Dry Sludge Research Articles

Rapid cultivation and enrichment of anammox bacteria solely using traditional activated sludge as inoculum and biocarrier in low-strength real sewage treatment

In low-ammonia sewage anammox process, cultivation and enrichment of anammox bacteria (AnAOB) is a challenge especially from traditional activated sludge. To this end, a novel strategy solely using activated sludge as inoculum and biocarrier in a dynamic fixed-bed reactor was proposed in this study. During 115-day operation, excellent performance was achieved with influent total inorganic nitrogen (TIN) and effluent TIN of 55.3 mg·L−1 and 4.1 mg·L−1, respectively. Rapid enrichment of AnAOB (doubling time: 8.5 days) was demonstrated by augmented specific anammox activity (trace value to 1.85 mg N·g VSS−1·h−1) and increased hzsB gene number (106 to 109 copies·g−1 dry sludge), with predominance of Candidatus_Brocadia. Large-flocs aggregate was the primary habitat for AnAOB with highest abundance and capacity. The distinctive sludge properties, symbiotic microbial interactions and dynamic operation scheme facilitated AnAOB growth and retention. This study provides a simple, economic and workable approach for the start-up of mainstream anammox process.

Improvement of water recovery from phosphate sludge at the M'Dhilla Mine, Tunisia.

In Tunisia, phosphate beneficiation from ores by the Gafsa Phosphate Company (GPC) is a water-intensive process that generates large amounts of sludge. Responsible mining minimizes water use and prioritizes its recycling and reuse to limit the impact on water resources. Recovery of water from the phosphate sludge (PS) using the densification with adapted flocculants-a low-energy consuming process-is plausible for efficient management of water resources. The objective of this study was to improve low-cost water recovery from PS produced at the M'Dhilla Mine plant operated by GPC in Tunisia. Representative samples of PS were first collected and characterized for physicochemical and mineralogical properties. To maximize water recovery, densification based on flocculation was then performed using two different flocculants (Slim Floc vs Flomin905) with different doses (0.1g/L vs 0.3g/L), consumption (100-1200g per ton of dry sludge g/tds), sludge concentration (50g/L vs 60g/L), and settling time (15-1200s). Results showed that PS particles were fine-grained and contained carbonates, silicates, and significant residual fluorapatite (59%) that could be valorized. Up to 91% of water was recovered using anionic flocculant Flomin 905. These findings show an improvement of 24% relative to the current water recovery at M'Dhilla plant (66%) while using a flocculant dose three times lower than the conventional flocculent Sim Floc (0.1g/L vs 0.3g/L). The best sludge settlement conditions were obtained with 0.1 g/L Flomin 905 at 600g/tds and 10min of settling time. The densification process using Flomin 905 proved efficient in maximizing water recovery (91%) with a consumption of flocculant that could be decreased by up to 70% annually in comparison with Slim Floc, thus decreasing treatment costs by 63%. Results will help to prevent exhaustion of groundwater resources and limit land exploitation while decreasing the volume of settling ponds.

Enhanced Sewage Sludge Drying with a Modified Solar Greenhouse

This work reports the results obtained with an innovative configuration of a closed-static solar greenhouse for sludge drying. The novelty of the solar greenhouse configuration consisted in using a forced ventilation system to provide hot air for sludge drying and the utilization of solar irradiation for energy supply. Wet sewage sludge (97% humidity) was successfully dried up to a residual humidity close to 5% after 25 days during wintertime. The increase of the airflow rate supplied under the sludge bed improved the sludge drying rate. Moreover, the fraction of volatile suspended solids decreased from 70% to 41% after 13 days, indicating that air supply promoted the simultaneous stabilization of the sludge as a side-effect to the drying process. Overall, the specific energy consumption per ton of evaporated water was estimated to approximately 450 kWh/t, resulting in about 55% of energy demand lower than a conventional thermal drying system, while using only free solar energy. The achieved high weight reduction of up to 99% implies a noticeable reduction of the excess sludge handling costs, indicating that solar greenhouse drying is a highly interesting opportunity for sludge drying in medium-small sized WWTPs.

