Application of microfiltration–nanofiltration combined technology for drinking water advanced treatment in a large-scale engineering project

People's demand of drinking water quality is gradually increasing while the national water quality standard is getting stricter, meaning that the drinking water produced through conventional water treatment technology, which is combined by coagulation, sedimentation, filtration, and disinfection, can barely meet their requirements anymore (Huang et al. 2019; Couto et al. 2020). Some water treatment plants start to improve the produced water quality by adding advanced treatment technologies including ozone–biological activated carbon technology, membrane separation technology, and optically catalyzed oxidation. Among all the mentioned technologies, nanofiltration (NF) technology receives more attention for the reason of its stable produced water quality and high water treatment efficiency (Costa & Pinho 2006; Shen et al. 2020; Wang et al. 2020a).

There are two remarkable advantages of the NF membrane in practice. Firstly, it can intercept monomolecular and micromolecular organic matters in drinking water and therefore eliminate their harm to body health. Secondly, it can effectively reduce water hardness by removing Ca²⁺, Mg²⁺, SO₄²⁻, and NO₃⁻, while beneficial elements including Na⁺, K⁺, and other microelements are selectively reserved in the drinking water (Reiss et al. 1999; Lesimple et al. 2020; Wang & Wei 2020; Zhang et al. 2020). On the other hand, the NF membrane always gets gradually fouled during the operation period and then impacts the performance of the total system. For solving the fouling problem caused by the fluctuated water quality after conventional drinking water treatment, insoluble matters should be sufficiently removed before the raw water enters the NF system (Shahriari & Hosseini 2020; Wang et al. 2020b). The pressurized-pot MF system applies highly intensive filters, such as polypropylene (PP) filters, to alleviate the fouling of NF membrane. This kind of filter is selected on account of its long service life, stable filtration accuracy, and low investment cost (Ozbey-Unal et al. 2020). However, there have not been enough studies or even one actual engineering project in China about effectively combining the pressurized-pot MF and NF technology and achieving a steadily-operating water purification system with high produced-water quality.

This research compares and analyzes the performance of pressurized-pot MF system and ultrafiltration (UF) membrane, which are, respectively, regarded as the pretreatment technologies of the NF system through pilot testing. The testing result demonstrates the feasibility of combined technology of NF with pressurized-pot MF for the practical application in engineering. Based on the pilot testing results, a new water purification process, which is sequentially combined by conventional purification technology, pressurized-pot MF, and NF, is creatively applied into upgrading and reforming the large-scale engineering project of drinking water advanced treatment for the first time in China.

The water-supply scale of Zhangjiagang Third Water Plant, Jiangsu Province, China is 200,000 m³/d, and its conventional drinking water treatment technology is combined by coagulation, sedimentation, filtration, and chlorination disinfection before the reformation project of advanced treatment. The produced-water quality has already been capable of meeting the requirements of Hygienic Standard for Drinking Water (GB5749-2006, China) with conventional treatment technology under routine conditions of raw water. Nevertheless, the water plant should additionally improve the daily-supplied water quality and possess emergency-disposal abilities in order to guarantee the water-supply security of Zhangjiagang city. Considering that the original technology is hardly satisfactory anymore, the upgraded water purification process, which is sequentially combined by conventional drinking water treatment technology, pressurized-pot MF and NF, is designed and applied based on the raw water quality. The main target of this project covers the extinction of water microbes, the controlling of organic substances and disinfection by-products (DBP), the improvement of drinking water mouthfeel, and enhanced reactions to water contamination emergencies. The NF-produced water, whose designed water-production scale is 100,000 m³/d, can be blended with the sand-filtered water in proportion of 1:1, which leads to a total water-supply scale of 200,000 m³/d.

