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In Japan, Kasumigaura Lake, Biawako Lake and many other closed water bodies are very serious in water quality. We attribute the water pollution to that the suspended solids flow into closed water body from surrounding environment. Floatation and supply of oxygen by micro-bubble, mild flow up of water by screw pump and biological filtration/oxidation were applied. These technologies were combined in developing a novel system for the purification of lake water. HRT was 10 minute, and 40 ton/d of lake water was treated. Thirsty percent of suspended solids (SS), equivalent to 3.53 mg-SS/L-lake water, was removed. COD decreased from 4.5 mg/L to 2.5 mg/L. The transparency was improved 36%. This system is in self-completion.
Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 233 - Improvement of Water Quality by Removal of Suspended Solids and High-speed Biological Filtration in Closed Water Body Masayuki Fukagawa*, Tsuyoshi IMAI**, Takashi Tanaka*** and Tetsuhiko Fujisato**** * Ube National College of Technology, Tokiwadai 2-14-2, Ube, Yamaguchi 755-8555, Japan ** Department of Civil and Environmental Engineering, Yamaguchi University Tokiwadai 2-16-1, Ube, Yamaguchi 755-8611, Japan ***Ube Industry Co. Ltd. Tsumazaki Kaisaku Ube, Yamaguchi 759-0925, Japan **** Bubble Tank Co. Ltd. Fujimagari Ube, Yamaguchi 755-0057, Japan Abstract In Japan, Kasumigaura Lake, Biawako Lake and many other closed water bodies are very serious in water quality. We attribute the water pollution to that the suspended solids flow into closed water body from surrounding environment. Floatation and supply of oxygen by micro-bubble, mild flow up of water by screw pump and biological filtration/oxidation were applied. These technologies were combined in developing a novel system for the purification of lake water. HRT was 10 minute, and 40 ton/d of lake water was treated. Thirsty percent of suspended solids (SS), equivalent to 3.53 mg-SS/L-lake water, was removed. COD decreased from 4.5 mg/L to 2.5 mg/L. The transparency was improved 36%. This system is in self-completion. Keywords Micro-bubble; Biological filtration; Suspended solids; Water quality; Closed water bodies Introduction Pollution of lakes and marshes is very serious not only in Japan [1] but also in the world [2,3]. Now various methods are being tried, but hardly successful. For example, the absorption of nutritional salts by plants [4], keeping aerobic condition by aeration [5] and other processes. In current study, we propose a new system that has following characteristics. First, it can catch various particles that are regarded as pollutants; second, organic particles caught can be degraded biologically in this system; third, it can remove nutrients; fourth, this system can be kept at high speed enough for enormous treatment of water; and last, it is able to remove sludge effectively. The system in this study should satisfy these requirements shown as Figure 1. Tokiwako Lake, the typical closed water body, is the theme park in Ube city. There are many swans and sculptures. The area is 0.83 km 2 , water storage is 3.7 million m 3 , and HRT is 280 day. Therefore, water pollution easily occurs in Tokiwako Lake. Feed residues and excreta of swans are pollutant sources. Organic sludge is settled over the bottom of lake, forming a non-oxygen condition. The experiment was carried out for the water quality improvement in Tokiwako Lake. This paper is an interim report concerning the removal of suspended particles and improvement of the transparency and COD of lake water. Materials and method Pre-experiment was carried out with about 100 ton of water tank for the design of the pilot plant. The outline of the equipment is shown in Figure 2. It was floated and fixed in the tank. Lake water is drawn up along with Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 234 - micro-bubble by screw pump in the inner tube. The size of micro-bubble is estimated to be less than 40×10 -6 m. The overflow water is introduced to the middle tank for biological filtration. And then, lighter particles are floated and heavier particles are settled at the bottom of this tank. Lacelike plastic is used as biological medium. Water flows down and out of the lower part in this middle tank. The upflow water in the outer tank overflows and is discharged. Rotation number of screw, number of micro-bubble generators and reflux ratio to the inner tube from the middle tank could be given in this pre-experiment. upflow third tank upflow downflow first tank second tank inflow