Sludge Anaerobic Digestion Research Articles

A comparative study on polyaluminum chloride (PACl) and Moringa oleifera (MO) chemically enhanced primary treatment (CEPT) in enhanced biogas production: anaerobic digestion performance and the Gompertz model.

A comparative study was performed to estimate biogas production from sludge produced by organic and inorganic chemically enhanced primary treatments (CEPTs). To this end, the effects of two coagulants, polyaluminum chloride (PACl) and Moringa oleifera (MO), on CEPT and biogas production in anaerobic digestion were surveyed within an incubation period of 24 days. The optimal dosage and pH of PACl and MO were optimized in terms of sCOD, TSS and VS parameters in the CEPT process. Next, the digestion performance of anaerobic digestion reactors fed with sludge obtained from PACl and MO coagulants at a batch mesophilic reactor (37 ± 1 °C) was surveyed from the biogas production, volatile solid reduction (VSR) and Gompertz model. At the optimal conditions (pH = 7 and dosage = 5 mg L-1), the removal efficiency of COD, TSS and VS in CEPT assisted with PACL was 63, 81 and 56%, respectively. Moreover, CEPT assisted with MO led to the removal efficiency of COD, TSS and VS until 55, 68 and 25%, respectively. The highest methane yield (0.598 L gVS removed-1) was obtained in an anaerobic digestion reactor with sludge from the MO coagulant. The anaerobic digestion of CEPT sludge instead of primary sludge resulted in higher sCOD removal efficiency, and 43-50% of sCOD was observed compared with the removal of 32% for the primary sludge. Furthermore, the high coefficient of determination (R2) demonstrated the trustworthy predictive precision of the modified Gompertz model with actual data. The combination of CEPT and anaerobic digestion, especially using natural coagulants, provides a cost-effective and practical way to increase BMP from primary sludge.

Effect of Fe3O4 nanoparticles on anaerobic digestion of municipal wastewater sludge

Anaerobic digestion of waste activated sludge often achieves less than 50% destruction of volatile suspended solids and stabilization of chemical oxygen demand to methane gas. Although the impact of Fe3O4 nanoparticles on anaerobic digestion performance was investigated in past studies, this study focused directly on the impacts of the size and concentration of the Fe3O4 nanoparticles. Fe3O4 nanoparticles with two different ranges -- (12–18)-nm and (50–100)-nm -- and different concentrations were evaluated for their impact on anaerobic digestion of combined wastewater sludges (waste activated sludge + primary sludge). The volume of methane produced and the concentrations of volatile suspended solids, total chemical oxygen demand, and Fe2+ were measured during Biochemical Methane Potential (BMP) experiments. Adding Fe3O4 nanoparticles enhanced solids digestion and methanogenesis, and the impact was greater for methanogenesis. Adding 120 mg/L of the smaller nanoparticles had the greatest impact: a 1.7-fold increase in the methane yield, 26% increase in volatile suspended solids destruction, and 35% increase in total chemical oxygen demand removal, compared with the control. The smaller nanoparticles, which had higher impacts on anaerobic-digestion performance than the larger nanoparticles, led to greater release of soluble Fe2+, which may have decreased sulfide inhibition of methanogenesis, increased the rate of solids hydrolysis, or increased inter-species electron transport. A modified Gompertz model quantified how adding 120 mg/L of the (12–18)-nm nanoparticles increased the rate and extent of methanogenesis.

Formation of Recalcitrant Compounds during Anaerobic Digestion of Thermally Pre-Treated Sludge: A Critical Macromolecular and Structural Study.

Thermal hydrolysis, when used as pre-treatment, enhances the anaerobic digestion of sewage sludge; moreover, due to the high temperature normally applied, undesirable recalcitrant compounds via Maillard reactions may also be formed. However, although the appearance of these recalcitrant compounds is widely reported, more information on the formation, structure, and fate of these compounds is still needed. This study was focused on understanding the amount and whereabouts of such compounds during the anaerobic digestion process with thermal pre-treatment in soluble and total phase and advance in its structural identification by analyzing their infrared (IR) spectra. It was found that, even with the improved methane production and COD degradation, at 165 °C for 30 min, humic-like compounds are formed which could not be degraded at the anaerobic digestion step. These compounds account for 25% of the original sludge. Infrared spectroscopy proved to be a powerful technique, permitting their differentiation from the natural humic-like compounds. This research provides new information about the structure of melanoidins at every stage of the thermal hydrolysis pre-treatment and how they contribute to the dissolved organic nitrogen.

