Waste water treatment by micro air and ozone bubbles

Transcription

1 International Review of Mechanical Engineering (I.RE.M.E.), Vol. xx, n. x Waste water by micro air and bubbles Lau Hao Wen 1, Patththil Madhav Menon 2, Ng Kim Choon 3 Abstract In this study, a novel physical separation method using a flotation system that employs micron sized air and bubbles is designed and tested to replace conventional dissolved air flotation (DAF) or clarification of water by settling. The bubbles produced are fine and turn the water milky white. Micro air bubbles remove fine coagulated but suspended particles from the waste water, achieving a separation efficiency of up to 99% of oil and grease and suspended solids. The removal of fine suspended particles from the water reduced Total Suspended Solids (TSS), Bio-Oxygen demand (BOD) and Chemical-Oxygen Demand (COD) levels in the water, requiring less chemicals in downstream processes. The water was subsequently treated using high concentration and micron sized. Test results show that both micro air flotation and have achieved high reduction of oil and grease, TSS, BOD and COD. The water is then filtered using a micro-filtration membrane, and the water fouls the membrane. The waste water was replaced with PUB tap water, and micro air and bubbles are backwashed separately through the membrane. Significant increase in the permeate flow is observed, implying that the micro air and bubbles are effective for fouling removal. Keywords: Micro air bubbles, microfiltration, micro bubbles, waste water I. Introduction Wastewater is a potential water source if its quality satisfies the criteria for reuse or disposal to the environment. Flotation is a conventional method which is effective for removing oil & grease and clarification for removal of suspended solids from the waste water. The flotation process essentially draws upon the principles of generating micro bubbles of size ranging from μm with a mean of 60 μm [1]. They attach to the particles in the contact zone, floating the bubble-particle to the surface of the water in the separation zone to form a sludge layer, and the sludge is skimmed off from the surface subsequently. Treated water is then extracted, ready for disposal or secondary. In wastewater practice, Dissolved Air Flotation (DAF) is commonly known with dissolved air the source of micro bubbles. However, there are limitations in the existing DAF: the saturator tank and air compressor, used for dissolving air into the water, are inefficient due to low mass transfer efficiency of air into the water (hence, low gas to liquid ratio) and high energy consumption of air compressors. The bubble generator often gets clogged and hence high maintenance costs are incurred. In this paper, we describe a new approach that would replace the compressor and saturation tank with a compact and yet energy efficient two-phase pump and flash mixer for dissolving the air / gas into the water at high pressure and producing micro bubbles when the saturated water is depressurized via a valve. No bubble generator is required in this new test apparatus. Capital investments and operational costs are reduced significantly with the replacement of the air compressor and saturator [2]. There has been increasing use of for water systems due to its effectiveness in oxidation, decolouration, odour removal and disinfection purposes. The combined direct oxidation by and indirection oxidation by hydroxyl radicals [3] make an effective agent for water applications to improve water quality without any hazardous byproducts of oxidation. Therefore, is used as the secondary to complement flotation in the new test facility and it disinfects the water before passing it to the micropore membrane process. From literature, DAF method has successfully removed up to 90% of emulsified and dissolved oil with air flotation [4] whilst in another study, flotation alone has removed 98% of algae and suspended solids [5]. We believe that the presence of micro bubbles would have efficiency in excess of 98% - a stark improvement over the existing DAF. With, it could remove membrane fouling and either maintain or increase permeate flux yield in a micro filtration resistant membrane from the rated capacity. [6]. II. Materials and Methods The first part of the experiments focuses on using air Manuscript received January 2007, revised January 2007