micro-bubble pumping floatation dissolution of oxygen floatation bio-degradation sedimentation arrangements of flow outflow Figure 1 Schematic diagram for improvement of water quality by removal of SS and biological filtration in closed water bodies Ｍ a b c a: screw motor b: biological carrier c: micro-bubble generator d: deposit d Figure 2 Schematic diagram of experimental equipment The actual experiment was carried out using a pilot plant. The height of this plant is 1.5 m and diameter is 0.5 m. A 1000-ton of closed water body and a raft of 20×20 m 2 were prepared at Tokiwako Lake. The maximum water depth of experimental area is 3.4 m, minimum is 1.4 m, and average is 2.3 m. Water depth changes around 0.4 m (up and down) by various reasons. This plant was floated and fixed at the center of the raft. Lake water was Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 235 - introduced at 1.5 m depth and was treated at 40 ton/d, HRT was 10 minute and reflux ratio was 1.0. Air was supplied at 43.2 L/d as micro-bubbles at the inlet of the equipment. All these bubbles were dissolved in this plant. Experiment was carried out from 29 August 2002 to 17 October 2002. During this period, one typhoon, one heavy rain and one windy day were met. Little influence of these disturbances was found. Some sampling points were selected. They are the outlet of plant, the outside of experimental area, 5 points of horizontal level and 0.5, 1.0, 1.5 m of vertical distance (water depth) at each horizontal point, respectively. Turbidity, transparency, SS, VSS, COD, TOC, temperature, ORP, and pH were measured in each point by standard methods [6]. Pore size is 1µm. In this paper, SS, transparency and COD at the outlet of plant and the outside of experimental area will be reported and discussed. Experimental results Variation of SS concentration Variations of SS concentration are shown in Figure 3. SS concentration at the outlet of equipment was compared with SS concentration at the outside of experimental area. Figure 3 shows that SS was considerably reduced. Slow increase in SS concentration at the outlet of the equipment is attributed to the washout of bio-film. On 50th day since the experiment started, SS concentration at the outside of experimental area decreased suddenly because water flew into Tokiwako Lake through the irrigation canal. 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 35 40 45 50 Time[d] SS Conc.[mg/L] outlet of equipment out of experimental area(depth：1.5m） Figure 3 Variation of SS concentration Water quality was improved about 30% on SS concentration on 50th day. The removal of SS is shown in Figure 4. The attached bio-film was 0.71 kg, while the sludge settled on the bottom of the middle tank was 2.83 kg. SS was removed 3.54 kg from 1000 ton of water every 50 days, about 30 % of involved SS in the experimental water area. Thus both the decrease in SS concentration and removal of SS are very well similar. Organic content in attached bio-film was about 20 w%, and that in the sludge settled on the bottom was about 13 w%. There were many small animals around the lacelike biological medium, and a certain ecosystem might have been made. Since T-P in the sludge was very little, it could not be removed as the sludge. typhoon heavy rain strong wind Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 236 - 2828 710 0 500 1000 1500 2000 2500 3000 Recovery SS[g] deposit attachment Figure 4 Removal SS in experimental period Variation of transparency Transparency is a very important factor in this city park with water’s edge. Variations of transparency at the outlet of equipment and the outside of experimental area are shown in Figure 5. Transparency was measured since 13th day after the experiment started. Transparency on 20th day became bad within and beyond the experimental area due to heavy rain. Transparency was improved about 40% on 50th day. 10 20 30 40 50 60 70 0 5 10 15 20 25 30 35 40 45 50 Time[d] Transparency[cm] outlet of equipment out of experimental area Figure 5 Variation of Transparency Variation of COD concentration Variation of COD concentrations at the outlet of equipment and the outside of experimental area are shown in Figure 6. COD concentration was improved remarkably from 15th day and reached 2.6 mg/L on 35th day. The growth of bio-film was observed for one week after the experiment started. After that, COD could be satisfactorily degraded biologically. Although the COD concentration of water in Tokiwako Lake is usually about 7∼8 mg/L, it was about 5 mg/L in this experiment. typhoon heavy rain strong wind Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 237 - 2 3 4 5 6 0 5 10 15 20 25 30 35 40 45 50 Time[d] COD Concent.