Our research object is to show water purification ability of biofilm attached to river bed. we report the findings of our reserch on chemical and biological composition and formation condition of biofilm on river bed of the urban rivers of Sapporo city. We summarize the findings as follows;
(1) The average chemical composition of biofilms attached to cobbles is carbon, about 60%, and nitrogen, about 7%. It is close to the composition of ordinary micro-organisms.
(2) The algae with diatoms and green algae as the main constituents are dominant on biofilm on river bed. The proportion of aerobic bacteria shows an increasing tendency as the amount of biofilm g ains more in quantity.
(3) The water velocity and water quality have much influence on removal rate of nutrients.
(4) We could confirm the removal of nutrients by biofilm on river bed as one way of water purification.
(5) The removal rate of nitrogen by algae is the largest under light condition. In the case of phosphorus, there is no difference under light and dark conditions, and it seems to have a removing system besides biological activities.
(6) The concentration of phosphorus in water controls the removal rate of nutrients of biofilm on river bed.
(7) Others.

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A blue green alga, Chroococcus sp., was cultivated continuously and the kinetics of metabolism were determined for major nurients.
Bottom mud from a small agricultural pond in Ube city was taken into a cultivation jar and algae were precultivated. Alga, Chroococcus sp., was transfered to a continuous cultivator which is of the net volume of 13.8 dm³, thermostated at 30°C, and applied light with 8 white spotlights of 200W. The culture medium contained NaHCO₃, NaNO₃, KH₂PO₄, MgSO₄, CaCl₂, FeCl3 and EDTA. The pH value was kept at 8.0. The alga cell concentration was kept at 3, 000 to 4, 000 mg/dm³. Under these condition, the alga growth rate was 11.4 g-dry biomass/m2-light applied area/day.
The uptake rates, which were determined with batch method, for hydrogen carbonate, nitrate and phosphate ions in light were 3.4 × 10^-2, 5.3 × 10^-3 and 5.8 × 10^-4 g/g-dry biomass/day and the uptake rate for oxygen in the dark was 2.7 × 10^-3 and the excreting rate for phosphate ion was 8.2 × 10^-6. The relative ratio of nutrient uptake rate with batch method was reasonably in agreement not only with that calculated from mass balance in continuous cultivation but with element content ratio of alga.

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A change in self-purification and bioproductivity caused by reclamation was studied quantitatively in Yamaguchi bay. For the purpose of evaluation, the baywas divided into two parts: the reclaimed area and the remaining water body.
Through the analysis of field survey data collected before and after reclamation, we estimated self-purification and bioproductivity capacities lost completely in the reclaimed area. The same criteria for the remaining water body were estimated by ecosystem model simulation including fishes. Self-purification was defined as a change in amounts of COD, DO, TN, TP; and bioproductivity was defined as a change in amount of biomass (such as primary products, benthos, and fishes) at each stage of the food chain. In this study, a new ecosystem model taking into account fishes and with a capacity to evaluate reclamation effects on the ecosystem is shown.

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Decomposition rates of wastewater sludge were investigated in the acidogenic phase of anaerobic digestion. The activity of methanogenic bacteria was inhibited with iodopropane to prevent the decomposition of volatile organic acids produced, and time-cocentration variations of volatile solids and organic acids were observed. The decomposition rate of fresh sludge (mixed sludge) was compared with the decomposition rates of primary sludge and waste activated sludge. Degradable organic materials contained in fresh sludge may be divided into two constituents which are fastly degradable organic materials (S1) and slowly degradable organic materials (S2). Observed values of decomposition rates of S1 and S2 correspond to the decomposition rates of primary sludge and waste activated sludge. Their decomposition rate equations were expressed by a Monod type rate equation. VS reductions of the primary sludge, fresh sludge and waste activated sludge were 47%, 40% and 38% respectively.

