Aerobic granular biomass systemsA challenge for a better wastewater treatment and sludge management

Resumen

The development of the consumer society is associated to an increase of pollution generation. One of the most important effects is the production of wastewater as a consequence of the human activity (agriculture, industry, business, households, etc.). The discharge of this wastewater without treatment into water bodies can provoke the disappearance of life forms, the loss of the biodiversity, the distortion of water systems, the over-fertilisation and affect the drinking water supplies, which can produce problems in the public health. The treatment of the produced wastewater is therefore necessary to reduce these drawbacks and guarantee its adequate disposal into the environment. However as a consequence of the wastewater treatment another residue is generated, ¿sludge¿, that also needs to be managed. The biological wastewater treatment is normally accomplished by Conventional Activated Sludge (CAS) systems, which generally require large surface areas for implantation and the presence of biomass separation units due to the poor settling properties of the sludge. Systems based on aerobic granular biomass are an alternative to conventional ones because of their smaller footprint compared to that of the CAS ones. This is due to the fact that the reactor design (large height/diameter ratio) and the properties of the biomass (good settling properties) make unnecessary the construction of secondary settlers. The high concentration of biomass achieved with these systems make it possible to treat large organic and nitrogen loads, which contributes to reduce the necessary volume of the reactor. Furthermore the aggregation of the biomass in granules allows the development of aerobic, anoxic and anaerobic conditions where microbial populations are able to perform different biological processes. This means that in a single unit the simultaneous carbon, nitrogen and phosphorus removal is feasible. Another advantage of the aerobic granular technology respect to the CAS one is the low sludge production of the first one. The management of the sludge produced as a consequence of the wastewater treatment processes accounts for up to 50% of the operational costs of a Wastewater Treatment Plant (WWTP). Furthermore the European regulations applicable to this sludge management are becoming more stringent. Therefore nowadays new technologies are under development in order to, first, reduce the sludge production and, second, improve its further treatment. At this point the aerobic granular biomass based systems are one of the new technologies proposed as substitutes of the CAS ones due to their compact configuration. The first works that reported the formation of aerobic granular biomass date from the 90¿s. So far research has been focused in the improvement of the different aspects related to the full scale application of this technology. It is known that a number of factors affect the granulation process, being one of them the type of substrate. Up to date, the results from several research works seem to indicate that the formation of aerobic granules is possible treating different substrates but evidences show that the microbial diversity and the structure of mature granules are closely related to them. The development of aerobic granular biomass has been studied treating different synthetic mediums and industrial wastewaters, which indicates that it is possible to grow aerobic granules with complex substrates. This is of interest for the industries with implantation surface limitations for the WWTP installation. This limitation is more evident in the case of those industries located near the coast, which are normally dedicated to the processing of seafood (canning, aquaculture, etc.). In this sense in Chapter 3 the development of aerobic granular biomass with the effluent from this type of industry has been studied, paying special attention to the characteristics of the formed aerobic granules and to the achieved efficiencies of organic matter and nitrogen removal. The stability of the aerobic granular biomass to sudden changes of the applied load was also studied, since the effluents from the seafood industry are characterized by a high variability in composition. The influence of different specific compounds on the process, not only the determination of the usefulness of the technology applied to a specific effluent, also is important. The effects of the presence of compounds like phenol, pyridine, heavy metals and dyes have been studied on aerobic granular systems. However the effects of the presence of coagulant-flocculant reagents, commonly used in the WWTPs as pre-treatment for solids separation before the biological processes, have not been studied so far. As the industrial effluent used in Chapter 3 was produced in a pre-treatment unit where coagulant and flocculant reagents were added, the obtained results seem to indicate that the residual concentration of these compounds could affect the properties of the aerobic granular biomass. For this reason in Chapter 4 the effects of these compounds on aerobic granular systems were tested using as effluent a synthetic medium to avoid the interference of another complex substances present in the seafood industrial effluent. The aerobic granular systems are operated in Sequencing Batch Reactors (SBRs) with a short initial feeding phase followed by an aerobic reaction phase that allows submitting the biomass to feast-famine regimes, where the feast period is short and the famine one long. These conditions are suitable to select the appropriate microorganisms to obtain compact and dense granules. However an excessively long famine period may not be necessary and lead to an extra energy consumption and low reactor capacity. Furthermore the nitrogen removal efficiency is limited by the denitrification process due to the high dissolved oxygen (DO) concentration obtained along the famine phase. Therefore in Chapter 5 other cycle configurations were tested in order to establish the length and the