2 flotation and to improve the water quality in terms of oil and grease, Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), and Chemical Oxygen Demand (COD). A coagulant is usually added to the water to break the oil emulsions as well as to charge neutralize the particles in the water. Pin flocs are formed in the water and with the aid of surface tension forces, micro-bubbles of air or are readily attached to the particles, giving the necessary buoyancy for particles to float. Initial trials are conducted to determine both the suitable coagulant type and their optimal concentrations that is most effective for the floc formation and improvement of water quality. A coagulant made up of highly charged cationic polymers and both organic and inorganic in nature is chosen for the actual testing. The chosen coagulant would have the best overall performance in reducing oil and grease, TSS, BOD and COD levels, removal of colour, and least volume of flocs produced. After air flotation, the treated water is diluted with PUB (Public Utilities Board) tap water in a ratio of 1:3 and put back into the main drum for. 3 g/hr and 180 g/nm 3 of are produced for 3 hours, and the treated water is circulated through the system. Separate water samples after air flotation and are retrieved and sent to an external laboratory for chemical analysis. After, the water is then pumped through the microfiltration membrane. This membrane is made of polyvinylidenefluoride (PVDF) substrate and of pore size 0.1 μm. Pressure is maintained at 6 bars, volume flow rate = 0.8 m 3 /h, velocity = m/s. Reynolds s Number = 10,941. The flow in the membrane is turbulent, required for permeate to be forced through the pores of the membrane. Permeate flow through the membrane is measured. After microfiltration, most of the turbidity is removed, and the water is clear. The membrane was fouled after use with treated water. An experiment is done with PUB tap water to investigate the effect of micro air and bubbles on the permeate flow rate. The waste water was replaced with PUB tap water, and micro bubbles were backwashed to remove the fouling in the membrane, giving an initial permeate flow of 0.12 m 3 /h. Micro air and bubbles were then introduced and removed after stipulated time periods, and the flow rate is recorded. Fig.1. Raw waste water Fig. 3. Connections of flash reactor, pump and PVC column Fig. 2. Trials with coagulants in the laboratory The waste water used for the experiment is oily waste water from the washing of ship tanker's hull. Air flotation is done by batches of 30 litres of water in the 4 diameter 5.2 m high PVC column. When enough micro air bubbles are produced, the pump is switched off, and around 15 minutes are required for the micro air bubbles to lift the flocs to the surface of the water. Once there are no more bubbles, the scum at the top and the treated water will be removed separately through the tap off points. The process gets repeated until all 150 litres of water have been treated by air flotation.

3 TABLE 1b WATER QUALITY AFTER AIR FLOTATION AND OZONE TREATMENT COD, BOD, Raw water 1, Discharge limit [7] Air flotation with coagulant Subsequent Fig. 4. PVC column and its tap-off points Membrane filtration * Results not multiplied by a factor of 3. Fig. 5. Micro filtration membrane III. Results and Analysis TABLE 1a WATER QUALITY AFTER AIR FLOATATION AND OZONE TREATMENT Oil and grease TSS, Raw water 1,098-3,640 - Discharge limit Air flotation with coagulant Subsequent Membrane filtration <5* <5* 99.9 The waste water used for this study is heavily polluted water from the washing of tanks of a crude tanker which contains oil and dissolved solids, as shown by the first row of data presented in TABLES 1a and 1b. The oil and grease in the water is measured at 1,098 mg/l, TSS is recorded at 3,640 mg/l as well as high in the COD. At this state, the waste water is not suitable for discharge into the drains or sewer as it cannot meet the local guidelines for sewer discharge. Detailed limits for water discharge (according to PUB, Singapore) are shown in the row 2 of TABLES 1a and 1b. Using a coagulant, at 75 ppm, & mixing it with a mechanical mixer, pin flocs are formed within the coagulant mixing tank which also acted as the waste water holding tank. The waste was then pumped through the system. For a period of 15 minutes, the entire batch of 30 l waste water is treated by flotation. Sludge formed at the top of the contact column, as shown in Fig. 6 which is then removed separately from the treated water. The quality of water is found to improve significantly: 99.0% for the oil and grease, 98.6% for TSS, 61.2% for COD and 11.7% for BOD. Ozone is introduced via the same bubble generatorpump system and further improvement in the water quality has been observed (results for all parameters except oil and grease are multiplied by 3): 99.5% for oil and grease, 99.4% for TSS, 84.8% for COD and 55.1% for BOD (see TABLES 1a and 1b). There is a stark reduction in the COD and BOD levels due to the oxidation effects of. Further is performed passing the water to a 0.1 μm filtration membrane. Even though it appears from Fig. 5 that the permeate is clear, chemical analysis showed that the water quality has worsened, caused possibly by the contamination in the receiving drums or in the plastic