Recirculation of activated sludge for coagulant synthesis under hydrothermal conditions.

A hypothesis was proposed that the activated sludge was converted into hydrochar full of phenolic hydroxyl and then was made into coagulant by graft copolymerization. The results show that under the addition of HCl, the content of phenolic hydroxyl on the surface of hydrochar (SBC) under hydrothermal conditions increased sharply, up to 1.586mmol/g, showing that HCl dosage of 0.10g/g dry sludge and holding time of 4h was recommended. Under graft copolymerization with the addition of DMC, the coagulant was synthesized. Based on the analysis by FTIR, XPS, zeta potential, etc., the possible synthesis route of coagulant from SBC was that phenolic hydroxyl on SBC was activated by the initiator and then the polymerization between SBC and DMC was triggered. The optimal grafting conditions are gotten. It was named as SBCHCl0.10g, 4h-g-DMC0.7. The removal by SBCHCl0.10g, 4h-g-DMC0.7 on COD and turbidity in domestic wastewater is up to 69% and 93%, respectively. The component of COD indicated that almost all particulate COD and most of colloidal COD are removed. On the contrary, the removal on dissolved COD can be neglected. Most of NH3-N and P is kept in the wastewater. This is in favor of subsequent reuse and biological treatment.

The GHG mitigation opportunity of sludge management in China

Greenhouse gas (GHG) mitigation in wastewater treatment sector is indispensable in China's carbon neutral target. As an important component of wastewater system, sludge generation is rapidly increased with the acceleration of urbanization in China. It is crucial to investigate the carbon footprint of various sludge management strategies and quantify the potential optimization of GHG reduction effect at national scale. Therefore, this study conducted a comprehensive analysis of sludge distribution and GHG profiles of various sludge systems. The overall dry sludge generation in China is 12.15 Mt, with spatial resolution at city level. Different sludge treatment options were categorized into four types: energy recovery, nutrient recovery (e.g. phosphorus and nitrogen), material valorisation (e.g. brick, biochar) and conventional disposal. With various sludge treatment options, the GHG profile of annual sludge management in China ranges from −35.86 Mt/year to 57.11 Mt/year. The best GHG mitigation can be achieved through energy recovery by co-incineration system and the greatest reduction opportunity is concentrated in highly urbanized regions, such as Yangtze River Delta, Pearl River Delta, and Beijing-Tianjin-Hebei urban agglomerations.

Carbon neutral and techno-economic analysis for sewage treatment plants

Sewage treatment plants consume great lots of power and result in a large amount of indirect carbon dioxide emissions. Four typical energy conservation and emission reduction technologies were selected for sewage treatment plants, included reclaimed water source heat pump for heating, reclaimed water source heat pump for sludge drying, sludge anaerobic digestion technology with a biogas cogeneration system and photovoltaic power generation. The carbon neutral, energy and economic analysis model were established for the four typical technologies. The results show that reclaimed water source heat pump can achieve a higher carbon neutrality effect than sludge anaerobic digestion technology with a biogas cogeneration system. The reclaimed water source heat pump for sludge drying can recover a large amount of waste heat. The payback period of photovoltaic power generation is more sensitive to the electricity price changes, while reclaimed water source heat pump for heating is more sensitive to the heat price changes. From the discussion, Z-value have a greater effect on the reclaimed water source heat pump for heating. With the treatment load of the sewage plants increases, the reclaimed water source heat pump can achieve a higher carbon neutrality effect. The application of photovoltaic power generation needs to consider the geographical location, and it can achieve higher carbon neutral efficiency in rich solar energy resources areas. A combined district heating method with reclaimed water source heat pump is proposed to expand the district heating radius of the reclaimed water source heat pump, and improve waste heat recovery efficiency in sewage. • Green technology for sewage treatment plants are studied by carbon neutral, energy and economic analysis. • Multi-factor and sensitivity analysis of technical methods are established. • Sludge drying is a better technical route in sewage treatment plants.

