Sludge Solids Research Articles

Process Design for Biohydrogen Production from Waste Materials and Its Application

Biohydrogen is regarded as an attractive future clean energy carrier due to its high energy content and environmentally friendly conversion. Biohydrogen reactor is widely used in studies concerning the anaerobic co-digestion of food waste, sewage sludge, wastewater and other organic solids. Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material (biomass or waste feedstock) in the absence of oxygen to produce biogas, which may generate electricity and heat, or can be processed into renewable natural gas and transportation fuels. This review article explains the scientific processes of anaerobic digestion process such as hydrolysis, acidogenesis, acetogenesis and hydrogenesis as well as methods to produce biohydrogen gas such as fermentation and biophotolysis for the waste management technology and sources of renewable energy and concludes with solutions that may allow anaerobic digestion to become more widely adopted throughout the developing countries to control the waste management system.

Adsorptive removal of chromium(VI) using Cu/Fe impregnated activated carbon prepared from solid sludge

Adsorptive removal of chromium(VI) using Cu/Fe impregnated activated carbon prepared from solid sludge

Comparison of energy-based disposal schemes for municipal solid waste and sewage sludge: Based on two life cycle assessment methods

Comparison of energy-based disposal schemes for municipal solid waste and sewage sludge: Based on two life cycle assessment methods

Fundamental Studies on CO2 Sequestration of Concrete Slurry Water Using Supercritical CO2

To prevent drastic climate change due to global warming, it is necessary to transition to a carbon-neutral society by reducing greenhouse gas emissions in all industrial sectors. This study aims to prepare measures to reduce the greenhouse gas in the cement industry, which is a large source of greenhouse gas emissions. The research uses supercritical CO2 carbonation to develop a carbon utilization fixation technology that uses concrete slurry water generated via concrete production as a new CO2 fixation source. Experiments were conducted using this concrete slurry water and supernatant water under different conditions of temperature (40 and 80 °C), pressure (100 and 150 bar), and reaction time (10 and 30 min). The results showed that reaction for 10 min was sufficient for complete carbonation at a sludge solids content of 5%. However, reaction products of supernatant water could not be identified due to the presence of Ca(HCO3)2 as an aqueous solution, warranting further research.

Report of Antibiotic Resistance in Urban and Rural Wastewaters from West Bengal, India

Aims: This study aims at comparative identification of antibiotic resistance patterns in bacteria isolated from samples collected from rural environment (LS) and urban environments (SS). Metagenomic profiling gave us insights into the microbial abundance of the two samples. This study focused on culture-based methods for complete identification of antibiotic resistant isolates and estimation of comparative antibiotic resistance among the two samples.
 Study Design: Untreated medical waste and anthropogenic waste disposal can lead to the propagation of different antibiotic resistant strains in wastewater environments both in urban and rural set ups which provide an insight towards this study approach mentioned in the methodology segment.
 Place and Duration of Study: Sewer system of a medical facility located in Purulia, India was the collection site for liquid sludge. Solid sludge and associated wastewater were collected in vicinity of a large urban medical facility from central Kolkata, India.
 Methodology: Physico-chemical properties were analyzed followed by microbiological and biochemical characterization. The antibiotic resistance patterns were determined by Kirby-Bauer disc diffusion assay. Potent multidrug resistant isolates were identified using 16srRNA gene amplification followed by Phylogenetic profiling, using CLC Genomics workbench.
 Results: We observed maximum resistance in an E. coli isolate which was resistant up to 22 antibiotics. Combined data for resistance from urban and rural samples were found to exhibit 83.9% resistance to beta lactams, 85.7% to macrolides, 44.2% to fluoroquinolones, 50% to glycopeptides and cephalosporins, 35.7 % to carbapenems and sulfonamides, 28.5 % to tetracycline, and 23.8 % to aminoglycosides.
 Conclusion: The high prevalence of antibiotic-resistant bacteria harbouring diverse resistance traits across samples indicated towards probable horizontal gene transfer across environmental niches. This study can prove to be useful to understand and map the patterns of resistance and stringently apply the counter measures related to public health practices.

