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

combined flow then proceeds to the oxidation ditch where it is mixed with recirculating flow in the oxidation ditch I channels. At nonnal design velocities, the volume of mixed liquor flowing in the channel is between 50 and 100 times the influent flow. This recirculating flow pattern and the resulting I dilution of influent prevents short- circuiting and provides a consistent reliable effluent quality. An amount of flow equal to the influent plus RAS is continuously displaced over the effluent I weIr. An additional feature of the MLE process using an oxidation ditch configuration is the plug flow I mode of operation, which is achieved within the basin channels. One benefit of the plug flow feature is the flocculation of mixed liquor solids, which occurs in the basin channels after the I aeration zone as the mixed liquor approaches the effluent weir. Bio- flocculation helps improve settling and clarification in the final clarifier. I The aerator layout used with the oxidation ditch configured MLE process makes it possible to I use fewer aerators, reducing maintenance requirements. The aerator layout provides an opportunity to enclose the aeration zones for the prevention of potential mist. i: F:\2001\ Ol. 0387. 000\Report\ R-4\ 041403 FlNAL\ SEC 4. DOC HAI#01.0387. 000 4- 31 041403 c  The anoxic basin is positioned upstream trom the oxidation ditch so as to receive mixed liquor I'll! recycle trom the oxidation ditch which contains high concentrations of nitrates fonned trom the oxidation of ammonia. In the anoxic basin, nitrates in the internal recycle are mixed with the influent wastewater and clarifier return sludge. Since this basin is mixed and not aerated, the I bacteria will utilize the oxygen in nitrate nitrogen for their oxygen source while consuming a portion ofthe influent BOD. I Internal recycle trom the oxidation ditchto the anoxic basin is achieved by means of a side channel adjacent to the inlet to the aeration zone. Flow is forced into this channel by means of I the velocity head generated in the oxidation ditch channels. At the entrance of the recycle channel, a control gage allows for adjustment ofthe flow. Flow is returned to the oxidation ditch I by means of another side channel adjacent to the outlet of the aeration zone. Control of internal recycle through the internal recycle channel is achieved by means of a manually operated flow I control gate. The gate, located at the entrance of the channel, allows adjustment for a wide range of recycle flow. I The MLE process was sized with a solids retention time ( SRT) of 10 days and a hydraulic detention time ( HDT) of 12 hours. This sludge age is selected to ensure that the effluent I requirements for this installation are met on a consistent basis. This also provides stable operation that will be relatively unaffected by diurnal load variations. The long hydraulic I detention time that results will help attenuate nonnal fluctuations in the influent, which translates to process stability and forgiveness. I This level of A MLSS concentration of 3, 000 mg/l was assumed for sizing the MLE system. MLSS is considered prudent for gaining maximum economy in the aeration basin structure ( i.e. I volume) while not compromising operating flexibility of the installation or solids loading capacity of the final clarifiers. Selecting a 3, 000 mg/l design MLSS will preserve process I flexibility since the MLE process is capable of operating at MLSS levels of over 8, 000 mg/l. Only clarifier solids loading capacity would limit the maximum operable MLSS level. I Two ( 2) basins are recommended which will provide good operational flexibility and the close I proximity of the aeration equipment for this layout simplifies construction, piping, electrical and installation. It was detennined that 150 HP aerators are necessary for each basin. i. F:\2001\ 01.0387. 000\Report\ R-4\041403 FINAL\ SEC 4. DOC HAI#01.0387. 000 4- 32 041403 iI.  The conversion to the oxidation ditch activated sludge process will require recycle pumping from the secondary clarifiers. This recycle is necessary to maintain the microorganisms in the III activated sludge process. Therefore, this will require the addition of recycle pumps, recycle basin, yard piping and control equipment to direct the RAS to the process basin. 4. 8 CAPITAL COSTS I The probable opinion of capital costs for the upgrades to the City's WWTP incorporating an oxidation ditch process is summarized on Table 4- 6. As with the other reviewed alternatives, I these costs are based on unit prices obtained from recent bid experience with upgrades to similar wastewater treatment facilities, as well as infonnation obtained from manufacturers of the I equipment to be supplied. As indicated on this table, the total probable opinion of cost for the upgrades to the City' s WWTP is estimated to be approximately $4, 262, 500. I 4. 9 SCHEDULE AND SEQUENCING I Based on the proposed plan described above it is anticipated that the overall program would be completed within 2 years from inception, with no downtime of the facilities required. I Construction will account for 18- months of this total, with the last 2- months of construction being reserved for start- up and operational modifications to bring the new facilities on- line. I Based on our experience the construction of the new facilities can proceed without impacting the existing facilities. I Once the construction of the recommended improvements is completed, the necessary yard piping to connect the new process structures would be connected, the equipment tested and certified, and the aeration basin seeded. A portion of the influent flow will then be diverted to new process tankage to acclimate the biomass in the unit. Once the new process is acclimated, Ii the existing RBC facilities will then be decommissioned and equipment removed, and all influent wastewater will flow through the new process tankage for treatment. III II. iIII F:\2001\ 01.0387. 000\Report\ R-4\ 041403 FINAL\ SEC 4.DOC HAI#01.0387. 000 4- 33 041403 II"' iii TABLE 4- 6 CITY OF FORT WALTON BEACH WASTEWATER TREATMENT PLANT PROBABLE OPINION OF CAPITAL COSTS MLE PROCESS ALTERNATIVE I Description of Work Cost I RBC Equipment Removal and Disposal 20, 000 Flow EQ Mixers/Rails/ Supports 150, 000 Oxidation Ditch Structure 1, 600,000 Additional Excavation to Maintain Hydraulic Profile 250, 000 Mechanical Aerators ( 2 - 150 HP) 500,000 Electrical Modifications 250, 000 I. Recycle Wetwell 110, 000 Recycle Pumps 115, 000 l1li Yard Piping 350, 000 Influent & Effluent Weir Gates ( includes recycle wetwell) 25, 000 Generator ( supplemental capacity) 80. 000 I SUB- TOTAL 3, 410, 000 I Engineering, Contingency, Pennitting @ 25% 852.500 TOTAL 4. 262. 500 I I I I I. F:\2001 \ Ol. 0387. 000\Report\ R-4\ 041403 FINAL\table 4- 6.doc HAI #01.0387. 000 4- 34 041403 IJIIIII IIiI IJIIIII j iii SECTION 5 filii PROPOSED WWTP CAPITAL IMPROVEMENTS II 5. 1 GENERAL DESCRIPTION I Previous sections of this report discussed the City' s existing service area, treatment facilities, historical wastewater characteristics, projected wastewater characteristics, and alternative I secondary treatment process options. The reviewed alternatives to improve the secondary treatment portion of the facilities included: I 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- AerotorThl system. I 3. Abandoning the RBC process and replacing it with a conventional suspended growth activated sludge process ( MLE Oxidation Ditch). I A weighted analysis was preformed and it was determined the MLE oxidation ditch alternative was the most viable option to replace the City' s existing RBC treatment process. Although the I MLE oxidation ditch system was the recommended alternative, the City desired to further III! investigate the option of replacing the existing RBC process with new RBC equipment. The initial capital expenditures associated with installing new RBC units were determined to be substantially less than that of the MLE oxidation ditch alternative. However, each of these r II. treatment alternatives has unique requirements separate from the secondary process, which impact the total capital expenditures. This section analyzes the total treatment plant process pi i' costs associated with the RBC and MLE oxidation ditch alternatives. ill Major components to the City wastewater treatment process