A Novel Sewage Treatment System by a Combination of UASB Reactor and DHS (Downflow Hanging Sponge)Reactor for Developing Countries (UASB法およびDHSリアクターから構成される発展途上国のための新規下水処理システムの開発)

氏名 イザルル マクダル
学位の種類 博士(工学)
学位記番号 博甲第228号
学位授与の日付 平成13年3月26日
学位論文題目 A Novel Sewage Treatment System by a Combination of UASB Reactor and DHS (Downflow Hanging Sponge) Reactor for Developing Countries (UASB法およびDHSリアクターから構成される開発途上国のための新規下水処理システムの開発)
 主査 助教授 大橋 晶良
 副査 教授 原田 秀樹
 副査 教授 桃井 清至
 副査 助教授 小松 俊哉
 副査 長岡工業高等専門学校助教授 荒木 信夫
 副査 京都大学助教授 藤井 滋穂

平成12(2000)年度博士論文題名一覧] [博士論文題名一覧]に戻る.








 High urbanization rates in most of developing countries stresses the need for sanitation, as there is clear relationship between lack of adequate sanitation and poor public health. Due to limited financial resources, developing countries face an enormous problem to develop their environmental health infrastructure. Therefore, the problem of municipal wastewater in developing countries requires an urgent attention. The challenge to recover the municipal wastewater problem in developing countries can be translated into assignments to search for technologies that are more effective. A novel concepts and systems are needed to be developed that address the urban sanitation problems in more cost-effective and simple ways.
 Upflow anaerobic sludge blanket (UASB) technology may be most attractive option for sewage treatment in developing countries, because it can be used at small or large scale, is technically simple, lower in cost (no need of aeration), yield less excess sludge and energy is produced instead of being consumed. However, UASB is pre-treatment unit in which the effluent needs further treatment to remove the residual organics and nutrients. Searching for an alternative process of UASB post-treatment, for application in developing countries, a novel aerobic reactor named the downflow hanging sponge-cube (DHS) reactor has been proposed (Machdar et al. 1997). The proposed cube-type DHS polyurethane foam (sponge) in air. The sponges act as a biomass immobilizer. Influent wastewater is supplied at the top of the reactor and it naturally permeates by means of reactor has a unique design concept, constructed by hanging a specific dimension of gravity into the sponge. The most important feature of the DHS is no requirement of external intended aeration and maintaining a very long SRT.
 The objective of this study is to investigate the feasibility of the proposed system, i.e., UASB pre-treatment unit followed by DHS post-treatment unit, with an emphasis on the nitrification and denitrification behaviors in DHS post-treatment unit. UASB reactor having a working volume of 155 litre consisting a 120-litre column portion and a 35-litre gas-solid separator was used in this study. The effluent from UASB unit (6-h HRT) was fed to cube-type DHS. To accumulate operational know-how, three cube-type DHS units (R-1 through R-3) were hanged in parallel operated at different modes. Each of DHS unit was 2 m in vertical length, composed of 90 sponge cubes (each 1.5 x 1.5 x 1.5 cm) connected diagonally in series with each other. DHS R-1 unit was operated as a one-through reactor at a 2.1 ml.min-1 flow rate. DHS R-2 unit received the same feeding rate as R-1, but the effluent from R-2 unit was recycled to the top of R-2 itself at 1:1 ratio. DHS R-3 unit had recirculation, just as R-2 unit did, but the upper-half of R-3 total height was kept in anoxic condition to enhance denitrification. The proposed system was operated by receiving actual sewage under practical conditions. UASB and DHS units were maintained at 25℃ to resemble annual average ambient temperature in most of developing countries in sub-tropical regions.
 Independent of operational mode, three DHS units showed no significant difference in their performance. The whole system (UASB unit plus cube-type DHS unit) proved to be an excellent reactor, achieving over 94% of total-COD (unfiltered-COD) removal, 81% of soluble-COD removal, and nearly perfect SS removal and total-BOD removal at the overall HRT of 8.3 h (6 h in UASB and 2.3 h in DHS unit). Furthermore, the cube-type DHS unit did 73-78% nitrification and some extent of denitrification.
