Study on the effects of organic matter characteristics on the residual aluminum and flocs in coagulation processes

Formation of Aluminum-Fluoride Complexes Compromises Defluoridation Efficiency of Aluminum-Based Coagulation

Aluminum-based coagulation has long been regarded as a reliable and cost-effective process for the defluoridation of industrial effluents. However, such a well-recognized viewpoint is challenged by the underestimation of fluoride levels in treated effluents. Herein, we developed a systematic protocol to distinguish different fluoride species, including free F-, exchangeable fluoride (EF), and nonexchangeable fluoride (NEF). We demonstrated that EF forms complexes with octahedral aluminum (AlO6) on the surface of polyaluminum and can be exchanged with (1,2-cyclohexylenedinitrilo)-tetraacetic acid (CDTA). However, NEF is incorporated with tetrahedral aluminum (AlO4) at the core of polyaluminum, as confirmed by 19F/23Al NMR and ESI-MS analysis, and cannot be exchanged with CDTA due to steric hindrance. Increasing the aluminum coagulant dosage effectively reduced free F- levels in photovoltaic and electroplating effluents to below 1 mg/L. However, the total fluoride content, with over half in the form of EF and NEF, was above 2 mg/L, exceeding the discharge limit regulated by many local governments of China. Furthermore, both EF and NEF can gradually transform to free F- in natural waters. Our findings indicate that aluminum-based coagulation inevitably accompanies the formation of substantial amounts of EF and NEF, compromising its defluoridation efficiency toward industrial effluents.

Compositional and structural identification of organic matter contributing to high residual soluble aluminum after coagulation

Understanding the complexation of aluminum (Al) with dissolved organic matter (DOM) is of great significance for the control of residual Al in drinking water after treatment. Here, we used high-resolution and accurate mass measurements to identify the composition and structure of DOM contributing to the formation of soluble organically-bound Al during coagulation at near neutral pH (pH 7.50). The results showed that the organic compounds contributing to soluble organically-bound Al were primarily phenolic compounds and aliphatic compounds. Among them, phenolic compounds with a sulfonic acid group could greatly enhance the hydrolysis of polymeric Al and the formation of high concentrations of monomeric/oligomeric Al-DOM complexes. These organic molecules had a mass-to-charge ratio concentrated below 350. Based on the assumption that oxygen-containing functional groups providing unsaturation in the molecular structure were carboxyl groups, it was inferred that the maximum number of carboxyl groups in phenolic compounds and aliphatic compounds was concentrated between 1-2 and 2-4, respectively. The presence of these molecules was responsible for soluble organically-bound Al accounting for over 80 % of the total soluble Al in the supernatant after coagulation in this study. These findings deepen the understanding of the complexation of Al with DOM. In drinking water treatment plants, the combination of coagulation with processes that can remove such characteristic organics is beneficial for controlling residual Al.

A techno-economical assessment of treatment by coagulation-flocculation with aluminum and iron-bases coagulants of landfill leachate membrane concentrates

Landfill leachate treatment involved with the membrane bioreactor (MBR) combined with membrane treatment via nanofiltration (NF) and/or reverse osmosis (RO) is widely used in Turkey. This treatment produces landfill leachate membrane concentrates (LLMCs) with an undesirably high concentration of contaminants. In the study, two different nanofiltration concentrates of leachate were coagulated. Coagulant dosages from 0.10 to 5.0 g of Me³⁺/L (Me³⁺: Al³⁺ or Fe³⁺), and the pH values ranged from 4.0 to 8.0 and 3.0-9.0 for Al-based and Fe-based coagulants, respectively. The most efficient pH values were 5.0 and 4.0 for Al³⁺ and Fe³⁺, respectively. These pH values are lower than those known to be effective in coagulants. The reason for this is the presence of humic substances in the wastewater. The cost of Fe₂(SO₄)_3.xH₂O was the lowest than other coagulants at the end of the cost analyses obtained from İstanbul region landfill leachate NF concentrate (NFCL-1) and Kocaeli region landfill leachate NF concentrate (NFCL-2). Under optimum conditions, the costs for NFCL-1 and NFCL-2 were calculated as 0.55 and 0.46 $/removed kg COD, respectively.

Enhancement of thioethers removal by pre-oxidation-coagulation: Effects of background organic matter

Swampy/septic odor caused by thioethers has become the main taste and odor (T&O) problem in drinking water of China. Improving its removal performance by commonly traditional water treatment process is significant. In our study, we have found that pre-oxidation could modify the background dissolved organic matter (DOM) properties and thus improve the coagulation performance of thioethers, increasing the coagulation removal rates by 1.5-3 times. Particularly, after pre-ozonation only protein-like substances remained, and thioethers removal was 1.5 times higher than that after pre-chlorination (only coagulation not including oxidation). Compared with humic acid (HA), the thioethers compounds removal efficiencies under bovine serum albumin (BSA) as background DOM was increased by 0.3-3 times. Through Freundlich model analysis, the binding strength of BSA (K_F = 20.712, at 298 K) to dimethyl disulfide (DMDS) was enhanced by 60 % compared to HA (K_F = 12.778, at 298 K). According to thermodynamic parameters, the binding effect between HA/BSA and thioethers compounds was mainly van der Waals forces and hydrogen bond. BSA with more amino structure and oxygen groups was more easily to adsorb DMDS through hydrogen bond and thus achieved better coagulation performance. Therefore, pre-ozonation combined with coagulation was suggested to be more suitable for thioethers compounds control.