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Solubilization of waste activated sludge during thermal pretreatment was experimentally studied. Thermal pretreatment increases biodegradability and gas production of sludge in anaerobic digestion process. In this study, a laboratory scale experiment was conducted to clear the mechanism of thermal pretreatment of sludge. Solubilization of sludge solids, and changes in constituents of organic materials, especialy nitrogenous materials and volatile fatty acids were analyzed. The solubilization mechanism of sludge solids by thermal pretreatment was made, and model of the reaction was proposed. Solubilization of waste activated sludge was promoted with an increase of thermal pretreatment temperature. Relationship between solubilization ratio and pretreatment temperature was approximated by a logistic function with respect to COD and proteins, and approximated by a second order polynomial with respect to carbohydrates. The total solubilization was almost governed by proteins. The liquid phase analysis shows that proteins was hydrolyzed as protein→amino acid→anmonia+volatle acid. The simulation by the model shows that the effect of holding time on solubilization was small over 30 minutes.

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Currently, more than fifty per cent of sewage sludge generated in municipal wastewatertreatment plants has been incinerated with mainly heavy oil in Japan. The increase inenergy costs over ten years has necessitated the improvements of current incinerationysystems to make them more energ efficient. The objectives of this study are as follows:
to investigate thermal characteristics of dewatered sewage cake.
to review features of incinerationsystems.
to survey process performances of incinerators in wastewater treatment plants.
to evaluate operating conditions of autogeneous combustion of sewage sludge, and
to evaluate economical aspects of autogeneous comnustion system.

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This investigation was carried out for the purpose of developing a newmethod for advancing the nitrogen removal capacity based on the extended aerationprocess by varying its operation system.
An existing plant which was designed as the extended aeration process wasmodified to the biological nitrification-denitrification process using endogenousrespiration for denitrification. This process was a sequential carbonoxidation-nitrification-denitrification system. The possibility of upgrading thenitrogen removal efficiency, the phenomenon of nitrogen removal in this process and the optimum operation condition for removing nitrogen were studied.
Through the results of this study, it was verified that the nitrogen removal wasachieved at a high level of efficiency without either any facility expansion or theaddition of organic carbon source for the denitrification. In the operation of thisprocess, it was necessary to know the minimum dissolved oxygen concentration in thenitrification tank, in which ammonia nitrogen did not remain, in order to maintainthe denitrification reaction at a high rate. And it was also clearly shown thatmaintaining the MLSS concentration in the low temperature period higher than that inthe high temperature period was necessary to prolong the sludge retention time fornitrifier reproduction.

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Bench-scale experiments were conducted to study the treatment characteristicsof the intermittent aeration process for nitrogen removal. Two bench-scale plants, one with completely mixed and another with plug flow, were used to estimate the influence of the mixing conditions on the nitrogen removal efficiency as well as the treatment characteristics of other compounds. The effect of the hydraulic retentiontime on the nitrogen removal efficiency and the response of the intermittent aeration process to varying hydraulic and nitrogen loading were also investigated to find theoptiium operational conditions for removing nitrogen.
Although the significant difference was not observed in the nitrogen removal efficiency between the completely mixed 2: ant and the plug flaw plant, it was found thatthe organic substrate was used directly for denitrification in the completely mixedplant, while the denitrification of the plug flow plant was occurred through endogenousre spiration. Nitrogen removal efficiency of more than 70% could be obtained inthe completely mixed plant even in the winter season under the hydraulic retentiontime of 10 hours. It was possible to provide a countermeasure for the varyinghy draulic and nitrogen loading by changing the intermittent aeration mode. Furthermoreit was also found that the control of the dissolved oxgen concentration of theaeration period was very important for nitrification and denitrification in thisprocess.

