Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Influence of saline stress in alternating pulses on aerobic granulation and resource production using different inoculum sources

Aerobic granular sludge (AGS) is a promising technology for wastewater treatment, particularly for its ability to recover valuable resources such as polyhydroxyalkanoates, alginate-like exopolysaccharide, and phosphorus. However, achieving stable granule formation remains a significant challenge. Research has shown that the addition of salt can accelerate the granulation process and enhance bioresource production. The source of the seed biomass is also critical for the system's success, with most AGS studies using activated sludge as the inoculum. This study aims to compare granulation, reactor performance, and bioresource recovery outcomes using inocula from different sources while also evaluating the impact of saline stress. Four sequential batch reactors were monitored, differing in the type of inoculum sludge (biomass from an aerated biofilter or activated sludge systems) and the presence of NaCl in the feed. The saline feed alternated between cycles containing 5 gNaCl/L and conventional feed without NaCl. Osmotic pressure was found to favor granulation and solids accumulation in both types of biomasses. Reactors inoculated with activated sludge and subjected to salt addition achieved complete granulation more rapidly. In contrast, reactors inoculated with submerged aerated biofilter sludge exhibited higher solids concentrations. All systems demonstrated excellent chemical oxygen demand removal, with activated sludge reactors showing superior performance in ammonia and total nitrogen removal and bioresources recovery. Salt addition stimulated the production of extracellular polymeric substances and amino acids such as tyrosine and tryptophan while reducing the intensity of fulvic acid-like substances, irrespective of the inoculum type.

Potential for curdlan recovery from aerobic granular sludge wastewater treatment systems - A review

The aerobic granular sludge (AGS) biotechnology has been explored for wastewater treatment for over two decades. AGS is gaining increased interest due to its enhanced treatment performance ability and the potential for resource recovery from AGS-based wastewater treatment systems. Resource recovery from AGS is a promising approach to sustainable wastewater treatment and attaining a circular economy in the wastewater management industry. Currently, research is at an advanced stage on recovering value-added resources such as phosphorus, polyhydroxyalkanoates, alginate-like exopolysaccharides, and tryptophan from waste aerobic granules. Recently, other value-added resources, including curdlan, have been identified in the aerobic granule matrix, and this may increase the sustainability of biotechnology in the wastewater industry. This paper provides an overview of AGS resource recovery potential. In particular, the potential for enhanced curdlan biosynthesis in the granule matrix and its recovery from AGS wastewater treatment systems is outlined.

PHA and EPS production from industrial wastewater by conventional activated sludge, membrane bioreactor and aerobic granular sludge technologies: A comprehensive comparison

The present study has focused on the mainstream integration of polyhydroxyalkanoate (PHA) production with industrial wastewater treatment by exploiting three different technologies all operating in sequencing batch reactors (SBR): conventional activated sludge (AS-SBR), membrane bioreactor (AS-MBR) and aerobic granular sludge (AGS). A full aerobic feast/famine strategy was adopted to obtain enrichment of biomass with PHA-storing bacteria. All the systems were operated at different organic loading (OLR) rate equal to 1-2-3 kgCOD/m3∙d in three respective experimental periods. The AS-MBR showed the better and stable carbon removal performance, whereas the effluent quality of the AS-SBR and AGS deteriorated at high OLR. Biomass enrichment with PHA-storing bacteria was successfully obtained in all the systems. The AS-MBR improved the PHA productivity with increasing OLR (max 35% w/w), whereas the AS-SBR reduced the PHA content (max 20% w/w) above an OLR threshold of 2 kgCOD/m3∙d. In contrast, in the AGS the increase of OLR resulted in a significant decrease in PHA productivity (max 14% w/w) and a concomitant increase of extracellular polymers (EPS) production (max 75% w/w). Results demonstrated that organic carbon was mainly driven towards the intracellular storage pathway in the AS-SBR (max yield 51%) and MBR (max yield 61%), whereas additional stressors in AGS (e.g., hydraulic selection pressure, shear forces) induced bacteria to channel the COD into extracellular storage compounds (max yield 50%) necessary to maintain the granule's structure. The results of the present study indicated that full-aerobic feast/famine strategy was more suitable for flocculent sludge-based technologies, although biofilm-like systems could open new scenarios for other biopolymers recovery (e.g., EPS). Moreover, the AS-MBR resulted the most suitable technology for the integration of PHA production in a mainstream industrial wastewater treatment plant, considering the greater process stability and the potential reclamation of the treated wastewater.

