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Waitsfield Centralized Sewer

A response to the 90% Preliminary Engineering Report (PER) prepared for the Town of Waitsfield

Waitsfield Wastewater Planning Project documents

The current state of the PER does not fully discuss several key aspects of the alternatives presented. The following sections are the broad category of comments.

Growth, Long Term Planning

There has been a rapid change in the pace of development in the Mad River Valley since 2020, and any large capital project should be designed with the needs of its current users as well as how the community sees itself in 20 years, if not longer. Before choosing what kind of wastewater system will be used, a clear vision of what Waitsfield will look like in 2043 should be established. The ultimate solution will flow directly from this vision. Some questions that could be considered include:

Where should concentrated development occur?
Does it make sense to keep existing wastewater disposal fields in Waitsfield Village or Irasville operational, or would their real estate be better served by another use (e.g. housing)?
How far out should the wastewater collection system extend?
Should the water system be expanded alongside the sewer system?

Collection Sewer

The project area is hilly, with many crests and dips along Route 100 from Munn Field to Tremblay Road. Irasville is the high point in the system at an approximate elevation of 760' which divides the sewershed in half. This poses a challenge to conveying wastewater, requiring the use of pumps.

The collection sewer layout shown uses 5 pump stations and one dosing siphon to convey wastewater from the northernmost extent of the service area at the Waitsfield Elementary School to Munn field. Gravity sewers are used to route wastewater to these pump stations; for much of the project length, gravity sewer and force main exist side-by-side. While it may be necessary in some circumstances to do this, alternatives should be consider that minimize the need.

Another disadvantage to a combination of gravity sewer and force main is that connections are only possible in the areas with gravity sewer. For example, the proposed collection system is unable to provide any sewer service to users between Fiddler's Green and the Munn Field.

One potential alternative that could be investigated are low pressure sewers. These systems require each connection to the sewer system to have their own small wastewater pump station. All users connect to a single force main that discharges at the wastewater treatment facility. The force main can use HDPE pipe, which is able to flex and turn. Burial depth of the force main can be consistent, saving on excavation costs. Several factors should be considered when comparing this approach to a conventional collection system.

Advantages

Cost savings from only installing one sewer in the roadway
Cost savings from no large-scale septic tank/pump station assemblies required
Reduced O&M burden without a gravity sewer system to maintain
Ability to provide more users access to the sewer system
Can be installed via direction drilling to provide additional flexibility

Disadvantages

Each user must have their own pump station
Pump stations typically use grinder pumps, which have a higher maintenance burden than effluent (solids handling) pumps

References

EPA fact sheet: pressure sewers

Loxahatchee River District (FL) low pressure system

example vendor of low pressure sewer systems

Treatment

Moving Bed Biological Reactor (MBBR)

Membrane Bioreactor (MBR)

Advantages

Very high treatment capacity
Low capital cost
Small system footprint

Disadvantages

Higher O&M costs (power)
System maintenance can be more "hands-on"

Disposal

Munn Field Characteristics

Soil types: Colton gravelly sandy loam, 3 - 8% slopes; Waitsfield silt loam, 0 - 3% slopes
693 linear feet of disposal area
100,000 square feet of disposal area

Preliminary Disposal Field Sizing

IDR table 19 application rate = 0.35 gpd/sq ft (firm soil) - 0.70 gpd/sq ft (very friable soil)
Tertiary treated effluent application rate: 3x standard application rate
Maximum potential disposal field capacity: 105,000 gpd - 210,000 gpd

References

VT Indirect Discharge Rules (2019)

VT DEC Report: Onsite Wastewater Disposal Systems (2001)

VT DEC Prescriptive Desktop Mounding Analysis (2003)

Drip irrigation

Spray application (Table 23):

15,516 gpd/acre (0.356 gpd/sq ft) @ 10/10 BOD/TSS