosphorous are removed from wastewater using energy. However, this concept should be changed in the next years in order to improve these systems in terms of economic feasibility and environmental impact. In this way, wastewater must be seen as a renewable and recoverable source of energy, resources and water. This is feasible by means of the application of new treatment systems and technologies. In addition, the new wastewater treatments should require smaller implantation areas and produce lower amounts of sub-products, as greenhouse gases or excess sludge, compared to conventional ones. According to this modern approach, the present thesis has been focused on the study of some alternatives to improve the WWTPs by the application of new technologies. In this way, the main improvements expected from this thesis are related to: - The reduction in the sludge production in the biological processes used to remove organic matter and nitrogen from the wastewater. - The improvement of the settleability of the biomass in the biological reactors. - The treatment of wastewater in order to facilitate its application as fertilizer and the recovery of nutrients from the wastewater. - The decrease of energy requirements for the wastewater treatment to achieve the energy self-sufficiency. In this way, in Chapter 3, the aerobic granulation technology was applied to the treatment of swine slurry. This technology allows reducing the excess sludge production in the WWTPs and the requirements of area needed for its implantation by the formation of granular biomass with good settling properties. Aerobic granulation was mainly obtained in Sequencing Batch Reactors (SBRs) as it was the case of the pilot plant scale reactor used in Chapter 3. Nevertheless, in Chapter 4, the possibility of obtaining this kind of good settling biomass in a continuous reactor was researched. In Chapter 5, ammonia sulphate and struvite, nutrient rich minerals, were recovered from the sludge liquid fraction and separately collected urine. This combination allowed the recovery of nitrogen and phosphorus from wastewater streams. Finally, in Chapter 6, the applicability of the Anammox (ANaerobic AMMonium OXidation) process for the nitrogen removal from the water line of WWTPs was researched, with the aim of increasing the energy self-sufficiency of the wastewater treatment. The main contents and the specific objectives corresponding to each chapter of the present thesis will be described in more detail in the following paragraphs. In Chapter 1, an overview of the state of the art of the future challenges related to the wastewater treatment, and some of the novel technologies and new solutions which have emerged in the last years, are presented. Special attention is paid to the new technologies applied to nutrients removal and recovery, and aerobic granular biomass, which are studied in the present thesis. An actualized literature review about the performed studies up to date in these fields is presented. In Chapter 2, a description of the analytical methodologies applied to determine the conventional parameters used in the thesis for the wastewater and biomass characterization is provided. Some usual parameters like the pH, dissolved oxygen (DO), chemical oxygen demand (COD) or solids concentration were measured following the instructions of the Standard Methods. The Fluorescent in situ Hybridization (FISH) technique, applied to the identification of the microbial populations involved in the biological processes, is briefly described. Other analyses were performed after being adapted to the research of this thesis, like the characterization of aerobic granular biomass by Sludge Volume Index (SVI), the determination of the biomass density of the aerobic granules, the application of the digital image analysis to determine the morphology and the average diameter of the granules or the measurement of their Poly-Hydroxy-Alkanoates (PHA) concentration. The calculations made to determine parameters which allowed the analysis of the obtained results used through the thesis are also presented in this chapter. The specific analytical methods and calculations used in a single part of the work, and the different experimental setups are described in the corresponding chapter. With respect to the aerobic granulation, most of the studies concerning the physical properties and performance of aerobic granular biomass were carried out in laboratory scale SBRs. These reactors are operated in sequential cycles that comprise: filling, reaction, settling and withdrawal phases. The cycles are characterized by the short length of the filling and settling phases, aiming to the development of biomass in the shape of granules in aerobic conditions. However, a relevant number of studies performed in pilot or full scale systems are not available yet. Furthermore, when working with industrial wastewater, the fluctuations of inlet feeding characteristics can difficult the achievement of stable