Influence of Acids and Alkali as Additives on Hydrothermally Treating Sewage Sludge: Effect on Phosphorus Recovery, Yield, and Energy Value of Hydrochar

Alkaline hydrothermal cracking effect and substance transformation characteristics of caprolactam-containing sludge

Caprolactam is a crucial chemical intermediate, but its wastewater treatment process generates a significant amount of caprolactam-containing sludge. This study represented the first exploration of the effects of alkaline hydrothermal technology on the cracking and transformation of substances in this sludge. The cracking effect of caprolactam-containing sludge during hydrothermal treatment increased with rising reaction temperature and longer reaction time. With NaOH dosage in hydrothermal treatment increasing from 0 to 2 wt%, the volatile suspended solids (VSS) removal rate of the sludge increased from 44.5% to 74.8%, soluble chemical oxygen demand (SCOD) in the cracking liquid increased from 12772 mg/L to 22976 mg/L, and ammonia nitrogen concentration increased from 398.0 mg/L to 851.2 mg/L. However, the addition of Ca(OH)2 did not significantly affect the changes of sludge suspended solids, VSS and SCOD concentration, but increased the leaching of ammonia nitrogen (up to 745.0 mg/L). This was due to the secondary flocculation of Ca2+, which rebound with dissolved non-proteinaceous organic matter. Increasing temperature, reaction time, and alkaline dosage all enhanced the fluorescence intensity of dissolved organic matter (DOM). Moreover, higher reaction temperature and alkaline dosage reduced the proportion of proteinaceous products in DOM while increasing the proportions of fulvic acids, soluble microbial metabolites, and humic acid-like substances. The study provided crucial theoretical support for engineering application of this technology.

Guidelines for efficient nitrogen preservation in sewage sludge-based fertilizers

This study explores sustainable methods to mitigate nitrogen (N) loss in agriculture amid rising food demands and limited arable land. It examines sewage sludge (SS) as an alternative to synthetic N fertilizers. SS is rich in nitrogen (4.21 ± 0.42 %) and phosphorus (3.60 ± 0.72 %), making it suitable for nutrient recovery and soil enhancement. Unfavorable sludge management methods result in the loss of 950,000 tons of nitrogen, meeting almost 10 % of the EU's nitrogen fertilization demand. This research evaluates SS treatment methods, including chemical conversion, thermal treatment, and biological composting, focusing on nitrogen conservation efficiency. Results show nitrogen loss during hydrolysis is minimized at pH 4 to 8 but increases significantly as ammonia (NH3) at pH 9 to 11, ranging from 4.2 % to 9 %. Neutralizing the hydrolysate is crucial; using solid KOH resulted in 13.5 % nitrogen loss, 11 times more than using slightly alkaline ash (1.22 %). Adding ash during drying reduced nitrogen emissions by 30 % compared to traditional drying at 105 °C. Improving the C/N ratio with food residues reduced nitrogen losses by 46.3 % during composting. These findings highlight the importance of pH control in chemical processes and temperature regulation in thermal treatments. Adding residues from other processes, such as biomass combustion waste, enhances SS processing conditions. Understanding nitrogen retention mechanisms is crucial for the environmental sustainability of SS usage. Efficient nitrogen retention strategies improve the fertilization value of SS and reduce its environmental footprint by lowering greenhouse gas emissions, particularly ammonia. Reducing nitrogen loss during SS treatment significantly lowers ammonia emissions, a major contributor to greenhouse gas emissions. These results help determine optimal methods for managing and processing SS to minimize emissions and increase agricultural usability.

Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO 3 - and NH 4 + in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH 4 + concentration was observed. Conclusions: HTC allows for the production of hydrochar as a potential fertilizer material that requires further processing. Adjusting HTC conditions enhanced P-recovery in the hydrochar, while retrieving higher nitrate concentrations in the liquid product. Optimizing HTC for the production of qualified hydrochars requires further treatment of Cr content, studying the availability of P in the products and enhancing the hydrochar yield for economic feasibility.

Hydrothermal carbonization combined with thermochemical treatment of sewage sludge: Effects of MgCl2 on the migration of phosphorus and heavy metal

Phosphorus (P) is a non-regenerative and finite raw material. Due to its decreasing availability, and to protect the environment, recycling methods are needed. With the focus on closing nutrient cycles, sewage sludge (SS) is a potential source for P recovery. The objective of this study was to produce a mineral P-reach fertilizer. For this purpose, the treatment of SS in a multi-stage process, consisting of a hydrothermal carbonization (HTC) and thermochemical post-treatment was examined and compared with a direct thermochemical treatment. The focus was on the transformation of P and the migration of the heavy metals during the processes. In addition, the role of MgCl₂ as an additive was examined. During the HTC, most of the P remained in the HTC-char, so that the P content increased in the HTC-char compared with the SS. The addition of MgCl₂ to the process resulted in lower transportation rates of P in the liquid phase and higher P solubilities in water, citric acid, and alkalic ammonium citrate out of the solid phase. The thermochemical treatment of SS and the HTC-chars further concentrated P in the ash. Retention rates of >97% were achieved, and P₂O₅ contents in the ash were as high as ∼16 wt-%. The presence of the additive resulted in (i) higher retention rates of P in the ashes (ii) higher P-solubility and (iii) higher removal rates of easily volatile heavy metals such as Pb and Zn, and the treatment of HTC-char favored these effects compared with the direct treatment of SS.

