Hydrochar – Livestock and Poultry Environmental Learning Community

Purpose

Dairy manure was once considered a waste, but it can be transformed into a valuable resource. As demand for sustainable waste management grows, innovative ways for converting dairy manure are being actively researched to enhance both dairy productivity and environmental sustainability. One such method, hydrothermal carbonization (HTC), has recently garnered significant attention due to its ability to convert wet biomass into value-added products. HTC involves treating wet biomass, such as dairy manure with high water content, at moderate temperatures (180℃-250℃) and pressure. The outcome of HTC is hydrochar, a solid product with high carbon and nutrient content.

Hydrochar has strong potential as a means of soil amendment, carbon sequestration and/or biofuel. Our lab scale experiments showed that hydrochar retains more than 90% phosphorus (P) from dairy manure. For hydrochar production to become a viable technology for dairy farms, a continuous system is essential. Such a system would offer numerous benefits, including increased production, enhanced efficiency, and greater potential for commercialization. The purpose of this study is to design a pre-commercial conceptual process for the continuous production of hydrochar from dairy manure.

What Did We Do?

Manure management consists of collecting manure from the floor to utilize it in the best possible way. Most dairy farms treat manure through anaerobic digestion to produce energy, separate the solids for use as a bedding material, and/or apply directly to field applications. To explore alternative ways of handling the large quantities of manure in a quick chemical method and recycling nutrients back to the cropland, dairy manure is processed into P-rich hydrochar via an HTC process. Based on the results of our laboratory experiments, a conceptual process was developed, which is capable of treating dairy manure from a mid-size farm with 1,000 lactating cows and equates to 38,000 tons of manure per year with 8-10% solids. The process design includes engineering designing details of manure preparation and handling, feeding and discharge mechanisms, main equipment (such as HTC reactor and heat exchangers), heating and temperature controls, and schemes for post-HTC process wastewater (post-water) handling. Figure 1 is the schematic of the conceptual process with major process equipment, where the thick, black lines indicate the flow of dairy manure slurry containing solids, while the thin, blue lines represent the flow of post-processed water.

Firstly, dairy manure collected from the dairy barns (approx. 10% solids) is stored in a storage tank (T-101) before being pumped into the feeding tank (T-102), where it is heated to 167°F (75°C) by the recycled post-water from preheater I (E-201) through internal heating coils. The feeding tank is equipped with a marine-style impeller for agitation to maintain solid suspension. Two preheaters (E-201 and E-202) are used to further heat the slurry to the required HTC temperature before entering the reactor (R-301). Preheater I is a shell-and-tube heat exchanger to heat the slurry up to 320°F (160°C) by heat recovery using the hot post-water from post-water tank (T-304). Preheater II is a tubular electric heater and is to finish the last stage of heating to 437°F (225°C). A continuously stirred tank reactor (CSTR) with agitation is the main equipment to thermochemically process dairy manure into hydrochar. After a 30-minute retention time in the reactor, the resulting product mixture is collected in the receiving tank/separator (T-302). Then the hot post processed-water is separated from the solid (the wet hydrochar cake) and collected in a storage tank (T-304) before being used as a heating medium for heat recovery. The wet hydrochar cake coming out of decanter centrifuge (T-303) is dewatered through an air-drying unit (C-305) to a water content of 12% or less, which can be used directly for land applications or packaged and transported to other markets.

Figure 1 Schematic of the conceptual process with major process equipment.

What Have We Learned?

Continuous hydrochar production holds great potential for recycling phosphorus from dairy manure back into the cropland as a soil amendment and for sequestrating carbon back to the soil. The conceptual process represents a significant step towards practically promoting this alternative manure treatment technology and creating a value-added product for nutrient cycling. This process is capable of producing approximately 5 million pounds (2,300 metric tons) of air-dried hydrochar per year, a yield of about 60% of the solid matter from dairy manure, and with a phosphorus concentration of approx. 1.4 lb/100 lb. Hydrochar is hydrophobic and can be sufficiently dried by ambient air. The air dried hydrochar contains a moisture content of 12% or less (as low as 5% per laboratory results due to hydrochar’s hydrophobic characteristics) and is suitable for long term storage and/or distance transportation. Because the raw, wet dairy manure can be processed directly from the farm without any pretreatment, the HTC process offers a good possibility for a cost-effective waste management alternative while producing valuable hydrochar for phosphorus recycling.

