A. Boudjadja, M. Messahel and H. Pauc
En Algérie, l'eau revêt un caractère stratégique du fait de sa rareté et d'un cycle naturellement perturbé et déséquilibré. Qu'il s'agisse de l'eau souterraine ou de l'eau de surface, les ressources sont limitées et, compte tenu des problèmes démographiques et de l'occupation de l'espace (sachant que près de 60% de la population algérienne sont concentrés dans la frange septentrionale du territoire qui ne représente que le dixième de la surface totale du pays), d'importants efforts sont nécessaires en matière d'urbanisation intégrée et de gestion rigoureuse dans l'exploitation des réserves, si on veut atteindre la satisfaction des besoins à l'horizon 2010. S'y ajoutent des problèmes de faible mobilisation et de mauvais recyclage par manque de maîtrise des stations d'épuration et l'envasement des retenues.
Les 11 bassins versants exoréiques de l'Algérie du Nord couvrant une surface de 130 000 km2 présentent des potentialités en eaux de surface de 11.109 m3 dont seulement 2,13.109 sont mobilisés par les 98 barrages en fonctionnement en 1995. Les besoins estimés à l'horizon 2010 évalués à 9,384. 109 m3 se décomposent en 1,524.109 m3 pour l'eau potable (AEP), 0,23.109 m3 pour l'industrie (AEI) et 7,63.109 m3 pour l'agriculture (AEA) (tableau 6). Sachant que les réserves en eaux souterraines sont évaluées à 1,25.109 m3, le renforcement de la mobilisation des eaux de surface par 3,834. 109 m3 et des eaux souterraines par le captage de 0,33.109 m3 supplémentaires constituent la projection faite en 1995 pour l'horizon 2010 par les différentes institutions publiques intervenant dans la mobilisation et la gestion des ressources en eaux. Cet objectif est déjà fortement entamé en 2002 car fixé dans un contexte de cloisonnement des différentes structures (Hydraulique et Agriculture) et de difficile circulation de l'information entre elles. Il surévalue les capacités de réalisations des barrages, les surfaces à mettre en valeur dans le cadre du programme de développement agricole ainsi que les dotation par habitant en matière d'eau potable. Cette situation constitue un facteur aggravant la difficulté d'une appréhension correcte aussi bien des besoins que des différentes projections sur l'avenir. Les données présentées dans cette synthèse montrent qu'il est désormais impératif que l'aménagement du territoire tienne compte des quantités disponibles afin de rationaliser l'utilisation, la protection de la qualité et la récupération par le recyclage.
La qualité chimique des eaux de l'Algérie du Nord est appréciée par les teneurs en nitrates et en chlorures des aquifères côtiers. Cependant, le développement économique et social conduit à une dégradation rapide de cette qualité des eaux, ce qui incite lourdement à œuvrer pour un meilleur recyclage et une meilleur protection des ressources. A défaut, la sanction serait la non satisfaction des besoins en eaux potable, d'irrigation et industrielle. Malheureusement, il semble que l'écart entre disponibilité et besoins soit difficile à réduire. Le but de cet article est de tenter une revue des causes du manque d'eau et des facteurs aggravants.
In Algeria, water has a strategic place in economic development, because of its scarcity and a perturbed, non-equilibrated natural cycle. Because of the limited water resources, and the need to meet the demands for the desired quantity and quality of water by the year 2010, planners must develop reasonable alternatives that take into account multiple purposes and objectives. In addition to the growing demography, 60% of the population resides in the northern range of Algeria, which represents one tenth of the total area of the country. Furthermore, inadequate water treatment due to a lack of appropriate technology, siltation of existing dams and limited capacities for water storage hamper the decision-making process.
The projected data for year 2010 show structural difficulties and organizational inadequacies that are linked to the multiple operating structures in charge of collecting and conveying the information. The other major difficulty is based on the quality of the information (e.g., empirical methods and a lack of data). For example, the evaporation rate is estimated by the traditional method of a Colorado pan in which the data are mostly discontinuous. The reservoir silt load is evaluated by irregular bathymetric measurements and the results are extrapolated over a year, sometimes from one year to the next. The volume of water losses during water transport and distribution is evaluated for only a very small portion of the network and is then generalized to the entire drinking water and irrigation system.