Zhangjiagang Third Water Plant purifies raw water from the Yangtze River whose quality meets the requirement of II class surface water defined by the Environmental Quality Standard for Surface Water (GB3838-2002, China). After the reformation of advanced treatment, the NF membrane workshop draws raw water from sand-filtered products, whose temperature maintains between 4.0 and 30.0 °C, pH between 6.0 and 9, turbidity lower than 0.5 NTU, COD (chemical oxygen demand) value lower than 3.0 mg/L, NH₄-N content lower than 0.5 mg/L, and fluorine content lower than 1.0 mg/L. All other water quality indexes meet the requirement of III class surface water defined by Environmental Quality Standard for Surface Water (GB3838-2002, China).

After the reformation of advanced treatment, the produced-water quality of Zhangjiagang Third Water Plant sufficiently meets the requirement of Hygienic Standard for Drinking Water (GB5749-2006, China), while the produced water gains finer mouthfeel and the water plant gains a higher capability of replying to water-contamination emergencies. The quality of NF-produced water is shown in Table 1.

The major technological process of this project is shown in Figure 1. The sand-filtered water stored in the buffer tank for NF is elevated by the water-supply pump and then dosed with reductant and antiscalant before entering the MF system. The pressurized-pot MF system is capable of removing large particle impurities and keeping the SDI (silt density index) value of produced water lower than 5 for the purpose of protecting NF membrane components. The produced water of the MF system is subsequently pressurized by the booster pump to reach the operating pressure of the NF system before entering the NF unit. The produced water of the NF system then enters the product tank.

The pilot testing compares and analyzes the performance of pressurized-pot MF system and UF membrane which are regarded as pretreatment technologies, aiming at testifying the feasibility of combined technology of NF with pressurized-pot MF for the practical application in engineering,

The raw water used for the pilot testing is the sand-filtered water from Zhangjiagang Water Plant. The highly intensified PP filter is selected for the pressurized-pot MF system, while the polyvinyl chloride composite membrane is selected for the UF system and the polypiperazine amide membrane is selected for the NF system. The setup parameters of the mentioned devices are shown in Table 2, and the chemical agencies needed for the testing are shown in Table 3.

The water temperature, electric conductivity, flow rate, and water pressure are, respectively, monitored by E + H online thermometer, E + H online pH/ORP (oxidation-reduction potential) meter, E + H electromagnetic flowmeter, and E + H pressure sensor. The other water quality indexes are tested by the methods prescribed by Standard Examination Method for Drinking Water (GB/T 5750, China).

For optimizing the pressurized-pot MF system, relevant parameters are, respectively, adjusted within eight stages based on the setup parameters. Then, the performance of pressurized-pot MF is analyzed and compared with the performance of the UF membrane as pretreatment technologies.

As the status shown in Figure 2, the quality of raw water entering the pressurized-pot MF is relatively stable since it has already experienced sand filtration. The SDI value of raw water mostly remains between 3.5 and 5.5, while the SDI value of produced water keeps lower than 5 with an average value about 3. The variation trend of both SDI curve manifested in Figure 2 indicates that the SDI value of produced water of pressurized-pot MF is barely affected by the parameter changes among different stages.

The variation trend of the pressure of raw water and produced water of pressurized-pot MF is shown in Figure 3, while transmembrane pressure (TMP) is equal to the former value subtracted by the latter value. During the first stage, the TMP value increases regularly when the inflow rate of raw water is 3.2 m³/h, at the same time keeping lower than 1 bar. For the second stage, the variation trend of TMP value becomes milder when the inflow rate of raw water is reduced to 2.3 m³/h and the interval between chemical cleanings is elongated. During the third stage, the TMP value increases rapidly earlier because of water quality deterioration by accident and becomes moderately again later, while the inflow rate of raw water in this period is raised back to 3.2 m³/h. From stage 4 to 8, the slope of TMP does not change apparently even if several parameters are adjusted as shown in Table 4. This result indicates that the variation slope of TMP of pressurized-pot MF is mainly influenced by the inflow rate of raw water, while other parameter changes have little effect.

