Sewage Sludge Management Research Articles

Production and Destination of Sewage Sludge in the Piemonte Region (Italy): The Results of a Survey for a Future Sustainable Management.

The management of sewage sludge originated from municipal wastewater treatment plants (WWTPs) is an urgent issue. In 2019, the local authority of the Piemonte region started a survey with the aim of collecting recent data concerning wastewater and sludge management in the WWTPs located in its own territory. The survey’s results revealed that 60% of the sludge (51,000 t, as dry substance, d.s.) produced by the local WWTPs was recovered or disposed of outside of the region, and a similar amount of sludge was recovered in agriculture directly or after composting. The increase in the costs to accommodate sewage sludge in recovery or disposal plants, followed to a recent Italian Sentence (27958/2017), and the more and more stringent requirements fixed by lots of European countries for the application of sludge in agriculture, are pushing the Piemonte region authority to re-organize its own network for sludge management, with solutions based onto proximity and diversification. Whether the provisions of the current German legislation are applied in the future also in Italy, approx. 90% of sewage sludge produced into the Piemonte region should be incinerated, with a subsequent step of phosphorous recovery. The new regional plan, according to the Regional Address Deed, should consider a diversification of sludge treatment and recovery practices. On this basis, a need for new plants for around 40,000 t d.s./y could be planned.

Re-thinking sludge management within the Sustainable Development Goal 6.2

Sustainable Development Goal 6.2 of the Agenda 2030 is addressed to achieve access to adequate and equitable sanitation and hygiene for all. Further, one of the major principles underlying the Lisbon Charter is that sustainable and safe management of wastewaters are fundamental to the health of communities and their sustainable socio-economic development. Within this framework, the management of sewage sludge in a sustainable manner is one of the critical issues facing modern society, due to the growing rise of its production, increasing difficulties in properly locating treatment and disposal works, and complying with more stringent environmental quality requirements, thus requiring a re-thinking of its management. This paper discusses major concepts and principles for re-thinking the sludge management in order to achieve the above-mentioned objectives.

Artificial Neural Networks in Modeling of Dewaterability of Sewage Sludge

Mechanical dewatering is a key process in the management of sewage sludge. However, the drainage efficiency depends on a number of factors, from the type and dose of the conditioning agent to the parameters of the drainage device. The selection of appropriate methods and parameters of conditioning and dewatering of sewage sludge is the task of laboratory work. This work can be accelerated through the use of artificial neural network (ANNs). The paper discusses the possibilities of using ANNs in predicting the dewatering efficiency of physically conditioned sludge. The input variables were only four parameters characterizing the conditioning methods and the dewatering method by centrifugation. These were the dose of the sludge skeleton builders (cement, gypsum, fly ash, and liquid glass), sonication parameters (sonication amplitude and time), and relative centrifugal force. Dewatering efficiency parameters such as sludge hydration and separation factor were output variables. Due to the nature of the research problem, two nonlinear networks were selected: a multilayer perceptron and a radial neural network. Based on the results of the prediction of artificial neural networks, it was found that these networks can be used to forecast the effectiveness of municipal sludge dewatering. The prediction error did not exceed 1.0% of the real value. ANN can therefore be useful in optimizing the dewatering process. In the case of the conducted research, it was the optimization of the sludge dewatering efficiency as a function of the type and parameters of conditioning factors. Therefore, it is possible to predict the dewatering efficiency of sludge that has not been tested in the laboratory, for example, with the use of other doses of physical conditioner. However, the condition for correct prediction and optimization was the use of a large dataset in the neural network training process.

