What Does It Mean to Have High Quality Biosolids?

Water Environment & Reuse Foundation research project looks to define and help enable production of High Quality Biosolids that the public will readily accept as valued products.

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For years, Class A Exceptional Quality status has been a major goal for clean-water agencies seeking beneficial use of their biosolids.

Class A EQ means the ability to apply biosolids to land and distribute the material to community members essentially with few restrictions. But what happens when, for whatever reason, a biosolids product rated as Class A EQ fails to win acceptance from the public? Stated in another way, what constitutes a High Quality Biosolids product that the general public readily accepts and considers valuable?

Answering that question is the goal of the High Quality Biosolids from Wastewater research project that’s being conducted by the Water Environment & Reuse Foundation. Its aims since startup two years ago include defining the criteria that can make biosolids a true high-quality product, documenting the effectiveness of High Quality Biosolids in meeting product requirements, and developing guidance to identify and assess markets for the high-quality materials.

William E. “Bill” Toffey, executive director of the Mid-Atlantic Biosolids Association, is a member of the project team. He talked about the research program and the quest for High Quality Biosolids in an interview with Treatment Plant Operator.

TPO: What is the nature of the team working on this research, and what is your role?

Toffey: The lead principal investigators are Trudy Johnston, principal of the Material Matters specialty consulting firm, and Christopher Peot, residuals manager with DC Water. The team includes researchers from Virginia Tech, the University of Maryland, Penn State University and Kansas State University. There is also a group of advisors. As one of the team members, I’m responsible for the communications component of the project.

TPO: Why is there a need for research on High Quality Biosolids?

Toffey: The federal 503 regulations set minimum standards that apply to Class A EQ biosolids, essentially a process for further reducing pathogens, meeting a vector attraction reduction standard, and meeting the higher of two pollutant concentration standards. These standards are designed to protect public health and the environment, but they do not relate to the public acceptability of products used for purposes like landscaping, horticulture and residential gardening. We’re trying to develop effective measurement approaches to help wastewater agencies classify and characterize their biosolids products for these uses.

TPO: Aren’t there already a number of well-accepted and established biosolids products?

Toffey: There are several such programs, and the classic one is Milwaukee’s Milorganite, where they developed demand for the product, branded it and labeled it for specific uses. The other side of the coin is that there is no particular set of standards by which to distinguish products that are as stable and easy to handle as Milorganite. Therefore, agencies seeking to mirror the success of Milorganite have difficulty procuring the needed equipment and designing comparable programs.

TPO: What makes some biosolids products that attain Class A EQ status less than ideal in terms of public acceptance?

Toffey: Their moisture content may be high. They may be lumpy. They may be smelly — that’s the biggest issue. We’ve confirmed that a poorly digested, heat-dried biosolids product has potential to attract flies if it is rewetted. So while drying may yield Class A EQ biosolids, that’s not sufficient for creating a product that can be defined as high quality.  

TPO: What is the consequence of Class A EQ products that fail to meet the criteria for public acceptance?

Toffey: One problem is that in states where there have been failures to produce biosolids that have public acceptance, regulators are wary of giving the freedom to distribute the product. There is a tendency among those state agencies to still require various steps designed to avoid odor nuisances and public complaints. They may require a state permit, restrictive setbacks, or nutrient management plans. For a wastewater agency that has invested heavily in equipment to achieve Class A EQ, it’s disappointing to still have their biosolids regulated in a manner very similar to when they had a Class B material.

TPO: Is your team taking lessons from agencies that market biosolids successfully?

Toffey: Yes. Tacoma (Washington) TAGRO, for example, is a partnering agency in our project. We examined TAGRO for characteristics that when measured demonstrate suitability and acceptability. Odor is certainly part of that, so we have an odor panel. We also measure emissions by taking gas samples off the product and making objective assessments. In Tacoma, although the material comes out of their digesters in apparent compliance with Class A EQ standards, they nevertheless have additional processing. They mix it with a combination of bark and sand, and they let it cure before they ship it out. That additional processing step causes changes to the biosolids. We’re trying to understand what those changes are and how they might apply to other products.

TPO: Is vector attraction a significant issue?

Toffey: Yes. We worked with the research lab of Dr. Ludec Zurek at Kansas State, who is a specialist in entomology and microbiology and does vector attraction reduction studies. He explores the role of houseflies in spreading diseases. He subjected various Class A EQ biosolids to a test of whether they attracted flies. Some products did, and others did not. We’re trying to identify the physical characteristics of biosolids to which flies just aren’t attracted.

TPO: Based on your investigation, what will it take to produce biosolids that are truly high quality?

Toffey: We’ve pretty well concluded that some form of digestion results in better products but that some additional step is needed even after anaerobic digestion to reduce the potential for unacceptable odors. That’s true even when thermal hydrolysis has been applied as a preprocessing step to digestion. There is just enough lingering protein ready to decompose into odorous compounds so some amount of additional curing or composting is useful.

TPO: So digestion alone doesn’t seem to be doing the job when it comes to odor reduction?

Toffey: We’re seeing that the amount of volatile solids destruction that occurs during digestion is an important measure in terms of the stability of the final product. To meet national standards, you need to have 38 percent volatile solids destruction, but that is not a high hurdle. We’re showing that substantially higher volatile solids destruction is necessary in order to bring the odor potential down to a lower level.

TPO: As a practical matter, to the extent that standards are developed for High Quality Biosolids, how do you see that playing out in terms of compliance?

Toffey: The 503 regulations were fixed back in 1993, and we’re stuck with a regulatory system that doesn’t easily accommodate change. Instead, I foresee an industry initiative similar to the way the U.S. Composting Council came up with test methods for composting, and the American Biogas Council is developing standards for digestate management. I envision our industry coming together around methods we can deploy on a voluntary basis to take biosolids to a level beyond the 503 regulations. Wastewater facilities would have processing standards enabling them on a consistent basis to deliver a product customers could rely upon, whether they be professional landscapers, companies that make soil products, or residential customers. Everyone in the value chain needs some assurance that the process is under control and reliable.

TPO: Where do High Quality Biosolids fit in the bigger picture in which many agencies still operate successful beneficial use programs with Class B material?

Toffey: There is still a significant role for Class B biosolids cake in the evaluation of options, even for large wastewater systems. The particulars for each city are very different in terms of the amount of agricultural land available, the nature of the wastewater system, and how much room there is at the treatment plant for processing. One thing that’s happening is that the gap between creating a Class A and Class B product is not as wide as it once was. As new equipment is introduced for advanced digestion, as competition for thermal hydrolysis rises, and as energy efficiencies are brought into drying technologies, the cost of producing Class A biosolids can come down, and it becomes easier for agencies to go to Class A.

TPO: What do you see as the ultimate benefit of this research project?

Toffey: I believe that if there were commonly agreed to measurements applied to biosolids that could be shown to produce high-quality products suitable for landscaping and other recycling, everyone involved would benefit. Equipment manufacturers could then design their processes to meet the standards. Regulators could modify their regulations to incorporate the standards. Treatment plants could apply the standards to process control.

TPO: How do you envision the future of biosolids recycling?

Toffey: One area we’re looking at is how effective biosolids can be as an ingredient in specialty soils designed to meet urban soil needs. There is growing interest in biosolids-based soil products because they are effective when used in green infrastructure, such as rooftop gardens and stormwater detention basins. In California, there is a trend toward using biosolids to restore fire-ravaged landscapes. To the extent that we can explain the characteristics that make High Quality Biosolids appropriate as ingredients in urban soils, we can help develop demand for those products being created by wastewater agencies.



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