The eleven watersheds of northern Algeria encompass an area of about 130,000 km2 and offer a surface water potential of approximately 11×109 cubic meters (m3) in which only 2.13×109 m3 were collected by 98 functional dams in 1995. According to the forecasts made in 1995, the creation of new reservoirs (17 dams are actually under construction, 25 are about to start construction, and 810 sites for small reservoirs are in the study phase) will increase the mobilized volume of water to about 5.89×109 m3. The silt load for 1995 as estimated by the National Dams Agency (ANB) for all the operational reservoirs corresponded to about 0.02×109 m3. Watershed erosion and the rapid degradation of the forest cover will raise this silt load to 0.482×109 m3. This situation is very alarming for the water resources and consequently in December 2001, the exploitation of major dams (Keddara, Fergoug, Oued Fodda, Ghrib, Beni Amrane, Ksob, Foum el Gherza, Foum el Gueiss) was temporary interrupted. Operations at the Ain Dalia and Hamman Grouz reservoirs were also interrupted in October 2002 due to silt load problems.
The water losses from the distribution network, which were estimated around 1.193×109 m3 in 1995, will be reduced to about 0.532×109 m3 in the year 2010 by special renovation programs. The volume of water lost by evaporation represented 0.427×109 m3 in 1995 and will double for year 2010 to reach 0.883×109 m3, according to the ANB.
In 1995, groundwater was pumped at a volume of about 0.849×109 m3 from an estimated total reserve of 1.256×109 m3. The urgent program of installing an additional 2000 wells will raise the extracted volume to 1.180×109 m3 by the year 2010. In the northern portion of the country, the water needs will evolve in the following manner :
· from 0.791×109 to 1.524×109 m3 per year for drinking water ;
· from 1.216×109 to 7.630×109 m3 per year for agricultural needs ;
· from 0.120×109 to 0.230×109 m3 per year for industry.
It seems clear that the forecast made in 1995 is infeasible and unrealistic. This prediction overevaluates the financial and technical capacities to carry out all the planned projects and also overestimates the land surface for the agricultural development program. The state of water resources and their distribution for the year 2002 already indicate depletion. Accordingly, the following incentive plans were proposed :
· Urgent programs for providing potable drinking water from treated seawater for coastal big cities (Algers, Oran, Arzew, and Annaba). This alternative has been implemented starting September 2002 for Annaba and Arzew.
· Encouraging and developing agricultural crops that consume less water and encouraging use of modern techniques for irrigation (drop irrigation).
· Activating the construction of the 800 planned watershed reservoirs. · Implementing a program for treating and recycling waste waters.
The groundwater quality analysis in the northern portion of Algeria, based on the nitrate and chloride concentrations, led to the following conclusion :
· Increasing nitrate concentrations for groundwater samples (up to 175 mg×L-1 for Chlef, 200mg×L-1 for Sidi Bel Abbes, and values exceeding the permissible level of 45 mg×L-1 for over half of the sampling points throughout the Mitidja).
· The dilution effect during flood events does not overcome the contamination of groundwater by the nitrates.
· Most of the coastal aquifers are contaminated by seawater intrusion. The chloride concentration values are about 3,650 mg×L-1 for Algiers region (Mazafran) and sometimes exceed 4,000 mg×L-1 in the eastern portion of the country (Bourchaid, Kissir).
· Water samples from dams and large reservoirs were not contaminated by nitrates because of the absence of agricultural activities in the watershed.
The degraded water quality and its scarcity complicate the problem of water supply that is already critical in the region.
In conclusion, the gap between water needs and the available water resource is very difficult to close. The goal of this article is to review some of the effects of water shortage and its components. The present article emphasizes the need to implement a change in the decision-making process for controlling and managing water. Improved management of water supplies must not only take into account the available water but also has to optimize and rationalize its use. An urgent planning program should also be developed for treating and recycling waste water and for protecting the environment.