Utilization of digested sewage sludge in lactic acid fermentation

The management of sewage sludge is mostly limited to anaerobic digestion, incineration of digestate and recovery of phosphorous. In terms of resource efficiency, it is recommended to make use of the potential of all organic compounds. Nitrogen compounds, for instance, can find application as nutrients in biotechnological processes. To follow this approach, sewage sludge collected after anaerobic digestion, which had carbon and nitrogen contents of 35.9% (w/w) and 5.6% (w/w), respectively, was first hydrolyzed using 0-1% (w/w) sulphuric acid for 15 minutes at 121°C and the hydrolysate used as nitrogen source in lactic acid fermentation. Even though the focus was on a recovery of nitrogen compounds, the hydrolytic treatment with 1% (v/v) sulphuric acid resulted in a release of 28 mg g-1 glucose. Because of the complex composition of the obtained hydrolysate it was not possible to quantify the released organic nitrogen compounds. Lactic acid fermentations, however, revealed that the concentration of organic nitrogen compounds was sufficient to efficiently convert 10 g L-1 of added glucose into 9 g L-1 lactic acid, and thus it is expected that digested sewage sludge may be an alternative nitrogen source in lactic acid fermentation, possibly combined with the utilization of a carbon-rich feedstock. Such a utilization approach goes beyond the conventional management strategies of digestated sewage sludge and allows a material utilization even after anaerobic digestion.

Anaerobic co-digestion of food waste, poultry litter and sewage sludge: seasonal performance under ambient condition and model evaluation

ABSTRACT With the increasing population, food waste, sewage sludge, and poultry litter management problem are scaling up even in low-income countries. The management of these wastes has therefore been challenging. Anaerobic digestion of food waste alone is not very stable due to its acidic nature and high degradability whereas sewage sludge and poultry litter have low biochemical methane potential and a high nitrogen concentration. Co-digestion of suitably selected substrate leads to enhanced biogas production potential, system stability due to synergetic effects, and resolving the problem of waste management in the vicinity in a holistic approach. However, these wastes have varying characteristics and composition, in terms of carbon-to-nitrogen (C/N) ratio, pH, and alkalinity. In addition, millions of the rural household bio-digesters operating in low-income countries are working under ambient conditions and are primarily unheated. Therefore, there is a need of research to assess the viability of biogas production of co-digestion of all above substrates in an optimal mixing ratio operating in an ambient temperature condition. In this study, food waste (FW), sewage sludge (SS), and poultry litter (PL) were co-digested at ratios (SS: PL: FW: 3:2:1, 2:1:1, 1:1:1) with 8% total solid (TS) content at ambient temperature in summer and winter seasons. Biogas yield was highest with mixing ratio of 2:1:1 with the values of 640 L/kgVS in summer while it gave extremely low biogas yield of 106 L/kgVS in winter. The 2:1:1 mixture also had the highest methane composition of nearly 65% as well as the highest VS removal efficiency of 60%, making it the most viable option for increasing biogas production. Mathematical modeling results using Gompertz model and first order model predicted well with R2 value ranging from 0.91 to 0.98, which upheld the experimentally obtained values. Findings from this study suggest that co-digestion substrates (SS:PL:FW) mixing ratio of 2:1:1 is an optimized ratio among the studied co-substrate ratio for enhanced biogas production. Furthermore, winter biogas yield is almost one sixth of summer biogas yield, which means use of temperature enhancement techniques to anaerobic digesters operating at ambient conditions would be essential to increase the biogas yield during winter.

Sewage Sludge Thermal Treatment Technologies with a Focus on Phosphorus Recovery: A Review

Phosphorus presents a limited, irreplaceable and essential nutrient necessary for the growth of organisms. There is an increasing effort to recover phosphorus from production waste streams. Sewage sludge presents an important source of phosphorus but also contains organic pollutants and heavy metals. Thermal treatment technologies seem to be a promising option to treat sewage sludge and obtain ash/char from which high recovery rate of phosphorus can be reached. In this review, sewage sludge management options in compliance with EU legal requirements are first reviewed. Follows, an overview of sewage sludge thermal treatment technologies including incineration, hydrothermal carbonisation, pyrolysis and gasification, for the purpose of phosphorus recapture. We summarize recent advances in thermal treatment processes of sewage sludge and phosphorus recovery, identify challenges and knowledge gaps. Thermochemical methods proved to have many advantages over pure wet chemical methods for phosphorus recovery. The review provides the foundation for future research aimed at achieving efficient, economic and environmental sustainable recapture of phosphorus from sludge thermal treatment products.

