The Sense to Smell Good

I heard my favorite environmental psychologist quoted on a Sunday morning podcast, The Daily Sunday Read. The January 31st podcast, “The Forgotten Sense,” was a radio story version of a New York Times article by Brooke Jarvis “What Can Covid-19 Teach Us About the Mysteries of Smell?”  That psychologist, Pam Dalton, is with the Monell Chemical Senses Center in Philadelphia, an organization that figures prominently in these two items, and she was tapped to discuss how terrifying to Covid-19 victims is the loss of smell, a symptom called anosmia, or, even more peculiarly, distorted smell perception (dysosmia) and phantom smells (phantosmia). I am thinking dysosmia might not be a wholly distressing phenomenon if biosolids were to smell like chocolate. But for Dr. Dalton, who advises widely on bad smells from composting, the issue with biosolids is that its smell provokes, not a craving for chocolate, but fear and danger. We biosolids practitioners need to have the sense to make biosolids smell good.

Dr. Dalton has been a “go-to” scientists in the field of biosolids malodors. Two decades ago, she was a keynote speaker at a biosolids conference. In her work on hog operations, she had shown that women of childbearing age were ten-fold more sensitive to fecal odors than male counterparts, particularly older workers in the biosolids business. She also had drawn attention to the phenomenon of “idiopathic environmental intolerance” (IEI), (Chemosensory function and response in idiopathic environmental intolerance), in which inhaled compounds, even non-irritant and non-toxic airborne chemicals, induce physiological reactions, or “dis-ease.” Further, she had shown that adverse “pre-knowledge” of the odorant negatively affects reactions, as in inducing asthma (Asthma and odors: The role of risk perception in asthma exacerbation).  She amused the audience with the story of a secret, and failed, Allied spy tool of World War II, a sulfurous fecal-odor spray aimed to distract German soldiers, a classified blend called “Who Me?” I followed up her lecture with a personal dive into IEI, with a paper “Biosolids as a Cause of Somatic Disease,”  in which I underscored the urgent need for biosolids generators to assume responsibility for how their product might offense and thereby invoke panic reactions.

The importance of understanding odors from biosolids cake was heightened in the late 1990’s by anecdotal reports that the new generation of dewatering equipment was making stinky biosolids. Centrifuges running at high speeds and torque, creating thereby high shear on sludge floc, were producing biosolids cake of amplified odor intensity. Conference papers at our national conferences confirmed this observation (Impact of high shear solids processing on production of volatile sulfur compounds from anaerobically digested biosolids (2002)). What is more, the shear effect seemed to cause, too, regrowth of the pathogenic indicator E Coli (Effect of Dewatering on Fecal Coliforms and Methanogens in Anaerobically Digested Biosolids), which was particularly alarming in thermophically digested biosolids, otherwise classified a “Class A EQ biosolids” (Regrowth of Fecal Coliforms in Class A Biosolids). The importance of understanding the connection between solids treatment processes and biosolids cake odors resulted in a multi-year, multi-phase odor research project, now available in four volumes from the Water Research Foundation. The key report is Identifying and Controlling Odor in Municipal Wastewater Environment Phase II: Impacts of In-plant Parameters on Biosolids Odor Quality. But even through all these investigations, high speed centrifuges enjoyed a strong market, as the need for higher cake solids was greater than the need for low odor biosolids. The challenge of biosolids odors was not solved by this unprecedented research effort.

Odors are a “management” challenge when recycling “ordinary” digested biosolids to farmlands, but odors are market killer when “Class A EQ biosolids” emit odors that offend the buying customer. Although odor emissions from biosolids composting operations have been studied and reported in literally hundreds of conference papers (for instance, Indoor Composting Facility Odor Control: Perception, Process, and Performance), I observed, even from the start, consultants and public agencies paid little attention to the odor quality of the “Class A EQ biosolids” compost from a user viewpoint, among other attributes  (Biosolids Compost: Research Needs for Making a Great Product). When thermal dryers became popular for producing “Class A EQ biosolids,” a truism was observed “stinky biosolids into the dryer, stinky biosolids out of the dryer” (Evaluation of Odor Characteristics of Heat-Dried Biosolids Product). Advanced alkaline stabilization, also a technology for “Class A EQ biosolids” could not uniformly produce an odor-free product, though efforts to find a right combination of liming ingredients and mixing equipment could vastly improve product quality (Examination of Mechanisms for Odor Compound Generation During Lime Stabilization).  We knew it in our hearts, but often failed to voice it out loud, “Class A EQ biosolids” is a term that is neither synonymous with “consumer-ready” biosolids, nor is it a class that is absent offensive, market killing odors.

If our industry were any ordinary, consumer-oriented manufacturing enterprise, we would jump at the opportunity to define odor standards consistent with consumer preferences. Even so, this has proved tough for soil and mulch products. The U.S. Composting Council has its Test Methods for the Examination of Compost and Composting, which is the backbone of its Seal of Testing Assurance for compost producers. The American Biogas Council issued its Digestate Standard Testing & Certification in October 2016.  The Mulch & Soil Council has its Uniform Voluntary Product Guidelines for Horticultural Mulches, Growing Media and Landscape Soils.  None of these groups attempt to objectively distinguish good smelling from bad smelling products. And “sour mulch” remains no small issue, as revealed in the article Beware Of Toxic Mulch: “if it has a pungent odor similar to vinegar, rotten eggs or silage, don’t spread it around your plants. Chances are good you’ve got “toxic” or “sour” mulch.”

