Sustainable Sludge Management Research Articles

Multiscale experimental insights into vacuum drying of sludge for enhanced energy efficiency and emission control

This study provides a comprehensive analysis of the vacuum drying process for sludge drying, with a focus on optimizing energy efficiency and emission control. The study used both lab-scale static and pilot-scale vacuum drying systems to test various parameters like vacuum levels, heat source temperatures, and sludge thicknesses. The results indicated that optimal drying conditions were achieved at a vacuum level of −0.06 MPa, a heat temperature of 140 °C, and a sludge thickness of 3.4 mm, where the drying rate reaches 0.13278 g·g−1·min−1. The study underscores the significant influence of vacuum level, temperature, and sludge thickness on drying rates. The Page model was used to analyze drying kinetics, elucidating how changes in these parameters affect drying characteristics. Furthermore, the study also examined the pollutant emissions and energy efficiency at the pilot scale. It found that high vacuum environments could efficiently dry sludge using low-temperature heat source, leading to average energy consumption per unit evaporation of 3020.29 kJ/kg, which is lower compared to traditional methods. By harnessing low-grade industrial waste heat, this can be further reduced to 875.76 kJ/kg. This study offers valuable insights for sustainable sludge management systems, highlighting the environmental and economic benefits of vacuum drying technology. The detailed experimental approach and thorough analysis make a significant contribution to the field of the sludge drying.

Bacterial community composition and diversity under increasing loading rates in reed wetland for sludge treatment

Efficient and sustainable sludge management is a significant environmental and health challenge. Sludge-treatment reed beds (STRBs) are widely recognized as a cost-effective, highly efficient, and environmentally friendly solution for sludge treatment and dewatering. This study investigated the bacterial community composition and diversity in pilot-scale STRBs operating at different sludge loading rates (75, 100, and 125 ​kg ​m2 year−1). 16S rRNA V4 DNA sequencing was used to assess the diversity of the bacterial communities within the sludge samples. The relative abundance of prokaryotic taxa was affected by all treatments. As the sludge loads increased, the Shannon entropy and evenness diversity also increased for the STRBs and unplanted beds. Interestingly, the presence of reeds resulted in significantly lower Shannon and evenness indices than unplanted beds, regardless of the sludge. Additionally, the correlation network analysis revealed distinct microbial clusters with distinct responses to reeds and sludge loads. Principal component analysis evidenced an association between cluster 5 and organic matter decomposition, primarily at higher sludge doses, while clusters 4 and 6 were related to sludge decomposition at lower doses. Additionally, cluster 4 was associated with nutrient removal. The formation of distinct microbial niches was linked to sludge stabilization and nutrient removal and was influenced by both sludge loading rates and the presence of reeds. Future research can leverage these findings to innovate pollutant removal and ecosystem services for sludge treatment, thus advancing sustainable sludge management and environmental preservation.

Acidification and alkali precipitation for phosphorus recovery from municipal digested sludge

AbstractMunicipal digested sludge (MDS) has considerable phosphorus content that is between 1% and 15% in dry matter and it is a potential source for phosphorus recovery. A high amount of MDS is generated from sewage treatment plants in large cities and has many environmental concerns with its organic and inorganic constituents. In this study, phosphorus was recovered by precipitation with Ca(OH)2 and NaOH. Acid pretreatment was applied to MDS with H2SO4 and 58.7% of phosphorus was extracted from the solid phase to the liquid phase at pH 2. Up to 99% of dissolved phosphorus in the liquid phase was recovered with Ca(OH)2 and NaOH. Almost similar phosphorus precipitation rates were achieved with both bases but higher phosphorus contents (17% as P2O5) in precipitates were obtained by NaOH. The product obtained in this study can serve as a high‐quality raw material substitute for the low‐grade phosphate rocks (<15% P2O5) commonly utilized in the phosphate industry. Furthermore, this research presents a practical, environmentally friendly and cost‐effective approach for sustainable sludge management and phosphorus production.

