Closing the Loop: Biosolids and Sustainable Food Production

LL.M. Candidate Maranda White, who has supported the Food Recovery Project as a Graduate Assistant researcher during the 2014-15 academic year, joins us as guest blogger for a frank discussion of the other food-related waste stream — the one we rarely discuss in polite company: human excreta.  

In Rebuilding the Foodshed, Philip Ackerman-Leist exposes the “eight million ton gorilla” that is keeping local agricultural systems from being self-sustaining: human excreta. In the United States alone, it is estimated that eight million dry tons of excreta, termed biosolids, are produced annually. Though these biosolids are brimming with nitrogen, phosphorus, and carbon—all necessary to nourish our nutrient impoverished soils—municipalities are often having them landfilled, incinerated, or land-applied to non-agricultural fields. In most other circumstances, the unnecessary waste of such a valuable resource would engender well-deserved criticism. However, even amongst academics and environmental professionals, the issue of human excreta remains taboo. As a result, municipal wastewater treatment systems have gone relatively unscrutinized, at least in regard to their potential as a nutrient resource. Until now, that is. 


Wastewater Treatment Systems

In the United States, the biggest roadblock to utilizing biosolids as a nutrient resource is our wastewater treatment systems themselves. Unlike elsewhere in the world, in the States, everything that gets washed down the drain, including urine, feces, chemicals, medications, toilet paper, baby wipes, and stormwater, is combined at the wastewater treatment plant and then collectively treated. As a result, the biosolids produced by this system can be contaminated with pathogens, heavy metals, and potentially dangerous chemicals.

To make biosolids a viable nutrient resource for sustainable food production systems, municipal wastewater treatment plants must pursue ecological sanitation, which involves the development of sanitary systems for processing, treating, and recycling human excreta. Depending on the chosen structure, these sanitary systems will either divert urine away from feces, so that they are never mixed, separate the urine from the feces after they have been mixed, or combine them and process them together as one resource. Urine, in and of itself, is a valuable nutrient resource that is generally sterile, allowing agricultural operations to utilize it immediately after collection. Fecal matter, in contrast, can be contaminated with pathogens and therefore requires further treatment through anaerobic digestion or aerobic composting before it is safe to handle or use as an agricultural amendment.

Anaerobic Digestion

AD Diagram from

Simply put, anaerobic digestion is the process whereby microorganisms breakdown organic matter, in this case, human excrement, in the absence of oxygen. The process begins with pre-treatment, where undesirable objects or inert materials are removed and water added to improve efficiency and digestate quality. Following pre-treatment, the biosolids are typically fed into one of three types of digesters: covered anaerobic lagoon, plug flow digester, or complex mix digester. Each consists of a covered or self-enclosed tank that seals in the biosolids, microorganisms, and heat to allow digestion to take place. Anaerobic digestion itself can be broken down into three distinct stages: hydrolysis, volatile acid fermentation, and methane formation. During hydrolysis, certain microorganisms use water to cleave chemical bonds and break down proteins, fats, and other chemicals into smaller soluble molecules. Once hydrolysis is complete, volatile acid fermentation converts those smaller soluble molecules into organic acids. Finally, those organic acids are converted into carbon dioxide and methane. The end-products of anaerobic digestion are biogas, which can be used as an energy source, and digestate, which is separated into liquid and solid products. The liquids are used as an agricultural amendment, while the solids are then composted for later agricultural use.

Aerobic Composting

Much like anaerobic digestion, composting can be simply defined as the microbiological decomposition of organic matter. In this case, however, decomposition takes place where oxygen is present. Currently, there are three common methods of composting human excreta: aerated static pile, windrow, and in-vessel. Each method begins by mixing dehydrated biosolids with a bulking agent to improve porosity and add carbon. When utilizing the aerated static pile method, the mixture is then piled into long mounds over a bed of pipes through which air is actively pumped, to ensure that the compost is sufficiently aerated. The windrow method is similar to the aerated static pile method, in that the mixture is again piled into long mounds. However, instead of utilizing pipes to aerate the beds, the windrow method requires the mixture to be turned periodically to ensure sufficient oxygenation. Finally, the in-vessel method requires the mixture to be contained in a silo, tunnel, or other vessel where augers are used to aerate and turn the mixture, and air is pumped in to ensure sufficient oxygenation. No matter the chosen composting method, the increased microbial activity raises the internal temperature of the compost enough to eradicate unwanted pathogens. Therefore, once the process is complete, the compost is suitable for use as an agricultural amendment and is an excellent source of organic matter and certain macronutrients.

Some municipalities have already begun utilizing anaerobic digestion and aerobic composting for the treatment of biosolids, with an eye toward marketing the end product. In Cambridge, Ohio, for example, the City established a windrow composting operation, both as a sustainable alternative to landfilling its biosolids and as a cost-saving measure. Utilizing wood chips provided by local tree-service companies and finished compost as a bulking agent, the City is able to compost a small portion of the three hundred dry tons of biosolids produced annually by its Water Pollution Control Center. The remaining biosolids are sent off to a private anaerobic digestion facility. However, the City plans to compost increasing amounts of its biosolids once it can expand the market for its end product.

Laws, Regulations, and Ordinances

As per the requirements of the Clean Water Act, 33 U.S.C. § 1251 et seq., the Environmental Protection Agency (EPA) has promulgated regulations governing biosolids. The Standards for the Use or Disposal of Sewage Sludge, found at 40 CFR § 503, allow for the agricultural use of both compost and digestate produced using biosolids provided that they have been treated for a sufficient length of time at a specific temperature to ensure a virtual absence of pathogens. However, negative public perception of these products can lead to actual or perceived prohibitions against their use. For example, the organic standards promulgated as part of the USDA’s National Organic Program strictly prohibit the use of sewage sludge in organic production and handling. The regulations define sewage sludge as both “[a] solid, semisolid, or liquid residue generated during the treatment of domestic sewage in a treatment works,” and any “material derived from sewage sludge.” As both digestate and compost are “materials derived from sewage sludge,” the organic standards appear to prohibit their use.

To facilitate the expanded utilization of biosolids-based compost or digestate, the organic standards should be amended to allow for their use, provided the end product complies with the requirements of 40 CFR § 503. If there are lingering safety concerns, the organic standards could set limits on the percentage of biosolids used as feedstock for composting or anaerobic digestion, and require the remaining feedstock be comprised of organic food waste or animal manure, to ensure the virtual absence of pathogens and dilute heavy metals or other undesirables. State laws or local ordinances that also prohibit the use of biosolids-based compost or digestate could be similarly amended to allow for their use, provided that the end product complies with the organic standards. In this way, federal, state, and local laws can help expand the market for these products and encourage municipalities to adopt ecological sanitation.

Without addressing the “eight million ton gorilla,” local food systems can never be truly sustainable. Biosolids are an inevitable result of food production. Refusing to address their potential as viable nutrient resources has led to their increasingly indefensible waste.  It’s time to break the taboo around this issue and work toward creating sustainable, closed loop, food production systems.

Maranda - Headshot

Maranda White is an LL.M. Candidate in the Agricultural and Food Law program at the University of Arkansas. She received her J.D. in 2014 from the University of Arkansas School of Law, and holds both a B.S. and an M.S. in Environmental Science. She is currently a clerk at McMath Woods P.A., specializing in Environmental Law.


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