Distribution and transformation behaviors of heavy metals and phosphorus during hydrothermal carbonization of sewage sludge

Pressurized electro-osmotic dewatering treatment of sludge: focusing on the influences on nutrients for agricultural application

Sewage sludge requires effective dewatering and high nutrients retention before disposal for agricultural application. Pressurized electro-osmotic dewatering (PEOD) process with low energy consumption can effectively remove water from sludge, but the influences of PEOD process on nutrients for agricultural application still lacks in-depth research. In this study, the influences of PEOD process on nutrients for agricultural application were investigated, including organic matter, nitrogen, phosphorus, potassium and silicon contents. Layered experiments were conducted to investigate the layered variation of nutrients in sludge and to understand the potential change mechanisms. The experimental results showed that PEOD process caused small losses (<10%) of organic matter and total phosphorus (TP) in sludge, but caused 11.2-18.4% loss of total nitrogen (TN). PEOD process also caused 18.6-27.0% loss of total potassium (TK) and over 80% loss of available potassium in sludge, and could weaken the potential salt damage during the agricultural application of sludge. Furthermore, the available phosphorus content of sludge in the anode area increased significantly after the PEOD process, indicating that PEOD process could enhance the phosphorus bioavailability of sludge in the anode area. Besides, PEOD process caused a slight loss of silicon components in sludge, but improved the long-term silicon dissolution and release ability of sludge. This work could expand the knowledge about the influences of PEOD process on sludge nutrients and provide scientific guidance for the agricultural application of PEOD sludge.

The effect of an acidic environment during the hydrothermal carbonization of sewage sludge on solid and liquid products: The fate of heavy metals, phosphorus and other compounds

The pH of sewage sludge is a crucial factor during the hydrothermal carbonization process that influences the characteristics of the resulting products and migration of certain compounds from the solid to liquid phase. Accordingly, this work is focused on examining the pH impact during the HTC process, in particular, pH equals 2, 3, 4, 5 and 6 on the individual hydrothermally carbonized products generated at 200 °C and 2 h residence time. For this reason, the chemical and physical indicators describing the post-processing liquid and hydrochar were determined. For instance, it was observed that the phosphorus content detected in the liquid, derived at pH2, rose significantly by 80%. Furthermore, decreasing the pH of sewage sludge had a significant impact on the ash content and the calorific value of the hydrochar. Additionally, changes in the specific surface area of hydrochar were noticed: pH = 5 and pH = 6 showed an increase of 20-30%, while for lower pH values a decrease of c.a. 26% was achieved. The distribution of heavy metals between the obtained fractions in the HTC process (solid and liquid) indicated that 92 to almost 100% of the tested heavy metals were transferred to the hydrochar. A significant effect of pH on the distribution between these fractions was observed only for Zn and Ni. For instance, for pH = 2, Zn and Ni in post-processing liquid were 34% and 29%, respectively. In addition, the sequential extraction of heavy metals from hydrochar was also performed in order to identify mobile and non-mobile phases. It was noticed that the acidic environment favours a higher amount of mobile heavy metals in hydrochar. The largest effect was observed for Cd, Pb, Cr and Cu, for which, at pH = 2, their respective amounts in the mobile fraction were 2.7; 3.6; 1.8; 6.2 times higher, compared to the hydrochar without pH correction.

Enhanced energy and nutrient recovery via hydrothermal carbonisation of sewage sludge: Effect of process parameters

Integration of waste management with energy and resource recovery is being widely explored to achieve sustainability. To achieve this, sewage sludge was treated with hydrothermal carbonisation (HTC) at temperatures ranging from 180 °C-260 °C with an increment of 20 °C for three different duration of 1 h, 3 h, and 5 h. The energy and resource recovery potential of the HTC treatment was evaluated through of hydrochar (HC) and process water (PW) properties. Dehydration and decarboxylation reactions resulted in reduced H/C and O/C atomic ratios of 1.35 and 0.45 respectively in HC-260-3, exhibiting peat-like propertied. The calorific value of HC-260-5 was enhanced to 5.9 MJ/kg (increase of 25.8 %) due to the combined effect of H/C and O/C atomic ratios, increased volatile organics and fixed carbon. A maximum energy recovery efficiency of 82.44 % was realised at 240 °C for 3 h rendering it the optimal process condition to ensure energy enrichment. Thermogravimetric analysis (TGA) of HC samples indicated an enhanced combustion behaviour with an increased HTC severity. The elevated levels of volatile fatty acids (VFAs) in PW (maximum 2296 mg/L) made it viable for energy recovery in anaerobic digestion units. Additionally, the PW contains significant concentrations of N and P (2091.68 mg/L and 40.51 mg/L, respectively), indicating enhanced resource/nutrient recovery potential.