Ultrasound-enhanced peracetic acid oxidation improves short-chain fatty acids production from waste activated sludge

This study proposed a new technology, i.e., ultrasound (US)-enhanced peracetic acid (PAA) pretreatment, to enhance resource recovery from anaerobic sludge fermentation. Results show that the optimum operation condition was achieved with US-enhanced 10 mg PAA/g TSS (total suspended solids), leading to a remarkable 31.72-fold increase in short-chain fatty acids (SCFAs) production compared to the control group (with PAA contribution in SCFAs deducted). Mechanism explorations prove that US-enhanced PAA pretreatment significantly promoted sludge disruption and organic matter release, creating a substantial amount of soluble substances (i.e., tyrosine-like and tryptophan-like substances) for subsequent fermentation. Fourier transform infrared (FTIR) analysis indicates that US-enhanced PAA pretreatment induced alterations in the protein secondary structure and facilitated its transformation. Quenching experiments imply that the contribution of reactive species during sludge oxidation followed the order: 1O2 >OH > CH3C(O)O/CH3C(O)OO. These reactive species played essential roles in the inherent mechanisms responsible for sludge disintegration and organic conversion. On the other hand, the analysis of key enzyme activities reveals a severe inhibition of the enzymes associated with SCFAs consumption. Further microbial community investigation confirms the enrichment of acidogenic microorganisms, such as Clostridium_sensu_stricto_10 sp., norank_f__norank_o__Bacteroidales sp., etc. Overall, US-enhanced PAA pretreatment can facilitate energy recovery and enhance the efficiency of anaerobic fermentation, offering significant benefits in sustainable sludge management.

Enhanced anaerobic digestion of waste activated sludge by alkaline protease-catalyzing hydrolysis: Role and significance of initial pH adjustment

Anaerobic digestion has been regarded as a preferred technology for carbon/energy recovery from waste activated sludge via volatile fatty acids (VFAs) and methane production. Although the enzyme pretreatment has been investigated, specific roles of initial pH adjustment on alkaline protease (AP) catalysis and its influence on anaerobic digestion have been rarely reported. This work demonstrated that the increasing initial pH displayed positive mutual promotion of AP-catalyzing hydrolysis for facilitating sludge solid reduction and biodegradable organic matter solubilization. The release of carbon source and phosphorus were also promoted, while the soluble NH4+-N content was decreased at increasing alkaline conditions, which was beneficial for hydrolysate bio-utilization. Nevertheless, initial pH = 10 was the optimum condition for maximizing VFAs and methane recovery after AP-catalyzing sludge hydrolysis. Under such condition, the sludge solid reduction and sludge hydrolysis rate constants were 0.7981 h−1 and 1.1288 h−1, respectively, which were 6.40 and 2.45 times higher than those in the control. Meanwhile, the biochemical acidogenic potential of 447.0 mg COD VFAs/g VSS and the estimated biochemical methane potential of 437.6 mL CH4/g VSS were achievable. The identification of the mutual promotion distribution revealed that initial pH adjustment presented more close association with anaerobic digestion than AP catalysis and its contribution was amplified with increasing pH. Apparently, such AP catalysis with initial pH adjustment is a promising technology for sustainable sludge management with economic benefits and carbon-emission reduction.

Sustainability analysis of implementing sludge reduction in overall sludge management process: Where do we stand?

Sustainable sludge management has significance due to the great potential of recovering energy and resources, potentially achieving carbon neutrality and energy positivity in the process. However, whether the sludge reduction strategy really benefits the aim of a sustainable sludge management process requires a holistic analysis. In this study, nine scenarios involving different sludge management strategies with or without sludge reduction methods were environmentally and economically assessed to clarify the necessity of adopting a sludge reduction or not. Results reveal that direct sludge incineration without prior in-plant sludge reduction generates the least environmental impacts (less than 25–120 %), but it increases operation costs by 103–110 % compared to landfilling with prior in-plant sludge reduction. Chemical Oxygen Demand flows indicate that direct sludge incineration is superior in converting organic matter into energy compared to employing sludge reduction followed by landfilling or land application. This converted energy offsets environmental impacts from electricity consumption, but these electricity benefits are insignificant in the overall cost. Case studies suggest that direct sludge incineration could facilitate potential nutrient and energy recovery, especially for metropolis. While sludge reduction strategies are more suited for developing regions relying on landfills or land application, to lower the economic burdens. The findings of this study tend to shed light on the decision-making of adopting sludge reduction strategies and sustainable sludge management.

