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    Biodigestion
     
    The City of Grand Rapids Sanitary Service Area is experiencing tremendous growth and the Water Resource Recovery Facility (WRRF) is taking proactive steps to address the increasing loads. The Sewer System had three primary options for addressing increased loadings;
     
    1. Reduce discharge limits on surcharge customers.
    2. Increase the capacity of Water Resource Recovery Facility (WRRF).
    3. Develop alternate methods to provide treatment services.
     
    A review of these options indicates that identifying alternative methods to provide treatment services is essential to limiting the impact on growth of industrial users, avoid significant investment to increase secondary treatment capacity of the WRRF, as well as maintaining an operation that supports the City’s commitment to the environment.
     
    As part of our approach to identify alternate methods, the WRRF is taking the following two steps:
     
    1. A pipeline local to the WRRF is planned to be installed for surcharge customers to discharge into a concentrated waste tank.  Customers will also be allowed to transport concentrated waste for direct discharge to the tank.  The concentrated waste then can be introduced to the WRRF during off peak hours to level out the diurnal flow pattern of current pollutant loadings.  A surcharge credit will be included in the 2016 Rate Study and applied to participating customers based on a portion of the overall savings resulting in the leveling of plant loads.  The current schedule places the system in service no later than December 2017.
    2.  A bio-digestion system is planned to be used to manage the higher strength wastes. The projected customer growth in the area, from food processing, brewing, and general economic expansion, would exceed the capacity of the current system within the next few years without the introduction of bio-digestion.  To address additional growth in a sustainable manner, bio-digestion is planned to be used to manage these higher strength wastes.  This approach reduces Biochemical Oxygen Demand (BOD) loadings directly on the WRRF and moves the reduced loading to recycle streams.   The biogas produced by recycling the available carbon (methane) created with the digestion system will be used to produce energy, primarily electricity that can then be used to power and heat the buildings and operations at WRRF and to operate the digesters.   The overall impact on revenue requirements for the system is expected to be a slight reduction.  The digesters are expected to be online by the end of 2018 in order to accommodate growth.
     
    Phosphorous recovery is a part of the WRRF’s commitment to operating a world-class facility, reducing costs, and protecting the environment. Phosphorus is one of the building blocks of all life. Every living cell requires it. In fact, plants need phosphorus to grow just as much as they need water. Insufficient phosphorus in the soil can result in a decreased crop yield. That is why we often apply phosphate fertilizers to farm crops.
     
    One source of phosphorus is through mining. This is not the best option since it’s found only in limited supply and mining it can have a negative impact on the environment. Phosphate strip mines carve up the land, disrupting sediment, releasing toxic chemicals into the atmosphere, and producing toxins in the process to separate it from the rock.
     
    A better solution is to recover phosphorus from digested sewage sludge during the wastewater treatment process. This has the added benefit of keeping the phosphate out of the watershed, which can lead to eutrophication of the water system and can lead to algae blooms which harm fish and other aquatic life.
     
    Phosphorus recovery becomes an option when phosphorus levels in the recycle stream exceed 150 mg/l and increasing cost effective as the concentration increases.   Dr. Lina Belia, of Primodal Inc., has completed a preliminary model of the future state of the internal return stream of the Water Resource Recovery Facility (WRRF).   The model predicts future concentrations in the recycle stream, when bio-digestion is implemented, will likely be in the 1000 mg/l range.   At the level projected the phosphorus will come out of solution and creating a struvite coating in pipes and on equipment resulting in excessive maintenance costs.
     
    How does a Digester Work?  Here is a short video from Michigan State University.  The City’s will not have manure going to them.
     
    What are the project costs?  The digesters are expected to be about 34 million dollars.
     
    Why did I read that it is 40 million dollars?  A pipe is being ran down Market Ave to collect local high strength wastes, initially from Coca Cola, Founders and SET Environmental.  The cost of the pipe is in the 4 million dollar range.  Other current or future high strength waste customers in the area will be able to utilize it.  The construction on the pipeline will be completed by the end of 2017.
     
    At the projected concentration levels, implementing phosphorus recovery becomes very cost effective and lower overall operating costs.   The total return on investment is dependent on the amount of liquid land applied biosolids in the future.   The total investment will be in the 6 million dollar range. 
     

    How will the project be funded?  It will be funded by issuing Municipal Bonds in 2017.  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.   The cost of the system is included as depreciation in the Rate Study (see page S-51).  There is a Citizen's Guide to the Water Sewer Rate Study which provides information on the how the Rate Study works.  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.

    Will the Digesters have Odor Issues? No, appropriate Odor Control is designed into the system.  A properly designed system will not produce odors.