Sustainable (Low Energy) Digester Linear Motion Mixing – “Show Me The Data”

The City of Austin (City) is committed to being a leader in sustainability and green infrastructure. This is a key component of its mission to achieve sigificant reductions in carbon footprint by 2020. The Hornsby Bend Biosolids Management Plant (HBBMP) Digester Improvements and Sustainability Project (Project) developed by the City with consulting services by CH2M HILL is in direct alignment with many of the City's sustainability and green infrastructure goals.

CH2M HILL prepared a detailed analysis to determine greenhouse (GHG) emissions before and after the implementation of the project facilities to make the City's case as a green infrastructure project for the American Recovery and Reinvestment Act (ARRA). As a result the City achieved state funding priority for this fast-tracked project. The project is expected to increase the net energy savings for the plant by 73 percent in year 2012, and by 74 percent in year 2030. The savings account for improvements to the digester mixing systems through the use of linear motion mixers (LMMs), increased digester gas production with subsequent increases in electricity and heat energy captured, and energy savings from decreased fossil fuel requirements. The total carbon footprint reduction from all sources due to the project is estimated to be 7,636 metric tons (8,400 tons) of CO2 emitted (TCO2e)/year in year 2012 (103 percent reduction) and 12,363 metric tons (13,600 tons) TCO2e/year in year 2030 (119 percent reduction).

The LMMs have been recently released to the wastewater market and there is only one wastewater treatment plant (WWTP) with operating LMMs: The Ina Road WRF in Tucson AZ. The Ina Road WRF has one LMM that has been operating for seven years and three LMMs that have been operating for three years. Plant personnel are reportedly very happy with the LMMs and have had no operational issues with them. CH2M HILL is in the process of specifying four more LMMs at the Ina Road WRF.

LMMs that were installed as part of the City's HBBMP Project are simple and shown to be low in maintenance at other WWTPs. The LMMs require 50 to 70 percent less energy than other potential digester mixing systems and are consistent with the sustainability goals of the City. The manuscript includes a summary of the full-scale performance testing results for the LMMs for both the thickened biosolids blend tank (TBBT) including solids profile, temperature profile and lithium dye testing. The lithium dye testing included an 8 hour initial mixing period and sampling events followed by daily samples for up to 40 days to simulate two solids retention time (SRTs). Observations to downstream processes as a result of the new mixers are also presented. In addition, computation fluid dynamics (CFD) and operational and performance data for LMM are presented from the wastewater treatment plants in Salt Lake City, UT; Sidney, OH; Tucson, AZ.