Essais d'épuration des eaux usées de Marrakech par la jacinthe d'eau (Charges organique, bactérienne et parasitologique) Marrakesh wastewater purification experiment utilizing water hyacinths (organic, bacterial and parasitological loads)

Résumé

Cette étude est destinée à tester expérimentalement les capacités d'épuration des eaux usées par lagunage à macrophytes (jacinthe d'eau : Eichhornia crassipes), sous les conditions climatiques de Marrakech.

L'installation fonctionne en continu avec un débit constant à l'entrée de 10 l/min. La charge admise est de 40 g DCO/M²/j.

Sous l'aspect de la production de biomasse végétale, les effluents domestiques constituent un bon substrat nutritionnel. Les taux de croissance et les productions obtenues montrent dans l'ensemble une excellent adaptation d'Eichhornia crassipesà ce milieu. Le maximum de biomasse et de productivité ont été obtenu en période estivale et sont respectivement de: 40 kg MF/m² et 38,6 MS/m^2/j. Il s'est avéré également que la jacinthe d'eau est persistante toute l'année sous le climat méditerranéen aride de Marrakech.

L'épuration des eaux usées domestiques par lagunage à macrophyles aboutit à des rendements satisfaisants surtout en période estivale où on obtient un abattement de 87 % de la DCO et une réduction de 95 % des MEST.

Sur te plan sanitaire, l'abattement de la charge bactérienne exprimée par les bactéries témoins de contamination fécale peut atteindre jusqu'à 2ULog pour un temps de séjour théorique très court (7 jours).

Ce système e par ailleurs fourni des abattement de 100 % des oeufs d'helminthes parasites au niveau de l'eau épurée.

Mots-clés:

• Jacinthe d'eau,
• eaux usées domestiques,
• épuration,
• oeufs d'helminthes,
• nématodes

Abstract

The aim of the present study is to experimentaly test the capacities of the mater hyacinth (Eichhornia crassipes) in order to purify wastewater under Manakesh climatic conditions.

The experiment was carried al wastewater spreading zone of Marrakesh pretraitement.

The experimant's installation is made of two lined water yacinth ponds that receive domestic wastewater.

The proposed process is a continuous system with a constant flow rate of 10 l/mn. The theoritical retention time was estimated to 7 days. The allowed load is 40 g COD/m²/day. Macrophytic biomass was observed for both ponds during the experimental period (Match, 1986 - February, 1987). Parameters of organic, bacterial and parasitological loads are studied in order to determine the system efficiency under arid climate.

Obtained results show the water hyacinth ability to adapt to Marrakesh climatic conditions. The number of plants doubled at 12 days, this is coherent with results obtained by PENFOUND (1956), BOCK (1969), WESTLAKE (1963, 1975) and SCULTHORPE (1967). Maximum biomass level and productivity were achieved during the summer period : 40 kg WW/m²/day and 38,6 g DW/m²/day respectively. Biomass and productivity obtained under arid climate are similar to results obtained by WOOTEN and DODD (1976), and by DINGES (1976) under subtropical conditions, and higher than chose obtained by JOHN (1985) under temperate climate. The growth period of water hyacinth is estimated to 9 months at Marrakesh, 10 months at subtropical climate (WOLVERTON and MC DONALD, 1976) and limited to 6 months under cold climate (COPELLI et al., 1982; DUBOIS, 1983; SAUZE, 1983; DE CASABIANCA, 1985). Temperature is considered as a limited growth factor of water yacinth. According to FRANCOIS et al. (1977), the water hyacinth growth was stopped when the temperature is lower than 10 °C. Linder Marrakesh arid climate, the temperature is always higher than 10 °C. It was also found that the water hyacinth survive all a year around in the arid climate of Marrakesh.

Domestic sewage purification by water hyacinth leads to satisfactory efficiency during the summer concerning reduction of COD: 87 % and TSS : 95 %. This phenomenon may be jointed to the retention time wich was lengthed (9,4 days) in the summer, and the higher biomass density of water hyacinths in this one. The purifying action of floating macrophytes (Eichharnia crassipes) is physical and biological. The root system stabilizes the medium thus favoring sedimentation of TSS and particulate COD both on the bottom of the tank and by trapping in the root hairs. Elimination of COD is realized by means of the action of bacteria which are present, by sedimentation of particulate COD and root filtration.

The biological action of the plants is not an important mechanism for COD elimination. The system efficiency is low at the winter and the reduction of COD and TSS have not exceed 60 % and 82 % respectively because the degenering of the water hyacinths.

From sanitary point of view, bacterial load reduction expressed by control faecal contamination bacteria achieved 2 log Units for a short theoritical retention time (7 days). This is higher than the result obtained by DUBOIS (1985). Two hypothesis are given to explain reduction of bacterial load by water hyacinths :1) the bacteria are sedimented or trapped in the root hairs of the water hyacinths whith TSS. 2) Water hyacinths may have a capacity to secrete a chemical substance wich could have bactericid or bacteriostatic effect. The improvement of retention time and the addition of one or two supplementary ponds will probably lead to better results. Moreover, this process had also reduced parasitical helminth eggs to undetectable levels (100 %). The parasitical helminth eggs distinguisched at domestic sewage received by the experimental installation, are Taenia, Hymenolepis, Trichuris and Ascaris geints. Their total number vary tram 0 to 120 eggs/l with a mean of 32. Other types of eggs could be encountred generally in waste water as : Toxocara, Oxyure, Capillaria and Taxoascaris (FOX and FITZGERALD,1976) but was not detected by our technique. No helminth eggs were found in purified wastewater descended through water hyacinth ponds. This phenomenon is explained by supposing that the helminth eggs are present in the effluent but it was the detection limit of the employed technique (Bailenger method), or there is no eggs really at the effluent because of their higher specific weight. Ascaris, Taenia and Trichuris eggs have a sedimentation rate of 0,65 m/h, 0,26 m/h and 1,5 m/h respectively (FEACHEM et al.,1983). The eggs sedimented rapidly in the water hyacinths ponds involving a transfer of the effluent pollution to the sediment. Several authors affirmed that the stabilization ponds are an effective means to reduce parasitical helminth eggs of the wastewaters (GLOYNA, 1972; KOWAL, 1985). Hence, if the parasitical risk could be controled in the purified water (effluent), particular attention should be given to sediments. It is also important to point out, that no parasitical nematode is found at the influent. Nematofauna associated to wastewater and roots of water hyacinth, was represented by bacteria consumer nematode. The abundance of nematode is definite by the existence of bacterial food in the wastewaters (CALAWAY, 1963; SHIEMER, 1976). The principal genus determined are Rhabditis sp, Plectus sp. and Mononchoïdes sp. It appears that Rhabditis genus, is dominant in the first pond (94,7 %) of the nematode population. However, the two genus Rhabditis sp. and Plectus sp. are dominant in the second one and represent 50 % and 49 % respectively. The presence of Plectidae in the second basin indicates that is less loaded (ZULINI, 1976). However, under the arid climate conditions of Marrakesh, the process based on water hyacinth for wastewater purification, is faced with two major problems : first, the water loss by evapotranspiration reachs 60 % during the summer time under arid climate of Marrakesh. The development of Mousquito particularly in the summer, constitutes the second problem of our proposed process. Moreover, front economical point of view, the water hyacinths show a good productivity in the summer under arid climate and could be exploited in several field.

Keywords:

• Water hyacinth,
• domestic sewage,
• purification,
• nematode helminth eggs and climate