A nanofiltration (NF) membrane pilot plant was tested to treat water from a spring located in the Tula Valley. Conventional physicochemical parameters and the pathogenic content were analyzed at the inlet and outlet (permeate) of the process. Ninety-five per cent removal of heterotrophic bacteria was obtained by the membrane and complete removal of all other pathogens was achieved. The membrane process performed better than the municipal chlorination facility. The NF process also removed most of the organic matter and removal of emerging pollutants varied from 5 to 100%, depending on the compound. A softening process prevented the membrane system from clogging, and continuous operation was carried out for more than 1 month with minimal maintenance.

INTRODUCTION

The government of Mexico City is facing a challenging future to supply fresh water to the rapidly growing population. To address this issue, and alleviate the city's water demand, additional non-conventional sources have been considered (Jiménez & Chávez 2004). One of these options is to reuse water from the aquifer of the Tula Valley, which is currently being recharged with untreated wastewater. This valley is located 40 km to the north of Mexico City, and for many decades (since 1890), it has received the bulk of the wastewater generated by Mexico City. This wastewater has been reused for irrigation without treatment for more than 100 years, and the practice has allowed the valley to become a very productive agricultural area, together with massive groundwater recharge. Given the amount of water that the valley has historically received (>50 m3/s on average), many springs have appeared on the lower parts of the valley where they have been used by local communities as their only source of water for human consumption. One of the main springs that is currently used for water supply, Cerro Colorado Spring, provides 600 L/s of water to a population of 130,000 inhabitants, with only chlorination as treatment. Based on prior studies, water travel times from the irrigated fields to the springs are estimated to be 3–5 days with groundwater velocities of 0.02–6 m/d (BGS 1998). Although soil is capable of acting as a purifying barrier efficiently removing a significant quantity of water pollutants (Jiménez & Chávez 2004), it has been demonstrated that bacteria, such as fecal coliforms, may remain in spring water (Gallegos et al. 1999). In fact, data from sampling campaigns of Tula Valley springs developed from 1997 to 2011 (Table 1) have shown the presence of fecal indicator and pathogenic microorganisms, even though spring water comes from wastewater that has been filtered through the irrigated soils.

Table 1

Summary of microbial, micropollutant, and basic quality parameter concentrations found in spring water from the Tula Valley between 1997 and 2011 (adapted from Chávez et al. 2011; Jiménez et al. 2012)

