Nassau County Department of Public Works, Cleaning, Repair and Addition of Digesters and Storage Tanks

Posted on June 18, 2015

Client: Nassau County Department of Public Works
Location: Cedar Creek Water Pollution Control Plant
Project Type: Cleaning and Repair of Digesters and Storage Tanks and Addition of New Tanks
Project Period: 1986 - 1992
D&B Project Manager: Steven A. Fangmann, P.E., BCEE
Project Cost: $25,000,000

Project Description

Design and Construction Phase Services, Construction Scope of Work, Packaging and Delivery

Steven Fangmann, P.E. directed and managed the Design and CM of the removal of contents and cleaning of the original two (2) primary digesters and one secondary digesters and one sludge storage tank at the Cedar Creek Water Pollution Control Plant (WPCP), in Wantagh, New York. The work also included the construction of an additional four tanks and related mechanical equipment.

CC-aerialThere are now four (4) primary, three (3) secondary and two (2) storage tanks at Cedar Creek as a result of the various project included. The project also included 5 heat exchangers along with related sludge piping. The tank cleaning consists of the removal of all the digester contents from the lower portion of the digesters and storage tanks. The sludge and wash water was pumped into a temporary belt filter press system for the purposes of dewatering. The contractor for this project was then required to pipe the dewatered contents of the digester to sludge handling vehicles for off site disposal. In addition, one (1) heat exchanger and four (4) gas recirculation compressors were replaced. Painting of tank covers was also performed as required.

The Cedar Creek WPCP Cleaning and Repair of Existing Digesters and Storage Tanks and Addition of Four Tanks project involved the cleaning of the existing digesters and storage tanks, heat exchangers, associated piping and repair work. The four Primary Digesters are the key to the sludge treatment process at Cedar Creek.

These tanks not only provide “required” sludge stabilization but the production of energy. This energy is in the form of digester gas which is approximately 60% natural gas. 

This gas is treated and utilized in the facilities power plant engines and boilers. The waste heat from the engines is utilized to heat the digesters to process the sludge. This “Green” technology was maintained during construction. Additional treatment is accomplished in the three (3) Secondary Digesters followed by storage in the Sludge Storage Tanks. One of each tank was kept in operation at all times.

Estimated Budget at Time of Bid/ Actual Construction Cost
$25,000,000
Project Schedule
1986 – 1992

Specific Techniques or Unique Methods employed to explore design options, determine estimated construction costs, and/or monitor construction.
Specific Key Construction Components and Previous Lessons Learned

  • The digesters must be kept in operation. The schedule must clearly define when the tanks go in and out of service. Typically, after the first primary digester is completed and in service with gas production; additional primary digesters can be taken out for cleaning. Per the County’s specification for this project, the rehabilitated digester must operate satisfactorily for 21 days before the next digester is taken out of service. In the past it is understood that a “new” fully functioning digester will outperform by a wide margin the tank taken out of service. An opportunity to advance the job by having “two” primary’s out of service can be reviewed. This will be dependent on many factors with plant operations being the most significant.
  • Dewatering must be maintained. Although originally designed to withstand being empty through the use of floor valves (groundwater relief); these valves have been found to clog. The danger of the floor of the digesters cracking and leaking cannot be allowed. It is imperative that the proper permits be received by the Contractor for this operation from the NYSDEC and that a discharge location be designated by the County for the groundwater permit.
  • Odor Containment. A plan must be developed by the Contractor and proper sequencing be included in the CPM Schedule. This plan must identify when odors can be generated and how they will be controlled.
  • Health and Safety. Digester Gas mixed with air is volatile and when confined will be explosive. There are no minimum measures to be taken to control this. Measures must be taken to never allow such conditions to take place. Also, no confined space can be entered without proper monitoring and equipment. This must be maintained even when everyone believes that it is safe. Monitoring must be maintained and all other confined space requirements and certifications met.
  • Dewatering of Sludge Measures. Again, a complete plan for removing water from the cleanout material must be in place to prevent odors and spills. A truck route of approved roads must be maintained by the Contractor. A disposal location of the clean out material must be identified. Appropriate manifests/tickets must be managed.
  • Struvite in Piping. When internal sludge or gas piping is exposed to the atmosphere, attention must be given to the proper handling since struvite buildup in the pipe can ignite or smolder when taken from an anaerobic to aerobic condition. A plan for the proper handling must be received from the Contractor.
  • Concrete Repair. The concrete will need to be repaired at joints and seals.
  • Valves. Various valves and internal gaskets will need to be repaired within the tanks.
  • Painting. All of the tank covers, ceilings and roofs will be painted under this project.

Additional CM Components

The project will be constructed under one General Construction Contract with a contract duration of thirty three (33) months. The work must be performed while maintaining both process and non-process plant facilities in continuous operation during the entire contract duration.

Key items for the Construction Management team on this project are:

  • Review detailed working plan developed by the contractor.
  • Track the submittal and review of all shop drawings and procurement of equipment items to minimize the impact to the project schedule.
  • Have the Contractor develop and cost load the CPM schedule within the first 60 calendar days of the NTP, for D&B to review.
  • Project Progress Meetings may have to be held on a weekly or bi-weekly basis at the start of the project to expedite the contractor’s progress on shop drawing submittals and the development of the detailed working plan.
  • After each tank cleaning, perform timely inspection (within one week) to determine the extent of repair work.
  • Odor Control and operation of the temporary portable belt filter press must be monitored closely for compliance with the specifications.
  • Confined space entry requirements must be strictly enforced for both the contractor’s and Construction Manager’s work force.
  • Record keeping involving field measurements and documenting time and material work is important to verify accurate payments to the contractor.

In accordance with the County RFP, it is anticipated that construction will commence in early 2015 and be complete in late 2017.

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