Socio-Economic, Technical and Environmental Indicators for Sustainable Sewage Sludge Management and LEAP Analysis of Emissions Reduction

The waste management sector is transitioning from a dirty and undesirable industry towards a green and sustainable future where energy and materials are recycled. Recycling has potential in sewage sludge treatment, where energy and highly valuable nutrients can be recovered through innovative and sustainable sludge management. Although there are many technologies and techniques already used for sewage sludge, the indicators for their sustainability are not developed. In terms of sustainable and innovative sewage sludge treatment, usually, only techno-economic aspects of existing and current technologies are considered. We explore the existing indicators and propose new indicators for sustainable and innovative sewage sludge treatment technologies. The indicators are differentiated into four main categories: technical, social, environmental and economic, where specific indicators are explained, followed by a description of their impact on sustainability. We also consider a case study using the LEAP tool, which considered GHG emissions when utilizing sewage sludge as an energy feedstock to replace existing fossil fuels in the energy mix in several scenarios. The results showed a significant emissions reduction when sludge is used—37.6% and 90.9% in 2030 and 2050, respectively.

Application of Financial Flow Simulator (eSOSView™) for Analyzing Financial Viability and Developing a Sustainable Fecal Sludge Management Business Model in Kushtia, Bangladesh

To achieve SDG 6.2.1 (a) on safely managed sanitation services, several financial flow models (FFMs) and business models for the sanitation value chain have been implemented in Bangladesh and elsewhere; however, there is limited research on financial viability and sustainability of business models. Bangladesh has attained 99% sanitation coverage, mostly with onsite sanitation systems; however, the country is facing a second-generation sanitation challenge, fecal sludge management, encompassing the entire sanitation chain. Kushtia Municipality in Bangladesh is entirely served by onsite systems; the fecal sludge emptying service is provided by the municipality, and the fecal sludge treatment plant is managed by a private entity. This study investigated sustainability of FFMs in Kushtia by using the financial, institutional, environmental, technical, and social (FIETS) sustainability approach and applying the financial flow simulator (eSOSView™) tool to analyze financial viability. Several criteria in each aspect of the FIETS approach were developed, scored, and validated by stakeholders to determine sustainability. The study found that the financial aspect is the most important criteria for sustainability and “modified parallel tax and discharge fee” is the most sustainable business model for Kushtia.

Developing a sustainability-oriented multi-criteria game theoretical decision analysis framework: A case study of sludge management

Decision-making process can be influenced by many factors, including the interests and preferences of stakeholders, their interactions, as well as the considered criteria. This study constructs a methodology framework based on game theory and multi-criteria decision making (MCDM) approach to address the decision-making problem with conflicting interests of different parties for sustainable sludge management. The proposed framework compares the overall sustainability performances of the alternatives for the corresponding stakeholder based on the life cycle sustainability impacts and the weight of each criterion. The weights of considered criteria are determined by the opinions of different groups of stakeholders. Then, game theory is applied to assist the stakeholders share the costs and benefits and guide them to reach a consensus on the final selection for the sludge management technology. A case study applying the proposed framework to analyze the game between sludge treatment facility and the government was carried out. Four different sludge valorization technologies were selected as the alternative strategies for both players, including incineration for electricity production followed by landfill (S1), incineration for power generation followed by cement production (S2), biogas from sludge digestion for electricity generation by fuel cells (S3) and biogas from sludge digestion for electricity generation by combustion (S4). Results show that both the sludge treatment facility and the government may mutually benefit from S3 if the sludge treatment facility pays a tipping fee of $0.19–7.59 per kWh net electricity generation. Sensitivity analysis was also carried out to study the influence of weighting variations and parameter uncertainty on the final strategy selection and results revealed the stability of the proposed framework. The outcome of the framework can contribute to the sustainable decision-making process for the involved players and reach an agreement more efficiently.

