Natural Organic Matter Removal Efficiency Research Articles

Removal of Natural Organic Matter (NOM) from Aqueous Solutions by Multi-Walled Carbon Nanotube Modification with Magnetic Fe3O4 Nanoparticles

Backgroundand Aim. Natural organic matter (NOM) has become one of the most serious environmental problems due to its persistence in aqueous solutions and the risk of carcinogenesis. In this study, the removal efficiencies of real and synthetic humic acid (HA) by multi-walled carbon nanotubes (MWCNTs) coated with iron oxide were evaluated. Materials and Methods. The MWCNs were synthesized and coated with iron oxide. In addition, the effects of pH, contact time, mixing speed, and adsorbent dose on the removal efficiency of NOM by MWCNTs-Fe3O4 were studied. Then, the removal efficiency of NOM from real samples was investigated at optimal conditions. The MWCNT-Fe3O4 was characterized by scanning electron microscopy (SEM) test and X-ray diffraction (XRD), respectively. Data analysis was performed using Minitab software based on the Taguchi method. Results. The results showed that MWCNTs were coated with Fe3O4. The SEM test shows particle (MWCNTs-Fe3O4) size in the range of 48–143 nm, and the particles have uniform spherical shapes. Enix software was used to identify the phase in this sample. The conditions including p H = 3 , mixing speed = 120 rpm, adsorbent dosage = 1.5 g·L−1, and contact time = 90 minutes were selected as optimal for NOM adsorption. The mean removal efficiencies of NOM in synthetic samples at 5, 10, and 20 mg·L−1 concentrations were 86.6%, 84.87%, and 95.41%, respectively. In addition, the mean removal efficiency of NOM in Choghakhor Wetland was 77%. Conclusion. Our findings demonstrated that the MWCNTs-Fe3O4 can be potentially used as an adsorbent for removing natural organic matter (HA) from aqueous solutions.

A molecular-level mechanism analysis of PFS coagulation behaviors: Differences in natural organic matter and algal organic matter

Coagulation is generally considered as the key process to remove dissolved organic matters in water treatment plants, of which natural organic matter (NOM) and algal organic matter (AOM) are two commonly seen species. However, the removal efficiency of NOM and AOM by coagulation varies a lot. Here we unraveled the possible reasons for the different performance on NOM and AOM removal from molecule-level insights. With van Krevelen diagram, we observed that removed components in NOM and AOM were located in similar areas (high O/C and low H/C), indicating that polyferric sulfate (PFS) coagulation is prone to remove oxygen-containing and unsaturated components. Further, the results showed that NOM components are mainly composed of C, H, and O, with high O/C and low H/C, which therefore are easier to be removed (51%). However, ∼50% AOM components are N-containing, which mainly located in areas of low O/C and high H/C, leading to the relatively poor coagulation performance (19%). With further data processing of the Fourier transform ion cyclotron resonance (FT-ICR) results, we found that molecules with higher oxidized state or more carboxyl groups were preferentially removed by PFS coagulation. This study reveals the PFS coagulation mechanism of NOM and AOM, and provides theoretical foundations for enhanced coagulation in practical applications.

Revealing the effect of anion exchange resin conditioning on the pH and natural organic matter model compounds removal mechanisms

Natural organic matter (NOM) negatively affects water quality and can cause serious issues during water treatment processes. Anion exchange resins (AER) have already proven their effectiveness to remove a major part of this NOM. However, due to its complexity, understanding and improving NOM removal remains challenging. This study aimed to provide in-depth insights in NOM removal with macroporous styrenic weak (WBA), strong (SBA) and combined weak/strong basic AER. Batch experiments were performed with resins in two industrially relevant counter ion forms: OH- and free base form for SBA and WBA resins respectively, using NaOH conditioning, and Cl- form resins using NaCl and HCl conditioning for SBA and WBA resins respectively. Synthetic water containing model compounds for different NOM fractions was used, focussing on the biopolymer fraction which is typically poorly removed by classical ion exchange processes and is a known disinfection by-product precursor in drinking water. The applied resin conditioning procedure influenced the equilibrium pH and consequently NOM removal efficiency and mechanisms. When ion exchange is the dominant driving force for NOM removal, the Cl- form proved to be most efficient. For uncharged NOM containing hydroxyl groups, SBA resins in the OH- form are preferable due to H-bonding. Adsorption of aromatic NOM through π-π interactions with the polystyrene backbone was not affected by the resin conditioning. This work clearly demonstrates that not only the AER type, but also the conditioning procedure and the corresponding counter ion form play a key role in the design of successful NOM removal processes.

