Fort Walton Beach Council agenda — 04-29-03 workshop agenda — reading text

to three ( 3) primary alternatives for the City' s consideration. The reviewed alternatives take into consideration current and projected II wastewater flows within the City' s wastewater service area, ease of operation, site layout, capital II I F:\2001\ 01.0387. 000\Report\ R-4\ 041403 FINAL\SEC 4. DOC HAI#01.0387.000 4-4 041403 Ii fIIIIII III costs, and operational costs among other issues. Considering several available treatment processes, there are essentially three ( 3) feasible alternatives available to the City, which include: 1. Rehabilitating the existing RBCs and replacing damaged units. I 2. Retrofitting the RBC basins with an advanced fixed film process, such as the WesTech STM- Aerotor"" system. II 3. Replacing the RBC process with a conventional suspended growth activated I sludge process. I Summarized herein are the findings of our analysis and an evaluation of the alternatives available to the City with regard to upgrading their WWTP to improve the overall efficiency and I.'." I perfonnance of the facility. I 4. 2. 1 Alternative 1 - Rehabilitate Existing RBCs The existing RBC units are reaching the end of their service life and the units have been plagued l" with a history of structural and mechanical problems, such as mechanical failure of the drive fill units, bearings, and frame assemblies. These mechanical failures have resulted in component jz..; III interference with the concrete basins, which has caused damage to the structure. Previous repairs to the RBC concrete structure have been substandard and the structure requires substantial ill. improvements to support new drive units, frames, and media. II The RBC structures which support the disk media includes a rotating steel shaft supported by the II walls of the concrete process basin. The fixed film media is partially submerged in the filii wastewater and rotates at approximately 1 to 5 revolutions per minute ( rpm). Biological growth becomes attached to the surface of the disks and eventually fonns a slime layer over the entire I' ll wetted surface area of the disks. The rotation of the disks alternately contacts the biomass with III the organic material in the wastewater and then with the atmosphere for adsorption of oxygen. The disk rotation affects oxygen transfer and maintains the biomass in an aerobic condition. The rotation also is the mechanism for removing excess solids from the disks by the shearing forces it creates and maintaining the sloughed solids in suspension so they can be carried from the unit to a clarifier. Figure 4- 2 illustrates the City' s WWTP RBC arrangement and the drive units and motors are shown in Figure 4- 3. j lit lilt F:\2001 \ 0 1.0387. 000\Report\ R-4\ 041 403 FINAL\ SEC 4. DOC HAI#01.0387.000 4- 5 041403 II  RBCs utilize a low operating food to micro- organism ( F1M) ratio, which pennits them to withstand hydraulic and organic surges effectively. The effect of staging in this plug flow system helps eliminate short- circuiting and lessens the effects of shock loadings on the system. At a flow of 4. 50 MGD, the hydraulic loading for the existing system is in the range of 1.25 to I''''.' i!! 2. 25 gpd/sq. ft. Further, assuming a CBODs removal of 30- percent across the primary clarifiers, to 2. 63 lbs. then organic loading of the existing units appears to be in the range of 1.46 I CBODslday/ sf at a flow of 4.50 MOD. Both of these loadings are within the generally accepted I values for municipal WWTPs. I To utilize the existing RBC process, several items were addressed to determine the feasibility of this alternative. This alternative requires replacement of the failing RBC shafts, mechanical drive units, media, and rehabilitation of the concrete structure to bring the existing facilities to an acceptable level of service. In addition, this alternative includes the installation of torque sensing III devices for the individual RBC shafts to signal shaft rotation stress when a malfunction occurs within the assembly. It is anticipated that the installation of a torque- sensing device should reduce damage to the units by disrupting power to the motors and subsequently stopping rotation II when a malfunction occurs. In addition to the biological treatment considerations described above, the final clarifier loadings must be addressed. The generally accepted design guidance document entitled " Recommended Standards for Wastewater Facilities, Great Lakes- Upper Mississippi River Board of State Public Health and Environmental Managers" sets forth suggested peaking factors and pennissible loadings for various unit operations. This document is commonly known as " 10 State Standards" and it is widely recognized and referred in F AC Chapter 62- 600 of the Florida Administrative Code ( F AC). Based on the peaking factor graph shown in this reference, a peaking factor of 2. 50 appears to be appropriate for a WWTP with a design AADF of 4. 50 MOD. This results in a III projected peak hourly flow of 11.25 MOD. Currently, there are two ( 2) 70- foot diameter clarifiers which provide a combined surface area of 7, 696 square feet ( sf). At a PHF of 11.25 MOD, the overflow rate for the existing clarifier units will equal 1, 462 gpd/sf. For a clarifier handling flow from a fixed growth process, 10 State Standards indicates that the overflow rates at PHF should not exceed 1, 200 gpd/sf. In view of this finding, it appears that a third 70- foot will be necessary when the AADF approaches 3. 50 MOD. i F:\2001\ 01.0387. 000\Report\ R-4\ 041403 FINAL\ SEC 4. DOC HAI#01.0387. 000 4- 6 041403 l~ c: o m s gJc: . u..... c: m. 01110 OmZ Ecl1l 1110,::: tI)- m mc... en;: Q)- S 0<( O~ I: L 00 m a: c: tI) j;( w t.) 2 FIGURE 4- 3 ill 4.2. 1.1 Capital Costs II The probable opinion of capital costs for Alternative 1 is summarized in Table 4- 1. These costs are based on unit prices obtained from recent bid experience as well as information obtained from manufacturers of the equipment to be supplied. As such, I the prices indicated in this table, reflect the anticipated costs if the project were currently bid. As indicated on this table, the total probable opinion of cost for the upgrades to the City' s WWTP RBC process is estimated to be approximately 2, 650, 000. i 4.2. 1. 2 Scheduling and Sequencing Based on the proposed plan described above it is anticipated that the overall program would be completed within 26- months from inception of the project. Furthermore, no downtime of the facilities should be required, based on the projected influent flows and the arrangement of the existing tankage. I is that the design associated with this alternative would take It anticipated I approximately 4 to 6 months to complete, including review periods and permitting. It is anticipated that bidding, review of bids, and recommendation for award will require i. an additional2- months to complete. Once construction is underway, the time to reach substantial completion is estimated to be approximately 16- months. During the last 2- months, start- up and operational modifications to the system would take place. 11" ill r Ii i III' II Iii I F:\2001\ 01.0387. 000\Report\ R-4\ 041403 FINAL\ SEC 4. DOC HAI#01.0387.000 4- 9 041403 i~:;, 0 III TABLE 4- 1 III CITY OF FORT WALTON BEACH II WASTEWATER TREATMENT PLANT PROBABLE OPINION OF CAPITAL COSTS filii REHABILITATE EXISTING RBC PROCESS Cost Description of Work I 620,000 High Density RBC Units with Shafts ( stage one) Standard Density RBC Units with Shafts ( stage two) 880, 000 IT;;~ Gear Boxes, Motors, Controls 150, 000 200, 000 Repair Basin Structure Influent & Effluent Weir Gates 40, 000 Aeration System (piping and diffusers) 20,000 150, 000 Centrifugal Blowers ( 3- 75 Hp) DO Monitoring System 20, 000 40. 000 Ii Additional Generator Capacity SUB- TOTAL $ 2, 120, 000 II II Engineering, Contingency, Pennitting