Amino acid preparation and recovery from refractory sludge by the oxidative acid hydrolysis process

ABSTRACT After the anaerobic digestion of excess sludge, dissolved organic matter is absorbed and used, but the treatment of refractory sludge is a headache. The oxidative acid (performic acid and hydrochloric acid) hydrolysis process can effectively prepare amino acids from refractory sludge. During the preparation process, insoluble proteins in sludge were turned into soluble proteins and peptides. All of them eventually hydrolyse into amino acids. The optimum conditions in the single-factor experiment were as follows: a temperature of 110°C, a reaction time of 24 h, and a hydrochloric acid (HCl) concentration of 6 M. The results showed that the maximum total yield of amino acids from refractory sludge was 94.76%. In the orthogonal experiment, the maximum total yield of amino acids was 97.20% under the optimum conditions of a temperature of 113.45°C, a reaction time of 26.79 h, and 5.92 M HCl. The recovery rate of purity amino acids was 17.16 g per 100 g of dry sludge. The recovery rate of the hydrochloric acid was approximately 70%. There were 17 kinds of amino acids in the hydrolysate, which could be used as deodorants, food additives, preservatives, and corrosion inhibitors. This new technology is expected to be very effective in the treatment of refractory sludge.

Variations in the extraction and clean-up efficiency of DNA during anaerobic digestion of wastewater sludge at mesophilic and thermophilic temperatures

The effect of anaerobic sludge digestion on the isolation of DNA and clean-up efficiency of obtained DNA isolates was studied. Influent and digestate of a continuous full-scale reactor were sampled twice. Also, batch digesters were operated at 35, 45 and 55 °C, namely, AnD.35, AnD.45 and AnD.55. The batch reactors were sampled at 3-to-4-day time intervals during the digestion. A sharp decline in the extraction yield was observed over time for the batch reactors. In AnD.55, the extraction yield declined by 96 % in 12 days. The purity of the DNA isolates was assessed by purity ratios, which included absorption at 260 nm to that at 280 nm (Abs260/280) and to that at 230 nm (Abs260/230). Abs260/230 before clean-up declined as a result of digestion in all reactors, the most severe case being AnD.55 decreasing from 1.91 to 0.47 in nine days. The efficiency of clean-up was evaluated by analysing DNA recovery and changes in the purity ratios. AnD.35 and the full-scale reactor had the highest recoveries (≥ 80 %). Furthermore, a 4.1-fold increase was observed in Abs260/230. Additionally, non-linear correlations were found between purity parameters and the DNA concentration (R2 > 0.7). The observations of the current investigation strongly indicate that the change in DNA extraction yield was at least partly due to the change in DNA extraction efficiency and not just the change in the actual DNA content. This is an important consideration for the quantification of biomass, specific genes, and microbial populations, where DNA isolation plays a key role.

In-situ sulfite treatment enhanced the production of short-chain fatty acids from waste activated sludge in the side-stream anaerobic fermentation

Sulfite-based technology could enhance methane production from anaerobic sludge digestion. However, its potential for in-situ direct sludge treatment without anaerobic sludge addition in the side-stream remains unclear. This study investigated the feasibility of using in-situ sulfite treating sludge for short-chain fatty acids (SCFAs) production via anaerobic fermentation of waste activated sludge (WAS) as a side-stream treatment. In-situ sulfite direct sludge treatment enhanced SCFAs and acetic acid production by 2.03 and 4.89 times at 500 mg S/L compared to the control. With in-situ sulfite treatment, WAS hydrolysis and acidification were enhanced while methanogenesis was spontaneously hindered. The in-situ sulfite treatment inactivated pathogens and improved the sludge dewatering properties. The relative abundances of SCFAs-production microbial were stimulated, facilitating the sludge bioconversion. The produced SCFAs from in-situ sulfite side-stream treatment could be applied as an “internal carbon source” to enhance biological nutrient removal to improve economic and environmental value from sludge treatment.

