Sewer capacity set for growth

Seeley Lake Sewer Board members have been hearing discussion in the community about the capacity of the wastewater treatment system designed to serve the Seeley Lake Sewer District. There seems to be a lack of information about the capacity.

Great West Engineering wrote a Technical Memorandum as a design report to submit to the Department of Environmental Quality (DEQ). The preferred alternative chosen by the District Board was a sequential batch reactor (SBR) with groundwater infiltration galleries. In the Design Flow section of Technical Memorandum (Chapter 1). The engineering document their calculations for the designed capacity of the system.

There is no way to monitor existing wastewater flows from individual septic systems. Great West used a nationally accepted method of using data from the public water system to establish an average. Three years of water records were used in the analysis and seasonal changes and irrigation water flow were factored into the formulas to establish an average annual flow per lot. Developed lots not connected to the Seeley Lake Water District and undeveloped lots were assigned the same per lot usage to plan for future needs. Non-residential users like restaurants and schools were also identified and a flow rate was established for those lots.

It cannot be determined at this time how the additional and vacant lots in the Sewer District will be developed so design presumed approximately 30% of future development on vacant lots will be non-residential. Non-residential flows are higher than residential giving even more capacity than may be needed.

In order to provide a safety factor - adequate capacity for future growth, account for uncertainty in the estimation methodology and to provide a conservative basis of design, average day design flows were established at 93,000 gallons per day when Phases 1 & 2 are connected, 21,300 gallons more than existing estimates. The capacity jumps to 154,000 gallons per day when Phases 3 & 4 are connected even though the analysis shows only 102,600 gallons per day are generated by existing use. So, the system as designed will have the capacity for approximately 1/3 more flow if it is needed.

One reason the Sewer Board chose this system design is because of the ability to add additional groundwater infiltration galleries when even more capacity is needed. The Treatment Plant can be modified and there is room for additional galleries (drain fields). The design has been completed with future additions in mind. The SBR tanks have been oriented to add treatment capacity and the drain fields have been designed to easily add disposal.

The mainlines and lift stations were designed to handle additional effluent in the future. Those costs are included in the General Obligation Bond on the ballot Feb. 23.

Jean Curtiss, District Manager Seeley Lake Sewer District, can be reached via email jcurtiss53@gmail.com or call 406-370-0393.