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Experimental studies were devoted to determining the feasibility of autotrophic denitrification with Thiobacillus denitrificans as a nitrogen removal process for wastewater treatment.Elemental sulfur (S⁰)-acclimated activated sludge, which has the ability of autotrophic denitrification using S⁰ as electron donors, was prepared by fill and draw cultivation method. The kinetic constants for S⁰ denitrification reaction by S⁰-acclimated activated sludge under NO₃-N limiting growth condition were determined to be Y=0.33 mg-TOC/mg-NO₃-N and b=0.058 day^-1, respectively.High percentages of denitrification over 95% and rates of denitrification ranging from 0.19 to 0.24mg-NO₃-N/mg-T0C·day were obtained in the continuous denitrification experiments by S⁰-acclimated activated sludge.

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In order to make clear the rate determining step of the activated sludgeprocess, the batch reactor experiments were operated. Peptone and mixture of freeamino acids (the compositions of amino acids were arranged so as to be same as ofpeptone) were selected as primary substrate. Each of substrates was supplied tothe activated sludge which was preliminarily acclimated to peptone in the batchreactor and the changes of the substrate concentration were traced.
The removal of peptone followed first order reaction kinetics, however that ofamino acids mixture followed zero order kinetics. The requisite time for 90% removalof peptone was more than twice of that of amino acids mixture. It was, therfore, concluded that the hydrolysis process of peptone was the time determining step.The amino acids were classified into two groups. The first group was composed ofthe essential amino acids, the maximum removal velocities of which were notinfluenced by the state of substrate (monomer or polymer). The second group wascomposed of the amino acids which were deaminated to the intermediates in the TCAcycle, that velocities of which were higher in amino acids mixture than in peptone.

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The comparative study was made to select better chemical for simultaneous phosphorusremoval in nitrified liquor recycling nitification-denitrification process. Alminum sulfate (alum) andpolyalminum chloride (PAC) were exzamined because of their popularity as the chemicals for simultaneousphosphorus removal in Japan. Experimental works were carried out for a year in Kyotoadvanced wastewater treatment pilot plant, using 2 series biological treatment facilities. Both alumand PAC gave excellent results upon the phosphorus removal, but some differences were observed innitrification and slude characteristics. Nitrification was deteriorated more severely with alum than PAC. In the alum dosed series, occasional stops of the nitrification were observed especially in winterseason, resulting from sudden drops of the water temperature. But in the PAC dosed series, suchphenomena were not observed even in winter season. Regarding the thickenability and dewaterabilityof the generated sludge, the alum dosed sludge exhibited a little better properties than the PAC dosed one.

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This paper is concerned with the behavior of air bubble plumedriven by the source such as a diffuser plate.We measured the meanvelocities and streamwi se component of the turbulent fluctuations with a miniature propeller flow meter. In addition, the local volume fractions σ which are occupied by the air are measured with a conductivity probe. The conductivity probe is sensitive to changes in the conductivity of the mediumat the probe tip. As the bubbly water stream moves upward, it entrains morewater and spreads laterally. It is shown experimentaly that the stream whichis driven by the source has a minimum plume width at a cartain level near the source. Furthermore, it is found that the interaction of air bubbles play very important role in the region close to the source. On the basis of this fact, the diffusivity depending on the interaction of air bubbles is assumedproportional to the centerline fraction (σ₀) ⁿ. The results of the analysisby the use of moment transformations of momentum and bouyancy equations are compared with our experimental data.

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The numerical simulation of the nocturnal urban boundary layer (UBL) was conducted byusing the second order turbulence closure model in order to clarify the formation mechanism of thenocturnal UBL. The mechanical turbulence generation by the urban buildings was incorporated into the model equations by the form drag concept (building surface area density). Simulation results werecompared with the field observation data at Sapporo, Japan. The numerical simulation showed that i) the nocturnal UBL was generated by the downward transport of sensible heat from the elevatedinversion due to the mechanically-generated turbulence of the urban building rather than the artificialheat emission, ii) the average elevated inversion base height Z_h was approximately twice the averagebuilding height and the height Z_h was proportional to the building area density, iii) above the Z_h, thetypical temperature “cross-over” region was formed when the urban area exists even if there is noartificial heat emission, iv) the Reynolds stress ‹uw› shows the nearly uniform profile, while, sensibleheat flux ‹wθ› had the negative peak at the inversion base height. This negative value of‹wθ›at the elevated inversion base can be considered as a heating source for the formation of the nocturnal UBL.

