The increasing global population and urbanization have led to significant challenges in waste management, particularly concerning vacuum blackwater (VBW), which is the wastewater generated from vacuum toilets. Traditional treatment methods, such as landfilling and composting, often fall short in terms of efficiency and sustainability. Anaerobic digestion (AD) has emerged as a promising alternative, offering benefits such as biogas production and digestate generation. However, the performance of AD can be influenced by various factors, including the composition of the feedstock, pH levels, and the presence of inhibitors. This review investigates the effects of calcium oxide (CaO)-modified biochar (BC) as an additive in AD of VBW. Modifying BC with CaO enhances its alkalinity, nutrient retention, and adsorption capacity, creating a more favorable environment for microorganisms and promoting biogas production, which serves as a valuable source of heat, fuel and electricity. Additionally, the digestate can be processed through plasma pyrolysis to ensure the complete destruction of pathogens while promoting resource utilization. Plasma pyrolysis operates at extremely high temperatures, effectively sterilizing the digestate and eliminating both pathogens and harmful contaminants. This process not only guarantees the safety of the end products, but also transforms organic materials into valuable outputs such as syngas and slag. The syngas produced is a versatile energy carrier that can be utilized as a source of hydrogen, electricity, and heat, making it a valuable resource for various applications, including fuel cells and power generation. Furthermore, the slag has potential for reuse as an additive in the AD process or as a biofertilizer to enhance soil properties. This study aims to provide insights into the benefits of using modified BC as a co-substrate in AD systems. The findings will contribute to the development of more sustainable and efficient waste management strategies, addressing the challenges associated with VBW treatment while promoting renewable energy production.

Department of Biological and Health Sciences Pak Austria Fachhochschule Institute of Applied Sciences and Technology Khanpur Road Haripur 22621 Pakistan

Faculty of Education Southwest University Chongqing 400715 China

Institute of Environmental Science Shanxi University Taiyuan 030006 China

Orlen Unicre a s Revolucňí 1521 84 400 01 Ústí nad Labem Czech Republic

School of Civil and Environmental Engineering Nanchang Institute of Science and Technology Nanchang 330108 China

School of Economics and Management Nanchang Institute of Science and Technology Nanchang 330100 China

School of Education Nanchang Institute of Science and Technology Nanchang 330108 China

Zobrazit více v PubMed

Oliveira T.J.J., da Fonseca Santiago A., da Silva Lanna M.C., Fongaro G., Milagres N.L., Cunha T.R., Corrêa A.L.I. Rural blackwater treatment by a full-scale Brazilian Biodigester Septic Tank: Microbial indicators and pathogen removal efficiency. Environ. Sci. Pollut. Res. Int. 2021;28:23235–23242. doi: 10.1007/s11356-020-12229-2. PubMed DOI

Orozco R.L., Iturriaga M.H., Tamplin M.L., Fratamico P.M., Call J.E., Luchansky J.B., Escartin E.F. Animal and environmental impact on the presence and distribution of Salmonella and Escherichia coli in hydroponic tomato greenhouses. J. Food Prot. 2008;71:676–683. doi: 10.4315/0362-028x-71.4.676. PubMed DOI

Kowalczyk K., Kłapeć T. Contamination of soil with eggs of geohelminths Ascaris spp., Trichuris spp., Toxocara spp. in Poland—Potential source of health risk in farmers. Ann. Parasitol. 2020;66:433–440. doi: 10.17420/ap6604.283. PubMed DOI

Mustafa S.A., Al-Rudainy A.J., Salman N.M. Effect of environmental pollutants on fish health: An overview. Egypt. J. Aquat. Res. 2024;50:225–233. doi: 10.1016/j.ejar.2024.02.006. DOI

World Bank Group . Air Transport—Annual Report 2022. Volume 1 World Bank Group; Washington, DC, USA: 2022.

