Biosolids-Derived Fertilizers: Pros, Cons, and Sewage Treatment Biosolids Post-Incineration Ash as Fertilizer: A Successful Project in Northeast Ohio

 

     Biosolids are the separated solid end components of the wastewater treatment process that are physically and chemically treated. The result is semi-solid nutrient-rich biosolids. In most cases, biosolids are basically treated sewage sludge. Biosolids have long been used as a land application treatment that increases soil fertility but there are many potential problems with the chemical content of the biosolids such as high levels of heavy metals, PFAS, dioxins, organic chemicals, and microplastics. Human pathogens are also a concern since some thrive in sewage and can be difficult to eliminate. They also contain phosphorus and carbon that are good for plants and soil. That is one reason why land application of biosolids is the fate of most biosolids as shown below. It should be remembered that it is not just human bodily wastes that end up at sewage treatment plants but whatever is dumped down a sink, including known illegal dumping. This may include toxic chemicals.

     Biosolids are either land-applied or disposed of via incineration or landfilling. The share of each is shown below. The ash left over from incineration is also typically landfilled.

 

     There are different classes of biosolids based on how the wastewater is treated or utilized. The EPA's classification scheme of Class B, Class A, and Class A-Exceptional Quality is shown below.

     The different types are given below based on a Canadian study of potential emerging contaminants from biosolids.

 

Anaerobic Digestion: Micro-organisms decompose the sludge in the absence of oxygen either at mesophilic (at 35 °C) or thermophilic (between 50° and 57 °C) temperatures.

Aerobic Digestion: Micro-organisms decompose the sludge in the presence of oxygen either at ambient and mesophilic (10 °C to 40 °C) or auto-thermal (40 °C to 80 °C) temperatures.

Composting: A biological process where organic matter decomposes to produce humus after the addition of some dry bulking material such as sawdust, wood chips, or shredded yard waste under controlled aerobic conditions.

Alkaline Treatment: The sludge is mixed with alkaline materials such as lime or cement kiln dust, or incinerator fly ash and maintained at pH above 12 for 24 hours (for Class B) or at temperature 70 °C for 30 minutes (for Class A).

 Heat Drying: Either convention or conduction dryers are used to dry the biosolids

 Dewatering: The separation of the water from biosolids is done to obtain a semi-solid or solid product by using a dewatering technologies (centrifuges, belt filter presses, plate and frame filter presses, and drying beds and lagoons).

      Some methods of biosolids composting are shown below.

     Human pathogens found in sewage wastewater include fecal coliform bacteria such as E. Coli, Salmonella sp. bacteria, enteric viruses, and viable helminth ova. The presence of these ‘indicator organisms’ suggests that the waste may be unsafe. Some have been known to survive the wastewater treatment process. EPA requires that there be no detectible level of such organisms in biosolids for land application. Concentration limits and loading rates for metals are shown below.

     Biosolids are used as a fertilizer in agriculture as well as in forests for timber land. They are also used as an additive to condition soil. They are used on reclamation sites to increase the rate of new vegetative growth. Certifiably the cleanest biosolids, Class A,  are approved for home lawn and garden use.

     The U.S. EPA requires all publicly owned treatment works (POTWs) to produce annual reports summarizing waste management practices and pollution monitoring data including contaminant levels in their waste. Nine states (Arizona, Idaho, Michigan, Ohio, Oklahoma, South Dakota, Texas, Utah, and Wisconsin) are authorized through the National Pollutant Discharge Elimination System (NPDES) Program to be the permitting authority for biosolids.

     Land application may be done in different ways and the table below compares their attributes for reclaimed land application. The land application method often depends on the consistency of the biosolids, whether they are mostly liquid or whether they are more solid granules or pellets.  

 

EPA also explains biosolids odors:

 

“Biosolids may emit a distinctive odor depending on the treatment process and methods used. The odorous compounds generated and detected most often are ammonia, amines, and reduced sulfur-containing compounds. Meteorological conditions such as wind speed and direction, relative humidity, and temperature can impact nuisance odors. The presence of biosolids odors does not mean that the biosolids pose harm to human health and the environment.”