The fate of heavy metals in excess sludge during disintegration by discharge plasma

• Plasma oxidation promoted heavy metals releasing from solid sludge. • The ratio of organic bounded metals decreased upon plasma treatment. • Contamination degree and environmental risk were weakened after plasma treatment. • Floc destruction and EPS change significantly affected the fate of heavy metals. Heavy metals accumulate in active sludge, and their fate and environmental risks might change during disintegration, which is essential for the subsequent treatment of the active sludge. In this study, the distribution of heavy metals (Zn, Cr, Cd, Ni, and Pb), transformation of their speciation fractions and leaching potentials were explored. Meanwhile, to explore the reasons for the changes, the evolution in morphology of sludge flocs and extracellular polymer substances (EPS) were also examined. The results showed that accompanied by the destruction of sludge flocs and the release of inner organic materials, large numbers of wrapped heavy metals were released from the solid phase into the liquid phase under discharge plasma treatment, especially for those organic bounded metals. As a consequence, the ratio of oxidizable fractions (F3) gradually decreased. The contamination degree and environmental risks of heavy metals were evaluated based on the fraction contents, and it was concluded that discharge plasma treatment could be in favor of reducing the contamination degree and environmental risks. In addition, the leaching contents of Cr decreased after plasma oxidation, while Pb, Zn, Ni and Cd contents increased; but were far below the limit values.

Enhanced Separation of Oil and Solids in Oily Sludge by Froth Flotation at Normal Temperature

Oily sludge (OS) contains a large number of hazardous materials, and froth flotation can achieve oil recovery and non-hazardous disposal of OS simultaneously. The influence of flotation parameters on OS treatment and the flotation mechanism were studied. OS samples were taken from Shengli Oilfield in May 2017 (OSS) and May 2020 (OST), respectively. Results showed that Na2SiO3 was the suitable flotation reagent treating OSS and OST, which could reduce the viscosity between oil and solids. Increasing flotation time, impeller speed and the ratio of liquid to OS could enhance the pulp shear effect, facilitate the formation of bubble and reduce pulp viscosity, respectively. Under the optimized parameters, the oil content of OST residue could be reduced to 1.2%, and that of OSS could be reduced to 0.6% because of OSS with low heavy oil components and wide solid particle size distribution. Orthogonal experimental results showed that the impeller speed was the most significant factor of all parameters for OSS and OST, and it could produce shear force to decrease the intensity of C-H bonds and destabilize the OS. The oil content of residue could be reduced effectively in the temperature range of 24–45 °C under the action of high impeller speed.

Evaluation of Physical and Chemical Properties of Poultry Sludge and its Suitability for Reuse in Agricultural and Non-Agricultural Applications

Disposal of poultry sludge is one of the great challenges facing cities because of very strict requirements for landfilling and the scarcity of space for landfills. The present study was therefore aimed at evaluating the physical and chemical properties of poultry sludge and its suitability for reuse in agricultural and non-agricultural applications. Three samples were collected from sludge at the wastewater treatment plant of Al-Thuraya slaughterhouse in Al-Mafraq District, Jordan. The physical and biochemical properties of these samples were analyzed. Also, elemental composition and heat value were determined. The results indicated that poultry sludge had a slightly alkaline pH and a total moisture content of 20%, as well as an average total solid of 80%. The dry solid sludge had a volatile solid content of 94.9% and 5.1% of ash. Also, dry sludge had a high protein content (62 %) followed by carbohydrate (20%) and fiber (17%), with fat being around 1%. The major elements in the sludge were carbon (65.5%) followed by nitrogen (16%), phosphorous (5 %) and sulfur (2%). Heavy metal concentrations in dry sludge ranged from 0.01 to 2 mg/kg. These heavy metal concentrations were well below the safe limits recommended by legislators for sludge used as a fertilizer. The findings from this study revealed that dry poultry sludge offers a wide range of potential uses as fertilizer, animal feed and a source of energy, and it should be considered as a potentially valuable and sustainable resource rather than a waste product.

