On-Farm Anaerobic Digester: Frequently Asked Questions (FAQs)

During and after the first offering of the Animal Waste Biogas Digester Operator Training (Kenansville, 10/11/2024), we received the following questions. A panel of experts was invited by Amanda Hatcher and Mahmoud Sharara to a Zoom conference call to provide their perspectives answering these questions. Panelists reviewed their answers before publication (panel participants are listed below in alphabetical order).

Note: If you have a question that was not answered below, please use our FAQs form HERE and we will add it with the answers to this page.

First offering of Animal Waste Biogas Digester Operator Training on October 11, 2024 (Kenansville, NC)

Questions RE: Permitting

Do I need a separate permit for the anaerobic digester?

Before installing a digester, a permit holder must apply for and receive approval for waste system modification. Typically, that involves a request for coverage under the farm digester general permit. In this case, the permit changes from a general animal waste permit to a farm digester general permit. If the proposed digester does not meet parameters for digester general permit, it will need to apply for and receive approval for an individual permit. A third case is if an operator intends to add a cover to an existing lagoon to exclude rainfall without planning to use the biogas for energy. In this case, they will not be permitted under the general digester permit. Rather, they will apply for a modification to their existing general waste permit. The reason here is that the law specifies that biogas must be used as a renewable energy source to meet the statutory definition of a farm digester system to be covered by the digester general permit.

What is the difference between a general permit and individual permit for digesters?

The difference between the general and individual permits for digesters depends on how the system is operated. General permits are used for a defined class or set of facilities, and any facility that does not meet the criteria would need to have an individual permit. Individual permits are also used when additional conditions/requirements are needed. For instance, a digester that will receive feedstock other than, or in addition to, manure generated on farm is ineligible for permitting under the general permit. That is because co-digestion is not permitted under the general digester permit.

Who approves a digester where manure and other feedstocks are co-mingled (co-digestion systems)?

The Division of Waste Management (DWM) permits co-digestion and centralized digesters – depending on the type of treatment unit. If the system is on an animal operations and exclusively receives manures, then it is permitted by the Animal Feeding Operations (AFO) Program in the Division of Water Resources (DWR). Historically, when a digester processed more than 50% of its feedstock from manure, it was permitted by the DWR. However, as DWR undergoes the rule readoption process starting in 2025 and get completed by 2028, some of these rules and guidelines may be changing. So, stay tuned.

When should the technical specialist sign and submit the updated CAWMP certification if a digester is being added: before construction or after project completion?

Historically, this form was submitted after additions/modifications were made to traditional systems. From the DWR perspective, CAWMP certification forms can be updated after the digester construction. The form does have a place to certify both design and installation.

How will a digester built to aggregate and treat manure from multiple farms be permitted?

A shared or joint manure digester, that pools manures from facilities with different owners, must be permitted after the farms are combined under one permit. In such cases, individual owners may choose to form a combined ownership structure, such as a limited liability company (LLC), to join operations and receive permitting. Owners can choose to detail inner works of their agreement within that entity.

How are digesters permitted on farms that operate under a federal permit (NPDES permit)?

There are no federal general permits available for digesters. In NC, NPDES permitted farms that wish to add a digester would apply for an Individual NPDES permit. A NC cattle farm that operates under the NPDES has received permitting for digester addition through the individual NPDES permit.

What are the sampling requirements per my permit after installing a digester?

Additional sampling requirements are spelled out under section III. 23. In the permit. Digester influent and effluent need to be sampled, analyzed, and reported on a quarterly basis for eight quarters (two years). Sampling to begin no later than six months after an engineer certifies digester system installation. The analysis needs to include the nitrogen series (total kjeldahl nitrogen, nitrate nitrogen, ammonium nitrogen), plus pH, and agronomic sample package. DWR uses this data to develop an observational record to track and understand how digesters may change effluent on the farm. These sampling requirements are in addition to the typical lagoon effluent sampling requirements associated with nutrient management plans (NMPs).

Covered lagoon digester on NC swine farm

Questions RE: Operation & Maintenance

Why are digesters designed for 40 days of treatment? And what happens to the manure afterwards?

