Big Ox Energy in South Sioux City, Nebraska, is an industrial facility that processes food waste and high-strength waste, along with industrial and residential wastewater to produce biogas energy.
The facility operates what is called an anaerobic digester as a preliminary wastewater treatment system. The digester system produces the biogas (methane, carbon dioxide and hydrogen sulfide mixture, and other trace gases) that is then converted to methane gas. The methane gas is sent through a pipeline to other industries that use it as fuel. Turning waste into energy offers tremendous environmental benefits when the system works as designed.
During normal operations, the facility has approximately 6,200 pounds of biogas on site. The amount of gas is an estimate, based on overhead space in the digester and associated transfer duct/pipe. The facility is also equipped with an odor scrubber for hydrogen sulfide removal from various on-site sources and a flare, which is used for gas combustion during times when the scrubber is not in operation. Big Ox Energy can produce as much at 1.3 billion standard cubic feet of biogas per year.
The facility also receives high-strength liquid industrial wastes including, but not limited to, paunch manure (partially-digested material taken from an animal at the time of slaughter). Incoming wastewater (approximately 2 million gallons per day) is delivered to the facility through the sewer system force main from nearby food processing facilities. The facility also receives some wastewater by truck. The wastewater is treated and then discharged from the facility to the South Sioux City sanitary sewer system, which then flows via force main to the Sioux City, Iowa, wastewater treatment plant for further treatment. Force mains are pipelines that convey wastewater under pressure from the discharge side of a pump or pneumatic ejector to a discharge point.
Solids produced from the anaerobic digestion process are dewatered through a centrifuge and screw press. A screw press separates liquids from solids. The screw press squeezes the material against a screen or filter, and the liquid is collected through the screen. The solids were previously hauled to local landfills for disposal; however, in December 2018, the facility began to store the solids on-site before processing.
EPA and Nebraska Department of Environmental Quality Share Regulatory Oversight
EPA and the Nebraska Department of Environmental Quality (NDEQ) share regulatory oversight of the facility. For example, NDEQ is generally the lead agency on air and water permitting, whereas EPA is responsible for chemical accident provisions of the Clean Air Act section 112(r) and the Emergency Planning Community Right-to-Know Act.
NDEQ and EPA have performed multiple investigations of the facility. During these inspections and investigations, NDEQ and EPA identified areas of concern and notified Big Ox Energy personnel of potential violations. In addition, the city of Sioux City, Iowa, has issued the Big Ox Energy (BOE) facility notice of multiple violations from its wastewater discharge permit. EPA continues to coordinate with NDEQ and the city of Sioux City to address areas of environmental noncompliance.
The BOE facility is currently permitted under the Nebraska General National Pollutant Discharge Elimination System (NPDES) Permit No. NER910002 for stormwater discharges associated with industrial processes. The NPDES permit was issued by NDEQ in 2016, prior to the facility's start of operations.
In addition, the facility is permitted as a Significant Industrial User under a wastewater discharge permit from the city of Sioux City's pretreatment program.
Operational History and EPA Actions
The BOE facility in South Sioux City, Nebraska, began accepting industrial wastewater in September 2016. In October 2016, following public odor complaints, the facility stopped accepting industrial wastewater, and in November 2016, stopped discharging wastewater to control sulfides in the municipal sewer system. In December 2016, a temporary lift station and new force main were installed to route the facility's effluent away from residential areas of South Sioux City. In the summer of 2017, a permanent municipal sewer force main was installed that routed wastewater directly to the Sioux City, Iowa, wastewater treatment facility.
In 2017, EPA conducted inspections of the pretreatment and industrial stormwater activities of the facility, pursuant to section 309 of the Clean Water Act. In 2018, EPA conducted an additional industrial stormwater inspection.
On Feb. 13 and 15, 2017, representatives of EPA Region 7 inspected the Big Ox Energy - Siouxland, LLC facility in Dakota City, Nebraska. The inspection was conducted to determine compliance with the Clean Air Act section 112(r), specifically with the Risk Management Program, 40 CFR Part 68, General Duty Clause, and the Emergency Planning Community-Right–To-Know Act.
In December 2017, EPA issued an order to Big Ox Energy, requiring it to come into compliance with the Clean Air Act after a 2016 chemical release resulted in the hospitalization of one employee.
In the spring and summer of 2018, NDEQ received multiple odor complaints that reported effluent overflows from the digester overtopping the facility's roof and onto the ground. BOE constructed temporary containment structures to hold the spills/releases on site. Digester overflows continued to occur intermittently from December 2017 through February 2019. In June 2018, the agency conducted mobile air monitoring at the BOE facility.
On Nov. 13 and 14, 2018, representatives of EPA Region 7 inspected the Big Ox Energy - Siouxland, LLC facility in Dakota City, Nebraska. This inspection was also conducted to determine compliance with the Clean Air Act section 112(r), specifically with the Risk Management Program, 40 CFR Part 68, General Duty Clause, and the Emergency Planning Community-Right–To-Know Act.
Repair an expansion joint
- Repair pressure relief devices
- Repair instrumentation
- Identify and address all release points
- Repair digester mixers
- Develop an air monitoring plan
- Develop a plan to inform the public in the event of:
- A release resulting in evacuation of any part of the facility,
- A release of hydrogen sulfide over 50 parts per million (ppm), OR
- Release of methane at 10 percent lower explosive limit.
On Feb. 12, 2019, EPA issued a unilateral order for Big Ox Energy - Siouxland, LLC to address compliance with the requirements of the Clean Air Act section 112(r)(1), 42 U.S.C. §7412(r)(1), in order to prevent any further releases of hydrogen sulfide, methane or biogas from the flare bypass and the digesters, and to minimize the consequences of any release that does occur which required a Compliance Plan.
- NDEQ maintains an extensive document repository related to the BOE facility in South Sioux City
ERT Report Summary (PDF)(3 pp, 202 K,
This document summarizes and provides context to the emissions monitoring report developed by the EPA Environmental Response Team (ERT) from the November 2018 air monitoring event in South Sioux City, Nebraska.
Big Ox Energy - Siouxland, LLC Consent Agreement and Final Order (PDF)(15 pp, 360 K,
Consent Agreement and Final Order between EPA and Big Ox Energy - Siouxland, LLC