With the proliferation of farm and WWTP AD facilities, comes the increased demand for clean off-farm/non-waste water organics that have increased biogas potential and an associated tip fee. In order to accept these types of feedstocks a facility will either need to invest in its own pre-processing system to clean the feedstocks or source a reliable supply of pre-processed feedstock slurry.
As the organics demand in a market increase and supplies of available clean feedstocks become scarce, existing and new AD operators are having to accept more contaminated feedstocks. The capex & opex for building and operating an organics pre-processing facility at an existing WWTP or farm-based digester can be prohibitive for smaller scale AD operations.
For the purposes of this discussion “pre-processing” means the process which treats the incoming raw SSO and other organic rich waste streams to remove the bulk of non-digestible contaminants and produce a pumpable slurry. This usually involves some type of size reduction (i.e. shredder/bag opener), potentially some kind of screening system & metal removal, the actual contaminant removal system (e.g. hammermill, hydraulic pressing etc.) and a pumping system for transporting the organic slurry to a holding tank or into a tanker truck.
As a result of the above factors, the industry is moving towards centralized organics pre-processing of commercial and residential SSO at transfer stations and the transportation of the “clean” slurry to one or several local existing digestion facilities (e.g. WWTP ADs, Farm ADs) than can have excess capacity and lower cost structures including easier and lower cost digestate disposal options.
We can see this in many markets across the continent e.g. Ontario, Boston, New York, RI and soon California.
Existing WWTP and especially farm AD facilities are always looking to secure long-term supply agreements for high biogas value food waste with tip fees. The waste collection companies are looking to dispose of the organic portion of their waste as inexpensively as possible without having to invest and develop large scale AD facilities.
Some key considerations when selecting a pre-processing system should be:
- Contaminant removal efficiency i.e. what % of the total contaminants in the incoming feedstock are removed and not present in the slurry.
- Flexibility of feedstock i.e. System should be able to process efficiently a wide range of feedstocks with varying degrees and types of contaminants.
- Minimal water use for achieving good organic/contaminant separation
- Minimal production of additional, small contaminant fragments that will find their way into the slurry, through the high speed/high impact of some pre-processing systems e.g. hammermills
- Industrial material handling systems to minimize manpower requirements and to enable 24 hr. operation if desired.
- Ease of maintenance – both planned and unplanned eg. blockages
The second part of the solution is to ensure that the receiving digestion facilities have the necessary infrastructure in place to deal with the increased contaminants (both sinking grit and floating plastics) that are inevitably part, to varying degrees, of any organic slurry generated from commercial and residential food waste.
Simple fixes such as having a removable ½ inch screen on the inlet to the storage tank at the digestion facility can eliminate the typical strange contaminants seen in FOGs from making their way into pumps and ultimately the digestion tanks.
More effective solutions for dealing with the problematic smaller contaminants that make their way into the digesters is to have either existing or new digesters outfitted with grit and light plastic removal systems such as those the Yield sells and installs in North America.
Even small percentages of grit & plastics in the received slurry can create havoc with downstream pumps, valves and digesters. Sedimentation over time of the grit contaminants can cause significant reductions in digester volume, hence biogas production, as well as, damage to agitators.
There will be many of these facilities being designed, installed and operational over the next 3-6 months. I invite anyone seriously considering this new model for the introduction of food waste into existing digesters to visit several sites utilizing a variety of pre-processing system and get real feedback from the operators on their efficacy and cost effectiveness.
We also suggest that you to visit the digestion end points and speak to the existing AD plant operators to really understand the impacts they see from receiving increased food waste at their facilities.