Evaluation of the Use of Sewage Sludge Biochar as a Soil Amendment—A Review

Impact of temperature and residence time on sewage sludge pyrolysis for combined carbon sequestration and energy production

Environmental challenges related to sewage sludge call for urgent sustainable management of this resource. Sludge pyrolysis might be considered as a sustainable technology and is anticipated to support measures for mitigating climate change through carbon sequestration. The end products of the process have various applications, including the agricultural utilization of biochar, as well as the energy exploitation of bio-oil and syngas. In this research, sewage sludge was pyrolyzed at 500 °C, 600 °C, 750 °C, and 850 °C. At each temperature, pyrolysis was explored at 1hr, 2hrs, and 3hrs residence times. The ratio (H/Corg)at was tapped to imply organic carbon stability and carbon sequestration potential. Optimum operating conditions were achieved at 750 °C and 2hrs residence time. Produced biochar had (H/Corg)at ratio of 0.54, while nutrients' contents based on dry weight were 3.99%, 3.2%, and 0.6% for total nitrogen (TN), total phosphorus (TP), and total potassium (TK), respectively. Electrical conductivity of biochar was lesser than the feed sludge. Heavy metals in biochar aligned with the recommended values of the International Biochar Initiative. Heat content of condensable and non-condensable volatiles was sufficient to maintain the temperature of the furnace provided that PYREG process is considered. However, additional energy source is demanded for sludge drying.

Unlocking the potential of biochar in the remediation of soils contaminated with heavy metals for sustainable agriculture

Agricultural soils contaminated with heavy metals (HMs) impose a threat to the environmental and to human health. Amendment with biochar could be an eco-friendly and cost-effective option to decrease HMs in contaminated soil. This paper reviews the application of biochar as a soil amendment to immobilise HMs in contaminated soil. We discuss the technologies of its preparation, their specific properties, and effect on the bioavailability of HMs. Biochar stabilises HMs in contaminated soil, enhance the overall quality of the contaminated soil, and significantly reduce HM uptake by plants, making it an option in soil remediation for HM contamination. Biochar enhances the physical (e.g. bulk density, soil structure, water holding capacity), chemical (e.g. cation exchange capacity, pH, nutrient availability, ion exchange, complexes), and biological properties (e.g. microbial abundance, enzymatic activities) of contaminated soil. Biochar also enhances soil fertility, improves plant growth, and reduces the plant availability of HMs. Various field studies have shown that biochar application reduces the bioavailability of HMs from contaminated soil while increasing crop yield. The review highlights the positive effects of biochar by reducing HM bioavailability in contaminated soils. Future work is recommended to ensure that biochars offer a safe and sustainable solution to remediate soils contaminated with HMs.

Enhanced Adsorptive Removal of Tetracycline by Phosphomolybdic Acid-Modified Low-Temperature Sludge Biochar

Increasing the adsorption capacity and reducing the energy consumption of sludge biochar during preparation is important. In this study, a new modification method was developed to prepare phosphomolybdic acid-modified sludge biochar through the low-temperature pyrolysis of sewage sludge using phosphomolybdic acid as a modifier. Tetracycline was used to assess the adsorption performance of sludge biochar, and phosphomolybdic acid-modified sludge biochar was prepared at different temperatures. The results showed that the adsorption capacity of sludge biochar improved from 84.49 to 120.86 mg/g through modification with phosphomolybdic acid at 200 °C. The maximum adsorption capacities of phosphomolybdic acid-modified sludge biochar (200 °C pyrolysis temperature) at 298, 308, and 318 K were 283.87, 421.39, and 545.48 mg/g, respectively. Both liquid film and intraparticle diffusion were the main rate-limiting steps of tetracycline adsorption by phosphomolybdic acid-modified sludge biochar. Furthermore, the adsorption of tetracycline by phosphomolybdic acid-modified sludge biochar was mainly attributed to π-π interactions, electrostatic interactions, hydrogen bonding, and pore filling.

