Mixed Sludge Research Articles

Study on failure characteristics of baking-free bricks with wrap-shell aggregate

Using sediment at the bottom of rivers and lakes as raw materials to prepare non-fired wrap shell aggregate (WSLAs) and using wrap shell aggregates to replace natural crushed stones to prepare baking-free bricks is a means of resource utilization. In this paper, using harmless treated sludge and other cementitious material mix as the core and cement as the shell, the wrap shell aggregate baking-free bricks were prepared and its damage characteristics were studied. By means of physical test and image processing, the mechanical properties of the baking-free brick under different aggregate substitution rate and grading conditions were analyzed. At the same time, the existing image analysis method is compared to select the most suitable method to study the damage characteristics of the baking-free brick, and further explore the meso-mechanism of the baking-free brick and the development and evolution of the crack under different substitution rates. The main research contents and conclusions are as follows: (1) Based on image processing technology, Multifractal spectroscopy and Grey Level Co-occurrence Matrix are faster, lower cost, and more accurate image analysis methods. These two methods make the development and evolution of cracks more convincing through quantitative analysis when exploring the damage characteristics of the wrap shell aggregate baking-free brick. (2) The fracture width Δa, maximum peak value and Δf are linear with the substitution rate, indicating that with the increase of the substitution rate, the fracture diversity is higher, the fracture complexity is higher, and the fracture propagation is wider and richer. (3) When GLCM analysis is used, the correlation between fracture characteristic values and the substitution rate is also linear, and the results show that the energy and correlation decrease with the increase of WSLAs substitution rate. Entropy and contrast increase with the increase of WSLAs substitution rate. (4) Both multifractal spectrum and gray level co-occurrence matrix can well reflect the fracture characteristics under different CAWS substitution rates. From the perspective of fracture complexity, the entropy linear correlation degree of gray level co-occurrence matrix is better than that of multifractal spectrum, which can simply and intuitively reflect the fracture expansion degree. The grayscale level co-existence matrix divides the crack expansion form into four different levels for more detailed research. For crack propagation, the two methods can be compared and verified at the same time to provide reference value for further engineering applications.

Genome-resolved correlation mapping links microbial community structure to metabolic interactions driving methane production from wastewater

Anaerobic digestion of municipal mixed sludge produces methane that can be converted into renewable natural gas. To improve economics of this microbial mediated process, metabolic interactions catalyzing biomass conversion to energy need to be identified. Here, we present a two-year time series associating microbial metabolism and physicochemistry in a full-scale wastewater treatment plant. By creating a co-occurrence network with thousands of time-resolved microbial populations from over 100 samples spanning four operating configurations, known and novel microbial consortia with potential to drive methane production were identified. Interactions between these populations were further resolved in relation to specific process configurations by mapping metagenome assembled genomes and cognate gene expression data onto the network. Prominent interactions included transcriptionally active Methanolinea methanogens and syntrophic benzoate oxidizing Syntrophorhabdus, as well as a Methanoregulaceae population and putative syntrophic acetate oxidizing bacteria affiliated with Bateroidetes (Tenuifilaceae) expressing the glycine cleavage bypass of the Wood–Ljungdahl pathway.

Enhancement of anaerobic ammonia oxidation process composed of mixed sludge without additional deoxidation by sulfur autotrophic denitrification under mainstream conditions

The practical application of anaerobic ammonia oxidation (anammox) process is hindered because of its strict demand of an anaerobic environment. Due to the slow growth cycle of anammox bacteria and the high cost of wastewater deoxidation, it is difficult to improve the contribution rate of denitrification in anammox pathway. Sulfur oxidizing bacteria (SOB) consume oxygen in the process of oxidizing sulfur elements to provide a better anaerobic environment for anammox bacteria, and sulfur autotrophic denitrification can also remove nitrate produced during the anammox process for deep nitrogen removal. The analysis of cell hydrophobicity and EPS secondary structure were consistent, which showed that the biofilm of A-SAD (anammox- sulfur autotrophic denitrification) system exhibited stronger adhesion and a more stable structure. The functional genes of ammonia oxidation (amoA) and nitrite oxidation (nxrA) were reduced by more than 90 %, implying that SAD provided a favorable anaerobic environment for anammox and reduced the cost of deoxidation in actual operation. The relative abundance of Candidatus_Kuenenia increased nearly threefold, becoming the dominant anammox bacteria in low substrate environment, while other anammox bacteria almost disappeared. The increase of the relative abundance of Thiobacillus and Desulfovibrio were beneficial to the transformation of S0 into bio-sulfur (Bio-S0) through redox, which provided the possibility for the recovery and utilization of sulfur. This experiment provides a comprehensive understanding of the nitrogen and sulfur conversion mechanism for the deoxidation and sulfur recovery of anammox in practical applications.

