In the world of composting, selecting the right combination of feedstocks is critical. Feedstocks like biosolids, food waste, yard waste, and other organics all have unique characteristics that influence the composting process and the final product. While biosolids have often been considered a standalone feedstock, there are reasons why commercial or municipal composters may be interested in combining biosolids with food waste and yard waste.
If you are endeavoring to make a feedstock mix that utilizes biosolids, understanding the trade-offs of mixing these materials is essential for optimizing efficiency, meeting regulatory and certification standards, and producing a high-quality compost. Here’s a look at some key factors to consider when blending biosolids with feedstocks like green waste and food residuals.
Biosolids and food waste offer a natural synergy in composting. Biosolids tend to arrive alkaline, while food wastes and fresh green wastes are often acidic. When mixed, these opposing pH levels help neutralize each other, creating a more balanced environment for microbial activity (composting is generally optimized between a pH of 6.5-8). This balance supports faster and more effective composting, resulting in a stable final product with fewer potential odors. Our resources pages offer more in-depth analysis about odor management and pH levels in compost.
While biosolids typically have near-zero contamination physical levels due to strict wastewater treatment standards, post-consumer food waste and yard wastes often contain higher levels of physical contaminants like plastics or metals. Yard wastes or manures may contain residual herbicides and pesticides that are resistant to degradation through composting. Mixing the feedstocks can inadvertently introduce contamination into a clean biosolids derived compost. This trade-off requires careful screening and preprocessing of food waste and green waste to maintain the integrity of the final product.
Biosolids often require the use of polymers during dewatering, which can disqualify compost from achieving organic certification. On the other hand, food waste, when sourced and processed correctly, could potentially qualify for organic certification, increasing the market value of the compost. If certification is a priority, feedstock separation may be necessary to preserve eligibility for organic status.
Combining feedstocks offers clear advantages in system design and operational efficiency. Infrastructure sizing often targets a composting zone fill rate of fewer than three days to maintain consistent material age and temperature control. If a fill rate is too slow, the back of the zone will have been composting for days before the last material arrives, creating temperature gradients that complicate management. While re-mixing the pile could help address this issue, it adds operational costs.
Combining biosolids with other feedstocks can meet the < 3-day fill rate target more efficiently, potentially reducing the number of composting zones that would need to be built to handle smaller, separated piles. Streamlining loading through combining feedstocks can lower project costs and simplify operations, especially for facilities handling smaller waste streams that may struggle to achieve sufficient throughput on their own.
When considering whether to mix or separate feedstocks, the ultimate decision often depends on the project’s goals and stakeholder priorities. For example, if maximizing organic certification is critical, separating feedstocks may be the best approach. However, if cost efficiency and scalability are top priorities, combining feedstocks could offer significant benefits.
As always, operators should keep an eye on their mix composition, and how the properties align with industry accepted best management practices for moisture, C/N ratio, and density. Food waste, biosolids, and seasonal green wastes (think leaves and grass), can all be dense, wet, and nitrogen-rich. Planning to utilize enough carbonaceous bulking material will be essential to a feedstock mix’s success. We share more on BMP’s here.
Blending feedstocks like biosolids, food waste, and yard waste comes with trade-offs, but thoughtful planning can turn these challenges into opportunities. By balancing the biological, operational, and market factors, composting facilities may be able to realize efficiencies that improve cost-effectiveness, maximize environmental sustainability, and align with the needs of their communities to divert organics from landfill. At ECS, we specialize in helping facilities navigate these complexities to deliver reliable and efficient composting solutions.