Based on the adjustment of the inflow rate of raw water and the variation trend of TMP, the interval between chemical cleanings is also slightly modulated. The intervals are 9 days for the first stage, 13 days for the second stage, 9 days for the third stage, 5 days for the fourth stage, 7 days for the fifth and sixth stages, 6 days for the seventh stage, and 8 days for the eighth stage. Generally speaking, the interval is modulated around 7 days.

As the statistics shown in Table 5, the amount of electricity or agent consumed by pressurized-pot MF does not show significant difference when the inflow rate of raw water and chemical-cleaning interval are adjusted simultaneously, which means this type of reasonable adjustment is allowed in practical engineering.

As shown in Figure 4, the functioning status of the NF system is generally stable after being, respectively, pretreated by pressurized-pot MF and UF. The temperature of raw water entering the NF system remains between 10 and 32 °C, while pH value between 8 and 8.5. The fluctuation range of the ORP value of UF-produced water is relatively large while still keeping lower than 300, and the ORP value of MF-produced water is relatively steady. The ORP value is positively related with the content of oxidative matters in water, which means that a lower ORP value is more beneficial to the service life of NF membrane.

As shown in Figure 5, the inflow rate of NF remains close to 3.2 m³/h, while the water-production rate of NF remains close to 2.9 m³/h. According to the ratio of mentioned values, the recovery rate of the NF system varies closely around 90% under both pretreatment technologies. The obtained recovery rate meets the anticipation of the pilot testing.

As shown in Figure 6, the inflow pressure of the NF system remains lower than 5 bar, while the TMP value of NF membrane remains lower than 1 bar, which is gained through the inflow pressure subtracting the concentrated water pressure. In the middle of testing, the TMP value of NF membrane rises violently as the result of the rapid growth of aquatic microbes in summer, which increases the speed of membrane fouling. Such growth of microorganisms can be properly inhibited by dosing sodium hypochlorite at the front end of the NF system, while the byproduct of the disinfection process, mainly consisting of residual chlorine, can be reduced by sodium hydrogen sulfite which is added ensuingly. The concentration of sodium hypochlorite is empirically controlled between 0.2 and 1.0 mg/L in practical engineering and can be moderately adjusted according to the variation of raw water quality.

Except for this kind of special situation, the NF system functions normally under both pretreatment technologies with fine performance of NF membrane.

For the surface water with micropollution, the key point of contaminant control is focusing on monomolecular organic matters, micromolecular organic matters, TOC and pigments (Bi et al. 2016). The terminal removal rate of COD and UV254 (which can characterize the content of humus) of combined technology comprising pressurized-pot MF or UF can both be over 90%. And the removal rate of DBP, which includes several types of halohydrocarbons such as chloroform, bromoform and carbon tetrachloride, can also reach 70–80% (as shown in Figure 7). Such removal capability for organic contaminants is sufficiently helpful to further improve the drinking water quality on the basis of conventionally produced drinking water.

As shown in Figure 8, the removal rate of pigments including chlorophylls (Chl) and phycocyanobilin (Phy) of combined technology is generally over 70%, which means that both combined technologies have nice effects on removing pigments from drinking water.

The overall salt rejection rate is characterized by the electric conductivity difference between raw water and produced water, which is then divided by the electric conductivity of raw water. As shown in Figure 9, the salt rejection rate of both combined technologies is approximately between 20 and 40%, which is quite limited. This circumstance occurs because the NF membrane selected for pilot testing is targeted specifically at organic substances, while its interception ability for monovalent ions such as Na⁺ and K⁺ is relatively lower and its interception ability for fluorides and nitrates does barely exist. Both combined technologies can remove over 80% of Fe³⁺, Mn²⁺ and Al³⁺, 20–60% of water hardness, which is characterized by the content of Ca²⁺ and Mg²⁺, and only about 20% of Na⁺ and K⁺ (as shown in Figures 10 and 11). The mineralization unit is not necessary to be arranged since the combined technology can leave over most of the beneficial minerals in drinking water.