Activité de la phosphatase alcaline bactérienne à l'interface eau-sédiment au sein du réservoir Sahela (Maroc)
B. Alaoui Mhamdi, S. M. Raoui, M. Alaoui Mhamdi and K. Derraz
Les variations temporelles des concentrations en phosphore, celles des abondances bactériennes et de l'activité de la phosphatase alcaline (APA) ont été estimées in situ en chambre benthique placée en deux points du réservoir Sahela (Maroc) durant les mois de septembre et octobre 98. En période de faible oxygénation, nous avons enregistré des élévations relativement faibles des concentrations en orthophosphates (de 0,020 à 0,035 mg. l-1 au point 1 et 0,015 à 0,025 mg. l-1 au point 2) par rapport au phosphore total (0,080 à 0,100 mg. l-1 au point 1 et de 0,035 à 0,040 mg. l-1 au point 2). À cette période, les abondances bactériennes et l'APA montrent des valeurs maximales (8. 106 bact. ml-1 et 0,323 mmol.PNP l-1.h-1 au point 1 et 6.106 bact. ml-1 et 0,438 mmol.PNP l-1.h-1 au point 2 respectivement). L'apparition des conditions anoxiques et la diminution du pH favorisaient la dissolution du phosphore particulaire et la libération du phosphore réactif soluble. Cette libération s'accompagne d'une élévation des abondances de bactéries anaérobies (de 5.106 à 9,2. 106 bact. ml-1 au point 1 et de 3,8.106 à 7,2. 106 bact. ml-1 au point 2) et une diminution progressive d'APA (de 0,200 à 0,025 mmol.PNP l-1.h-1 au point 1 et de 0,125 à 0,077 mmol.PNP l-1.h-1 au point 2). Ce relargage du phosphore à partir du sédiment est accentué par les rejets domestiques et industriels de la ville de Taounate, ce qui accélère le processus d'eutrophisation de ce réservoir.
Temporal variations of phosphorus concentrations, bacterial abundance and alkaline phosphatase activity (APA) were estimated in situ in a benthic chamber. The chamber used has a surface of 0.4m2 and a volume of 90 l ; it resembles those used in oceanography, with a tube connecting the interior of the chamber to the lake surface. The water in the chamber was permanently mixed by an electric agitation system. The chamber was placed at two points in the Sahela reservoir (Morocco). Point 1 was located near Guelta El Haila, a site that receives both domestic and industrial effluent, and point 2 was located in the centre of the reservoir. During the two incubations, eight samples were taken over 24 d in September and October 1998 from point 1, and seven samples were taken from point 2 over 29 d in October. After each sampling, an equal volume of water was injected into the chamber to avoid bubble formation.
Under low dissolved oxygen concentrations in the benthic chamber, we noticed a relatively small elevation in orthophosphate concentrations (from 0.020 to 0.035 mg×L-1 at point 1 and from 0.015 to 0.025 mg×L-1 at point 2) in relation to total phosphorus (from 0.080 to 0.100 mg×L-1 and from 0.035 to 0.040 mg×L-1 at points 1 and 2 respectively). The low residual oxygenation of hypolimnic layer allowed the oxidation of iron, manganese and led to their binding to phosphorus released from the interstitial water. Dissociation of calcium-phosphorus complexes as a result of a pH decrease may have contributed to phosphorus release. The orthophosphate concentrations were relatively low, a situation that favours the synthesis of alkaline phosphatase by aerobic bacteria and facultative aerobic bacteria. In this period, the bacterial abundance and APA were comparable and showed the maximal values (8. 106 bact. ml-1 and 0.323 mmol. PNP l-1.h-1 (phosphate nitrophenol) in point 1 and 6. 106 bact. ml-1and 0.438 mmol. PNP l-1.h-1 in point 2 respectively). The lack of a significant correlation between total APA and bacterial abundance at the two sampling points is probably due to the decrease of aerobic bacteria followed by a repopulation with anaerobic bacteria. However, the relative contribution of two bacterial populations that have different sizes, as at points 1 and 2, showed that the majority of total APA was produced by bacteria that are attached to organic matter. Therefore we suggest that attached bacteria contribute more than free bacteria to APA production.