Simultaneous treatment of epichlorohydrin wastewater and polyhydroxyalkanoate recovery by halophilic aerobic granular sludge highly enriched by Halomonas sp

The treatment of epichlorohydrin (ECH) wastewater exists chances for achieving cleaner production. This study initially employed moderately halophilic aerobic granular sludge (HAGS) to treat ECH wastewater, and the resulting HAGS was utilized to recover polyhydroxyalkanoate (PHA). During the acclimation process of HAGS, the chemical oxygen demand removal efficiency stabilized at 70 %. Moreover, due to the high enrichment of Halomonas sp. (relative abundance of 86 ± 0.50 %), the maximum PHA content of wasted HAGS was 52.67 wt% in the fermentation process. Simultaneously, the utilization of nuclear magnetic resonance spectroscopy (1H and 13C spectra) and fourier transform infrared spectroscopy for the structural analysis of polymers revealed that polyhydroxybutyrate was the predominant substance extracted from HAGS. In this study, the innovative use of highly enriched HAGS for treating ECH wastewater and simultaneously recovering PHA not only enables the efficient biological treatment of ECH wastewater but also realizes resource recovery of ECH wastewater.

Granule formation mechanism, key influencing factors, and resource recycling in aerobic granular sludge (AGS) wastewater treatment: A review

The high-efficiency and additionally economic benefits generated from aerobic granular sludge (AGS) wastewater treatment have led to its increasing popularity among academics and industrial players. The AGS process can recycle high value-added biomaterials including extracellular polymeric substances (EPS), sodium alginate-like external polymer (ALE), polyhydroxyfatty acid (PHA), and phosphorus (P), etc., which can serve various fields including agriculture, construction, and chemical while removing pollutants from wastewaters. The effects of various key operation parameters on formation and structural stability of AGS are comprehensively summarized. The degradable metabolism of typical pollutants and corresponding microbial diversity and succession in the AGS wastewater treatment system are also discussed, especially with a focus on emerging contaminants removal. In addition, recent attempts for potentially effective production of high value-added biomaterials from AGS are proposed, particularly concerning improving the yield, quality, and application of these biomaterials. This review aims to provide a reference for in-depth research on the AGS process, suggesting a new alternative for wastewater treatment recycling.

Calcium Alginate Production through Forward Osmosis with Reverse Solute Diffusion and Mechanism Analysis

Calcium alginate (Ca-Alg) is a novel target product for recovering alginate from aerobic granular sludge. A novel Ca-Alg production method was proposed herein where Ca-Alg was formed in a sodium alginate (SA) feed solution (FS) and concentrated via forward osmosis (FO) with Ca²⁺ reverse osmosis using a draw solution of CaCl₂. An abnormal reverse solute diffusion was observed, with the average reverse solute flux (RSF) decreasing with increasing CaCl₂ concentrations, while the average RSF increased with increasing alginate concentrations. The RSF of Ca²⁺ in FS decreased continuously as the FO progressed, using 1.0 g/L SA as the FS, while it increased initially and later decreased using 2.0 and 3.0 g/L SA as the FS. These results were attributed to the Ca-Alg recovery production (CARP) formed on the FO membrane surface on the feed side, and the percentage of Ca²⁺ in CARP to total Ca²⁺ reverse osmosis reached 36.28%. Scanning electron microscopy and energy dispersive spectroscopy also verified CARP existence and its Ca²⁺ content. The thin film composite FO membrane with a supporting polysulfone electrospinning nanofiber membrane layer showed high water flux and RSF of Ca²⁺, which was proposed as a novel FO membrane for Ca-Alg production via the FO process with Ca²⁺ reverse diffusion. Four mechanisms including molecular sieve role, electrification of colloids, osmotic pressure of ions in CARP, and FO membrane structure were proposed to control the Ca-Alg production. Thus, the results provide further insights into Ca-Alg production via FO along with Ca²⁺ reverse osmosis.