The sulfate occupies the major part of bivalent salts in raw water. As shown in Figure 12, both combined technologies perform remarkable removal rate for sulfates which is over 95%. The high removal rate of sulfates is guaranteed as a result of the prominent ability of NF membrane for intercepting ions with higher relative molecular mass.

The odorous substances consist of organic matters and inorganic salts in contaminated water which emit irritant smells, including ammonia, H₂S, CS₂, DMDS (dimethyl disulfide), TMA (trimethylamine), and styrene (Arola et al. 2021). After being treated by the secondary and tertiary treatment technologies of sewage plant as well as the conventional treatment process of the drinking water plant, there are usually only a few odorous substances left behind. Both combined technologies are able to further intercept the rest of odorous substances, whose removal rate is between 50 and 70% as shown in Figure 13.

According to the conclusions drawn from the pilot testing, it is ensured that the NF system can function well with the pretreatment of both pressurized-pot MF and UF, consuming a similar amount of electricity and agent. The UF system as pretreatment technology can certainly provide raw water for NF of better conditions, but it does not make obvious improvement to the functioning of the NF system.

In previous engineering projects of drinking water advanced treatment, the UF system is usually applied as the front-end pretreatment unit for the NF system in order to reduce the turbidity and SDI value of sand-filtered water as well as intercept the possibly left sands when the sand filtration device is damaged, protecting the NF membrane from being seriously fouled or scratched (Fan et al. 2020).

The pilot testing proves that the pressurized-pot MF system is capable of performing similar functions when the quality of raw water is at a relatively high level. The pressurized-pot MF is featured by its lower price and easily being backflushed by water or chemical agencies, which cuts down both preliminary investment cost and following cost of device maintenance. In consequence, the pressurized-pot MF can replace the UF system as the pretreatment unit for the NF system when the raw water quality is at a relatively high level.

The system of the water-supply pump is equipped with two pump-groups, each of which is installed with five commonly used centrifugal pumps controlled by frequency converter. The whole system is connected to one main drain.

The MF system is equipped with two series, each of which is installed with five sets with 220 filters, several flowmeters and several manual butterfly valves in each set. Each series is connected to one main drain. The backflushing pump system appended to the MF system is equipped with two pump-groups, each of which is corresponding to one series and installed with one horizontal end-suction centrifugal pump for common use and another one as standby. The water used for backflushing is drawn from the produced-water pipe of NF system. Considering the possible impact of oil on filters, the air-backflushing system appended to the MF system applies two groups of screw air compressors.

The NF system is equipped with two series, each of which is installed with five membrane stacks with a certain quantity of membrane shells in each stack. Each membrane shell contains seven NF membrane components, and the quantity of membrane shell is decided by the designed membrane permeability. Ten booster pumps are appended to the NF system, each of which is corresponding to one membrane stack and controlled by the frequency controller. Each membrane stack is divided into three segments, where the intermediary booster pump controlled by the frequency controller is installed between the first and second segments. There is no recycling system for concentrated water correlated with the NF system. The massively flushing pump system appended to the NF system is equipped with two pump-groups, each of which is corresponding to one series and installed with one horizontal end-suction centrifugal pump for common use and another one as standby. The water used for flushing is drawn from the produced-water pipe of the NF system.

One automatic agent-dosing system is connected with the NF system, which is equipped with one set of reductant-dosing device including two tanks, two commonly used pumps and two standby pumps, and one set of antiscalant-dosing device including one tank, 10 commonly used pumps and one standby pump. One automatic chemical in-place cleaning system is also prepared for both MF system and NF system, which is constituted by one tank, one commonly used pump, one standby pump and one ultrafilter.

See Tables 6 and 7.