Impact of the modernized technology on the quality of water supplied to the extended distribution system of the city of Pozna\u0144

The paper presents the results of a long-term study covering the development, implementation and operation of the second stage of water treatment (i.e. ozonation and subsequent granular active carbon (GAC) filtration) in the “Mosina” water treatment station supplying drinking water to the city of Poznań. The basis for the modernisation of the system was the high reactivity of the natural organic matter (NOM) present in the treated water with the disinfectant (in this case chlorine dioxide) resulting in an increased demand for ClO2 and reduced microbiological stability of the water. During the study it was shown that simple carboxylic acids are generated during ozonation and their presence can be an indicator of the microbiological stability of the treated water. However, these compounds are effectively removed from water during filtration through biologically active GAC filters. It was also shown that the best and cheapest parameter allowing to control water quality at individual stages of its treatment is UV absorbance, which shows reactive components of NOM removal efficiency in the technological sequence. The effectiveness of the modernisation of the technological system was evaluated on the basis of the disinfectant demand as well as on the basis of selected carboxylic acids concentration in the intake points on the water supply network fed with water from the WTS “Mosina”. At the last stage of the study, it was observed that the concentration of carboxylic acids in the distribution network was significantly reduced and stabilised, and a low dose of chlorine dioxide did not cause their re-formation. As a result of the modernisation, a new balance was achieved between the disinfectants used and the NOM compounds present in the treated water. Thus, the results confirmed that properly conducted pilot studies are a required element in planning of modernisation changes for water supply facilities.

Operating Bicarbonate-Form versus Chloride-Form Ion Exchange Resins without Regeneration for Natural Organic Matter Removal

Ion exchange (IX) is a promising drinking water treatment process for natural organic matter (NOM) removal. However, standard IX processes require frequent regenerations with concentrated NaCl solution, producing a brine that requires costly and complicated disposal methods. To alleviate the burden of IX brine, we previously proposed operating IX with infrequent regeneration to favor biomass development on the resins and thus benefit NOM removal through biomass contribution, a process referred to as biological ion exchange (BIEX). The objective of the present study is to evaluate the performance of BIEX filtration for NOM removal from primary IX to complete exhaustion using bicarbonate-form and chloride-form IX resins. Parallel pilot-scale bicarbonate-form and chloride-form BIEX were fed with surface water (dissolved organic carbon (DOC) = 7 mg C/L) for 9 months without regeneration. The results demonstrated that bicarbonate-form BIEX achieved a marginally lower DOC removal (median: 49% vs 53%), a higher biodegradable DOC (BDOC) removal (average: 50% vs 33%) and a similar disinfection byproduct precursor removal compared to chloride-form BIEX. Overall, BIEX filtration using bicarbonate-form and chloride-form IX resins offers a similar NOM removal efficiency and eases spent brine management.

Heterogeneous Fenton-like reaction followed by GAC filtration improved removal efficiency of NOM and DBPs without adjusting pH