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A one dimensional simulation model of suspended sediment transport in a tidal river was developed with erosion, deposition, and thickening processes of sediments, and inflow from tributaries. This model uses the explicit leap-frog method and its lower end boundary of the river is extended into the sea to close the boundary for.calculation. Laboratory experiments were performed to determine erosional and depositional rates of sediments and to study the sediment thickening process in the river under various concentrations of chlorinity and suspended solids. Numerical simulation results with the parameter values obtained experimentally did not show good agreement with observed data. Modifying the parameter valuesaccording to physical phenomena was necessary to obtain good agreement in between. After the modification, computation results during a fortnightly cycle explain satisfactorily the sediment transport phenomena in this river.

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An ecological model of Lake Yunoko, a dimictic lake, was developed to obtain quantitative information on the nutrient cycle in Lake Yunoko, particularly the sediment-waterinteractions and to assess various programs for eutrophication control. A multi-component dynamic model for nutrients in the bottom sediment-water system was incorporated into the model, one dimensional water temperature-water quality-ecological model. Good agreement between the model calculations and observed nutrients, chlorophyll-a, and dissolved oxygen concentrations in the water and nutrients concentrations in the bottom sediment was noted. The agreement between the model calculations and the measured nutrient flux from bottom sediment during the summer stagnation period was good. The effectiveness of eutrophication control programson chlorophyll-a concentration in the surface water and dissolved oxygen concentration in the bottom water was assessed using the calibrated model. Sediment dredging decreased nutrient flux from bottom sediment. As a result, chlorophyll-a concentrations in the surface water at autumncirculation period decreased in comparison with those of the control simulation. However, sediment dredging had not an effect on the depression of nutrient release flux after 4 years as compared with the control simulation. The depletion of dissolved oxygen in the bottom water during the summer stagnation period was scarcely restored by dredging only. Artificial circulation increased dissolved oxgen in the bottom water more than 5.2 mg 1^-1 throughout the year, whereas chlorophyll-a concentration in the surface water scarcely decreased compared with the control simulation.In the case of 50 percent reduction of external phosphorus loading from the wastewater treatment facility, the annual average chlorophyll-a concentration in the surface water decreased to 79 percentof that of the control simulation for the 4 th year.

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The transfer characteristics of water or non-adsorptive and desorptive solute in the unsaturated layer of solid wastes, which consist of incinerator residue andone including rice straw, are investigated using tritium (³H) as tracer material.
The results obtained from this study are as follows.
(1) The breakthrough curves from the series of experiments can not be explained by dispersion equation.
(2) The flow mechanism of water and the solute in the solid wastes is expressed by two layer flow model of stream region and stagnant region-
(3) The equation of two layer flow model is approximated by dispersion equation for sufficiently large values of Peclet number (Pe), and dispersion coefficient of unsaturated flow in the waste layer is expressed by flow rate per unit area and three parametersin the equation of two layer flow model.

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The precipitation and complexation scheme of cadmium in sanitary landfilllayer is studied by investigating the reaction equilibrium among Cd²⁺, OH^-, Cl^- and CO_32-. First, the scheme and equilibrium constants of the reactions among those species in pure water was determined. From these experiments, it is found that not only the precipitaion reactions but also the complexation reactions should be taken into accounton estimating the total concentration of dissolved cadmium, [Cdt] and so a set ofequations are obtained for calculating [Cdt]. The equilibrium constants in this work agree well with published. Secondly, the same reactions were observed in leachate, which had been sampled form three sanitary landfill sites. The observed [Cdt] with various concentrations of OH^-, Cl^- and CO_32- are estimated compatively well by using the set of equations above mentioned. Finally, based on these results, the variation of [Cdt] and other dissolved species in various conditions of pH, Cl^- and CO₃^2- are considered and so it is found that the minimum [Cdt] in the range of pH 7 to 11 isabout 0.2 mg/l.