operation conditions. Thus, in Chapter 3, the startup and performance of a granular aerobic pilot plant scale SBR reactor, with a working volume of 100 L and treating the wastewater from a pig farm was studied. Since the availability to withstand shock loadings is stated as one of the characteristics of the aerobic granular biomass, the objective of this work was to test the effect of this high variability in the feeding composition on the performance of a pilot-scale granular SBR treating swine slurry. This information is indispensable in order to carry out the scale up and application of these systems to industrial level, since this is the last purpose of the design of new technologies. The pig slurry used in this research was characterized by its highly variable composition, in terms of organic matter and nitrogen content, and C/N ratio. This variability was further increased by the dilution with tap water. In a first stage, the performance of the pilot plant during the starting up and the process of granular biomass formation were studied. In a later stage, the reactor and granular biomass capacities of withstanding the variable fed conditions were researched. Granulation process was achieved in the reactor, and the physical properties of the biomass remained rather stable during the operational period of more than 300 days. The granular biomass obtained in the reactor showed excellent settling properties, with the sludge volume index values varying between 27 and 60 mL/g TSS. The good settling properties facilitate the granular biomass separation from the effluent and its retaining inside the reactor. The first aerobic granules were obtained after 9 days of operation, and their average size during the operation time varied between 2.0 and 2.8 mm. The reactor showed a good biomass retention capacity to select for granular biomass. In this way, solids concentrations from 5 to 12 g VSS/L were achieved. However, its efficiency to retain the solids contained in the fed pig slurry was low and as a consequence, the solids concentration in the effluent of the reactor was similar to that measured in the influent. Organic matter removal efficiencies were not affected by the fluctuations of the applied organic loading rates (OLR) but by the non-biodegradable fraction of the swine slurry. For this reason, the average organic matter removal efficiencies never reached values higher than 80%. Ammonia load was mainly oxidized to nitrite and, in this way, the ammonia removal efficiency was around 76%, even with the high variability of the wastewater fed to the reactor. However, denitrification was practically not observed during the experimental period. In order to evaluate the performance of the reactor, cycle measurements were carried out on different days of operation. The biodegradable organic matter was easily removed in the first minutes of the aeration phase in all cycles, during the so called feast period. During this time, a reduction in the DO concentration due to the quick organic matter oxidation was measured. Once the biodegradable organic matter was consumed, the DO concentration in the bulk liquid started to increase (famine period), while the fraction of non biodegradable COD remained unaltered. When the organic matter was depleted, nitrate and nitrite concentrations increased in the bulk liquid, but total nitrogen concentration remained almost constant. This observation suggests that simultaneous nitrification-denitrification processes did not occur during aeration phase. Concentrations of PHAs measured in the biomass confirmed that bacteria stored organic matter in the form of these compounds during feast period and consumed them during first minutes of the famine period. FISH technique was applied for the characterization of the obtained biomass in order to follow the evolution of the microbial populations during the granulation process, and during the operation of the reactor once the granules were formed. The detected bacterial populations indicated an evolution from those present in the inoculated sludge to those composing the aerobic granules. Filamentous organisms were present primarily in the inoculums, while they were washed out from the system as aerobic granules developed. The nitrifying microbial population was mainly composed by members of Nitrosomonas spp. as ammonia oxidizing bacteria, and a smaller amount of nitrite oxidizing bacteria belonging to the phylum Nitrospirae, in correspondence with the nitrite accumulation observed in the reactor. The application of the concepts of the granular SBR technology for the upgrading of existing WWTPs could be limited by the different required operational conditions and the geometry of both column-type SBR and conventional activated sludge reactors. Transforming a continuous system into an SBR suitable to obtain aerobic granules is sometimes difficult. Thus, the objective of the Chapter 4 was to define the appropriate operational conditions to develop