Bilan du carbone dans le lagunage anaérobie appliqué sous climat méditerranéen Mass balance of carbon in anaerobic ponds applied under a mediterranean climate

Résumé

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.

Mots-clés:

• Lagunage anaérobie,
• biogaz,
• accumulation des boues,
• bilan du carbone,
• performance épuratoire

Abstract

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 × e^0.1203T, 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.

Keywords:

• Anaerobic pond,
• biogas,
• carbon mass balance,
• sludge accumulation,
• removal performance