This paper explores the removal of natural organic matter (NOM) and disinfection by-products (DBPs) in drinking water by heterogeneous Fenton-like oxidation and granular activated carbon (GAC) filtration in a pilot scale test without adjusting pH of the water. pH of the used water in this study was from 7.5 to 8.7. Comparing with conventional water treatment processes, heterogeneous Fenton-like reaction and GAC filtration increased the removal rate of dissolved organic carbon (DOC), NOM with different apparent molecular weight (AMW) (F1-F7) and with different compositions (C1-C5) to 75.11%, 88.65% and 58.73%, respectively. Moreover, heterogeneous Fenton-like reaction and GAC filtration increased the removal rate of DBPs to 88.18%. The multivariate statistical analysis indicated that haloacetic acids (HAAs) and haloketones (HKs) mainly came from fulvic-like and humic-like NOM with AMW lower than 1.5 K Da. These substances with AMW between 1.5 K Da and 3.5 K Da were the main precursors of trihalomethanes (THMs). The main precursors of halonitriles (HANs) were soluble microbial byproduct-like (SMP-like), tyrosine-like and tryptophan-like substance with the AMW between 3.5 K Da and 4.5 K Da. Overall, without adjusting pH of water, heterogeneous Fenton-like reaction and GAC filtration improved the removal efficiency of NOM and DBPs, and controlled the genotoxicity of drinking water.

Removal of natural organic matter in waters using hydrodynamic cavitation and hydrogen peroxide (HC-H2O2)

The presence of natural organic matter (NOM) in water does not present direct risk to the human body or to the environment. However, its presence along with other pollutants can lead to countless issues and damage human health and the environment. The hydrodynamic cavitation (HC) phenomenon started being used in the early 21st century as a process capable of treating supply-water and wastewater based on pollutant and pathogen degradation. Process effectiveness increases when it is combined to chemical agents, creating an advanced oxidation process (AOP). Although several studies have presented broaden applications for the HC process, its use for NOM removal from supply-water was not yet assessed; therefore, it remains a gap in scientific knowledge. The aim of the current study is to assess HC potential in NOM removal. In order to do so, the experiments were carried out in bench scale hydrodynamic cavitation system operated at batch model within 15-min duration period-of-time. In addition, decantation experiments (24-h period-of-time) were performed in order to check HC influence on molecules found in reaction medium after the exposure of NOM to the phenomenon. NOM was produced by a synthetic humic acid (HA) matrix at fixed concentration of 100 ppm. In total, 16 experiments were carried out; each experiment was featured by the following pair: pH (2.6, 3.0, 3.5 and 5.5) and hydrogen peroxide (0, 1, 5 and 30 mL). The best removal efficiencies (34%-36%) were observed in the most acidic pH ranges (2.6-3.0) at H2O2 concentration of 15mL. Results have presented high NOM removal efficiency (approximately 90%) after decantation at the most acidic pH ranges, as well. It can be explained by the fact that hydrodynamic cavitation in acid solution can break molecular structures suspended in the liquid medium, which favors decantation. Based on the present study, hydrodynamic cavitation with hydrogen peroxide addition can remove NOM from water; moreover, pH control is an essential factor for process development.

Comparative removal efficiencies of natural organic matter by conventional drinking water treatment plants in Zimbabwe and South Africa.

Natural organic matter (NOM) influences the quality and treatability of drinking water; therefore, its removal is paramount. A few studies exist on NOM removal in developing countries, and comparative studies are even fewer globally. This study compared the removal efficiencies for bulk NOM and biodegradable organic carbon (BDOC) fractions of drinking water treatment plants in Zimbabwe (Z) and South Africa (S). NOM removal efficiency at the coagulation stage of plant Z and plant S was 11% and 13%, respectively. The fluorescence index (FI) for the raw water feeding plant Z (1.66) indicated a mixture of both microbial and terrestrially derived NOM, whereas for plant S the FI (4.08) showed terrestrially derived NOM. Based on the log-transformed absorbance at the disinfection stage, plant S had a 58% greater opportunity to produce disinfection by-products than plant Z. The BDOC results for plant Z showed humic fractions were the major substrates for bacterial assimilation, whereas the heterotrophic bacteria in plant S were not particularly selective toward DOC fractions. Overall, the plants had comparable NOM removal performances. PRACTITIONER POINTS: NOM removal efficiency at the coagulation stage of plant Z and plant S was 11% and 13%, respectively. Plant Z had a mixture of both microbial and terrestrially derived NOM, whereas plant S had terrestrially derived NOM. Plant S had a 58% greater opportunity to produce disinfection by-products than plant Z. Humic fractions were the major substrates for bacterial assimilation for plant Z, whereas the heterotrophic bacteria in plant S were not selective towards DOC fractions.