Polyhexamethylene biguanidine used as a new type sewage sludge conditioning agent: Effect on sludge dewaterability and mechanism

Dewatering is the basic procedure of sludge treatment and disposal, and environmentally friendly and efficient sludge conditioning methods are urgently needed. Polyhexamethylene biguanide (PHMB), a broad-spectrum germicide used in daily life and medicine, was proposed as a sludge conditioning reagent in this paper, and its effect on waste activated sludge (WAS) dewaterability was studied for the first time. Results showed that PHMB can improve sludge dewatering performance, and capillary suction time (CST) and water content (Wc) of dewatered sludge cake was reduced by 78.11% and 13.37% with 100 mg PHMB/g dry sludge (DS). Further investigation revealed that the sludge properties changed pronouncedly after PHMB conditioning, the bound water content decreased from 1.58 g/g DS to 1.29 g/g DS, the particle size (D50) increased from 34.3 μm to 39.2 μm, the zeta potential increased from −20.96 mV to −3.36 mV, and the flowability increased whilst the viscosity decreased. When the dose of PHMB was lower than 50 mg/g DS, it mainly reacted with extracellular polymeric substance (EPS), resulting in a decrease in its content, which was also manifested by the decrease of molecular weights. However, when the dose reached 100 mg/g DS, PHMB would disrupt the cytomembranes of microorganisms and release cellular contents, reflected by a corresponding growth of EPS contents and the intensity of Fourier transform infrared (FTIR) spectrum. And the scanning electron microscope (SEM) images showed that PHMB conditioning made cracks and holes on sludge microstructures. The key mechanism of PHMB improving sludge dewaterability was inferred as “organic molecules disrupting” and “sludge particles flocculating”. These findings demonstrate that PHMB is promising to be a novel, effective, and environmentally friendly sludge conditioning reagent.

Synthesis and Weak Hydrogelling Properties of a Salt Resistance Copolymer Based on Fumaric Acid Sludge and Its Application in Oil Well Drilling Fluids

Fumaric acid sludge (FAS) by-produced from phthalic anhydride production wastewater treatment contains a large amount of refractory organic compounds with a complex composition, which will cause environmental pollution unless it is treated in a deep, harmless manner. FAS included saturated carboxylic acid, more than 60%, and unsaturated carboxylic acid, close to 30%, which accounted for the total mass of dry sludge. A new oil well drilling fluid filtrate loss reducer, poly(AM-AMPS-FAS) (PAAF), was synthesized by copolymerizing FAS with acrylamide (AM) and 2-acrylamide-2-methyl propane sulfonic acid (AMPS). Without a refining requirement for FAS, it can be used as a polymerizable free radical monomer for the synthesis of PAAF after a simple drying process. The copolymer PAAF synthesis process was studied, and the optimal monomer mass ratio was determined to be AM:AMPS:FAS = 1:1:1. The temperature resistance of the synthesized PAAF was significantly improved when 5% sodium silicate was added as a cross-linking agent. The structural characterization and evaluation of temperature and complex saline resistance performance of PAAF were carried out. The FT-IR results show that the structure of PAAF contained amide groups and sulfonic acid groups. The TGA results show that PAAF has good temperature resistance. As an oilfield filtrate loss reducer, the cost-effective copolymer PAAF not only has excellent temperature and complex saline resistance, the API filtration loss (FL) was only 13.2 mL/30 min after 16 h of hot rolling and aging at 150 °C in the complex saline-based mud, which is smaller compared with other filtrate loss reducer copolymers, but it also has little effect on the rheological properties of drilling fluid.