One complication for defining odors in organic matter products is that the media (water, air or soil), the concentration (dilute /concentrated), and the context (under your arm / in your biosolids) can hugely affect human perceptions of nuisance odors. For instance, geosmin is a natural compound that is a huge “taste and smell” nuisance in public drinking water systems in springtime using surface water sources (Geosmin and 2-Methylisoborneol from Cyanobacteria in Three Water Supply Systems), but it is the same compound release by the soil in spring rains, and companies are trying to harness it for perfumes (Earth Perfume and the Scent of Rain). When biosolids odor research was first launched, the odorants were likened to bad breath and stinky feet (Identifying and Controlling Odor in the Municipal Wastewater Environment - Health Effects of Biosolids Odors: A Literature Review and Analysis).  

Though odor control in wastewater and biosolids is mostly a side-concern, odor control from arm pits, dirty clothes, cat boxes, etc., is big business, and technology is constantly advancing.  The ingredients that make Febreze Pet Odor Eliminator effective, Hydroxypropyl Cyclodextrin, may also work with bad breath (Estimation of association constants between oral malodor components and various native and derivatized cyclodextrins). The kinds of body-derived fatty substances that contribute to biosolids digestion challenges contribute, too, to the challenges of designing effective laundry detergents (This is why your workout clothes still stink after being washed.) And, additionally, while bad odors are often top-of-mind to us biosolids practitioners, in the wider world it is ironically the good odors, the vast array of fragrances, that pose the far larger domain of concern for human and environmental safety (check the work of The Research Institute for Fragrance Materials (RIFM) Environmental Program and its ingredient safety assessment).

You would think by now we would know better than to let up on advancing the science of biosolids odor emissions and mitigation. Yet the current efforts seem meager. Great work has come out recently from Australia, with a research focus by Ruth Fisher, yielding such important journal articles in 2019 as Review of the effects of wastewater biosolids stabilization processes on odor emissions and Importance of 2,4,6-Trichloroanisole (TCA) as an odorant in the emissions from anaerobically stabilized dewatered biosolids. But I was reminded of a capstone essay by one of my heroes of biosolids, the U.K.'s Tim Evans, of how soon we let up on the gas when driving innovation. In his essay Forty Years in the Dark Stuff, Evans explains how he had developed a program modeled on the food industry's HACCP (Hazard Analysis Critical Control Points) because "when examining the hazards that needed to be managed, teams always agree that odour from land application and from treatment was the most public vulnerability." But, in the end, Evans explained "the CATNAP culture triumphed again." CATNAP is "Cheapest Available Technology Narrowly Avoiding Prosecution."

Yet, the wastewater profession did commit a just a few years back to a major research project examining odor qualities of different Class A EQ biosolids products. This was the High Quality Biosolids from Wastewater report. This has been available from the Water Research Foundation since 2020 (PROJECT NO. NTRY7R15/4823), and the project team, of which I was part, and the principal of which was Trudy Johnston, Material Matters, presented the key findings to a workshop at the Residuals and Biosolids Conference in 2018, in Phoenix, AZ. 

The key findings of High Quality Biosolids from Wastewater are usefully summarized, for the tantalizing evidence that further research, improved treatment efforts, and innovation in technology can yield significant improvements to the odor qualities of Class A EQ biosolids products. Here are the highlights:

The High Quality Biosolids from Wastewater project showed that biosolids odors can be objectively quantified and that “good” products can be distinguished from the “not-so-good.” Various Class A EQ biosolids products show statistically significant differences in odor characteristics. Even biosolids from the same facility can show significant odor differences. The report showed that odor potential may be predicted, based on treatment processes and selected chemical properties. Lower odors are associated with dewatering by presses versus thermal drying or centrifugation. Centrifuges release odor-generating compounds, and rewetting of thermally dried product reactivates microbial activity. Biosolids products blended with wood products and containing high iron (elemental iron higher than 8%) were also lower in odor potential. Wood product not only dilute and absorb odor-causing chemicals in biosolids but contribute to their biodegradation. The biosolids products blended with wood had the lowest odor intensity and the most pleasant smell (“hedonic tone”). Iron seems to render volatile organic sulfide compounds less biodegradable, thereby reducing odorous mercaptan release. Treatment processes that effectively degrade volatile solids produce a biosolids that has a low respiration rate, as can be verified by respirometer (the Xylem Oxi-Top was used). 

Post processing management of biosolids products has a strong effect on odor qualities. In addition to wood blending, extended storage and/or periodic turning of biosolids piles reduce odors with minimal infrastructure cost, if real estate is available to accommodate the footprint. Aging prior to distribution and marketing reduced odors in the biosolids product, perhaps by allowing off-gassing of odorous compounds prior to marketing, but also giving time for additional biodegradation. Potential space and equipment requirements are some of the tradeoffs if a facility intends its biosolids products to go beyond meeting minimum standards for Class A EQ and instead meet consumer standards in high-value markets.

Odors associated with Class A EQ biosolids are not negligible, unavoidable, nor unimportant. The HQB from Wastewater results demonstrate that specific steps can be taken with biosolids to create a product with low odors, with a positive or neutral hedonic tone, and with a positive value in the marketplace. Just as psychologists and physicians were open to surprise in the effects of the SARS-CoV-2 virus on perception of odors, we in the practice of biosolids can be open to surprise at the effects of wood fibers, iron amendment, aging and aeration on the perception of odors, in this case the odors in biosolids products. So, let’s get to the task of making a low odor biosolids, so that finally we show we have the Sense to Smell Good.