Tracking and analysis of DBP precursors' properties by fluorescence spectrometry of dissolved organic matter

Enhanced denitrification of dispersed swine wastewater using Ca(OH)2-pretreated rice straw as a solid carbon source

In a pilot-scale packed bed reactor, the denitrification performance and microbial community structure of the dispersed swine wastewater treatment using calcium hydroxide (Ca(OH)₂) pretreated rice straw as a carbon source were investigated. In a Ca(OH)₂-pretreated rice straw supported denitrification system (Ca(OH)₂-RS), the removal efficiency of NO₃^--N was 96.39% at an influent NO₃^--N load of 0.04 kg/(m³•d). Meanwhile, there was no obvious accumulation of NO₂^--N or chemical oxygen demand (COD) in the effluent of Ca(OH)₂-RS. The contents of soluble microbial byproduct-like substances and tryptophan-like substances in the effluent of Ca(OH)₂-RS were reduced by 46.2% and 43.4%, respectively, compared with the influent. Overall, the Ca(OH)₂-pretreated rice straw system had a strong resistance to fluctuations in water quality conditions, such as influent NO₃^--N and COD concentrations. According to the microbial assay results, the Ca(OH)₂ pretreatment enriched more denitrifying bacteria. Among them, Proteobacteria (42.33%) and Bacteroidetes (35.28%) were the dominant bacteria. Moreover, the main denitrifying functional bacteria, generanorank_f_Saprospiraceae (13.32%), norank_f_Porphyromonadaceae (4.22%), and Flavobacterium (3.25%), were enriched in Ca(OH)₂-RS. This suggested that using Ca(OH)₂-pretreated rice straw as a carbon source was a stable and efficient technology to enhance the denitrification performance of dispersed swine wastewater.

Molecular transformation of dissolved organic matter and the formation of disinfection byproducts in full-scale surface water treatment processes

Dissolved organic matters (DOM) have important effects on the performance of surface water treatment processes and may convert into disinfection by-products (DBPs) during disinfection. In this work, the transformation of DOM and the chlorinated DBPs (Cl-DBPs) formation in two different full-scale surface water treatment processes (process 1: prechlorination-coagulation-precipitation-filtration; process 2: coagulation-precipitation-post-disinfection-filtration) were comparatively investigated at molecular scale. The results showed that coagulation preferentially removed unsaturated (H/C < 1.0 and DBE > 17) and oxidized (O/C > 0.5) compounds containing more carboxyl groups. Therefore, prechlorination produced more Cl-DBPs with H/C < 1.0 and O/C > 0.5 than post-disinfection. However, the algal in the influent produced many reduced molecules (O/C < 0.5) without prechlorination, and these compounds were more reactive with disinfectants. Sand filtration was ineffective in DOM removal, while microorganisms in the filter produced high molecular weight (MW) substances that were involved in the Cl-DBPs formation, causing the generation of higher MW Cl-DBPs under post-disinfection. Furthermore, the CHO molecules with high O atom number and the CHON molecules containing one N atom were the main Cl-DBPs precursors in both surface water treatment processes. In consideration of the putative Cl-DBPs precursors and their reaction pathways, the precursors with higher unsaturation degree and aromaticity were prone to produce Cl-DBPs through addition reactions, while that with higher saturation degree tended to form Cl-DBPs through substitution reactions. These findings are useful to optimize the treatment processes to ensure the safety of water quality.