Evaluation of impact of sludge types and solids content on sludge treatment using microwave enhanced advanced oxidation process

ABSTRACT The microwave enhanced advanced oxidation process (MW-AOP) has been applied to pre-treat different sludge types and high solids content. Secondary sludge not only had the highest solids and nutrient content but also yielded higher treatment efficiency than primary or mixed sludge. In the case of secondary sludge with 4% total solids (TS), the total suspended solids (TSS) concentration was reduced by 32% while soluble chemical oxygen demand concentration increased from 1% to 40% after treatment at 110°C. A high level of nutrient release was also achieved; about 65% total phosphate (TP) solubilized at 110°C. The degree of secondary sludge disintegration was dictated by temperature and hydrogen peroxide dosage. The optimal operating temperature for the system was 110°C, and sludge containing TS up to 8% was treated effectively. Secondary sludge with 8% TS had a TSS reduction of 41% after treatment at 110°C while COD solubilization was about 45%; about 55% TP was solubilized at 10 min holding time. Treatment of sludge with higher solids content would allow for handling larger amounts of sludge at a given period and reduce heating cost per unit of treated sludge. The inter-relationship between the degree of sludge disintegration and changes in chemical and physical properties was also clearly demonstrated here. The treated sludge would be an ideal substrate for anaerobic digestion or phosphorous recovery processes. High levels of nutrients (phosphorus and nitrogen) and metal release, and solids disintegration from sludge containing high solids content would make subsequent resource recovery processes more effective and economical.

Closing global P cycles: The effect of dewatered fish sludge and manure solids as P fertiliser.

The aim of this study was to contribute to closing global phosphorus (P) cycles by investigating and explaining the effect of fish sludge (feed residues and faeces of farmed fish) and manure solids as P fertiliser. Phosphorus quality in 14 filtered and/or dried, composted, separated or pyrolysed products based on fish sludge or cattle or swine manure was studied by sequential chemical fractionation and in two two-year growth trials, a pot experiment with barley (Hordeum vulgare) and a field experiment with spring wheat (Triticum aestivum). In fish sludge, P was mainly solubilised in the HCl fraction (66±10%), commonly being associated with slowly soluble calcium phosphates, and mean relative agronomic efficiency (RAE) of fish sludge products during the first year of the pot experiment was only 47±24%. Low immediate P availability was not compensated for during the second year. Thus efforts are needed to optimise the P effects if fish sludge is to be transformed from a waste into a valuable fertiliser. In manure solids, P was mainly soluble in H2O and 0.5M NaHCO3 (72±14%), commonly being associated with plant-available P, and mean RAE during the first year of the pot experiment was 77±19%. Biochars based on fish sludge or manure had low concentrations of soluble P and low P fertilisation effects, confirming that treatment processes other than pyrolysis should be chosen for P-rich waste resources to allow efficient P recycling. The field experiment supported the results of the pot experiment, but provided little additional information.

Fate and removal of silver nanoparticles during sludge conditioning and their impact on soil health after simulated land application

The growing use of silver nanoparticles (AgNPs) in personal care products and clothing has increased their concentrations in wastewater and subsequently in sludge raising concerns about their fate and toxicity during wastewater treatment and after land application of sludge. This research investigated the fate and removal of AgNPs during chemical conditioning of anaerobically digested sludge and their impact on soil bacteria and health after land application. Ferric chloride (FeCl3), alum (Al2 (SO4)3 • (14–18) H2O), and synthetic (polyacrylamide) polymer were used for sludge conditioning. All conditioners effectively removed AgNPs from the liquid phase and concentrated them in sludge solids. Concentration analyses showed that out of 53.0 mg/L of silver in the sludge, only 0.1 to 0.003 mg/L of silver remained in the sludge supernatant after conditioning and 12 to 20% of this value were particulates. Morphological analyses also showed that AgNPs went through physical, chemical, and morphological changes in sludge that were not observed in nanopure water and the resulting floc structures and the incorporation of nanoparticles were different for each conditioner. The impact of conditioned AgNPs on the biological activities of soil was evaluated by investigating its impact on the presence of five important phyla (Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria). The results showed that AgNPs at a concentration of 20 mg AgNPs/g soil had a minimal impact on the presence and diversity of the assessed phyla. Also, using different chemicals for sludge conditioning resulted in different growth behavior of studied phyla. This study provides new insight into how the presence of AgNPs and different chemicals used for sludge conditioning might impact the soil biological activities and hence plant growth. The study also provides a solid basis for further research in the risk assessment of nanoparticle toxicity in biosolids amended soils.

