Clean The Digester? Maybe next year . . .

In the summer of 2015, The City of Beloit, Wisconsin Water Resources Division began the process of draining their 1.3 MG No.1 anaerobic digester for the purposes of cleaning and performing any necessary mechanical maintenance. Based on staff’s previous experience with draining and cleaning digester No. 2 in 2011, a relatively quick turnaround was expected and it would be back on-line before too long. While staff dewatered approximately four-fifths of the digester contents the project was bid and mobilization occurred soon after. Once on-site, the contractor estimated the project would take approximately four days to remove the remaining biosolids. The contractor would insert a submersible pump, pump to 6000 gallon transport trailers and recycle on approved application sites. Things went well until they encountered the expected struvite accumulated near the bottom and center of the tank. While estimates prior to pumping ranged wildly, it was expected to land in the 100 to 200 cubic yard range and they would simply add water to facilitate pumping. We were wrong. Way wrong. The final tally was approximately 850 cubic yards of compact struvite – and the struvite did not want to move. Several suggestions and brainstorming sessions later, it was decided to modify a skid steer loader to fit into the digester and offload cubic yard by cubic yard with an overhead crane-assisted bucket brigade. While we hoped to continue the beneficial reuse of the product, time constraints were working against us so we opted to recycle to a local landfill. However, due to the landfill’s daily cover protocols and good neighbor policy, we were allowed to offload only two trailers each day and only before 1:30 p.m. In order to keep the struvite removal process moving forward, we used areas of on-site impervious surface to temporarily store the product until it could be moved to the landfill. After a little over two months, a project that was expected to take “a few days” was finally over. We licked our wounds, set a plan to assure that no digester gets left behind and mandated that a three to five year cleaning protocol will be the order of the day.