Biosolids Dust is a Big Deal

Dust is a big deal! Viruses have so commanded the front stage of our news media that you may have missed other big “dust” stories. China is having a BIG dust problem. The article “Apocalyptic skies as Beijing hit by worst sandstorm in a decade” (March 15, 2021) explains winds off Mongolia are carrying a dense cloud of dust to Beijing. This is an issue with serious health and environmental implications (Characterization of the composition of dust fallout and identification of dust sources in arid and semiarid North China. The newspapers are happy to remind us that early Spring brings tree pollen, a phenomenon steeped in science  Pollen calendars and maps of allergenic pollen in North America. Two weeks of dry weather in the mid-Atlantic, and wildfires are releasing unhealthy soot: “Wildland firefighter smoke exposure and risk of lung cancer and cardiovascular disease mortality.” While the dangers of dust-borne lead in homes is well established (Children’s Lead Exposure: A Multimedia Modeling Analysis to Guide Public Health Decision-Making), household dust contains many more compounds of concern, many of which we see also in our wastewater and biosolids, such as flame retardants and phthalates: Tracing the chemistry of household dust The sloughing skin that joins cloth fibers in the household dust. Worse yet, some of the dust may be radioactive (Health Implications of Fallout from Nuclear Weapons Testing through 1961). On a lighter note, we also learned in a January 2021 news article that amidst the load of anthropogenic dust, which has greatly accumulated in my house during the pandemic lockdown, is some very ancient dust (7 billion-year-old stardust is oldest material found on Earth), older than our solar system. Yes, dust is a big deal for scientists, inhabitants, and life in the world.

Dust is also important for us biosolids practitioners, too, but you would not know that from the paucity of professional news coverage and research on the topic. The Water Environment Federation has brought into its website all papers and articles from several decades of conferences and publications. These are found in Access Water. In this large database, 4,660 articles on biosolids are catalogued. While some 700 articles mention biosolids dust when dealing with treatment plant processes, to protect from fire, explosion and worker injury, only two papers treat dust as a key characteristic of biosolids products deserving consideration in the choice of treatment processes. The first paper, from the 2017 WEF Residuals and Biosolids Conference, is  Not All Dryer Products are Created Equal, and the second is from WEFTEC 2019, Worthless Dust or Valuable Resource? Drying Thermally Hydrolyzed Solids the Right Way.

The first paper, by Material Matter’s Lisa Challenger, makes the case for a eyes-wide-open approach to selecting technology for the desired end-use of the product. Challenger underscores the point that technologies identical in terms of Part 503 regulatory compliance (Pathogen Reduction PFRP Class A – Alternative 5 and Vector Attraction Reduction – Option 8) yield end products that are opposites in their suitability for distribution and marketing. The marketable heat-dried biosolids was a digested biosolids processed in a rotary kiln direct dryer and the non-marketable dried biosolids was an undigested biosolids dried in an indirect paddle-type dryer. In the second case, high dust, low density, and intense odors doomed the product’s use as a fertilizer, despite regulatory compliance with national standards.

The second paper, by HDR’s Stephanie Spalding and Sebastian Smoot, examines three attributes of biosolids product quality -- energy content, friability, and bulk density -- against various combinations of equipment and process trains and of user requirements. Too few case studies permitted the authors conclusive answers, but several themes were suggested by nine cases, and dustiness of the product was a key concern. High dustiness followed several process features: thermal hydrolysis of the entire solids flow, the use of iron as a coagulant, a drying process that agitated the solids, and post-treatment handling by truck and land application equipment. One or more of these features could yield dust that discouraged customer acceptance. 

Dust in biosolids products may be a problem for a variety of reasons, but human health effects are primary. If there were a “canary in the coal mine” for risks from biosolids dust exposure it would be treatment plant operators. I had not held much concern, ever since the Philadelphia Water Department was one of 4 compost facilities in a NIOSH health study. This lead to “Respiratory Exposure Hazards in Composting” which determined: “Very high levels of dust, endotoxins, (1-3)-β -D-glucan and ammonia were measured in compost facilities depending on the location, activity and enclosure. Exposure appeared to be correlated with few respiratory health parameters, although no significant objective pulmonary function differences were detected between the study groups.” I drew from this the premature conclusion that community exposure to biosolids compost dust would be benign.

Since that workplace study of the late 1990s, new tools have become available for measuring and characterizing “dust.” Researchers have sharpened their understanding of the characteristics of airborne biological particles, especially with genomic tools for identifying microbes. Dust of the kind from biosolids is more specifically defined as a bioaerosol: “microbial fragments, constituents of cells and airborne biological particles that can consist of fungi, bacteria, pollen, fragments, constituents, particulate matter (PM10), and by-products of cells, that may be viable or nonviable.” 