Separation characteristics of cations and natural organic matter in electrodialysis

Natural organic matter (NOM) in water can cause the formation of disinfection byproducts (DBPs) when NOM reacts with a disinfectant such as chlorine. Nanofiltration (NF) and reverse osmosis (RO) are effective treatment methods to remove NOM. However, the simultaneous presence of both NOM and divalent cations such as Ca2+ can enhance fouling in NF and RO. Therefore, an appropriate pretreatment to remove such cations before NF or RO would delay fouling and perhaps improve water recovery and NOM removal efficiency. Electrodialysis (ED) is potentially an excellent pretreatment method because it separates inorganic ions well using ion exchange membranes (IEMs) but does not separate NOM well because of its size and consequent low diffusivity. This research focused on ion and NOM separation with ED using three different types of commercially available IEM pairs, two different commercially available NOM sources, and six different influents at 500 mg/L total dissolved solids in the presence or absence of both hardness (Ca2+ and Mg2+) and NOM. The target waters of interest in this research were drinking water supplies that require little or no desalination but substantial removal of NOM; after NOM separation in NF or RO, some or all of the ED concentrate can be added to the NF/RO permeate to create the finished water prior to disinfection. The results indicate that ED can effectively separate alkaline earth metal ions without separation of NOM. Cation exchange membranes with high exchange capacity, high selectivity for divalent cations are desirable, and both anion and cation exchange membranes should have low affinity for organic adsorption. The process would work particularly well for NOM with relatively low hydrophobicity, since NOM adsorption to most surfaces increases with hydrophobicity.

ADSORPSİYON VE İYON DEĞİŞİMİ PROSESLERİYLE İÇME SULARINDAN DOĞAL ORGANİK MADDE GİDERİMİ

Organik maddelerin kompleks heterojen bir karışımı olan doğal organik madde (DOM), su kalitesi üzerinde neden olduğu olumsuz etkilerinin yanında klorla reaksiyonu sonucunda trihalometan (THM) ve haloasetik asit (HAA) gibi dezenfeksiyon yan ürünlerini (DYÜ) oluşturarak insanlar için önemli bir sağlık riski oluşturmaktadır.Bunun yanında DOM’un konsantrasyonu ve kompozisyondaki büyük değişkenlik, su arıtma tesislerinde DOM’un arıtımını zorlaştırmaktadır.Bu güne kadar içme sularından DOM giderimi için çeşitli arıtma metotları araştırılmıştır. Bu metotlar arasında adsorpsiyon, DOM giderimi için en çok çalışılan ve uygulanan proseslerden biridir.İyon değişimi de DOM giderimi için adsorpsiyona alternatif bir proses olarak ortaya çıkmıştır.Büyük bir kısmı negatif yüklü fraksiyonlardan oluşan DOM’lar, anyon değiştirici reçinelerle giderilebilir. Bu çalışmada, DOM’un içme sularından giderimi için kullanılan adsorpsiyon ve iyon değişimi proseslerinin performansıyla ilgili çeşitli araştırmacılar tarafından ortaya konan bilgiler derlenerek sunulmaktadır.Bu bağlamda DOM gideriminde kullanılan orijinal ve yüzeyleri farklı yöntemlerle modifiye edilmiş çeşitli adsorbentler incelenmiş, bu adsorbentlerin DOM giderme etkinlikleri ve bunu etkileyen faktörler irdelenmiştir.Çalışmada ayrıca iyon değiştirici reçinelerin kullanımı, reçine yapısının performans üzerindeki etkisi gibi konulara değinilmiş ve çözünmüş organik karbon (ÇOK) giderimi için özellikle tasarlanan manyetik iyon değiştirici (MIEX), akışkan yataklı iyon değiştirici (FIX) ve askıda iyon değiştirici (SIX) gibi farklı iyon değiştirme proseslerine yer verilmiştir.

