A comprehensive evaluation method for sludge pyrolysis and adsorption process in the treatment of coking wastewater

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

DOTA functionalized adsorbent DOTA@Sludge@Chitosan derived from recycled shrimp shells and sludge and its application for lead and chromium removal from water

DOTA@Sludge@Chitosan was synthesized by a facile treatment using DOTA (1,4,7,10-tetraazacyclododecane-N,N',N,N'-tetraacetic acid) to modify dry sludge and chitosan in an acidic solution. The performance of developed DOTA@Sludge@Chitosan was investigated for the adsorptive removal of Cr6+ and Pb2+ from water. Characterization studies showed that the materials possess a large surface area (52.009 m2/g), pore volume (0.069 cm3/g), and abundant functional groups of amino and hydroxyl. The prepared material showed a synergetic effect due to carboxylic acid and sludge, effectively removing Cr6+ and Pb2+. It reached 329.4 mg/g (Pb2+) and 273.3 mg/g (Cr6+) at 20 °C, much higher than commercial activated carbon. The regeneration of the adsorbent was tested for six adsorption and desorption cycles. The results demonstrate that the DOTA@Sludge@Chitosan adsorbent well-maintained high adsorption capacity attributed to its stability, making it a promising adsorbent for heavy metals removal from industrial effluent.

Adsorption of pyrolysis oil model compound (phenol) with plasma-modified hydro-chars and mechanism exploration

Phenol is one of the important ingredients of pyrolysis oil, contributing to the high biotoxicity of pyrolysis oil. To promote the degradation and conversion of phenol during anaerobic digestion, cheap hydro-chars with high phenol adsorption capacity were produced. The phenol adsorption capabilities of the plain hydro-char, plasma modified hydro-char at 25 °C (HC-NH3-P-25) and 500 °C (HC-NH3-P-500) were evaluated, and their adsorption kinetics and thermodynamics were explored. Experimental results indicate that the phenol adsorption capability of HC-NH3-P-500 was the highest. The phenol adsorption kinetics of all samples followed the pseudo-second-order equation and interparticle diffusion model, indicating that the adsorption rate of phenol was controlled by interparticle diffusion and chemistry adsorption simultaneously. By DFT calculations, π-π stacking and hydrogen bond are the main interactions for phenol adsorption. It was observed that an enriched graphite N content decreased the average vertical distance between hydro-chars and phenol in π-π stacking complex, from 3.5120 to 3.4532 Å, causing an increase in the negative adsorption energy between phenol and hydro-char from 13.9330 to 23.4181 kJ/mol. For hydrogen bond complex, the average vertical distance decreased from 3.4885 to 3.3386 Å due to the increase in graphite N content; causing the corresponding negative adsorption energy increased from 19.0233 to 19.9517 kJ/mol. Additionally, the presence of graphite N in the hydro-char created a positive diffusion region and enhanced the electron density between hydro-char and phenol. Analyses suggest that enriched graphite N contributed to the adsorption complex stability, resulting in an improved phenol adsorption capacity.

Physicochemical pre- and post-treatment of coking wastewater combined for energy recovery and reduced environmental risk

In this study, physicochemical pre- and post-treatment of highly polluting coking wastewater (CWW) for the removal of refractory compounds and recovery of high-energy substances/components was investigated. An economic optimization model targeting the development of a cost-effective and sustainable treatment technology was proposed. At the post-treatment stage, powdered activated carbon (PAC) was used to separate the refractory and toxic pollutants from the bio-treated CWW, with the adsorption capacity ranging from 50 to 120 mg chemical oxygen demand (COD) g^-1 PAC. Then, the spent PAC, together with a coagulant, was reused in the pre-treatment of highly concentrated raw CWW, which lifted the adsorption capacity to 800-1200 mg COD g^-1 PAC. Results showed that the adsorbent's high selectivity towards macromolecular and complicated pollutants could remove 25-65 % of COD in both CWW flows. Analysis of pollutants' molecular weight distribution and GC-MS indicated a good affinity between PAC and high-energy pollutants (phenolic compounds and alkanes), which could transfer 144,555 kJ m^-3 of energy from CWW to the adsorption-coagulation sludge. The economic optimization model suggested that the cost of the adsorbent was compensated by the net benefits of energy recovery and that profit was achieved when the PAC price was less than 5562 CNY t^-1. The proposed two-stage PAC/coagulant approach offers a way to sustainable water quality and sludge management, plus energy recycling, in CWW treatment. It may also be applied to the treatment of other industrial wastewaters.