MinimumMaximumAverageSDn
Biological parameters 
 Fecal streptococci (CFU/100 mL) 0.5 21.0 7.9 0.5 18 
 Somatic bacteriophages (PFU/L) 21.0 24.0 22.5 21 17 
 Fecal coliforms (CFU/100 mL) 11.4 930.0 210.4 11.4 25 
 Total coliforms (CFU/100 mL) 38.0 2,566 608.2 38 25 
Escherichia coli (CFU/100 mL) 13.4 19.7 16.7 13.4 19 
 Enterococci (CFU/100 mL) 2.7 11.7 8.6 2.7 18 
Giardia spp. (cysts/L) ND ND ND 19 
Clostridium perfringens (CFU/100 mL) 0.0 3.6 1.8 19 
Salmonella spp. (CFU/mL) ND ND ND 19 
Entamoeba histolytica (cysts/L) ND ND ND 16 
Shigella spp. (CFU/mL) ND ND ND 16 
 Helminth eggs (eggs/L) ND ND ND 22 
Micropollutants (ng/L) 
 4-n-nonylphenol 1.82 16 6.13 1.82 19 
 Triclosan 0.76 8.9 2.68 0.76 19 
 Bisphenol-A 0.81 2.15 1.54 0.81 19 
 Butylbenzylphthalate (BuBeP) 0.95 16.7 6.64 0.95 19 
 Bis-2-ethylhexylphthalate (DEHP) 15.8 70.4 52.23 15.8 19 
 Estrone 0.165 0.26 0.23 0.16 19 
 Clofibric acid 0.18 0.09 19 
 Ketoprofen 0.17 0.08 19 
 Gemfibrozil 0.06 0.03 19 
 Ibuprofen 1.5 0.46 19 
 Salicylic acid 0.29 11.4 6.19 0.29 19 
 Naproxen 0.99 0.51 19 
 Diclofenac 0.4 0.1 19 
 Carbamazepine 7.7 5.1 19 
Basic parameters 
 Turbidity, NTU 0.0 5.0 0.9 0.0 24 
 Total dissolved solids (TDS) (mg/L) 12.0 11,128 1,789 12.0 28 
 Chemical oxygen demand (COD) (mg/L) 1.9 49.0 22.5 1.9 19 
 Total organic carbon, TOC (mg/L) 1.0 32.0 12.3 1.0 18 
 Total hardness (mg CaCO3/L) 48.0 1,009 404.8 48.0 25 
MinimumMaximumAverageSDn
Biological parameters 
 Fecal streptococci (CFU/100 mL) 0.5 21.0 7.9 0.5 18 
 Somatic bacteriophages (PFU/L) 21.0 24.0 22.5 21 17 
 Fecal coliforms (CFU/100 mL) 11.4 930.0 210.4 11.4 25 
 Total coliforms (CFU/100 mL) 38.0 2,566 608.2 38 25 
Escherichia coli (CFU/100 mL) 13.4 19.7 16.7 13.4 19 
 Enterococci (CFU/100 mL) 2.7 11.7 8.6 2.7 18 
Giardia spp. (cysts/L) ND ND ND 19 
Clostridium perfringens (CFU/100 mL) 0.0 3.6 1.8 19 
Salmonella spp. (CFU/mL) ND ND ND 19 
Entamoeba histolytica (cysts/L) ND ND ND 16 
Shigella spp. (CFU/mL) ND ND ND 16 
 Helminth eggs (eggs/L) ND ND ND 22 
Micropollutants (ng/L) 
 4-n-nonylphenol 1.82 16 6.13 1.82 19 
 Triclosan 0.76 8.9 2.68 0.76 19 
 Bisphenol-A 0.81 2.15 1.54 0.81 19 
 Butylbenzylphthalate (BuBeP) 0.95 16.7 6.64 0.95 19 
 Bis-2-ethylhexylphthalate (DEHP) 15.8 70.4 52.23 15.8 19 
 Estrone 0.165 0.26 0.23 0.16 19 
 Clofibric acid 0.18 0.09 19 
 Ketoprofen 0.17 0.08 19 
 Gemfibrozil 0.06 0.03 19 
 Ibuprofen 1.5 0.46 19 
 Salicylic acid 0.29 11.4 6.19 0.29 19 
 Naproxen 0.99 0.51 19 
 Diclofenac 0.4 0.1 19 
 Carbamazepine 7.7 5.1 19 
Basic parameters 
 Turbidity, NTU 0.0 5.0 0.9 0.0 24 
 Total dissolved solids (TDS) (mg/L) 12.0 11,128 1,789 12.0 28 
 Chemical oxygen demand (COD) (mg/L) 1.9 49.0 22.5 1.9 19 
 Total organic carbon, TOC (mg/L) 1.0 32.0 12.3 1.0 18 
 Total hardness (mg CaCO3/L) 48.0 1,009 404.8 48.0 25 

These levels of pathogens indicate that treatment in addition to the typical disinfection procedure is needed before distribution to the final users, with pathogens in reused water being the main concern for health regulators, farmers, and general public (Toze 2006). Moreover, some regulated parameters are currently at concentrations above the recommended limits for human consumption (EU 1998; US EPA 2008; WHO 2011; NOM-127-SSA1-1994 – mod. 2000). These and other non-regulated organic pollutants (Table 1) represent additional health risks.

To face the potential health risks posed by Cerro Colorado spring water, the WHO (2011) recommendation based on the use of multiple barriers applies. It suggests setting barriers from catchment to consumer, and it is particularly concerned with securing the microbial safety of drinking water supplies. It advises that disinfection efficacy may be unsatisfactory against some pathogens and appropriate treatment processes as barriers should be used in conjunction with disinfection. In general, their regulations are designed to ensure safe drinking water using multiple barriers.

