A preliminary evaluation of turbidity removal by natural coagulants indigenous to Venezuela

Comparative analysis of new natural coagulant extracts for turbidity removal in water systems

The ability of two plant materials, lupin beans and rice straw, to serve as sources of coagulation-active components for water treatment was investigated using synthetic turbid water. The functional groups, surficial architecture, and elemental composition of lupin beans and rice straw were determined using FTIR, scanning electron microscope, and X-ray (EDAX). The point of zero charges, zeta potential, and particle size distribution of aqueous extracts were measured. The optimal dosage of 50 mg/L of the lupin bean extract achieved 86.4% turbidity removal at initial turbidity of 500 NTU, pH 7, and 25 °C. Similarly, rice straw extract achieved 63.3% removal under the same conditions. The coagulation efficiency of both extracts improved with higher initial turbidity and temperatures between 20 and 40 °C, and their performance was unaffected by pH variations. Combining natural coagulants with alum reduced alum usage by 25-50%, achieving up to 98.7% turbidity removal. These results suggest that the use of natural coagulants can effectively lower treatment costs and reduce environmental impacts associated with chemical coagulants.

Evaluating the effectiveness of coagulation-flocculation treatment on a wastewater from the moroccan leather tanning industry : An ecological approach

The removal of pollutants from tannery wastewaters, which is renowned for its substantial volumes, intricate composition, and considerable hazards to human health and the environment, is a prominent research area in the field of water treatment. The aim of this study is to employ a bio-coagulant derived from Parkinsonia aculeata seeds and a bio-flocculant derived from Hibiscus esculentus to minimise the concentration of pollutants in the combined wastewater originating from tanneries. In the course of the research, a thorough physicochemical analysis of the coagulating and flocculating agents, Parkinsonia aculeata (PA) and Hibiscus esculentus (HE), was performed using techniques such as XRD (X-ray diffraction), FTIR (Fourier-transform infrared spectroscopy), and SEM-EDS (scanning electron microscopy-energy dispersive X-ray spectroscopy). This analysis aimed to determine the composition and characteristics of these biomasses. Subsequently, a comprehensive overview was conducted to summarize the various factors that influence the treatment of tannery wastewater through coagulation/flocculation. This was accomplished by manipulating the target factors and observing their impact on the removal of specific physicochemical parameters such as chemical oxygen demand (COD), electrical conductivity (EC), total chromium (Cr) and Optical density (OD). The variables that were established include pH, dosage of coagulant and flocculant, as well as the speed and duration of agitation in both the fast and slow mixing stages. The experiments were carried out while taking into account the optimal parameters, leading to the near-complete removal of all analyzed pollutants. The optimal requirements for the Parkinsonia aculeata-Hibiscus esculentus Coagulation Flocculation System involve adjusting the pH to 8, choosing concentrations of approximately 1.25 g L-1 and 0.6 g L-1 for the coagulant and flocculant respectively, maintaining a fast speed of 170 rpm for 3 min while keeping the slow agitation at around 30 rpm for 20 min. The removal rates achieved after treating tannery wastewater using the PA-HE coagulant-flocculant combination demonstrate high efficacy, with values reaching approximately 100% for TSS, 98.71% for BOD5, 99.93% for COD, 98.88% for NH4+, 98.21% for NO3-, 90.32% for NO2-, 93.13% for SO42-, 95.44% for PO43-, 96.08% for OD and 60% for total chromium. These results indicate the successful removal of a wide range of pollutants from tannery wastewater through the PA-HE treatment method. In predicting the CF treatment approach, PCA has been employed to preprocess the input data and determine the key variables that impact the process. This can streamline the modeling process and enhance the precision of the predictions.

Usage of Cicer Arietinum as a local and eco-friendly natural coagulant in sewage treatment and its ability to increase the formation of floc process

Cicer Arietinum (CA) or chickpea seeds were used as a local natural coagulant, cheap and cultivable which available in Egypt that can be used to reduce turbidity from wastewater, especially sewage water instead of chemical coagulant that causes different diseases like intestinal constipation, loss of memory, convulsions, so this paper represents the use of chickpea as a natural coagulant and eco-friendly in the environment because it assumed to be safe for the human health and efficient in sewage treatment 6, So the researchers advices now to use natural coagulant as coagulant aids which has a higher ability to raise the consistency of floc and prevent of the coagulation operation. The optimum removal conditions that applied on the research were temperature =250C, pH= 3, Contact time=120 min, agitation speed for 2 minutes =80 rpm (rapid mixing), (CA) dosage is 90 mg/L, and (95.89%) turbidity reduction was achieved on the studied area.