     A 2016 study published in Chemosphere evaluated the cadmium-phosphorus (Cd-P) ratios in biosolids vs. biosolids ash, or combustion residuals. Cadmium is a toxic heavy metal and it is desirable to keep its levels down. Some conclusions from the abstract are given below:

 

“Combustion of biosolids improved the Cd/P ratio in ashes by 2–5 times, compared with the initial biosolids. The low Cd content in ashes (4–9 mg Cd (kg P)−1) makes this material a particularly attractive alternative to mineral fertilizers. Significantly higher pore water P (as well as total N) was measured in soils containing biosolids, but plants produced a higher biomass in soil fertilized with ashes. The K and Ca amendments prior to biosolids combustion generally decreased the total Cd in ash, but had little effect on P and Cd uptake and biomass growth. Similarly, the combustion temperature had negligible effect on these factors as well.”

 

     Another study published in the June 2023 issue of the Science of the Total Environment reviewed the challenges and opportunities of biosolids-derived fertilizers. The highlights given below indicate that there is considerable opportunity for biosolids, particularly contaminant-free biosolids, to fertilize plants, condition soil, reduce incineration ash, and increase landfill space.

 

•Land application of biosolids is a cost-effective way to reuse nutrient in soils.

•Ever changing nature of biosolids contaminants dictates regulatory guidelines.

•Nutrient content in biosolids provides an understanding of baseline agronomic value.

•Extractive technologies can recover and purify valuable constituents from biosolids.

•Prospects for novel granulated fertilisers derived from biosolids are significant.

 

     Emerging technologies like chemical extraction of desired constituents of biosolids before thermal processing are being considered more and more. This may be desirable since nitrogen and carbon compounds are destroyed during combustion. Nitrogen, phosphorous, and humic substances may be recovered in this way. The authors think this will eventually become a future trend in wastewater treatment plants. It would further the circular economy capabilities of biosolids. The graphic below from the paper compares current practices to a low-value end-use and a high-value end-use. The high-value end-use makes use of mineral and chemical recovery.

   

     EPA notes some important advantages of biosolids-derived fertilizers over traditional synthetic fertilizers:

“The nutrients in the biosolids offer several advantages over those in inorganic fertilizers because they are organic and are released slowly to growing plants. These organic forms of nutrients are less water soluble and, therefore, less likely to leach into groundwater or run off into surface waters.”

EPA goes on to tout the overall the advantages of biosolids:

* Biosolids are a recycled product, use of which does not deplete non-renewable resources such as phosphorous.

* The nutrients in biosolids are not as soluble as those in chemical fertilizers and are therefore released more slowly.

* Biosolids appliers are required to maintain setbacks from water resources and are often subject to more stringent soil conservation and erosion control practices, nutrient management, and record keeping and reporting requirements than farmers who use only chemical fertilizers or manures.

* Biosolids are closely monitored.

* The organic matter in biosolids improves soil properties for optimum plant growth, including tilth, friability, fertility and water holding capacity. They also decrease the need for pesticide use.

 

Incineration of Biosolids and Biosolids Ash-to-Soil

     Incineration of biosolids involves two steps: adequate evaporation of water content, and combustion. 65-75% of the biosolids are combustible which means that biosolids ash volumes are considerably less than non-combusted biosolids. The ash is more inert than the original biosolids which may still be reactive. The combustible content means that there is no need for additional fuel except for start-up of the incinerator. The incinerated ash may have other uses such as a filler for bricks and concrete, sub-base materials for road construction, daily landfill cover (after pelletization), and as an “ingredient in footing at athletic facilities, including baseball diamonds, and equestrian facilities, such as race tracks and arenas.”