granular biomass in continuous stirred tank reactors (CSTR) with geometry similar to the conventional activated sludge reactors used in the WWTPs, which height to diameter ratio is usually around 1. The production of aerobic granules in a continuous reactor would open a new perspective to the application of this technology for the improvement of already existing WWTPs. In order to achieve the aerobic granular biomass development, a biomass selection system, based on the reactor hydrodynamics and progressive shortening of the hydraulic retention time (HRT), was utilized in the reactor. This device consisted of a tube semi-submerged in the liquid media through which the effluent of the reactor was discharged. In this tube, an upflow velocity of around 10 m/h was fixed, and consequently, particles with a settling velocity smaller than this fixed upflow velocity were washed out from the reactor, while the solids with good settling properties were retained. Hydraulic retention times of 6, 3 and 1 hour were tested in reactors of 3 and 6 L. Aerobic granular-like biomass was formed in the reactor when an HRT of 1 hour and a hydraulic pressure of 10 m/h of settling velocity in the effluent discharge tube were applied. On the other hand, floccular biomass accumulated in the reactor when an HRT of 3 and 6 hours were used. The granules presented an average diameter as high as 7 mm and a large settling velocity of around 36-48 m/h. These values were remarkably higher than the typical values of settleability of activated sludge and were comparable to those from aerobic granules formed in SBRs. However, the SVI and density values were worse compared to those corresponding to the aerobic granules formed in SBRs. During the formation of the aggregates, the SVI10 value decreased gradually from 700 mL/g TSS to 127 mL/g TSS, while SVI corresponding to aerobic granules cultivated in SBR reactors can vary around 30-40 mL/g TSS. Biomass density of the aggregates in the continuous reactor varied between 7 and 11 g VSS/Lgranule. These values are relatively low compared to that of 43.5 g VSS/Lgranule corresponding to granules formed in SBRs. The biomass produced in this continuous system will be more easily separated from the liquid phase, reducing the need of big settlers, and facilitating the management and disposal of the sludge produced during wastewater treatment. Regarding organic matter and nitrogen removals, OLRs as high as 4.8-6.0 g COD/L¿d were treated with removal percentages of around 60%, due to the presence of a non-biodegradable COD fraction. Nitrogen removal varied between 10 and 15%, and it can be attributed completely to biomass growth, while nitrification and denitrification processes did not occur in the reactor. To our knowledge, this aerobic granular-like biomass was the first reported to have been formed in a continuous CSTR reactor with geometry similar to the activated sludge reactors, with a height to diameter ratio around 1. The objectives of Chapter 5 were the evaluation of the pretreatment of the separately collected urine and its further co-treatment in an air stripping system. In this air stripping reactor, the urine was mixed with the sludge liquid from an anaerobic digester. The experiments performed in this work provided preliminary results, and more experiments are needed in order to test the viability of the system, but first results were promising. In this chapter, a novel ammonia stripping system was used to treat the supernatant from an anaerobic digester. This liquid stream in the WWTPs is characterized by a relatively low flow and a high concentration of nitrogen. This system was operated at full scale in a municipal WWTP, combined to a previous CO2 pre-stripper and to a subsequent ammonia sorber unit. With this combination, the ammonia was recovered as an ammonia sulphate solution at the end of the treatment. This nitrogen rich solution was sold to local farmers who used it as fertilizer. In this way, a waste product was turned into a valuable product. With respect to the system operation, the presence of the CO2 pre-stripper, which reduced the consumption of chemicals in the overall system, increased the pH of the liquid and facilitated the deprotonation of the ammonia. Consequently, the CO2 pre-stripper increased the nitrogen recovery in the sorber unit, while allowed reducing 50% of the NaOH consumption. The efficiency of this system was further increased when pre-treated urine was added to the supernatant liquid. Urine was collected separately by means of No-mix toilets and dry urinals. Then, it was pre-treated in a reactor in order to remove and recover the phosphorus which was present in high concentrations in the urine. Struvite, a phosphorus rich mineral used as fertilizer, precipitated in the reactor after magnesium oxide was added to the collected urine. Struvite crystals with an average size of 42 to 80 µm were formed in the reactor. The first experiments showed the feasibility of