Recent Achievements in Polymer Bio-Based Flocculants for Water Treatment

Polymer flocculants are used to promote solid-liquid separation processes in potable water and wastewater treatment. Recently, bio-based flocculants have received a lot of attention due to their superior advantages over conventional synthetic polymers or inorganic agents. Among natural polymers, polysaccharides show many benefits such as biodegradability, non-toxicity, ability to undergo different chemical modifications, and wide accessibility from renewable sources. The following article provides an overview of bio-based flocculants and their potential application in water treatment, which may be an indication to look for safer alternatives compared to synthetic polymers. Based on the recent literature, a new approach in searching for biopolymer flocculants sources, flocculation mechanisms, test methods, and factors affecting this process are presented. Particular attention is paid to flocculants based on starch, cellulose, chitosan, and their derivatives because they are low-cost and ecological materials, accepted in industrial practice. New trends in water treatment technology, including biosynthetic polymers, nanobioflocculants, and stimulant-responsive flocculants are also considered.

Coagulating potential of Iranian oak (Quercus Branti) extract as a natural coagulant in turbidity removal from water

Because of the presence of tannin in the molecular structure of oak extract, this substance is used as a natural coagulant to remove turbidity from water. The aim of this study was to determine the efficiency of this coagulant alone and in combination with polyaluminium chloride (PACl) in turbidity removal from water under optimal conditions. In this experimental study, Iranian oak extract was prepared by maceration method using ethanol 96% as an extractor. Kaolin was used to prepare synthetic turbid water samples. Using the jar test, the optimum concentrations of oak extract and PACl were determined in various concentrations of initial turbidity and pH. Moreover, the central composite design (CCD) method was utilized to design experiments and RSM was applied for analyzing the obtained results. Optimum concentrations of oak extract and PACl were 62.6 mg/L and 52.6 mg/L, respectively. An increase in initial turbidity and pH led to an increase in turbidity removal by the two coagulants. The efficiency of turbidity removal by oak extract and PACl was 63.5% and 66.5%, respectively. The simultaneous application of oak extract and polyaluminium chloride increased removal efficiency (85%) and reduced the total organic carbon concentration (TOC) in water (42.3%). The results showed that the simultaneous application of Iranian oak extract and polyaluminium chloride had an acceptable performance in removing turbidity from water.

Pine cone extract as natural coagulant for purification of turbid water

Turbidity is a characteristic impurity of groundwater in Pakistan. Turbid water is not suitable for drinking purposes. The main objective of this study is to reduce water turbidity using natural coagulant, extracted from pine cones. The coagulation activity of coagulant is tested using synthetic turbid water. Coagulant activity is affected by various factors such as coagulant dose, water turbidity, pH, extract density and settling time. The optimum coagulant dose and water turbidities are fixed; 0.5 ml/L, 67, and 75 NTU, respectively. The highest coagulation activities are observed at pH values 2 and 12. Further, coagulation activity of pine cone extract is maximized to 82% when its density is 1.8 g/cm³. Moreover, most of the coagulation activity takes place in the first hour. The results recommend the potential use of pine cone extract for turbid water purification.

A fluorescence-based assessment of the fate of organic matter in water treated using crude/purified Hibiscus seeds as coagulant in drinking water treatment

This study used fluorescence excitation-emission matrices (EEMs) analysis to investigate the characteristics of natural organic matter (NOM) in treated water using okra crude extract (OCE), sabdariffa crude extract (SCE) and kenaf crude extract (KCE) as coagulants. In addition, an assessment of the impact of purified okra protein (POP), purified sabdariffa protein (PSP) and purified kenaf protein (PKP) was undertaken. The performance evaluation of these coagulants in terms of increase or decrease in dissolved organic carbon (DOC) was compared with Peak T fluorescence intensity observed at excitation wavelength 220-230 nm, and emission wavelength 340-360 nm. Fluorescence analysis of water treated with the crude extracts identified the removal of DOC in peaks A and C region whereas the increase in DOC from the protein was predominantly found in peaks T and B region. Furthermore, it was observed that the purified proteins were noted to be capable of reducing the DOC concentration in raw water where all fluorophores were not detected. The application of OCE, SCE and KCE yielded an increase in DOC of 65, 61 and 55% respectively, corresponding to increases of 65, 29 and 54% in peak T fluorescence intensities, at 100 mg/l dose. Furthermore, DOC concentration was reduced by 25, 24 and 18% using POP, PSP and PKP respectively as coagulants with corresponding decreases in fluorescence intensity of 46%, 44 and 36% in POP, PSP and PKP, at a lower dose of 0.1 mg/l. Therefore, it is clear that Peak T fluorescence intensity could be used to characterise organic matter in treated water using natural extracts to assess final water quality.