distribution of the operational cycle which allow maximizing the treatment capacity and the nitrogen removal efficiency, respectively. Although the production of sludge with the aerobic granular systems, where organic matter and nitrogen are simultaneously removed, is lower than with the CAS process, its treatment is still necessary. However, nowadays, no much research about its possible post treatment is available. This lack of information is due to the fact that the aerobic granular systems are at the moment under study at pilot scale. In order to scale up the process, and previously to the industrial application, the study of the potential treatability of this type of sludge is of great interest. The anaerobic digestion is widely used for the treatment of the sewage sludge generated in WWTPs because it allows stabilizing its organic content reducing the solids and producing biogas that can be recovery as energy. However the anaerobic degradation of the excess sludge produced in the CAS process (Waste Activated Sludge, WAS) is normally limited by the hydrolysis step rate and the maximum achievable percentage values are between 30-50%. To enhance the anaerobic digestion of the WAS a pre-treatment can be applied (thermal, mechanical, chemical or a combination of them), which leads to a destruction of the sludge microbial matrix (flocs) to improve the hydrolysis rate. Since the sludge produced in the aerobic granular systems has a similar nature to the WAS (coming from a biological treatment) the use of the anaerobic digestion can be also an interesting solution to treat it, as well as the application of a pre-treatment before the anaerobic step, which could contribute to mitigate the possible effect of the state of aggregation of the biomass in granules. Therefore in Chapters 6 and 7 the objective was to study the feasibility of the anaerobic digestion to treat the Aerobic Granular Sludge (AGS), first performing batch experiments (Chapter 6) and then with the operation of a continuous anaerobic digester at laboratory scale (Chapter 7). The application of a thermal pre-treatment to improve the anaerobic digestion of AGS also was studied in both chapters. In this context the present thesis is framed in the field of wastewater treatment by the application of the aerobic granular technology (Chapters 3, 4 and 5) and the posterior treatment of the granular sludge generated by anaerobic digestion (Chapters 6 and 7). The main contents of each of the chapters comprising this thesis and the achieved objectives are detailed in the following sections: In Chapter 1 an overview about the wastewater treatment processes is provided, focused on the CAS and the bottlenecks associated to the application of this old technology (high surface requirements, low load capacity and large amounts of sludge production with poor settling properties). The aerobic granular systems are presented as a possible alternative to solve part of these drawbacks. A review of the factors affecting the formation of aerobic granular biomass and the applications of this technology are presented. Finally the basic aspects of the sludge anaerobic digestion are described with special attention paid to the microbiology of the process and the different available pre-treatment methods. In Chapter 2 the materials and methods used in the thesis are described. They comprised the analysis of the liquid, gas and solid phases. Some conventional parameters like the pH, dissolved oxygen (DO), chemical oxygen demand (COD), biogas composition or solids concentration were measured following the instructions of the Standard Methods. Another analysis were performed after being adapted to the research of this thesis, like the characterization of aerobic granular biomass by Sludge Volume Index (SVI), determination of the granules density, the application of the digital image analysis to determine the morphology and the average granule diameter and the measurement of the Poly-Hydroxy-Alkanoates (PHA) concentration. The Fluorescent in situ Hybridization (FISH) technique, applied to the identification of the microbial populations involved in the biological processes, especially in the anaerobic digestion, is also described. In Chapter 3 the start-up and operation of an aerobic granular SBR treating the industrial effluent from a seafood processing plant were studied. This industrial effluent was characterized by a high variable composition due to the different products processed in the plant (squid, prawn, hake, etc.). Furthermore this effluent came from a previous physical-chemical pre-treatment where coagulant and flocculant reagents were added. The operation of the aerobic granular SBR was divided in two stages. In the first one (days 0-295) the development of the aerobic granules was promoted using applied Organic Loading Rates (OLRs) between 2 and 4 kg CODS/m3¿d. The granulation process took place after 130 days of operation and the developed aerobic granular biomass exhibited good settling properties, with values of the SVI of 35 mL/g TSS and the density of 60 g VSS/Lgranule around day 170 of operation. The granules presented an average diameter between 2-3 mm, however the presence of high residual concentrations of coagulant-flocculant reagents might be the responsible for the increase in the average granule diameter to 11.0 mm occurred in few days, which led to the disintegration of the aerobic granules. The efficiency of organic matter removal was of 90% and, although the conditions for nitrogen removal were not optimized, percentages of 65% and 30% for ammonia and total nitrogen removal, respectively, were achieved. In the second stage (days 296-330) the stability of the aerobic granular biomass when variable OLRs were applied (from 3 to 13 kg CODS/m3¿d) was tested in order to approximate the operation of the SBR to the real conditions in the industry. The stability tests with variable OLRs showed that the organic matter removal efficiency was not affected but the granules physical properties and the nitrogen removal efficiency experienced a detrimental effect. In Chapter 4 two aerobic granular