[mg/L] outlet of equipment out of experimental area Figure 6 Variation of COD concentration Discussion As this experiment was carried out in the field, some disturbances such as typhoon, heavy rain and windy day were met. These disturbances had influence on temperature, SS, turbidity and the inflow of water, but they had hardly affected the factors except transparency. Two roles of micro-bubbles are floating suspended particles and supplying of oxygen to bio-filter. As the micro-bubbles can ascend by Stokes’ equation or Allen’s equation, they can supply oxygen to bio-filter. DO was kept in saturation. The role of screw pump can keep very small animals alive. Since the ecosystem is made around biological medium, organic particles could be degraded biologically at high speed. HRT was set up at 10 minutes in this experiment, but the optimum time has not been determined. Sludge should be removed to prevent from the outflow of sludge, and the bio-filter was cleaned once two week. SS in Tokiwako Lake is almost inorganics like clay. Therefore the contents of organics and T-P were very low in treated sludge. Other processes should be considered for T-P removal. Transparency was improved largely. It is generally considered related to turbidity, but the results of experiment showed that it was related to SS concentration. The transparency of Tokiwako Lake was considered that it might depend on SS over 10 -6 m rather than particles under 10 -6 m. As the bio-filter grew, COD concentration decreased. Particles were caught by bio-filter and were taken in the small animals, and thus it is thought that high-speed biodegradation might be carried out. It is thought that the COD value obtained in this experiment, 2.6 mg/L, is probably the limit in the biological treatment. Attaching particles to bio-filter promotes the improvement of transparency, since the bio-filter may coagulate particles and easily remove SS. typhoon heavy rain strong wind Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 238 - Conclusion The experiment on improving water quality was carried out by removal of SS and biological filtration in the closed water body, Tokiwako Lake in Ube city. The developed system is useful for improvement of water quality in closed water body. Following results could be obtained from the experiment. 1. Micro-bubble is very useful for floatation and supplying oxygen. 2. Screw pump is very useful for keeping alive very small animals, and organics may be degrades biologically at high speed by the ecosystem formed around the biological medium. HRT is 10 minute although the optimum HRT is not determined yet. 3. It was found in Tokiwako Lake that the transparency did not depend on turbidity but SS. The characteristics of suspended particles are thought related to that. 4. Forming an ecosystem may reduce COD. The proposed system is different from the usual concept, because pollutants in the water body are degraded, separated and removed directly on site. This system has not yet been completed. The process on phosphate fixation will be added in the improved system, and the removal of nitrogen should be considered, too. References [1] Department of Water Environment, Ministry of the Environment, Japan. July 2002. Outline of Administration of Water Environment P20. (In Japanese) [2] M. Soto-Jiménez, F. Páez-Osuna and A. C. Ruiz-Fernández. Geochemical evidences of the anthropogenic alteration of trace metal composition of the sediments of Chiricahueto marsh (SE Gulf of California), Environmental Pollution, 125(3), 2003, 423-432 [3] S. S. Lau and L. M. Chu. The significance of sediment contamination in a coastal wetland, Hong Kong, China, Water Research, 34(2), 2000, 379-386. [4] C. N. Mulligan, R. N. Yong and B. F. Gibbs. Remediation technologies for metal-contaminated soils and groundwater: an evaluation, Engineering Geology, 60(1-4), 193-207. [5] Robert H. Kadlec and Robert L. Knight. Treatment Wetlands. CRC Lewis Publishers. 1996. [6] APHA, AWWA and WPCF: Standard methods for the examination of water and wastewater, 18th ed., America Public Health Association. Washington, D.C., 1992. . irrigation canal. 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 35 40 45 50 Time[d] SS Conc.[mg/L] outlet of equipment out of experimental area(depth 1. 5m） Figure 3 Variation. Environment Technology, Vol .1, No.2, 2003 - 235 - introduced at 1. 5 m depth and was treated at 40 ton/d, HRT was 10 minute and reflux ratio was 1. 0. Air was supplied