Choosing between Composting and Anaerobic Digestion: soil, fuel or both?

Many communities, businesses and institutions are considering whether they should focus on composting of discarded organic materials or anaerobically digesting (A.D.) those materials. There has been a lot of interest in pursuing A.D. (or “Biodigestion”) in recent years, as it has the attraction of recovering energy from discarded materials, while operating at biological temperatures and recovering also a solid organic output that may be used as a soil improver, so this approach is acceptable to real Zero Waste advocates.

We’d like to provide arguments for a more informed decision-making process. There are a number of key factors that should be considered when evaluating whether to compost or digest, such as:

  • In general, composting should be considered as a priority option over A.D., to return discarded organic materials back to soils and due to its intrinsic robustness, and scalability, which typically makes it suitable also for small-scale, low-tech, or temporary approaches in communities businesses and institutions, pending decisions on larger scale sites.
  • The Zero Waste Hierarchy of Highest and Best Use recommends the following for pursuing options, in this order:
    • Support and expand composting as close to the generator as possible (prioritizing home or on site or local composting wherever possible)
    • Whenever home/decentralized composting is not possible, consider industrial composting, or if local conditions require/allow, anaerobic digestion.
    • If conditions allow, recover energy using only systems that operate at Biological Temperature and Pressure (The ambient temperature and pressure that occurs naturally without the use of added energy, or in any case not above 100°C to change it such as anaerobic digestion).
  • Biodigestion or A.D. may be a good alternative when composting may not be successful or there are critical issues in planning, siting and operating. A.D. is the next step to be considered whenever home or industrial-scale composting is not possible. For example, many larger cities may generate larger amounts of food scraps than yard trimmings, which may make the mixture not as conducive to good management of composting.
  • The main focus must be on improving the soil. As long as A.D. is giving back to the soil, it’s acceptable, and it’s just an alternative way to composting. If A.D. is used with clean organics from separate collection, it produces both a soil amendment and renewable energy. To avoid “dilution” of pollutants, need to check quality not only of the end product, but also of input material, so that any “dilution” of pollutants be avoided (as might happen with some low-quality biosolids/sludges).
  • A.D. should not be pursued solely to produce energy with the residue (“digestate”) being landfilled. In Europe, the EU Commission stipulates that A.D. only counts toward meeting recycling targets if it produces a material fit for use on soils and farmlands. For the moment, “fitness for purpose” depends on legislation in each of the nations for defining what’s fit for use. The EUis working to develop a common EU standard for what’s “fit”. Separate collection of organics should be one of the prerequisites, so that only clean feedstocks are processed and turned into high-quality soil improvers.
  • In some locations multiple A.D. systems in one geographic area are being implemented to obtain incentives for energy production. Without subsidies, A.D. typically can’t compete with composting, due to larger capital costs. Typically, investments related to A.D. systems cost 1.5x – 5x as much as composting systems. In Europe, there are very high subsidies for A.D. (up to 240 Euros per megawatthour). If these subsidies are used to develop A.D. for crops raised for energy, that promotes land grabbing by agri-business and heightens the problems of raising crops for fuel instead of food, which is a social justice problem.
  • ZWIA therefore recommends eliminating any subsidies to the use of A.D. ZWIA supports investing in A.D. only if the facilities are planned primarily to treat organics discarded from residents or businesses (and not agricultural energy crops). A problem with subsidies going to A.D. is that there are a limited amount of dollars available to spend on things for a community. If communities invest in composting instead of A.D., they would not have to invest so much money. Conserving resources also includes conserving cash. Even though with all the subsidies the larger costs for A.D. are competitive with composting, they are still larger capital expenditures that are balanced by the subsidies.
  • A.D. produces biomethane. Use of this biomethane to replace gasoline fuel (automotive) or natural gas (domestic) is preferable to the use of combined heat and power (CHP) units at a centralized process site to produce electricity and heat for district heating or reheating the biomass for digestion. Biomethane produced fuels don’t keep the impacts on site and avoid concentrated burning in a single place. Provides replacement fuels for cars or trucks, or add to the grid for natural gas.
  • A.D. and composting may also be appropriate for alternative treatment for residual waste in Materials Recovery and Biological Treatment (MRBT) systems. The B in MRBT could be aerobic or anaerobic biological treatment. It must be remarked, though, that the goal in this case is to stabilize before landfilling, not to produce a quality soil amendment. The benefit is flexibility, since over time, less capacity may be used for stabilization prior to landfilling, and more capacity for the clean organics coming from separate collection. That is why ZWIA supports MRBT as a flexible treatment system for residuals.
  • Both for “clean” organics back to the soil, or “dirty” organics in residuals for stabilization before landfilling, ZWIA recommends that, also after A.D., the final step should be aerobic composting, to ensure the minimization of fermentability of materials. If digestate is used from A.D. directly back to the soil, it may release fugitive emissions of methane and ammonia, cause phytotoxicity and leakage of nitrogen. Post-composting of digestate creates a soil amendment that is basically the same quality as coming directly from composting. Hence, A.D. must be seen basically as a possible replacement (if and when need be) for the early stages of composting, so that we keep C in the form of a renewable energy source (biogas/biomethane) instead of releasing it as CO2. The final stage should anyway be aerobic, and aimed at a similar goal (production of well stabilized soil improvers in the case of clean organics, proper minimization of fermentability before landfilling, in the case of dirty organics still included in residual waste)

For more insights, see the ZWIA Zero Waste Hierarchy of Highest and Best Use.

Frequently Asked Questions

ZWIA favors biomethane to replace gasoline (automotive) or natural gas (domestic) to avoid concentrated burning in a combined heat and power (CHP) unit at the process site to produce electricity and heat for district heating or reheating the biomass for digestion. Biomethane doesn’t keep impacts on site. Provides replacement fuels for cars or trucks, or add to the grid for natural gas.

Need to meet air emissions for burning in whatever application is used. In Europe there are some working groups to come up with common standards among nations.

Just as with composting, both must prove that they are good quality and meet the standards for use in agriculture.

When it comes to biosolids and sludge, quality must be checked on inputs besides the output in order to avoid “dilution” during co-digestion with food scraps and yard trimmings. Typically, agroindustrial sludge tends to be high quality (but there may be exceptions) whereas sewage sludge from WWTPs often includes relatively high concentration of pollutants. This may be different in rural vs. urban areas, and depending on whether any “pollution prevention program” is in place (e.g. setting limits for discharges of wastewater by industry into sewage).