 In situ nitrification and denitrification rate were determined for each DHS reactor by feeding defined synthetic wastewater. Ammonia oxidation in all three DHS units had four to six times higher potentials than actual in situ rate (sewage-fed rate). No substantial difference in ammonia-oxidation activity and nitrite-oxidation activity was observed among three DHS reactors. In all three DHS units nitrite oxidation activities were comparable to or slightly higher than ammonia oxidation activities, accounting for the observation that nitrite was not present in the effluents. An interesting finding is DHS reactor exhibited relatively high denitrification activity, nearly a half as large that of as ammonia oxidation activity.
 The average concentration of accumulated biomass along cube-type DHS height was 20.000 mgSS.L-sponge-1 and the ratio of VSS/SS was 0.76 in the respective DHS units. A comparison to activated sludge process having sludge concentration between 1,500 and 10,000 mgSS.L-1, suggests that DHS reactor configuration be advantageous for maintaining a very long SRT. Consequently, cube-type DHS configuration enhanced proliferation of slow-growing nitrifiers.

 SEM observation clearly demonstrated two distinctive manners of biomass retainment within DHS sponge-cubes. The outer surface of biofilm accumulated onto cube surface, displaying individual cells were embedded into dense polymer-like matrix. A slightly inner distance from the surface of this type of thick biofilm would readily become DO depleted zone, possibly providing denitrifiers with favorable habitat.
 From these studies it can be concluded that cube-type DHS configuration seems advantageous as a post-treatment unit of UASB (treating sewage) effluent. However, in a practical viewpoint, cube-type DHS seemed to have some drawbacks in such points as area requirement and distribution of influent. Therefore, to improve these drawbacks, the second generation DHS reactor called curtain-type DHS was developed (Machdar et al., 2000). The curtain-type DHS was constructed using a right-triangular prism polyurethane foams (prism-side: 3 cm and length: 75 cm). The foams were attached on both surfaces of a vertical, acrylic plastic, rectangular-sheet (75 cm in width and 200 cm in length. DHS reactor was initially seeded with activated sludge mixed liquor from Nagaoka Sewage Treatment Plant for one day prior to start-up.
 The objective of this study is to evaluate efficiency of organic removal in the combination of UASB and curtain-type DHS process. During three-year experiment, two different configuration of DHS were distinguished i.e. 2-m height DHS (phase-1) and 4-m height DHS (phase-2). To maintain an identical HRT (2 h) for both DHS configurations, the feeding flow rate to phase-2 was set at twice as that of phase-1.
 The whole system (UASB unit and DHS unit) exhibited a very satisfactory organic removal, achieving 83-84% of total-COD removal, 95-97% of total-BOD removal, and 68-79% of SS removal. The final total-BOD produced from the proposed system (4-8mgBOD.L-1) is far below the BOD discharge standard to surface water in most of developing countries (20-60 mgBOD.L-1). Moreover, the system was comparable in the reduction of fecal coliform to that of activated sludge process.
 Determination of sludge activities of the 4-m height DHS at the middle portion (200cm) revealed that the nitrite oxidation rate (0.03 kgN.kgVSS-1.d-1) exceed the ammonia oxidation rate (0.01 kgN.kgVSS-1.d-1), its similar to phenomenon observed in the 2-m height. The higher nitrite oxidation rate (0.07 kgN.kgVSS-1.d-1) than ammonia oxidation rate (0.05 kgN.kgVSS-1.d-1) was also obtained in the lower portion (400 cm). This result is accounting for the observation that nitrite was not present in the effluents. Regarding denitrification, the activity obtained at the order of 0.02 kgN.kgVSS-1.d-1 in the upper and in the middle portion. The denitrification activity in the lower portion decreased approximately a half of activity of the previous portion, following the increase in DO.
