Going the Extra Mile in Evaluating Digester Mixing Technologies for Existing Digesters

APPLICABILITY Optimized mixing improves digestion stability and increases digestion capacity. Our objectives were to evaluate nine mixing technologies for installation on existing digesters at a California WRRF, determine what retrofit modifications would be required and associated costs, and ultimately select a mixing technology for the digesters. Understanding available mixing options for existing digesters and how to evaluate them in a manner that defines retrofit requirements allows for better feasibility assessment and cost estimates than typical evaluations. DEMONSTRATED RESULTS/CONCLUSIONS Introduction The mixing systems for two existing digesters at a California WRRF were the gas lift 'eductor tube' type system which have been working well for the plant since initial installation. However, the utility wanted to determine if changing to a different mixing technology could reduce energy consumption while maintaining good digestion and the costs to implement such a change. Nine mixing technologies were considered in a phased evaluation that began with a technology screening/initial cost comparison to identify the three most beneficial options for the utility followed by a detailed structural analysis to finetune understanding of retrofit requirements and overall costs for those three options. The digesters are prestressed concrete tanks with column-supported concrete covers, so the tanks have unique structural constraints relative to retrofitting. Therefore, a finite element analysis (FEA) was performed on the three screened technologies to better assess feasibility, understand the extent of structural modifications required for installation, and the costs to implement the mixing system at the WRRF. Nine mixing technologies were considered in a phased evaluation that began with a technology screening/initial cost comparison to identify the three most beneficial options for the utility followed by a detailed structural analysis to finetune understanding of retrofit requirements and overall costs for those three options. The digesters are prestressed concrete tanks with column-supported concrete covers, so the tanks have unique structural constraints relative to retrofitting. Therefore, a finite element analysis (FEA) was performed on the three screened technologies to better assess feasibility, understand the extent of structural modifications required for installation, and the costs to implement the mixing system at the WRRF. Methodology The nine technologies and associated representative system manufacturers that were considered for the initial screening and cost comparison are listed below: Gas Recirculation: - Lance type (manufacturer supporting technology not identified/responsive). - Bottom mounted diffusers (Enviromix). - Gas lift 'Eductor Tube' (Walker Process). - Gas piston 'Bubble Gun' (Suez/Veolia). Mechanical: - Submersible high solids axial impeller mixers (Anaergia). - Vertical shaft agitators (Philadelphia Mixing Solutions). - Linear motion (LM) mixing (Ovivo). - Hydraulic draft tubes - externally mounted and roof mounted (Ovivo). Hydraulic: - Pumped mixing systems (Vaughan). Equipment proposals solicited from the vendors included the recommended number of mixing units for each digester, system layout, cut sheets, motor sizes, ancillary equipment needed, installation lists, and costs. Screening level evaluation criteria included the following factors: —Economic o Conceptual Level Capital Costs o Operation & Maintenance Costs o 20-Year Life Cycle Costs — Non-Economic: o Footprint o Performance o Number of installations o Operating history o Retrofit requirements and feasibility o Constructability o O&M requirements o Sensitivity to debris o Suitability for co-digestion o Position relative to industry trends Based on the initial evaluation, the following systems were deemed infeasible: lance type gas mixing due to outdated technology and externally mounted hydraulic draft tubes and pumped mixing systems due to large penetrations required in the pre-stressed digester walls. The remaining technologies were then considered relative to the non-economic criteria, capital costs, and life cycle costs. The cost estimates at this stage of the evaluation were high level and included base assumptions regarding retrofit requirements for implementation. From that analysis, four more technologies were screened out due to high life cycle cost. The three remaining technologies included: gas lift mixers (similar to existing), submersible axial impellers, and LM mixers. Advantages, disadvantages, and the initial 20-year life cycle cost for each of these three technologies following the screening level evaluation are displayed in Table 1. The cost for the gas lift system assumed replacement in-kind of existing eductor tubes but maintaining existing compressors which remain in good condition. Each of these remaining technologies involve elements that are fully or partially roof-mounted. The axial impeller and LM mixer technologies would impart new static and dynamic loads on the roof structure and the axial impellers would require new, large openings to be cut into the roofs. Furthermore, while the axial impellers can be supported off the walls and floor, the LM mixers are usually supported entirely by the roof. Both digesters have column-supported concrete roofs with heavy rebar reinforcement along the column line grid as indicated in yellow and green in Figure 1. Equipment installed in the blue areas of the figure would be most feasible. However, additional structural analysis was necessary to truly assess impacts for the axial impellers and LM mixers and their associated static and dynamic loads. To understand and fully vet feasibility and implementation cost of these technologies, FEA was performed for both digesters. In addition, the vendor-recommended number of axial impellers was lower for each digester than on other studies for similar applications. The vendor reassessed their recommendation, adding one more mixer on each tank. The technology evaluation and cost estimates were then updated with the more refined assessment of structural and electrical impacts and equipment proposals. Results An example of the FEA results for the LM mixer for both digesters is shown in Figure 2. These results showed that the loads imparted by this mixing system, would require significant structural upgrades including additional bracing and concrete beams between columns. The FEA for the axial impeller option indicated a need for additional reinforcement at new roof openings, but no more significant modifications. However, with the increased mixing units, this technology's capital and power costs increased. Previously assumed modifications were found to be unnecessary for the gas lift option, reducing the technology's capital costs. After updating the cost estimates, the previous initial results changed, making gas lift mixing the most economical option. Table 2 shows the final life cycle cost estimates for the three technologies. Conclusions The phased evaluation with detailed structural assessment confirmed that the existing gas lift technology is the best option for this utility even though it consumes higher power than other options. Results for other WRRFs with differently constructed digesters, power costs, or drivers for change may vary. Investigating the potential impacts on existing digesters from changing technologies can yield different conclusions than typical evaluations that stop at the initial step described above. CONSEQUENCES Our results show that technology evaluation requires thorough vetting and consideration of impacts on existing tanks. The mechanical, electrical, and structural requirements for implementation should be investigated to thoroughly assess option feasibility and cost. Furthermore, energy savings alone may not warrant change if the existing system is effective and well-functioning. RELEVANCE To reduce costs and improve performance, WRRFs are focused on optimization and fully leveraging existing infrastructure. While newer, efficient technologies are promising, thoroughly evaluating options against a utility's specific drivers and investigating the impacts of various technologies on existing infrastructure is essential to defensible decision-making. There is no one-size-fits-all solution.