Converting coastal silt into subgrade soil with biochar as reinforcing agent, CO2 adsorbent, and carbon sequestrating material

Large amounts of coastal silt produced annually is urgent to be treated with a feasible strategy. This study converted it into subgrade soil by cement solidification for resource utilization. Biochar was used as exogenous additive for enhancing compressive strength of the product, simultaneously achieving carbon sequestration. Three biochars derived from peanut shells (PSBC), cow dung (CDBC) and sewage sludge (SSBC) at 300 °C, 500 °C and 700 °C pyrolysis, were added into raw materials with 1%, 2% and 5%, respectively. All biochars significantly improved the compressive strength of the subgrade soil by 20-110%. Biochar catalyzed cement hydration reactions to produce more Ca(OH)2, CaCO3 and calcium silicate hydrates (C-S-H gel). The catalytic capacity of different biochars followed the order of SSBC > PSBC > CDBC. Addition of 2% SSBC500 induced the greatest increase in 28 d-strength from only 1.0 MPa-2.1 MPa, which was due to that 500 °C biochar had a suitable specific surface area and porosity. Biochar facilitated CO2 capture (absorption) during the hydration reactions at the initial 48 h with 55-70 mg g-1. The high alkalinity and water holding capacity of biochar contributed to the absorption of CO2; the high content of minerals in SSBC compared to CDBC and PSBC promoted chemical conversion of CO2 to carbonate. Besides, the biochar itself as carbon rich material was encapsulated in the subgrade soil, which can be regarded as a long-term carbon sequestration strategy. Carbon budget analysis demonstrated that converting one ton dry silt into subgrade soil with addition of 2% biochar could increase CO2 sequestration from 11 kg to 36-94 kg. This study proposes a novel strategy of using biochar to strengthen the subgrade soil simultaneously achieve long-term carbon sequestration.

Lipid extraction in the primary sludge generated from urban wastewater treatment: characteristics and seasonal composition analysis

A seasonal study of the lipid composition of a primary sludge (dry and dewatered base) obtained from an urban wastewater treatment plant located in Aguascalientes (Mexico) is reported. This study assessed the variability in sludge composition to establish its potential as a raw material for biodiesel production. Lipid recovery was achieved by extraction using two solvents. Hexane was employed for lipid extraction from dry sludge, whereas hexane and ethyl butyrate were used for comparison with dewatered sludge. The formation (%) of fatty acid methyl esters (biodiesel) was determined using extracted lipids. The extraction results from the dry sludge showed 14 and 6% of recovered lipids and their conversion to biodiesel, respectively. For the dewatered sludge, the lipid recovery and biodiesel formation were 17.4 and 60% using hexane, and 23 and 77% for ethyl butyrate, respectively, on a dry basis. Statistical data indicated that lipid recovery depended on the physicochemical characteristics of sewage sludge, which were related to seasonal changes, population activities, and changes in plant configuration, among other factors. These variables must be considered in the design of large-scale extraction equipment for the application and commercial exploitation of biomass waste in biofuel production.

Pyrolysis-A tool in the wastewater solids handling portfolio, not a silver bullet: Benefits, drawbacks, and future directions

Pyrolysis is the process whereby carbonaceous materials, such as biosolids, are heated between 400°C and 900°C in the absence of oxygen. Three main products are generated: a solid product called biochar, a py-liquid that consists of aqueous phase and non-aqueous phase liquid, and py-gas. The biochar holds value as a beneficial soil amendment and sequesters carbon. The py-liquid is potentially hazardous and needs to be dealt with (including potentially reducing it on-site via catalysis or thermal oxidation). Py-gas can be used on-site for energy recovery. Pyrolysis has gained recent interest due to concern over per- and polyfluoroalkyl substances (PFAS) in biosolids. Although pyrolysis can remove PFAS from biosolids, it has been shown to produce PFAS that reside in py-liquid, and the fate in py-gas remains a knowledge gap. More research is needed to help close the PFAS and fluorine mass balance through pyrolysis influent and effluent products because pyrolysis alone does not destroy all PFAS. The moisture content of biosolids substantially affects the energy balance for pyrolysis. Utilities that already produce a dried biosolids product are in a better position to install pyrolysis. Pyrolysis has both defined benefits (solids reduction, PFAS removal from biosolids, and biochar production) as well as remaining questions (the fate of PFAS in py-gas and py-liquid, mass balance on nutrients, and py-liquid handling options) that will be answered through more pilot and full-scale demonstrations. Regulations and local policies (such as carbon sequestration credits) could affect pyrolysis implementation. Pyrolysis should be considered as an option in the biosolids stabilization toolbox with application being based on individual circumstances of a utility (e.g., energy, moisture content of biosolids, PFAS). PRACTITIONER POINTS: Pyrolysis has known benefits but limited full-scale operational data. Pyrolysis removes PFAS from biochar, but PFAS fate in gas phase is unknown. Moisture content of influent feed solids affects energy balance of pyrolysis. Policy on PFAS, carbon sequestration, or renewable energy could impact pyrolysis.

