Elimination de l'azote et du phosphore dans un lagunage à haut rendement

Abstract

L'objectif de cette étude est de comprendre le fonctionnement épuratoire de l'écosystème particulier que constitue le lagunage à haut rendement (LHR) afin de déterminer les principaux facteurs responsables de l'élimination de l'azote et du phosphore. Sur un bassin de 48 m 2 , alimenté eu eaux usées domestiques préalablement traitées pendant une semaine dans un bassin primaire ont été suivies selon un rythme hebdomadaire les formes carbonées, azotées et phosphorées dis-soutes et particulaires, les variables caractéristiques de l'activité photosynthétiques (chlorophylle a, pH et 0 2 ) et les données climatiques (rayonnement solaire et température). Une analyse en composantes principales réalisée sur l'ensemble des résultats a montré, d'une part l'opposition des variables climatiques et photosynthétiques aux formes minérales de l'azote et du phosphore (N-NH 4 et P-PO 4 ) et d'autre part l'influence de la charge organique sur le fonctionnement du système. L'évolution des formes azotées et phosphorées présente un effet saisonnier marqué. En hiver ou lors de surcharge organique importante, une augmentation du temps de séjour peut améliorer les rendements épuratoires. L'évolution de la matière organique dissoute est indépendante du cycle saisonnier. Au cours des deux années du suivi on observe une adaptation progressive de l'écosystème à la dégradation de la matière organique.Dans le lagunage à haut rendement l'élimination de l'azote et du phosphore est liée principalement à l'activité algale, qui entraîne une assimilation biologique et une élévation du pH, responsables des phénomènes chimiques de volatilisation de l'azote ammoniacal et de précipitation de phosphate de calcium.High rait algal ponds (HRAP) for wastewater treatment have been the locus of a lot of attention since their creation by Professor W.J. OSWALD in the early 1960' s. These aquatic ecosystems are hypereutrophic because of massive nutrient introduction with the wastewater influent. This kind of pond is very different from oxidation ponds because al short residence times (2 to 12 days), shallow depths (0.30 to 0.60 m) and constant mechanical mixing which improves algal growth. This study has been carried out in a small town in the department of Herault (France). The objective is to establish the efficiencies and mechanisms for nitrogen and phosphorus remval from secondary domestic wastewater by treatment by HRAP. A high rate algal pond of 48 m 2 surface area and 35 cm depth, constantly mixed with paddle wheels was studied ( fig . 1). Residence limes changed from 2 te 12 days with solar radiations and influent loading. The samples were collected once a week from the in- and outflow at 3 p.m. - Dissolved and particulate COD, varions nitrogen and phosphorus compounds, suspended solide, chlorophyll-a, pH, dissolved oxygen, temperature and irradiation were determined. The results of principal component analysis show a negative correlation between (1) the climatic and photosynthetic parameters and (2) inorganic nitrogen and phosphorus compounds, as well as the influence of organic loading on the HRAP efficiency. The chronological clustering analysis was performed on the data taking into account the discontinuities of effluent autrient contents ( fig . 4). During the first year, from February to September 1988, good climatic conditions were shown to favour photosynthetic activity and consequently a good ammonia and orthophosphate removal. Front September 1988 to February 1989, the effluent was characterized by high ammonia and orthophosphate levels because of unfavourable climatic conditions and low photosynthetic activity. During the second year, the objective was to improve removal efficiencies ; the residence time was changed with solar radiations and influent loading. The results of the chronological clustering analysis of ammonia and orthophosphate levels showed only one sequence (February 1989-January1990) because the seasonal variations of nutrient removal were attenuated. The optimal residence time was found to be 4 days in summer and 12 days in winter. The evolution of dissolved chemical oxygen demand is independent of seasonal conditions but decreases with the maturity of the system (< 50 mg . l -1 ). The mass balance of nitrogen and phosphorus compounds has been established ( fig . 5). In high rate algal pond influent, the main nitrogen form is ammonia. In the effluent the main nitrogen form is suspended organic nitrogen. The oxidized forms (NO 2 - et NO 3 ) present very low levels ; nitrification is a minor process in the system. Ammonia is reduced by both assimilation and stripping ; suspended organic nitrogen in the effluent is an indicator of assimilation by algal biomass and loss of nitrogen is an indicator of ammonia stripping. With regard to directive 91/271 EEC concerning domestic wastewater treatment, partial algal separation is necessary in order to achieve 15 mg • l -1 total nitrogen, which is the limit for zones sensitive to eutrophication. The residual phosphate levels are controlled by pH, which is responsible for instantaneous calcium phosphate precipitation. As the dissolved calcium concentrations are high in the calcareous region near the site (Ca = 125 mg • l -1 ), the pH of the water will be the main contraint on phosphate elimination in the HRAP. Theoretical calculations indicate that the precipitate is probably an amorphous tricalcium phosphate in pseudoequilibrium ( fig .6). Phosphate concentrations are much higher titan the values expected for calcium hydroxyapatite in equilibrium (MOUTIN et al ., 1992). The limit of 2 mg • l -1 for phosphores can only be attained when pH reaches or exceeds 9. Nitrogen and phosphorus removal is correlated mainly with phytoplanktonic activity, which controls biological nutrients assimilation and pH levels. High pH values are responsable for ammonia stripping and calcium phosphate precipitation.