Wu H., Wang G., Li L., Gao Z., Wang M., Wang J., Zhang Z., Wang A., Tian X., Li J. Partial nitritation and nitrogen removal of vacuum toilet wastewater from high-speed trains in a sequential batch reactor. Chemosphere. 2023;329:138657. doi: 10.1016/j.chemosphere.2023.138657. PubMed DOI

Arifan F., Abdullah A., Sumardiono S. Effectiveness Analysis of Anaerobic Digestion Method in Making Biogas from Animal Manure and Tofu Liquid Waste. J. Ilmu Teknol. Has. Ternak. 2021;16:84–94. doi: 10.21776/ub.jitek.2021.016.02.2. DOI

Bolzonella D., Fatone F., Gottardo M., Frison N. Nutrients recovery from anaerobic digestate of agro-waste: Techno-economic assessment of full scale applications. J. Environ. Manag. 2018;216:111–119. doi: 10.1016/j.jenvman.2017.08.026. PubMed DOI

Nag R., Auer A., Nolan S., Russell L., Markey B.K., Whyte P., O’Flaherty V., Bolton D., Fenton O., Richards K.G., et al. Evaluation of pathogen concentration in anaerobic digestate using a predictive modelling approach (ADRISK) Sci. Total Environ. 2021;800:149574. doi: 10.1016/j.scitotenv.2021.149574. PubMed DOI

Ahmad M., Lee S.S., Dou X., Mohan D., Sung J.K., Yang J.E., Ok Y.S. Effects of pyrolysis temperature on soybean stover- and peanut shell-derived biochar properties and TCE adsorption in water. Bioresour. Technol. 2012;118:536–544. doi: 10.1016/j.biortech.2012.05.042. PubMed DOI

Devi P., Eskicioglu C. Effects of biochar on anaerobic digestion: A review. Environ. Chem. Lett. 2024;22:2845–2886. doi: 10.1007/s10311-024-01766-8. DOI

Ding J., Zhen F., Kong X., Hu Y., Zhang Y., Gong L. Effect of Biochar in Modulating Anaerobic Digestion Performance and Microbial Structure Community of Different Inoculum Sources. Fermentation. 2024;10:151. doi: 10.3390/fermentation10030151. DOI

Viaene J., Peiren N., Vandamme D., Lataf A., Cuypers A., Debeer L., Vandecasteele B. Science of the Total Environment Application of biochar to anaerobic digestion versus digestate: Effects on N emissions and C stability. Sci. Total Environ. 2024;915:170124. doi: 10.1016/j.scitotenv.2024.170124. PubMed DOI

Lee J., Lee S., Park Y.K. Reduction of odor-causing compounds in wastewater using biochar: A review. Bioresour. Technol. 2023;385:129419. doi: 10.1016/j.biortech.2023.129419. PubMed DOI

Chiang P.F., Zhang T.L., Maurice N.J., Claire M.J., Gentil B., Memon A.G., Giwa A.S. Impacts of Polyvinyl Alcohol and Chitosan-Modified Biochar on the Anaerobic Digestion of Sewage Sludge and Valuable Resource Recovery. Processes. 2024;12:1987. doi: 10.3390/pr12091987. DOI

Zhang M., Wang Y. Effects of Fe-Mn-modified biochar addition on anaerobic digestion of sewage sludge: Biomethane production, heavy metal speciation and performance stability. Bioresour. Technol. 2020;313:123695. doi: 10.1016/j.biortech.2020.123695. PubMed DOI

Wang J., Wang S. Preparation, modification and environmental application of biochar: A review. J. Clean. Prod. 2019;227:1002–1022. doi: 10.1016/j.jclepro.2019.04.282. DOI

Khater E.S., Bahnasawy A., Hamouda R., Sabahy A., Abbas W., Morsy O.M. Biochar production under different pyrolysis temperatures with different types of agricultural wastes. Sci. Rep. 2024;14:2625. doi: 10.1038/s41598-024-52336-5. PubMed DOI PMC

Chiang P.F., Claire M.J., Han S., Maurice N.J., Giwa A.S. Effectiveness of Torrefaction By-Products as Additive in Vacuum Blackwater under Anaerobic Digestion and Economic Significance. Processes. 2023;11:3330. doi: 10.3390/pr11123330. DOI

Elhenawy Y., Fouad K., Bassyouni M., Al-Qabandi O.A., Majozi T. Yield and energy outputs analysis of sawdust biomass pyrolysis. Energy Convers. Manag. X. 2024;22:100583. doi: 10.1016/j.ecmx.2024.100583. DOI

