ADVANCED TREATMENT OF RAINWATER USING METAL MEMBRANE COMBINED WITH OZONATION Ree Ho Kim 1*, Sangho Lee 1, Seog-Ku Kim 1, and Jong-Oh Kim 2 1 Korea Institute of Construction Technology, 2311 Deahwa-Dong, Ilsan-Gu, Goyang-Si, Gyeonggi-Do, 411-712, South Korea 2 Department of Civil Engineering, Kangnung National University, 12 Gangneung Deahangno, Gangneung-Si, Gangwon-Do, 21-72, South Korea Abstract This paper outlines a metal membrane system for removing contaminants from rainwater. The system consists of submerged metal membranes in a tank and an aerator or an ozone generator. Contaminated rainwater is introduced into the tank after an initial screening of solids and permeate is removed after passing through the membrane. Ozone bubbling as well as aeration in the feed side were applied to reduce membrane fouling, destruct organic pollutants, and inactivate microorganisms. The metal membrane appears to be suitable to be used with ozonation because of its excellent chemical stability. Experiments were performed to compare filtration characteristics of rainwater in storage tank, roof runoff, and roof garden runoff and to investigate the effect of ozone injection on the increase in transmembrane pressure and membrane fouling. * Corresponding author: Tel) 82-31-91-34; Fax) 82-31-91-291; Email) rhkim@kict.re.kr
Introduction Growing interests in society for saving water resources have led to different attempts to use or recycle rainwater [1-4]. However, rainwater in the urban area contains substantial amounts of contaminants including dust, particles, halogenated organics [5], heavy metal [6], ions [7, 8], pathogenic microorganisms [9], and endocrine disruptors [5] that cause problems in direct use or discharge. Therefore, a method to effectively remove contaminants from rainwater is critical for rainwater harvesting in urban area. A novel technology that has potential to clarify rainwater is metal membrane filtration [1]. Metal membrane technology hold great promise for rainwater treatment because it has unique advantages over polymeric microfilters [11]: Metal membrane is durable to high pressure up to 1 MPa, high temperature up to 35 o C, outer shock power, and chemical oxidation such as ozonation. Unlike polymeric microfilters, lifetime for metal membrane is long enough to minimize the maintenance cost. Metal membrane can be stored in dry forms, which makes it more attractive than polymeric membranes because rainwater filtration may operate intermittently. This work focuses on the development of a novel treatment system for contaminated rainwater from buildings using metal membranes. A submerged type of metal membrane system was investigated to remove contaminants effectively while maintaining high flux and low transmembrane pressure. In situ injection of ozone was also attempted to minimize membrane fouling and to improve filterability of rainwater [1]. Experiments Rainwater collected from different locations was used in this study as illustrated in Figure 1. These include supernatant from a storage tank; runoff from a roof of the building; and runoff from a roof garden. Table 1 compares the compositions of three rainwaters from a building. The supernatant from storage tank has the best water quality, while the runoff from roof garden contains substantial amounts of particulates (turbidity), ions (conductivity), nutrients (nitrogen and phosphate), and heavy metals. A schematic diagram of the submerged metal membrane system used in this study is shown in Figure 2. The cylindrical reactor had a working volume of 5 L and the feed solution was mixed by aeration at the bottom. A metal membrane module made of stainless steel was
immersed and suspended vertically in the reactor. Metal membrane with 1 m and 5 m were compared and the characteristics of membranes are summarized in Table 2. Permeate from the membrane module was pulled by a peristaltic pump. The flux was monitored by collecting permeate on a graduated cylinder. The transmembrane pressure was continuously measured using a pressure gauge. Total recycle mode, where both the retentate from the membrane filtration loop and permeate were recycled into the tank, was adopted to keep the reactor volume constant during the operation time. Ozone was generated using a laboratory ozone generator (Trigen Ozonia). Spectrophotometric methods of Hach [12] using DR-4 spectrophotometer were adapted to measure the ionic concentrations and turbidity. Conductivity, ph, were also measured and automatically corrected for temperature influence. Results and Discussions To determine the optimum operating conditions for membrane filtration of rainwater, transmembrane pressure across the membrane was monitored with a stepwise increase of permeate flux. Fig.8 shows the transmembrane pressure versus time behavior during the filtration of the tank supernatant using the 5 m and 1 m membranes, respectively. In both cases, the transmembrane pressures changed similarly from 5 L/m 2 -hr to 3 L/m 2 -hr. Figure 4 compares the filtration characteristics of three rainwaters using metal membranes. Transmembrane pressure did not increase for supernatant filtration for both membranes. However, a rapid increase in transmembrane pressure with filtration time was observed when filtering runoff rainwaters using the 1 m membrane while no increase in transmembrane pressure was shown for the 5 m membrane. This is because most of the foulants that block the 1 m membrane are small enough to pass through the 5 m membrane. During the filtration of runoff waters, the permeate from 5 m membrane has same water quality as the input water. This suggests that the 5 m membrane is not suitable for clarifying the runoff rainwater. The transmembrane pressure with ozone bubbling was compared with that with aeration (without ozone) in Figure 5. The increasing rate of transmembrane pressure with ozone bubbling was much smaller than that without ozone bubbling. This is probably because ozone
in water destruct organic matters and destabilize colloids. These effects have occasionally been referred to as microflocculation/ozone-induced particle destabilization/ coagulation effects of ozone in water treatment. Several proposed hypotheses (metal humates complex, particle aggregation via bridging reaction, disrupted stabilizing organic coatings on particles, in-site production of coagulant, etc.) were well summarized by ref [13]. Similar reduction in ultrafiltration membrane fouling with pre-ozonation due to ozone-induced particle destabilization was reported by other researchers [14]. Conclusion A metal membrane system combined with aeration or ozone injection was investigated as a novel method to remove contaminants from rainwater. The following conclusions can be drawn: (1) Metal membranes appear to be efficient to treat rainwater from storage tank. (2) The membrane with the pore size of 5 m appears to be ineffective to clarify the runoffs from roof and roof garden because of its large pore size. (3) Ozone injection significantly reduces the increase in transmembrane pressure due to membrane fouling. References 1. Kim, R.H. Rainwater utilization for urban establishment of new paradigm,, F 41-44. in A Joint Conference with Korea Society of Water and Wastewater and Korea Society on Water Quality. 22. 2. Kim, R.H., Utilization of ground water and rainwater in urban area. Geoenvironment, 21: p. 217-241. 3. Kim, R.H., Development trend of rainwater utilization. Construction Technology Review. 211: p. 23-28. 4. Kim, R.H., Rainwater utilization and functional changes in building roof. Construction Technology Review. 22: p. 13-19. 5. Dorfler, U. and I. Scheunert, S-Triazine herbicides in rainwater with special reference
to the situation in germany. Chemosphere, 1997. 35(1/2): p. 77-85. 6. Reimann, C., et al., Rainwater composition in eight arctic catchment in northern europe (Finland, Norway, and Russia). Atmospheric Environment, 1997. 31(2): p. 159-17. 7. Sanusi, A., et al., Chemical composition of rainwater in eastern France. Atmospheric Environment, 1996. 3(1): p. 59-71. 8. Sequeira, R. and C.C. Lai, Small-scale spatial variability in the representative ionic composition of rainwater within urban Hong Kong. Atmospheric Environment, 1998. 32(2): p. 133-144. 9. Albrechtsen, H.-J., Microbiological investigations of rainwater and graywater collected for toilet flushing. Water Science and Technology, 22. 46(6-7): p. 311-316. 1. Kim, R.-H., et al., Advanced treatment apparatus and method for rainwater using metal membrane combined with ozonation, in 1-23-3388. 23: Korea. 11. KIM, J.-O. and I. SOMIYA, Innovative Fouling Control by Intermittent Back-ozonation in Metal Membrane Micro Filtration System. 22. 12. Hach, Hach Water Analysis Handbook. 2nd ed. 1992, Colorado, USA: Hach Company. 13. Rekhow, D.A., P.C. Singer, and R.R. Trusell. Ozone as a Coagulant Aid. in Annual AWWA Conference Proceedings. 1986. Denver, Colorado: AWWA. 14. Hyung, H., et al., Effect of Preozonation on Flux and Water Quality in Ozonation- Ultrafiltration Hybrid System for Water Treatment. Ozone Science and Technology, 2. 22: p. 637-652.
Table 1. Composition of Rainwaters in Different Sources Supernatant Runoff from from Storage Roof Tank Runoff from Roof Garden Turbidity (FAU) 9 137.5 6 ph 7.79 6.9 7.76 Conductivity (S/cm) 97.1 147 632 Total phosphate (mg/l).45.7 4 Total Nitrogen (mg/l) 1 2 9 Iron (mg/l).34.38 3.2 Copper (mg/l).2.49 1.7 Zinc (mg/l).52.39.46 Table 2. Parameters for the Submerged Metal Membrane Systems Parameter Outer Radius (r o ) Nominal Pore Radius (r i ) Filter Area Length (L) Value.7 m 1 m, 5 m.222 m Membrane Area (A m ) 9.76 1-3 m 2 Membrane Resistance (R m ) 5.5 1 8 m -1
Building Green Roof Runoff from Green Roof Roof Runoff Utilization Rainwater storage tank Figure 1. Three types of rainwater from buildings: Roof runoff, runoff from green roof, and rainwater in storage tank. Pressure Gauge Peristaltic Pump Graduated Cylinder Metal Membrane Feed Tank Air or Ozone Air Blower or Ozone Generator Figure 2. Schematics of metal membrane filtration system.
.2 :2 35 3 Transmembrane Pressure (atm).15.1.5 Flux Pressure 25 2 15 1 Flux (L/m2-hr) 5 5 1 15 2 25 3 35 Filtration time (min) (a) 5 m Membrane.2 :2 35 3 Transmembrane Pressure (atm).15.1.5 Flux Pressure 25 2 15 1 Flux (L/m2-hr) 5 5 1 15 2 25 3 35 Filtration time (min) (b) 1 m Membrane Figure 3. Dependence of transmembrane pressure on permeate flux for metal membranes. Conditions: aeration rate = 5 ml/min
Transmembrane Pressure (atm).2.15.1.5 :2 Roof garden runoff Roof runoff Supernatant in storage tank 2 4 6 8 Filtration time (min) (a) 5 m Membrane.2 :2 Transmembrane Pressure (atm).15.1.5 Roof garden Runoff Roof runoff Supernatant in storage tank 1 2 3 4 5 6 7 Filtration time (min) (b) 1 m Membrane Figure 4. Effect of input water characteristics on transmembrane pressure for metal membranes. Conditions: flux = 2 L/m 2 -hr; aeration rate = 5 ml/min
Transmembrane Pressure (atm).2.15.1.5 without Ozonation :2 with Ozonation 2 4 6 8 Filtration time (min) Figure 5. Effect of ozonation on membrane filtration characteristics for roof garden runoff. Conditions: membrane pore size, 1 m; flux, 2 L/m 2 -hr; aeration rate, 5 ml/min, ozone dose, 5 mg/hr.