Current research shows that organic waste treatment can be a significant source of bioaerosol exposures. The article Methods for Bioaerosol Characterization: Limits and Perspectives for Human Health Risk Assessment in Organic Waste Treatment describes “composting biomarkers” for identifying a “causality process between chronic bioaerosol exposure and disease onset, and finally, on defining common exposure limits.” Advances in microbiology expands the range of microbes exposures associated with wastewater treatment (Evaluation of Bioaerosol Bacterial Components of a Wastewater Treatment Plant Through an Integrate Approach and In Vivo Assessment): “next generation sequencing analysis was used also to identify the uncultivable species that were not detected by the culture dependent-method.” As new measurement tools are added, the range of potential risks seems to enlarge. In The size distribution of airborne bacteria and human pathogenic bacteria in a commercial composting plant “Seven out of eight HPB [human pathogenic bacteria] with a small geometric mean aerodynamic diameter had a high concentration in composting areas.”

Yet, while tools for measurement have improved, the attribution of risk levels has lagged. In Bioaerosol exposure from composting facilities and health outcomes in workers and in the community: A systematic review update the authors conclude “there is insufficient evidence to provide a quantitative comment on the risk to nearby residents from exposure to compost bioaerosols.” This kind of open issue is itself an issue, particularly from the viewpoint of environmental justice. The article Characterising populations living close to intensive farming and composting facilities in England observes that with regard to high exposures to bioaerosols from intensive farming “few people (0.01 %) live very close to these sites and tend to be older people. Close to composting facilities, populations are more likely to be urban and more deprived.” The key here is that science is in the early stages of exploring the human health risks of bioaerosols.

Low dust is a prized attribute of heat-dried biosolids pellets. In the WEF Conference paper Toronto’s Pelletizer Facility – A New Start, the “new start” included the aspiration that “dust production, resulting from friction during transport and handling, will be very low with the biosolids pellet product.” To this attribute was also that of hardness: “pellet hardness is… slightly higher than that of chemical fertilizers… [such that] handling and spreading of the product is relatively easy and dust-free.”

While durability and dustiness are key attributes, no article in WEF Access Water discuss measurements of these attributes. Chemical fertilizer and wood pellet industries are keen to prevent pellets from turning to dust, a property they term durability. Hence, these industries deploy a “product durability index” and measure this with “product durability testers.” The PDI is “a standardized parameter for specifying the ability of the fuel pellets to resist degradation caused by shipping and handling.” For, under $4,000 you, too, can own a “Two Compartment Pellet Durability Tester.” This tester subjects pellets to a tumbler that simulates conveyance and transport handling, and test results are reported as a percentage of pellet mass that degrades into dusty particles. This sounds as though this device ought to have a place in measuring durability and dryness of biosolids pellets, but it does not.

Though durability and dustiness seem to be secondary objectives in choice of treatment technologies, various pre-drying and post-drying options at the plant can modify these product attributes. Fine screening and digestion are treatment steps that reduce fibers and low-density organic matter, and subsequent dried product is denser than undigested biosolids. Post-drying screening is another step, and Challenger reports: “screenings are recycled back to the head of the dryer and blended with the cake product to avoid the “sticky” phase of the biosolids product typically returning and blending fine particles into the cake feeding the dryers, results in a denser product. “ 

What is more, the world stands ready to help with durability and dustiness. Many manufacturers provide granulators that could help us create a durable pellet, such as a Compost Pellet Machines and a Powder Granulator Machines; you can even buy on Amazon a Feed Pellet Machine. Though granulated products may still be dusty and odorous, you can add to it a coating. Surface Chemists of Florida can customize a coating for dried biosolids; its SurPhase FLOW promises to “preserve your product’s integrity.” Similarly, ArrMaz can design a special DUSTROL? or GALORYL? dust control coating for biosolids. Yet, these machines and coatings are an on-going expense to fix a situation that might have been otherwise avoided with better technology selection upfront.

The matter of control of biosolids dust is no light matter. Biosolids products that lack durability, that fail to withstand transport and land spreading and that consequently pose a risk of bioaerosol release are unlikely to be part of an economical, sustainable program. Each component of treatment, from screening, to digestion, to dewatering, to subsequent stabilization, warrants evaluation for its contribution to the “product’s integrity.” Just as the SARS-CoV-2 virus has raised global fears about invisible particles in the air we breathe, our industry cannot afford to be a source of invisible particles that raise public fears. We ought to recalibrate our focus on technologies that minimize dust and bioaerosol releases in response to new health concerns and scientific capabilities, because Biosolids Dust is a Big Deal.