City launching major biosolids project

The City of Sisters is not letting waste go to waste.

The Sisters City Council last week awarded a public-improvement contract in the amount of $153,092 to Clear Harbors Environmental Services of California for the Biosolid Removal Project at the City’s wastewater treatment plant.

The project is part of the 2016 Wastewater Capital Facilities Plan and was estimated to occur in 2021 — 20 years from when the plant was started up. Establishment of the City sewer system was approved by the voters in 1997-98. Mayor Steve Wilson acquired the 160 acres at the end of Locust Street from the U.S. Forest Service for locating the sewage treatment plant.

The removal of the biosolids required the creation of a Biosolids Management Plan (BMP) to determine the disposal method and location of the disposal site. Staff developed the BMP and received Department of Environmental Quality approval last November. Three bids were received from companies in Iowa, California, and Washington, all coming in under the budgeted $250,000.

Biosolids removal includes the removal of “sludge,” or the material that remains in the ponds after treatment. These biosolids accumulate in the ponds and reduce their capacity over time. The City’s treatment plant is comprised of three ponds with the 2.1-acre, 10-foot-deep primary lagoon, or Pond One, doing the majority of the treatment, utilizing six aerators. Pond One is where the biosolids are collected and where the dredging will take place.

Pond Two provides additional treatment to capture suspended solids prior to discharging to the large 16-foot-deep holding pond (Pond Three) that the City irrigates out of in the summer.

While the dredging is taking place in Pond One, incoming flows can be diverted to Pond Two while the biosolids are removed out of Pond One. Then the clear water off the top of the biosolids layer will be decanted.

The biosolids, which are two to three feet deep, will be removed using an adjustable dredge that will pump the biosolids into mixing tanks where lime will be added to adjust the pH to 12.0 or higher, which will kill all pathogens and reduce odor. The treated biosolids will then be tested and pumped into a tanker and hauled out to the existing effluent field on 50 acres of the Lazy Z Ranch property owned by the City. The estimated quantity of biosolids is 500,000 gallons, to be hauled using 12,000-gallon tank trucks in approximately 42 loads.

The application of the biosolids will provide fertilizer which will be disced into the soil, then a crop of orchard grass will be planted to take up the nutrients. The crop will be irrigated with the City’s effluent from Pond Three. The City is looking at contracting with a local farmer to harvest the crop this summer. If the market price for the grass is high enough, the City looks to make a little money on the sale of the crop. The planting and harvesting are undertaken to avoid the build-up of nutrients in the soil.

The contractor estimated that it will take 15 days to dredge, treat, and haul all the biosolids. The application site cannot be frozen, so it is anticipated that the start date will be in March.

This is a maintenance project that does not provide additional capacity to the plant; therefore, System Development Charges to fund the project cannot be used. The Sewer Operating Fund for FY 2020/21 will pay for the full costs of the project.

When asked how long before this operation would have to be undertaken again, Public Works Director Paul Bertagna estimated that, given Sisters’ rapid growth, it will probably be another 15 years. As a point of comparison, he shared that the City of Portland sends multiple trucks every day filled with treated biosolids up Highway 84 in the Columbia Gorge to be applied to poplar farms in Eastern Oregon.