Based on the principle of multiple barriers, a nanofiltration (NF) membrane system was selected, considering it has been previously proposed as an effective technology to eliminate micropollutants (Kimura et al. 2003). This process operates with lower costs than reverse osmosis systems, and it was thought that such systems might also be used to remove the pathogenic content of the spring water. This paper addresses the application of an NF membrane unit to improve the water quality (in terms of microbial, basic parameters, and micropollutants) and compares it to that of the chlorinated water quality supply from the Cerro Colorado Spring in the Tula Valley (Figure 1).

Figure 1

Location of the Tula Valley and the Cerro Colorado Spring, Mexico.

Figure 1

Location of the Tula Valley and the Cerro Colorado Spring, Mexico.

MATERIALS AND METHODS

The pilot plant (11.4 m3/d) included a pumping system, a prefilter (polycarbonate with a 5 μm pore size), a softening unit (30,000 grains of ion exchange resin for reducing potential scaling), and the NF module. The system consisted of a membrane unit with cylindrical housings built to fit with 4040 spiral wound membranes. The effective transfer area was 7.6 m2 (NF270 membrane, Dow Chemical Company, cut-off size 170–200 Da). The NF membrane was selected after laboratory trials on a pilot cell (Osmonics SEPA CF). The pilot plant had an operating control pressure valve and the instrumentation required to measure flow, pH, TDS, as well as pressure along the system. The operating pressure was 170 psi. Recovery (permeate) was maintained at more than 50% of the influent for 2 months of continuous operation. The plant was installed next to the Cerro Colorado Spring (Figure 2).

Figure 2

Flow diagram (left) and actual installation of the NF pilot plant (right) at the Cerro Colorado Spring.

Figure 2

Flow diagram (left) and actual installation of the NF pilot plant (right) at the Cerro Colorado Spring.

Basic parameters such as total solids, total dissolved solids (TDS), pH, conductivity, and total hardness were monitored according to Standard Methods (APHA, AWWA, WPCF 1998); chemical oxygen demand (COD) was determined according to the HACH Water Analysis Handbook (Method 8000 Reactor Digestion Method); and total organic carbon (TOC) with a Shimadzu TOC5050 carbon analyzer. Samples were taken in triplicate at four process points: inlet (raw spring water), softened spring water, membrane permeate, and membrane reject. Also, during operation, pH and TDS were monitored online for process control.

For microbial characterization, two samples were taken along the NF system operation with 2 weeks in between (days 15 and 30 of operation) to quantify the content of fecal coliforms, Enterococcus faecalis, Salmonella spp., spores of Clostridium perfringens, bacteriophages, cysts of Giardia spp., oocysts of Cryptosporidium spp., and helminth eggs at three sampling points: raw spring water, membrane permeate, and chlorinated spring water. Analytical techniques used were the following (APHA, AWWA, WPCF (1998), unless otherwise indicated): fecal coliforms using membrane filter 9222D method; E. faecalis using membrane filter 9230C method; Salmonella spp. using membrane filter 9260D method; C. perfringens (spores) using the UK National Standard Method (Health Protection Agency 2004); bacteriophages using the dual layer method ISO 10705 with Escherichia coli WG5 (Grabow & Cobrough 1986); Giardia spp. and Cryptosporidium spp. using the epifluorescence method 1623 (APHA 1998; US EPA 1999); and helminth eggs using the US EPA (1992) method. The concentration of emerging pollutants in the liquid samples was determined by gas chromatography–mass spectrometry; the preparation and analysis of liquid samples were carried out following the procedure proposed and validated by Gibson et al. (2007).

RESULTS AND DISCUSSION

The NF membrane unit was operated as described previously, over a period of 2 months, which demonstrated an adequate performance of the equipment. The softening system was installed prior to the membrane, to reduce the high calcium hardness of the spring water and decrease the potential for scaling of the membrane. Water hardness was reduced by 53% (Table 2) before entering the NF membrane. Softening performed by ion exchange exhibited a slight retention of organic matter, indicated by a reduction of 26% of COD and 4% of TOC. NF removed most of the remaining organic matter, achieving 100 and 97% removal of COD and TOC, respectively. Overall, the pilot plant removed 54% of TDS; 50% of total solids; and 92% of total hardness. Permeate water met the criteria for drinking water indicated in Table 2, achieving adequate physicochemical levels with respect to the parameters analyzed and thus did not need a remineralization step as reverse osmosis generally requires, given the permeate conductivity values of 558 μS/cm. These results were obtained with the NF system operating at 170 psi and an average permeate flow of 15 L/min, yielding 1.97 L/min m2 of continuous flux along more than 1 month of continuous operation. Considering the size of the pilot plant and the period of operation (2 months), there was no need for a clean-in-place system, and thus, the membrane was not cleaned during this period, but flux remained stable with a 20% reduction during the overall operational period. However, for a full operating system, cleaning must be performed to remove slight but expected fouling and scaling of the membrane and increase its life service.