Exploration of novel high-temperature heavy metals adsorbent for sludge incineration process: Experiments and theoretical calculations

Adsorption is an effective and environment-friendly method to control the pollution of volatile and semi-volatile heavy metals (HMs) produced during municipal sludge incineration. In this study, the effect of a novel high-temperature adsorbent (attapulgite) on the migration and transformation mechanisms of HMs (Cr, Cu, Zn, Ni, and Pb) was investigated and compared with the conventional adsorbent (kaolin). The results of experiments and characterization showed attapulgite possessed improved adsorption effects than kaolin due to the smaller crystal distortion and more active sites reserved at 850 ℃. Furthermore, it was found that volatile HMs (Pb, and Zn) tend to be retained in the fly ash, while low-volatile HMs were adsorbed in the bottom slag by Si-Al-based absorbents. Meanwhile, the analytical results of BCR and TCLP tests showed that the proportion of unstable fraction of HMs in the adsorption products of attapulgite was much lower than that of raw dry sludge, and exhibited better leaching stability than kaolin. Theoretical calculations revealed that the adsorption of typical heavy metal forms at high temperatures by attapulgite exhibited a tight chemical adsorption, furthermore, the heavy metal oxides were more susceptible to removal by attapulgite (1 1 0) surface than the corresponding heavy metal chlorides. This work demonstrated the effectiveness of attapulgite in controlling heavy metals in furnaces and provided a new perspective on the development of this field.

An efficient heterogeneous solid acid catalyst derived from sewage sludge for the catalytic transformation of sludge into biodiesel: Preparation, characterization, and arylation process modeling

Homogeneous and heterogeneous catalysts for biodiesel production are facing challenges such as corrosion, leaching of active sites, and separation difficulties. Herein, a safe, chemically stable, and high-recoverable heterogeneous solid acid catalyst was fabricated and successfully evaluated in the catalytic transesterification of sludge-derived lipids. To this end, sludge-based granular activated carbon (SBGAC) was prepared and arylated with 4-sulfobenzenediazonium chloride in a reducing agent-free reaction. Hybrid models of the adaptive neuro-fuzzy inference system (ANFIS) with evolutionary algorithms such as the genetic algorithm (GA-FIS), particle swarm optimization (PSO-FIS), and ant colony optimization (ACO-FIS) were developed to model and optimize the empirical relationship between arylation parameters. The highest PhSO3H density (1.36 mmol/g) was obtained at 70 °C, 7.6 min arylation time, and a mass ratio of acid to SBGAC of 9.83 using the PSO-FIS model, which was ∼26% more than the value obtained without optimization. Loading 20 wt% of this catalyst in the transesterification of sludge-originated lipid at 70 °C yielded 17.34% (w/w dry sludge) biodiesel after 14 h. Coating the catalyst granules on the polystyrene grains was significantly enhanced its recoverability from the reaction mixture. Moreover, the spent catalyst exhibited good chemical stability and reusability after the ninth transesterification cycle.

Impact of a methane emission tax on circular economy scenarios in small wastewater treatment plants.

This paper analyzes the impact of methane emissions taxation on the recovery of the investments required for implementing technologies that use biogas energy in small wastewater treatment plants (WWTPs) in Brazil. It is based on the hypothesis that the adoption of a national methane emission tax policy would encourage small WWTPs to become sustainable power plants. The procedure involved 173 anaerobic plants to analyze: (a) methane production; (b) available useful energy; (c) investments and avoided costs for implementing STHIL system (thermal drying sludge) and motor generator (electricity generation); (d) financial impact for two scenarios (C1: no emissions tax; C2: with tax). Positive environmental and financial results were observed for WWTPs, varying according to the period of time analyzed for both technologies. Investments must be made in cogeneration in anaerobic WWTPs for achieving satisfactory results. Taxation must not be viewed simply as a punitive instrument; on the contrary, it should be seen as a tool to encourage continuous process improvement. The circular economy may support the enlargement of the wastewater collection and treatment system, guaranteeing widespread sanitation conditions in urban areas. However, the actual implementation of a methane emission tax in Brazil still requires many rounds of discussion among sanitation companies, government, and civil society, to establish emission limits, and unit taxes, as well as to consolidate a carbon trade to follow through with this decision in the sanitation sector.