Agricultural Use of Sewage Sludge as a Threat of Microplastic (MP) Spread in the Environment and the Role of Governance

Sewage sludge from wastewater treatment plants is commonly used as a soil amendment on agricultural land. Unfortunately, more and more research indicates that large amounts of microplastic (MP) are re-introduced to soil with sludge. This study aims to present the impact MP has on soil ecosystems, global trends in agricultural sludge management resulting from a high concentration of micropollutants in sludge, and finally propose a high-level strategy for sustainable sludge management. This strategy is mostly dedicated to the European Union and involves multiple stakeholders and the links between each of them to achieve appropriate sludge management to avoid soil pollution with MP. Governance, Technology, Consumer Acceptance, and Economy and Commercial Viability is explored in depth. To the author’s knowledge, this is the first paper to discuss these topics in the context of a changing agricultural scene and identifies ways of which sewage sludge can limit MP pollution whilst still fitting into a circular economy. As total elimination of agriculturally used sludge is not a viable option, more stringent regulation on sludge quality before its use is necessary, especially on contaminant concentrations. This includes MPs limits, to improve sludge quality, in turn improving soil health. More alternative management options for sludge that does not meet land usage requirements are necessary and will be explored in this study. Overall, the combination of factors discussed will inevitably lead to more emphasis on sewage sludge management, therefore it can be expected that the information presented in this review will be of high demand and importance for sludge producers and serves as a comprehensive foundation for researchers to build off.

Phosphorus recovery from municipal sludge-derived ash and hydrochar through wet-chemical technology: A review towards sustainable waste management

• Sludge-derived ash and hydrochar are promising waste sources for P recovery. • P recovery from hydrochar is significant for implementing hydrothermal liquefaction. • Feasibilities and challenges of wet-chemical methods for P recovery are identified. • Sequential extraction is practical and effective to address metal interference. • The integration of wastewater treatment and resource recovery is prospected. Phosphorus (P) is a non-renewable resource, and its recovery and recycling are necessary for meeting future P demands and environmental conservation. Ash and hydrochar from incineration and hydrothermal liquefaction (HTL) of municipal sludge, respectively, represent promising sources for efficient P recovery. The full-scale application of wet-chemical techniques has proven their technical feasibility for P recovery from sludge-derived ash. HTL for sludge treatment has received significant attention for producing biocrude with net-positive energy recovery. P recovery from the solid by-product of sludge HTL, hydrochar, is a critical step in holistic sustainable sludge management. This review aims to guide P recovery from sludge-derived ash and hydrochar by presenting recent advances in wet-chemical extraction and precipitation. By comparing their characteristics, ash and hydrochar derived from municipal sludge exhibit similar feasibilities and challenges for P recovery. Extraction is recognized as the critical step for P recovery. The advantages and disadvantages of various P extraction approaches are provided. Acidic extraction achieves high efficiency (up to 100%) but requires removal of co-extracted heavy metals. Alkaline extraction prevents metals contamination but shows low efficiency (<70%). Sequential extraction is first identified as an important advance in this review, with promising P extraction efficiencies (70–91%) and low metal interferences. Three possible fertilizers (struvite, calcium phosphates, and vivianite) are recommended through the final precipitation, given their pros and cons of recovery. Ultimately, this review highlights the need for a wastewater biorefinery integrating wastewater treatment, HTL (energy recovery), and nutrient recovery for sustainable sludge management.

Sustainable sludge management - its use for remediation of degraded lands

The sludge resulted from the urban wastewater treatment plants, can be used, from a circular economy, as a fertilizer for agricultural and degraded lands, for horticulture and forestry. This paper aims to propose an alternative for the use of this type of sludge in a sustainable way, with a low impact on the environment. The study consisted of experiments done to recover the stabilized sludge from the urban wastewater treatment plants in the process of degraded land rehabilitation, using the energetic willow, Salix sp. The experimental results showed that the use of stabilized sludge as fertilizer for naturally and anthropogenic degraded lands determined the improvement of nutritional characteristics of the lands cultivated with Salix sp., and allowed the growth, development, and maintenance of this plant. Taking into consideration the characteristics of the cultures and the quantities of metals accumulated by Salix sp., it can be concluded that these plants were adapted to the environmental conditions and the substrate toxically composition. The results obtained on two degraded lands indicate that Salix sp. can be used for its introduction in the economic circuit, both for salty soils and ash dumps.