4 bottles used for sending samples for testing to external laboratory Graph of Permeate flow rate (m3/h) against Time (min) for tap water P erm eate flo w rate(m 3/h) Micro air bubbles on Micro air bubbles off Micro bubbles on Micro bubbles off Time (min) Fig. 9. Graph of Permeate flow rate against time Raw water Fig. 6. The scum layer at the surface of the water after the injection of micro air bubbles. After air flotation Before treatmen t (1:3 dilution) After Fig. 7. A qualitative comparison of water Quality after respective s. After membrane The final is the use of the 0.1 μm PVDF membranes. It is observed from Fig. 8 that the permeate flow decreases rapidly from approximately its rated flow of 0.06 m 3 /h to m 3 /h in a time interval of 90 minutes. The fouled PVDF membrane is then cleaned with ozonated PUB tap water and the permeate flow was restored to 0.12 m 3 /h almost twice that of the rated flow rate, as shown by the starting flow rate of Fig. 6. With only tap water (no bubbles) pumped through the membrane, the permeate flow rate reduces and at 40 minutes, air micro-bubbles are introduced and this arrested the reduction in the permeate flow. However, the permeate flow rate continued to decrease after the micro air bubbles supply ceased after 100 minutes. At 160 minutes, micro bubbles are then introduced and the permeate flow increased sharply reaching 0.15 m 3 /h, 2.5 times the rated flow. The tests show the positive effects of micro air and bubbles on the permeate flow rates, due to the scoring effect of the micro bubbles against the membrane and hence removing the clogged materials in them. High oxidation potential of has also led to a sharp increase of permeate flow. Permeate flow (m3/h) Graph of Permate flow (m3/hr) against Time (min) for waste water (without micro bubbles) Time (min) Rated flow Waste water (without micro bubble) Fig. 8. Graph of Permeate flow rate against time for waste water (without micro bubbles) IV. Conclusion A new flotation system that employs micro air and bubbles has been tested successfully with heavily polluted water. The novel method is simple and hence it has substantial cost saving over the conventional approach such as the DAF. The micro bubbles are responsible for the high reduction of oil and grease, TSS, BOD and COD produced in the system apparatus without the use of chemicals except a mild dose of coagulant. Test results show that complements flotation and give rise to a large reduction in COD and BOD. Using it as secondary after air flotation would reduce operational costs and hence make water more cost effective. Tests with the micro filtration membrane have yielded positive results in fouling removing and

5 enhanced performance with micro bubbles. There exists immense potential in reducing maintenance cost of membrane filtration and has great ability for disinfection. References Chapters in Books: [1] Edzwald, J.K., Chapter 6: Dissolved air flotation in drinking water. (USA: Elsevier Ltd, pp ). Books: [2] David, H., Water Treatment Unit Processes, Physical and chemical. (CRC Press, 2006). [3] Lin, S.H. & Yeh, K.L., Looking to treat wastewater? Try. (Chem. Eng., 1993). Journal Papers: [4] Li, X. B., Liu, J.T., Wang, Y. T., Wang, C.Y., and Zhou, X. H., Separation of oil from wastewater by column flotation. J China Univ Miniing & Technol, Vol. 17, No. 4, pp , food industry for disinfection and shelf life extension. Recycling and water recovery and develop applications using microbubble system. He may be contacted at pmmenon@tech.com.sg. Professor Kim Choon NG obtained the BSc. (Hons) and PhD from the Strathclyde University (UK) in 1975 and 1980, respectively, and joined the ME Dept. of NUS in His research interests are in solar energy applications, chillers and heat pumps, adsorption cooling and desalination, co-generation systems analysis and testing, He has over two hundreds peer reviewed journals and conference publications. He is active in professional services, serving as associate editors in three international journals and a member of the examination sub-committee of the professional engineering board of Singapore. He is associate editors to 3 internal journals, namely the Heat Transfer Engineering, Journal of Process Mechanical Engineering and the International Review Journal in Mechanical Engineering. He has published 1 book and 7 patents and two business licenses for the patents. [5] Betzer, N., Argaman, Y., and Kott, Y., Effluent and algae recovery by -induced flotation. Water Research, Vol. 14, pp , [6] You, S. H., Tseng, D. H., Hsu, W. C., Effect and mechanism of ultrafiltration membrane fouling removal by ozonation. Desalination 202, pp , Lau Hao Wen 2 Pattathil Madhav Menon 3 Ng Kim Choon Authors information Lau Hao Wen is born in Singapore. He obtained his Bachelor degree in Mechanical Engineering at National University of Singapore in June He was involved in this study for his final year dissertation. Pattathil Madhav Menon graduated from the Regional Engineering College (REC), Calicut, India, now called National Institute of Technology (NIT) in Mechanical Engineering (B.Sc) in He later on went on to receive a Post Graduate Diploma in Environmental Engineering in 1999 from the National University of Singapore. He has extensive experience in the industry (27 years) having worked for Indian Sugar and General Engineering Corporation, India, Hindustan Hydraulics Pte Ltd, India, Indomag Steel technology Ltd., India, McAlister and Company Ltd, Singapore, Intraco Ltd., Singapore and OzoneCarbon Technologies Pte Ltd, Singapore. He has now set up his own technology company Ozone Tech (Singapore) Pte Ltd., Singapore to develop clean technologies in the field of environmental engineering using, carbon, UV and microbubble gas liquid mixing technology for air, water, waste water, drinking water, cooling water, gas scrubbing and for agri food applications. His areas of research interest are for HVAC and cold rooms, for the