Thermodynamics, kinetics and thermal decomposition characteristics of sewage sludge during slow pyrolysis

Pyrolysis has shown great potential for sewage sludge valorisation and management by producing value-added chemicals. Although the product process yields are extensively studied, a few studies exist without consensus on the kinetic properties of sewage sludge pyrolysis. As a result, a study to investigate the thermal decomposition characteristics of Gauteng sewage sludge (GSS) at various heating rates (10, 20, and 30 °C/min), its pyrolysis kinetic parameters, reaction mechanism and thermodynamic properties was meticulously conducted. The results show that sewage sludge decomposition occurs in three stages, whereby the main decomposition (active pyrolysis) takes place at 150–570 °C. Fourier transform infrared spectroscopy (FTIR) analysis results confirm progression of thermal decomposition of GSS and drive off volatile compounds and formation of aromatic structures during TGA studies of GSS. An increase in heating rate shifts the characteristic temperatures towards higher temperatures with the highest decomposition rate of 1.10%/min.mg at 30 °C/min. The activation energies of GSS pyrolysis were calculated using Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose and Starink methods and averaged as 225.92, 218.04 and 218.97 kJ/mol, respectively. GSS pyrolysis involves complex reaction chemistry with high reactivity whereby reactions that follow third order and three-dimensional diffusion-reaction mechanisms dominated the process. However, these mechanisms cannot be used explicitly to define the global pyrolysis kinetics due to the occurrence of multiple simultaneous reactions. The obtained thermodynamic and kinetic data will advance and amplify the design, simulation and optimisation of global energy pyrolysis units for production of value-added chemicals.

Study on some nutrients in urban sewage vermicompost (NASSAJI small town) by rice Straw as compared in international standard

Background and Objective: the aim of this survey was prevent the environmental pollutant, urban sewage sludge management and vermicompost production by beneficial usage of rice straw as an agricultural trash which burn in arable rice grounds. Method: This study conduct to evaluate the nutrition matter consist of nitrogen, phosphorus, potash in (21±4˚c) temperature and pH (7±0.5) in urban sewage management, and provided the suitable vermicompost by pure dehydrate urban sewage mixed in rice straw as compared with rice straw and manure complex and pure dehydrate urban sewage (control; PDUS) containing E. feothida from three replication in 8 week. At the end of the test amount of nutrition matter was compared between treatments in (p<0.05) value. Findings: Results shown, significant difference between treatment and reduced the amount of nitrogen, phosphorus, potash (p<0.05).So, the best level of nitrogen (1.21±022)% ,phosphorus(0.69±0.03)% and potash(0.07±0.01)% was determined in rice chaff and manure complex treatment. After this treatment, the PDUS mixed by rice chaff group had best point than control group innutrition matter except in amount of potassium. In potassium level after rice chaff and manure complex, control group was set in second station and the lower point was in dehydrate urban sewage mixed by rice chaff group. Discussion and Conclusion: Followed our results, we can say that rice chaff was more effective to produce the urban sewage vermicompost. Also this is good agricultural manure cause to best nutritional value and it could be help us in urban sewage management.