Production of an iron-coated adsorbent for arsenic removal by hydrothermal carbonization of olive pomace: Effect of the feedwater pH

The removal of arsenic from water by adsorption is currently hindered by the elevated cost of conventional adsorbent materials. To overcome this limit, an innovative iron-coated adsorbent was produced by hydrothermal carbonization (170 °C, 30 min) of olive pomace, an inexpensive byproduct of the olive oil production. Hydrothermal carbonization experiments were performed starting from olive pomace dispersions in solutions with acidic, neutral and alkaline pH, in presence and absence of FeCl₃. Acidic conditions improved the carbonization, ensuring reduced H/C and O/C ratios, and increased the adsorbent stability. However, acidic pH yielded unsatisfactory iron coating, with only 32% of the iron dissolved in the initial solution transferred to the produced hydrochar. Under alkaline pH, 96% of the iron in the feedwater was, in contrast, stably dispersed over the hydrochar surface, giving the highest maximum arsenic adsorption capacity (4.1 mg/g). However, alkaline pH promoted biomass hydrolysis, causing the loss of 60% and 87% of the total C and N, respectively, and reducing the stability of the produced hydrochar. A two-stage process was tested to overcome these issues, including hydrothermal carbonization under acidic pH with FeCl₃, followed by the addition of NaOH. This process prevented biomass hydrolysis yielding a stable hydrochar. However, as compared to the one-stage alkaline synthesis, the two-stage process produced an hydrochar with reduced arsenic adsorption capacity (1.4 mg/g), indicating that biomass hydrolysis could positively influence hydrochar adsorption characteristics, possibly by increasing the specific surface area. Indications are then provided on how to optimize the two-stage process in order to produce a hydrochar with both satisfactory stability and arsenic adsorption capacity.

Application and Mechanism of Sludge-Based Activated Carbon for Phenol and Cyanide Removal from Bio-Treated Effluent of Coking Wastewater

The toxic pollutants phenol and cyanide in the bio-treated effluent of coking wastewater still need advanced treatment to meet environmental requirements. In this study, activated carbon prepared from municipal sludge and bamboo waste (SBAC) was used for simultaneous adsorption of phenol and cyanide from bio-treated effluent of coking wastewater. The results showed that the optimum removal efficiencies of volatile phenol (69.7%) and total cyanide (80.1%) were observed at a SBAC dosage of 8 g/L, a pH value of 8.0, and a contact time of 80 min. The physical and chemical properties of SBAC were analyzed using Brunauer–Emmett–Teller (BET) surface area (SBET), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy. SBAC had high SBET (289.58 m2/g) and rich mesoporous structure (average pore diameter of 3.688 nm), and carboxylic groups on SBAC surfaces were enhanced due to the addition of bamboo waste. In addition, a kinetic model of pseudo-first-order fitted well with the experimental data of volatile phenol, while the adsorption of total cyanide onto the SBAC was better described by a pseudo-second-order kinetic model.

Ultrafast synthesis of magnetic hollow carbon nanospheres for the adsorption of quinoline from coking wastewater

Magnetic hollow carbon nanospheres (MHCNSs) with a uniform particle size (40 nm) were ultrafast synthesized for removing quinoline from coking wastewater.

Heterogeneous biochars from agriculture residues and coal fly ash for the removal of heavy metals from coking wastewater

While we have started down the path towards a global transition to a green economy, as with most things we began with the "low-hanging fruit," such that increasingly difficult material and chemical conversions remain. Coking is one such example; it is unlikely that steel production will transition away from using coking coal anytime in the near future, such that coking wastewater remains a global environmental challenge. However, we can develop greener methods and materials to treat such waste. The present work demonstrates how wheat straw, an abundant agricultural residue, can be co-pyrolyzed and co-activated with coal fly ash to produce a high surface area biochar. Coal fly ash has previously been shown to promote devolatilization and deoxygenation of pyrolyzed biofuels. This work shows how coal fly ash increases microporosity as well as aromaticity of the surface functional groups, while decreasing carbonyl but preserving or only slightly decreasing ketones and carboxylic acids. CO2-activation of 5 and 10 wt% fly ash with wheat straw blends yields heterogeneous biochars with adsorption capacities upwards of 170 mgmetal gchar -1, with 5 wt% blends showing higher capacity and adsorption uptake rates than the 0 or 10 wt% blends. The adsorption of the four heavy metals ions (Ni2+, Co2+, Zn2+, and Mn2+) was chemical in nature, with cobalt preferentially adsorbing to the char surface. The overall adsorption rate is limited by an initial rapid uptake to fill available surface adsorption sites.