Pyrite-enhanced Sludge Digestion via Stimulation of Direct Interspecies Electron Transfer between Syntrophic Propionate- and Sulfur-oxidizing Bacteria and Methanogens: Genome-centric Metagenomics Evidence

The supplementation of conductive materials (CMs) is a promising strategy to improve the methane (CH4) yield from anaerobic digestion (AD) of sewage sludge. However, most CMs are relatively costly, and the mechanisms by which they influence microbial functional traits (e.g., direct interspecies electron transfer (DIET) and extracellular polymeric substances (EPS)) remain poorly understood, which limits the application of CMs for enhancing CH4 production. Here, we investigated the effects of micro- and milli-scale particles of pyrite (an iron-based (semi)conductive waste, supplemented at doses of 5–40 g/L) on sludge digestion, and the underlying mechanisms. The results indicated that CH4 production rate and yield were improved mostly (by 20.8 % and 37.8 %, respectively) by the addition of 5 g/L micro-scale pyrite. The hydrolysis, acidification, and acetoclastic methanogenesis steps were accordingly promoted by 17.0–98.2 %, and the corresponding activities of key enzymes were elevated by 27.2–33.4 %. Pyrite enriched the populations of Anaerolineae and Burkholderiales, which are putative EPS producers according to genome-centric metagenomics, thus boosting the secretion of redox-active components in EPS and facilitating extracellular electron transport. Interestingly, pyrite may mediate “dual-drive” DIET, in which bin 116 (Syntrophobacteraceae sp.) and bin 2 (Thiobacillaceae sp.) directly transfer the electrons released by the oxidation of propionate and sulfide, respectively, to methanogen bin 28 (Methanothrix soehngenii). These results provide deep insights into the highly favorable performance of pyrite-supplemented AD systems and the microbial mechanisms. The proposed pyrite-mediated dual-drive DIET could expand the diversity of electron-donating bacteria and provide a novel microbial strategy for the simultaneous recovery of CH4 and elemental sulfur from waste organic streams in CM-supplemented AD systems.

A review of the impact of conductive materials on antibiotic resistance genes during the anaerobic digestion of sewage sludge and animal manure

The urgent need to reduce the environmental burden of antibiotic resistance genes (ARGs) has become even more apparent as concerted efforts are made globally to tackle the dissemination of antimicrobial resistance. Concerning levels of ARGs abound in sewage sludge and animal manure, and their inadequate attenuation during conventional anaerobic digestion (AD) compromises the safety of the digestate, a nutrient-rich by-product of AD commonly recycled to agricultural land for improvement of soil quality. Exogenous ARGs introduced into the natural environment via the land application of digestate can be transferred from innocuous environmental bacteria to clinically relevant bacteria by horizontal gene transfer (HGT) and may eventually reach humans through food, water, and air. This review, therefore, discusses the prospects of using carbon- and iron-based conductive materials (CMs) as additives to mitigate the proliferation of ARGs during the AD of sewage sludge and animal manure. The review spotlights the core mechanisms underpinning the influence of CMs on the resistome profile, the steps to maximize ARG attenuation using CMs, and the current knowledge gaps. Data and information gathered indicate that CMs can profoundly reduce the abundance of ARGs in the digestate by easing selective pressure on ARGs, altering microbial community structure, and diminishing HGT.

The role of microbial ecology in improving the performance of anaerobic digestion of sewage sludge.