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The production process of soluble organic pollutants is studied by using two small scale lysimeters, i.e., the water saturated refuse layer (W. S. L.) and the gravitationally dewatered refuse layer (G. D. L.). The used refuse is an artificial one, i. e., the model for mixed domestic refuse, which contains many components such asgarbage, papers, textiles, eic., The temperature of the lysimeters was controlled atca. 20°C and the anaerobic condition was kept inside the lysimeters. The small amount of digested sewage sludge was injected for promoting methane fermentation.
1) Methane fermentation began at 125th day together with the increase of pH to ca. 5 and decayed at ca. 700th day. The maximum gas production rate of each lysimeterwas respectively ca. 4 (W. S. L.) and 2 (G. D. L.) mol/(m³d). And so the BOD of leachate from both lysimeters was initially ca.10000 mg/l, but rapidly decreased to ca. 10 mg/l by methane fermentation.
2) In the initial stage within 150-200th day, garbage was gradually decomposed in proportion to the undecomposed amount of garbage. It is suggested that cellulose decomposition was occured and this process is limiting in the solubilization of refuse after 200th day. And so the decomposed amount of refuse in each lysimeter attained to ca. thrice (W. S. L.) and twice (G. D. L.) of that of garbage as Carbon content.

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Experimental Studies have been undertaken on Hg vapour (H^°g) removal by pulse-corona-induced plasma chemical process (PPCP) of pulse voltage of a 100 nsrise time. The gases which have H^°g initial concentrations of 0.5mg/Nm³ or higher than added to room air or exhaust gas from a municipal refuse incinerator were led todischarge cell. The treated gas was led to gas washing bottle, and there oxidized Hg was removed. Dependence of pulse frequency (f) ranging 50 to 250Hz on H^°g removalefficiency (η) has been examined, and measurements concerning Hg removal mechanismhave been made. When waveform of pulse voltage and gas residence time were identical, the relation between η and f was approximately explained byη = 1-exp (-kf). Then, highη was obtained with increasing frequency. The pulse corona power requiredto gain the same η was only 10-30% of direct current one.η little decreased with increasing HLl conc., of even 2mg/Nm³ for exhaust gas.η little increased with increasing HCL conc., decreased with increasing CO conc., and increased markedly with increasing HCl conc. It's considered that O₃, O, and active species by HCl dissociation, etc. contributed to H^°g oxidation.

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Mechanism of a daily fluctuation of collected amount of municipal solid waste is studied in terms of a deviation D from moving average which is obtained by data normalizing procedure.
Assume that some portion of residents don't take their garbage out to garbage stations on one collection day. Those “reserved” waste must be taken out on the next collection day together with waste generated during collection interval. So a daily increase of collected amount of waste always takes place in consequence of a sudden decrease, and both incremental amounts are equal in quantity each other. Such a pattern of the fluctuation is remarkably observed on national holidays with large deviation D. Brief explanation is given to that phenomenon in relation to residents' behavior. The fluctuation pattern is also observed when the collection day isn't holiday, and rainfall is confirmed to be a leading cause of the sudden decrease.

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Total human intake in various exposure situation derived from consumption of alternatives of chlordane is calculated by using of multi-media environmental fate model. The magnitudeof ingested chemicals is converted into risk considering the ADI value of chemicals concerned. Alternative chemicals are evaluated in terms of risk potential, cost of them, benefit of life time extension of the wooden house. Major results obtained are as follows:
(1) Any alternative chemicals have low risk compared to chlordane from exposure analysis in tesed real environment.
(2) Tetrachlorvinphos and tripropyl isocyanurate are superior to chlordane from the view point of risk exposed to residents and workers but others are not by virtue of indoor exposure analysis. Residents pay more to meet their need for anti-termite treatment. Workers and residents receive more health risk from phoxim pyridaphenthion compared to that from chlordane.
(3) Multi-criteria analysis shows that tripropyl isocyanurate, tetrachlorvinphos and phoximare superior to others in terms of human risk, cost and benefit. Some criteria which are includedin MPD (Minimum Premarketing set of Data) are found to be inclined torwards a risk-emphasized evaluation.

