5.5. SR 06-07-1993 - Laserfiche WebLink

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Mr. Patrick Klaers June 3, 1993 Page Two In summary, Otsego should pay 1.5 million dollars for the construction and $2.00 per 1,000 gallons of sewage treated at Elk River. DIGESTER REHABILITATION A structural and mechanical inspection of the existing primary anaerobic digester was performed on May 17, 1993, following the digester cleaning. We were surprised to find that, after 15 years of service, the digester is still in good condition. To prolong the service life of the existing digester and to be operational with future WWTP expansions, we recommend the following remedial repairs. The interior inspection of the steel dome revealed several areas of total coating failure with approximately 15 % to 25 % steel plate section loss from corrosion Due to the highly corrosive environment inside the digester, these exposed areas of steel will continue to corrode at an accelerated rate causing potentially dangerous structural conditions throughout the dome if it is left unattended. It is recommended that the interior of the dome be completely sand blasted and recoated with a high build coal tar epoxy. The steel dome interior surface restoration work would cost approximately $9,000, but will extend the service life another 15-20 years. If such preventive repair is not implemented, it will eventually cost the City more than $120,000 for a new steel dome replacement. The reinforced concrete walls and floor slab are structurally sound. However, there are some areas with spalled and pitted concrete. The deterioration of the concrete will continue at a much slower and less damaging rate than that of the steel dome. We feel at this time that the recoating of the walls and floor slab are not critical to the future operation of the digester tank. It is recommended that future wall and floor inspections be scheduled with any future digester cleanings to monitor and evaluate it's performance uncoated. Although minor corrosion is also found on the interior piping, repair is not recommended at this time since it will be cheaper to replace it in the future than to sand blast and recoat it now. The existing interior piping appears sound and does not require replacement at the present time. The piping is either ductile iron pipe or cast iron pipe. An additional 15-year service life for these pipes can be expected. Other immediate rehabilitation needs include steel dome anchorage repair, a 6" pipe support, a new sewage gas pipe connection, and an additional 24-inch access manway, which will provide conformance with OSHA confined space entry requirements and additional ventilation during the interior surface preparation and coating, as shown in Table 1. The total cost for this work is estimated at $12,600. Since the digester interior was just thoroughly cleaned for inspection and is now clean enough for repair work, it is recommended to perform this repair work now to avoid another $18,000 digester cleaning cost to the City when the WWTP is expanded. It is our understanding that the digester can be temporarily out of service and repaired sometime during the next several months.