The use of next-generation diagnostic tools to optimise the anaerobic digestion of municipal sewage sludge has the potential to increase renewable natural gas recovery, improve the reuse of biosolid fertilisers and help operators expand circular economies globally. This review aims to provide perspectives on the role of microbial ecology in improving digester performance in wastewater treatment plants, highlighting that a systems biology approach is fundamental for monitoring mesophilic anaerobic sewage sludge in continuously stirred reactor tanks. We further highlight the potential applications arising from investigations into sludge ecology. The principal limitation for improvements in methane recoveries or in process stability of anaerobic digestion, especially after pre-treatment or during co-digestion, are ecological knowledge gaps related to the front-end metabolism (hydrolysis and fermentation). Operational problems such as stable biological foaming are a key problem, for which ecological markers are a suitable approach. However, no biomarkers exist yet to assist in monitoring and management of clade-specific foaming potentials along with other risks, such as pollutants and pathogens. Fundamental ecological principles apply to anaerobic digestion, which presents opportunities to predict and manipulate reactor functions. The path ahead for mapping ecological markers on process endpoints and risk factors of anaerobic digestion will involve numerical ecology, an expanding field that employs metrics derived from alpha, beta, phylogenetic, taxonomic, and functional diversity, as well as from phenotypes or life strategies derived from genetic potentials. In contrast to addressing operational issues (as noted above), which are effectively addressed by whole population or individual biomarkers, broad improvement and optimisation of function will require enhancement of hydrolysis and acidogenic processes. This will require a discovery-based approach, which will involve integrative research involving the proteome and metabolome. This will utilise, but overcome current limitations of DNA-centric approaches, and likely have broad application outside the specific field of anaerobic digestion.

Quantification of free and esterified long-chain fatty acids without extraction by high-performance liquid chromatography in anaerobic digestion sludge

Quantification of free and esterified long-chain fatty acids without extraction by high-performance liquid chromatography in anaerobic digestion sludge

Machine learning-assisted exploration for carbon neutrality potential of municipal sludge recycling via hydrothermal carbonization

In the context of advocating carbon neutrality, there are new requirements for sustainable management of municipal sludge (MS). Hydrothermal carbonization (HTC) is a promising technology to deal with high-moisture MS considering its low energy consumption (without drying pretreatment) and value-added products (i.e., hydrochar). This study applied machine learning (ML) methods to conduct a holistic assessment with higher heating value (HHV) of hydrochar, carbon recovery (CR), and energy recovery (ER) as model targets, yielding accurate prediction models with R2 of 0.983, 0.844 and 0.858, respectively. Furthermore, MS properties showed positive (e.g., carbon content, HHV) and negative (e.g., ash content, O/C, and N/C) influences on the hydrochar HHV. By comparison, HTC parameters play a critical role for CR (51.7%) and ER (52.5%) prediction. The primary sludge was an optimal HTC feedstock while anaerobic digestion sludge had the lowest potential. This study provided a comprehensive reference for sustainable MS treatment and industrial application.

Impacts of magnetic biochar from reed straw on anaerobic digestion of pigment sludge: Biomethane production and the transformation of heavy metals speciation

The effects of magnetic biochar (MBC) on the anaerobic digestion efficiency of pigment sludge and the passivation of heavy metals Cd, Pb were investigated. MBC participated in the dual direct interspecies electron transfer effects of biochar and Fe3O4, improved the anaerobic digestion efficiency. Compared with the control group, the maximum volatile fatty acids (VFAs) concentration, the VS (volatile solids) removal rate, cumulative methane production, average daily methane production, and average daily methane production rate increased by 51.4 %, 37.8 %, 56.3 %, 56.3 %, 37.2 %, respectively. Moreover, MBC addition signally reduced the distribution rate of the bioavailable state of heavy metals in the pigment sludge and improved the passivation efficiency. Compared with the feedstock, the mass ratios of exchangeable and reducible Cr were reduced by 43.6 % and 61.6 %, and the corresponding ratios of Pb were reduced by 41.0 % and 59.2 %, respectively. The results indicate MBC could improve the efficiency of the anaerobic digestion of pigment sludge and reduce the bioavailability of heavy metals Cd and Pb.

Coexistence of microplastics alters the inhibitory effect of antibiotics on sludge anaerobic digestion