     The two main methods of incineration are multiple hearth furnace (MHF) technology and fluidized bed furnace (FBF) technology. Both have advantages and disadvantages. Back in 1993 when a higher share of biosolids was incinerated EPA noted that in 1993, 343 biosolids incinerators were in operation in the U.S. and 80% of them were MHFs and 20 percent were FBFs. Some comparisons are given in the table below.

 Northeast Ohio Regional Sewer District Biosolids Ash Land Application Project

     Six years ago, a project was initiated in the Northeast Ohio Regional Sewer District to apply incinerated biosolids ash to soil for fertilization. Previously, the biosolids ash was stored in lagoons or ponds at the Southerly Wastewater Treatment plant. The sandy slurry eventually becomes a solid that looks like red clay which would later be dug out and shipped to a landfill. This project which began in 2018 was the first of its kind. Has it worked? According to the District's Robin Halperin, it has:

“It's worked fantastic; seven years later, we have reused 100% of our ash, and we have not landfilled anything. We've also saved an estimated $6.5 million by not landfilling it, and we reduced our carbon emissions by 96%.”

The solidified sludge is taken to Kurtz Brothers Landscape Supply which has called the project a “win-win for them, their customers who use the soil and, in a roundabout way, Lake Erie.” Since it is less likely to runoff than regular synthetic fertilizer it can help protect the lake from issues like harmful algae blooms. The District is also looking into a reuse application for the grit that is removed at the early stages of wastewater treatment. The grit is sand, rock, and gravel that finds its way to the treatment plant. It needs to be cleaned but after that, it can be reused, they say. They point out that while not all WWTPs have biosolids ash, they all have grit that must be removed because it can damage pipes and pumps. Before the project was initiated it took about a decade of testing and permitting before the Ohio EPA approved the project. The decreased CO2 emissions are a result of being able to truck the biosolids to the nearby landscaping firm instead of taking them to the more distant landfill. 

 

References:

Turning solid waste ash into fertilized soil proves win-win for Northeast Ohio Regional Sewer District. John Kosich. News 5 Cleveland. August 20, 2024. Turning solid waste ash into fertilized soil proves win-win NEORSD (news5cleveland.com)

Biosolids-derived fertilisers: A review of challenges and opportunities. Serhiy Marchuk, Stephan Tait, Payel Sinha, Peter Harris, Diogenes L. Antille, and Bernadette K. McCabe. Science of The Total Environment. Volume 875, 1 June 2023, 162555. Biosolids-derived fertilisers: A review of challenges and opportunities - ScienceDirect

Using Biosolids for Reclamation/Remediation of Disturbed Soils. Sally Brown and Chuck Henry. University of Washington. (U.S. EPA.) May 2015. biosolidswhitepaper-uwash.pdf

Phosphorus and cadmium availability in soil fertilized with biosolids and ashes. Jurate Kumpiene, Evelina Brännvall, Martin Wolters, Nils Skoglund, Stasys Čirba, and Vladislovas Česlovas Aksamitauskas. Chemosphere. Volume 151, May 2016, Pages 124-132. Phosphorus and cadmium availability in soil fertilized with biosolids and ashes - ScienceDirect

Basic Information about Biosolids. U.S. EPA. Basic Information about Biosolids | US EPA

Land Application of Biosolids. U.S. EPA. Land Application of Biosolids | US EPA

Hydromantis, Inc. (2010). "Emerging substances of concern in biosolids: concentrations and effects of treatment processes" (PDF). Canadian Council of Ministers of the Environment. 2018. Wayback Machine (archive.org)

Biosolids. Wikipedia. Biosolids - Wikipedia

Biosolids Technology Fact Sheet: Land Application of Biosolids. U.S. EPA. Biosolids Technology Fact Sheet: Land Application of Biosolids (epa.gov)

Biosolids Technology Fact Sheet: Use of Incineration for Biosolids Management. U.S. EPA. P:\WTC-Studies\EPA Contract\04 Fact Sheets\Option Year 2 - 742092\Deliverables-Final\FINAL_Incineration for Biosolids 2.wpd