Feasibility of hydrothermal liquefaction in phosphorus-recovery from wastewater sludges

A new process train is proposed to stabilize wastewater sludges and to recover calcium phosphate (CaP) from them via hydrothermal liquefaction (HTL). The HTL process concentrates most of the P-content of wastewater sludges into a solid residue referred to as HTL-char. Acid leaching of HTL-char to elute P followed by a 3-step precipitation process is demonstrated for recovering high-quality CaP. Leaching HTL-char with 0.4 M HCl at a liquid:solid ratio of 20 for 0.5 h at 60 °C enabled >99 % elution of P. The 3-step precipitation process resulted in 60–89 % recovery of the P-content of HTL-char as CaP, yielding 15–26 g of CaP/kg dry sludge. Yield of CaP could be increased further if the raw sludge mix is dewatered to increase its solid content to ∼5 wt% prior to HTL. Inductively coupled plasma optical emission spectrophotometer results confirmed that the 9 regulated heavy metals in the recovered CaP were below the limits defined by the U.S. Environmental Protection Agency and the European Environmental Agency. Resource and energy costs associated with the proposed process are found to be comparable to those reported for the five patented processes for recovering P from anaerobically digested sludge (17.9 vs 21.2 ± 12.7 €/kg P recovered).

Ligneous amendments increase soil organic carbon content in fine-textured boreal soils and modulate N2O emissions.

Organic soil amendments are used to improve soil quality and mitigate climate change. However, their effects on soil structure, nutrient and water retention as well as greenhouse gas (GHG) emissions are still poorly understood. The purpose of this study was to determine the residual effects of a single field application of four ligneous soil amendments on soil structure and GHG emissions. We conducted a laboratory incubation experiment using soil samples collected from an ongoing soil-amendment field experiment at Qvidja Farm in south-west Finland, two years after a single application of four ligneous biomasses. Specifically, two biochars (willow and spruce) produced via slow pyrolysis, and two mixed pulp sludges from paper industry side-streams were applied at a rate of 9-22 Mg ha-1 mixed in the top 0.1 m soil layer. An unamended fertilized soil was used as a control. The laboratory incubation lasted for 33 days, during which the samples were kept at room temperature (21°C) and at 20%, 40%, 70% or 100% water holding capacity. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured periodically after 1, 5, 12, 20 and 33 days of incubation. The application of ligneous soil amendments increased the pH of the sampled soils by 0.4-0.8 units, whereas the effects on soil organic carbon content and soil structure varied between treatments. The GHG exchange was dominated by CO2 emissions, which were mainly unaffected by the soil amendment treatments. The contribution of soil CH4 exchange was negligible (nearly no emissions) compared to soil CO2 and N2O emissions. The soil N2O emissions exhibited a positive exponential relationship with soil moisture. Overall, the soil amendments reduced N2O emissions on average by 13%, 64%, 28%, and 37%, at the four soil moisture levels, respectively. Furthermore, the variation in N2O emissions between the amendments correlated positively with their liming effect. More specifically, the potential for the pulp sludge treatments to modulate N2O emissions was evident only in response to high water contents. This tendency to modulate N2O emissions was attributed to their capacity to increase soil pH and influence soil processes by persisting in the soil long after their application.

