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Two for the Price of One: Increasing Digester Capacity while Reducing Filamentous Bulking through Co-thickening at Tacoma's CTP

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As many wastewater treatment facilities are striving to increase the sustainable operation of their facilities, they turn to increasing process efficiency and effectiveness while maximizing the use of existing infrastructure. This approach requires innovative, holistic solutions and “out of the box” thinking. The City of Tacoma (City) had what appeared to be two significant but separate process constraints at their Central Treatment Plant (CTP), one associated with the digestion process, the other with the liquid stream. After consideration of the overall plant process, it was found that fairly straight-forward modifications to one existing process could address both issues.

The CTP is a secondary treatment facility that uses high purity oxygen (HPO) activated sludge units for biological oxidation of wastewater. The secondary treatment system is currently designed for peak flows of up to 90 million gallons per day (mgd). Flows beyond 90 mgd receive advanced primary treatment and disinfection but no secondary treatment. Waste activated sludge (WAS) is pumped to the dissolved air flotation tanks (DAFT) for thickening, with underflow returned to the head of the HPO tanks. Thickened WAS is combined with the primary sludge from the CTP and primary sludge that is trucked in from the North End Treatment Plant (NETP) and mixed in a Blended Sludge Storage Tank. The solids are then heated and processed through autothermal aerobic digesters. The aerobically digested solids then flow through a series of 3-stage temperature-phased anaerobic digesters for further stabilization. The process produces Class A biosolids which the City of Tacoma markets as TAGRO.

It is anticipated that future loadings will increase due to increased service area and the potential addition of organic food waste. Also, the City has identified a long term goal to promote the generation of green energy through cogeneration. To facilitate this, hydraulic flow to anaerobic digesters must be reduced in order to increase capacity and increase gas production. Concurrent with its energy and capacity goals, the City identified the need to minimize the formation of volatile fatty acids in the primary clarifiers, which would reduce the formation of filaments in the secondary system. Typically, volatile acid reductions are achieved by increasing the solids removal rate from the primary clarifiers. However, increasing the solids removal rate would increase the hydraulic load to the digesters and further reduce capacity. After careful analysis, it was determined that an upgrade to the existing DAFTs to allow for co-thickening of primary and secondary solids would provide a cost effective solution to remove these two process constraints. Co-thickening in the DAFTs would allow the inventory in the primary clarifiers to be kept at a minimum, reducing VFA production. The enhanced thickening of both primary and secondary solids would also reduce the hydraulic load to the digesters, recovering capacity for other uses.

Brown and Caldwell (BC) worked with the City to design modifications to four existing DAFTs to support co-thickening of WAS and primary solids at the CTP. The design team evaluated the DAFTs to identify process, mechanical, structural, electrical and instrumentation constraints of the system. By using this approach, the reuse of existing equipment and infrastructure could be maximized while providing an effective means of thickening raw solids. A capacity evaluation of the existing DAFTs, confirmed with stress testing, showed that there is sufficient capacity to accommodate co-thickening through the year 2027, with all four DAFTs in service. The capacity of three DAFTs is projected to last through the year 2013.

Pre-design for the CTP solids system included an option to pump—in various combinations—primary sludge, thickened waste activated sludge (TWAS) from the DAFTs, and sludge from a new solids receiving station (where sludge from the NETP is discharged) to the blended solids storage tank (BSST). In the future, liquid food wastes might also be discharged to the solids receiving station, and this may require modification to the piping system of the receiving station. The contents of the BSST are mixed and pumped through new water-to-sludge and sludge-to-sludge heat exchangers prior to being discharged to the auto-thermal aerobic digesters, the first stage of the digestion process. A business case evaluation (BCE) model, developed by BC, was used to facilitate major design decisions. The major design elements, some of which required BCE analysis, for the DAFT upgrade are listed below:

Raw Sludge Conveyance to the DAFTs — There were several viable routes for conveyance of raw sludge from the primary clarifiers to the DAFT units. Each route was evaluated, and it was determined that a new six inch glass lined pipe should be installed through the existing gallery and pipe chase – along the same route as an existing eight inch pipe to be demolished. This will provide CTP with the most reliable performance and long-term ease of maintenance.

Air Saturation System — The existing air saturator equipment is undersized for the co-thickening upgrade. This was confirmed through field efficiency testing. From test results, the air-to-solids (A/S) ratio was calculated to be 0.0062 pounds of air per pounds of solids (lb air/lb solids) and 0.0055 lb air/lb solids for Air Saturator #2 and #4, respectively. A recommended minimum A/S ratio for co-thickening in DAFTs is 0.015 lb/lb. Based on this criteria, the current system, as it was operated during testing, only achieves one-third of the recommended A/S ratio. The existing air saturator equipment will be replaced with new vendor-supplied tanks and recirculation pumps.

Underflow Sludge Conveyance — An increase in the amount of accumulated bottom sludge is anticipated with co-thickening due to the addition of primary sludge to the DAFTs. Conveyance of this material, either by pumping or gravity flow, to the headworks is required for degritting. After hydraulic investigation, it was determined that gravity flow of underflow sludge to the headworks will be maintained, and new pumps are not required.

DAFT Odor Control — The addition of primary solids to the WAS prior to thickening will result in an increase of on-site nuisance odors relative to the current condition. Identified mitigation steps include the installation of flat aluminum covers over all four DAFTs, plus matting over the effluent channel. The aluminum covers will be ducted to route the foul air exhaust to the existing odor scrubbing units (wet scrubbers), which also service the room air. Caustic soda will be added to the existing hypochlorite odor control system to better maintain a constant, optimum pH in the wet scrubber and maximize hydrogen sulfide removal.

Replacement of TWAS Pumps — The existing progressive cavity pumps are undersized for the projected 20-year loads of WAS and primary sludge. New, larger progressive cavity pumps were selected as the lowest life-cycle cost alternative.

Process Automation — Several options for process automation are recommended by BC for implementation. Increasing the level of process control and monitoring should improve performance and efficiency; while potentially reducing long-term operating costs.

By taking a holistic view of the operations of the CTP, a single upgrade was identified that would increase digester capacity, reduce filamentous bulking and allow for the potential implementation of co-digestion and co-generation at CTP. Furthermore, costs were reduced, and plant footprint was conserved by maximizing the reuse of existing infrastructure. This project is an excellent example of how problem solving can be achieved by a multi-disciplinary team using an integrated approach, maximizing benefits to utilities.
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Keywords: DAFT; WAS; air saturation; co-thickening; dissolved air flotation thickening; odor control; primary sludge; thickened sludge

Document Type: Research Article

Publication date: 2010-01-01

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