Future Plans

Upon completing this continuous flow process design, we will conduct a techno-economic assessment (TEA) to provide insights into the system’s economic feasibility, cost structure, and profitability. The TEA study will also offer a better perspective on the economic viability, technical challenges, and potential profitability of adopting and investing in the continuous hydrochar production system from dairy manure for waste management and nutrient cycling.

Authors

Presenting author

Imran Hussain Mahdy, Graduate Student (Ph.D.), University of Idaho

Corresponding author

Brian He, Professor, University of Idaho, bhe@uidaho.edu

Acknowledgements

USDA AFRI, UADA NIFA and Idaho Agricultural Experiment Station are acknowledged for their financial support through Sustainable Agricultural Systems (SAS) program (Grant 2020-69012-31871), and hatch project of IDA0-1716 (Accession number1012741).

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2025. Title of presentation. Waste to Worth. Boise, ID. April 7–11, 2025. URL of this page. Accessed on: today’s date.

Purpose

To address the depletion of phosphorus resources and the environmental issues associated with phosphorus enriched runoff from the application of raw manure, a strategic and sustainable approach is to recycle phosphorus from dairy manure using innovative and efficient methods. Hydrothermal carbonization (HTC) has been considered one of the sustainable techniques, which can transform dairy manure into phosphorus-enriched hydrochar at relatively low temperatures, typically ranging from 180 to 250 °C. This process not only recovers valuable phosphorus but also converts organic waste into a stable, nutrient-rich product that can be used as a soil amendment or phosphorus fertilizer.

Despite the massive experimental activity performed to characterize the HTC process, the design and development of a validated bench-scale model is crucial for scaling up. While numerous studies have explored the HTC process at the laboratory level, only a limited number of studies have assessed the technical feasibility and performance of implementing this process on an industrial scale. In this context, the purpose of this study was to provide a detailed and systematic examination of phosphorus recovery from dairy manure using a lab-scale HTC reactor and illustrated the basis of the design of a bench-scale processor and evaluated its performance in terms of hydrochar yield (HY) and phosphorus recovery (PR).

What Did We Do?

Fig. 1. Scale-up of the hydrothermal carbonization (HTC) reactor from lab-scale to bench-scale. (a) Lab-scale HTC reactor with a 300 mL capacity, used for small-batch experiments. (b) Scaled-up bench-scale HTC reactor with a 9 L capacity.

In this study, the HTC of raw dairy manure with a 7.9% of total solids was first conducted using the lab-scale reactor to optimize the process parameters, including temperature and reaction time, and then scaled up at a scale of 30 times larger (Fig. 1). The HTC-derived hydrochar samples were named according to the temperature and reaction time. For example, HC200-30 represents the hydrochar sample obtained at 200 °C and 30 min. The scaled-up reactor was designed and operated at the optimized conditions obtained from the lab-scale study, which was 225 ºC and 60 min of reaction time. The HY, which also reflects the mass reduction of dairy manure (on a dry weight basis), and PR were the two main parameters evaluating the HTC of dairy manure. We additionally evaluated the energy required for hydrochar processing in both lab- and bench-scale processors.

The HY and PR expressed as a percentage were determined by the following equations:

What Have We Learned?

Fig. 2 shows the effects of HTC processing temperature and reaction time on the HY and PR using the lab-scale reactor. It was observed that the HY decreased gradually as the processing temperature and time increased, which is attributed to the temperature and time dependent degradation of organic matter during HTC. The highest PR was observed at 225 ºC and 60 min.

Fig. 2. Hydrochar yield (%) and phosphorus recovery (%) efficiency under different hydrothermal carbonization conditions.

As shown in Fig. 3, the scale-up of the hydrothermal carbonization (HTC) process demonstrated that HY and PR remained consistent between lab-scale and bench-scale systems, indicating that the transition to a larger reactor did not compromise HY or PR. Notably, the energy input per mass of hydrochar was significantly reduced in the bench-scale system, improving overall energy efficiency. These findings indicate that scaling up HTC can enhance process feasibility while maintaining similar nutrient recovery.

Fig. 3. Comparison between lab-scale and bench-scale HTC systems. (a) Hydrochar yield (%), (b) phosphorus recovery (%), (c) Energy input (kwh/kg-HC), and (d) specifications of the scale-up reactor.

Acknowledgements

This work is supported partially by USDA NIFA (award number 2021-67022-35504) and the University of Idaho P3R1 grant.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2025. Title of presentation. Waste to Worth. Boise, ID. April 7-11, 2025. URL of this page. Accessed on: today’s date.