Evaluating Biochar Blending During Curing of THP Class A Biosolids: Impacts on Product Quality, Odor, Handleability, and PFAS Concentrations

Evaluating Biochar Blending During Curing of THP Class A Biosolids: Impacts on Product Quality, Odor, Handleability, and PFAS Concentrations Background The Hampton Roads Sanitation District (HRSD) Atlantic Treatment Plant produces Class A/EQ biosolids using thermal hydrolysis followed by mesophilic anaerobic digestion. The resulting thermal hydrolysis process (THP) cake is nutrient-rich but can present challenges related to odor generation, moisture content, and product handleability during curing, storage, and distribution. HRSD has implemented a windrow 'curing' process to dry the product, reduce ammonia compounds, and improve odor and stabilization of the cured product. However, site odors associated with the curing process have been observed. To address both site and product odor challenges and improve the physical and chemical properties of Class A biosolids - including odor, moisture content, and availability of metals and organic compounds - HRSD, in collaboration with Material Matters (MM), conducted a pilot-scale blending trial incorporating wood-based biochar into the THP cake prior to curing. Biochar, a carbon-rich product made from the pyrolysis of biomass, is known for its adsorptive capacity, moisture retention characteristics, and potential to mitigate odors while stabilizing nutrients and pollutants. This pilot-scale blending trial aimed to determine whether varying biochar addition rates could improve overall product quality and reduce potential challenges associated with product storage and beneficial use. It was hypothesized that increasing the proportion of biochar added to THP cake, combined with the established curing process, would result in lower moisture and ammonia concentrations, and improve site and product odor, nutrient and pollutant concentrations, and handling characteristics. Objectives The research objectives are described below: 1. Determine the ideal range of biochar addition ratios relative to fresh cake biosolids on a dry weight basis to achieve odor and pollutant reduction; 2. Assess physical, chemical, and biological characteristics present in cured biosolids end-products; and 3. Provide recommendations for optimal biochar amendment ratio based on trial findings. Methodology A pilot-scale blending and curing trial was conducted using five biosolids piles (approximately 6 wet tons each): one unamended control (0% biochar) and four amended piles with biochar addition rates of 2%, 5%, 8%, and 10% on a dry weight basis (Figure 1). Initial blending ensured even distribution of biochar. All piles were managed under HRSD's standard six-week curing protocol (windrows turned three times per week, located on a paved, covered storage pad). Environmental exposure and management conditions were consistent across all piles to minimize variability unrelated to amendment rate. Composite solids sampling was performed immediately after blending (week 0), mid-curing (week 3), and at the end of curing (week 6). Analytical parameters included nutrients, metals, moisture content, soluble salts, maturity indicators, product odor characterization, handleability metrics, and per- and polyfluoroalkyl substances (PFAS). Following the six-week curing period, each product was moved to individual storage piles for up to ten months. Solids Sampling and General Analytical Parameters - At each sampling event, composite solids samples were collected from each pile using stainless steel scoops. Samples were obtained from seven evenly distributed locations within each pile, with four scoops collected per location (i.e., 28 scoops total per pile). The scoops from each pile were thoroughly homogenized prior to sub-sampling for laboratory analysis. Subsamples were submitted to accredited laboratories for analysis of nutrients, metals, moisture content, soluble salts, and compost maturity indicators, consistent with standard biosolids and compost characterization protocols. PFAS Sampling - PFAS analysis was conducted on solids collected from the 0% (control) and both 5% and 10% biochar-amended piles to evaluate potential effects of biochar addition on PFAS concentrations during curing. Odor Sampling - Odor sampling was conducted during three curing events to evaluate the effect of biochar amendment rate on biosolids odor potential. Odor samples were collected using a composite headspace sampling approach designed to minimize spatial variability within each pile. For each sampling event, composite solids from each pile were prepared as described above and placed into individual 25-L glass wide-mouth bubblers. Each bubbler contained homogenized material representing all pile locations. Following removal of solids subsamples for chemical and physical analysis, the remaining material in each bubbler was sealed with a Teflon transfer lid equipped with dedicated flux and sample tubing. Bagged gas samples were collected by fluxing the bubbler headspace with nitrogen gas at a rate of 5 L/min (e.g., fluxing rate) while simultaneously drawing sample air at 2.5 L/min (e.g., sampling flow rate) into 10-L sampling bags using SKC pumps. Samples were collected via a vacuum chamber system to prevent contamination and maintain consistent flow rates. Each bag was pre-conditioned through multiple fill-and-purge cycles prior to final sample collection to minimize background odors. Handleability - Handleability of each cured product was evaluated qualitatively and quantitatively at week 0 (immediately after blending), week 3 (mid-curing), and at week 6 (end of curing). Observations were recorded by operations staff using standardized evaluation criteria (rating on a scale of 1 to 4; with 4 indicating the most favorable score for that factor) to allow comparison across amendment rates. Findings and Current Status: Product Quality - Biochar-amended piles exhibited higher ammonium-N fractions and elevated solute salt concentrations relative to the control, consistent with biochar's adsorptive properties and its potential to retain ammonium during the curing process. These changes may influence nitrogen availability following land application, depending on crop demand, soil conditions, and application rate. Increased ammonium-N may be advantageous for beneficial use scenarios requiring readily plant-available nitrogen, while potentially limiting suitability for end uses the prioritize controlled nitrogen release or have low tolerance for elevated soluble salts and ammonium-related phytotoxicity. Other measured constituents, including nutrients and regulated metals, generally reflected dilution effects associated with biochar addition (Figure 2). Product Odor - Product odor improved substantially across all piles as a result of aerobic curing, indicating that curing was the primary driver of odor reduction, supported by both odor detection thresholds (Figure 3) and selected-ion flow-tube mass spectrometry (SIFT-MS) results (Figure 4). While biochar-amended piles exhibited odor profiles comparable to or slightly improved relative to the control at certain time points, overall results provided limited evidence that biochar addition at rates up to 10% produced measurable odor reductions beyond those achieved through curing alone. Product Handleability - Handleability improved moderately with curing across all piles. Improvements included reduced smear and increased friability as moisture content declined. PFAS Concentrations - PFAS results showed substantial variability over time, with evidence suggesting precursor transformation during aerobic curing. While biochar-amended piles generally exhibited lower PFAS concentrations than the control, reductions were likely reflective of dilution or sampling variability. Additional Research - HRSD staff are currently conducting additional research to understand the impact of medium to long-term product storage on the cured piles with respect to physical and chemical changes in the end-product with potential to impact market outlet acceptance. Conclusion Trial findings provide practical insights for utilities considering biochar as a biosolids amendment within existing curing operations. The preliminary findings of the trial suggest biochar addition (up to 10% by dry weight) to fresh THP cake biosolids does not drastically improve odor or physical product characteristics beyond the benefits of curing alone. Findings highlight the need for further research evaluating long-term storage after curing, higher amendment rates, potential impacts on PFAS mobility and leaching behavior, and assessing the replicability of these initial observations.