The beginning of anoxic conditions and the decrease in pH favored the dissolution of particulate phosphorus and the liberation of reactive phosphorus. We noted an elevation in orthophosphate concentrations (from 0.035 to 0.050 mg×L-1 at point 1 and stabilised at 0.025 mg×L-1 at point 2) and total phosphorus (from 0.100 to 0.150 mg×L-1 at point 1 and from 0.040 to 0.050 mg×L-1 at point 2). This liberation followed an increase in anaerobic bacterial abundance (from 5×106 to 9.2×106 bact×mL-1 at point 1 and from 3.8×106 to 7.2×106 bact×mL-1 at point 2) and the progressive decrease in APA (from 0.200 to 0.025 mmol PNP L-1 ×h-1 at point 1 and from 0.125 to 0.077 mmol PNP L-1 ×h-1 at point 2). The anaerobic bacteria did not activate their alkaline phosphatase and the synthesis of their enzyme was progressively inhibited by a de-repression phenomenon caused by high phosphorus concentrations.
We conclude that low oxygen and a decrease in pH favoured the release of phosphorus by dissolution of chemical complexes: calcium-phosphorus, iron-phosphorus, manganese-phosphorus and aluminium-phosphorus. This release is also the result of bacterial phosphatase activity to which fixed bacteria contributed the most. The release of phosphorus from water-sediment interface is further amplified by phosphate import from domestic and industrial waste waters originating from Taounate, which accelerates the eutrophication process in this reservoir.
A. Kellil and D. Bensafia
La présente étude porte sur la réduction des phosphates d'une eau synthétique à faible turbidité par filtration directe sur lit de sable. L'effet de plusieurs paramètres (vitesse de filtration, dose de sulfate d'aluminium et intensité d'agitation) sur la performance du procédé a été étudié. Des efficacités de rétention de phosphore supérieures à 80 % ont été obtenues pour des vitesses de filtration de 5 et 10 m/h. La filtration en ligne est applicable à 5 m/h mais devient beaucoup moins efficace à 10 m/h. Le rapport massique Al/P=2 est optimal et indépendant de la vitesse de filtration ainsi que de l'intensité d'agitation dans le floculateur.
Phosphates and nitrates are the main nutrients responsible for disturbing aquatic ecosystem balance. In general, high levels of phosphate and nitrogen cause increases in algal proliferation, which has negative effects on water quality - the phenomenon is known as eutrophication. While nitrogen is in great quantities in the atmosphere, the sources of phosphorus are limited to two main sources: domestic waste waters and chemical fertilizers. Phosphorus is often known as the limiting factor for the growth of the algae in the aquatic areas.
The usual biological processes (activated sludge) are not very efficient in the removal of phosphorus from waste water (efficiency of less than 50%). Alternative biological processes require certain conditions that are difficult to create in practice. For that reason, we thought it useful to explore another alternative, based on a combination of the precipitation-flocculation of phosphorus by metallic cations brought by alum (aluminium sulfate) and direct filtration through a sand bed.
The aim of this work was to study the effect of filtration velocity, mass ratio (Al/P), and velocity gradient (G) on phosphorus removal efficiency in the flocculator. To this effect, a pilot-scale unit was set up. It consisted of one glass column (2 m long, with an inner diameter of 30 mm) that was filled with sand (0.85 m). The volumetric mass of sand and the porosity of the bed were 2.62 g/cm3 and 0.44 respectively.
The synthetic water used was obtained by dissolution of potassium dihydrogen-phosphate in tap water. The characteristics of this water were as follows: turbidity (varied from 0.5 to 1 NTU); pH 7-8 ; electric conductivity of 450 -550 µS/cm; temperature fixed at 25 °C; a phosphorus concentration 2 mg/L. The pH of coagulation was adjusted to 6.5 and the flocculation time was 10 min. The filtration cycle was arbitrarily fixed at 9 h. The first sample was taken 15 min after the start of the experiment and subsequent sampling was carried out every hour. Once the filtration phase was terminated, the filter was back-washed (combined air-water). The efficiency of phosphorus removal by direct filtration was determined by measurements of residual phosphorus, final turbidity and total head loss in the bed.
The results obtained show that :
- Phosphorus removal efficiency is strongly related to the filtration velocity.
