Survey for the City of Raleigh of existing THP biosolids characteristics and end-use

1) Introduction & Objectives The City of Raleigh, NC (Raleigh) is currently constructing new anaerobic digestion facilities including pre-digestion thermal hydrolysis process (THP) at its Neuse River Resource Recovery Facility. Raleigh has run a successful Class A limed cake program and land applied aerobically digested Class B liquid solids, but the new Class A THP biosolids cake will be a significantly different product. To inform their marketing plans, Raleigh wanted to gain insight from the four existing THP facilities in the U.S. The objectives of this paper are to: Determine the current status of marketing and product development efforts, and lessons learned, -Better understand the characteristics of their products, -Estimate heavy metal and nutrient concentrations for Raleigh THP cake. 2) Methods Details of the four water resource recovery facilities (WRRFs) in the U.S. with operating THP units are shown in Table 1. Telephone interviews were conducted with key staff from each utility covering a range of questions about biosolids treatment and end-use management before and after installing THP. Survey respondents were asked about changes in product characteristics that are attributed to THP, regulatory approvals, odors, additional cake processing, branding and marketing efforts. Utilities also submitted corresponding biosolids lab data. [Table 1. Existing THP Facilities] 3) Results A) Marketing efforts Raleigh is expecting the vast majority of its THP material to be distributed to agriculture initially. While Raleigh has experience marketing limed Class A material, THP cake will be a significantly new product in their region. Market familiarity exists with other Class A biosolids (e.g., compost, pellets) but Raleigh will need to educate former and potential future users on the benefits of applying Class A cake. To inform their marketing approach, Raleigh wanted to have a better understanding of the marketing efforts of facilities with similar material. These are summarized in Table 2. Of note, Medina does not plan to market its material due to the low cost of its land application program (~$15/wet ton), making it difficult to realize a return on investment through marketing. By contrast, DC Water has a well-established marketing program, currently consisting of 5-6 FTEs. [Table 2. Marketing Efforts of Existing THP Facilities] B) Product development efforts Raleigh is exploring options for post-dewatering processing to manufacture products with higher market value and a longer period of use than agricultural land application. The additional processing steps for Class A dewatered cake that have been presented previously are: 1) soil blending, 2) curing, 3) partial drying, 4) composting (Brower, 2018; Smoot, 2021). Blending and curing are generating the most interest among current and future THP facilities. In addition to blending performed on-site by staff, DC Water sells Bloom to soil blenders in the area, which incorporate the material into their blends. HRSD has on-going research looking at blending water treatment residuals with THP cake, with a particular aim to immobilize and/or dilute PFAS, and in the past completed market research pertaining to THP cake and soil blends. The curing process was developed at DC Water, with the assistance of R. Alexander Associates, Inc. (Brower, 2018). Dewatered cake is laid out in long piles and turned periodically, similar to windrow composting. Over the course of several weeks, if protected from precipitation, the less stable organic matter in the biosolids decomposes and the material dries out, creating a product that is easier to handle and is lower odor and more stable when incorporated in soil. HRSD has an on-going curing pilot and is looking to upgrade existing facilities to facilitate on-site curing. C) Heavy metals The heavy metal data obtained from the existing THP facilities before and after installing THP is shown in Table 3. The DC Water data is most illustrative of a change, similar to Raleigh, of a facility going from no anaerobic digestion to THP with anaerobic digestion with the same solids. [Table 3. Heavy metal concentration in biosolids before and after installing THP (mg/kg, dry)] Many of the metals in the DC Water cake increased around 150%, meaning a 2.5-times increase. Table 4 shows the average and maximum concentrations for Raleigh solids from a period in 2020-2021, and those values with a 150% increase. These values are compared to federal and state Exceptional Quality limits. Even the maximum heavy metal values when more than doubled are still no more than 51% of the EQ limit, so Raleigh should not be at risk of exceeding EQ standards. [Table 4. Raleigh Dewatered Cake Part 503 Heavy Metal Concentrations, Apr '20 - Mar '21 (mg/kg)] D) Nutrients Data from other THP plants were used to estimate likely nutrient values in Raleigh cake. Table 5 summarizes previously published nutrient data from WWRFs in Europe, along with data gathered from the existing U.S. installations. [Table 5. Typical properties of thermally hydrolyzed digested biosolids cake from municipal water resource recovery facilities] Based on these data, a range of likely characteristics were estimated for the Raleigh cake (last line of Table 5). HRSD and Oakland County were used to estimate TKN and ammonia as these plants have the most similar treatment to Raleigh. 4) Lessons Learned Marketing Based on the experience of the four existing THP facilities, markets exist for THP biosolids, particularly in agriculture. While cost savings and revenue are possible, resources must be dedicated to marketing efforts and related market conditions must be considered. DC Water is saving well over $1M per year (compared with contracted land application) but estimates 5-6 FTEs support this effort. Medina County is seeing competition for their THP solids, but since they contract their biosolids management they are not able to obtain value reflecting that demand. Product Development Class A designation opens up opportunities for THP solids, including soil blending. DC Water has expanded its blending facility and HRSD is conducting research on blends. DC Water has also found a market for Bloom as a feedstock for external soil blenders. Curing, which Raleigh is exploring, produces a product with the highest market value. Most outlets for Cured Bloom are in bulk, but it is a testament to the quality of the material that it is able to be bagged and has a retail price higher than other bagged amendments. Heavy Metals While the effect of THP on heavy metals through digestion is unclear, the experience of the existing facilities show that heavy metals will not typically concentrate more than would be expected from the conversion of solid organic matter to biogas. Nutrients Existing THP facilities, in the U.S. and Europe, provide a useful guide for typical nutrient levels in THP biosolids; however, each facility is different and nutrient interactions are complex so the true levels will not be known until the digesters reach steady state. Raleigh's cake is estimated to have an NPK of 1.5-2-0 on a wet-weight basis. 5) Conclusions The uncertainty in nutrient levels (a range of +/- 0.5% for nitrogen on a wet-weight basis) can have a dramatic impact in the field. A dewatered cake with a nitrogen concentration of 1% would be applied at a significantly higher application rate than a cake with a nitrogen concentration of 2%. These values are not absolute even for existing facilities. For instance, for Medina cake in 2021 the total Kjeldahl nitrogen and ammonia concentrations varied 40% and 230%, respectively. From a preliminary analysis, it does not appear Raleigh will have an issue meeting Exceptional Quality requirements. The Water Environment Federation (WEF) has a Research and Innovation Committee (RIC) focus group on Improving Pre-digestion Hydrolysis with a sub-group on End-Product Considerations. In initial discussions, this group has expressed interest in exploring the effect of THP on product characteristics, nutrients and heavy metals in more detail. The preliminary results in this abstract could be the starting point for a more detailed analysis of more extensive data from these utilities before and after installing THP to refine these results. Based on the experience of the four existing THP facilities, markets exist for THP biosolids, particularly in agriculture. While cost savings and revenue are possible, resources must be dedicated to marketing efforts. Additional processing, such as curing, can increase market value and open up additional outlets to support broader diversification of end uses.