Applying ultrasound plus electrokinetics to enhance sludge dewatering

Most sludges are dewatered using mechanical dewatering systems that can bring the weight percentage of the sludge solids up to a maximum of about 40 TS%. This study examined a new way to enhance pressure-filter dewatering by adding ultrasound and electrokinetics. The ultrasound adds a “stripping action” to free up bound water, and the electrokinetics adds a “pumping action” to push the freed-up water out of the sludge. This new enhanced dewatering process specifically targets bio and sewage sludges. The development work reported here, however, makes use of a sludge surrogate to determine (1) how to best optimize the enhanced dewatering process, and (2) just how much more water can be economically removed. The results demonstrated a substantial dewatering improvement to 55 TS%. A follow-up paper will demonstrate how this optimized ultrasound- and electrokinetics-enhanced dewatering process performs with bio and sewage sludges.

Numerical simulation analysis on the characteristics of municipal domestic waste with high calorific value and sludge co-combustion

The combustion characteristics of a high calorific value domestic waste blend with municipal sludge of 350 t/d, 750 t/d, and 900 t/d in three kinds of mainstream mechanical waste incinerators in Guangdong were studied by numerical simulation method, and the model was verified with the operating data. Based on the “3 T” principle of incinerator, the combustion characteristics of mixed fuel with high calorific value municipal solid waste and sludge in different incinerators were analyzed. The results showed that when the maximum blending ratios of sludge in the 350 t/d, 750 t/d, and 900 t/d waste incinerators were 13%, 17%, and 10%, respectively, and the residence times of flue gas in the temperature region above 1123 K were 4.18 s, 3.68 s, and 6.36 s, respectively. The 750 t/d waste incinerator adopts a spiral secondary air injection mode, which is beneficial to the realization of “3 T” principle. The secondary air of the 350 t/d waste incinerator adopts the hedge injection mode, the first flue velocity is larger, and the flue gas residence time is shorter. 900 t/d waste incinerator combined with spiral and hedge injection, the temperature at the exit of one flue decreased by 61.12 K and 132.62 K compared to 0% working condition for 10% and 17% sludge blending, respectively, and the oxygen content increased from 2.97% to 3.91% and 5.63%. The appropriate primary and secondary air ratio and effective secondary air configuration can adjust the incinerator to be in the best operating condition.

Optimization of energy-water-waste nexus at district level: A techno-economic approach

The United Nations (UN) Sustainable Development Goals (SDG) includes sustainable clean water and clean energy supply. Contrary to the typical dogma, wastewater is now a valuable source to recover certain resources as the effluent stream is also applicable to be reused and/or reclaimed, minimizing freshwater consumption. Hence, clean water and clean energy can be obtained from the integration works. However, the attempt to establish a tripartite energy-water-waste nexus at the district level is yet to be made, hence requiring the use of the mathematical approach to perform simultaneous optimization of the nexus. This study develops a mathematical model to optimize the nexus for a centralized facility, which is applicable at the district level. The sources are from domestic and industrial wastewaters and multiple contaminants are considered for optimization. The recovery processes are intended to recover biogas, phosphorus, nitrogen, metal, digestate, solid sludge, and microalgae oil from the wastewater streams, while supplying reused and/or reclaimed water for various application options. Connection items and segregation tanks are also included in the model, as well as the techno-economics equations. The model is formulated as mixed-integer non-linear programming (MINLP). As multiple resources can be recovered in the facility, a case study is conducted. A total of 3.2 MW of renewable energy can be recovered, and freshwater consumption is reduced by 66%. Supply water selling price, industrial processing fee value, and maximum length of the main transmission pipeline are the important factors affecting the project's economics. As there are no subsidies set, the study provides insight on how the tripartite nexus can be symbiotically integrated at the district level.

Factors influencing recovery of SARS-CoV-2 RNA in raw sewage and wastewater sludge using polyethylene glycol-based concentration method.

