EGG-SHAPED DIGESTERS – EXPERIENCE OF A PROCESS UPSET

Egg-shaped digesters have been promoted as free from maintenance expenses that plague conventional digesters. In 2001, however, OSP suffered digestion process upset that was exacerbated by clogging and mixing problems that developed because of deferred maintenance and the accumulation of grit. A Nocardioform bloom in the secondary system was unable to be brought under control because pumping and mixing problems reduced the digester active volume and thus substantially limited the amount of feed that could be introduced. Biological foam from the WAS appeared to correlate with substantial digester foaming problems that led to the uncontrolled transfer of digester contents to the belt press day tank and poor dewaterability. In addition, episodic foaming caused interruption in gas collection and affected the cogeneration process. Only by putting a stand-by digester in service and by clearing the accumulated grit and debris in two of the remaining digesters could the mixing lines be reopened and full-digester volume utilized. The versatility of the egg-shaped digester design allowed the maintenance staff to clear grit through the bottom withdrawal piping and the staff was able to complete the cleaning job in less than three weeks (480 man-hours).Historically, the digestion system has been under loaded, achieving over 61% volatile solids reduction and averaging 83 mg/L volatile acids and a VA/ALK ratio of 0.02. During the peak of the upset conditions, the volatile acids reached over 1900 mg/l and the VA/ALK ratio increased to 0.28. The volatile solids reduction dropped to around 14%. The sour conditions that developed during the upset period quickly subsided once the pumping and mixing problems were addressed. Because of the reduced wasting during the digester upset, Nocardioform filaments predominated in the mixed liquor and substantial foaming problems persisted in both the secondary system and in the digesters until wasting rates could bring the filament counts to historic levels (≤1.0 × 106 intersections/g VSS).The paper describes in detail the causes of the OSP digesters upset, the impact to upstream and downstream processes, and the corrective measures taken.