The NF system used in this project is mainly targeted at the removal of organic micropollutants in surface water, including TOC, UV254, pesticide, herbicide and precursory of THM. The content of TDS (total dissolved solid), sulfates, Ca²⁺, Mg²⁺, and alkalinity in micropolluted surface water is much lower compared with that in traditional brackish water (Orecki et al. 2004; Owusu-Agyeman et al. 2019). Therefore, the overall recovery rate of the NF system can reach 90%, while the concentration polarization at the end of NF membrane is effectively controlled, since the saturability of CaSO₄ or CaCO₃ at the side of concentrated water is relatively low and the Langelier Saturation Index (LSI) of concentrated water is lower than zero after being dosed with antiscalants.

The NF system used in this project is arranged in double layers, where the membrane stacks are on the facility layer and the valve brackets are on the pipe-gallery layer. This arrangement is able to ensure the stability and flexibility of NF functioning, since the facilities and matched valves can be operated or maintained separately without cross interference and the occupied area can also be effectively limited. Unattended operation can be applied to the whole system with guaranteed security and reliability with the assistance of building information modeling (BIM) system, which will be introduced in detail below.

The project creatively introduces the BIM system to NF-involved practical engineering. The BIM system is established on the base of three-dimensional digital technology, which models and digitalizes the process of construction. The statistics, word records, images and other visual data or information, which are collected during the planning period, designing period, construction period, operation period and maintenance period, are all digitalized in the BIM system. In this way, the mentioned periods are able to be visualized, informationized, and systematized (Arola et al. 2021).

After establishing the BIM system (Figure 14), a digital water plant conforming to the tangible water plant, which is able to function as a tool for asset and operation management during the prospective operation period, can be achieved simultaneously. The value of the BIM system in this project mainly manifests during the following periods:

• For the designing period, a three-dimensional model of prospective facilities can be designed and created through the BIM system, which can also be referred during the subsequent periods. The BIM system can also be used for simulating crash tests in order to discover the possible deficiencies and then improve the engineering quality.

• For the construction period, information about device arrival, installment, and debugging can all be uploaded to the BIM system for checking and controlling the construction schedule. The BIM system is also helpful to the installment process by locating the relevant devices.

• For the operation period, the three-dimensional model created through the BIM system can be correlated with the SCADA (Supervisory Control and Data Acquisition) of tangible water plant and then displays live operating statistics. The accomplished model assembles designing information, construction information, and operation information together, which completely conforms to the tangible water plant, and provides statistics foundation for the following operation management and asset management.

The evaluated electricity consumption and agent consumption of the pressurized-pot MF system and the NF system are shown in Tables 8 and 9.

According to the tables above, the electricity consumption of pressurized-pot MF is evaluated as 0.003 kWh/ton of water, while the electricity consumption of NF is 0.197 kWh/ton of water. The consumptions of NaClO, NaOH, citric acid, and NaHSO₃ for pressurized-pot MF are, respectively, 0.00029, 0.00026, 0.00029, and 0.00076 kg/ton of water, while the consumptions of NaHSO₃ and antiscalant for NF are, respectively, 0.0034 and 0.0017 kg/ton of water.

1. The NF system is able to function finely with high permeability and satisfactory quality of produced water when pressurized-pot MF is applied as its pretreatment technology.

2. The combined technology of NF with pressurized-pot MF performs over 90% removal rate for organic substances and humus (UV254) and 70–80% removal rate for DBP including chloroform, bromoform, and carbon tetrachloride. In addition, the combined technology also shows 70% minimum removal rate for pigments including Chl and Phy, 20–60% removal rate for water hardness, over 95% removal rate for sulfates which occupies the major part of bivalent salts, and 50–70% removal rate for odorous substances.

3. The recovery rate of the NF system applied in the project is able to reach 90%, and the predicted power consumption of pressurized-pot MF system and NF system is, respectively, 0.003 and 0.197 kWh/ton of water. The consumptions of NaClO, NaOH, citric acid and NaHSO₃ for pressurized-pot MF are, respectively, 0.00029, 0.00026, 0.00029 and 0.00076 kg/ton of water, while the consumptions of NaHSO₃ and antiscalant for NF are, respectively, 0.0034 and 0.0017 kg/ton of water.

All relevant data are included in the paper or its Supplementary Information.