Development, current state and future trends of sludge management in China: Based on exploratory data and CO2-equivaient emissions analysis

This study statistically reported the current state of sludge treatment/disposal in China from the aspects of sources, technical routes, geographical distribution, and development by using observational data after 1978. By the end of 2019, 5476 municipal wastewater treatment plants were operating in China, leading to an annual sludge productivity of 39.04 million tons (80% water content). Overall, 29.3% of the sludge in China was disposed via land application, followed by incineration (26.7%) and sanitary landfills (20.1%). Incineration, compost, thermal hydrolysis and anerobic digestion were the mainstream technologies for sludge treatment in China, with capacities of 27,122, 11,250, 8342 and 6944 t/d in 2019, respectively. Incineration and drying were preferentially constructed in East China. In contrast, sludge compost was most frequently used in Northeast China (46.5%), East China (22.4%) and Central China (12.8%), while anaerobic digestion in East China, North China and Central China. The capacities of sludge facilities exhibited a sharp increase in 2009–2019, with an overall greenhouse gas emissions in China in 2019 reached 108.18 × 108 kg CO2-equivaient emissions, and the four main technical routes contributed as: incineration (45.11%) > sanitary landfills (23.04%) > land utilization (17.64%) > building materials (14.21%). Challenges and existing problems of sludge disposal in China, including high CO2 emissions, unbalanced regional development, low stabilization and land utilization levels, were discussed. Finally, suggestions regarding potential technical and administrative measures in China, and sustainable sludge management for developing countries, were also given.

Scheduled Faecal Sludge Service (SFSS) for Sustainable Faecal Sludge Management in Cilacap

In accordance with the 6’th Sustainable Development Goals (SDG's), that ensurin availability and sustainable management of water and sanitation for all and with the launch of the 100-0-100 movement by Indonesian Government, which is a program to achieve the 100% accesss for water, 0% for slums, and 100% accesss to proper sanitation, Cilacap government need to work harder to achieve 100% accesss to proper sanitation especially in faecal sludge management that still needs to be improved. More than 98% households in Cilacap are using on-site sanitation system, without periodicaly emptied, and its causing problems in public health, and has impact for environmental and economic nearby communities. This study was held in Cilacap, a municipal in Central Java, Indonesia, using secondary data and literature review. This study aims to analyze feseability of the plan of implementation Scheduled Faecal Sludge Service (SFSS). Based on analysis, Cilacap are feasible to implementing SFSS as long as Cilacap could ensure to repair all gaps that shown in Gap analysis. Data of sludge service areas in cilacap, operating patterns, SFSS implementation procedures, and financial calculations must be made on preparation stage of SFSS.

Surfactant bilayer on chitosan bead surface for enhanced Ni(II) adsorption

Surfactant bilayer formation on charged solid surfaces under suitably chosen conditions and their utilization for adsolubilizing the inorganic ions with improved efficiency is an established phenomenon. Such bilayer formation on solid substrates such as alumina, silica, carbon etc., although is more common, however, organic biomaterials for such purpose is not explored. Chitosan modifications through derivatization or composite formation (with bentonite, alumina, silica, and even with surfactant) are known already for pollutant removal with higher efficiency. This is the first time we explore here the formed bilayer on chitosan bead for the removal of Ni(II) from water. Chitosan bead surface was modified with sodium dodecyl sulfate (SDS) at different concentrations (1500 and 6000 mg L−1) to prepare different kinds of adsorbents, which were used to remove Ni(II) from the water environment. This is a new kind of approach which involves the ‘adsolubilization’ phenomenon. Detailed studies on adsorption kinetics, effect of pH, variation of adsorbent dose, evaluation of thermodynamic parameters, interference of common ions and desorption phenomenon were performed on the removal of Ni(II) from aqueous media. The adsorption followed pseudo-second order kinetics and Langmuir isotherm model. It was also found that intra-particle diffusion subsisted in the Ni(II) adsorption process, but that it was not the only rate-limiting step. Thermodynamic studies indicated that the adsorption was endothermic and spontaneous in nature. Based on the kinetic and thermodynamic studies, it could be concluded that, the adsorption is governed by both physisorption and chemisorption mechanisms. On treatment with dilute HNO3 or H2SO4, 100% desorption of Ni(II) was accomplished. The recovered Ni(II) either can be reused, or after the Ni(II) adsorption onto the surfactant-modified chitosan beads the material can be converted to a suitable catalyst. This leads to a sustainable sludge management procedure.