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Methane fermentative bacteria and denitrifying bacteria were enriched with a synthetic medium, and a biofilm was formed on solid support surfaces in a completely mixed reactor. In the medium, glucose, peptone and sodium nitrate were used as carbon and nitrogen sources.
By using the biofilm reactor, characteristics of the anaerobic treatment of an aqueo us mixture of organic matter and nitrate were studied experimentally, and effects of the C/N ratio on removal efficiencies of TOC and nitrate and effluent characteristics were examined in batch wise and/or continuous operations.
Organic m atter and nitrate were removed simultaneously in the anaerobic biofilm reactor, in which methane fermentative bacteria and denitrifying bacteria symbiosed. In the simultaneous treatment, acetate which is an intermediate metabolic product in methane fermentation were effectivelly utilized by denitrifying bacteria as hydrogen donor. Competition between methanogens and denitrifying bacteria existed for the utilization of acetate. Removal efficiencies of TOC and nitrogen for contiuous operations were greater than 94% and 65%, respectively. Both the values of ORP and pH increased with the increment of nitrate concentration. Under high nitrate loading conditions, methane gas production decreased.

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Two continuous flow experiments were conducted using self-immobilized methanogenic cells reactors, i. e., up flow anaerobic sludge blanket reactors to investigate its process performance. The first experiments were made at 25°C both with a mixture of starch and sucrose as a substrate at a influent COD of 3, 000 mg·l^-1 (refer to as Run 1) and with molasses at a influent COD of 6, 000 and 2, 200 mg·l^-1(Run 2). Maximum allowable loading rates were evaluated to be 25 kgCOD·m^-3·day^-1 (corresponding to HRT of 2.9 hrs) for Run 1, and to be 14 and 20 kgCOD·m^-3·day^-1 for a influent COD of 6, 000 and 2, 200 mg·l^-1, respectively, for Run 2. The second experiments were made at 35°C both using a mixture of volatile fatty acids at a influent COD of 5, 000 mg·^-1 (Run 3) and acetate at a influent COD of 3, 000 mg·^-1 (Run 4), resulting that maximum loading rates were achieved at 70 (corresponding to HRT of 1.7 hrs) for Run 3 and at 25 kgCOD·m^-3·day^-1 (HRT of 2.9 hrs) for Run 4, respectively.
Comparison of microbial activity between granular sludges cultivated in Runs 1 to 4 indicated that methanogenic activity, ATP pool and F₄₂₀ content were varied in the range of 0.3-1.7 kgCH₄-COD. kgVSS^-1·day^-1, 0.046-0.355 mg·gVSS^-1 and 0.032-0.39 mg·gVSS^-1, respectively, which were 4-20 times, 1.1-87 times and 2-26 times greater than those of the original innocula of digested sewage sludge. Morphological observation of granular sludges by scanning and transmission electron microscopes revealed that both filamentous growth of methanothrix genus methanogens and excretion of extracellular biopolymer by acidogens take significant roles in granule formation.

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Lake Biwa, the raw water source of the Osaka municipal water works, has been suffered from the musty odor occurrence for the past seven years. The causative compounds have been identified 2-methylisoborneol (2-MIB) and geosmin. The maximum concentration of 2-MIB and geosmin detected so far at the outlet of lake Biwa was 313ng/l and 620ng/l respectively. The threshold odor concentration of 2-MIB and geosmin in drinking water was determined 4ng/l and 100ng/l respectively, according to the triangular taste and odor testing methods. These compounds in raw water colud not be removed efficiently by the conventional water treatment processes even with an addition of powdered activated carbon. Ozonation coupled with granular activated carbon filtration could remove them by almost 100%.

