Prevention, detection, control options, and the role of anaerobic digesters
March 17, 2015 by Steven Sell
An Internet search for manure-related spills can lead to an exhausting list of news articles and images of damage.
In Wisconsin alone, the volume of reported manure spilled during 2013 was the most since pre-2007, indicating the problem is not on a downward trend. With each and every manure spill comes increased scrutiny by the public on manure management – this perception also affects the anaerobic digestion (AD) industry in both positive and negative ways. As the number of manure spills on both large farms and those without digesters increases, so does favorability of the AD industry.
The AD industry is an established solution for nutrient management that also provides energy production in ways that are much less detrimental than conventional energy production. Positive mentality towards AD on farms is mainly due to the added manure storage onsite that comes with an installation, as well as the control over manure volume and flow. Anaerobic digestion systems help solve manure treatment issues by removing some of the biological oxygen demand of the waste stream which is the main contributor of downstream eutrophication. Because of this, it is plausible to say a spill of digester treated effluent would be less harmful to the environment than raw manure. The fertilizer value is not lost through the digestion process and effluent is still suitable for land application. In addition, anaerobic digestion systems produce biogas as a useful energy source that can be combusted to create electricity or heat.
However, because farms with AD are more closely monitored, a spill occurrence results in highly touted, bad press that significantly impacts the public’s perception. Photos and images can provide lasting impact on brands or digester types that leave readers or viewers with the thought that AD causes more environmental damage than assistance. This article aims to provide a better understanding of the levels of prevention, detection, and control that are in place for manure handling on farms and AD facilities.
Determining the cause of previous manure spills or leaks is the first level of understanding. Evaluation of spills in Iowa by the Iowa Environmental Council and Iowa Farm Bureau found most spills occur during transportation of manure and are caused by human error or equipment failure. The development of manure spills is affected by numerous factors, including, but not limited to: snow fall, cold temperatures, storage volume, farm size, and conveyance method. Wisconsin’s 2013-2014 winter was filled with especially harsh temperatures and snowfall, putting more strain on piping and liquid storage.
The first level of protection against manure spills is prevention. Manure spills are preventable with proper design, operation, and upkeep. However, as a factor of safety, multiple levels of protection should be used. Prevention can be as simple as minimizing water use and addition to manure supply; this will decrease total volume of manure as well as the likelihood of the manure to flow. Treatment of runoff or wash water in a separate area will help minimize the volume of high-strength wastewater. In addition to volume reduction, farm managers should strive for minimal transportation from production to enduse. Doing so limits the duration inside piping and vehicles and will decrease the potential impact area. Regardless of manure transport type, manure must be stored in some manner. Storage units should be professionally designed and evaluated on a regular basis for performance. When it comes to anaerobic digestion, manure is most often stored in constructed tanks.
Properly constructed tanks should be visually inspected on a regular basis to ensure structural integrity. Commonly, tank levels will vary as manure is treated during an anaerobic digestion process. Using level sensors with redundancy or backup will help keep readings accurate in the event of a unit failure. Today’s technology allows operators to receive immediate updates on AD process output and function. Plant errors and warnings can be connected to phone dial-outs or on site-displays to notify changes in process performance. These readings can provide the following key information:
• High-level readings – may indicate pumping or addition to the tank is occurring too rapidly and the outlet to the tank is blocked or malfunctioning. Control logic should be set in place to shutdown feeding to the tank if this occurs. Some examples of sensors that work well for high level readings are float sensors or capacitance probes set at the correct height.
• Low-level readings – may indicate pumping or removal of material from the tank is occurring too rapidly and the inflow is not working properly. It could also indicate a leak in the tank at some location. Some examples of sensors that work well for low-level readings are float sensors or capacitance probes set at the correct height.
• Continuous level transmitters – this type of measurement displays height or change in height of the storage vessel. Radar, sonar, pressure, or capacitance sensors may be used to obtain liquid height within the tank. Monitoring the level change over time will show how rapid the level is increasing or decreasing and could help pinpoint the type of error.
Liquid manure is often conveyed via pumps, drag hose, or piping. Since this step involves transport of manure underground or in exposed conditions – and sometimes long distances – the risk of leakage remains high. Simple flow and pressure sensors can be installed on suction and discharge sides of pumps to detect piping and pump performance; this helps indicate pipe blockages or leaks as a leak on the discharge side can be detected by a zero gauge pressure or a lack of flow in a down stream flow meter. Control logic can be setup to immediately shutdown this type of situation, preventing large spill volumes.
Proper control and operation of manure handling systems is the first line of defense against large manure spills. Operators should be equipped with safety plans and emergency procedures. However, with today’s advancements in facility controls and software, operators can be notified of plant errors by dial-out and can control the plant remotely via phone or Internet connection. This is a unique feature with the potential to decrease leak time through proper and immediate response to isolate areas.
Unfortunately, no matter how efficient plant operations may be, structural failure is always a possibility. For this reason, each state will inspect and require a containment zone. This level of inspection will typically be handled by the natural resources permitting agency and will require all storage structures to be properly lined to prevent groundwater leakage, be surrounded by a secondary containment structure that can hold all the liquid within the vessel in the event of a rupture, and provide a safe spillway to reroute excess contaminated water away from environmental habitat. Manure storage tanks or anaerobic digestion reactors can be equipped with overflow piping to convey spills to properly sized and constructed storage ponds. Overflows can be equipped with simple flow sensors to provide operators alerts that an extraordinary event has occurred.
There are numerous manure spill response plans established by each state’s Natural Resources Department, a list of guidelines from the United States Geological Survey – Natural Resources Conservation Services (USGS NRCS), and other organizations to help keep these organized. However, the best response plan starts with manure spill prevention.
Steven Sell is a biologist and application engineer with BIOFerm Energy Systems