Integration of hydrothermal carbonization and aqueous phase reforming for energy recovery from sewage sludge

This study investigates the energy recovery from sewage sludge by combining hydrothermal carbonization and aqueous phase reforming of the generated process water. The hydrothermal treatment was carried out at 170 °C and 230 °C for 1 h, and produced a hydrochar with adequate physicochemical properties, such as a higher heating value (20–24 MJ kg−1) than sludge, improved fuel ratio (0.2–0.5) and a broad comprehensive combustion index (5.8 × 10−7 to 10.0 × 10−7 min−2 °C−3). Aqueous phase reforming of the process water was carried out at 220 °C and 25–29 bar for 4 h. A maximum H2 yield of 98.7 mmol H2 gTOC−1 was achieved using a bimetallic Pt/Rh catalyst supported on carbon black and for the process water obtained at the lowest carbonization temperature. The integration of hydrothermal carbonization and aqueous phase reforming processes is an interesting alternative for the management and energy recovery of sewage sludge, which allows reaching a potential energy recovery of 17.3 MJ per kg of dry sewage sludge, equivalent to 93.5 % gross energy recovery, under optimized conditions.

Occurrence of Microplastics in Waste Sludge of Wastewater Treatment Plants: Comparison between Membrane Bioreactor (MBR) and Conventional Activated Sludge (CAS) Technologies.

In this study, the presence of microplastics in the sludge of three wastewater treatment plants (WWTPs) was examined. The investigated WWTPs operated based on a conventional activated sludge (CAS) process, with (W1) or without (W2) primary clarification, and a membrane bioreactor process (MBR) (W3). The microplastics (MPs) concentration in the samples of W3 was approximately 81.1 ± 4.2 × 103 particles/kg dry sludge, whereas MPs concentrations in W1 and W2 were 46.0 ± 14.8 × 103 particles/kg dry sludge and 36.0 ± 5.2 × 103 particles/kg dry sludge, respectively. Moreover, MPs mainly consisted of fragments (66–68%) in the CAS plants, whereas the fractions of MPs shapes in the MBR sludge were more evenly distributed, although fiber (47%) was the most abundant fraction. Furthermore, samples from the MBR showed a greater diversity in MPs composition. Indeed, all the main polyesters (i.e., textile fibers and polyethylene terephthalate), polyolefins (i.e., polyethylene and polypropylene) and rubber (i.e., polybutadiene) were observed, whereas only polybutadiene, cellulose acetate and polyester were detected in the CAS plants. These findings confirmed that MPs from wastewater are transferred and concentrated in the waste sludge. This is a critical finding since sludge disposal could become a new pathway for microplastic release into the environment and because MPs might affect the fouling behavior of the membrane.

Microplastics removal from a primary settler tank in a wastewater treatment plant and estimations of contamination onto European agricultural land via sewage sludge recycling