The Potential of Recycling Used Engine Oil for Biogas Generation by Co-Digestion with Animals’ Manure: Experimental and Kinetic Study

This study investigates the potential of biogas recovery from used engine oil (UEO) by co-digestion with animals’ manure, including cow dung (CD), poultry manure (PM), and cattle manure (CM). The experimental work was carried out in anaerobic biodigesters at mesophilic conditions (37°C). Two groups of biodigesters were prepared. Each group consisted of 4 digesters. UEO was the main component in the first group of biodigesters with and without inoculum, whereby a mix of UEO and petroleum refinery oily sludge (ROS) was the component in the second group of biodigesters. The results revealed that for UEO-based biodigesters, maximum biogas production was 0.98, 1.23, 1.93, and 0 ml/g VS from UEO±CD, UEO±CM, UEO±PM, and UEO, respectively, whereby, for the UEO=ROS-based biodigesters, maximum biogas production was 3.49, 2.47, 3.64 and 2.44 ml/g VS from UEO+ROS±CD, UEO+ROS±CM, UEO+ROS±PM, and UEO+ROS, respectively. These results indicated that UEO was not feasible and efficient for biogas recovery since biogas production was very low in the first group of biodigesters compared to its recovery in the second group. A modified Gompertz model was applied to study the kinetics of the bio-digestion process. Measured and predicted values of biogas generation were fitted well with determination coefficients higher than 0.92.

Aerobic granulation in an oxidation ditch using the residual sludge after extracting slime-extracellular polymer substances

Sludge granulation in continuous flow reactors (CFRs) is still of great challenge with the hardly obtained hydraulic selection pressure. A novel method was investigated for cultivating granular sludge in an oxidation ditch by recycling the residual sludge after extracting slime-extracellular polymer substances (slime-EPS). Compared to the raw sludge, bioactivities of the residual sludge slightly decreased based on endogenous, autotrophic, and heterotrophic respiration rates (SOURe, SOURA, and SOURH). When the slime-EPS extracted sludge was returned to the main reactor, the SOURe reduced and maintained stable later. For the first and second recycling, SOURA increased from 0.80 to 1.69 mgO2/gSS/h, and 0.16 to 1.21 mgO2/gSS/h, respectively, while SOURH exhibited initial decrease, subsequent increase and later stable trends. Moreover, total nitrogen in effluent decreased from 8.09 to 7.22 mg/L, and the protein-to-polysaccharide concentration ratio increased from 0.49 to 1.55. In addition, the mixed sludge of raw and residual sludge exhibited higher coaggregation capacity with decreased supernatant turbidity, triggering the granulation process. After 94-day operations, granules larger than 200 μm accounted for 32 % in the oxidation ditch, where neither hydraulic selection pressures nor feast-famine phases were provided. Therefore, returning the slime-EPS extracted sludge to the CFRs is a promising approach for aerobic granulation.

Volatile fatty acid production from primary and secondary sludges to support efficient nutrient management

Enhanced hydrolysis of sludges during fermentation is an important factor to achieve solubilization of complex carbon sources and increase the amount of soluble COD that microorganisms could use as food during biological nutrient removal processes. This research shows that a combination of mixing, bioaugmentation, and co-fermentation can be used to increase the hydrolysis of sludges and enhanced the production of volatile fatty acids (VFA). Mixing of primary sludge (PS) at 350 revolutions per minute (RPM) during fermentation increased the hydrolysis of the sludge and increased the soluble chemical oxygen demand (sCOD) by 72% compared to no mixing. Mixing also increased the production of VFA by 60% compared to no mixing conditions. PS hydrolysis was also evaluated using bioaugmentation with the bacteria Bacillus amyloliquefacients, a known producer of the biosurfactant surfactin. Results showed that bioaugmentation enhanced the hydrolysis of the PS by increasing the amount of soluble carbohydrates and soluble proteins present in the form of sCOD. Methanogenesis experiments performed with co-fermentation of decanted primary sludge (PS) and raw waste-activated sludge (WAS) at 75:25 and 50:50 ratios displayed a decreased in production of total biogas by 25.58% and 20.95% and a reduction on methane production by 20.00% and 28.76% respectively, compared to co-fermentation of raw sludges. Compared to fermentation of the sludges separately, co-fermentation of PS and WAS increased the production of VFA and it was determined that 50:50 was the optimum co-fermentation ratio for production of VFA while reducing the reintroduction of nutrients produced during the fermentation process to BNR processes.