Table 2

Average physicochemical parameter concentrations determined for the raw spring water and the NF process streams

Process streamConductivity (μS/cm)TDS (mg/L)pHTS (mg/L)Total hardness (CaCO3mg/L)COD (mg/L)TOC (mg/L)
 Raw spring water 1,195 827 7.25 1,172 698 54 25 
 Softening 1,159 833 7.27 1,190 330 40 24 
 Permeate 558 380 7.09 591 27 0.7 
 Reject 1,741 1,222 7.35 1,724 586 50 80 
 Removal (softening + NF) 53% 54% 2% 50% 96% 100% 97% 
National and international criteria for drinking water 
WHO (2011)         
EU (1998)  2,500 500 6.5–9.5     
US EPA (2008)   500 6.5–8.5     
 Mexican legislation 1,000 500 6.5–8.5  500  2a 
Process streamConductivity (μS/cm)TDS (mg/L)pHTS (mg/L)Total hardness (CaCO3mg/L)COD (mg/L)TOC (mg/L)
 Raw spring water 1,195 827 7.25 1,172 698 54 25 
 Softening 1,159 833 7.27 1,190 330 40 24 
 Permeate 558 380 7.09 591 27 0.7 
 Reject 1,741 1,222 7.35 1,724 586 50 80 
 Removal (softening + NF) 53% 54% 2% 50% 96% 100% 97% 
National and international criteria for drinking water 
WHO (2011)         
EU (1998)  2,500 500 6.5–9.5     
US EPA (2008)   500 6.5–8.5     
 Mexican legislation 1,000 500 6.5–8.5  500  2a 

TDS: total dissolved solids; TS: total solids; COD: chemical oxygen demand; TOC: total organic carbon.

aLimit for direct reinjection to aquifer (DOF 2009).

Regarding the reject of the membrane, the high salt and organic matter concentration (1,222 mg/L of TDS and 80 mg/L of TOC) makes it a stream that should be treated separately to complete the treatment process. The operating conditions tested in this work could be optimized to increase the permeate flow by modifying the working pressure or proposing a different arrangement of the NF membranes for a full-scale facility. Moreover, management of NF reject is seldom reported in the literature, and thus, practical experience needs to be documented. Many authors (Kosutic Kunst 2002; Van der Bruggen et al. 2003; Radjenovíc et al. 2008; Bolong et al. 2009; Acero et al. 2010) have concluded that it is possible to concentrate organic recalcitrant compounds, many of which are emerging pollutants (Kümmerer 2011). Also, the bulk of microorganisms removed after NF will concentrate in reject water, and thus, this needs to be inactivated. Currently, the literature has not addressed their removal or degradation in reject water, but advanced oxidation processes such as photocatalysis and electrooxidation are being studied to treat the membrane reject.

In addition to physicochemical parameters, the system exhibited good performance as a barrier against the pathogen content of the raw spring water. As shown in Table 3, spring water has better quality than raw wastewater after being filtered through the soil (soil–aquifer treatment system, SAT), but some bacteria and protozoa are still present in spring water. In contrast, bacteriophages and helminth eggs were absent from samples taken at days 15 and 30 of operation.

