WWTP Plan Update

2020 WWTP Facilities Plan Update

Primary Objectives

1. Evaluate alternatives to the existing outfall and the associated mixing zone study required by the NPDES permit.

2. Evaluate options to proactively and sustainably increase nutrient removal.

3. Minimize the carbon footprint of proposed improvements (upfront and long-term).

4. Support ecosystem resilience and diversity to enhance available ecosystem services.

5. Identify solids handling improvements which could reduce disposal costs in the future, when needed.

6. Minimize rate increases.

Basic Planning Information

• 2019 City Population: 470 people

• Forecasted 2040 City Population: 586 people (1.1% OPFP annual residential growth)

• Buildable lands will not constrain growth

WWTP Flow and Loading Information

2040 forecasted growth will not exceed WWTP design criteria:

Regulatory Requirements

Current NPDES Permit requirements:

 • Secondary treatment standards for BOD or TSS (20/30mg/L monthly/weekly summer, 30/45winter)

• TMDL for Temperature in the Columbia and Lower Snake Rivers (EPA 2020, currently out for public comment)

• Nutrient removal is not currently required but agencies are increasingly implementing nutrient limits

• Outfall mixing zone study required during next permit cycle

• Maintaining a direct discharge will likely require substantial modification

• Inherent risks with direct discharge

• Recommendation is an indirect discharge due to cost, risks and other factors

Key Findings of Hydrogeological Investigations

UPRR monitoring wells show:

1. Well levels directly follow Columbia River level, typical of hyporheic waters

2. Gradient of hyporheic water appears to flow northeast, away from Rock Creek

Preliminary geologic investigations show:

1. The site is underlain with permeable material

2. Shallow infiltration tests demonstrated a very high rate of infiltration

3. The site appears suitable for an indirect discharge

Solids Handling System

UPRR monitoring wells show:

1. Wasted sludge is stored and periodically hauled offsite for treatment

2. 2x per week hauling to Hood River WWTP, increasing as plant loading increases

3. Potential cost escalation due to labor for hauling and offsite treatment

Future solids handling system

1. Thicken all waste sludge to reduce volume

2. Repurpose existing tankage to equalize and store sludge

3. Sludge thickening can increase ultimate disposal option


Needs to be considered during the planning period:

• Current permit requires existing outfall to be addressed, likely requiring extension or replacement if a direct discharge is maintained

•A future temperature limit will likely be established based on the pending TMDL

• Nutrient removal is not currently required though maybe in the future

• WWTP capacity is adequate through the 20-year planning period but some maintenance or replacement of existing equipment items is likely

• The existing solids handling system limits disposal options


• Seek DEQ approval for an indirect discharge in lieu of extending the existing outfall

• Consider solids handling improvements and other equipment replacements with any large project

• Consider other sustainable technologies, such as EBPR, that could proactively be implemented