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

250, 000 1M Influent and Effluent Weir Gates ( includes Recycle Wetwell) 25. 000 SUB- TOTAL 2, 351, 000 1M 587. 750 Engineering, Contingency, Pennitting @ 25% TOTAL 2. 938. 750 III III' ill III II i III F:\2001 \ Ol. 0387. 000\Report\ R-4\ 041403 FINAL\table 4- 2. doc HAl #01.0387. 000 4- 16 041403  Construction can proceed on the new facilities without impacting the operation of the existing RBC system. Once the construction of the recommended improvements are filii completed, tested and certified, and the WesTech STM- AerotorTM process seeded, the yard piping to connect the new process structures would be connected. A portion of filii the influent flow would be diverted to new process tankage to acclimate the biomass in the unit. Once the new process is acclimated, the old RBC facilities would then be decommissioned, equipment removed, and all influent wastewater will flow through the new process tankage for treatment. 4.2. 3 Alternative 3 - Replace Existing RBCs with an Activated Sludge Process A survey was conducted of all permitted WWTP' s in the State of Florida as of July 2001 that had pennitted treatment capacities of 1. 0 MGD and greater, and is provided in Appendix C. As indicated in this survey there is a total of 283 WWTP' s that meet these criteria. Of that total e., trickling filter, RBC' s, there are only five ( 5) facilities that use the fixed film technology ( i. etc.) and the remaining 278 facilities use the activated sludge technology as the primary means of ill wastewater treatment. It is important to note that only two ( 2) ofthe facilities used RBC' s as the primary means of wastewater treatment. There are a number of activated sludge processes that can be used to replace the existing RBCs. As such, there are a number of methods to provide the necessary oxygen to maintain the process. Based on our evaluation of the various alternatives available for the City, a Modified Ludzack- Ettinger ( MLE) process with mechanical aeration is considered the preferred alternative. This type of process is commonly referred to as an oxidation ditch system. In an oxidation ditch system, the raw wastewater and mixed liquor suspended solids ( MLSS) are The conveyed around an oval concrete structure using radial low- speed mechanical aerators. II. recommended process utilizes the MLE configuration. This process was selected for its ease of filii operation, lower maintenance costs, successful track record, and ability to meet more stringent f. future permit requirements. In addition , the MLE process has provisions to control nitrate III ditch/MLE through variation of the internal recycle. Figure 4- 6 shows one ( 1) type of oxidation process. The MLE process consists of either vertically mounted, low speed surface aerators or fine bubble 1M diffusion and mixers in an aeration basin with partitions used to establish a continuous channel. filii ill F:\2001\ Ol. 0387. 000\Report\ R-4\041403 FINAL\ SEC 4. DOC HAI#01.0387.000 4- 17 041403  For this alternative, vertically mounted, low speed surface aerators were chosen as the preferred method of aeration in an oxidation ditch tank configuration. A flow velocity sufficient to maintain the suspension of mixed liquor in the oxidation ditch channels is achieved by the pumping action of the aerators. The pumping action is created by lining up the aerators with the partition wall in a manner such that the aerator pumps mixed liquor from the upstream channel II into the aeration zone where it is completely mixed and forced into the downstream channel. Wastewater and return activated sludge enter the anoxic section the MLE process upstream of the oxidation ditch where it is mixed with return activated sludge and internal mixed liquor recycle. This combined flow then proceeds to the oxidation ditch where it is mixed with recirculating flow in the oxidation ditch channels. At normal 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 dilution of influent prevents short- circuiting and provides a consistent reliable effluent quality. An amount of flow equal to the influent plus return activated sludge is continuously discharged over the eft1uent weir. An additional feature of the oxidation ditch is the quasi- plug flow 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 aeration zone as the mixed liquor approaches the effluent weir. Bio- flocculation helps improve settling and clarification in the final clarifier. The aerator layout used with the oxidation ditch configuration makes it possible to use fewer aerators, reducing maintenance requirements. The aerator layout provides an opportunity to enclose the aeration zones for the prevention of potential mist. II As previously stated, the anoxic basin is positioned upstream from the oxidation ditch so as to receive mixed liquor recycle from the oxidation ditch which contains high concentrations of nitrates formed from the oxidation of ammonia. In the anoxic basin, nitrate in the internal i. recycle are mixed with the influent wastewater and clarifier return sludge. Since this basin is mixed and not aerated, the bacteria will utilize the oxygen in nitrate nitrogen for their oxygen source while consuming a portion of the influent BOD. III Internal recycle from the oxidation ditch to the anoxic basin is achieved by means of a side III channel adjacent to the inlet to the aeration zone. Flow is forced into this channel by means of F:\2001 \ Ol. 0387. 000\Report\ R-4\ 041403 FINAL\SEC 4. DOC HAI#01.0387. 000 4- 19 041403 III lilt the velocity head generated in the Carrousel channels. At the entrance of the recycle channel, a control gate allows for adjustment of the flow. Flow is returned to the oxidation ditch by means III of another side channel adjacent to the outlet of the aeration zone. Control of internal recycle lilt through the internal return channel is achieved by means of a manually operated flow control gate, which allows adjustment for a wide range of recycle flow. The MLE process was sized with an aerobic solids retention time ( SRT) of approximately 10 ill This sludge age will ensure days and an aerobic hydraulic detention time ( HDT) of 12 hours. that the effluent requirements for this installation are met on a consistent basis and also provides stable operation that will be relatively unaffected by diurnal load variations. Further, a 10- day aerobic SRT will ensure that nitrification is achieved even at high flow and low temperatures. i A MLSS concentration of 3, 000 mg/1 was assumed for sizing the MLE process. This level of r MLSS is considered prudent for gaining maximum economy in the aeration basin structure ( i.e. volume) while not compromising operating flexibility of the installation or solids loading filii capacity of the final clarifiers. At a flow of 4. 50 MOD, the F/M ratio for the Carrousel basin is expected to be in the range of 0. 13 and the organic loading will be approximately 25 Ibs./ l000 fe. These values are not fully consistent with an " extended aeration" mode of operation, but they iii do represent a very conservatively designed " conventional" system. As previously stated, an anoxic zone will be provided upstream of each oxidation ditch to control nitrate. Each basin has been sized to provide a hydraulic retention time of 4 hours based on AADF. Several different predictive relationships can be used to estimate the effectiveness of the lilt anoxic zone with respect to nitrate removal. In the case of the Ft. Walton Beach WWTP, the filii proposed treatment concept should result in an effluent nitrate concentration in the 7 to 10 mg/l range without the need for any chemical addition. Two ( 2) process trains are recommended, which will provide good operational flexibility and the III close proximity of the aeration equipment for this layout simplifies construction, piping, a.. electrical and installation. It was detennined that 150 HP aerators are necessary for each basin. As with the WesTech STM- AerotorThl system discussed in Alternative 2, Alternative 3 is an II activated sludge process which will require recycle pumping of RAS from the secondary III clarifiers and the addition of a third clarifier when AADFs approach 3. 50 MOD. The RAS is a I ill necessary component in an activated sludge process and will require the addition of recycle lilt F:\2001 \ 01.0387. 000\Report\ R-4\ 041403 FINAL\ SEC 4. DOC HAI#01.0387. 000 4- 20 041403 ill II pumps, recycle wet well, yard piping, and control equipment to direct the RAS to the process basin. Additional yard piping will be required to bring the newly constructed oxidation ditch H online. Other components that will be incorporated into the design of the oxidation ditch system are the necessary instrumentation and controls. These features are key elements that can have a III profound effect on the perfonnance of the system and operational costs. 4. 2. 3. 1 Capital Costs Ii I! II X