Efficient natural organic matter removal from water using nano-MgO coupled with microfiltration membrane separation

Excess natural organic matter (NOM) in water not only lead to unpleasant black color and dissolved oxygen depletion in wastewater and natural water body but also causes carcinogenic chlorinated organic byproduct during drinking water chlorine disinfection. We try to develop a novel cost-effective and green technology for water NOM removal. In our simulated NOM removal process using humic acid (HA) as typical organic matter, we find that mesoporous nano-MgO performs an abnormally high NOM removal capacity (1260 mg-HA/g-MgO, or 446 mgC/g-MgO) when coupled with microfiltration membrane separation, which can’t be illustrated by traditional adsorption mechanism. Actually, Mg2+ from dissolved Mg(OH)2 contributes ∼ 92% NOM removal via coagulation while Mg(OH)2 is responsible for the residue ∼ 8% via adsorption. MgO serves as a two-in-one coagulant and adsorbent. The MgO treatment process is highly pH sensitive and weak acidic condition is favored for high NOM removal efficiency. MgO can be regenerated for more than 10 circulations by annealing Mg(OH)2/Mg-NOM composite at 500 °C, so that our MgO recycling process will be sustainable without the need of continuous chemical purchase. More importantly, no solid waste is generated in this novel process. This MgO-recycling NOM-removal process is simple, efficient, and sustainable for water NOM removal and will be significant in promoting novel sustainable technologies for NOM- or HA-related water remediation and treatment while minimizing the generation of solid waste.

Combined process of visible light irradiation photocatalysis-coagulation enhances natural organic matter removal: Optimization of influencing factors and mechanism

Photocatalysis or coagulation process has been separately studied for the removal of natural organic matter (NOM) from surface water, and the reaction mechanism of organic fractionations of treated effluent has been clarified. Although coagulation-based process has been extensively studied for NOM removal, few attempts have been made based on a combined process that can not only efficiently degrade NOM, but also enhance the coagulation performance. The hypothesis is that a combination of photocatalysis and coagulation process in a single unit might potentially enhance NOM removal efficiency. This is because the nano-photocatalyst can serve as both pre-oxidant and flocculation core. Herein, a combined process of visible light photocatalysis prior to coagulation is conducted, with the transformation efficiency of the organics fractionations revealed. The as-prepared nano-sized photocatalyst consisting of Bi2O3-TiO2 (molar Bi/Ti ratio 0.04:1) had a stronger visible light absorption ability. Compared to coagulation alone, the combined photocatalysis-coagulation process greatly improved NOM removal, with synergistic effect, while the pre-photocatalysis reduced the required dosage of polyaluminium chloride (PACl) coagulant and increased the optimal pH of solution. A mathematical model optimized by response surface methodology of the combined process was developed that produced a good correlation between predicted and experimental values. The optimal conditions were calculated as follow: 2.0 g/L photocatalyst, a photocatalytic reaction time of 10 min 18 sec, 0.04 mM PACl coagulant, and a pH of 4.2. These conditions particularly enhanced removal of acidic hydrophobic substance, humic-/protein-like substances and fractions with molecular weight between 3 and 44 kDa. The XRD spectra and SEM images of the produced flocs identified that these flocs consisted of a core of photocatalyst, which could contribute to an effective flocculation and enhanced NOM removal.

Removal of natural organic matter from Lake Terkos by EC process: Studying on removal mechanism by floc size and zeta potential measurement and characterization by HPSEC method