Table 3

Comparative biological indicators and parasite content determined for irrigation raw wastewater (data from Jiménez et al. 2004; Chávez et al. 2012), Cerro Colorado spring water (raw water), NF permeate, and the municipal chlorination system

FcSfSsHBCpBpGCHe
SampleOd(CFU/100 mL)PFU/LCysts/LOocysts/LEgg/L
Raw wastewater NA 5.0 × 107 7.7 × 105 4.4 × 109  3.4 × 105 1.8 × 103 5.5 × 102  14 
Raw spring water 15 9.7 ± 2.1 2.7 ± 1.2 1.0 ± 0.0 42.7 ± 8.3 5.3 ± 1.2 ND 3.5 ± 0.7 5.5 ± 0.7 ND 
30 14.3 ± 2.9 1.7 ± 0.6 1.7 ± 1.2 33.0 ± 6.6 9.7 ± 1.5 ND 2.5 ± 0.7 3.0 ± 0.0 ND 
Permeate 15 ND ND ND 2.3 ± 0.6 ND ND ND ND ND 
 30 ND ND ND 2.0 ± 0.0 ND ND ND ND ND 
Chlorination 15 ND ND ND ND ND ND 2.0 ± 0.0 ND ND 
 30 ND ND ND ND ND ND 0.5 ± 0.5 0.5 ± 0.5 ND 
WHO (2011)  NA          
US EPA (2008)  NA ND   5 × 104  a a a  
EU (1998)          
Mexican legislationb  ND      
  TOC (mg/L) Nitrates (mg/L) Sodium (mg/L) Sulfates (mg/L) TDS (mg/L) Hardness (mgCaCO3/L) Turbidity (UTN) Chlorides (mg/L) Conductivity 
Permeate NA 0.78 16.53 97.6 5.4 380 27 0.5  558 
Chlorination NA  16.7 242.8 148 1,163 404 154 1,573 
FcSfSsHBCpBpGCHe
SampleOd(CFU/100 mL)PFU/LCysts/LOocysts/LEgg/L
Raw wastewater NA 5.0 × 107 7.7 × 105 4.4 × 109  3.4 × 105 1.8 × 103 5.5 × 102  14 
Raw spring water 15 9.7 ± 2.1 2.7 ± 1.2 1.0 ± 0.0 42.7 ± 8.3 5.3 ± 1.2 ND 3.5 ± 0.7 5.5 ± 0.7 ND 
30 14.3 ± 2.9 1.7 ± 0.6 1.7 ± 1.2 33.0 ± 6.6 9.7 ± 1.5 ND 2.5 ± 0.7 3.0 ± 0.0 ND 
Permeate 15 ND ND ND 2.3 ± 0.6 ND ND ND ND ND 
 30 ND ND ND 2.0 ± 0.0 ND ND ND ND ND 
Chlorination 15 ND ND ND ND ND ND 2.0 ± 0.0 ND ND 
 30 ND ND ND ND ND ND 0.5 ± 0.5 0.5 ± 0.5 ND 
WHO (2011)  NA          
US EPA (2008)  NA ND   5 × 104  a a a  
EU (1998)          
Mexican legislationb  ND      
  TOC (mg/L) Nitrates (mg/L) Sodium (mg/L) Sulfates (mg/L) TDS (mg/L) Hardness (mgCaCO3/L) Turbidity (UTN) Chlorides (mg/L) Conductivity 
Permeate NA 0.78 16.53 97.6 5.4 380 27 0.5  558 
Chlorination NA  16.7 242.8 148 1,163 404 154 1,573 

Fc: fecal coliforms; Sf: Streptococcus faecalis; Ss: Salmonella spp.; HB: heterotrophic bacteria; Cp: Clostridium perfringens; Bp: bacteriophages; G: Giardia spp.; C: Cryptosporidium spp.; He: helminth eggs; Od: operational day; ND: not detected; NA: not applied.

a99.9% removal during the filtration process.

Bacteriophages are removed after wastewater is applied for irrigation, because soil organic matter provides hydrophobic binding sites for virus attachment (Schijven & Hassanizadeh 2000). In this respect, the role of organic matter in soil adsorption has also been reported for emerging pollutants (Gibson et al. 2010), suggesting that the use of wastewater for irrigation provides a large amount of suspended and dissolved organic matter that influences sorption. In the case of helminth eggs, these structures are easily retained by soil particles as their size ranges between 20 and 80 μm and they do not usually reach groundwater. In contrast, bacteria may still contaminate groundwater as travel times of wastewater through soil do not allow complete attenuation (BGS 1998) and together with protozoa they have smaller sizes (about 1 and 4–12 μm, respectively).