Emerging plant-based flocculation treatment of phosphate clay: case study from Metlaoui-Gafsa (southwest Tunisia)

The objective of this study was to investigate the ability of cactus Opuntia ficus-indica, used as a natural flocculant in the treatment of phosphate clay as an industrial wastewater. It is a new process that has proven very promising in the removal of high turbidity from industrial effluents. The volume of clear water recovered (supernatant) is one of the characteristic features which allow us to judge the effectiveness of this bioflocculant. The following of the settling velocity on the one hand and the analysis of supernatant and sludge on the other hand allow us to compare the cactus-based flocculant effectiveness with that of a chemical flocculant (CF) of anionic polyacrylamide nature. The optimum pH required for maximum settling velocity was found to be 12. Obtained results indicated that after 900 seconds of settling, the highest volume of clear supernatant was obtained with the natural flocculant (740 mL/L), against 666 mL/L obtained with CF. Several analyses on recovered clear water (pH, turbidity, Cu, Zn, Fe, Cd and Cr) and on dry sludge (P₂O₅, CaO, MgO, organic C, SiO₂ and Cd) are highlighted.

Investigating the characteristic strength of flocs formed from crude and purified Hibiscus extracts in water treatment

The growth, breakage and re-growth of flocs formed using crude and purified seed extracts of Okra (OK), Sabdariffa (SB) and Kenaf (KE) as coagulants and coagulant aids was assessed. The results showed floc size increased from 300 μm when aluminium sulphate (AS) was used as a coagulant to between 696 μm and 722 μm with the addition of 50 mg/l of OK, KE and SB crude samples as coagulant aids. Similarly, an increase in floc size was observed when each of the purified proteins was used as coagulant aid at doses of between 0.123 and 0.74 mg/l. The largest floc sizes of 741 μm, 460 μm and 571 μm were obtained with a 0.123 mg/l dose of purified Okra protein (POP), purified Sabdariffa (PSP) and purified Kenaf (PKP) respectively. Further coagulant aid addition from 0.123 to 0.74 mg/l resulted in a decrease in floc size and strength in POP and PSP. However, an increase in floc strength and reduced d50 size was observed in PKP at a dose of 0.74 mg/l. Flocs produced when using purified and crude extract samples as coagulant aids exhibited high recovery factors and strength. However, flocs exhibited greater recovery post-breakage when the extracts were used as a primary coagulant. It was observed that the combination of purified proteins and AS improved floc size, strength and recovery factors. Therefore, the applications of Hibiscus seeds in either crude or purified form increases floc growth, strength, recoverability and can also reduce the cost associated with the import of AS in developing countries.

Assessing the application and downstream effects of pulsed mode ultrasound as a pre-treatment for alum coagulation

The application of pulsed mode ultrasound (PMU) as a pre-treatment for alum coagulation was investigated at various alum dosages and pH levels. The effects of the treatments on turbidity and dissolved organic carbon (DOC) removal and residual Al were evaluated. Response surface methodology (RSM) was utilized to optimize the operating conditions of the applied treatments. The results showed that PMU pre-treatment increased turbidity and DOC removal percentages from maximum of 96.6% and 43% to 98.8% and 52%, respectively. It also helped decrease the minimum residual Al from 0.100 to 0.094 ppm. The multiple response optimization was carried out using the desirability function. A desirability value of >0.97 estimated respective turbidity removal, DOC removal and Al residual of 89.24%, 45.66% and ∼ 0.1 ppm for coagulation (control) and 90.61%, >55% and ∼ 0 for coagulation preceded by PMU. These figures were validated via confirmatory experiments. PMU pre-treatment increased total coliform removal from 80% to >98% and decreased trihalomethane formation potential (THMFP) from 250 to 200 ppb CH3Cl. Additionally, PMU application prior to coagulation improved the settleability of sludge due to the degassing effects. The results of this study confirms that PMU pre-treatment can significantly improve coagulation performance.