Progress in the interaction of dissolved organic matter and microbes (1991-2020): a bibliometric review

Dissolved organic matter (DOM) and microbes are key in the planetary carbon cycle, and research on them can lead to a better understanding of the global carbon cycle and an improved ability to cope with environmental challenges. Several papers have reviewed one or several aspects of the interaction of DOM and microbes, but no overall review has been performed. Here, we bibliometrically analyzed all publications from the Web of Science on DOM and microbes (1991-2020). The results showed that studies on DOM and microbes grew exponentially during this period; the USA contributed the most to the total publications, and China has had the fastest increasing rate since 2010. Moreover, we used the Latent Dirichlet Allocation model to identify topics and determine their (cold or hot) trends by analyzing the abstracts of 9851 publications related to DOM and microbes. A total of 96 topics were extracted, and these topics that are related to the source, composition, and removal path of DOM and the temporal-spatial patterns of DOM and microbes consistently rose from 1991 to 2020. Most studies have used accurate and rapid methods combined with microbiological genetic approaches to study the interaction of DOM and microbes in terrestrial and aquatic ecosystems. The results also showed that the impacts of climate change and land use on the interaction of DOM and microbes, and topics related to human health have received considerable attention. In the future, the interaction mechanism of DOM and microbes and its response to environmental change should be further elucidated.

Study on the Control of Dichloroacetonitrile Generation by Two-Point Influent Activated Carbon-Quartz Sand Biofilter

Aiming at the problem of highly toxic Nitrogenous disinfection by-products (N-DBPs) produced by disinfection in the process of drinking water, two-point influent activated carbon-quartz sand biofilter, activated carbon-quartz sand biofilter, and quartz sand biofilter are selected. This study takes typical N-DBPs Dichloroacetonitrile (DCAN) as the research object and aromatic amino acid Tyrosine (Tyr), an important precursor of DCAN, as the model precursor. By measuring the changes of conventional pollutants in different biofilters, and the changes of Tyr, the output DCAN formation potential of the biofilters, this article investigates the control of DCAN generation of the two-point influent activated carbon-quartz sand biofilter. The results show that the average Tyr removal rate of the three biofilters during steady operation is 73%, 50%, and 20%, respectively, while the average effluent DCAN generation potential removal rate is 78%, 52%, and 23%, respectively. The two-point influent activated carbon-sand biofilter features the highest removal rate. The two-point water intake improves the hypoxia problem of the lower filter material of the activated carbon-quartz sand biofilter, and at the same time, the soluble microbial products produced by microbial metabolism can be reduced by an appropriate carbon sand ratio, which is better than traditional quartz sand filters and activated carbon-quartz sand biofilters in the performance of controlling the precursors of N-DBPs.

A comparison of removal efficiencies of conventional drinking water treatment and advanced treatment equipped with ozone-biological activated carbon process

ABSTRACTUsing raw water from a shallow water supply reservoir located in the lower Yangtze River region, the removal efficiencies of conventional treatment on dissolved organic matter (DOM) and disinfection by-products formation potential (DBPFP) were compared with an advanced treatment that equipped with ozone-biological activated carbon (O₃-BAC) process. The results showed that the advanced treatment was more efficient than the conventional treatment at removing dissolved organic carbon (DOC; 40-67% removal), UV₂₅₄ (61-81% removal), the trihalomethane formation potential (THMFP; 37-70% removal) and the haloacetic acid formation potential (HAAFP; 35-89% removal). The sand filter in the conventional treatment process was identified as the main contributor to decreasing DOC, UV₂₅₄ and DBPFP. The O₃-BAC in advanced treatment was found to decrease THMFP and HAAFP, with removal rates of 17-40% and 22-59%, respectively. To improve the water quality of effluents, advanced treatment with O₃-BAC should be used to treat raw water from the shallow water supply reservoir in lower Yangtze River. However, the increased DBPFP yield, which is proportional to the potential health risks, should not be ignored.

