Conference Agenda

Biogas generated from anaerobic digestion typically requires gas conditioning before it is used to generate energy. The H₂S concentrations in the digester gas at the Budd Inlet Treatment Plant typically range from 950 to 1,050 ppmv, which is approximately double the average inlet design concentration of the existing H₂S removal system, resulting in increased O&M costs.

Full-scale piloting of micro-aeration technology (MA) was undertaken to test its feasibility to reduce H₂S from the biogas, as limited experience is available of this new technology in full-scale digesters at wastewater resource recovery facilities (WRRF). The full-scale tests were undertaken during a one-year period under different configurations and generated the following findings:

• Reducing the H₂S concentration to 200 ppmv (or a removal efficiency of approximately 80 %) is technical feasible in the current digesters.

• Injecting micro-aeration air in the sludge heating recirculation stream is more effective than injecting in the bottom of the digester.

• Measuring the ORP of the sludge continuously was reasonably successful after several installation modifications, but is not recommended for MA process control because of accumulation of debris on the ORP sensor requiring frequent (daily) cleaning.

• The biogas production and sludge volatile solids reduction are not negatively affected by the MA process, while there is some evidence that the dewaterability of the digested sludge may slightly increase as a result of micro-aeration.

In summary, the full-scale pilot tests showed that this new process-integrated MA technology has potential using minimum plant upgrading while contributing to the sustainability and economic efficiency of the energy recovery process of waste sludge digestion at WRRFs.

Clackamas Water Environment Services (WES) completed a collection system master plan (CSMP) in 2019. One of the findings of the CSMP is that it is more cost effective system wide over the next 20 years to reduce I/I in 19 key basins rather than increase the conveyance and treatment capacity of the system to accommodate the extraneous wet weather flows. The collection system is comprised of multiple jurisdictions or member communities as well as WES owned infrastructure. Every jurisdiction contains a basin in which it was found to be more cost effective to reduce I/I rather than increase the downstream system capacity.

WES and the City of Gladstone (City) both have jurisdiction over portions of the collection system within the 19 key basins, and have immediate capacity needs to reduce the I/I. The good working relationship between WES and the City made it possible for the two entities to establish an IGA and enter into a contract with a consulting engineering firm to address the I/I in both jurisdictions simultaneously. The consulting contract will identify the sources of the I/I and design rehabilitation projects to remove it.

The joint project provides efficiency and cost savings to both of the partner entities. Project management costs are reduced since there is only one project not two. There is economy of scale and reduced mobilization costs when combining the field investigation activities. There are cost savings in the designs when applying the same standards across multiple sets of construction documents. These are just a few of the financial benefits of working together for a common goal.

The I/I identification activities are currently underway and should be completed in June 2021. At that time, the project will move into the phase of designing rehabilitation projects to remove the I/I identified. It is anticipated upon completion of the designs each partner will bid out their own construction projects.