Enhancement of digestates dewaterability by CTAB combined with CFA pretreatment

Dataset on the solid-liquid separation of anaerobic digestate by means of wood ash-based treatment

Wood bottom and fly ashes were added to the anaerobic digestate using sulfuric, hydrochloric, nitric, and lactic acids, as pH conditioners and sorption activating agents. Minimum (pH of zero-point charge), mild, and severe acidification of the samples were tested. The solid-liquid separation achieved was accounted visually and with the measurement of the masses of water-soluble extract and water-insoluble material isolated. The average mass of the blend was 36.61 ± 0.68 g, including the extractant agent that was prepared with the commercial acids and ultrapure milli-Q^Ⓡ water. During the 144-h incubation of the mixtures at 20 °C and 0.17 x g, the shares of the solid and liquid were determined by centrifugation of the destructive samples at 3,130.40 x g for 5 minutes and 3-µm filtration of the supernatant. Before weighing the water-insoluble material, both the pellet that remained in the tube and the filter cake were dried at 70 °C until reaching constant weight. There was a significant increase in the amount of water-insoluble phase of the wood bottom ash due to the activation with lactic and sulfuric acids. The treatment of the wood fly ash and the agrowaste digestate with the hydrochloric acid showed an increase in the formation of water-soluble extract, in direct relation with the acidic pH of the blend. The characterization of the pH of the WS extract was performed with a Mettler Toledo^Ⓡ Seven Compact^TM S220 pH/Ion meter. The conditions of this process can be further optimized and each of the fractions can be characterized, in terms of nutrient content, to confirm the efficiency of the separation. More complex and ambitious processes can be designed for combining the wood ash and the anaerobic digestate. The performance of this treatment involving wood ashes and commercial acids can be extrapolated to other type of organic manures with a moisture content of approximately 95%, to improve their management in terms of reducing the cost of storage and transportation for land application below £5 per tonne.

Circularity of Bioenergy Residues: Acidification of Anaerobic Digestate Prior to Addition of Wood Ash

The present study investigated the acidification treatment of an agrowaste digestate and a food waste digestate, which is necessary before the addition of the wood ashes to attain the pH of zero point of charge in the blend intended to behave as a slow-release fertilizer. The 336-h acidification treatments of the 2.39 ± 0.35 g of digestates were performed with high and low doses of four commercial acids (sulfuric, hydrochloric, nitric, and lactic acids) in 50-mL capped Corning® tubes. For analytical purposes, after the incubation, ultrapure milli-Q® water was added at a rate of 10 mL for each gram of digestate to create a water-soluble phase that allowed the measurement of the pH and the electric conductivity. The results showed that the optimum dose and type of acid were very dependent on the nature of the anaerobic digestate. The maximum buffer capacity of the agrowaste digestate was 0.07 mmol H+-H2SO4/g, but this increased by adding the food waste digestate with a greater content of ammoniacal nitrogen. The agrowaste digestate with a greater content of undigested fiber was more easily oxidized by nitric acid. On the other hand, sulfuric acid oxidized the food waste digestate to a greater extent than the other acids did. Since a high dose of acid was required to achieve a greater efficiency in the solid–liquid separation, which would ease any subsequent handling of the digestates, hydrochloric acid was considered to be the most suitable acid. Lactic acid promoted the growth of filamentous microbes in the agrowaste digestate and microbial colonies in the food waste digestate, which is an indication of the poor preservation of the organic matter under these conditions.

Manufacturing of a Granular Fertilizer Based on Organic Slurry and Hardening Agent

The manufacturing of a granular fertilizer based on organic slurry (OS) and sorptive materials aims to enhance the circular economy. This article describes a technology that was conceived after appraising the chemistry of the anaerobic digestate and wood ash and the synergies of combining both materials. The information available in the literature about similar materials such as cattle slurry and lime was also considered to build a better understanding of the underlying science. The processes and machinery designed were optimized from the points of view of energy and material consumption, cost of storage, transportation and land application. The system was sized to process 1 tonne of OS (97% moisture) in a 10 h batch-shift, consuming 140 kg of wood pellets and 0.55 kW of electricity for the fan blowing preheated air. The results of the market research allowed us to reach the most economically viable routes for the commercialization of granular fertilizers. Based on the financial study, an initial investment of GBP 20,000 is needed to successfully implement the value proposition and business plan. The wide adoption of the composite fertilizer improves the management of the OS and reduces the contamination of air, soil, and water derived from intensive agricultural practices.