Microplastics and antibiotics are emerging pollutants that inevitably enter the sludge anaerobic digestion system. Current studies mainly focused on the individual effects of microplastics or antibiotics on methane generation. However, there may also be complex reactions between them and their combine effects on anaerobic digestion remain unclear. This study revealed the combined influencing mechanism of polyamide and ofloxacin through batch and semi-continuous tests. Brief exposure to ofloxacin was detrimental to methane production. The coexistence of polyamide enabled partial adsorption of ofloxacin, which effectively mitigates the inhibitory effect of ofloxacin at low concentrations. Polyamide also facilitated the colonization of hydrolytic bacteria and acidogens, such as Hydrogenispora, and promoted the conversion of organic matter to acetate. However, polyamide could not effectively alleviate the inhibitory effect of excess ofloxacin on anaerobic digestion. Ofloxacin at high concentrations stimulated the release of humic acid, reduced methane production by more than 80%, and changed the predominant methanogenesis pathway from acetoclastic methanogenesis to hydrogenotrophic methanogenesis. Acidogenesis and methanogenesis pathways were restructured to further explore the effect of ofloxacin on the metabolic potentials of microorganisms. Metagenomic analysis revealed that most of functional genes associated with glycolysis, amino acid metabolism and methanogenesis were down regulated under long-term stress of high concentration of ofloxacin. This study elucidated the microbial process of organic matter conversion inhibition by ofloxacin, shed light on the combined effects of polyamide and ofloxacin on anaerobic digestion and provided new insights for assessing the impact of multiple pollutants on the safe disposal of sludge.

Anaerobic digestion of vinasse and water treatment plant sludge increases methane production and stability of UASB reactors

Studies are still needed to increase the stability and efficiency of methane production from vinasse. Therefore, operations strategies, such as the anaerobic digestion with one or more wastes and adding micronutrients, especially iron, become attractive. The performance of two treatment systems, each one composed of two UASB reactors in series, operated under mesophilic (R1M and R2M) and thermophilic (R1T and R2T) temperature conditions, was evaluated in the anaerobic digestion of vinasse (ADV). First, the reactors were operated with the effluent recirculation and increasing organic loading rate (OLR) up to 20 g CODtotal L−1d−1 in the R1M and R1T. Then, the anaerobic digestion of vinasse and water treatment plant (WTP) sludge (ADVS) was performed in the proportions of 25:75 to 50:50 (% v/v) in both systems. In the ADV, applying the highest OLR, the mesophilic and thermophilic reactors instabilities happened. The ADVS of over 35% of WTP sludge promoted the recovery of the mesophilic and thermophilic UASB reactors with significantly reduced total volatile acids and increased alkalinity and biogas production. The higher average values of the volumetric methane production (VMP) occurred in the ADVS at 50% of WTP; in the R1M and R1T, they were 3.23 and 3.00 L CH4 L−1d−1, respectively. In the ADV, the thermophilic system presented higher VMP concerning the mesophilic for OLR up to 15 g CODtotal L−1d−1. For higher OLR, the mesophilic system showed better carrying capacity and stability. The ADVS with above 35% of WTP sludge promoted similar benefits in the two systems, with no significant differences in CODtotal removal and VMP. Therefore, adding iron by WTP sludge in ADVS improves methane production and increases the stability of UASB reactors under mesophilic and thermophilic conditions.

Insights into the resistance of different extracellular polymeric substance (EPS) layers to the fermentation environment in sludge anaerobic digestion

• EPS layer delamination had great promotion for the methane production. • The semi-rigid structure of sludge flocs was damaged after EPS layer delamination. • EPS layer delamination reduced metal contents and the apparent activation energy. Extracellular polymeric substance (EPS) is key in maintaining microbial cell stability and agglomeration in sludge, which may play as a protective layer for cells. This study tried to assess the digestive performances of sludge after delamination of different EPS layers, aiming to understand the resistance of different EPS layers to the external environment. Results indicated that, after EPS layer delamination, the cumulative methane yield in anaerobic digestion (AD) was improved from 148.8 to 199.9 mL/g COD. The total methane yield for the EPS layers and EPS extracted sludge increased by 32.2% compared to the raw sludge. Moreover, with the delamination of EPS layers, the apparent activation energy (AAE) of the sludge samples decreased, and the interactions between metals and organic substances were destroyed, resulting in lower energy barrier in organic solubilization. Additionally, the treatment changed the distribution of organic substances in sludge, and the residual organic substances presented higher theoretical methane production (TMP). Therefore, both the hydrolytic efficiency and the conversion efficiency of organic substances to methane in AD were improved. By understanding the resistance effects of different EPS layers to the fermentation environment, this study will help us develop more targeted methods in sludge pretreatment for improving its digestive performance.