Comparative life cycle assessment of sludge management: A case study of Xiamen, China

Sludge management poses a great challenge to China's urbanization process as the number of wastewater treatment plants increases and the discharge standards are strengthened. China's sludge production rate is still low compared with developed countries but is undergoing a rapid growth due to increasing urbanization. Sludge management requires a systematic solution which integrates environmental effectiveness, social acceptability, and economic affordability based on a life cycle approach. In this study, we assess the environmental and economic performance of hydrothermal-pyrolysis technology (HPT), an emerging technology for sludge disposal. Climate change, human toxicity, costs, beneficial use of sludge and land occupation are used as indicators to compare HPT with conventional disposal methods, including incineration with two water contents, landfill, and compost. A particle swarm optimization (PSO) model based on the LCA results (PSO-LCA) was built to find a sustainable sludge management system in Xiamen. HPT was the most favorable scenario for sludge disposal overall, but given capacity restrictions and a trade-off between indicators, the optimal disposal proportion was for landfill to account for 9.3% of total sludge disposal, incineration with water content of 80% to account for 35.9%, HPT to account for 28.9% and compost to account for the remaining 25.9%. As further dewatering contributes a significant fraction to human toxicity effects and economic performance, the amount of dewatering should be selected based on the subsequent disposal process. The PSO-LCA method provides scientific evidence to inform decision makers to find a sustainable sludge management plan in accordance with wastewater features and socioeconomic conditions in Chinese cities.

Characterization and constructive utilization of sludge produced in clari-flocculation unit of water treatment plant

All water treatment plants produce waste/residue amid the treatment of raw water. This study selectively investigates the clariflocculator sludge for its physicochemical characteristics and potential reuse options. Sieve analysis, XRF, SEM, XRD, FTIR, and TG-DTA instrumental techniques have been used to characterize the sludge sample. Results show that clariflocculator sludge contains about 78% fine sand having grain size range 150–75 μm. SiO2, Al2O3, Fe2O3 and CaO constitute the maximum percentage of chemical compounds present in the sludge and quartz is the main crystalline phase of the sludge. Recycling and reuse of this sludge, especially, as fine sand in preparing mortar, concrete mix and other civil engineering products would pave the way for constructive utilization with safe and sustainable sludge management strategies.

Dairy wastewater treatment using water treatment sludge as coagulant: a novel treatment approach

The dairy industry is among the most polluting industries as it produces large volume of wastewater that may adversely affect the environment if discharged untreated. Dairy wastewater is characterized by high COD, BOD and nutrient levels. In this study, water treatment sludge was used as a coagulant for the treatment of synthetic dairy wastewater in the pH range of 4–10. Turbidity, COD, BOD, TSS and TDS removals from the synthetic dairy wastewater were found to be around 93, 65, 67, 84 and 85%, respectively, at the optimum conditions. Water treatment sludge was found to perform even better than other conventional coagulants used for the same. Results showed that it has the potential to substitute the conventional coagulants partially or fully in the primary treatment of dairy wastewater. The utilization of water treatment sludge at dairy wastewater treatment plants would provide sustainable sludge management and cost-effective dairy wastewater treatment.