Analysis of Adapted Sewage Sludge Treatment and Disposal Routes in Bujumbura, Burundi

The disparities found in studies on sewage sludge management, aggravated by the high financial investment required, may lead to decision-making uncertainty and hesitancy. This research, basing on seven different sludge treatment and disposal routes that are widely used in China (thickening-anaerobic digestion-dewatering-land application, thickening-condi- tioning-dewatering-sanitary landfill; thickening-conditioning-dewatering- anaerobic digestion-land application, thickening-conditioning-dewatering- drying-incineration; thickening-conditioning-dewatering-drying-land application; thickening-conditioning-dewatering-incineration-sanitary landfill and thickening-conditioning-dewatering-drying-sintering-building materials), and a contextual assessment of major influencing indicators in Bujumbura through a review of priorities defined in the country National Development Plan 2018-2027 and other relevant reports from the government and different local and international organizations, selected five key factors such as energy recovery, local policies and priorities, weather conditions, environmental impact rate, and capital investment, and analyzed which routes are more adapted for this city. Thus, an assessment matrix for suitability of each of the seven routes to each factor was developed to summarize the advantages and disadvantages for each scenario. The analysis results demonstrated that the financial capacity excludes incineration process. Energy recovery-oriented routes are likely to be particularly attractive due to their potential of producing electricity which is scarce in this city. The treatment should apply drying before disposal, a cost-free process using natural drying to reduce the cost and eventual environmental impact during sludge transportation to the disposal site. Landfilling is preferable over land application, to prevent the potential risk posed by the geographic position and topographic condition of the city. The study concluded that a route using anaerobic digestion, drying and landfilling would be a good choice for the area studied.

Oxy-steam co-gasification of sewage sludge and woody biomass for bio-methane production: an experimental and numerical approach

Sewage sludge management is a current problem of extreme concern in Europe. Till now, the most frequent route for sludge management has been incineration or the reuse by shedding on agricultural land both directly or after composting. However, this last method is critically reconsidered lately owing to the increasing contamination of sewage sludge by chemicals, heavy metals, residual organic oils, etc.. In the present study, the performance of a steam-oxy gasification plant has been analyzed. The plant allows the thermo-chemical conversion of sewage sludge, together with woody biomass, into a gas mixture (syngas) composed by basic chemicals (synthetic hydrocarbons), suitable for subsequent industrial production of very high added-value products such as methane. In particular, a numerical model of methanation plant, composed by a downdraft gasifier, a syngas cleaning system and a Sabatier reactor, has been created within the commercial code Aspen-One and validated with the results obtained from a real scale air gasifier fed with sewage sludge and woody biomass. First results showed that the steam-oxy co-gasification process has a high energy conversion efficiency that reaches roughly 80% (i.e. produced syngas calorific value vs the dried feedstock calorific value) with an overall methane yield of about 20% (by mass) of the feedstock utilized.

Standards, rules, and analytical techniques for sewage sludge management

This article discusses the most common sewage sludge management techniques, as well as the unit activities and procedures that relate to them. Reclamation and adaptation of lands to specific needs; plant cultivation not intended for human consumption or food production; use in agriculture; use in construction; recovery of phosphorus, rare earth metals, or fats and use in industry; production of combustible pellets, granulates, or other usable materials such as absorbents; and storage on treatment territory Stabilization processing results in the creation of materials that may be contaminated with a range of organic chemicals. Because this method of management often involves dumping processed sludge on the ground, it has the potential to contaminate soil with unknown organic substances. Thermal processing of raw sewage sludge, on the other hand, virtually eliminates this possibility. The vast majority of organic stuff is converted to a simple, mineralized state. In this instance, the most serious concern is heavy metal poisoning of sewage sludge ash. The identification of heavy metals in ashes is, however, considerably easier than that of organic molecules. Chemical analysis may be extremely helpful in determining the environmental safety of sewage sludge that has been treated and handled. As a result, there are many analytical approaches in use that are likely to aid in the process of developing and implementing innovative economically and ecologically sound methods of sewage sludge reuse. The method of technical sewage sludge use at Gdansk's Wastewater Treatment Plant "Wschod" is also explained. The technical line has just been updated.