Rafiq M.K., Bachmann R.T., Rafiq M.T., Shang Z., Joseph S., Long R.L. Influence of pyrolysis temperature on physico-chemical properties of corn stover (Zea mays L.) biochar and feasibility for carbon capture and energy balance. PLoS ONE. 2016;11:e0156894. doi: 10.1371/journal.pone.0156894. PubMed DOI PMC

Gai X., Wang H., Liu J., Zhai L., Liu S., Ren T., Liu H. Effects of feedstock and pyrolysis temperature on biochar adsorption of ammonium and nitrate. PLoS ONE. 2014;9:e113888. doi: 10.1371/journal.pone.0113888. PubMed DOI PMC

Lian F., Sun B., Song Z., Zhu L., Qi X., Xing B. Physicochemical properties of herb-residue biochar and its sorption to ionizable antibiotic sulfamethoxazole. Chem. Eng. J. 2014;248:128–134. doi: 10.1016/j.cej.2014.03.021. DOI

Sun J., Lian F., Liu Z., Zhu L., Song Z. Biochars derived from various crop straws: Characterization and Cd(II) removal potential. Ecotoxicol. Environ. Saf. 2014;106:226–231. doi: 10.1016/j.ecoenv.2014.04.042. PubMed DOI

Devi P., Saroha A.K. Effect of pyrolysis temperature on polycyclic aromatic hydrocarbons toxicity and sorption behaviour of biochars prepared by pyrolysis of paper mill effluent treatment plant sludge. Bioresour. Technol. 2015;192:312–320. doi: 10.1016/j.biortech.2015.05.084. PubMed DOI

Li Y., Zhang J., Wen X., Mazarji M., Chen S., Liu Q., Zhao S., Feng L., Li G., Zhou H., et al. Advancing anaerobic digestion with MnO2-modified biochar: Insights into performance and mechanisms. Sci. Total Environ. 2024;954:176303. doi: 10.1016/j.scitotenv.2024.176303. PubMed DOI

Wang Q., Zhang X., Sun S., Wang Z., Cui D. Effect of CaO on Pyrolysis Products and Reaction Mechanisms of a Corn Stover. ACS Omega. 2020;5:10276–10287. doi: 10.1021/acsomega.9b03945. PubMed DOI PMC

Wang Y., Wang K., Zhou X., Dai B., Du D. Calcium oxide enhances the anaerobic co-digestion of excess sludge and plant waste: Performance and mechanism. Water Sci. Technol. 2024;90:1267–1279. doi: 10.2166/wst.2024.268. PubMed DOI

Yu F.C., Phalak N., Sun Z., Fan L.S. Activation strategies for calcium-based sorbents for CO2 capture: A perspective. Ind. Eng. Chem. Res. 2012;51:2133–2142. doi: 10.1021/ie200802y. DOI

Li B., Magoua Mbeugang C.F., Huang Y., Liu D., Wang Q., Zhang S. A review of CaO based catalysts for tar removal during biomass gasification. Energy. 2022;244:123172. doi: 10.1016/j.energy.2022.123172. DOI

Shen C., Wu G., Sun J., Hou J., Sun H., Ding K., Liu W., Zhang S. Controlled synthesis of innovative carbon-based CaO2 materials with boosted oxygen release performance in the aqueous environment. J. Environ. Chem. Eng. 2023;11:109616. doi: 10.1016/j.jece.2023.109616. DOI

Li J.-G., Xiong Z.-R., Zhang L.-H., Duan F. Interaction investigation of three forest waste biochars and CaO in the process of Ca-L/CARBONOx. Fuel. 2023;337:126841. doi: 10.1016/j.fuel.2022.126841. DOI

Hu A., Jiang Y., An J., Huang X., Elgarhy A.H., Cao H., Liu G. Novel Fe/Ca oxide co-embedded coconut shell biochar for phosphorus recovery from agricultural return flows. RSC Adv. 2024;14:27204–27214. doi: 10.1039/D4RA04795H. PubMed DOI PMC

Zhang H., Zhao J., Fu Z., Wang Y., Guan D., Xie J., Zhang Q., Liu Q., Wang D., Sun Y. Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion. Bioresour. Technol. 2023;387:129647. doi: 10.1016/j.biortech.2023.129647. PubMed DOI