"Green" nZVI-Biochar as Fenton Catalyst: Perspective of Closing-the-Loop in Wastewater Treatment

In the framework of wastewater treatment plants, sewage sludge can be directed to biochar production, which when coupled with an external iron source has the potential to be used as a carbon-iron composite material for treating various organic pollutants in advanced oxidation processes. In this research, "green" synthesized nano zero-valent iron (nZVI) supported on sewage sludge-based biochar (BC)-nZVI-BC was used in the Fenton process for the degradation of the recalcitrant organic molecule. In this way, the circular economy principles were supported within wastewater treatment with immediate loop closing; unlike previous papers, where only the water treatment was assessed, the authors proposed a new approach to wastewater treatment, combining solutions for both water and sludge. The following phases were implemented: synthesis and characterization of nano zero-valent iron supported on sewage sludge-based biochar (nZVI-BC); optimization of organic pollutant removal (Reactive Blue 4 as the model pollutant) by nZVI-BC in the Fenton process, using a Definitive Screening Design (DSD) model; reuse of the obtained Fenton sludge, as an additional catalytic material, under previously optimized conditions; and assessment of the exhausted Fenton sludge's ability to be used as a source of nutrients. nZVI-BC was used in the Fenton treatment for the degradation of Reactive Blue 4-a model substance containing a complex and stable anthraquinone structure. The DSD model proposes a high dye-removal efficiency of 95.02% under the following optimal conditions: [RB4] = 50 mg/L, [nZVI] = 200 mg/L, [H₂O₂] = 10 mM. pH correction was not performed (pH = 3.2). Afterwards, the remaining Fenton sludge, which was thermally treated (named FS_treated), was applied as a heterogeneous catalyst under the same optimal conditions with a near-complete organic molecule degradation (99.56% ± 0.15). It could be clearly noticed that the cumulative amount of released nutrients significantly increased with the number of leaching experiments. The highest cumulative amounts of released K, Ca, Mg, Na, and P were therefore observed at the fifth leaching cycle (6.40, 1.66, 1.12, 0.62, 0.48 and 58.2 mg/g, respectively). According to the nutrient release and toxic metal content, FS_treated proved to be viable for agricultural applications; these findings illustrated that the "green" synthesis of nZVI-BC not only provides innovative and efficient Fenton catalysts, but also constitutes a novel approach for the utilization of sewage sludge, supporting overall process sustainability.

Feasibility of Biochar Derived from Sewage Sludge to Promote Sustainable Agriculture and Mitigate GHG Emissions-A Review

Sewage sludge (SS) has been connected to a variety of global environmental problems. Assessing the risk of various disposal techniques can be quite useful in recommending appropriate management. The preparation of sewage sludge biochar (SSB) and its impacts on soil characteristics, plant health, nutrient leaching, and greenhouse gas emissions (GHGs) are critically reviewed in this study. Comparing the features of SSB obtained at various pyrolysis temperatures revealed changes in its elemental content. Lower hydrogen/carbon ratios in SSB generated at higher pyrolysis temperatures point to the existence of more aromatic carbon molecules. Additionally, the preparation of SSB has an increased ash content, a lower yield, and a higher surface area as a result of the rise in pyrolysis temperature. The worldwide potential of SS output and CO₂-equivalent emissions in 2050 were predicted as factors of global population and common disposal management in order to create a futuristic strategy and cope with the quantity of abundant global SS. According to estimations, the worldwide SS output and associated CO₂-eq emissions were around 115 million tons dry solid (Mt DS) and 14,139 teragrams (Tg), respectively, in 2020. This quantity will rise to about 138 Mt DS sewage sludge and 16985 Tg CO₂-eq emissions in 2050, a 20% increase. In this regard, developing and populous countries may support economic growth by utilizing low-cost methods for producing biochar and employing it in local agriculture. To completely comprehend the benefits and drawbacks of SSB as a soil supplement, further study on long-term field applications of SSB is required.