Biodigestion Program

Our Sanitary Service Area is growing. The Water Resource Recovery Facility (WRRF) is taking proactive steps to address increasing loads. The WRRF had three primary options to address these increases:

  1. Reduce the discharge limits on surcharge customers.
  2. Increase the WRRF's capacity.
  3. Develop alternate methods to provide treatment services.

The City moved forward with developing the following solutions:

WRFF local pipeline

We constructed a 2-mile-long, 10-inch waste transmission pipeline under Market Avenue SW. The pipeline allows surcharge customers to discharge concentrated waste. The waste flows into the WRRF at 1300 Market Ave. Currently, we hold waste from this pipeline until off peak hours. We then load balance the flow of the concentrated waste. This levels out current pollutant loading flow patterns at the WRRF.


We began construction of a biodigestion system in late 2017. When competed, this will manage municipal solids and higher strength wastes from the pipeline. We're planning to begin biodigester implementation in July 2020. Biodigestion will allow us to recycle the available carbon (methane) that the biodigester produces. We can use this to produce energy (electricity) to power and heat WRRF buildings and operations. We expect slight reductions in revenue requirements for the system.

Renewable Natural Gas (RNG) Conditioning System

RNG, or biomethane, is a high-quality gas that is interchangeable with natural gas. RNG can be used as vehicle fuel. Biogas that is produced during biodigestion can be refined to produce a higher quality product. The result is RNG. The sale of RNG produced by the biodigestion process will result in revenue. We estimate revenues of over $4 million annually when fully functional. We expect this system to be functioning soon after the biodigestion system comes online.

Phosphorous Recovery

The City must recover phosphorous from the biodigestion process. This is essential to prevent the build up of struvite in the biodigestion system. Struvite is made up of magnesium, ammonium, and phosphate. Removing phosphorous from the system will prevent the build up of struvite. If struvite is allowed to grow, it will clog up the pipes at WRRF. Therefore we must have a phosphorous recovery system in place. 

Video Example

You can view a short video from Michigan State University. The City’s biodigester will not have manure going to it.

Odor Prevention

No, appropriate odor control is designed into the system. A properly designed system will not produce odors. Carbon filters and a biofilter are included as part of the design.

Biodigester Cost

The biodigesters are expected to be about 76 million dollars.

When calculating the cost, these are some of the factors we considered:

  • Adjusted timing of the revenue from the sale of RNG
  • Reductions from the value engineering
  • Operations & maintenance
  • Capital reprioritization

Sewer Fund Impact

Revised financial projections depict how the Sewer Fund will

  • Remain financially stable
  • Produce a favorable rate impact for the community
  • Continue to provide a satisfactory debt coverage ratio.

Projected Savings

Projections show strong long-term value by reducing sewer rates 9% (30-year cumulative) to our residents. It is anticipated a bond of approximately $21 million will be required to complete this work. Staff will be bringing a Notice of Intent to sell bonds later this fall. These costs have been modeled into future rate impacts.

Municipal Bonds

It will be funded by issuing Municipal Bonds in 2018. The bonds will be repaid by the users of the sewer system.

How will this affect the sewer rates?

Rates are expected to be stable over the next five years. Project is expected to reduce operating costs by removing some loadings from the headworks of the WRRF, lowering solids volumes by 20%, producing electricity saving $600,000 and some heat recovery. The operating cost reductions will be slightly higher that the cost of the system. Placing the bonds, digester, other planned capital and change in operating costs into the rate study over the next 5 years has demostrated that rates will remain stable.

The cost of the system is included as depreciation in the annual Water/Sewer Rate Study. We created a Citizen's Guide to the Water Sewer Rate Study(PDF, 357KB). The document provides information on how the Rate Study works.

View the Most Recent Rate Study