Alginate-like polymers from full-scale aerobic granular sludge: content, recovery, characterization, and application for cadmium adsorption

Aerobic granular sludge (AGS) is a proven resource for the recovery of biopolymers like alginate-like polymers (ALP). This is the first report on the dynamics of ALP produced by AGS (ALP-AGS) in a full-scale wastewater treatment plant (WWTP), optimization of ALP recovery from AGS, and adsorption of cadmium (Cd2+) by ALP. Recovery of ALP was highest when using 120 mL of 0.2 M Na2CO3 at 70 °C for 45 min. Seasonal (1.5 years, over 3100 cycles) and intra-cycle changes in ALP-AGS in the WWTP were monitored. The ALP content in AGS increased in the transition period between winter and spring, reaching over 150 mg/g MLSS. In the batch reactor cycle, the ALP-AGS level peaked 2 h after the start of aeration (mean peak level: 120 mg/g MLSS), then decreased about two-fold by the end of the cycle. The ALP-AGS had a small surface area and a lamellar structure with crystalline outgrowths. The optimal conditions of Cd2+ adsorption with ALP were a dosage of 7.9 g d.m./L, a pH of 4-8, and an equilibrium time of 60 min. Carboxyl and hydroxyl groups were the key functional groups involved in Cd2+ adsorption. According to the Sips model, the maximum Cd2+ adsorption capacity of ALP-AGS was 29.5 mg/g d.m., which is similar to that of commercial alginate. AGS is a richer source of ALP than activated sludge, which ensures the cost-effectiveness of ALP recovery and increases the sustainability of wastewater treatment. Information on the chemical properties and yields of ALP from full-scale WWTPs is important for downstream applications with the recovered ALP.

Recovery of structural extracellular polymeric substances (sEPS) from aerobic granular sludge: Insights on biopolymers characterization and hydrogel properties for potential applications

Nowadays, wastewater treatment plants (WWTPs) are transforming into water resource recovery facilities (WRRFs) where the resource recovery from waste streams is pivotal. Aerobic granular sludge (AGS) is a novel technology applied for wastewater treatment. Extracellular polymeric substances (EPS) secreted by microorganisms promote the aggregation of bacterial cells into AGS and the structural fraction of EPS (sEPS) is responsible for the mechanical properties of AGS. sEPS can be extracted and recovered from waste AGS by physico-chemical methods and its characterization is to date of relevant concern to understand the properties in the perspective of potential applications. This study reports on: characterization of sEPS extracted and recovered from AGS; - formation and characterization of sEPS-based hydrogels. Briefly, sEPS were extracted by a thermo-alkaline process followed by an acidic precipitation. sEPS-based hydrogels were formed by a cross-linking process with a 2.5% w/w CaCl₂ solution. The following key-findings can be drawn: i) hydrogels can be formed starting from 1% w/w sEPS on, by diffusion of Ca²⁺ into sEPS network; ii) the Ca/C molar ratio of hydrogels decreased with increasing concentration of sEPS from 1 to 10% w/w; iii) the thermogravimetric and spectroscopic behaviours of sEPS show that the cross-linking reaction mainly involves the polysaccharidic fraction of biopolymers; iv) water-holding capacity up to 99 gH₂O/g_sEPS was registered for 1% w/w sEPS-based hydrogels, suggesting applications in several industrial sectors (i.e. chemical, paper, textile, agronomic, etc.); v) rheological results highlighted a solid-like behaviour (G'≫G") of sEPS-based hydrogels. The power-law fitting of G' vs. sEPS concentration suggests that the expansion of the sEPS network during cross-linking occurs through a percolative mechanism involving the initial formation of sEPS oligomers clusters followed by their interconnection towards the formation of 3D network. These findings provide additional information about the mechanisms of sEPS-based hydrogel formation and reveal the peculiar physico-chemical characteristics of sEPS which nowadays are increasingly gaining interest in the context of resource recovery.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.