Successive choline addition enhancing the methanogenesis of waste activated sludge anaerobic digestion: Insight from hydrophilicity, electrochemical performance and microbial community

Anaerobic digestion (AD) is a promising technology to treat waste-activated sludge, previous study proved that methane production could be enhanced with the addition of choline, this work aimed to solve the problem of rapid biodegradability of choline in the AD process by changing its dosing method. With 0.75 g/L as the optimal choline dosing concentration, experimental results showed that successive choline dosing during the first 3–6 days of AD (experimental groups, EGs) performed better than the single dosing. The accumulative biogas production in EGs was increased by 35.55–36.73%, which could be caused by the simultaneous promotion of hydrolysis-acidification and methanogenesis processes. Especially, the electron exchange capacity of digested sludge in EGs was increased by 16.71–34.58%. In addition, the surface Gibbs free energy (△GSL) of sludge in EGs was 105.51–172.21% higher (corresponding to stronger hydrophilicity and repulsion), which might help disperse sludge flocs and improve mass transfer efficiency, and the △GSL values were positively correlated with the accumulative methane production (R2 = 0.7029). Microbiological analysis showed that microbial communities in EGs were richer and Methanosaeta was regarded as the dominant species with 15.93–30.08% higher relative abundance with choline addition. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, EGs were found to be more active in metabolism clusters. Collectively, these findings demonstrated that successive choline dosing during the first 3–6 days is an effective and novel method to enhance methane production in AD process.

Comprehensive Review of Hydrothermal Pretreatment Parameters Affecting Fermentation and Anaerobic Digestion of Municipal Sludge

Municipal solid waste treatment and disposal have become one of the major concerns in waste management due to the excessive production of waste and higher levels of pollution. To address these challenges and protect the environment in sustainable ways, the hydrothermal pretreatment (HTP) technique coupled with anaerobic digestion (AD) becomes a preferred alternative technology that can be used for municipal solid waste stabilization and the production of renewable energy. However, the impact of HTP parameters such as temperature, retention time, pH, and solid content on the fermentation of TWAS is yet to be well studied and analyzed. Hence this study was conducted to review the effect of hydrothermal pretreatment of thickened waste-activated sludge (TWAS) on fermentation and anaerobic digestion processes. Many studies reported that fermentation of TWAS at pretreatment temperature ranges from 160 °C to 180 °C resulted in a 50% increase in volatile fatty acid (VFA) yields compared to no pretreatment. However, for the AD process, HTP in the range of 175 °C to 200 °C with a 30–60 min retention time was considered the optimal condition for higher biogas production, with 30% increase in biodegradability and greater than 55% increase in biogas production. Even though there is a direct relationship between increased HTP temperature and the hydrolysis of TWAS, a pretreatment temperature range beyond 200 °C alters the biogas production. The solid content (SC) of sludge plays a crucial role in HTP, where in practice up to 16% SC has been utilized for HTP. Further, a combined alkaline-HTP enhances the process performance.

Full-scale anaerobic digestion of sewage sludges: Fate evaluation of pharmaceuticals and main metabolites

The occurrence and behavior of pharmaceuticals especially in sewage sludge have attracted great attention nowadays due to the intensive application of sewage sludge in agriculture in terms of the potential adverse effects of pharmaceuticals. Although anaerobic digestion is a well-developed and widely applied treatment process in terms of stabilization of sewage sludges and removal of organics, the treatment performances of this technology for different pharmaceutical subgroups are still not well known. In this study, the occurrence and seasonal variations of 45 pharmaceutical compounds consisting of 27 parent compounds and 18 main metabolites from 10 different therapeutic groups were investigated both in thickened and digested sludge generated in a large-scale urban wastewater treatment plant (WWTP). In this way, the seasonal removal performances of full-scale anaerobic digesters for parent compounds and metabolites have been determined and compared simultaneously. While the total pharmaceutical concentrations in the thickened sludge varied between 21.1 and 177.6 μg/kg seasonally, it was determined in the range of 21.5 and 88.8 μg/kg in the digested sludge. In the thickened sludge, the predominant group was found as antibiotics (35.9 %), while in the digested sludge, it was determined as anticonvulsants (31.3 %). Besides, seasonal relationships of parent compounds and metabolites both in thickened and digested sludges were evaluated statistically in terms of both occurrence and removal. In general, although the capacities for sorption onto sewage sludges were found as different, removal patterns of both parent compounds and metabolites in anaerobic digesters have been determined as quite similar. The removal performance of the anaerobic digesters showed great differences based on the investigated pharmaceutical subgroups. Although varies seasonally, while antibiotic (up to 82.9 %), NSAID (up to 70.9 %), and nerve stimulant (up to 89.6 %) groups tended to be treated in anaerobic digesters, negative removal efficiencies were determined for anticonvulsant (up to −242.9 %), steroid hormone (up to −155.3 %) and antidepressant (up to −5010 %) groups. While the total pharmaceutical treatment performances of anaerobic sludge digesters varied between −78 % (in summer) and 65 % (in winter) seasonally, it has been determined as 28.5 % annually. Therefore, it has been determined in this study that the pharmaceutical treatment performance of full-scale anaerobic digesters is not effective enough.