Discharge and Temperature Controls of Dissolved Organic Matter (DOM) in a Forested Coastal Plain Stream

Streams in the southeastern United States Coastal Plains serve as an essential source of energy and nutrients for important estuarine ecosystems, and dissolved organic matter (DOM) exported from these streams can have profound impacts on the biogeochemical and ecological functions of fluvial networks. Here, we examined hydrological and temperature controls of DOM during low-flow periods from a forested stream located within the Coastal Plain physiographic region of Alabama, USA. We analyzed DOM via combining dissolved organic carbon (DOC) analysis, fluorescence excitation–emission matrix combined with parallel factor analysis (EEM-PARAFAC), and microbial degradation experiments. Four fluorescence components were identified: terrestrial humic-like DOM, microbial humic-like DOM, tyrosine-like DOM, and tryptophan-like DOM. Humic-like DOM accounted for ~70% of total fluorescence, and biodegradation experiments showed that it was less bioreactive than protein-like DOM that accounted for ~30% of total fluorescence. This observation indicates fluorescent DOM (FDOM) was controlled primarily by soil inputs and not substantially influenced by instream production and processing, suggesting that the bulk of FDOM in these streams is transported to downstream environments with limited in situ modification. Linear regression and redundancy analysis models identified that the seasonal variations in DOM were dictated primarily by hydrology and temperature. Overall, high discharge and shallow flow paths led to the enrichment of less-degraded DOM with higher percentages of microbial humic-like and tyrosine-like compounds, whereas high temperatures favored the accumulation of high-aromaticity, high-molecular-weight, terrestrial, humic-like compounds in stream water. The flux of DOC and four fluorescence components was driven primarily by water discharge. Thus, the instantaneous exports of both refractory humic-like DOM and reactive protein-like DOM were higher in wetter seasons (winter and spring). As high temperatures and severe precipitation are projected to become more prominent in the southeastern U.S. due to climate change, our findings have important implications for future changes in the amount, source, and composition of DOM in Coastal Plain streams and the associated impacts on downstream carbon and nutrient supplies and water quality.

Investigation into the content and formation of trihalomethanes and molecular change of dissolved organic matter from a typical water plant in south China

Trihalomethanes (THMs) are a class of disinfection by-products that were proved to have adverse effects to human health. Investigation into its content change and molecular composition variation of its main precursor, which is believed to be dissolved organic matter (DOM) during water purification process, can help understand the formation mechanism of THMs and optimize the processes in drinking water treatment plant (DWTP). This is of great significance to ensure the safety of urban water supply. In this study, detailed changes of THMs' content and formation potential were determined during the water purification process in summer and winter at a typical DWTP in south China. Specific molecular composition changes of DOM were also characterized by ultrahigh-resolution mass spectrometry, to comprehensively study its correlation with the formation of THMs in different water processing units and seasons. The result showed that chlorination will cause drastic changes of water quality and a sharp increase in the concentration of THMs (18.7 times in summer and 13.9 times in winter). Molecular-level characterization of DOM indicates that a range of lignin-like substance with lower O/C (< 0.5) and H/C (< 1.25) vanished and considerable amount of protein-like and tannins-like substance with higher H/C (> 1.25) and O/C (> 0.5) was formed after chlorination. Analysis of Cl-containing products demonstrated that a bulk of CHOCl₁ and CHOCl₂ compounds with moderate molecular weights were formed in both winter and summer. However, the newly formed CHOCl₁ molecules showed a relatively higher mass weight in summer (> 500 Da) compared to winter (300-500 Da). Seasonal differences also emerged in the result of correlation between the trihalomethanes formation potential and total organic carbon. The correlation coefficient in summer (0.500) was lower than that in winter (0.843). The results suggested that the exhaustive reaction and contribution of DOM to THMs may vary in different seasons.

Source identification and characteristics of dissolved organic matter and disinfection by-product formation potential using EEM-PARAFAC in the Manas River, China

