Context for Assessing Compost

StopWaste often fields questions about compost quality and safety, and many concerns can be alleviated with a better understanding of the regulated composting process and the "feedstocks" that are actually composted by facilities serving Alameda County.

Compared to other parts of the country and state, Northern California has decades of composting infrastructure, green waste and food scraps collection programs, and established markets for compost. Additionally, home composting education in the Bay Area has raised the general public's awareness of compost. In effect, our region has access to high quality composts that meet regulatory safety standards, and nearly all local commercially-produced composts are organic certified, as organic farmers are an important compost market in the state.

Compost quality needs to be discerned from compost variability. Composts can vary physically (coarse vs fine, moist vs dry) and chemically (nutrients, organic matter, pH, salts). Gardeners, landscape managers, farmers, and ranchers all have different preferences for those variable characteristics of composts, so composters often aim to make a compost that fits their local demand. For example, coarse composts high in carbon are excellent for mulching, erosion control, and firescaping, whereas fine composts high in nitrogen are desirable by gardeners, farmers, and parks to supply nutrients to their soil. Compost can also vary in its stability and maturity, with immature compost commonly identified by an ammonia-like odor, which reduces as the compost continues to cure overtime.

Commercial composters are required to regularly test their compost for metal concentrations, pathogens, and physical contaminants as set forth in California Code of Regulations "Title 14, Natural Resources Division 7, Chapter 3.1, Article 7." Additionally, the Seal of Testing Assurance (STA) Program creates a standardized format for laboratories to utilize for compost testing, of which nearly all local composters utilize.

Together, a compost's variable characteristics, as well as indicators of its quality and safety, can be assessed by reviewing compost laboratory analyses which can be requested from any compost producer or vendor.

The Commercial Composting Process

Brian Oliveira of Oliveira Enterprises Composting Facility provides a tour of his composting operation.

Certain misconceptions about compost quality can be addressed by clarifying the commercial composting process. StopWaste's "Compost Happens in Alameda County!" video showcases a local commercial composter "Oliveira Enterprises," highlighting the process typical of commercial composting facilities:

  • Collection and receiving: Organic materials arrive at the facility from curbside collection and commercial sources, and larger visible contaminants are removed
  • Grinding and mixing: Materials are processed into smaller pieces and mixed to achieve the right carbon-to-nitrogen balance necessary for decomposition
  • Active composting: Materials are arranged in windrows or enclosed vessels, accelerating microbial decomposition. Microbes release waste heat, which is insulated by the windrows, causing temperatures in the piles to rise. Composters must achieve hot composting temperatures of 131–155°F for a minimum of 15 days with at least 5 turning/mixing cycles (called the Process to Further Reduce Pathogens or PFRP), which kills pathogens and makes weed seeds unviable
  • Curing: After active composting, materials are cured for additional weeks-to-months to allow compost to stabilize (let the microbes settle down) and mature (let the microbes finish breaking down any phytotoxins)
  • Screening: Finished compost is screened to size (coarse vs fine) and to remove physical contaminants; compost that falls through the screen is considered finished compost, whereas materials that do not pass through the screen are referred to as "overs"
  • Testing: Finished compost products are tested for physical and chemical analysis, stability and maturity, pathogens, inert contamination, and heavy metals; regular compost testing is required of compost facilities in California, and test results must be provided when requested

Common FAQs about Commercial Composting

The primary feedstocks used by commercial composters include green waste (plant and tree trimmings) and food waste. These materials are collected through a variety of means such as green waste drop off facilities or municipal organics collection programs (your residential green bin or commercial organics cart). Commercial composters do not use trash or contents collected in landfill bins as a feedstock.

No for both. While manure is a great composting feedstock, nearly all commercial composters in our region do not accept nor utilize manure in their compost. There are local on-farm and on-site composting operations that use manure and make and market great composts that are safe to use. That said, when manure is used as a compost feedstock, it will not be "present" in the finished compost. Composting is a biological process that breaks down the feedstocks and changes their composition (decomposition). Additionally, composters must achieve the "Process to Further Reduce Pathogens" (PFRP) by maintaining the material at temperatures above 131 F while regularly mixing the compost pile over a 15 day period, which effectively kills pathogens including fecal coliforms. Compost is not manure, nor is not aged manure, and composted manure does not resemble raw manure. Human waste, or biosolids, are managed by waste water treatment plants. Some biosolids are composted aerobically, others are digested anaerobically, using either process to achieve PFRP and kill pathogens and fecal coliforms. However, biosolids compost is not marketed nor made available to most end users. Only certain agricultural markets make use of biosolids compost and use it at low application rates.

Finished compost, meaning its fully stable and mature, should smell earthy and pleasant like the forest floor. When commercial compost is delivered to your project, it can typically arrive immature, easily identified by steam rising from the pile (signaling instability - microbes are still active and respirating) and smelling of ammonia (signaling immaturity - the compost has not finished cycling nitrogen). Immature compost is not desirable and should be used carefully, as immature compost can "burn" young plants. That said, immature compost does not signal poor quality, it just signals its "age" as the compost will continue to cure towards a stable and mature material.

Dig Deeper

Understanding Contamination in Compost

Common contamination concerns in compost including physical inerts, compostable plastics, PFAS, and pesticide residues.

Understanding Compost Lab Analysis Reports

How to read and interpret compost lab analysis reports to evaluate product quality for your application.