Effect of Ca3(PO4)2 doped to basalt on the structure, electrical characteristics and spinnability

Ca3(PO4)2 is selected to be added to the basalt to simulate basalt mixed sludge in order to produce glass and continuous fiber. XRD and Raman are used to analyze basalt glass with varied Ca3(PO4)2 additions. Basalt glass is amorphous, according to XRD measurements. However, Ca3(PO4)2 alters the Raman spectra of basalt glass noticeably. Raman results show Ca3(PO4)2 increases the degree of polymerization of the glass, but the degree of polymerization decreases with the further increase of Ca3(PO4)2 content. The dielectric characteristics are tested at a frequency of 1 MHz. When the addition amount of Ca3(PO4)2 is 1 g, it has the lowest dielectric constant of 6.567. When the addition amount is 2 g, it has the lowest dielectric loss of 0.0026. The volume resistivity of the sample without Ca3(PO4)2 is 4.227 × 1013 Ω· cm (25 °C). With the increase of Ca3(PO4)2 content, the volume resistivity decreases, reaching the lowest 1.78 × 1013 Ω·cm when the addition amount is 2 g (25 °C). With the increase of Ca3(PO4)2 content, the fiber diameter becomes larger, the crack decreases and the fracture frequency decrease. So the sludge can be considered as an addition in the continuous fiber production.

Sludge Treatment Wetland for Treating Microalgae Digestate Grown in Agricultural Runoff: A Technical, Economic, and Environmental Assessment

The management and disposal of wastewater treatment sludge can be a costly and resource-intensive process. To provide a cost-effective and sustainable alternative, Sludge Treatment Wetlands (STW) have emerged as a viable solution for enhancing sludge quality through dewatering and biodegradation. In this study, the effectiveness of a full-scale STW for stabilizing and dewatering digested microalgal biomass from a domestic and agricultural wastewater treatment system was evaluated. The properties of the treated digestate in the STW were assessed after 35 weeks of operation and a resting period of 4 weeks. The dry matter content was found to be 12.8%, and the average macronutrient content was K: 3.8 mg/g DW, P: 4.9 mg/g DW, and Ca: 95 mg/g DW. The highest contents of micronutrients were for Fe: 7.8 mg/g DW and Mg: 7.6 mg/g DW, while heavy metals and pathogen contents were below the EC limits for sewage sludge reuse in agriculture. The STW was found to be a cost-effective and environmentally friendly option for treating mixed wastewater-based sludge for land application. The STW outperformed reference systems using centrifuge dewatering techniques, particularly in terms of eutrophication potential and acidification potential. However, the STW’s economic performance was slightly worse than that of the dewatering system in terms of unit production cost. This study is the first in the literature to investigate the use of STW for treating digested microalgae and its possible reuse in arable land, suggesting that STW infrastructures have great potential for the development of sustainable and eco-friendly sludge treatment technologies.

Anaerobic co-digestion of elephant dung and biological pretreated Napier grass: Synergistic effect and kinetics of methane production

Elephant dung (ED) and Napier grass (NP) are used as waste-to-energy resources that help in waste management and reduce greenhouse gas emissions, in accordance with Sustainable Development Goal 13 (SDG 13). In this study, anaerobic co-digestion of ED and NP (with and without biological pretreatment) was conducted under various mixing ratios. The biological pretreatment of NP was performed using the microbial activator super LDD1 (mixed culture microbial sludge) enzyme before further biogas production. The results confirmed that LDD1 enzymatic microbes increased methane production capacity by 1.95 times compared to untreated NP. Moreover, the data revealed that a NP/ED ratio of 1:1 with a 14-day pretreatment duration yielded the highest cumulative methane production of 234.8 ± 5.9 mL-CH4/g-VS, a 99.2% increase. The synergistic index results showed that substrate pretreatment had the most significant effect on methane yield, with the maximum synergistic index equaling 1.53. The Gompertz model outperformed the Logistic model in determining the reaction's kinetic parameters, with R2 > 0.995.