SBRs were operated in order to know the effect of the coagulant-flocculant reagents on the formation and characteristics of aerobic granules, since the results obtained in Chapter 3 seem to indicate that these compounds, frequently used in the primary treatment, can affect them. Both SBRs were fed with a synthetic medium and one of them was supplemented with a residual amount of coagulant-flocculant reagents (CF), while the other served as control. The principal observed effect was a worse biomass retention capacity of the reactor fed with CF compared to the control one, which implied lower solids concentration (4.5 vs. 7.9 g VSS/L) and higher SVI (70 vs. 40 mL/g TSS). The granules obtained in the CF reactor presented also a higher average diameter (5.0 vs. 2.3 mm). Another observation was that in the CF reactor the maximum oxygen consumption rate decreased respect to the control but the removal efficiencies of organic matter (90%) and nitrogen (60%) were not affected. In Chapter 5 the distribution of the operational cycle of the aerobic granular SBR was changed respect to the distribution used in Chapters 3 and 4 in order to improve the treatment capacity and the nitrogen removal. In a first experiment the mature aerobic granular biomass of the control reactor of Chapter 4 was submitted to a progressive decrease in the famine/feast ratio by decreasing the length of the aerobic phase. The aim of this cycle change was to increase the treatment capacity of the system. The results obtained showed that to reduce the famine/feast ratio from 10 to 5 was possible increasing the treated OLR and Nitrogen Loading Rate (NLR) in the system around 33%. The removal efficiencies of organic matter (97%) and nitrogen (64%) were not affected while a slight detriment of the granules characteristics was produced. In a second experiment mature aerobic granular biomass was taken from a pilot plant (100 L) treating pig manure and used to inoculate two SBRs at laboratory scale. In these SBRs an initial anoxic phase, previous to the aerobic one, was implemented in order to improve the nitrogen removal efficiency respect to the operation of the pilot plant (with only an aerobic phase). The results obtained showed that an anoxic phase of 30 min previous to the aerobic one with a pulse-fed mode increased the removal efficiency of the nitrogen in the pig manure from 20 to 60%. However the cycle configuration comprising a continuous feeding simultaneous to an anoxic phase of 60 min not only did not enhance the nitrogen removal efficiency but also worsened the ammonia oxidation one. In Chapter 6 the effects of the thermal pre-treatment on the macroscopic and biochemical characteristics of the AGS were studied, as well as the anaerobic BioDegradability (BD) enhancement after this pre-treatment application. The biochemical characterization of the samples and the results obtained from the batch anaerobic tests were also used to validate a model that allows estimating the anaerobic biodegradability of the aerobic granular biomass. To achieve these objectives two AGS samples from aerobic granular pilot plants (100 L) were studied: one from a reactor fed with pig manure (AGS1) and another from a reactor fed with a synthetic medium to simulate an urban wastewater (AGS2). The pre-treatment temperatures were tested in a range between 60-210 ºC for AGS1 and 170-210 ºC for AGS2 and applied during 20 minutes. The results obtained with the untreated AGS samples showed that their anaerobic BD, of 33% for AGS1 and 49% for AGS2, was similar to that reported for a WAS (30-50%). The thermal pre-treatment before the anaerobic digestion enhanced the BD of AGS1 respect to the untreated sludge for all the temperatures assayed: 21% at 60 ºC; 42% at 90 ºC; 64% at 115 ºC; 82% at 140 ºC; 88% at 170 ºC; 70% at 190 ºC and 58% at 210 ºC. For AGS2 the thermal pre-treatment did not enhance the anaerobic BD at 170 ºC and improvement of only 14% and 18% were achieved at 190 and 210 ºC, respectively. In Chapter 7 the feasibility of the application of continuous anaerobic digestion of AGS was studied. In this case the AGS sample was collected from the same aerobic granular pilot plant than AGS1 in Chapter 6. The anaerobic digester had a useful volume of 5 L and was operated in the mesophilic range (35 ºC) in three different operational stages. The first one corresponded with the anaerobic digestion of raw AGS, where the obtained values of BD (44%) and solids reduction (32%) were in the range than those reported for WAS. The anaerobic BD in this case was higher than that obtained in the batch tests (33%) in Chapter 6, probably due to changes in the AGS sample, since samples used in both experiments were collected from the pilot plant with one year of difference. The second stage corresponded with the anaerobic digestion of thermal pre-treated AGS. In this case the thermal pre-treatment was carried out at 130 ºC along 20 minutes. The temperature of pre-treatment was chosen according to the results obtained previously in the batch tests (Chapter 6). This thermal pre-treatment of the AGS enhanced the reactor performance 32% and 47% in terms of BD and solids reduction, respectively. In the third stage the anaerobic digester was fed with a mixture of thermal pre-treated AGS and raw Primary Sludge (PS), since the bibliography indicates that the pre-treatment of the last one is not necessary. This mixture produced an enhancement of 17% for the solids removal respect to only pre-treated AGS, while the BD decreased from 58% to 53%, which was due to the higher CODT/VS ratio of the mixture respect to that corresponding to the experiment with only pre-treated AGS. In this chapter microbial analysis with the FISH technique were done to know how the type of feeding influences the development of different microbial populations inside the anaerobic reactor. The main features observed were that the Archaea domain had a higher presence in comparison with the Bacteria domain. Furthermore the abundance of both domains decreased with the change on the feeding from raw AGS, to pre-treated AGS and its mixture with PS.