Mechanism insights into hydrothermal dewatering of food waste digestate for products valorization

Poor dewaterability is a bottleneck of the disposal of digestate from food waste (DFW). However, the dewatering mechanism remains unclear due to the complex composition of DFW. Understanding the dewatering mechanism, as well as the transformation of organic/inorganic matters is essential for the DFW management and valorization. In this study, the distribution, transformation, and complex interplay of organic and inorganic matters at different Hydrothermal treatment (HTT) temperatures were comprehensively analyzed to explore the hydrothermal dewatering mechanism of DFW. When HTT was conducted in the temperature range of 120-180 °C, the interstitial water was released as surface or free water because of membrane breaking and size reduction of the solid substrate. Releasing divalent cations increased the Zeta potential of the bulk solution. The weaker electrostatic repulsion between suspended particles made them easier to settle as the centrifugation cake. When the temperature of HTT was above 180 °C, polymerization and aromatization reactions took place gradually for organic matters, and the bound water was further removed. The generated humic substances were more hydrophobic than the raw material. In addition, the humic substance could combine with cationic metals, which decreased the zeta potential of the bulk solution but promoted the aggregation of nanoparticles and enhance the dewaterability of DFW.

Wood Ash Based Treatment of Anaerobic Digestate: State-of-the-Art and Possibilities

The problem of current agricultural practices is not limited to land management but also to the unsustainable consumption of essential nutrients for plants, such as phosphorus. This article focuses on the valorization of wood ash and anaerobic digestate for the preparation of a slow-release fertilizer. The underlying chemistry of the blend of these two materials is elucidated by analyzing the applications of the mixture. First, the feasibility of employing low doses (≤1 g total solids (TS) ash/g TS digestate) of wood ash is explained as a way to improve the composition of the feedstock of anaerobic digestion and enhance biogas production. Secondly, a detailed description concerning high doses of wood ash and their uses in the downstream processing of the anaerobic digestate to further enhance its stability is offered. Among all the physico-chemical phenomena involved, sorption processes are meticulously depicted, since they are responsible for nutrient recovery, dewatering, and self-hardening in preparing a granular fertilizer. Simple activation procedures (e.g., carbonization, carbonation, calcination, acidification, wash, milling, and sieving) are proposed to promote immobilization of the nutrients. Due to the limited information on the combined processing of wood ash and the anaerobic digestate, transformations of similar residues are additionally considered. Considering all the possible synergies in the anaerobic digestion and the downstream stages, a dose of ash of 5 g TS ash/g TS digestate is proposed for future experiments.

Optimizing sludge dewatering with a combined conditioner of Fenton's reagent and cationic surfactant

Enhancing sludge dewatering is of importance in reducing environmental burden and disposal costs. In this work, a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), was combined with Fenton's reagent for sludge dewatering. Results show that the Fenton-CTAB conditioning significantly promotes the sludge dewatering. Using combined techniques of response surface methodology and uniform design, dosages of Fe²⁺, H₂O₂, and CTAB for water content response were optimized to be 89, 276, and 233 mg/g dry solids (DS), respectively. The water content of sludge decreased from 79.0% to 66.8% under the optimal conditions. Compared with cationic polyacrylamide, the Fenton-CTAB system exhibited superior sludge dewatering performance. To gain insights into the mechanisms involved in sludge dewatering, the effects of Fenton-CTAB conditioning on the composition of extracellular polymeric substances (EPS) and the morphology of the sludge flocs were investigated. The decomposition of EPS into some dissolved organics and the release of proteins in tightly bound EPS facilitated the conversion of bound water to free water and further reduced the water content of sludge cake. After conditioning, morphology of sludge showed aggregation. Overall, the enhanced sludge dewatering by Fenton-CTAB treatment provides an efficient way for management of sewage sludge.

Rheology improvement in an osmotic membrane bioreactor for waste sludge anaerobic digestion and the implication on agitation energy consumption

Sludge rheology is an essential factor for anaerobic digestion (AD) processes to control the agitation energy consumption. In this study, the sludge rheology was characterized for an osmotic membrane bioreactor and a conventional sludge anaerobic digestion reactor as the solid content being increased from 3.5-3.7% to 7.5-7.7%. The flow curves were fitted using different rheological models and the mechanism was discussed. The sludge from the osmotic membrane bioreactor exhibited obviously better rheological properties than that of the conventional reactor at a solid content of 7.5-7.7%. Larger particles induced by less negative zeta potential and higher extracellular polymeric substances, together with the higher conductivity resulted by reverse salt flux in the osmotic membrane bioreactor, improved the sludge rheology due to reduced interactions between particles. As a result, the agitation energy consumption of the osmotic membrane bioreactor can save up to 34-39% compared with the conventional one at total solid content of 7.5-7.7%.