Bai Y., Huang R., Li S., Li X., Fan Q., Liu S., Hu L. Potential of Calcium-Modified Biochar for Soil Nutrient and Carbon Sequestration in Citrus Orchards. Agriculture. 2024;14:2222. doi: 10.3390/agriculture14122222. DOI

Liu Y., Tian L., Qiu W., Sun W., Pan C., Fan G., Cheng Q. Impact thermal and calcium oxide pretreatment on the anaerobic digestion of food waste: Performance and carbon emissions. Int. J. Environ. Sci. Technol. 2024 doi: 10.1007/s13762-024-05863-7. DOI

Tie H.O., Che Man H., Koyama M., Syukri F., Yusoff F.M., Toda T., Nakasaki K., Mohamed Ramli N. The effect of calcium hydroxide addition on enhancing ammonia recovery during thermophilic composting in a self-heated pilot-scale reactor. Waste Manag. 2023;166:194–202. doi: 10.1016/j.wasman.2023.04.046. PubMed DOI

Gao M., Zhang L., Florentino A.P., Liu Y. Performance of anaerobic treatment of blackwater collected from different toilet flushing systems: Can we achieve both energy recovery and water conservation? J. Hazard. Mater. 2019;365:44–52. doi: 10.1016/j.jhazmat.2018.10.055. PubMed DOI

Abdel-Shafy H.I., El-Khateeb M.A., Regelsberger M., El-Sheikh R., Shehata M. Integrated system for the treatment of blackwater and greywater via UASB and constructed wetland in Egypt. Desalin. Water Treat. 2009;8:272–278. doi: 10.5004/dwt.2009.788. DOI

Wasielewski S., Morandi C. Impacts of blackwater co-digestion upon biogas production in pilot-scale UASB and CSTR reactors; Proceedings of the 13th IWA Specialized Conference on Small Water and Wastewater Systems & 5th IWA Specialized Conference on Resources-Oriented Sanitation; Athenes, Greece. 14–16 September 2016.

Zhang L., Lim E.Y., Loh K.C., Ok Y.S., Lee J.T.E., Shen Y., Wang C.H., Dai Y., Tong Y.W. Biochar enhanced thermophilic anaerobic digestion of food waste: Focusing on biochar particle size, microbial community analysis and pilot-scale application. Energy Convers. Manag. 2020;209:112654. doi: 10.1016/j.enconman.2020.112654. DOI

Wang H., Larson R.A., Runge T. Impacts to hydrogen sulfide concentrations in biogas when poplar wood chips, steam treated wood chips, and biochar are added to manure-based anaerobic digestion systems. Bioresour. Technol. Reports. 2019;7:100232. doi: 10.1016/j.biteb.2019.100232. DOI

ali H.A., Faraj J.J., Hussien F.M. Effect of pH on biogas production during anaerobic digestion. J. Univ. Shanghai Sci. Technol. 2021;23:224–231. doi: 10.51201/JUSST/21/08369. DOI

Zhang M., Li J., Wang Y., Yang C. Impacts of different biochar types on the anaerobic digestion of sewage sludge. RSC Adv. 2019;9:42375–42386. doi: 10.1039/C9RA08700A. PubMed DOI PMC

Wang Z., Zhu C., Yi M., Yang Z., Feng Q., Wang S. Effect of the Ca2+ concentration on anaerobic digestion and microbial communities of granular sludge. BioResources. 2019;13:6062–6076. doi: 10.15376/biores.13.3.6062-6076. DOI

Guan R., Gu J., Wachemo A.C., Yuan H., Li X. Novel Insights into Anaerobic Digestion of Rice Straw Using Combined Pretreatment with CaO and the Liquid Fraction of Digestate: Anaerobic Digestion Performance and Kinetic Analysis. Energy Fuels. 2020;34:1119–1130. doi: 10.1021/acs.energyfuels.9b02104. DOI

Yang G., Jin K., Eraky M., Peng J., Li Q., Meng L., Zhang H., Ai P. Positive effect of Ca addition on the risk of Cu and Zn in digestate as biofertilizer. J. Environ. Chem. Eng. 2023;11:109633. doi: 10.1016/j.jece.2023.109633. DOI

