Sewer System Information

Sewer Basics

Duvall is a “consumption based” small municipal sewer system provider. Our costs fund basic sewer needs including:

  • Consumables and process materials (CPI increase annually)
  • Inflow and Infiltration - 25% of our flows are attributable to mostly rainwater entering the conveyance system
  • Personnel and benefits (increase annually)
  • Power to process (increase annually)
  • Monthly bill processing

Revenue to fund the sewer system includes monthly sewer fees and facilities connection charges:

  • 96.4% of monthly sewer fee revenue is residential
  • 3.6% of monthly sewer fee revenue is commercial
  • Monthly customer rates are balanced to provide a fair cost to both residential and commercial customers. New connections from development are also required to pay a one-time General Facility Charge (GFC) to buy into the system
  • Total revenues must be maintained to operate the sewer system

Sewer Cost Items Out of Our Control

  • Bond rating obligations previously pledged by Duvall. We guaranteed to increase rates if we could not maintain our debt and operational obligations
  • Previous debt
  • Requirements from permitting agencies which include stringent environmental / health regulations. The city is subject to federal pollution control regulations and must discharge its treated sewer waste into the Snoqualmie River in accordance with the terms of a National Pollutant Discharge Elimination System (NPDES) permit administered by the Washington State Department of Ecology (WDOE)

Sewer Capacities

  • 1.3 MGD* with 3 trains (serves approximately a population of 9,800)
  • 1.75 MGD with 4 trains (future expansion)
  • Current flows:
    • MGD Winter: 0.75MGD
    • Summer: 0.4

*MGD - Million Gallons per Day


2023 Rate Basics

Sewer monthly charge:

  • Base fee is $84.91
  • Commercial is $84.91 + $3.09 per 100-cubic-feet over 900 cubic-feet.
  • Commercial users do pay more based on consumption
  • Flows are irregular and we have too few customers to have a mathematically sustainable approach to base rates on consumption
  • Reduced Rate (based on income limit categories established by HUD) is 10% reduction of the base fee. 

Rate Methodology

Based on history and “equity” the rates have been created to try and balance the heavy residential load the city must treat relative to the very low commercial customers we serve. A slight tipping of the fees from residential to commercial does not provide straight across equity and has a dramatic change to the fee structure. Further pushing rates down will increase the costs for new connections which are already fairly high and may further curb development within Duvall.

The option of variable rates for all customers based directly on water consumption has been discussed in the past. However, this approach may penalize users with high irrigation or other non ‐ sewer water use. The revenue and expenditure flows for a small municipality or operator are critical and swings on a monthly basis of high and low charges make business very difficult to manage. In addition, because the sewer system costs are relatively fixed, fluctuations in flow ‐ based rates would require a higher contingency, frequent rate evaluations and possible rate changes, and possible higher rates to cover shortfalls. Smaller operators such as Duvall tend to charge against a fixed fee structure because of these concerns.

Related Documents and Reports