The microbial content of the raw spring water was removed successfully by the NF system with the exception of heterotrophic bacteria (plaque counting), which showed concentrations between 2.0 and 2.3 CFU/100 mL in the permeate. Nonetheless, mean removal was 1.24 logs and the concentration was well below the US EPA limit (5 × 104 CFU/100 mL; US EPA 2008). On the other hand, chlorination was effective against bacteria, but the presence of active Giardia and Cryptosporidium, while removed completely by NF, indicated the need for treatment in addition to the conventional disinfection process currently used on-site. These microorganisms represent a significant health risk, and thus, they must be completely removed before water is consumed. This can be accomplished by adding a filtration step before disinfection as required by the US EPA (2008) regulations, since protozoa are highly resistant to chlorination. Without this, significant health risks will prevail within the exposed population. The results obtained are consistent with other studies where NF was used to remove organic matter prior to disinfection, as well as bacteria and viruses present in source water (Patterson et al. 2012); these studies also recommended pilot testing to define final treatment membranes.

With respect to emerging pollutants, some compounds are present in spring water, as shown in Table 1. Even though limits for these pollutants have not been set, it is suggested that they are removed from drinking water sources before human consumption to reduce potential health risks (Nghiem et al. 2004). NF has proven useful in removing them from the Cerro Colorado Spring (Neira-Ruiz et al. 2012). NF was highly effective in removing emerging pollutants with the following selectivity: carbamazepine and diclofenac (100% removal), BuBeP (79%), gemfibrozil (76%), triclosan (68%), bisphenol-A and estradiol (66%), bis-2-ethylhexylphthalate (31%), and a marginal selectivity for salicylic acid (6%) and nonylphenol (5%). Thus, this process appears to be the best alternative to reduce salinity, organic matter, emerging pollutants, and microorganisms and produce safe water for human consumption. Moreover, NF has been recommended to improve performance, in terms of organic matter removal, in existing facilities (Durand-Bourlier et al. 2011).

The results of this study have clearly shown the capability of NF to effectively remove the pathogenic content of the spring water; and its performance was a distinct improvement in comparison with the site's current disinfection operation. During operation, the NF process was stable and it was demonstrated that continuous operation is possible for the spring water treatment with few maintenance periods. However, the pilot treatment could be further improved in terms of efficiency, for example, in terms of optimization of the operational pressure, or the most appropriate permeate/reject ratio in terms of costs. Additionally, reject stream treatment requires further study for membrane processes in order to achieve a treatment regime that minimizes environmental footprint.

Despite this, the effectiveness of the membrane process establishes the possibility of using this technology to treat the spring water of the Tula Valley, so it might be safely used by the local population and also for possible future consumption of the water back in Mexico City. The proposed process seems to be competitive by providing a reduction in chlorine demand given the removal of natural organic matter and pathogen content with the NF membrane. NF technology has become competitive over the last decade, and standard costs have already been established for such operations (Fane et al. 2011), being even lower than alternative treatment processes (Mohammad et al. 2007).

CONCLUSIONS

The NF process was able to reduce salinity (54% of TDS), total hardness (96%), organic matter (97% as TOC), and most of the microorganisms present in spring water from the Tula Valley, meeting international criteria for drinking water. Only a small fraction of the initial heterotrophic bacteria remained after the process but well below established limits. In addition, considering that NF is capable of removing emerging pollutants, its application for treating a non-conventional source has been demonstrated. This is of particular relevance considering that groundwater from the Tula Valley is being evaluated as a potential source for Mexico City.

However, according to the multiple barrier concept, NF must be coupled to complementary processes (e.g. activated carbon and disinfection) to reduce the risk of pathogens and emerging pollutants reaching consumers. The use of chlorine is still recommended to provide residual disinfection along the distribution system and to inactivate remaining bacteria. Further research must be carried out, including treatment of the membrane reject, to offer an integrated treatment solution.

ACKNOWLEDGEMENTS

The authors are thankful for the financial support provided by the Secretaría de Ciencia, Tecnología e Innovación del Distrito Federal (formerly ICyTDF) as project sponsor within project 0348 ICYTDF/63/2010 and ICYTDF/113/2012.

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