Wastewater treatment plants (WwTPs) remove microplastics (MPs) from municipal sewage flow, with the resulting bulk of MPs being concentrated within generated sewage sludge which is frequently recycled back onto agricultural land as accepted practice in many European countries as a sustainable fertiliser resource. This circular process means that MPs successfully removed from WwTPs are deposited into the soil and able to return into the natural watercourse by means of run-off or infiltration to groundwater. This study quantifies the removal efficiency of MPs with size ranging between 1000 and 5000 μm in a primary settlement tank (PST) at a WwTP serving a population equivalent of 300,000 and provides MP concentrations in the generated sewage sludge. Our study revealed that the proportion of MPs partitioning in a PST to settled sludge, floating scum and effluent was 96%, 4% and 0% respectively, implying 100% removal of MPs of 1000–5000 μm in size. The generated sewage sludge was estimated to contain concentrations of approximately 0.01 g of MPs or 24.7 MP particles per g of dry sewage sludge solid, equivalent to ∼1% of the sewage sludge weight. Using these figures and data from the European Commission and Eurostat, the potential yearly MP contamination onto soils throughout European nations is estimated to be equivalent to a mass of MPs ranging between 31,000 and 42,000 tonnes (considering MPs 1000–5000 μm in size) or 8.6×1013–7.1×1014 MP particles (considering MPs 25–5000 μm in size). An estimated maximum application rate of 4.8 g of MP/m2/yr or 11,489 MP particles/m2/yr, suggests that the practice of spreading sludge on agricultural land could potentially make them one of the largest global reservoirs of MP pollution. Hence, recycling raw sewage sludge onto agricultural soils should be reviewed to avoid introducing extreme MP pollution into the environment.

Combining sequential extraction and 3D fluorescence to investigate the behavior of antibiotic and polycyclic aromatic hydrocarbons during solar drying of sewage sludge

Solar drying and liming are commonly used for sludge treatment, but little is known about their efficiency on antibiotics and Polycyclic Aromatic Hydrocarbons (PAHs) removal. This study aimed to investigate the removal of antibiotics and PAHs during solar drying of Limed Sludge (LS) and Non-Limed Sludge (NLS). Thus, organic matter fractionation and 3D fluorescence were used to assess the accessibility and the complexity of organic matter. 2 experiments have been conducted using LS and NLS for 45 days of drying in a pilot scale tunnel. Physicochemical results indicated significant decrease of water content (90%) for both sludge samples within 15 days of drying. For both treatments, the removal of total organic carbon and total nitrogen was low and similar for both treatments. Through this study, it has been confirmed that liming and drying contributed to a strong modification of the organic matter quality with an increase of its accessibility. On the other hand, drying alone increased the less accessible compartments, while the presence of lime affected the interconnexion between the organic matter pools. 3D fluorescence confirmed the obtained results and indicated that LS leads to obtaining more simple molecules in the most accessible compartments, while NLS leads to obtaining more complex molecules in the less accessible compartments. In addition, solar radiations and leaching may contribute to the significant removal (p < 0.01) of roxithromycin, benzo(a)anthracene, chrysene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo(g, h, i) perylene in the presence of lime. Furthermore, the evolution of organic matter pools in terms of accessibility and complexity may drive the bioavailability of these pollutants, leading to their significant removal.

Climate-neutral waste management in The Russian Federation: New approach to sludge treatment on drying beds under climate change

Identification and ecological diagnostics of the influence of basic load parameters (the cumulative effect of air temperature, the amount of precipitation) is a fundamental aspect of the wastewater sludge treatment at drying beds. The positive dynamics of atmospheric precipitation and the long-term functioning of natural and technical systems for wastewater sludge treatment under the influence of excessive atmospheric moisture does not allow the treatment/drying of precipitation, which provokes soil pollution with subsequent diffusion of pollutants into groundwater, which leads to the degradation of the natural environment components interacting with drying beds. The article is devoted to the adaptation of the process of treatment/drying of wastewater sludge at drying beds. The method includes identification of the dynamics of climatic factors of a long-term chronological series, which makes it possible to predict the effect of atmospheric precipitation on the wastewater sludge drying. The costs for the implementation and subsequent use of the proposed method are absent or insignificant (in the conditions of an increase in usable area during the modernisation of existing drying beds) in comparison with the costs of well-known and widespread methods of deliquefaction.