Pre-aggregation of Al13 in optimizing coagulation for removal of humic acid

The effective removal of humic acid (HA) by coagulation has been extensively investigated for water treatments. However, the limitations of pH variation and excessive residual aluminum issues were still factors needed to be considered. In this study, to investigate the coagulation mechanism for removing HA by Al₁₃ and optimize Al₁₃ operation for removing HA, Al₁₃ and preformed Al₁₃ aggregates (Al₁₃agg) were applied to remove HA at different pH conditions. The results showed that preformed Al₁₃agg exhibited superior HA removal performance than Al₁₃ due to its wide pH range and low residual Al level. During coagulation, Al₁₃ and Al₁₃agg interacted with HA in their original status, but the DSlope_325-375 difference implied that the complexation capacity between HA and Al₁₃agg was stronger than Al₁₃. The new peaks of HPSEC representing larger molecular weight substances were formed under acidic and neutral conditions, which indicated that HA firstly aggregated into larger complexed molecules by interacting with Al₁₃ or its hydrolysates and was subsequently removed by forming large flocs which was completely different from Al₁₃agg situation. Therefore, the different coagulation mechanisms played the roles in HA removal for Al₁₃ and Al₁₃agg which were studied in this paper. It was believed that the complexation and charge neutralization effects dominated coagulation process for Al₁₃ while sweep flocculation and adsorption coagulation were main driving force for Al₁₃agg in HA removing. This work provides significant understanding of HA removal by Al₁₃ and Al₁₃agg coagulation, which can help to design and optimize the high efficiency coagulant based on Al polycations.

Interactions among low-molecular-weight organics, heavy metals, and Fe(III) during coagulation of landfill leachate nanofiltration concentrate

The generation of landfill leachate nanofiltration concentrate (LLNC) has been a dilemma for leachate treatment plants because it contains large amounts of refractory organics with low molecular weight (LMWO), as well as heavy metals (HMs), and is difficult to handle. The coagulation removal of LMWOs is a significant challenge, as is the removal of HMs bonded to LMWOs. In this study, coagulation through the dosing of FeCl₃ was used to remove LMWOs and HMs from LLNC. The results interestingly demonstrated that the removal rates of dissolved organic carbon (DOC), Cr, Ni, and As reached up to 84.1% ± 3.9%, 91.0 ± 1.1%, 73.1 ± 2.2%, and 96.9 ± 1.5%, respectively. The partition of LMWO components, as well as the interactions among the LMWOs, HMs, and Fe(III) were investigated to determine the mechanism behind the LMWO and HM removal. LMWOs with a high degree of humification, including humic and fulvic acid-like components, were preferentially removed through aggregation and electrostatic attraction originating from the specialistic adsorption of Fe₂(OH)₂⁴⁺ and Fe₃(OH)₄⁵⁺. In addition to being removed, a portion of these two components was dissociated into aromatic protein I, aromatic protein II, and soluble microbial by-product-like materials due to an acid effect and the formation of inner-sphere complexes. A redundancy analysis revealed that As, Cr, and Ni are mainly removed through the electrostatic attraction of Fe(III), bonding to humic substances and hydrophilic organics, respectively. The outcomes provide a new understanding on the coagulation removal of LMWOs and HMs.

Charge Neutralization Mechanism Efficiency in Water with High Color Turbidity Ratio Using Aluminium Sulfate and Flocculation Index

Most of the water supplied in Brazil comes from water streams that may have higher values for apparent color than turbidity. Treatability trials were performed for color and turbidity removal to evaluate the advantages of coagulation during the charge neutralization mechanism when compared to sweep flocculation for water with those characteristics. There were three types of trials: conventional Jar Test with raw water, with and without filtration, and in a pilot Water Treatment Plant, direct downward filtration, with synthetic water. Auxiliary equipment such as Continuous Flocculation Monitoring Equipment (CFME) and image analysis were used to evaluate the growth of flocs. In the Jar Tests experiments, similar levels of color (61.49%) and turbidity (61.30%) removal were observed, with a lower dosage of coagulant (46 mg/L) in the charge neutralization mechanism compared to the ones with sweep flocculation (58.22% for color and 54.73% for turbidity removal with 52 mg/L of aluminium sulfate). Similar results were found on pilot plant. At filtration rates of 180 and 300 m3 m−2 day−1, sweep flocculation mechanism had shorter filtration cycle (<5 m3) compared to other mechanisms. Therefore, a change in the operation of Water Treatment plants that use a coagulant dosage associated with sweep flocculation can bring advantages such as the reduction of coagulant consumption and sludge productions, as well as the increase of chlorine disinfection and filtration cycles.