Bo Hu.

2016-17 Rapid Ag: Reducing Sulfide Generation by Electrochemical Treatment of Liquid Manure in Deep-pit Manure Storage Systems

November 1, 2015

Principal Leader

Bo Hu

Department

Department of Bioproducts & Biosystems Engineering 

Funding Awarded

  • 2016 Fiscal Year: $67,491
  • 2017 Fiscal Year: $100,499 

The Problem

Deep-pit manure storage is the most widely used manure storage system in swine buildings in Minnesota and adjacent Midwestern states. With manure stored beneath the slatted floor, hydrogen sulfide (H2S) is continuously generated and emitted into the barn air, creating air quality and safety issues for both the farm workers and animals. 

Background

Deep-pit manure storage is the most widely used manure storage system in swine buildings in Minnesota and adjacent Midwestern states. With manure stored beneath the slatted floor, hydrogen sulfide (H2S) is continuously generated and emitted into the barn air, creating air quality and safety issues for both the farm workers and animals. During manure agitation (typically in the fall from Sept-Nov), the danger of exposure to dangerous concentrations of H2S is increased [1], because the agitation rapidly releases the trapped gas in manure that exponentially increases the hydrogen sulfide level in barn air [2, 3]. The level of acute H2S toxicity starts in the range of between 10 and 200 ppm (by volume), and olfactory fatigue occurs when its concentration nears 100 ppm. When the concentration goes beyond 500 ppm, respiratory paralysis leads to immediate unconsciousness and even death to exposed humans and animals [4]. A number of fatal incidents due to H2S exposure in animal barns have been recorded. Between 1975 and 2004, there were 14 human fatalities recorded in manure handling in Minnesota, ranked the first among all states in the United States [5]. In order to regulate H2S level, Occupational Safety and Health Administration (OSHA), Minnesota Pollution Control Agency, and Centers for Disease Control and Prevention either set H2S exposure standards or instituted programs to record and assess fatal injuries due to exposure. For example, OSHA (29 CFR 1910.1000) recommends a maximum level of 20 ppm of H2S over an 8 hour period in general industry. In Minnesota, the state administrative rule of MS 7009.0080 recommends an outside limit of 0.05 ppm with less than half an hour exposure that does not exceed two times per year.

At the same time, the air quality inside a swine barn can be very corrosive partially due to the high H2S generated from the manure stored in deep-pit facilities. Air in animal houses is often classified as highly to very highly corrosive, and air quality drastically deteriorates when the barn is not properly ventilated and the relative humidity level exceeds 85%. Long-term exposure to a relatively high level of H2S (greater than 1 ppm) decreases the expected service life of structural materials, equipment and electronic devices inside pig barns that are made from concrete [6], wood [7], steel, silver and copper [8, 9]. With recent development of the corn ethanol industry and wide applications of distiller's dried grains with solubles (DDGS) and other corn co-products as alternative ingredients to conventional pig feedstuff, issues related to H2S in swine barns are expected to become more common and severe because of the increased sulfur source in the feeds as well as in the excreted manure [10]. Addressing the hydrogen sulfide issues in swine barns is both emerging and urgent.  

The PI and Co-PIs of this proposal received supports from RARF and MnDRIVE and other funding sources to evaluate and improve air quality in swine barns and biogas quality in laboratory anaerobic digesters. There are multiple mitigation techniques to decrease the hydrogen sulfide level in both the indoor and ambient environment, such as dietary control, application of chemicals on manure, electrostatic precipitation, ventilation, treatment of exhaust air via biofilters, etc. However, most of these techniques only deal with H2S emission, and the economic viability of these techniques needs to be further assessed for applicability under swine production conditions. The proposed electrochemical method in this study shows strong advantages over the other methods. This method is expected to remove or greatly reduce sulfide generation in the stored liquid manure while requiring only minimum equipment installation and retrofit of the current storage pit with little maintenance requirement, low electricity consumption, and low chemicals usage. Our preliminary study from a batch anaerobic digesters laboratory study showed over 95% removal of hydrogen sulfide in the biogas produced by the adoption of the electrochemical systems. Another experiment based on a continuous operating anaerobic digester that lasted for nearly one year showed that an electrochemical technique reduced hydrogen sulfide in biogas by 83%. From the preliminary data, this electrochemical technology has shown the potential to be cost effective and only brought a marginal impact on the manure’s nutrients thus having little or no effect on the fertilizer value.  

Objectives

  1. To optimize the performance of the elements of electrochemical systems on sulfide removal in simulated manure storages, and to establish scale-up principles from lab to field study by the determination of critical parameters.

  2. To install a pilot-scale electrochemical system in a conventional deep-pit swine manure storage, and to evaluate the reduction in sulfide level and emission by comparing with a conventional manure storage.

  3. To conduct an economic analysis of implementing the electrochemical system in a deep-pit manure storage.