Low temperature catalytic oxidation of NO over sludge char under different pyrolysis activation conditions

Catalytic oxidation of NO (CONO) at ambient temperature over sludge char (SC) prepared from sewage sludge was studied in this paper. The influencing mechanism of sludge pyrolysis/activation parameters on the CONO were investigated for the SC prepared with different pyrolysis temperatures (600, 700, and 800 °C) and different mass ratios of dried sludge (DS) to KOH (DS/KOH = 4:1, 3:1, 2:1, and 1:1). The results indicated that both pyrolysis temperature and KOH activation had great influence on the catalytic performance of SC. With the increase of temperature from 600 to 800 °C, the NO conversion increased from 12% to 36%. However, the catalytic activity of KOH-activated SC increased and then decreased with the increase of KOH content. The SC prepared under the pyrolysis temperature of 700 °C and the DS/KOH mass ratio of 3:1 showed the best catalytic performance with NO conversion of 56%. The NO conversion further increased to 76.5% when the SC was reduced by H2. It was also found that there was a strong correlation between the catalytic activity and the specific surface area of the SCs. The reaction might follow the Eley-Rideal (E-R) mechanism.

Preparation of Citric Acid-Sewage Sludge Hydrochar and Its Adsorption Performance for Pb(II) in Aqueous Solution.

In order to seek the value-added utilization method of sewage sludge and develop low-cost and high-efficient adsorbents, a hydrochar was prepared by the co-hydrothermal carbonization of sewage sludge and citric acid and then characterized. The differences in Pb(II) adsorption performance between the citric acid–sewage sludge hydrochars (AHC) and the hydrochar prepared solely from sewage sludge (SSHC) were also investigated. When citric acid dose ratio (mass ratio of citric acid to dry sewage sludge) is 0.1, the obtained hydrohcar (AHC0.1) has the highest specific surface area (59.95 m2·g−1), the most abundant oxygen-containing functional groups, the lowest pHpzc (5.43), and the highest equilibrium adsorption capacity for Pb(II). The maximum adsorption capacity of AHC0.1 for Pb(II) is 60.88 mg·g−1 (298 K), which is approximately 1.3 times that of SSHC. The potential mechanisms can be electrostatic attraction, co-precipitation, complexation, and cation-π interaction. It was demonstrated that by incorporating citric acid into the hydrothermal carbonization, resource utilization of sewage sludge can be accomplished effectively.

Research on Sludge Drying Characteristics in a Double-Axis Paddle Dryer by Coupling Markov Chain and Penetration Theory

Research on Sludge Drying Characteristics in a Double-Axis Paddle Dryer by Coupling Markov Chain and Penetration Theory

Effects of the sludge physical-chemical properties on its microwave drying performance

Thermal drying is an effective sludge treatment method for dealing with large volumes of sludge. Microwave (MW) technology has been proposed as an effective and efficient technology for sludge drying. The physical-chemical properties of the sludge depend both on the origin of the sludge, as well as on the treatment process at which the sludge has been exposed. The physical-chemical properties of the sludge affect the performance and the subsequent valorisation and management of the sludge. This study evaluated the effect of certain physical-chemical properties of the sludge (moisture content, organic content, calorific value, porosity, hydrophobicity, and water-sludge molecular interaction, among others) on the MW sludge drying and energy performance. Four different types of sludge were evaluated collected from municipal wastewater treatment plants and septic tanks. The performance of the MW system was assessed by evaluating the sludge drying rates, exposure times, energy efficiencies and power input consumed by the MW system and linking the MW drying performance to the sludge physical-chemical properties. The results confirmed that MW drying substantially extends the constant drying period associated with unbound water evaporation, irrespective of the sludge sample evaluated. However, the duration and intensity were determined to depend on the dielectric properties of the sludge, particularly on the distribution of bound and free water. Sludge samples with a higher amount of free and loosely bound water absorbed and converted MW energy into heat more efficiently than sludge samples with a lower amount of free water. As a result, the sludge drying rates increased and the constant drying rate period prolonged; hence, leading to an increase in MW drying energy efficiency. The availability of free and loosely bound water molecules was favoured when hydrophobic compounds, e.g., oils and fats, were present in the sludge.