- A mass ratio Al/P=2 seems optimal for the two velocities tested (5 and 10 m/h). For the ratio Al/P=3, we can see an increase in head loss and an increased turbidity in the filtered water, without any improvement in phosphorus retention. The increased turbidity is caused by the formation of fine particles of aluminium hydroxide and/or aluminium orthophosphate.
- When the filtration velocity is 5 m/h, the mixing intensity (G) in the flocculator does not increase the removal efficiency of phosphorus. The efficiencies at G=0 (in line filtration) and G=50 s-1 are similar and development of head loss is essentially the same in both cases. Mixing in the flocculator becomes necessary when the filtration velocity is 10 m/h. In line filtration is much less efficient.
- Intense mixing (G>100 s-1) had no beneficial effect on phosphorus removal, but rather led to a decrease in filtration time.
Analyse de la relation de puissance : débit solide - débit liquide à l'échelle du bassin versant de l'Oued Wahrane (Algérie)
A. Benkhaled and B. Remini
Les flux de sédiments en suspension recueillis à l'exutoire d'un bassin versant sont étroitement liés aux débits liquides. Les modèles de relation liant ces deux paramètres reposent sur des concepts similaires faisant intervenir la loi de puissance Y=aXb. Bien que la validité d'une telle relation n'est pas à prouver, une analyse qualitative de la constante a et de l'exposant b a été effectuée pour un bassin versant situé en zone semi - aride : l'Oued Wahrane (Algérie). Les différentes échelles temporelles d'analyse utilisées ont permis d'expliquer le comportement des paramètres de l'équation en vue d'une meilleure compréhension de cette relation et de relier ces paramètres aux facteurs qui induisent le débit solide. L'objectif principal à trait aux aspects théoriques et pratiques du phénomène du transport solide en suspension. Les résultats ont permis d'identifier de manière assez concluante les modèles saisonniers. L'échelle saisonnière est très représentative du phénomène de transport solide dans le bassin versant. D'autres part, l'événement averse-crue s'est avéré l'unité hydrologique la plus adéquate pour les analyses en relation avec le transport solide.
Fluxes of suspended materials collected at gauge stations are closely related to flow discharges. In the absence of continuous recorded suspended sediment concentration data, hydrologists have used rating sediment transport curves to define the water discharge/suspended sediment relationship and to estimate (predict) suspended sediment concentrations for use in flux calculations. Although there are many methods for developing rating curves, the most common is a power function Y=aXb that relates suspended sediment concentration to water discharge. The discharge measurement is composed of independent variables and a and b are the intercept and slope of the rating curve, respectively. These rating curves are usually generated by least squares regression. Although the accuracy of this approach has been questioned, the applicability appears to be adequate for many purposes. In order to better understand the relationship, this paper reports a qualitative analysis of parameters a and b.
The average suspended sediment load for a cross-section of average stream discharge is the product of the average concentration of the suspended materials and the average flow discharge during the same period. The relation between sediment discharge and water discharge in Algeria rivers is unknown and difficult to assess. Some of these difficulties are related to sampling methods and a continuous record of suspended sediment concentrations is not available. This work will also be used to describe sedimentary dynamics specific to the basin during flood periods. The study was carried out on daily flow discharge data and the concentration of suspended materials collected from the gauging station on the Wahrane River (1972 to 1989). To determine the variation of the parameters a and b of the power relation, a methodology using regression models was used, based on an analysis on various scales of the parameters a and b of the equation of sediment transport in relation to the hydrological characteristics. The analysis reveals the evolution of the parameters a and b. Several temporal scales were used. An examination on an annual scale shows that it was possible to define a very strong relationship between a and the hydraulicity for the dry years and that indeed the factor a, indicating erodability, was well correlated with the hydraulicity during the dry period. The exponent b was correlated with hydraulicity only during the wet period. The relationship between a and mean rainfall was not significant. The seasonal scale revealed a very distinct effect.