The effect of an acid catalyst on the hydrothermal carbonization of sewage sludge

This research is focused on the addition of a catalyst, sulphuric acid (VI), to sewage sludge, and its effect on solid and liquid products resulting from the hydrothermal carbonization process. Consequently, for hydrochars, proximate and ultimate analyses, higher heating value and specific surface area were determined. Additionally, Fourier-transform infrared spectrophotometric and thermogravimetric analyses were conducted. The heavy metal contents in the ash composition of hydrochars were identified by X-ray fluorescence spectrometry. It was confirmed that the catalyst addition changed the structure as well as the physical and chemical properties of hydrochars and their ashes. Regarding post-processing water, both pH value and conductivity were determined and the element composition, including heavy metals, was conducted by the inductively coupled plasma optical emission and mass spectrometry analyses. It was found that the addition of the catalyst caused a decrease in heavy metal contents and an increase of phosphorus compound in filtrates.

Optimizing directional recovery of high-bioavailable phosphorus from human manure: Molecular-level understanding and assessment of application potential

Phosphorus (P) recovery from human manure (HM) is critical for food production security. For the first time, a one-step hydrothermal carbonation (HTC) treatment of HM was proposed in this study for the targeted high-bioavailable P recovery from P-rich hydrochars (PHCs) for direct soil application. Furthermore, the mechanism for the transformation of P speciation in the derived PHCs was also studied at the molecular level. A high portion of P (80.1∼89.3%) was retained in the solid phase after HTC treatment (120∼240°C) due to high metal contents. The decomposition of organophosphorus (OP) into high-bioavailable orthophosphate (Ortho-P) was accelerated when the HTC temperature was increased, reaching ∼97.1% at 210°C. In addition, due to the high content of Ca (40.45±2.37 g/kg) in HM, the HTC process promoted the conversion of low-bioavailable non-apatite inorganic (NAIP) into high-bioavailable apatite inorganic P (AP). In pot experiments with pea seedling growth, the application of newly obtained PHCs significantly promoted plant growth, including average wet/dry weight and plant height. Producing 1 ton of PHCs (210°C) with the same effective P content as agricultural-type calcium superphosphate could result in a net return of $58.69. More importantly, this pathway for P recovery is predicted to meet ∼38% of the current agricultural demand.

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.

Catalytic hydrothermal carbonization of wet organic solid waste: A review

Hydrothermal carbonization has gained attention in converting wet organic solid waste into hydrochar with many applications such as solid fuel, energy storage material precursor, fertilizer or soil conditioner. Recently, various catalysts such as organic and inorganic catalysts are employed to guide the properties of the hydrochar. This review presents a summarize and a critical discussion on types of catalysts, process parameters and catalytic mechanisms. The catalytic impact of carboxylic acids is related to their acidity level and the number of carboxylic groups. The catalysis level with strong mineral acids is likely related to the number of hydronium ions liberated from their hydrolysis. The impact of inorganic salts is determined by the Lewis acidity of the cation. The metallic ions in metallic salts may incorporate into the hydrochar and increase the ash of the hydrochar. The selection of catalysts for various applications of hydrochars and the environmental and the techno-economic aspects of the process are also presented. Although some catalysts might enhance the characteristics of hydrochar for various applications, these catalysts may also result in considerable carbon loss, particularly in the case of organic acid catalysts, which may potentially ruin the overall advantage of the process. Overall, depending on the expected application of the hydrochar, the type of catalyst and the amount of catalyst loading requires careful consideration. Some recommendations are made for future investigations to improve laboratory-scale process comprehension and understanding of pathways as well as to encourage widespread industrial adoption.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0-4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

Phosphorus transformation during the carbonaceous skeleton assisted thermal hydrolysis of sludge