 Determination of retained sludge concentration revealed the curtain-type DHS sponge-material contained the range of 25,000-36,000 mgSS per liter sponge-volume, with the VSS/SS ratio of 0.62-0.66 and the C/N ratio of 7.2-8.0. Since DHS reactor exhibited the high COD removal and superior ammonia oxidation efficiencies, the polyurethane sponges, as a biomass immobilizer used in the DHS unit, were through to be suitable for accumulating active biomass, including nitrifiers. SEM observations of the sponges visually showed that a certain amount of biomass was accumulated in the material throughout the operation. The entrapped biomass formed thick and dense aggregates in the inter-pores of the materials, involving several morph-types of cells and debris-like materials.
 Despite there was no forced aeration input to DHS unit, DO level of downward-flowing wastewater increased from zero at the inlet (corresponding to the UASB effluent) up to almost saturated at the exit. DO profile of sponge-inside located near DHS exit, demonstrated a distinctive DO gradient occurred along sponge-inward depth, suggesting coexistence of a nitrification region at the shallow sponge-inside and denitrification region at the deep sponge-inside. Both results explain eloquently the most important feature of curtain-type DHS reactor: no requirement of external aeration input.
 The total bacteria of DHS retained sludge laid between 1.4 x 107 (SD± 0.3 x 107) cells.mgVSS-1 and 5.6 x 107 (SD± 1.4 x 107) cells.mgVSS-1. However, according to the analysis of live-dead microbes, the sludge at the inlet point had a higher percentage of dead microbes (more than 50% of total cells) than those of the sludge at the middle and the lower portions. At the lower portion, the dead cells were obtained only between 33.3% (SD± 12.0%) and 31.2% (SD± 11.6%). In situ hybridization with Nso190 (Nitrosomonas spp.) probe showed a number of coccoid cells in DHS sludge. According to analysis, the sludge at the lower portion (400 cm), particularly the surface of the sponges, contained the highest number of Nitrosomonas spp. clusters approximately 4.07 x 103 (SD± 3.06 x 103) clusters.mgVSS-1. Except for the sludge at near the inlet point of the DHS bioreactor, Nsr1156-positive cells, probably classified as a member of the genus Nitrospira, were also found in 200-cm and 400-cm locations. According to analysis, Nitrospira spp. cells were counted nearly between 3.12 x 103 (SD± 0.12 x 103) clusters.mgVSS-1 and 6.18 x 103 (SD±0.87 x 103) clusters.mgVSS-1, which were very similar to the values obtained for Nso190 positive cells at similar location. The Nsr1156-positive cells were frequently detected in the DHS sludge than that of the case for NIT3 probe. This suggests that Nitrospira-like cells, might play a more significant role for nitrite oxidation in the DHS sludge consortia rather than Nitrobacter spp. Nitrobacter-like cells appeared only in the locations in which there is a relatively higher concentration of dissolved oxygen (4 to 5 mgDO.L-1) and a higher concentration of nitrite (15 mg.N.L-1 to 20 mg.N.L-1). This indicates that Nitrobacter species could occur only in the limited circumstances where the oxygen and nitrite concentrations were sufficiently high.
 From this study several conclusions could be drawn. Although no forced aeration was provided, curtain-type DHS could satisfactorily nitrify a UASB effluent at removal rates comparable with other nitrification systems. The favorable nitrogen removal in DHS was mainly influenced by sludge retainment and DO diffusion. It appears that DHS process might be having a longer SRT that is favorable for nitrifiers, which are known to have a slow growth rate. The DHS configuration provided retained biomass which was not persistently covered by liquid but they were directly exposed to air. This condition reduced liquid film resistance and consequently increased oxygen mass transfer to the biomass. The bacterial population as observed with specific probes offered a reliable information on nitrification in the DHS sludge consortia. The combination of UASB and curtain-type DHS reactor is a cost-effective sewage treatment system. The system possess distinguished advantages: neither requirement of external aeration input nor excess sludge production. The DHS reactor is simple to construct, is easy to operate and maintain. For practical implementation of the proposed system in developing countries, it is first necessary to determine technical and economical feasibility of the system by installing a pilot- or demonstration-scale-plant under actual conditions in tropical climate.