The performance of electrocoagulation (EC) process via aluminium and iron plate anodes were evaluated for removal of aquatic natural organic matter (NOM). The effect of current density (j), operation time (tEC), initial pH (pHi), charge loading and electrode types were investigated. The NOM removal efficiencies were measured with respect to dissolved organic carbon (DOC), UV254/VIS436 and specific UV-absorbance (SUVA) with initial value 6.58 mg DOC/L, 0.1363 1/cm, 0.004 1/cm and 2.07 L/(m mg), respectively. The experimental results showed that the effective removal of NOM (63.7% for DOC, 79.7% for UV254 and 76.7% for VIS436 (color) was obtained by iron electrode at original pHi of lake water (7.74±0.02) at 60 min. The adsorption capacity, qt, (mg DOC per g produced Al and Fe or coulomb) were calculated at a specific electrolysis time. At 6 mA/cm2 of optimum current density, 8.2 mg of DOC was removed per g in-situ produced Fe at original pHi of water. In addition the zeta potential and floc size analysis’ were used to explicate the removal mechanism of NOM. The results of the size exclusion chromatography showed that the affinity of EC process is higher for the aromatic fraction of NOM during electrolysis time.

The occurrence of natural organic matter in South African water treatment plants

Water supplied to customers must be safe and fit for purpose. It is thus important that the water from water treatment plants (WTPs) meets World Health Organization standards or local standards such as the South African National Standard, before distribution. This study focuses on the treatment of natural organic matter (NOM) in South African WTPs. The problems associated with NOM include: (1) it affects the organoleptic properties of water; (2) increases coagulant demand; (3) reacts with chlorine to form disinfection by-products and (4) causes membrane fouling. The aim of the study was to characterise the NOM composition of various water sources. Specifically, the objectives were: (1) to assess the character of NOM in selected regions of South Africa; and (2) to evaluate the NOM removal efficiencies of the selected WTPs. To achieve these, various characterisation techniques were used to determine parameters such as dissolved organic carbon, ultraviolet absorbance, specific ultraviolet absorbance, and dissolved organic matter, using fluorescence excitation-emission matrix spectroscopy. The character of NOM evolved throughout the treatment process as a result of coagulation, sedimentation and filtration processes. As indicated by SUVA, a decrease in the hydrophobic fraction of NOM from raw to final water was observed, except for plant C2 in Region C where a drastic increase in SUVA from 1.26–31.95 L/mg.m was observed, indicating an increase in the hydrophobic compounds in water. All WTPs were shown to have different removal efficiencies because of the character of NOM in the raw water and the coagulation conditions used.

The Combination of Coagulation and Adsorption for Controlling Ultra-Filtration Membrane Fouling in Water Treatment

Ultra-filtration technology has been increasingly used in drinking water treatment due to improvements in membrane performance and lowering of costs. However, membrane fouling is the main limitation in the application of ultra-filtration technology. In this study, we investigated the impact of four different pre-treatments: Coagulation, adsorption, coagulation followed by adsorption (C-A), and simultaneous coagulation and adsorption (C+A), on membrane fouling and natural organic matter removal efficiency. The results showed that adsorption process required a large amount of adsorbent and formed a dense cake layer on the membrane surface leading to severe membrane fouling. Compared to adsorption alone, the coagulation and C-A processes decreased the transmembrane pressure by 4.9 kPa. It was due to less accumulation of particles on the membrane surface. As for water quality, the C-A ultra-filtration process achieved the highest removal efficiencies of natural organic matter and disinfection by-product precursors. Therefore, the addition of adsorbent after coagulation is a potentially important approach for alleviating ultra-filtration membrane fouling and enhancing treatment performance.

Comparison of natural organic matter removal by ultrafiltration, granular activated carbon filtration and full scale conventional water treatment

Effective removal of natural organic matter (NOM) during water treatment is crucial as disinfection by-products (DBPs) such as trihalomethanes in the final drinking water are formed when residual NOM reacts with the disinfectant such as chlorine. This study compared the NOM removal efficiency by granular activated carbon (GAC) filtration, an ultrafiltration membrane and conventional water treatment plant. The NOM removal efficiency by GAC and submerged ultrafiltration process was conducted at pilot scale level and the conventional water treatment process assessment using a full scale water treatment plant. The performance of the three process train in removing NOM was conducted in parallel. On average, the conventional water treatment processes (coagulation, sedimentation and sand filtration) removed 61% and 24% NOM as indicated by UV254 and dissolved organic carbon (DOC) removal, respectively. A respective reduction of 73% and 25% of UV254 and DOC was achieved by ultrafiltration. The GAC filter column (preceded by a sand filter column) achieved UV254 removal of 86% and DOC removal of 28%. Conventional water treatment removed the hydrophobic high molecular weight (HMW) NOM fraction while GAC and ultrafiltration not only increased removal of HMW NOM, but substantially increased removal of the intermediate (IMW) and low molecular weight (LMW) NOM. Addition of GAC filtration to the conventional treatment process appeared superior to ultrafiltration and conventional treatment and increased the overall NOM removal efficiency.