Zhao W., Jeanne Huang J., Hua B., Huang Z., Droste R.L., Chen L., Wang B., Yang C., Yang S. A new strategy to recover from volatile fatty acid inhibition in anaerobic digestion by photosynthetic bacteria. Bioresour. Technol. 2020;311:123501. doi: 10.1016/j.biortech.2020.123501. PubMed DOI

Ahmad A., Ghufran R., Wahid Z.A. Role of calcium oxide in sludge granulation and methanogenesis for the treatment of palm oil mill effluent using UASB reactor. J. Hazard. Mater. 2011;198:40–48. doi: 10.1016/j.jhazmat.2011.10.008. PubMed DOI

Sun H., Yang Z., Zhou L., Papadakis V.G., Goula M.A., Liu G., Zhang Y., Wang W. Calcium ion can alleviate ammonia inhibition on anaerobic digestion via balanced-strengthening dehydrogenases and reinforcing protein-binding structure: Model evaluation and microbial characterization. Bioresour. Technol. 2022;354:127165. doi: 10.1016/j.biortech.2022.127165. PubMed DOI

Tang S., Yan F., Zheng C., Zhang Z. Novel Calcium Oxide-Enhancement Phosphorus Recycling Technique through Sewage Sludge Pyrolysis. ACS Sustain. Chem. Eng. 2018;6:9167–9177. doi: 10.1021/acssuschemeng.8b01492. DOI

Lee J.I., Oh J.S., Yoo S.C., Jho E.H., Lee C.G., Park S.J. Removal of phosphorus from water using calcium-rich organic waste and its potential as a fertilizer for rice growth. J. Environ. Chem. Eng. 2022;10:107367. doi: 10.1016/j.jece.2022.107367. DOI

Zhang X., Tong H., Zhang H., Chen C. Nitrogen oxides absorption on calcium hydroxide at low temperature. Ind. Eng. Chem. Res. 2008;47:3827–3833. doi: 10.1021/ie070660d. DOI

Guo S., Liu T., Hui J., Che D., Li X., Sun B., Li S. Effects of calcium oxide on nitrogen oxide precursor formation during sludge protein pyrolysis. Energy. 2019;189:116217. doi: 10.1016/j.energy.2019.116217. DOI

Giwa A.S., Maurice N.J., Luoyan A., Liu X., Yunlong Y., Hong Z. Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery. Heliyon. 2023;9:e19765. doi: 10.1016/j.heliyon.2023.e19765. PubMed DOI PMC

Tozsin G., Oztas T. Utilization of Steel Slag as a Soil Amendment and Mineral Fertilizer in Agriculture: A Review. Tarim Bilim. Derg. 2023;29:906–913. doi: 10.15832/ankutbd.1197239. DOI

Fiore M., Magi V., Viggiano A. Internal combustion engines powered by syngas: A review. Appl. Energy. 2020;276:115415. doi: 10.1016/j.apenergy.2020.115415. DOI

Galaly A.R. Sustainable Development Solutions for the Medical Waste Problem Using Thermal Plasmas. Sustainability. 2022;14:11045. doi: 10.3390/su141711045. DOI

Ramírez J., Deago E., James Rivas A.M.C. Effect of Biochar on Anaerobic Co-Digestion of Untreated Sewage Sludge with Municipal Organic Waste under Mesophilic Conditions. Energies. 2024;17:2393. doi: 10.3390/en17102393. DOI

Han F., Yun S., Zhang C., Xu H., Wang Z. Steel slag as accelerant in anaerobic digestion for nonhazardous treatment and digestate fertilizer utilization. Bioresour. Technol. 2019;282:331–338. doi: 10.1016/j.biortech.2019.03.029. PubMed DOI

Rida Galaly A., Van Oost G., Dawood N. Sustainable Plasma Gasification Treatment of Plastic Waste: Evaluating Environmental, Economic, and Strategic Dimensions. ACS Omega. 2024;9:21174–21186. doi: 10.1021/acsomega.4c01084. PubMed DOI PMC

Jin Q., Kirk M.F. pH as a primary control in environmental microbiology: 1. thermodynamic perspective. Front. Environ. Sci. 2018;6:21. doi: 10.3389/fenvs.2018.00021. DOI