The production of sediments was very strong in autumn and winter when sediment transport approached the very typical power model. The seasonal rainfall strongly explained the variations of a while the exponent b was related to peak discharge. The analysis carried out on a monthly scale essentially produced the same conclusions as the analysis on a seasonal scale. It was found that the variation of a was more significant in winter and autumn than in the other seasons, presumably because of the hydrologic and sedimentology activity during this period, and the nature and surface quality of the soil. The relation between parameter a and the monthly average rainfall was very good. Given that the storm is the temporal unit most adequate for hydrological analyses in relation to sediment transport, analysis of single hydrologic events in the Wahrane River yields more details on the variation of a and b. The study was extended to the decomposition of the rising and falling limbs of the hydrograms. The floods of March and October showed the evolution of the parameters a and b. First, the range of variation of a and b, which was narrow during the preceding analyses, was broader for the analysis of the rising limb of the hydrograms. Secondly, the values of a and b showed opposite temporal trends. The high values of b were particularly characteristic of the risings associated with the autumn, during which flow discharges are strongest. The risings of winter floods were marked by low values of a. February and December were characterized by lower values of a in the falling limb than in the rising limb and stronger values of b in falling limb. Lastly, analysis of the values of a with respect to the antecedent precipitations index, IPA, suggested that the highest values of a were related to a weak IPA and a strong soil erodability in the catchment, whereas low values of a would reflect soil saturation (i.e. a strong IPA).
The study showed that the power relationship between sediment load and stream flow was influenced by precipitation, runoff amount and soil moisture, and illustrated the complexity of solid transport.
J. Paing, J.-P. Sambuco, R. Costa, A. Rambaud and B. Picot
Ce travail a porté sur l'application du lagunage anaérobie pour le traitement primaire des eaux résiduaires urbaines sous climat méditerranéen. Il a été réalisé sur les lagunes anaérobies de l' Ecosite de Mèze (Hérault, France). Les lagunes anaérobies constituent un traitement primaire satisfaisant avec des rendements moyens de 55% pour les MES et 30% pour la DBO5, pour une faible emprise au sol. Le taux d'accumulation des boues est de seulement 0,017 m3 /EH.an, notamment du fait de l'efficacité de la dégradation anaérobie. L'équilibre du métabolisme anaérobie des boues est atteint après un an de fonctionnement. L'accumulation des boues se fait alors de façon saisonnière avec une forte accumulation en hiver et une digestion du stock en été. Cette évolution est liée à l'influence de la température sur la méthanogénèse. La production de biogaz (83% CH4) a pu être mesurée par des collecteurs à gaz mis au point pour cette étude et dépend également fortement de la température. Le bilan du carbone montre que 74% du carbone organique épuré est converti en CH4, 13% en carbone inorganique dissous et seulement 15% est stocké dans les boues. Toutefois, les lagunes anaérobies présentent un risque de créer des nuisances olfactives dues à l'émission de H2 S.
This project examined the application of anaerobic ponds for the primary treatment of urban wastewater under a Mediterranean climate. The objectives of the study were to determine removal performances and to study sludge accumulation and the production of biogas. Together, these results allowed us to calculate the carbon mass balance in the anaerobic ponds.
This work was carried out in the two large primary anaerobic ponds of the waste stabilization pond system at the Ecosite of Mèze (Hérault, France), treating domestic wastewater for 13,800 person-equivalents (PE). Anaerobic ponds were 5,000 m3 in volume, 3.1 m in depth and presented a retention time of 4.6 d with a mean volumetric organic loading of 83 g BOD5 /m3 ×d× The characteristics of the influent and effluent (including: suspended solids (SS); chemical oxygen demand (COD); biological oxygen demand (BOD5); bicarbonate (HCO3 -); total organic carbon (TOC); and volatile fatty acids (VFA)) were determined twice-monthly. The volume of sludge and its characteristics (including: SS; volatile solids (VS); TOC; and VFA) were measured monthly. The production of biogas and its composition (CH4, CO2, H2 S) were measured with gas collectors specially developed for this study.
Results showed that the anaerobic ponds constituted a good primary treatment with mean removal rates of 55% for SS, 30% for BOD and 22% for COD. Removal performances were relatively constant over the year. Indeed, removal rates were essentially due to the removal of particulate organic matter by sedimentation. Anaerobic degradation occurred essentially in the sludge layer and the removal of soluble COD was low.