the combined treatment system of supernatant liquid and urine in the ammonia stripping reactor including a CO2 pre-stripper. In this way, an increase of 10% in the liquid flux by the addition of the urine represented a 40% increase of the ammonia concentration in the inlet of the stripping unit. Even if the efficiency of the nitrogen removal was lower than the values reached during the optimization and simulations performed during the operation without urine addition, the achievement of these percentages generated a proportional increase in the fertilizer production. The fertilizer production rise partially covered the chemical and operational costs of the ammonia stripping system. In addition, operational problems due to the treated urine addition were not reported by the WWTP operators; however, the length of the experiments was too short to discard this possibility. In Chapter 6, the autotrophic nitrogen removal at low temperatures (15-20 ºC) was performed using a Completely Autotrophic Nitrogen Removal Over Nitrite (CANON) system with granular biomass. This system was operated to research its application to remove nitrogen from the water line of the WWTPs. The CANON system is the combination of the partial autotrophic nitritation, oxidation of half of the present ammonium to nitrite by the Ammonia Oxidizing Bacteria (AOB), and the Anammox process, where ammonia and nitrite are combined to produce nitrogen gas under oxygen limiting conditions. The Anammox processes have successfully been applied to rich ammonia streams at temperatures above 25 ºC, as it is the case of the supernatant from anaerobic sludge digesters. However, little information about the application of Anammox process at low temperature and nitrogen concentration, like it is the case of the main stream of the WWTP, is available. As advantages, the CANON system presents the fact that it requires less aeration energy and no organic carbon source for the conversion to nitrogen gas, compared to the conventional nitrification/denitrification processes. Furthermore, larger amounts of organic matter are available for methane production, which increase the energy recovery in the WWTPs, as the organic matter is not needed for heterotrophic denitrification. The use of slow growing autotrophic microorganisms, as it is the case of the Anammox bacteria, implies the use of reactor systems characterized by high biomass retention capacities. In this case, the biomass retention was enhanced through the use of biomass in the form of granules grown in an SBR system. AOB bacteria developed in the outer layers of the granules where oxygen and ammonia were present. On the other hand, Anammox bacteria grown in deeper layers, where oxygen was depleted and ammonia and nitrite were available. The CANON system was initially operated at 20 ºC and fed with moderate concentrations of ammonia (150-250 mg NH4+-N/L). Later the temperature of operation was decreased to 15 ºC. Finally, an influent containing low ammonia concentrations (50-80 mg NH4+-N/L) was applied in order to simulate the conditions of the water line of a municipal WWTP where the organic matter was previously removed. At 20 ºC, the biomass retention in the SBR was satisfactorily achieved, and solids concentrations of more than 12 g VSS/L were obtained. Nitrogen removal rate values of 0.45 g N/L¿d were achieved, reaching a 70% of nitrogen removal efficiency. However, when the process was operated at 15 ºC the low biomass growth and the difficulties to control the low dissolved oxygen concentrations in the bulk liquid implied the need of testing different reactor configurations. In addition, the low oxygen concentration, temperature and nitrogen load facilitated the development of a third microbial group, the nitrite oxidizing bacteria (NOB). The NOB compete with Anammox for the nitrite and with the AOB for the oxygen. The overcoming of these undesired microorganisms was a challenge in the last part of this research. In order to solve these drawbacks, the inclusion of mechanical stirring, the use of a reactor with a different height to diameter ratio, and variations in the settling time were tested. Two laboratory scale SBRs were used during the experimentation with a working volume of 1.5 L and 4.0 L, respectively. Three main factors should be pointed out as crucial to control the performance of the AOB and Anammox bacteria in a CANON system operated at low temperature and nitrogen load: 1) to achieve a high biomass retention; 2) to achieve an equilibrium between the AOB and Anammox activities and 3) to avoid the NOB development in the biomass. In a WWTP, the conventional activated sludge reactor could be substituted by a continuous aerobic granular reactor, where the organic matter would be removed, while the nitrogen would be removed in the Anammox based process. The different alternatives of treatment research in this thesis are expected to lead to more efficient and sustainable wastewater treatment systems.