Evaluation of bioenergy production and material flow in treating Japanese concentrated Johkasou sludge using high-solid anaerobic membrane bioreactor based on one-year operation

The treatment of waste sludge generated from on-site wastewater treatment facilities (Johkasou) applied extensively in Japan is a key concern. In this study, a high-solid anaerobic membrane bioreactor (AnMBR) was continuously operated throughout a year to investigate the bioenergy production, material flow and seasonal impacts of concentrated Johkasou sludge (CJoS). The CJoS has higher carbon-to-nitrogen ratio and lipid fraction, but less proteins than waste activated sludge (WAS). The biogas yield of CJoS by AnMBR ranged from 0.47 to 0.52 L/g-VSfed, which was higher than the anaerobic digestion of WAS and lower than mixed sewage sludge from centralized wastewater treatment facilities. The greatest biogas production rate was observed during summer (0.63 L/L/d), owing to the high total organic and solid content. However, winter CJoS had the highest biogas yield and carbon conversion rate owing to the conservation of soluble organic matter in low temperature and high digestibility. The material flow results revealed that 46.5–52.4% total COD and 50.2–58.0% carbon was converted to biogas. The energy recovery rates ranged from 8.69 to 9.96 kJ/g-VSfed and the system energy self-sufficiency rates ranged from 1.92 to 3.02. The optimal dewaterability of digested sludge was achieved when polyamidine flocculant dosage was 30 mg/g-MLSS and the corresponding sludge specific resistance to filtration was 2.26 × 1012 m/kg. The results demonstrated that AnMBR yielded high energy conversion efficiency without co-digestion and pretreatment processing.

Fouling mitigation in membrane bioreactors by nanobubble-assisted backwashing

Widespread application of membrane bioreactors (MBRs) is still difficult because of the problem of membrane fouling, which increases operational and maintenance costs. A novel approach to mitigate fouling in MBRs, nanobubble-assisted backwashing (NBB), is presented in this paper. Nanobubbles generated in this study had an average diameter of about 150 nm, and some of the nanobubbles could pass through the pores of microfiltration (MF) membranes used for MBRs. Therefore, nanobubbles introduced from the permeate side during backwashing might exhibit cleaning effects for membranes since applications of nanobubbles for cleaning of various materials such as protein-coated gold electrodes and stainless steel have been reported. The effectiveness of NBB was compared with that of tap water backwashing (TWB) and chemically enhanced backwashing (CEB) using 50 ppm NaClO. Experiments were carried out by using three bench-scale MBRs installed at a local wastewater treatment plant. The three MBRs were operated in parallel with identical operating conditions except for backwashing conditions. Polyethylene glycol (PEG) granular materials were inserted in each membrane tank to scour the membrane surface. Regardless of the variation of the feed, NBB always exhibited better cleaning performance than TWB, and the cleaning efficiency of NBB was sometimes comparable to that of CEB. The degree of reversible fouling observed with TWB was 424 % higher than that with NBB, indicating that NBB is much more effective than TWB for the control of reversible fouling. It was also found that NBB could mitigate irreversible fouling even in the absence of a thick protective layer on the surface of the membrane. It was attempted to reveal the mechanism by which NBB enhances the cleaning effect by carrying out comprehensive analysis of the distribution of filtration resistance, mixed liquor suspension and the fouling layer. Significant changes in microbial activity of the mixed liquor suspension and sludge properties in the MBRs were not observed when NBB was carried out. The results of the analysis suggested that NBB made the structure of the fouling layer more porous, possibly facilitating the removal of the fouling layer from the membrane.

Anaerobic digestion of aqueous product of co-hydrothermal liquefaction of beverage waste and sewage sludge: Reduction of toxicity and energy assessment

Beverage waste and sewage sludge are two of the most generated waste-based biomasses. Recently, the production of biocrude through the hydrothermal liquefaction (HTL) of both wastes independently or jointly resulted in the generation of an aqueous byproduct (ACP) of high dissolved organic carbon. The high dissolved organic carbon means that ACP can be digested anaerobically to recover more bioenergy products like methane. This study investigated the anaerobic digestion (AD) of ACP produced from co-HTL of beverage waste [winery rose lees (RL)/brewery trub (BT)] and primary sludge (PS) with reference to the quantity and energy content of methane produced as well as the suitability of digestate as an organic fertilizer. Results indicated that ACP generated from the HTL of combined RL and PS (RLPS) produced the highest specific methane yield of 317 mL/gCOD and energy recovery of 80.64% while ACP generated from PS produced the lowest specific methane yield of 26 mL/gCOD and energy recovery of 6.62%. Results also indicated the N/P ratio of the anaerobic digestate makes it suitable for fertilizer application although it needs to be supplemented by K, Ca, and Mg. The results of this study proved that ACP produced during the HTL processing of mixed sewage sludge and beverage waste generates higher methane yield than what could be generated if both wastes are not mixed. This study emphasized that efficient process optimization and energy recovery can be obtained in AD of HTL ACP using a mixed feedstock rather than single feedstocks.