The study of sludge characteristics showed that anaerobic digestion equilibrium was reached after one year of operation. The beginning of methanogenesis could be observed by the decrease in the concentration of volatile fatty acids. The accumulation of sludge showed seasonal variation with an important accumulation in winter and the digestion of the accumulated stock in summer. This evolution could be related to the influence of temperature on methanogenesis. The mean rate of sludge accumulation was only 3.8 g SS/P-E×d or 0.017 m3/P-E×yr. This rate was significantly lower than for the primary settling tank (50-60 g SS/P-E×d) and for the facultative ponds (0.085 m3 /P-E×yr) due to the intensive anaerobic degradation.
The production of biogas was measured by gas collectors specially developed for this study. The biogas contained essentially CH4 (83%); CO2 was less than 4% because it dissolved in the water column and was converted into bicarbonate alkalinity. The concentration of H2 S was less than 1% (between 75 and 4770 ppm) but was the cause of unpleasant odours. The biogas production rate was strongly dependent on temperature. A non-linear relationship was obtained (Ebiogas=4.8451 × e0.1203T, r2=0.92, n=16). The mean annual biogas production rate was calculated to be 49 L/m2 ×d. Seasonal variation in the biogas production rate could be related to seasonal variations in sludge accumulation.
The carbon mass balance showed that 74% of the removed organic carbon was converted into CH4, 13% into dissolved inorganic carbon (bicarbonates) and only 15% was stored in sludge. The mass balance was well equilibrated and did not show the entry of atmospheric CO2, which occurs in aerobic ponds where CO2 is used by algae to produce their cell biomass. In an anaerobic pond, the low production of sludge was due to the efficiency of the anaerobic degradation but also to the low internal biomass production.
To conclude, the use of a primary anaerobic pond was advantageous and permitted a reduction in the required surface area for a waste stabilization pond system. This process produced effluent for secondary treatment in a facultative pond with essentially the removal of the particulate organic matter. However, However, anaerobic ponds may cause odor problems linked to the emission of H2 S.
Production et transport des matières solides en suspension dans le bassin versant de la Haute-Tafna (Nord-Ouest Algérien)
A. Megnounif, A. Terfous and A. Bouanani
Dans ce travail on s'intéresse aux sédiments transportés en suspension par le cours d'eau Oued Tafna à Beni-Bahdel. Une analyse de la variabilité de la charge en suspension de l'eau quant aux fluctuations des régimes hydro-climatiques, pour la période allant de septembre 1988 à août 1993, montre que la production des sédiments est très forte en automne mais de faible ampleur au printemps. En été, des crues éphémères fortement chargées sont parfois observées. En automne, le flux des matières solides en suspension représente 44% du flux annuel estimé à 286000 tonnes, alors que les apports en eaux ne représentent que 17% de l'apport annuel moyen évalué à 30,9 millions de m3. Au printemps, malgré l'augmentation des débits (60% de l'apport total annuel), la charge en suspension diminue de façon significative.
Cependant, les sédiments ramenés par le cours d'eau sont produits soit par ruissellement superficiel sur les versants soit arrachés du lit et des berges du cours d'eau. On montre alors que la contribution de ces deux mécanismes d'érosion reste remarquablement constante d'une année à une autre. La production du ruissellement superficiel en matières solides représente alors 62% du flux annuel alors que l'érosion du cours d'eau contribue par 38%.
Knowledge of suspended sediment loads carried by rivers provides insight into the underlying erosion mechanisms. In Maghreb rivers there are few data available on suspended sediment transport (PROBST and SUCHET, 1992). In this context knowledge of suspended sediment loads in the Oued Sebdou River (Mediterranean Algeria), which is a main stream in the Upper-Tafna basin, is of interest. The purpose of this study was to determine periods of active erosion and estimate the contributions of slope erosion and channel erosion in this basin.
Situated in northwest Algeria, the Upper-Tafna basin covers an area of 256 km2 (Fig.1). Rising in Ouled Ouriach, the upper reaches of the river flow through Jurassic soils at altitudes up to 1400 m. These tributary streams join on the Sebdou plain (900 m), which is composed of Plio-quaternary alluviums. The soil is composed of calcareous-marnes, calcareous and Jurassic dolomites (BENEST, 1972; BENEST et al., 1999) until the Beni-Bahdel dam.