Wastewater surveillance for monitoring severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an important epidemiologic tool for the assessment of population-wide coronavirus disease 2019 (COVID-19). This tool can be successfully implemented only if SARS-CoV-2 RNA in wastewater can be accurately recovered and quantified. The lack of standardized procedure for wastewater virus analysis has resulted in varying SARS-CoV-2 concentrations for the same sample. This study reports the effect of 4 key factors-sample volume, percentage polyethylene glycol (PEG)-NaCl, incubation period, and storage duration at 4°C-on the recovery of spiked noninfectious SARS-CoV-2 RNA in raw sewage and sludge samples. N1 and N2 genes of SARS-CoV-2 were quantified using the reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and digital droplet PCR (RT-ddPCR) techniques. Results indicate that 1) for raw sewage, 50-ml sample volume, 30% PEG-NaCl addition, 6-h incubation, and sample analysis within 24 h of collection can result in much better RNA recovery (RT-qPCR: 72% for N1 and 82% for N2; RT-ddPCR: 55% for N1 and 85% for N2) when compared with commonly used PEG-based method; 2) for sludge, the sample analysis using raw sewage protocol and all other variations of each factor mostly resulted in false negatives for both N1 and N2. The absence of N1 and N2 suggests that sludge samples probably need a pretreatment step that releases RNA entrapped in sludge solids back into bulk solution. In conclusion, our modified PEG-based concentration method can cut down the analysis time at least by half, which in turn helps to implement early detection system for SARS-CoV-2 in wastewater.

Application of general third-order Gaussian distribution activation energy model in the combustion of solid waste

ABSTRACT Solid waste had a variety of components, and its complex and obscure combustion behavior brought challenges to calculate the kinetic parameters accurately. Therefore, the thermogravimetric experiments of municipal solid waste and sewage sludge were carried out and a general third-order Gaussian distributed activation energy model was built. The combustion components of solid waste could be divided into three categories: light volatile components, heavy volatile components, and fixed carbon. The kinetic model matched the experimental data well, the adjR2 was above 0.9994 and the global activation energy was within a reasonable range of 54.85 kJ/mol-178.04 kJ/mol. Local sensitivity analysis found that the average activation energy had a strong influence on the stability of the model, and the degree of influence was positively correlated with the content of combustion components. In addition, the model had the ability to predict experimental data under unknown conditions.

Optimization and evaluation of malathion removal by electrocoagulation process and sludge management

In this research work, the effect of various independent operating variables on the removal of malathion pesticide by electrocoagulation (EC) process was evaluated by single variable multivariable optimization. The various operating variables selected were reaction time (40–75 min), pH (6−9), applied voltage (8–20 V), and amount of electrolyte (8–20 ml). The effect of aluminium and iron electrode on the removal efficiency were compared and aluminium was found to be better performed, further RSM studies were carried out with this electrode. The optimum operational conditions were achieved as reaction time of 75 min, solution pH of 7.5, applied voltage of 15 V and the electrolyte addition of 15 ml. A removal efficiency of 85% was obtained at an energy consumption of kWh. by incorporating the interaction effects of different optimized variables. In this paper, the facts associated with the removal mechanism and the operational cost analyses were performed. The reaction behaviour has been discussed through mini reaction pathways and results were supported by SEM, EDAX and FTIR characterizations. End products of the EC process are the dissociated pesticide with metal hydroxide particles. This study also discusses the disposal of solid sludge with economic and environmental benefits used as admixture in the stabilization of expansive soils. The sludge fused soil properties were improved when compared to the plain soil properties and the classification of soil were changed from moderate to stiff soil. This indicates the possible use of sludge as stabilizing agent subsequently and it can address the disposal and environment concern related to the end products of EC process in economic way.Effect of this soil on the ground water pollution was also investigated.

Microwave-alkali treatment with water elutriation to enhance waste activated sludge solubilization for carbon sources and nutrients recovery