Vermi-Conversion of Anaerobic Sludges by Eisenia fetida Earthworms

Sludge management is considered a difficult and challenging task and is a priority of environmental policy. This study evaluates the transformation of the anaerobic sludge of agroindustrial wastes (cow manure, anaerobic sludge mixtures) directly to compost using Eisenia fetida earthworms (vermicomposting) in relation to sludge quality; moreover, it investigates the effects of different sludge compositions on vermicompost physicochemical properties. In particular, the biostabilization of anaerobic sludge (AS1) produced in excess from the wastewater treatment plant (WWTP) of Patras, Greece, and mixed anaerobic sludge (AS2) originating from the effluents of a laboratory anaerobic co-digestion system treating an agroindustrial waste mixture (olive mill wastewater, cheese whey, and liquid cow manure in a ratio of 55: 40: 5 (w/w), respectively) mixed with cow dung (CD) using Eisenia fetida earthworms was examined. Comparing the mixtures of CD-AS1 to CD-AS2, superior results were obtained with the use of AS2 since an increase in N-P-K was observed when either 10% (22%, 51.8%, and 2.4%, respectively) or 15% of AS2 (38.7%, 14.1%, and 8.1% respectively) was used. Although a reduction in earthworms’ growth was observed compared to 100% CD, during the vermicomposting of the CD-AS mixtures, 410 and 250 mg/earthworm was sustained in the mixtures of 85% CD-15% AS2 and 80% CD-20% AS2 after a period of 63 and 70 days of vermicomposting, respectively.

Hemoadhican, a Tissue Adhesion Hemostatic Material Independent of Blood Coagulation.

Uncontrolled hemorrhage is a leading cause of death, emphasizing the need for novel hemostatic agents. Here, a novel hemostatic polysaccharide hemoadhican (HD) is screened out by analyzing the rheological properties of screened material mixed blood sludges, which is prepared by mixing polysaccharide granules and whole blood to mimic the coagulation in vitro. HD is produced by a bacterial isolate Paenibacillus sp.1229, and the repeating units of HD are →)-α-L-Rhap-(1→3)-β-D-Glcp-(1→4)[4,6-ethylidene-α-D-Galp-(1→4)-α-D-Glcp-(1→3)]-α-D-Manp-(1→. Compared to chitosan and celox, HD achieves more effective hemostasis in animal models with mouse and rat femoral arteries, rat carotid arteries, and rabbit femoral arteries. Especially, HD maintains an excellent hemostatic capability in animals with heparin-induced hemorrhage diathesis. In vitro experiments show HD granules can quickly absorb a small amount of blood component to create a hemophobic blood sludge resistant to high pressure. The blood sludge firmly adheres to damaged tissue and efficiently repels blood. In vitro experiments show that HD does not actively trigger blood coagulation cascade and is independent of blood conditions including heparin treatment. In addition, HD moisturizes wounds and accelerates wound healing, exhibiting excellent biodegradability, and hemocompatibility. The results indicate that HD is a promising hemostatic material for treating traumatic hemorrhages and uncontrollable surgical bleeding.

Two-Dimensional Modelling of Sludge Heat and Mass Transfer in a Paddle Dryer

In this paper, we propose a concise but general two-dimensional model based on the penetration model in order to simulate the heat and mass transfer and drying kinetics of sludge in the paddle dryer. The sludge control equations were developed to simulate the heat and mass transfer of sludge and the penetration model was introduced to describe the mixing of sludge. Compared to the experimental results in the existing literature, the drying kinetics simulated simulated by the present model were in good agreement with the experimental data under various operation conditions. The effects of four key parameters on drying kinetics in the paddle dryer were studied. The results showed that the water content increased with the increase of sludge flowrate and the decrease of wall temperature and sludge density. Furthermore, the stirring velocity had little effect on the drying kinetics of the sludge. The present study contributes to the better understanding of the sludge drying kinetics and provides guidance for the optimal design of industrial paddle dryers.

