Résumés
Résumé
La qualité de l'effluent des systèmes de biotraitement par boues activées est souvent limitée par la performance de l'unité de décantation. Cette étude analyse les causes possibles des difficultés d'opération rencontrées dans la décantation des boues évacuées aux décanteurs secondaires d'une raffinerie de pétrole. Les objectifs visent à déterminer et à quantifier l'effet des paramètres d'opération qui affectent le fonctionnement du décanteur, en les reliant si possible à son mauvais fonctionnement, à évaluer de l'ajout d'alun et à proposer des méthodes correctives pour améliorer l'efficacité de décantation.
L'étude montre que :
- l'efficacité des décanteurs sera améliorée si le niveau d'oxygène dissous et la concentration en matières totales en suspension (MTS) dans le bassin d'aération sont respectivement maintenus entre 0,9 à 1,3 ppm et 4 300 à 4 800 mg/L;
- le pH de la liqueur mixte ne semble pas influencer la décantabitité;
- l'addition d'alun doit être évitée ou maintenue inférieure à 50 mg/L;
- l'agitation et les vitesses linéaires élevées dans les conduites d'alimentation des décanteurs doivent être évitées.
Mots-clés:
- Boues biologiques,
- décantation,
- traitements tertiaires,
- matières totales en suspension (MTS),
- alun
Abstract
The wastewater treatment systems built in the seventies for treating the effluents from oil refineries often fail to comply with new environmental standards. The present study focuses mainly on the problem of sludge settling in the clarifiers. The purpose of this work was to determine the effect of various parameters on the settling characteristics of the activated sludge, to evaluate the rote of alun as a flocculating agent and to characterize the operation of the clarifiers primarily as it relates to the settling problem.
The influence of the quantity of the alun added and of the stirring speed on the settling characteristics of the sludge were determined in jar tests. The effects, on the performance of the clarifiers, of the level of dissolved oxygen in the bioreactor, of the pH and of the concentration in total suspended solids (TSS) maintained in the mixed liquor, were also investigated.
The studies on the sludge settleability led to the following conclusions :
- the concentration in dissolved oxygen and the concentration in TSS in the aeration tank are two parameters which have a critical effect on the effluent quality. The optimal operating intervals for these parameters are from 0.9 to 1.3 ppm for dissolved oxygen and from 4 300 to 4 800 mg/L for the TSS;
- the pH of the mixed liquor is not a main factor and does not seem to have any effect on the sludge settleability;
- the addition of alum in excess of 50 mg/L has an adverse effect on the sludge settling characteristics. Addition of alum below this level, however, improves the clarity of the residual liquor. Nevertheless, the addition of this flocculating agent is not recommended;
- the high flowrates and violent mixing in the piping system used to feed the mixed liquor from the aeration tank to the clarifiers adversely affects the solid-liquid separation in the clarifiers.
By simply stopping the addition of the flocculating agent, maintaining the dissolved oxygen concentration between 1 to 2 ppm and the TSS between 4 300 to 4 800 mg/L in the wastewater treatment unit, it was possible to improve substantially the quality of the effluent and thus meet the environmental standards.
Keywords:
- Biological sludges,
- decantation,
- ternary treatments,
- total suspended solids,
- alun