(350a) Hydrothermal Treatment of CAFO Waste to Recover Nutrients to Prevent Nutrient Runoff and Watershed Pollution

Concentrated animal feeding operations (CAFOs) are an important part of the U.S. economy, providing the country with a substantial low-cost source of consumable meat and dairy products such as beef, pork, poultry, milk, and eggs. However, CAFOs are becoming a greater public/government concern due to the amount of animal manure (waste) generated by such facilities, resulting in both air and water pollution. The United States Department of Agriculture (USDA) estimates that 335 million tons of dry manure waste is annually generated by CAFOs. This waste stream contains significant quantities of nutrients, accounting for nitrogen and potassium totals of approximately 6.17 and 1.85 million metric tons, respectively.

Application of manure to pasture and crop land is the most prevalent CAFO waste management practice. This practice can be beneficial as it improves soil quality by replenishing important nutrients for plant health. However, the amount of applied manure along with nutrient mobility leads to runoff into local watersheds, causing a host of local/regional public health and economic issues. Nutrient runoff (especially P) leads to growth of harmful algal blooms, which has led to the loss of water supply to U.S. cities (due to elevated microcystin levels), loss of aquatic life, and over $100 million in economic damage since 2008.

Due to its distributed nature, nutrient runoff is one of the most prevalent and challenging environmental issues in the U.S. To address this issue, Ohio University (OHIO) is developing a supercritical water (SCW)-based modular process to be operated at CAFO sites in order to recover the nutrients as a solid product. This nutrient-rich solid product may be used as a fertilizer supplement for field application, with significantly reduced potential for runoff into local watersheds. In addition, the process produces a methane-rich gas product, which may be utilized at the CAFO site or upgraded into pipeline-quality natural gas. SCW provides an advantageous media for recovery of nutrients from animal manure; its lower fluid density and hydrogen bonding strength promotes precipitation of solid P components, and provides a gasification medium for undigested carbonaceous species.

To evaluate process potential, OHIO has been conducting both experimental investigations for treating dairy CAFO waste using a semi-continuous bench-scale reactor and process simulations/techno-economic assessments using Aspen Plus^TM. Major operating parameters being investigated include temperature (250-400°C), pressure (23-25 MPa), manure solids content (2.0-10 wt%), residence time (5-30 min), and oxidizing agent. The liquid and the slurry (liquid and solid contents) phases have been characterized using microwave-assisted acid digestion (AD) and inductively-coupled plasma (ICP) spectroscopy and ion chromatography (IC). The solid nutrient-rich product is collected after each trial and analyzed using AD, ICP, and IC, while the gas product is analyzed using mass spectroscopy. Aspen Plus^TM process simulations have been used to develop material and energy balances, heat integration strategies, and process costs. This presentation will review results from this study including nutrient recovery, solid nutrient composition, and product gas volume/composition with operating parameters and treatment costs associated with a 1,000 head dairy CAFO.