Analysis of heavy metal content in sewage sludge as an instrument of selecting new method of sustainable sewage sludge management

Analysis of heavy metal content in sewage sludge as an instrument of selecting new method of sustainable sewage sludge management

Assessment of diversity and composition of bacterial community in sludge treatment reed bed systems

Due to their low emission of odours and lack of the need to apply additional chemical agents, sludge treatment reed beds (STRBs) constitute an economically feasible and eco-friendly approach to sewage sludge management. Correctly designed and operated STRBs ensure effective reduction of the dry matter content coupled with the mineralisation of organic compounds. Successful operation of STRBs relies on complex interactions between the plants and microorganisms responsible for the decomposition of organic matter and nutrient cycling. While the biocenoses of wetland systems dedicated to wastewater treatment have been intensively investigated, in the case of sludge treatment applications, there is a deficit of available microbial data. The aim of this study was to explore the diversity and spatial distribution of the bacteria in three distinct STRBs which differ in maturation and feeding patterns. Analyses of the dry mass and organic matter content showed the general trend of the sludge stabilisation processes advancing through the bed depth, with the best performance in the Matured Continuous Feed (MCF) bed being noted. Samples from the MCF bed showed the statistically greatest biodiversity in relation to the other beds. Moreover, increased biodiversity of microorganisms was observed on the surface of the STRBs and the bottom zone of the MCF equipped with a passive aeration system, which proves the application of such solutions in order to enhance the performance of the process. The results of 16S rRNA gene sequencing revealed that Bacteroidetes, Proteobacteria and Firmicutes contributed approximately 80% of all identified sequences read. Network analysis revealed dominant role of Bacteroidetes in the formation of interspecies co-existence patterns. Nitrospira was the most abundant organism responsible for nitrogen metabolism in the STRBs.

Toxic metals in soil due to the land application of sewage sludge in China: Spatiotemporal variations and influencing factors

Land application has become a promising method for recycling energy and resources from sewage sludge; however, the changes that occur to the toxic metal concentrations in soil following the application of sewage sludge have been poorly investigated in China. The present study attempted to investigate the spatiotemporal variations of toxic metal concentrations in soil due to the land application of sewage sludge and the critical influencing factors. Overall, the results indicated that an increasing ratio of sewage sludge for land application, the concentrated disposal measures, and a shallower soil may lead to elevated toxic metal concentrations in soil. The worst scenario simulation showed that the cumulative discharge of toxic metals through sludge disposal were ranked as: Zn > Cu > Cr > Pb > Ni > As > Cd > Hg. After sewage sludge was applied to previously unaffected soil, i.e., background soil, the toxic metal concentrations in the soil increased annually over the period from 2006 to 2017. However, with respect to the affected soil, the concentrations of Zn and Cu increased, whereas the concentrations of As, Cd, Cr, and Pb decreased annually over the period from 2006 to 2017. The results indicate that, in practice, the selection of soil for sewage sludge disposal depends on the background and actual concentrations of toxic metals in a soil as well as the stress caused by the amount of sewage sludge application to cultivated land. We propose to use sewage sludge containing relatively lower concentrations of metals than the disposal soil for land application. Furthermore, land application of sewage sludge should be suited to local conditions in the future sewage sludge management.

Implementation of a Decision Support System for Sewage Sludge Management

In this work, a decision support system (DSS) coupled with wastewater treatment plant (WWTP) simulator tool that uses a hierarchical set of key performance indicators (KPIs) to provide an assessment of the performance of WWTP systems is presented. An assessment of different Scenarios in a real WWTP case study, each consisting of a different set of sludge line technologies and derived combinations, was successfully conducted with the developed DSS–WWTP simulator, based on Scenario simulation and hierarchical KPI analysis. The test carried out on the selected WWTP showed that although thermal valorisation and thermal hydrolysis showed similar (the best) economic viability, the latter showed additional benefits, including synergies related to improving the thermal balance of the overall WWTP even when considering other technologies. On the other hand, biogas-upgrading technologies allowed reduction of emissions, but with higher costs and thermal demands. The usage of this tool may allow the development of proposals for technological priorities as a pathway to the transition to circular economy based on the management criteria of the correspondent sanitation system.