The study zone is characterized by a semi-arid climate. Analysis of the rainfall data at the Beni-Bahdel station (X=34°42'33"; Y=01°29'48"; Z=660 m) from 1939-1940 to 1997-1998 (Fig. 2) demonstrates that the rain supply has decreased greatly since 1975, with more than an 18% decrease in rainfall amounts.
Hydrological data and methodology
The study used instantaneous water discharge and suspended sediment load values (1257 values) measured and supplied by the National Agency of Hydrologic Resources [ANRH]. Five annual water years have been investigated during the period from September 1988 to August 1993. The discharge values (QL, in m3/s) were determined from the rating curve for the measured heights of the water at the gauging station. The suspended load (g/l) was determined on water samples taken from the stream channel banks. The number of samples taken was adapted to the hydrological regime (i.e. every other day or during flood periods) and samples were collected as frequently as increments of 10 cm.
The suspended sediments were carried from the slopes into the river by surface runoff or by stream channel erosion caused by flowing water. In order to express the contribution of these two mechanisms, we applied hydrograph separation methods used by ETCHANCHU (1988), ETCHANCHU and PROBST (1986), KATTAN et al. (1987) and PROBST and BAZERBACH (1986).
Balance sheet of solid and liquid contributions
The annual liquid contributions from 1988 to 1993 were evaluated to be 30.9 million m3. The abrupt slopes of the basin cause water to move quickly with a lot of force. In turn a large amount of suspended sediments (estimated at 286,000 tons) is generated, which corresponds to a soil erosion rate of 1120 tons/km2/year. This value is comparable to those estimated by WALLING (1984) where the erosion rates ranged between 1000 and 5000 tons/km2 /year.
As seen in Table 1, the annual suspended sediment load varies dramatically. In 1990/91, the soil erosion rate was estimated to be 4283 tons per km2 ; this amount was 180 times higher than the sediment load in 1992/93 (24 tons per km2). Furthermore, there is no relationship between annual suspended sediment load and water discharge or annual rainfall. The annual precipitation (355 mm) was recorded during the year 1988/1989, and generated a soil erosion rate evaluated at 1072 tons per km2. For a similar rainfall, 345 mm recorded during the year 1992/93, the erosion rate was only about 24 tons/km2 /year.
Autumn liquid contributions represent only about 17% of the mean annual supply (Fig. 4). Furthermore the heavy rains produce higher peak flows as the rain encounters dry soil with less vegetation cover and produces large quantities of solid by heavy rain splash erosion. The mean seasonal suspended sediment concentration was higher representing approximately 44% of total annual production. Spring corresponds to a dilution in sediment concentration caused by a large groundwater contribution. The volume discharged by spring flow is about 60% of total annual river discharge. The solids transported during this period are mainly eroded from the stream banks and represent 36% of total annual sediment load.
Calculated values of monthly suspended sediment load and water supply are reported in Table 3. The monthly suspended sediment load carried by the river was very irregular. The majority (94%) of the total sediment load was transported in only three months, 45% in September, 30% in March and 19% in July. The monthly amounts varied from year to year, and this can be explained by the occurrence of floods (Table 4).
The river suspended sediments were carried from the slopes into the river by surface runoff water or by stream channel erosion. Using a hydrograph separation method, the contributions of these two mechanisms are summarized in Table 4. According to these values we observed that :
- The contribution of slope erosion processes represents an average of 62% of the total river-suspended sediment transport.
- The second contribution from channel erosion represents at least 38% of the total river transport. This percentage is comparable with those obtained by ETCHANCHU and PROBST (1986) for the Garonne River basin (30%). This contribution is higher than those estimated by KATTAN et al. (1987) (22% for the Senegal River basin) and by ROBINSON (1977) for some American rivers.
- This contribution was far less than those estimated by DUYSING (1985) for a forest stream in Luxemburg, where sediment produced by surface runoff was about 62% of annual suspended sediment loads.
- This estimate is comparable to those estimated by where the erosion rates range between 1000 and 5000 tons/km2/year.
- Although the annual amount of suspended sediment was variable, the relative contributions of the slope and channel erosion were less variable from year to year.
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