Utilization of plant-based natural coagulants as future alternatives towards sustainable water clarification

Rapid industrial developments coupled with surging population growth have complicated issues dealing with water scarcity as the quest for clean and sanitized water intensifies globally. Existing fresh water supplies could be contaminated with organic, inorganic and biological matters that have potential harm to the society. Turbidity in general is a measure of water cloudiness induced by such colloidal and suspended matters and is also one of the major criteria in raw water monitoring to meet the stipulated water quality guidelines. Turbidity reduction is often accomplished using chemical coagulants such as alum. The use of alum is widely associated with potential development of health issues and generation of voluminous sludge. Natural coagulants that are available in abundance can certainly be considered in addressing the drawbacks associated with the use of chemical coagulants. Twenty one types of plant-based natural coagulants categorized as fruit waste and others are identified and presented collectively with their research summary in this review. The barriers and prospects of commercialization of natural coagulants in near future are also discussed.

The use of a natural coagulant (Opuntia ficus-indica) in the removal for organic materials of textile effluents

The goal of this study was to investigate the activity of the coagulant extracted from the cactus Opuntia ficus-indica (OFI) in the process of coagulation/flocculation of textile effluents. Preliminary tests of a kaolinite suspension achieved maximum turbidity removal of 95 % using an NaCl extraction solution. Optimization assays were conducted with actual effluents using the response surface methodology (RSM) based on the Box-Behnken experimental design. The responses of the variables FeCl3, dosage, cactus dosage, and pH in the removal of COD and turbidity from both effluents were investigated. The optimum conditions determined for jeans washing laundry effluent were the following: FeCl3 160 mg L(-1), cactus dosage 2.60 mg L(-1), and pH 5.0. For the fabric dyeing effluent, the optimum conditions were the following: FeCl3 640 mg L(-1), cactus dosage 160 mg L(-1), and pH 6.0. Investigation of the effects of the storage time and temperature of the cactus O. ficus-indica showed that coagulation efficiency was not significantly affected for storage at room temperature for up to 4 days.

Toxicity assessment and modelling of Moringa oleifera seeds in water purification by whole cell bioreporter

Moringa oleifera has been used as a coagulation reagent for drinking water purification, especially in developing countries such as Malawi. This research revealed the cytoxicity and genotoxicity of M. oleifera by Acinetobacter bioreporter. The results indicated that significant cytoxicity effects were observed when the powdered M. oleifera seeds concentration is from 1 to 50 mg/L. Through direct contact, ethanolic-water extraction and hexane extraction, the toxic effects of hydrophobic and hydrophilic components in M. oleifera seeds were distinguished. It suggested that the hydrophobic lipids contributed to the dominant cytoxicity, consequently resulting in the dominant genotoxicity in the water-soluble fraction due to limited dissolution when the M. oleifera seeds granule concentration was from 10 to 1000 mg/L. Based on cytoxicity and genotoxicity model, the LC50 and LC90 of M. oleifera seeds were 8.5 mg/L and 300 mg/L respectively and their genotoxicity was equivalent to 8.3 mg mitomycin C per 1.0 g dry M. oleifera seed. The toxicity of M. oleifera has also remarkable synergistic effects, suggesting whole cell bioreporter as an appropriate and complementary tool to chemical analysis for environmental toxicity assessment.

Dragon Fruit Foliage Plant-Based Coagulant for Treatment of Concentrated Latex Effluent: Comparison of Treatment with Ferric Sulfate

The effectiveness of dragon fruit foliage as a natural coagulant for treatment of concentrated latex effluent was investigated and compared with ferric sulfate, a chemical coagulant. Dragon fruit is a round and often red-colored fruit with scales-like texture and is native to south American countries which is also cultivated and heavily marketed in southeast Asian countries. Its foliage represents a part of its overall plant system. Latex effluent is one of the main byproduct from rubber processing factories in Malaysia. Three main parameters investigated were chemical oxygen demand (COD), suspended solids (SS), and turbidity of effluent. Coagulation experiments using jar test were performed with a flocculation system where the effects of latex effluent pH as well as coagulation dosage on coagulation effectiveness were examined. The highest recorded COD, SS, and turbidity removal percentages for foliage were observed for effluent pH 10 at 94.7, 88.9, and 99.7%, respectively. It is concluded that the foliage showed tremendous potential as a natural coagulant for water treatment purposes. The foliage could be used in the pretreatment stage of Malaysian latex effluent prior to secondary treatment.