Ion exchange with macroporous polystyrene resins for the removal of natural organic matter

Abstract Due to the regulations on drinking water quality becoming stricter, it is important to remove natural organic matter (NOM) from water. The ion exchange process that uses synthetic resins is among the many processes that allow it. The suitability of two macroporous polystyrene resins: A100 and BD400FD for the removal of NOM from water and also the integrated process combining ion exchange with ultrafiltration were evaluated. The impact of process set-up and selected water parameters on the treatment effectiveness was compared. XAD resin fractionation was used to characterize removed NOM. The results showed that tested resins can be effectively used to remove NOM from water. Performing the ion exchange in a flow set-up resulted in better purification effects when compared to a batch set-up. The A100 resin was more effective. An increase in pH resulted in a slight decrease in NOM removal efficiency, while an increase in temperature increased it. The use of the integrated process resulted in significant improvements in water quality due to the combined effect. Fractional analysis showed a change in the treated water composition when compared to raw samples. Preferential removal of higher molecular weight hydrophobic fractions and also poor hydrophilic fractions elimination were observed.

A novel photodegradation approach for the efficient removal of natural organic matter (NOM) from water

The presence of natural organic matter (NOM) in water can negatively affect water quality if NOM is present in excessive amounts during certain stages of water treatment. It is known that NOM constituents serve as precursors for DBPs (disinfection by-products) during the disinfection step of the water treatment process. Removal of natural organic matter (NOM) before disinfection is therefore essential to control DBP formation; however, owing to the heterogeneity and complexity of NOM, most water treatment processes are unable to attain complete removal of NOM from water sources. There is therefore a great need to develop methods that can effectively degrade NOM into smaller and relatively harmless compounds that are easily removed during water treatment processes. In this study, multi-walled carbon nanotubes/nitrogen, palladium co-doped titanium dioxide (MWCNT/N, Pd co-doped TiO2) (0.5–5% MWCNTs) were fabricated for the enhancement of NOM degradation under visible-light illumination. Different MWCNT/N, Pd co-doped TiO2 (also referred to as CT) nanocomposites were synthesised via a modified sol-gel method and characterised using ultraviolet visible (UV-Vis), X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) Spectroscopy and energy-dispersive X-ray spectroscopy (EDS) analysis. The UV-Vis results showed that introducing the MWCNTs on the nanocomposites was accompanied by a red shift, with CT (0.5% MWCNTs) occurring at a longer wavelength than the others. The efficiency and effectiveness of the synthesised nanocomposites was quantified through measuring the NOM degradation rate in raw water samples collected from two South African water treatment plants. These two drinking water treatment plants were selected as the raw water treated in these plants contains different NOM fractions: the raw water treated at Midvaal (MV) Water Company contains transphilic NOM fractions, while that treated at Plettenberg Bay (P) Water Treatment Plant contains high amounts of hydrophobic NOM. The changes in the amount of NOM in a sample were monitored by measuring the UV absorbance at 254 nm (UVA254) as there is a strong correlation between absorbance at this wavelength and the aromatic content. The highest photocatalytic activity was observed with CT (0.5% MWCNTs) which attained 69.4% UVA254 reduction for MV and 97.7% UVA254 reduction for P raw waters. Compared to conventional methods applied by the two water treatment plants of interest, NOM removal efficiency (in terms of UVA254 reduction) by CT (0.5% MWCNTs) increased by up to 9.4% and 11.5% for MV and P water treatment plants, respectively. The enhanced photocatalytic activity of this CT nanocomposite (0.5% MWCNTs) is attributable to the large surface area of the synthesised nanocomposites which allows huge amounts of NOM to be adsorbed onto the surface of the TiO2, thereby increasing their photodegradation. It is also due to the synergistic effect of N and Pd which enhanced the photoactivity of the TiO2 in the MWCNT/N, Pd co-doped TiO2 composite. Overall, it is clear that the use of the conventional processes is not effective in removing NOM from water. Findings obtained show that CT (0.5% MWCNTs) nanocomposite is a promising future NOM removal method that could complement the available water treatment processes in enhancing the removal of the aromatic component of NOM in water.