Enhancing the Stability of Aerobic Granular Sludge Process Treating Municipal Wastewater by Adjusting Organic Loading Rate and Dissolved Oxygen Concentration

Aerobic granular sludge (AGS) application in treating municipal wastewater has been greatly restricted due to its low stability. It has been found that operation parameters have a great impact on stability. The organic loading rate (OLR) and dissolved oxygen (DO) concentration are two very important parameters that impact stability. In this study, the organic loading rate (OLR) and aeration rate were studied to verify their influence on AGS system stability, which is indicated by determining pollutant removal performance, including chemical oxygen demand (COD), ammonia nitrogen, and total nitrogen (TN). The physical and chemical property changes of AGS and the effects of pollutant removal during the formation of AGS were systematically investigated. The AGS was formed after about 25 days and remained stable for about 45–50 days. The AGS was light-yellow globular sludge with an average particle size of 1.25 mm and a sludge volume index (SVI) of 33.9 mL/g. The optimal condition was obtained at an OLR of 4.2 kg COD/m3·d, aeration rate of 4 L/min, and a hydraulic retention time (HRT) of 4 h. The corresponding removal efficiencies of COD, ammonia nitrogen, and TN were 94.1%, 98.4% and 74.1%, respectively. The study shows that the AGS system has great potential for pollutant removal from wastewater.

Carbon source affects the resource recovery in aerobic granular sludge systems treating wastewater

This study evaluated the influence of carbon sources on alginate-like exopolymers (ALE) and tryptophan (Trp) biosynthesis in the aerobic granular sludge (AGS). With acetate, the highest biopolymers levels, per gram of volatile suspended solids (VSS) (418.7 mgALE∙g^-1 and 4.1 mgTrp∙gVSS^-1), were found likely due to biomass loss throughout the operation, which resulted in lower sludge age (4-7 days) and shorter famine period. During granulation, encouraging results on ALE production were obtained with propionate (>250 mgALE∙gVSS^-1), significantly higher than those found with glycerol, glucose, and sucrose. Regarding tryptophan production, propionate and glycerol proved to be good substrates, although the content was still lower than acetate (1.6 mgTrp∙gVSS^-1). Granules fed with glucose showed the worst results compared to the other substrates (38.5 mgALE∙VSS^-1 and 0.6 mgTrp∙gVSS^-1) due to the filamentous microorganisms' abundance found. Therefore, this study provides insights to value the production of compounds of industrial interest in AGS systems.

Laboratory-scale study of a biodegradable microplastic polylactic acid stabilizing aerobic granular sludge system

The effects of microplastics on aerobic granular sludge technology are an emerging issue, although the impact of degradable microplastics (DMPs) on the aerobic granular system is still unexplored. In this study, degradable microplastic polylactic acid (DMP-PLA) was added at three concentrations (5, 15, 40 mg/L), which strengthened the granular stability and consequently stabilized pollutant removal compared to the control (without DMP-PLA). The experiment showed that adding DMP-PLA made cells secrete more extracellular polymeric substances [64.8 mg/g MLVSS (mixed liquor suspended solids)], particularly retaining β-D-glucopyranose polysaccharides in experimental group. In addition, abundant hydrogen bonds were also maintained. The reactor under the stress of DMP-PLA exhibited high pollutant removal efficiency (COD>88%, TP>91%, TIN>86%), indicating high performance of the microbes. Microbial analysis at the genus level indicated that Defuviicoccus and Candidatus_Competibacter were dominant after DMP-PLA addition, which identified denitrifying glycogen-accumulating organisms as beneficial for nitrogenous compound removal. Redundancy analysis showed that the abundance of Candidatus_Competibacter was positively related to the addition of DMP-PLA. This study demonstrated that DMP-PLA was feasibly employed in the aerobic granular water treatment process, and presents a new method to optimize the stability and extracellular secretion of the microbial community.