From municipal/industrial wastewater sludge and FOG to fertilizer: A proposal for economic sustainable sludge management

After a ban on the depositing of untreated sludge in landfills, the sludge from municipal and industrial water-treatment plants can be regarded as a problem. Waste products of the water treatment process can be a problem or an opportunity - a source for obtaining raw materials. In the European Union, raw sludge and fats, oil and grease (FOG) from municipal and industrial wastewater treatment plants (WWTP) cannot be deposited in any natural or controlled environment. For this reason, it must be processed (stabilized, dried) to be used later as a fertilizer, building material, or alternative fuel source suitable for co-incineration in high temperature furnaces (power plants or concrete plants). The processes of drying sludge, where heat and electricity are used, are energy consuming and economically unattractive. Beside energy efficiency, the main problem of sludge drying is in its variability of quality as a raw material. In addition to this, sludge can be contaminated by a number of organic and inorganic pollutants and organisms. Due to the presence or absence of pollutants, different end products can be economically interesting. For example, if the dried sludge contains coliform bacteria, viruses, helminths eggs or smaller quantities of heavy metals, it cannot be used as a fertilizer but can still be used as a fuel. The objectives of the current article is to present a batch-processing pilot device of sludge or digestate that allows the following: (1) low pressure and low temperature energy effective drying of from 10 to 40% remaining water content, (2) disinfection of pathogen (micro)organisms, (3) heavy metal reduction, (4) production of products of predetermined quality (e.g. containing different quantities of water; it can be used as a fertilizer, or if the percentage of water in the dry sludge is decreased to 10%, then the dried sludge can be used as a fuel with a calorific value similar to coal). An important feature is also the utilization of low-pressure technology to prevent odorous gasses from spreading into the environment. There are presented two new technologies:a) Sewage sludge or digestate drying in the vacuum chamber consumes approx. 1 kWh/dm3 of evaporated water and, therefore, reaches a price of 180–240 Euros/t Dry Matter (DM), andb) Heavy metals' reduction using adsorbing reaction with magnetite nanostructures can decrease the level of heavy metals in the sewage sludge or digestate up to 20% in one cycle, which can be repeated several times on the same sludge.The aim of the paper is to present a newly developed technology which can provide economic and safe use of moderate heavy metals polluted sewage sludge on agricultural lands as organic fertilizer and, therefore, returning the nutrients (nitrogen, phosphorous, potassium) back to the human food chain, instead of being incinerated or landfilled. The proposed drying technology is economically sustainable due to the low vacuum and temperature (35 °C–40 °C), that increases the efficiency of the heat pump (coefficient of performance 5–7,2) of the energy produced by the anaerobic digestion. Hence, the main emphasis is given to the development of: an efficient method for heavy metals' reduction in the sludge treatment chain by using chitosan covered magnetite nanoparticles, an efficient drying method in a vacuum with low temperature energy which can be exploited from sludge digestion to reduce organic matter, and an energy sustainable concept of sludge treatment, with the addition of fats, oil and grease (FOG) to produce enough biogas for sludge drying to produce fertilizer.

Predicting the apparent viscosity and yield stress of digested and secondary sludge mixtures

The legal banning of conventional sludge disposal methods such as landfill has led to a global movement towards achieving a sustainable sludge management strategy. Reusing sludge for energy production (biogas production) through the anaerobic digestion of sludge can provide a sustainable solution. However, for the optimum performance of digesters with minimal use of energy input, operating conditions must be regulated in accordance with the rheological characteristics of the sludge.If it is assumed that only secondary sludge enters the anaerobic digesters, an impact of variations to the solids concentration and volume fraction of each sludge type must be investigated to understand how the apparent viscosity and yield stress of the secondary and digested sludge mixture inside the digesters changes. In this study, five different total solids concentration of secondary and digested sludge were mixed at different digested sludge volume fractions ranging from 0 to 1.It was found that if secondary sludge was mixed with digested sludge at the same total solids concentration, the apparent viscosity and the yield stress of the mixture increased exponentially by increasing the volume fraction of digested sludge. However, if secondary sludge was added to digested sludge with a different solids concentration, the apparent viscosity and yield stress of the resulting mixed sludge was controlled by the concentrated sludge regardless of its type. Semi – empirical correlations were proposed to predict the apparent viscosity and yield stress of the mixed digested and secondary sludge.A master curve was also developed to predict the flow behaviour of sludge mixtures regardless of the total solid concentration and volume fraction of each sludge type within the studied solids concentration range of 1.4 and 7%TS. This model can be used for digesters optimization and design by predicting the rheology of sludge mixture inside digester.