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Inorganic ions besides nitrogen and phosphorus have influence on the eutrophication of lake. The secular increase of inorganic ion loadings from a river basin by the accumulated influence of the massive fertilizer injection in agricultural land and air pollution including acid rain are forecasted. The specific loadings of inorganic ions for ten river basins were estimated on the basis of weekly observation data for one year. From the correlation analysis between specific loadings of inorganic ions and area ratios of land-use types in river basins, most of inorganic ions showed high correlation coefficients for urban and agricultural areas. But nitrate ion only become a high correlation coefficient for farmland area. In the correlation analysis among inorganic ions, those except nitrate ion have high correlation coefficients with one another. The seasonal changes in concentrations and loadings of inorganic ions were chiefly ruled by discharge and slightly by water temperature. In the period of storm runoff the concentrations of most inorganic ions did not steeply decrease and those loadings increased fairly. Particularly, the increase of potassium concentration among the cation group at the biggest peak of discharge during a storm runoff was recognized as well as nitrate and sulfate ions among the anion group.

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This paper describes a new method to study nutrient flow out process from non-pointsource. We selected asphalt ground, bare ground, a grass slope, a road of an industrial area, and aforest as study basins. At each study basin, we observed runoff concentrations of nutrients (T-N, T-P, COD) and flow rate of runoff during several storms. Then we constructed a box model that containedfour compartments: suspended matter, dissolved matter, accumulated matter, and soil. Themodel had 9 paths which represented nutrient transfers between the compartments and each path hadunknown coefficients. Using the Newton-Raphson method, we got the optimal coefficient values anddiscovered the principal paths of nutrient transfer. The results are as follows.In each study basin, accumulation of suspended matter and its washout was an important factor.
In the case of bare ground and the grass slope, erosion was found to be important. T-P transfer wasfound to be important only in the paths concerning the suspended matter. On the other hand, T-Ntransfer appeared in both paths concerning the suspended and the dissolved matter.
Using these results, we presented the pollutegraph model for a general c atchment basin. Thevalue calculated by the model was in good accordance with the observed data.

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To elucidate the fundamental features of water qualities variations (BOD, COD, pH, SS, TN, TP etc.), statistical analyses were applied for the data obtained at 1, 235 sampling points in Japanese rivers. The magnitude of water quality variations defined by (C₇₅ -C₂₅)(C_X means xpercentile of C in each point) was nearly proportional to the median (C₅₀) in most of the water qualities. Better agreements were found in expressing the water quality variations by the varianceanalysis model with seasonal classification, compared with by the regression model to water discharge. The possitive correlation coefficients between pH and the saturation rate of DO etc. indicatedthe primary production in river waters. The degrees of agreement by the variance analysismodel and the correlation coefficients between some water qualities and water discharge variedwith every group of sampling points classified by the median of BOD.

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In order to facilitate communication on river water quality among local governments, professionals and the general public, many techniques for interpreting water quality have developed. One useful technique is water quality indices. In developing water quality management plan, waterquality is often difficult to evalute especially in terms of overall quality despite large amount ofdata, so indices can be used in decision making.
This article aims to present how to develop a comprehensive water quality index. Proposed indexingmethod consists of water quality parameters, weighting, value functions and aggregation process. Choice of parameters, weights and value function are based on a Delphi questionnaire technique. Seven experts in river water quality management answered the questionnaire repeatedly. Resultantindex CWQI (Comprehensive Water Quality Index) is applied to two rivers in order to assess annualtrend of water quality and to compare it's value with organic pollution parameter BOD. It is concludedthat a Delphi technique is useful to summarize the knowledge and experience of experts inriver water quality management and a developed water quality index CWQI is an appropriate toolfor evaluating overall river water quality.

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