ASSESSMENT OF THE POSSIBILITIES OF AGRICULTURAL USE OF SEWAGE SLUDGE FROM SMALL, MEDIUM AND LARGE WASTEWATER TREATMENT PLANTS

The management of sewage sludge, generated at various stages of wastewater treatment, is still a significant problem. One of the possibilities of using sewage sludge, which is a valuable source of nutrients (carbon, nitrogen, phosphorus), is its use for agricultural purposes. Sludge can be used for fertilising when it meets the requirements with regard to the content of heavy metals, the number of eggs of intestinal parasites and Salmonella bacteria, and when the amount of organic substances and fertilising components is sufficient. That is why it is so important to know exactly the characteristics of sewage sludge. This allows determination of whether it can be used as fertiliser in agriculture. The study assesses the agricultural use of sewage sludge from three mechanical and biological wastewater treatment plants of different PE, produced in 2016-2018. The analysis was carried out on the basis of the physicochemical parameters of the sludge. Particular attention was paid to the fertilisation and sanitary properties, as well as the presence of heavy metals, which were largely influenced by such factors as: the size of the wastewater treatment plant, the method of sewage sludge treatment and sludge management, and the nature of the catchment area. The concentrations of particular substances in sludge from wastewater treatment plants differed from one another, and sometimes these were very large differences. The conducted analysis showed that sewage sludge, from both small and large wastewater treatment plants, has a significant potential as agricultural fertiliser due to the presence of minerals such as nitrogen, phosphorus and calcium.

Utilization of fly ash amended sewage sludge as brick for sustainable building material with special emphasis on dimensional effect

Sewage sludge (SS) management has been regarded as a serious environmental and health issue around the globe, especially in developing countries like India, mainly due to the lack of proper regulatory criteria, and appropriate engineering infrastructures. The present research work demonstrates the utilization of Fly Ash (FA) in varying percentages (with an interval of 10%) with SS as brick for sustainable building material with special emphasis on dimensional Effect. The impact of dimensional effect is measured using compressive strength behaviour for a stress rate of 14 MPa/min of two different sizes of brick i.e. D1 and D2 of sizes 7 × 7x7 cm3 and 2 × 2x2 cm3, respectively. Owing to the sintering effect, the maximum strengths of 4.02 MPa and 7.6 MPa are noticed for a composition of 60% SS+40% FA for D1 and 40% SS + 60% FA for D2, respectively. According to the Indian Standards (IS) 13757: 1993, an exhibited strength of more than 3.5 MPa by any brick composition enables it to be used as a common building brick. In general, due to the formation of triangular wedges, the D2 bricks exhibited better strength for any mixing ratio of FA and SS. The added benefit in our study was lower thermal conductivity (0.48 Wm−1K−1) of the prepared fired brick implying its better insulation property. Our study uniquely highlights the possible utilization of FA for stabilizing SS for sustainable substitution of natural clay fired brick. Future study may further investigate the compositional effect of FA on the thermal and thermodynamic attributes of the bricks.

Hydrothermal carbonization of sewage sludge coupled with anaerobic digestion: Integrated approach for sludge management and energy recycling

Sewage sludge management is a major operational task that challenges sustainability in municipal wastewater treatment plants. On the other hand, the high level of organic carbon in sewage sludge suggests its utilization potential for renewable energy applications. This study investigates hydrothermal carbonization (HTC) of anaerobic sludge at increasing temperatures (200, 250 and 300 °C) and retention times (30, 60 and 120 min), providing a wide spectrum of reaction severities (logR0 = 4.4–8.0). The goal of the study was to quantify the energy recovery from hydrothermal co-products (i.e., hydrochar and process water) for their use as a solid fuel and a feedstock for anaerobic digestion (AD), respectively. The hydrochar yield was quantified and its quality was evaluated for elemental composition, higher heating value and combustion behavior using thermogravimetric analysis. The hydrothermal process water was analyzed for its chemical oxygen demand and biochemical methane potential. The hydrochar yield was negatively affected by reaction severity with the highest value of 73% at logR0 of 4.4 and the lowest value of 49% at logR0 of 8.0. On the other hand, the hydrochar quality was found to be positively influenced by reaction severity, which enhanced the reduction of oxygen by decarboxylation processes. The biomethane yields were also affected by the reaction severity, with the highest potential value of 227 mL CH4 g COD−1 at the lowest severity and the lowest potential value of 28 mL CH4 g COD−1 at the highest severity. An overall energy balance suggests that mild reaction severities can serve as a suitable platform returning more than 70% of the energy from anaerobic sludge using the HTC-AD process integration.