Behavior of trace metals during composting of mixed sewage sludge and tropical green waste: a combined EDTA kinetic and BCR sequential extraction study in New Caledonia.

The aim of the study was to assess the impact of composting on the release dynamics and partitioning of geogenic nickel (Ni), chromium (Cr)and anthropogenic copper (Cu) and zinc (Zn) in a mixture of sewage sludge and green waste in New Caledonia. In contrast to Cu and Zn, total concentrations of Ni and Cr were very high, tenfold the French regulation, due to their sourcing from Ni and Cr enrichedultramafic soils. The novel method used to assess the behavior of trace metals during composting involved combining EDTA kinetic extraction and BCR sequential extraction. BCR extraction revealed marked mobility of Cu and Zn: more than 30% of the total concentration of these trace metals was found in the mobile fractions (F1 + F2) whereas Ni and Cr were mainly found in the residual fraction (F4). Composting increased the proportion of the stable fractions (F3 + F4) of all four trace metals studied. Interestingly, only EDTA kinetic extraction was able to identify the increase in Cr mobility during composting, Cr mobility being driven by the more labile pool (Q1). However, the total mobilizable pool (Q1 + Q2) of Cr remained very low, < 1% of total Cr content. Among the four trace metals studied, only Ni showed significant mobility, the (Q1 + Q2) pool represented almost half the value given in the regulatory guidelines. This suggests possible environmental and ecological risks associated with spreading our type of compost that require further investigation. Beyond New Caledonia, our results also raise the question of the risks in other Ni-rich soils worldwide.

Effect of concrete mixing plant sludge powder on properties of ultra-high performance concrete

The output of concrete mixing plant sludge (CMPS) is large and increases year by year, which needs to be disposed urgently. The effect of CMPS powder replacing silica fume (SF) on the properties of ultra-high performance concrete (UHPC) was investigated through testing the fluidity, compressive strengths, shrinkage, pore structure, micromorphology and nanoscale properties. The obtained results indicated that the use of CMPS powder substituting SF improved the fluidity of UHPC; 20% CMPS powder replacing SF increased the compressive strengths of UHPC, but with the further increase of CMPS powder, the strengths of UHPC were reduced, and the compressive strength of UHPC including 60% CMPS powder was less than 100 MPa at 28 days; the incorporation of CMPS powder increased the shrinkage of UHPC, so it is necessary to pay attention to the cracking sensitivity of CMPS powder UHPC; the use of CMPS powder replacing 20% SF produced more calcium silicate hydrate and reduced porosity of UHPC, and had no remarkable effect on the interfacial transition zone of UHPC.

A modelling approach for the assessment of energy recovery and impact on the water line of sludge pre-treatments

A simple, easy-to-use, first-order model was elaborated to predict the methane production and the release of ammoniacal nitrogen (N–NH3) to the digestate in full-scale anaerobic digestion (AD) processes. The study used long-term, semi-continuous AD tests, carried out with samples of primary sludge (PS), raw waste activated sludge (WAS), WAS after a thermo-alkali pre-treatment (90 °C, 90 min, 4 g NaOH/100 g TS) and mixed sludge (PS/treated WAS), to calibrate and validate the model. The results of both the experimental activities and the phase of model tuning demonstrated that the proposed model was capable to provide reliable information to completely characterize the AD process, thus overcoming the limitations due to discontinuity of experimental tests. Furthermore, it was demonstrated that low-temperature thermo-alkali pre-treatments could increase the values of the model parameters, namely methane production after an infinite time (B0, +70%) and hydrolysis constant (k, +450%), and made them comparable to those obtained by the application of commercial, high-energy demanding treatments (e.g. Cambi). Finally, the issue concerning the release of N–NH3 to digestate was deemed to be very worthy to being investigated because, after pre-treatments, the cost for nitrogen removal in the water line, through the traditional processes of nitrification – denitrification, could increase even by 140%.