On the basis of the carbonaceous skeleton assisted thermal hydrolysis that we proposed to achieve efficient sludge dewatering, this work further explored phosphorus (P) transformation in the process. The results showed that during independent thermal hydrolysis in the temperature range of 120-240 °C, organic-P was first decomposed into soluble-P and particulate-P in liquid, and then combined with Ca, Fe, and Al to form more apatite-P (AP) and less non-apatite inorganic-P (NAIP). When the skeleton assisted the sludge thermal hydrolysis, the turning point of the hydrolysis temperature would reduce from 180 °C to 150 °C, at which the liquid-P began to decrease and the organic-P generally decomposed. Moreover, the increment in the content of AP halved while that of NAIP doubled compared to that in the process without the carbonaceous skeleton. These effects come from the exogenous components introduced by adding the skeleton, which were different from the sludge. Compared with the P-rich compound and metal elements that tend to bond with phosphate introduced by the skeleton, hemicellulose as a main organic component played a leading role in the different P transformations of AP and NAIP. The hemicellulose slightly increased the acidity of sludge products, thereby inhibiting AP production and promoting the production of recyclable NAIP. Overall, the carbonaceous skeleton assisted thermal hydrolysis was beneficial for P recovery with a very low filtrate loss rate.

Thermal treatment of sewage sludge: A comparative review of the conversion principle, recovery methods and bioavailability-predicting of phosphorus

Phosphorus is a nutrient that is essential to nature and human life and has attracted attention because of its very limited reserves. Dwindling phosphorus reserves and soaring prices have made the recovery of phosphorus from waste biosolids even more urgent. Waste activated sludge, as the final destination of most of the phosphorus in human domestic and industrial water, has been considered as a reliable source of phosphorus recovery. The thermal treatment method of sewage sludge is currently a relatively environmentally friendly disposal method, which mainly includes incineration, pyrolysis and hydrothermal carbonization. This paper reviews the methods for the recovery of different forms of phosphorus (wet chemical, thermochemical and electrodialysis) from solid products obtained from different sludge thermal treatment methods (incinerated sewage sludge ash, pyrolysis of sewage sludge char and hydrochar) and the bioavailability of the recovered phosphorus products. Incineration of sewage sludge is currently the most established and effective method for recovering phosphorus from the thermal treatment products of sewage sludge. One of the wet chemical methods has been applied on a commercial scale and is expected to be further developed for future industrial applications. Pyrolysis and hydrothermal carbonation still have many research gaps in this field. Based on their principles and laboratory performance, both of them have the potential to recover phosphorus and should be further explored.

Hydrothermal alkaline conversion of sewage sludge: optimization of process parameters and characterization of humic acid

Sewage sludge (SS) dewatering is a key step in sludge disposal, which plays an important role in reducing sludge volume, facilitating transportation and subsequent treatment. In this paper, a facile hydrothermal-alkaline treatment for SS was proposed, which can be used for sludge dewatering and humic acid (HA) recycling at the same time. Response surface methodology (RSM) was used to determine the optimal conditions, and a mathematical model was established to accurately predict the changes of sludge water content and the extraction rate of HA. Under the optimal conditions of 170 °C/42 min/0.05 (for hydrothermal temperature, hydrothermal time, and mass ratio of KOH to wet sludge, respectively), the water content decreased to 46.7% and the extraction rate of HA (with a purity of 96.2%) was 89.1%. The improvement of the dewatering performance effectively facilitates the subsequent disposal of the sludge. The hydrothermal-alkaline method not only realizes the efficient dehydration of the sludge, but also obtains HA from the sludge extract. The obtained HA has potential economic value in the fields of agriculture, biological medicine, environment, and the like.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

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

The high moisture content present in sewage sludge hinders the use of sewage sludge in incineration or energy application. This limitation of moisture present in sewage sludge can be obviated by using the hydrothermal carbonization (HTC) process. In sewage sludge management, the HTC process requires less energy compared to other conventional thermo–chemical management processes. The HTC process produces energy-rich hydrochar products and simultaneously enables phosphorus recovery. This study investigates the influence of organic acids, inorganic acid, and alkali as additives on phosphorus transformation, yield, proximate analysis and the heating value of subsequently produced hydrochar. The analysis includes various process temperatures (200 °C, 220 °C, and 240 °C) in the presence of deionized water, acids (0.1 M and 0.25 M; H2SO4, HCOOH, CH3COOH), and alkali (0.1 M and 0.25 M; NaOH) solutions as feed water. The results show that phosphorus leaching into the process-water, hydrochar yield, proximate analysis, and the heating value of produced hydrochar is pH- and temperature-dependent, and particularly significant in the presence of H2SO4. In contrast, utilization of H2SO4 and NaOH as an additive has a negative influence on the heating value of produced hydrochar.