Deep dewatering process of sludge by chemical conditioning and its potential influence on wastewater treatment plants

FeCl₃, quick lime, and cationic polyacrylamide (CPAM) were used for excess sludge conditioning from wastewater treatment plant (WWTP) and the dewatering performance by different chemical conditioners was investigated. Experimental results showed that FeCl₃ could make small and concentrated sludge particles. Furthermore, new mineral phase structures for building a dewatering framework were obtained by the addition of quick lime, and the coagulation capacity was enhanced by the formation of colloid hydroxyl polymer, which was induced due to the alkaline environment. In addition, the floc particle size significantly increased after the CPAM dosage. The bound water could be released with the stripping of tightly bound extracellular polymeric substance (EPS). Therefore, the dewatering performance and efficiencies were improved and subsequently the hypothetical sludge deep dewatering process was depicted in accordance with the variation of EPS. However, high-strength refractory organics in sludge filtrates caused by quick lime pyrolysis could lead to the unstable operation of the WWTP, because the relatively high concentrations of organic compounds with benzene were dominant in sludge dewatering filtrates.

Piggery wastewater treatment by aerobic granular sludge: Granulation process and antibiotics and antibiotic-resistant bacteria removal and transport

The aim of this work was to study the responses of aerobic granulation process to antibiotics and investigate the antibiotics and antibiotic-resistant bacteria (ARB) removal and transport. Results showed that aerobic granular sludge (AGS) was dominant in the bioreactor at day 45, and the relatively high protein content from tightly bound extracellular polymeric substances (TB-EPS) facilitated aerobic granulation and maintained biomass stabilization. The protein contents in EPS and TB-EPS were positively correlated with relative hydrophobicity, thereby improving the adsorption capacity among hydrophobic particles. The chemical oxygen demand (COD), NH₃-N, and total N removal efficiencies were 98.0%, 97.0%, and 92.4%, respectively. Five antibiotics, including kanamycin, tetracycline, ciprofloxacin, ampicillin, and erythromycin, were examined in piggery wastewater, with concentrations up to the concentration range of 29.4-44.1 µg/l, and the total antibiotics removal rate reached up to 88.4% ± 4.5%. A total of 5.2% of the total antibiotics were discharged from bioreactor, and 62.5% of the total antibiotics were degraded, and 32.3% of total antibiotics were adsorbed by aerobic granules. The presence of antibiotics rarely exhibited an influence on AGS formation, and the relatively high microbial activity of aerobic granules was beneficial to antibiotics removal. The ARB removal rate increased up to 89.4% ± 3.3%, but a large amount of ARB was enriched in aerobic granules.

Dewatering of drilling sludge by ultrasound assisted Fe(ii)-activated persulfate oxidation

An ultrasound assisted Fe(ii)-activated persulfate oxidation method was put forward to improve the dewaterability of drilling sludge in this research. The water content in the filter cake and specific resistance to filtration (SRF) were measured to evaluate the sludge dewaterability. Volatile suspended solids (VSS), transmittance of supernatant, microstructure, particle size distribution and zeta potential were tested to justify the proposed mechanism. The results showed that appropriate ultrasound assisted Fe(ii)-activated persulfate oxidation could not only further enhance the sludge dewaterability but also reduce the reaction time as well. The optimal conditions for this method were 1.6% sodium persulfate, 0.8% ferrous sulfate, 40 W ultrasonic power and 45 min reaction time. Reduction of VSS and an increase of transmittance were further achieved compared to oxidation alone. SEM results and the decrease of particle size after the treatment confirmed the disintegration of sludge flocs, which promoted the release of bound water. A synergistic effect mechanism of ultrasound and chemical oxidation was proposed, with ultrasonic cavitation disintegrating the flocs, exposing the interior organics and persulfate further oxidizing the released organics.

An ultrasound assisted Fe(ii)-activated persulfate oxidation method was put forward to improve the dewaterability of drilling sludge in this research.