Dissolved organic matter (DOM) is ubiquitous in natural water and reacts with disinfectants to form disinfection by-products (DBPs). The analysis of DOM in raw water is helpful in evaluating the formation potential of DBPs. However, there is relatively little research on the DOM identification of raw water in northern China. In this study, the sources and characteristics of DOM were investigated in the samples collected from the Manas River. Dissolved organic carbon (DOC), UV₂₅₄, specific ultraviolet absorbance, and fluorescence indices (fluorescence index, humification index, and biological index) were measured to characterize the DOM, and trihalomethanes (THMs) were quantified following formation potential tests with free chlorine. The maximum amount of total trihalomethane formation potential (THMsFP) was 225.57 μg L⁻¹. The DOM of the Manas River consisted of microorganisms and soil resources. The excitation–emission matrix combined with parallel factor analysis (EEM-PARAFAC) identified microbial humus (C1, 54%) and tryptophan-like protein (C2, 46%). PARAFAC components were evaluated as the precursor surrogate parameters of THMsFP. Additionally, the linear THMsFP correlation was stronger with C1 + C2 (r = 0.529, p < 0.01) than with C1 (r = 0.485, p < 0.01). Thus, C1 + C2 is an accurate THMsFP precursor surrogate parameter for the Manas River, and the use of fluorescence spectroscopy may be a robust alternative for predicting DOC removal.

Humic-like and protein-like components were identified by PARAFAC. THMs FP was significantly correlated with components C1 and C1 + C2. The source, types and humification degree of DOM affect the formation of DBPs.

A year-long cyclic pattern of dissolved organic matter in the tap water of a metropolitan city revealed by fluorescence spectroscopy

Delivering drinking water with stable quality in metropolitan cities is a big challenge. This study investigated the year-long dynamics of dissolved organic matter (DOM) in the tap water and source water of a metropolitan city in southern China using fluorescence spectroscopy. The DOM detected in the tap water, and source water of Shenzhen city was season and location-dependent. A year-long cyclic trend of DOM was found with predominate protein-like fluorescence in the dry season compared to the humic-like enriched DOM in the wet season. A general DOM pattern was estimated by measuring the shift in dominant fluorescence regions on the excitation-emission matrix (EEM). The difference in fluorescent DOM (FDOM) composition (in terms of the ratio of protein-like to humic-like fluorescence) was above 200% between wet and dry seasons. The taps associated with reservoirs receiving water from the eastern tributary of Dongjiang River showed significant changes in protein-like contents than the taps with source water originating from the western part of the river. This study highlights the importance of optimizing drinking water treatment plants' operational conditions after considering seasonal changes and source water characteristics.

Dissolved organic matter adsorption from surface waters by granular composites versus granular activated carbon columns: An applicable approach

Many new sorbents have been introduced as an alternative for granular activated carbon (GAC), the most common sorbent for dissolved organic matter (DOM) removal. In the current study, we developed an applicable granular composite based on a flocculant commonly employed for drinking water treatment adsorbed to montmorillonite. DOM adsorption from surface waters, Lake Kinneret and Suwannee River, with low and high specific ultraviolet absorption (SUVA), respectively, by composite and GAC columns, was studied. Adsorption of DOM from Suwannee River was significantly higher by the composite column, in comparison to the GAC column, while an opposite trend was obtained for the adsorption of DOM from Lake Kinneret. In-situ regeneration of the columns with a brine solution was extremely efficient and inefficient for the composite and GAC columns, respectively. Adsorption, of both waters, post-regeneration by the composite column was not compromised, while GAC effectiveness decreased. The opposite trend in DOM adsorption from Suwannee River and Lake Kinneret was explained by the different affinities of the sorbents towards various DOM molecules. Distinguishing between different DOM components adsorbed by GAC and the composite was supported by ¹³C NMR and direct pyrolysis-GC-MS measurements. Furthermore, we demonstrated that the kinetics and adsorption at the equilibrium of five organic molecules to the composite and GAC can be correlated to their chemical-physical properties. Indeed, combining the properties of both sorbents, by integrating them into a single column, yielded higher DOM removal than by the individual columns. Furthermore, since DOM removal by GAC and by the composite, increases, and decreases with temperature, respectively, the integrated column, mitigates the changes in removal, stabilizing the adsorption performance. Such an integrated filter may minimize additional seasonal and water quality fluctuations.