Brief Project Description
The Town of Orange currently owns and operates a wastewater treatment plant utilizing trickling filter technology, with two tiered permitted design flows: 0.75 MGD during dry weather (May-Dec.), and 1.5 MGD during wet weather (Jan.-Apr.). During the rulemaking to assign nutrient waste load allocations, the Town petitioned DEQ for allocations based on having a 3.0 MGD facility constructed and certified for operation by Jan. 1, 2010. Although the allocations based on the increased design flow were conditionally approved, in order to determine grant eligibility of the proposed expansion a review was made of the “reasonable and necessary” design flow for a twenty-year service life. Subsequent to adopting the regulation and based on review of several information sources - including recent flow history, population and development projections requiring new service - and plans for infiltration/inflow corrections, it was determined that a design flow of 2.0 MGD could be supported as “reasonable and necessary”. While the Town still intends to build an expanded facility with a design flow of 3.0 MGD, cost-share eligibility under this grant is limited to the prorated cost of a 2.0 MGD facility.
In proceeding with the Town’s Comprehensive plan, the new Chesapeake Bay Regulations and waste load allocations for nutrients adopted by the State Water Control Board were incorporated into the design and expansion of the facility. To maintain capacity needs and meet waste load allocations, the design of the facility has included Enhanced Nutrient Removal (ENR) technology to achieve annual average discharge concentrations not to exceed 0.30 mg/l total phosphorus and 4.0 mg/l total nitrogen.
As per the Preliminary Engineering Report, the following is a summary and description by unit process of the proposed expanded plant with ENR capability.
Primary Clarification - The existing primary clarifier will be kept in service to promote hydrolysis and acid fermentation which will increase volatile fatty acids entering the downstream anoxic zones, enhancing nutrient removal. The existing secondary clarifier will be converted to function as a primary clarifier (which will serve as an equalization basin to attenuate load and flow variations as well). Provisions are made to recycle sludge through the primary clarifiers for the elutriation of these compounds.
Biological (nutrient reduction) Process - The proposed biological process is the 4-stage Bardenpho system, which is a single sludge, suspended growth process, generally regarded to be a very reliable and efficient biological process for advanced wastewater treatment. The proposed unit will have a total volume of 1.5 million gallons based on an annual average daily flow of 3.0 MGD. The anticipated BOD load will be 4,620 lb/day or 38.5 lb/d/1000 ft3 based on 0.90 MG in the nitrification and re-aeration basins. The design simulation assumed winter conditions at 12 degrees C and summer conditions at 25 degrees C. Design mean cell residence time (solids retention time) used was 20 days for winter and 15 days for summer conditions.
Three basins will be provided with room to add one future basin. The nitrification basin in each train is divided into two compartments. Because the waste strength is higher in the first basin, more air will be required in the upstream basin. Analysis shows that more than half the BOD and 90 percent of TKN is oxidized in the first chamber with an air flow requirement 50 percent higher in the first basin than the second. Coarse bubble diffusers are specified to insure adequate mixing during low loading periods when lower air flow rates will be necessary. Pumping is provided for return mixed liquor recycle flow for each train.
The volume of the second anoxic chamber was set equal to the volume of the first anoxic zone. Because denitrification rates for endogenous respiration are much lower in the second anoxic basin, some additional carbon source, such as methanol, may need to be added during cold weather and has been included in the design. Additionally, each basin will be provided with three floating or submersible mixers in the anoxic zone.
Secondary Clarification -Three 68 foot diameter circular clarifiers will be constructed with room onsite for an additional unit in the future. Pumping will be provided for return activated sludge for each secondary clarifier pump station. Provisions will be made to provide rapid chemical mixing prior to clarification and filtration for the addition of coagulants, alum or ferric chloride for the removal of phosphorus or to aid in sedimentation, filtration, and dewatering of residuals
Solids handling - Solids from the biological nutrient removal process will be thickened prior to digestion. Dewatering of digested biological solids prior to offsite disposal will be achieved by a one-meter belt filter press. At full loading and flow, approximately 3,300 lb of 15 percent solids cake will be produced, with the press dewatering 3,400 lb/day digested sludge.
A total of three aerobic digesters are proposed with a total volume of 0.69 MG (20% of annual average daily flow is 0.6 MG). This will be accomplished by converting the existing anaerobic digester to aerobic (by removing the cover, heat exchangers, and gas piping and installing coarse bubble diffusers). Two new, additional digesters will also be constructed as part of the project.
Effluent Filtration – Tertiary effluent filtration will be achieved utilizing two (2) cloth media filter units operated in parallel. Each filter unit has twelve disks totaling 646 square feet of filtration surface area per filter, which results in a total of 1,292 square feet of filtration area for both filters. The woven cloth media has a nominal pore opening of 10 microns which produces a low TSS effluent under a wide range of loading conditions. Under average flow conditions, the hydraulic loading rate will be approximately 1.6 GPM/Ft2 at 3.0 MGD. At peak flow conditions, the hydraulic loading rate will be approximately 4.3 GPM/Ft2 at 8.0 MGD. Due to the configuration, the filter system will be fed via gravity flow; thus, no pumping will be required either before or after the filter system.
The total cost is estimated to be $21.4 million with about $9.6 million attributable to nutrient reduction technology' application of a 90% cost share to the eligible cost yields ~$7.05 million grant.