Effect of CaO and hydrothermal carbonization conditions on the fuel characteristics of rice husk hydrochars

The effects of hydrothermal temperature and catalyst concentration on the basic elements and combustion characteristics and kinetic parameters of hydrochars were investigated using ultimate analyzer and thermogravimetric method with rice husk as the research object and CaO as the additives. The results showed that: (1) the fixed carbon content of hydrochars gradually increased with the increase of hydrothermal temperature, whereas the volatile content gradually decreased. When CaO was added, the changes of fixed carbon and volatile fraction gradually decreased with the increase of hydrothermal temperature, and H/C atomic ratio increased to different degrees, which had a certain inhibitory effect on the degree of hydrothermal carbonization of rice husk. (2) The peak of the volatile fraction combustion section of hydrochars combustion derivative thermogravimetric curve was higher than that of the fixed carbon combustion section. CaO concentration has less effect on the volatile combustion section, and the combustion peak of the fixed carbon section is significantly reduced. (3) When the heating rate of the combustion test is accelerated, the ignition and burnout temperatures of the sample increase and the overall combustion curve shifts to the high temperature region. (4) The comprehensive combustion index S_N decreases with the increase in hydrothermal temperature. When the hydrothermal temperature is certain, the CaO concentration causes the S_N to increase and then decrease, which finally reduces the combustion performance of hydrochars. (5) The activation energy of the fixed carbon combustion section of hydrochars is lower than that of the volatile combustion section, and the activation energy of both volatile and fixed carbon combustion sections gradually decreases after adding CaO. The primary reaction kinetic model was used to describe the combustion kinetics of hydrochars, and the correlation coefficients (R²) were all above 0.92, and the results were reliable.

Polypyrrole/Magnetic/Tea Waste Composites for PO43− Ions Removal: Adsorption-Desorption, Kinetics, and Thermodynamics Studies

The polypyrrole (PPY/TW) and magnetic (MG/TW) composite with tea waste (TW) was prepared and used as an adsorbent for PO43− ions removal from aqueous media. The composite were characterized with SEM and FTIR techniques. Batch study was conducted to investigate the effect of different reaction parameters on the adsorption of PO43− ions. The native TW, PPY/TW, and MG/TW showed the PO43− ions removal of 7.2, 7.3, and 7.9 (mg/g), respectively, using 0.05 g adsorbent dose and 10 mg/L initial concentration of PO43− ions at pH of 6, 10, and 3, respectively, and equilibrium was reached in 90 min. Kinetics and isotherm models were employed on the PO43− ions adsorption data and PO43− ions adsorption followed the pseudo-second order kinetics, intraparticle diffusion, and Langmuir isotherm models. Thermodynamics analysis reveals an exothermic process and spontaneous adsorption of PO43− ions on the composites. Results revealed that the magnetic and polypyrrole composites with tea waste have auspicious potential as an adsorbent and this class of the composites can be utilized for the removal of PO43− ions from the effluents.

Hydrochar: A Promising Step Towards Achieving a Circular Economy and Sustainable Development Goals

The United Nations 17 Sustainable Development Goals (SDGs) are a universal call to action to end poverty, protect the environment, and improve the lives and prospects of everyone on this planet. However, progress on SDGs is currently lagging behind its 2030 target. The availability of water of adequate quality and quantity is considered as one of the most significant challenges in reaching that target. The concept of the ‘Circular Economy’ has been termed as a potential solution to fasten the rate of progress in achieving SDGs. One of the promising engineering solutions with applications in water treatment and promoting the concept of the circular economy is hydrochar. Compared to biochar, hydrochar research is still in its infancy in terms of optimization of production processes, custom design for specific applications, and knowledge of its water treatment potential. In this context, this paper critically reviews the role of hydrochar in contributing to achieving the SDGs and promoting a circular economy through water treatment and incorporating a waste-to-value approach. Additionally, key knowledge gaps in the production and utilization of engineered hydrochar are identified, and possible strategies are suggested to further enhance its water remediation potential and circular economy in the context of better natural resource management using hydrochar. Research on converting different waste biomass to valuable hydrochar based products need further development and optimization of parameters to fulfil its potential. Critical knowledge gaps also exist in the area of utilizing hydrochar for large-scale drinking water treatment to address SDG-6.

Special Issue on “Advanced Technology of Waste Treatment”

The protection of human health and the environment (representing the main reason for waste management), as well as the sustainable use of natural resources, requires chemical, biological, physical and thermal treatment of wastes [...]

Influence of Post- and Pre-Acid Treatment during Hydrothermal Carbonization of Sewage Sludge on P-Transformation and the Characteristics of Hydrochar

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H2SO4) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H2SO4, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H2SO4 in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar.