The Application of Flat Ultra-Filtration for Rainwater Reuse

Transcription

1 The Application of Flat Ultra-Filtration for Rainwater Reuse C. H. Liaw*, J. Kuo**, C.Y. Shih***, C. H. Wang*** * National Taiwan Ocean University, 2 Pei-Ning Road,Keelung 20224, Taiwan **New Century Membrane Co., Ltd. No. 110, Sec. 2, Gansu Rd., Situn District, Taichung City 40743, Taiwan ***Capital Engineering Co., Ltd, 8F, No. 20, Sec. 1, Hoping W. Rd. Taipei City 100, Taiwan Abstract Precipitation is a major source of valuable water resource on the Earth. The distribution, depth, and frequencies of precipitation are extraordinary due to the global climate change. Therefore, the level of application of precipitation depends on the functions of collecting and treatment systems. The filtering unit has been one of most important component of the water treatment procedures. However, filtering along is not enough to meet the standard. The latest technical features of membrane were applied in this research. The flat UF membrane with pore diameter of 30nm is used. The SS in the precipitation was removed and the total coliform was reduced from more than 16 MPN/100 ml to Not Detected (N.D.), and the application of recycled rainwater is broader. The ultimate target is to reach fully application of precipitation and to provide a convenient and cheap scheme of precipitation recycling. Keywords Precipitation recycling; filtration; UF membrane Introduction As the population of the world and the degree of urbanization continue to increase, the demand of water, especially in the cities, has gone up significantly. With all the rivers, lakes, and aquifers almost completely developed to meet these demands, peoples started to seek more unconventional sources of water, such as ocean, rain, and even reclaimed water. Rainwater recycling is a relatively small scale system to collect rainwater to be used on site. It reduces the need for the tap water and can also be pretty economical if the area has more than 254 mm of precipitation per year (Reynolds, 1990). Rooftop precipitation recycling has been used in many areas to supplement water use for decades.

2 An initial study had been done to examine the characteristic of the recycled rain. Rainwater was collected on the rooftop of the National Taipei University of Technology on three separate raining days. The precipitation was collected at the first few minutes of the raining event to simulate the worst case scenario. The samples were characterized as followed: Table 1. Characteristic of precipitation in Taipei Sample Total Coliform (CFU/100mL) S.S (mg/l) Turbidity (NTU) ph Conductivity (µs/cm) The samples were somewhat turbid, and contained some coliforms. The low ph might due to the dissolved CO 2, NO x, and SO x. Precipitation recycled from roofs can contain animal and bird feces, mosses and lichens, windblown dust, particulates from urban pollution, biocides, and inorganic ions from (Ca +, Mg 2+, Na +, K +, Cl -, SO 2-4 ), and dissolved gases (CO 2, NO x, SO x ). The concentration of these species and contaminants can be really high in the first rain after a dry spell Due to the above mentioned characteristics, precipitation is often not considered suitable for drinking without treatment. Untreated precipitation is usually used in a non-contact manner such as gardening, car washing, and flushing the toilet in the household level. Large scale precipitation recycling in urban areas can provide water for fire hydrants, increase soil moisture levels for urban greenery, to increase the groundwater table through artificial recharge, and to mitigate urban flooding. This research is aimed to investigate the quality of precipitation after being treated by the ultrafiltration membrane and see if the treated water can be used in other areas.. Methodology The UF membrane is effective in removing flocs, particles, proteins, microbial cells (e.g. Giardia and Cryptosporidium) that are larger than 0.05 µm, most organic matters, and hardness. In this experiment, a membrane sheet is tested in the batch mode in order to understand how the pilot plant will perform and what results will be expected based on the flux and rejection of solutes as well as understand the chemical cleaning

3 effect on the fouled membrane. Laboratory Setup and Equipment The bench-top US OPTISEP-CL simulation system; dimensions of the test sheet are 15 cm by 7.6 cm and the chamber are 14.5 cm by 6.5 cm. The flow rate is maintained at 10 ml/min, the flux is 37 GFD. Experimental Procedures A. The UF and accessories were used for conducting the experiment. B. The storage tank was filled with tested solution; the pressure difference across the membrane was kept constant to operate the membrane system at a constant flux. The filtrate was collected and its flow rate was recorded for determination of concentrations of dissolved substance and the filtrate volume. Figure 1. C. The removal efficiency or rejection is calculated as: R=1-C f /C o Where: C o = Initial concentration of solute in the storage tank. C f = Final concentration of solute in the storage tank. The UF was first tested for its flux; it was then subject to the high-concentration test to speed up the membrane fouling. The fouled UF membrane was washed and cleaned with alkaline solution. The rejection of conductivity and its operating performance were measured to be referenced for future operation of the pilot-scale plant. Results and Discussion The water collected is the first flush of the rain, therefore the quality of the precipitation before filtration has fairly high quantity of suspended solid (measured in turbidity), and coliforms, shown in Table 2. Table 2. The concentration of turbidity and coliforms before and after the treatment. Test Item Unit Before Filtration After Filtration

4 Turbidity NTU 4.4 <0.5 Total colony cfu/ml 700 N.D. Total coliform MPN/100ml >16 N.D. The turbidity in the test water is 4.4 NTU which is not suitable for a lot of applications. Since the test water is the first flush, the high level of suspended solids is most likely been from the roof and the particles in the atmosphere. After the test solution is processed by the UF, the turbidity in the permeate is reduced to less than 0.5 NTU. The total number of coliform before the treatment was 16 MPN/100ml. The bacteria were also brought down from the rooftop. Since the pore size of the UF membrane is 0.03 µm, which is smaller than the size of most bacteria, no coliform was detected in the permeate. On the other hand, the traditional sand filtration can still allows bacteria to pass through. Since no coliform was detected, no disinfection is needed if the treated water is going to be used immediately. If the water is going to be stored, small amount of HOCl can be added because the disinfecting ability of the residual chlorine. In this experiment, we chose to use the flat UF membrane because it has several advantages. First of all, the flat membrane are less likely to be damaged comparing to the hollow fiber membranes. The streamline of the influent is parallel to the membrane. As the suspended solids move through the hollow fibers, their Brownian Motions may damage or even cut through the wall of the fiber. On the other hand the streamline of the influent is perpendicular to the membrane in the flat membrane modules. The membrane is less likely to be damaged and lasts longer. The characteristic of the perpendicular streamline also allow the suspended solids to form a layer of cake on the flat UF membrane. That layer of cake can provide extra filter ability without blocking the flow of water. It also serves as a layer of protection for the membrane. However, as the cake builds up, more pressure is required to push the water though. Therefore, a periodic backwash is necessary. Comparing to the traditional sand filtration, the UF filtration has much less footprint, which allow much more modules to be installed and treats more rainwater in the same mount of area. Conclusions The recycled precipitation contained high amount of S.S, turbidity, and total coliforms initially. After being treated by the UF membrane, the turbidity was reduced from 4.4 to 1 NTU and no bacteria was detected. The cleaning ability of the UF

5 membrane is far superior to the traditional sand filtration or vortex filters. The treated water should be clean enough to be used in wider applications other than gardening and flushing the toilet. To add some residual disinfection ability during storage, a small amount of HOCl can be added. The choice of the flat UF membrane has several advantages. The flat membrane has longer lifetime than the hollow fiber membranes. The cake on top of the flat membrane can provide extra cleaning ability. The footprint of flat UF membrane is mush smaller than the sand filtration; more water can be treated with the given area. The result of this experiment can be used in the future study of pilot plant or commercial models of precipitation recycling systems. Reference Reynolds M. (1990). Earthship Volume 2: Systems and Components. Solar Survival Press. USA. Liaw, C.H et al. (2004~2005). Rainwater Harvesting Project in Offshore Islands. Taiwan Rainwater Catchment Systems Association, Water Resources Agency. Taiwan. Liaw, C.H et al. (2007). Research in Rainwater Harvesting Goals and Equipment Standards. Taiwan Rainwater Catchment Systems Association, Water Resources Agency. Taiwan. Liaw, C.H et al. (2008). Water Quality Analysis for Rooftop Rainwater and Performance Research and Development. Taiwan Rainwater Catchment Systems Association, Water Resources Agency. Taiwan. Liaw, C.H. and Y.L. Tsai, Optimum Storage Volume of Rooftop Rainwater Harvesting Systems for Domestic Use, Journal of American Water Resources Association (printing), Liaw, C. H., S.H. Chu, Y.L. Tsai, and W.Y. Chen, Development of Urban Rainwater Cistern Systems Technology, Engineering Science & Technology Bulletin, NSC 26:75-78, 1997.