Using fluorescence-parallel factor analysis for assessing disinfection by-product formation and natural organic matter removal efficiency in secondary treated synthetic drinking waters

Parallel factor (PARAFAC) analysis of fluorescence excitation-emission matrices (EEMs) was used to investigate the organic matter and DBP formation characteristics of untreated, primary treated (enhanced coagulation; EC) and secondary treated synthetic waters prepared using a Suwannee River natural organic matter (SR-NOM) isolate. The organic matter was characterised by four different fluorescence components; two humic acid-like (C1 and C2) and two protein-like (C3 and C4). Secondary treatment methods tested, following EC treatment, were; powdered activated carbon (PAC), granular activated carbon (GAC), 0.1% silver-impregnated activated carbon (SIAC), and MIEX® resin. Secondary treatments were more effective at removing natural organic matter (NOM) and fluorescent DBP-precursor components than EC alone. The formation of a suite of 17 DBPs including chlorinated, brominated and iodinated trihalomethanes (THMs), dihaloacetonitriles (DHANs), chloropropanones (CPs), chloral hydrate (CH) and trichloronitromethane (TCNM) was determined after chlorinating water sampled before and after each treatment step. Regression analysis was used to investigate the relationship between peak component fluorescence intensity (FMAX), DBP concentration and speciation, and more commonly used aggregate parameters such as DOC, UV254 and SUVA254. PARAFAC component 1 (C1) was in general a better predictor of DBP formation than other aggregate parameters, and was well correlated (R ≥ 0.80) with all detected DBPs except dibromochloromethane (DBCM) and dibromoacetonitrile (DBAN). These results indicate that the fluorescence-PARAFAC approach could provide a robust analytical tool for predicting DBP formation, and for evaluating the removal of NOM fractions relevant to DBP formation during water treatment.

Elimination of natural organic matter by electrocoagulation using bipolar and monopolar arrangements of iron and aluminum electrodes

The aim of this research was to evaluate the efficiency of electrocoagulation (EC) for the removal of natural organic matter (NOM) by using iron (Fe) and aluminum (Al) electrodes. The effects of several operational parameters such as initial pH (3–10), time of electrolysis (5–30 min), initial concentration of organic matter (10–50 mg NOM/L), current density (0.25–1.25 mA/cm2), type of electrode material (n = 4, 2 sides × 11 cm × 10 cm, wall thickness = 2 mm, distance between each electrode = 5 mm), and type of connection of electrodes (bipolar and monopolar configurations) were explored for the removal of NOM from synthetic humic acid solution in a 2 L laboratory-scale EC cells (A s/V = 0.110 cm−1). The optimum conditions for the process were identified as pH = 3 and 7, electrolysis time = 20 and 10 min for Fe and Al electrodes, respectively. Using both electrodes at current density = 0.25 mA/cm2 and initial concentration of organic matter = 50 mg/L, a NOM removal efficiency of almost 100% could be achieved in the bipolar mode. Based on the optimum conditions, specific reactor electrical energy consumptions were 14.90 kWh/kg Al (or 0.092 kWh/m3) and 2.88 kWh/kg Fe (or 0.11 kWh/m3). Specific electrode consumptions were obtained to be 0.0062 and 0.0382 kg/m3, and operating costs of the EC system were preliminary estimated at 0.057 and 0.119 $/m3 for Al and Fe electrodes, respectively.