Cyanophycin Granule Polypeptide: a Neglected High Value-Added Biopolymer, Synthesized in Activated Sludge on a Large Scale

Recovery of microbial synthetic polymers with high economic value and market demand in activated sludge has attracted extensive attention. This work analyzed the synthesis of cyanophycin granule peptide (CGP) in activated sludge and its adsorption capacity for heavy metals and dyes. The distribution and expression of synthetic genes for eight biopolymers in two wastewater treatment plants (WWTPs) were analyzed by metagenomics and metatranscriptomics. The results indicate that the abundance and expression level of CGP synthase (cphA) are similar to those of polyhydroxyalkanoate polymerase, implying high synthesis of CGP in activated sludges. CGP in activated sludge is mainly polymerized from aspartic acid and arginine, and its secondary structure is mainly β-sheet. The crude yields of CGP are as high as 104 ± 26 and 76 ± 13 mg/g dry sludge in winter and in summer, respectively, comparable to those of polyhydroxyalkanoate and alginate. CGP has a stronger adsorption capacity for anionic pollutants (Cr (VI) and methyl orange) than for cationic pollutants because it is rich in guanidine groups. This study highlights prospects for recovery and application of CGP from WWTPs. IMPORTANCE The conversion of organic pollutants into bioresources by activated sludge can reduce the carbon dioxide emission of wastewater treatment plants. Identification of new high value-added biopolymers produced by activated sludge is beneficial to recover bioresources. Cyanophycin granule polypeptide (CGP), first discovered in cyanobacteria, has unique chemical and material properties suitable for industrial food, medicine, cosmetics, water treatment, and agriculture applications. Here, we revealed for the first time that activated sludge has a remarkable ability to produce CGP. These findings could further facilitate the conversion of wastewater treatment plants into resource recycling plants.

From pollutant removal to resource recovery: A bibliometric analysis of municipal wastewater research in Europe

Municipal wastewaters are abundant low-strength streams that require adequate treatment and disposal to ensure public and environmental health. This study aims to provide a comprehensive summary of municipal wastewater research in Europe in the 2010s in the form of bibliometric analysis. The work was based on the Science Citation Index Expanded (Web of Science) and carried out using the R-package bibliometrix for bibliometric data analysis and the software VOSviewer for science mapping. Analysing a dataset of 5645 publications, we identified the most influential journals, countries, authors, institutions, and publications, and mapped the co-authorship and keyword co-occurrence networks. Spain had produced the most publications while Switzerland had the highest average citations per publication. China was the most collaborative country from outside of Europe. Analysis of the most cited articles revealed the popularity of micropollutant removal in European municipal wastewater research. The keyword analysis visualized a paradigm shift from pollutant removal towards resource recovery and circular economy. We found that current challenges of resource recovery from municipal wastewater come from both technical and non-technical (e.g., environmental, economic, and social) aspects. We also discussed future research opportunities that can tackle these challenges.

Effect of an Increased Particulate COD Load on the Aerobic Granular Sludge Process: A Full Scale Study

High concentrations of particulate COD (pCOD) in the influent of aerobic granular sludge (AGS) systems are often associated to small granule diameter and a large fraction of flocculent sludge. At high particulate concentrations even granule stability and process performance might be compromised. However, pilot- or full-scale studies focusing on the effect of real wastewater particulates on AGS are scarce. This study describes a 3-month period of increased particulate loading at a municipal AGS wastewater treatment plant. The pCOD concentration of the influent increased from 0.5 g COD/L to 1.3 g COD/L, by adding an untreated slaughterhouse wastewater source to the influent. Sludge concentration, waste sludge production and COD and nutrient removal performance were monitored. Furthermore, to investigate how the sludge acclimatises to a higher influent particulate content, lipase and protease hydrolytic activities were studied, as well as the microbial community composition of the sludge. The composition of the granule bed and nutrient removal efficiency did not change considerably by the increased pCOD. Interestingly, the biomass-specific hydrolytic activities of the sludge did not increase during the test period either. However, already during normal operation the aerobic granules and flocs exhibited a hydrolytic potential that exceeded the influent concentrations of proteins and lipids. Microbial community analysis also revealed a high proportion of putative hydrolysing and fermenting organisms in the sludge, both during normal operation and during the test period. The results of this study highlight the robustness of the full-scale AGS process, which can bear a substantial increase in the influent pCOD concentration during an extended period.