Is Biochar from the Torrefaction of Sewage Sludge Hazardous Waste?

Improved technologies are needed for sustainable management of sewage sludge (SS). The torrefaction (also known as biomass “roasting”) is considered a pretreatment of SS before use in agriculture. However, it is not known whether the torrefaction has the potential to decrease heavy metals’ (HMs) leachability and the SS toxicity. Thus, the aim of the study was to evaluate the influences of the SS torrefaction parameters (temperature and process time) on HM contents in biochar, HM leachability, and biochar toxicity, and compare them with raw SS. The experiments were designed in 18 combinations (six temperatures, 200, 220, 240, 260, 280, and 300 °C; and three process times—20, 40, 60 min). Standard tests were used to determine HMs content, leachability, and toxicity. Results indicated that the torrefaction did not increase (p < 0.05) the HM content in comparison to the raw SS. The leachability of Zn, Ni, Cu, Cr, and Mn from SS biochars was similar to raw SS. However, the degree of leachability varied significantly (p < 0.05) from as low as 0.1% for Cu to high as 16.7% for Cd. The leachability of Cd (<16.7%) and Pb (<11.9%) from biochars was higher than from raw SS (<6.1% and <2.4%, respectively). The leachability of Cd from SS biochar, in five torrefaction combinations, was higher than the threshold value for hazardous waste. It is recommended that site-specific decisions are made for torrefaction of SS with respect to its HM content, as the resulting biochar could be considered as hazardous waste, depending on the feedstock. Moreover, the biochar produced under the whole range of temperatures during 20 min retention time significantly (p < 0.05) increased the Daphnia magna Straus mobility inhibition by up to 100% in comparison to the biochar obtained during 40 and 60 min torrefaction. Taking into account the increased leachability of specific HMs and D. magna Straus mobility inhibition, biochar should be considered a potentially hazardous material. Future research should focus on biochar dosage as a fertilizer in relation to its toxicity. Additional research is warranted to focus on the optimization of SS torrefaction process parameters affecting the toxicity.

Techno-economic and life cycle assessment of an integrated hydrothermal carbonization system for sewage sludge

The need of more effective solutions for sewage sludge management is twofold: i) to alleviate the overall operating costs for wastewater treatment and ii) to ensure proper utilization/disposal considering environmental legislation. A comprehensive solution integrating hydrothermal carbonization and anaerobic digestion to treat sewage sludge was analyzed and compared with that of standalone anaerobic digestion. In order to evaluate its performance, a process simulation model in Aspen Plus® served as the main data source of the required input-output inventories to perform both techno-economic and life cycle assessment, which follow conventional methodological standards. It was observed that the integrated strategy generally reduces the environmental impacts compared to the standalone configuration due to the recovery of a hydrochar that could replace fossil fuels. In contrast, the standalone option only recovers a digestate that can generate even a higher impact than its counterfactual alternative. The integration of hydrothermal treatments showed an increase of 14% in the gross energy efficiency of the anaerobic digestion. However, economic concerns make the approach to require further optimization, since the estimated cost for the HTC option were 42% higher in comparison to the conventional anaerobic digestion alternative. Although there is a changing regulatory framework around sewage sludge, especially on nutrient recovery, hydrothermal treatments have been proven as a potentially sustainable route for sewage sludge treatment.