Upgrading Anaerobic Sludge Digestion by Using an Oil Refinery By-Product

Carbon-based conductive additives have been studied for their positive effects on anaerobic digestion (AD) using synthetic substrates, but their importance in wastewater sludge digestion has not been sufficiently explored. This research investigated and compared the effects of two conductive materials (graphene and petroleum coke) with and without trace metal supplementation. The results indicated that supplementing reactors with graphene and petroleum coke could significantly improve biogas production. The supplementation of 1 g/L petroleum coke and 2 g/L graphene, without trace metal addition, led to an increase in the biogas production by 19.10 ± 1.04% and 16.97 ± 5.00%, respectively. Thus, it can be concluded that petroleum coke, which is an oil refinery by-product, can be used to enhance biogas production in a similar way to other carbon-based conductive materials that are currently available on the market. Moreover, using petroleum coke and graphene, the average chemical oxygen demand (COD) removal was 42.84 ± 1.23% and 42.80 ± 0.45%, respectively, without the addition of trace metals. On the other hand, supplementation of the reactors with trace elements resulted in a COD removal of 34.65 ± 0.43% and 34.05 ± 0.45% using petroleum coke and graphene, respectively.

Removal of bacterial pathogens and antibiotic resistance bacteria by anaerobic sludge digestion with thermal hydrolysis pre-treatment and alkaline stabilization post-treatment

Primary sludge (PS) is associated with public health and environmental risks, so regulations focus on reducing the pathogenic and heavy metal contents of the treated material (biosolids), intended for soil amendments and land reclamation. The regulations set limits for Escherichia coli (or fecal coliforms), Salmonella spp., helminth eggs and enterovirus. However, the potential risk due to antibiotic resistant bacteria (ARB) and other human potential pathogenic bacteria (HPB) are not considered.In this work, three sludge treatment processes, having in common an anaerobic digestion step, were applied to assess the removal of regulated bacteria (fecal coliforms, Salmonella spp), ARB and HPB. The treatment arrangements, fed with PS from a full-scale wastewater treatment plant were: 1) Mesophilic anaerobic digestion followed by alkaline stabilization post-treatment (MAD-CaO); 2) Thermophilic anaerobic digestion (TAD) and, 3) Pre-treatment (mild thermo-hydrolysis) followed by TAD (PT-TAD). The results address the identification, quantification (colony forming units) and taxonomic characterization of ARB resistant to β-lactams and vancomycin, as well as the taxonomic characterization of HPB by sequencing with PacBio. In addition, quantification based on culture media of fecal coliforms and Salmonella spp. is presented.The capabilities and limitations of microbiological and metataxonomomic analyses based on PacBio sequencing are discussed, emphasizing that they complement each other. Genus Aeromonas, Acinetobacter, Citrobacter, Enterobacter, Escherichia, Klebsiella, Ochrobactrum, Pseudomonas and Raoultella, among others, were found in the PS, which are of clinical or environmental importance, being either HPB, HPB-ARB, or non-pathogenic ARB with the potentiality of horizontal gene transfer. Based on the analysis of fecal coliforms and Salmonella spp., the three processes produced class A (highest) biosolids, suitable for unrestricted agriculture applications. Mild thermo-hydrolisis was effective in decreasing ARB cultivability, but it reappeared after the following TAD. O. intermedium (HPB-ARB) was enriched in MAD and TAD while Laribacter hongkongensis (HPB) did persist after the applied treatments.