Coagulant properties of Moringa oleifera protein preparations: application to humic acid removal

This work aimed to characterize the coagulant properties of protein preparations from Moringa oleifera seeds in the removal of humic acids from water. Three distinct preparations were assayed, namely extract (seeds homogenized with 0.15 M NaCl), fraction (extract precipitated with 60% w/v ammonium sulphate) and cMoL (protein purified with guar gel column chromatography). The extract showed the highest coagulant activity in a protein concentration between 1 mg/L and 180 mg/L at pH 7.0. The zeta potential of the extract (-10 mV to -15 mV) was less negative than that of the humic acid (-41 mV to -42 mV) in a pH range between 5.0 and 8.0; thus, the mechanism that might be involved in this coagulation activity is adsorption and neutralization of charges. Reduction of total organic carbon (TOC) and dissolved organic carbon (DOC) was observed in water samples containing 9 mg/L carbon as humic acid when treated with 1 mg/L of the extract. A decrease in colour and in the aromatic content of the treated water was also observed. These results suggested that the extract from M. oleifera seeds in a low concentration (1 mg/L) can be an interesting natural alternative for removing humic acid from water in developing countries. The extract dose determined in the present study does not impart odour or colour to the treated water.

Reduction of turbidity of water using locally available natural coagulants

Turbidity imparts a great problem in water treatment. Moringa oleifera, Cicer arietinum, and Dolichos lablab were used as locally available natural coagulants in this study to reduce turbidity of synthetic water. The tests were carried out, using artificial turbid water with conventional jar test apparatus. Optimum mixing intensity and duration were determined. After dosing water-soluble extracts of Moringa oleifera, Cicer arietinum, and Dolichos lablab reduced turbidity to 5.9, 3.9, and 11.1 nephelometric turbidity unit (NTU), respectively, from 100 NTU and 5, 3.3, and 9.5, NTU, respectively, after dosing and filtration. Natural coagulants worked better with high, turbid, water compare to medium, or low, turbid, water. Highest turbidity reduction efficiency (95.89%) was found with Cicer arietinum. About 89 to 96% total coliform reduction were also found with natural coagulant treatment of turbid water. Using locally available natural coagulants, suitable, easier, and environment friendly options for water treatment were observed.

A preliminary study on Jatropha curcas as coagulant in wastewater treatment

Many coagulants, mainly inorganic, are widely used in conventional water and wastewater treatment. Recent studies reported the occurrence of some chronic diseases associated with residual coagulant in treated wastewater. The use of alternative coagulants which are biodegradable and environmentally friendly could alleviate the problem associated with these diseases. This work investigates the capability of Jatropha curcas seed and presscake (the residue left after oil extraction) to reduce the turbidity of wastewater through coagulation. The coagulant was prepared by dissolving Jatropha curcas seed and presscake powder into solution. Then jar tests were conducted on kaolin solution as the model wastewater. The Jatropha seed was found to be an effective coagulant with more than 96% of turbidity removal at pH 1-3 and pH 11-12. The highest turbidity removal was recorded at pH 3 using a dosage of 120 mg/L. The flocs formed using Jatropha were observed to be bigger and to sediment faster when compared with flocs formed using alum. The turbidity removal was high (>98%) at all turbidities (100 NTU to 8000 NTU), suggesting its suitability for a wide range of industrial wastewater. The performance of Jatropha presscake after extraction of oil was also comparable to the fresh seed and alum at highly acidic and highly alkaline conditions. The addition of Jatropha did not significantly affect the pH of the kaolin samples after treatment and the sludge volume produced was less in comparison to alum. These results strongly support the use of Jatropha curcas seed and presscake as a potential coagulant agent.

