Getting Your Money's Worth: Existing System Performance Testing and Optimization Leads to Cost Effective Design

The Water & Sewer Authority of Cabarrus County, NC (WSACC) owns and operates the 24 million gallons per day (mgd) Rocky River Regional Wastewater Treatment Plant (RRRWWTP). In order to maintain autogenous combustion of biosolids cake in the multiple hearth furnace (MHF) at the facility, and consequently increase revenue and reduce cost for auxiliary fuel, WSACC contracted with CH2M HILL for the design of an Imported Biosolids Handling Facility to accept imported biosolids from surrounding utilities. Design activities included an analysis and upgrade of the existing two-stage chemical odor control system at the facility to determine its capability to handle odorous air loads from existing and new sources.In the existing odor control system, sodium hydroxide is dosed to both Stage 1 and 2, while sodium hypochlorite is only dosed to Stage 2. The existing odor control system is rated for maximum hydrogen sulfide (H2S) concentration of 100 parts per million, volume basis (ppmv), average H2S concentration of 31.5 ppmv, and a base airflow treatment capacity rate of 2,700 cubic feet per minute (cfm). Modifications are available to increase the base airflow capacity to 4,200 cfm.The new Imported Biosolids Handling Facility will include receiving bins for imported dewatered sludge, extraction screw conveyors, a sludge mixer, and related piping. Foul air emissions from the new Imported Biosolids Handling Facility will be routed to an odor control facility.As part of design efforts, WSACC authorized CH2M HILL to evaluate the existing two-stage odor control system at the RRRWWTP with three primary objectives:1. To assess the current performance of that system under existing loads,2. To predict its performance under future loads3. To develop options to treat foul air loads from the new equipment with the existing odor control system, a new odor control system, or a combination thereof.Results from field testing included calculated airflow rates using velocity measured with a digital anemometer, H2S measured with a digital meter, and calculated removal performance for a variety of odor concentrations measured in the field using odor detection tubes. Odorants evaluated using odor detection tubes included amines, ammonia, dimethyl sulfide, H2S, and methyl mercaptan. During field testing efforts, it was determined that the existing odor control system performs well with lower inlet H2S concentrations (such as those from the gravity thickeners alone, up to 20 ppmv), but performs poorly with higher H2S concentrations (with gravity thickeners and dewatering centrifuges, up to 100 ppmv). The manuscript will include detailed results of field testing and evaluation efforts. After a discussion with WSACC about the odor control field investigation and the performance evaluation of the existing odor control system, three options for odor control improvements were identified:1. Retain existing odor control system, but pre-treat the foul air sources containing the heaviest loads of H2S using a new biological odor scrubber (biotower), prior to using the existing odor control system to treat the gravity thickener headspace exhaust plus the exhaust from the biotower as a second-stage odor scrubber.2. Replace existing odor control system with a new two-stage system for treatment of all solids-handling related odor sources, consisting of a biotower in the first stage followed by activated carbon for polishing in the second stage.3. Use the existing odor control system to treat existing loads plus new solids storage bunker and install new two-stage odor control system for new Solids Receiving Facilities, consisting of a biotower followed by activated carbon in the second stage.Estimates for capital cost, annual operation and maintenance (O&M) cost, and 10-year life cycle costs were developed for each option, with the results shown in Table 1.Following presentation of the results and operational considerations for each option, WSACC chose Option 1 as it involves substantially lower estimated capital cost, estimated annual O&M cost, and estimated 10-year life cycle cost than the other options evaluated. The biotower provides reduction of the highest H2S loads upstream of the existing odor control system. Therefore, Option 1 allows the existing odor control system to operate efficiently within its design range of inlet concentrations with only minor modifications. The combination of field testing results, analysis, and the resulting optimized use of the existing odor control system allow for the most cost effective treatment of odors from both existing sources and new sources constructed as part of the Imported Biosolids Handling Facility project, with construction starting in Fall 2011.