• The dissolution degree of COD was 12.38 ± 0.20% with MW treatment alone, 54.68 ± 0.28% by MA, and 55.58 ± 0.68% by MAW. • A VSS/TSS ratio reduction was 10.50% by alkali treatment, and 22.60% by MA. • Water elutriation after MA treatment improved TP and TN release more significantly. • High MW power inputs, long MW heating time, or high sludge solids concentration could cause gelatinization and carbonization. Effects of MW treatment, combined MW-alkali (MA) treatment, and combined MA-water elutriation (MAW) treatment on sludge solubilization were studied; this work aims at the recovery of carbon sources and nutrients. Effects of MW power input, MW heating time, NaOH dosages, alkali treatment durations, and sludge solids concentration on sludge solubilization were explored. These treatments produced good chemical oxygen demand (COD) dissolution degree outcomes, with optimal values of 12.38 ± 0.20% (MW treatment), 54.68 ± 0.28% (MA treatment), and 55.58 ± 0.68% (MAW treatment), respectively. Besides, MW treatment could disrupt sludge flocs and biomass cell membranes, with a 120 mg/L volatile fatty acids (VFAs) release (280 W for 5 min). A reduction of VSS/TSS ratio was 10.50% by alkali treatment, and 22.60% by MA treatment. Compared with MA treatment only, water elutriation after MA resulted in outstanding nutrients release, with optimal dissolution degrees increasing from 67.87 ± 0.52% to 81.87 ± 0.60% (TP), and 56.08 ± 0.94% to 73.73 ± 1.04% (TN), respectively. MW power inputs, MW heating duration, alkali treatment time, and especially NaOH dosages had significant effects on sludge solubilization, but solids concentration had a minor effect on sludge solubilization. Moreover, higher MW power input, longer MW heating time, and greater sludge solids concentration could cause gelatinization and even carbonization.

Microbial electrolysis cells for return flow: Simultaneous nitrogen and carbon removal

The concentration of solids in secondary sludge before anaerobic digestion in a wastewater treatment plant, bring about the production of a return flow, which contains high concentrations of all the common pollutant parameters. This return flow could unfavourably affect the performance of the processes and effluent quality of the waterline. Here, we report the utilisation of three similar microbial electrolysis cells reactors that performs simultaneous carbon and nitrogen removal to reduce the impact of the return flow in the plant. The result of the batch-fed (72 h) experiment showed COD and total nitrogen removal efficiencies that reached 90% and 80%, respectively, supporting the premise that return flows are suitable substrates for a bioelectrochemical treatment. The three reactors followed similar trends, showing good replicability and confirming the potential of MECs as a feasible technology for return flow treatment. Furthermore, when cathodic conversion efficiency was higher than 80%, the pure hydrogen production allows to recover the electric energy consumption, indicating that the system could be theoretically energy neutral.

Acidic aerobic digestion of anaerobically-digested sludge enabled by a novel ammonia-oxidizing bacterium

Anaerobic digestion is a commonly used process for the reduction and stabilization of wasted activated sludge generated in wastewater treatment plants. However, anaerobically-digested (AD) sludge is still a problematic waste stream due to its large volume and often poor quality. In this study, two aerobic digesters were set up to treat anaerobically-digested sludge, with one digester operated in self-generated acidic condition as the experimental reactor, and one at neutral pH as the control reactor. The acidic condition in the experimental reactor was driven by an inoculated special ammonia-oxidizing bacterium, ‘Candidatus Nitrosoglobus’, which can tolerate low pH. As a result of ammonium oxidation by Ca. Nitrosoglobus, the pH decreased to 4.8 ± 0.2 and nitrite accumulated to and stayed at 200.0 ± 17.2 mg N L−1, from which free nitrous acid (FNA) at 8.5 ± 1.8 mg HNO2N L−1 formed in-situ. As a combined effect of low pH and high concentration of FNA, the experimental reactor reduced the total solids (TS), volatile solids (VS) and non-volatile solids (NVS) in the AD sludge by 25.2 ± 7.0%, 29.8 ± 4.3%, and 22.6 ± 5.5%, respectively. In contrast, the control reactor without Ca. Nitrosoglobus inoculation (operated at a near-neutral pH of 6.8 ± 0.3 and no FNA formation) only reduced VS in the AD sludge by 10.4 ± 4.3%, along with negligible NVS reduction. Additionally, the acidic aerobic digestion in the experimental reactor significantly stabilized AD sludge, decreasing the specific oxygen uptake rate (SOUR) to 0.5 ± 0.1 mg O2 g−1VS h−1 and the most probable number (MPN) of Faecal Coliforms to 2.4 ± 0.1 log(MPN g−1TS), both of which meet USEPA standards for Class A biosolids. In comparison, the control reactor produced biosolids at Class B level only, with an SOUR of 1.8 ± 0.2 mg O2 g−1VS h−1 and a Faecal Coliforms MPN of 3.6 ± 0.1 log(MPN g−1TS). By reducing the volume and improving the quality of the AD sludge, the acidic aerobic digestion of AD sludge enabled by Ca. Nitrosoglobus has the potential to significantly save the sludge disposal costs in wastewater treatment.