Removal of water turbidity by natural coagulants obtained from chestnut and acorn

The ability of seed extracts of several species of chestnut and acorn to act as natural coagulants was tested using a synthetic turbid water. Active components were extracted from ground seeds of Horse chestnut and acorns of some species of family Fagaceae: Common oak, Turkey oak, Northern red oak and European chestnut. All investigated extracts had coagulation capabilities and their amounts depended on pH values and initial turbidities. The seed extracts from European chestnut and Common oak acorn were the most efficient expressing the highest coagulation activities, about 80% and 70%, respectively, in both low and medium investigated water turbidities at the lowest coagulant dose 0.5 ml/L.

Removal of Pb(II) from aqueous solution by seed powder of Prosopis juliflora DC

Biosorption potential of Prosopis juliflora seed powder (PJSP) for Pb(II) from aqueous solution was investigated. The effects of pH, contact time and different metal concentrations were studied in batch experiments. The maximum uptake of metal ions was obtained at pH 6.0. Adsorption equilibrium was established at 360 min. The pseudo-first-order and pseudo-second-order kinetic models were applied to study the kinetics of the biosorption processes. The pseudo-second-order kinetic model provided the best correlation (R(2)=0.9992) of the experimental data compared to the pseudo-first-order kinetic model. The maximum Pb(II) adsorbed was found to be 40.322 mg/g and it was found that the biosorption of Pb(II) on PJSP has correlated well (R(2)=0.9719) with the Langmuir equation compared to Freundlich isotherm equation (R(2)=0.9282) in the concentration range studied. Negative values of DeltaG indicated that the adsorption process was spontaneous and exothermic in nature. The FTIR study revealed the presence of various functional groups which are responsible for the adsorption process. The overall results show that PJSP can be envisaged as a vibrant, biosorbent for metal cleanup operations.

Toward understanding the efficacy and mechanism of Opuntia spp. as a natural coagulant for potential application in water treatment

Historically, there is evidence to suggest that communities in the developing world have used plant-based materials as one strategy for purifying drinking water. In this study, the coagulant properties of Opuntia spp., a species of cactus, are quantitatively evaluated for the first time. Opuntia spp. was evaluated for turbidity removal from synthetic water samples, and steps were made toward elucidating the underlying coagulation mechanism. In model turbid water using kaolin clay particles at pH 10, Opuntia spp. reduced turbidity by 98% for a range of initial turbidities. This is similar to the observed coagulation activities previously described for Moringa oleifera, a widely studied natural coagulant. Although it has been reported that Moringa oleifera predominantly operates through charge neutralization, comparison of zeta potential measurements and transmission electron microscopy images of flocs formed by Opuntia spp. suggest that these natural coagulants operate through different mechanisms. It is suggested that Opuntia spp. operates predominantly through a bridging coagulation mechanism. Once optimized, application of these readily available plants as a part of point-of-use water treatment technology may offer a practical, inexpensive, and appropriate solution for producing potable water in some developing communities.

TRATAMENTO ALTERNATIVO DA ÁGUA UTILIZANDO EXTRATO DE SEMENTE DE MORINGA OLEIFERA E RADIAÇÃO SOLAR

RESUMO O objetivo deste estudo foi avaliar a eficiência do uso do extrato de Moringa oleifera e da radiação solar na clarificação e desinfecção da água com diferentes níveis de turbidez. O experimento foi divido em duas etapas: (a) tratamento por exposição ao sol de amostras de água com diferentes níveis de turbidez e (b) utilização das sementes de M. oleifera para floculação e sedimentação clarificando a água e desinfecção da água clarificada por radiação solar. Nos dois protocolos foram realizados: colimetria, segundo substrato cromogênico; DBO, por medição manométrica; turbidez, por meio de turbidímetro. Os resultados comprovaram a eficiência da radiação solar na desinfecção da água com baixo e alto nível de turbidez, sendo que, para Escherichia coli, as maiores reduções ocorreram nas amostras com turbidez 30-40 UNT e após 12h de exposição. A adição do extrato de M. oleifera, nas amostras de baixa e alta turbidez, proporcionou elevadas reduções da turbidez em relação ao nível de turbidez inicial e potencializou a ação da luz solar na desinfecção da água. Propõe-se a adição de extratos de sementes de M. oleifera e radiação solar para maximizar a eliminação de E. coli das amostras de água turvas e assim minimizar o risco de transmissão hídrica de agentes patogênicos de origem intestinal.