Transforming THP biosolids cake using a new biodrying process

Utilization of the Thermal Hydrolysis Process (THP) to treat sludge before anaerobic digestion has been practiced since 1995. There are many benefits to this process including improved digestion performance, the ability to feed digesters at a higher solids concentration thereby reducing needed digestion volume by 50% or more, a reduction in the quantity of solids remaining after digestion, improved dewatering, and production of a low odor Class A exceptional quality (EQ) biosolids cake product (Barber, 2020). There are now more than 70 full scale THP plants operating worldwide with at least 20 more in construction or design that utilize this process.

One of the challenges that remains regarding implementation of the process is that even though a low odor Class A EQ biosolids is produced, the dewatered cake at 30% solids still looks like Class B biosolids cake and must be land applied on agricultural farmland, usually at a significant distance from the plant. Work has been done using simple windrowing to air dry thermally hydrolyzed biosolids cake to approximately 55-60% solids to produce a more soil-like material (Brower et.al., 2018). The air-dried biosolids product can then be more readily used in landscape and horticulture applications. However, the windrow air drying process can take 3-6 weeks or longer, requires a significant amount of covered land area, regular turning of the windrows to achieve the necessary level of drying, and includes risk of odor concerns due to the large area and frequent pile turning.

A simplified intensification process has been developed called Dune that takes advantage of the natural biological process to generate heat and to achieve drying of THP dewatered cake to 55-60% solids in less than 2 weeks (Figures 1 and 2). This process requires only a fraction of the space (approximately one third) required by windrow air drying while significantly reducing ammonia and odor emissions. The Dune process has been successfully demonstrated at field scale at two THP solids processing facilities: one in the US and one in the UK (Figures 3 and 4). A third field scale demonstration is in progress with the results being available before the transcript is due. The process uses a portion of previously biologically dried THP cake blended with fresh wet THP cake to produce a mixture that has adequate porosity and bulk density to allow for forced ventilation in an aerated static pile configuration.

Results of these field scale trials prove that sufficient energy remains within the dewatered THP cake to continue biological degradation with naturally occurring aerobic microorganisms present within the recycled dried THP cake. No external amendments are required. A major by-product of this continued biological degradation process is heat. By providing aeration to maintain aerobic conditions within the mass of the mixed materials, temperatures rapidly rise, and moisture is removed. The result is that the tonnage of the solids can be reduced by 50% which in turn will reduce costs of hauling and land application by 50% or more. In addition, the characteristics of the dried product are like a well stabilized compost or soil that can be utilized in horticultural applications such as landscaping, turf, and ornamental gardens instead of only agricultural farmland (Table 1). The product pH transitioned from slightly basic to just below neutral, the carbon dioxide respiration rate significantly reduced indicating more stability, and a slight decrease in volatile solids occurred showing further volatile solids reduction. These changes in the biosolids characteristics while maintaining Class A biosolids status allows for multiple products uses instead of only in agriculture, thus opening different product markets. Another benefit of the process is that by changing the conditions of the biosolids from anaerobic to aerobic, methane emissions from the stockpile of material are almost immediately stopped. Emissions testing of the process demonstrated a drastic reduction in methane emissions (Figure 5) and potential GHG reductions when compared to conventional storage of dewatered THP cake prior to land application.

The authors are performing a third field scale demonstration at another full scale THP facility in the first quarter of 2025. The testing results of that demonstration will be also reported in the final manuscript and presented at the conference. This presentation will provide a description of the Dune process and discuss results of the field scale demonstrations including the impact on product quality, product quantity, and methane emissions as compared to the conventional method of storing THP biosolids cake. O&M cost comparisons will also be provided showing the cost-benefit of full-scale application of the Dune drying intensification process to Class A THP cake compared to windrow drying or storage of cake without drying.