Using ultrafiltration/microfiltration membranes coupled with ultraviolet or reverse osmosis for water reuse in agricultural and industrial purposes

Abstract

Due to the water crisis that affects every part of the globe, the concept of circular economy is increasingly being applied through the act of water saving and reuse. In order to safely reuse water, wastewater should be sufficiently treated to yield good quality effluents that are compatible with different reuse purposes. Reclaimed wastewater decreases the pressure on natural water sources by acting as an alternative water source. Moreover, pollution in water resources is mitigated because the treated wastewater is no longer discharged into water bodies. Due to the fact that conventional treatment technologies are not sufficient to produce high quality effluents intended for reuse, advanced treatment technologies are being applied. Such technologies also include membrane systems. Membrane treatment is a physical process that uses semi-permeable membranes to produce high quality effluents; moreover, membrane systems require little space, are relatively energy efficient to conventional methods and can be controlled with automation. High pressure membranes such as reverse osmosis (RO) produce higher quality effluents than low pressure membranes such as microfiltration (MF) and ultrafiltration (UF) by means of having smaller pore size. Ultraviolet (UV) treatment is widely used for wastewater disinfection and its use is becoming more widespread due to its effectiveness, low cost, short contact time and not forming hazardous by-product formation. In this study, biologically treated municipal wastewater was further treated with MF and UF. Next, the effluents of these membrane systems were subjected to further treatment, through UV disinfection and RO. The effluent of the UV system was to be compared to agricultural reuse standards, while that of the RO system was to be compared to industrial reuse standards. In this case, the low pressure membranes, MF and UF, were served as pre-treatment prior to RO with the purpose of minimizing fouling. The main objectives of this study include offering reclaimed wastewater as an alternative water source, obtaining reclaimed wastewater suitable for agricultural reuse by subjecting biologically treated wastewater to MF/UF + UV, producing reclaimed wastewater suitable for industrial reuse by subjecting biologically treated wastewater to MF/UF + RO, and assessing the quality of the RO and UV effluents and comparing them to reuse standards. A laboratory-scale MF system with hollow fiber polysulfone (PSf) membranes with a pore size of 0.8 µm and membrane area of 0.032 m2 was operated. As for the UF system, hollow fiber polyvinylidene fluoride (PVDF) membranes with a pore size of 0.06 µm and membrane area of 0.032 m2 was used. The operating flux of both MF and UF was 20 L/m2. h. Moreover, the UV system consisted of a UV-C lamp with a UV light intensity of 50 mW/cm2 and UV dose of 30,000 mWh/cm2; the disinfection period was 1.5 minutes. Lastly, the RO system contained flat sheet polyamide thin film composite membranes with an area of 0.014 m2 and molecular weight cut-off (MWCO) of 100-200 Da. The pressure was set at 15 bar and the recovery rate was 25%. Samples were analyzed in order to compare the effluent quality to national and international water reuse standards. Within the scope of the study, Turkey's Technical Procedures Notice on Wastewater Treatment Facilities (AATTUT) "Annex 7 - Reuse Criteria for Treated Wastewater as Irrigation Water" (Turkey Ministry of Environment and Forestry, 2010) and "Technical Procedures on Wastewater Treatment Facilities" (Turkey Ministry of Environment, Urbanization and Climate Change, 2022), the annexes of United States Environmental Protection Agency (EPA) "Water Reuse Guide" (EPA, 2012) and the European Union (EU) "Minimum Requirements for Water Reuse Regulation" (European Commission, 2020) were considered. Experimental analyses was carried out in order to monitor the treatment performances of the laboratory scale systems. At the end of the study, scanning electron microscope (SEM) analysis for the used membranes was performed to reveal the fouling properties of the membranes and the factors causing fouling. Data was processed at the end of the study and treatment performance was evaluated and then compared to standards. MF and UF yielded effluents with TSS values below 10 mg/L. UF treatment achieved complete pathogen removal while MF only achieved an average log removal efficiency of 3.7, 1.04, 1.0 for total coliform, fecal coliform, and E. coli, respectively. Moreover, the UF system had better overall treatment performance than the MF system. No pathogens were detected in the effluents of MF-UV and UF-UV. The average BOD5 in UF-UV effluent was 15.3 mg/L while it was 18 mg/L in MF-UV effluent. Both MF-RO and UF-RO effluents achieved complete pathogen removal. In addition, there was a decrease in pH after RO treatment, effluent had an average pH of 6.59, while the influent pH was around 7.8. Moreover, unlike UV, there was a significant decrease in total dissolved solids (TDS) and conductivity (>90% removal efficiency) in RO treatment, and around 94% and 96% removal of total nitrogen (TN) and total phosphorus (TP) were achieved, respectively. Chemical oxygen demand (COD) and biological oxygen demand (BOD5) of RO effluent were <20 mg/L and <10 mg/L, respectively. The RO system was also capable of removing most ions and metals that were analyzed. Moreover, using MF and UF as pre-treatment for RO ensured minimal fouling of RO membranes and no significant increase in TMP was observed with a drastic reduction in flux. Flux in MF-RO varied from 5-18 L/m2.hr and varied from 8-23 L/m2.hr in UF-RO during the RO operation. When compared to agricultural standards, BOD5 values of the UV effluent were within EPA and AATTUT standards but not within EU standards (<10 mg/L). Moreover, TDS and conductivity did not meet any of the standards and thus this indicates that UV was not sufficient to yield effluents that are high enough in quality for agricultural reuse in terms of these two parameters. Alternatively, high pressure membrane treatment could be applied instead of UV in the case of agricultural reuse. For instance, in this study, NF was applied after MF and UF as an alternative to UV. Following the NF treatment, the results were then compared to agricultural reuse standards. NF system yielded effluents with a BOD5 concentration of less than 10 mg/L and also all standards were met for agricultural reuse. In addition, TDS and conductivity were reduced by NF treatment to values that are within irrigation water standards. However, Na+ and Cl- ions can be applied for slight to moderate restrictions for drip irrigation; as for surface irrigations, the effluent values are within severe restrictions. In the study, the NF effluent was found to be more suitable for the irrigation of crops with higher salt tolerance such as asparagus. This is because highly salt-tolerant crops are more likely to achieve full yield under slight to moderate restrictions for irrigation. All the other parameters are within standards making NF treatment suitable for agricultural reuse. On the other hand, MF-RO and UF-RO effluents were compared to industrial reuse standards. Complete pathogen removal was achieved by RO. Only Mn and Fe from the heavy metals analyzed had standards for cooling water and the values achieved by this study were within the range. Additional parameters such as hardness, alkalinity and silica were compared in the case of industrial reuse studies. Hardness and alkalinity were lower when RO was pretreated with UF instead of MF with an average UF-RO value of 4.0 CaCO3 mg/L and 4.5 CaCO3 mg/L, respectively. In comparison to UV and NF effluents, RO achieved much higher ion, TDS and conductivity removal. Overall, it was found that the RO effluent met the criteria to be used for cooling water.