In North Carolina, digesters are built with treatment volume equivalent to 40-day worth of manure volume. If you do not provide additional heat, the 40-day period is the amount of time needed for digester bacteria to convert most manure carbon into biogas. This period can increase or decrease depending on where you are; a warmer climate means a shorter period and vice versa. So, on any given day, as manure is being fed into the digester, an equivalent volume flows out from the opposite end as a digestate and enters the secondary lagoon. If operated correctly, the digestate leaving the digester has spent 40 days inside the digester.

How do you prevent liner damage during sludge removal? What does the owner need to ensure/consider for this process?

It is recommended the farm owner hires a contractor with experience removing sludge from plastic-lined structures. The farm owner needs to provide the sludge removal operator with a clear design plan showing depth, side-slopes, and any internal parts that may interfere with pumping.

From the perspective of the project developer, how could you judge that the digester is working the way it is supposed to?

There are three factors that we look for to make sure the digester is working properly. First, gas production needs to remain close to expectations based on time of year, and where animals are in their growth cycle. We track gas production through the amount of cover inflation. The second factor we look at is the concentration of methane in the gas. It needs to be at or greater than 65% of the gas volume, based on our experience. Finally, we look at the relative amount of volatile fatty acids to alkalinity to ensure it is below 0.3. This ratio tells us whether the digester operation is stable or it could suffer from acidic pH. A low pH in the digester reduces gas productivity or completely stops it. We also track the amount of total solids in the manure entering the digester compares to total solids leaving the digester. We found the swine manure in NC has a wealth of alkalinity, and thus, acidifying a digester not to be concern. From a business perspective, we are always looking for opportunities to optimize operations to increase efficiency and profitability.

How often would a grower with this system evaluate how the digester is behaving?

This differs depending on the details of the contract between the farm owner and the developers. Often, developers specifically spell out the responsibility for tracking performance and developing benchmarks. Tracking performance by sampling and data analysis is critical to detect if performance is lagging and to develop corrective action plans.

What happens to the sludge in the lagoon after I install a digester?

On farms where the existing lagoon is covered, we remove sludge first. On farms where a new digester will be built, we proactively collaborate with farmers to help them desludge the lagoon and find options to turn it into organic fertilizer. It is critical to do so economically to avoid financial burden to the producer. We do what we can to minimize sludge from accumulating in the new digester. We anticipate the existing lagoon to see the sludge accumulation stay the same or slightly decrease but we do not have a long track record of data to have a conclusive answer. We also try to design and construct new digesters to have additional access points, as well, that we hope will aid in future sludge removal. It is early to know exactly how sludge in the lagoon shifts after a digester is installed before it.

How often do I need to clean the digester sludge?

We use digester performance to guide the frequency of sludge removal. We found the smaller the digester, the sooner you may need to remove sludge. We typically include in the design some sludge storage volume but work to reduce its accumulation. Depending on the size, location, and design, a digester may need cleanout once per year to once every 20 years. More often, it is a 5 to 10 year time-horizon between cleanouts. We observed that more frequent sludge removal that are of smaller volumes are easier for most farmers to incorporate into their nutrient management plans, and recommend this approach. We typically look to NRCS technical standards for recommendations on sizing and operation of digesters (Code NRCS CPS 366). This standard includes a minimum retention time to ensure proper operation. The economics of the digester operations will dictate the sludge removal much more urgently than any regulatory requirements. We are seeing digester designs submitted for permitting that include a volume for sludge storage in the digester.

Will I need to remove the digester cover to remove the sludge?

We require digester designs submitted for permitting to include plans showing how sludge will be removed. There is no regulatory requirement on which option is used to manage sludge. Some meet this requirement by installing pipes at the bottom of the digester that allow for pumping sludge without removing covers. Others indicate they will cut part of the cover to gain access to the digester content to remove sludge then repair it to resume operations. There are different scenarios for sludge removal from covered lagoon digesters, but possibly yes. In cases where the digester is accessed through the cover, it is opened then resealed afterwards. In cases where pipes are installed inside the digester, there is no need to open the cover. These digesters need to be graded correctly to ensure solids can be removed easily. In cases where digesters are equipped with under-pipes, sludge can be removed more frequently; possibly once a year.

I am having problems with salt buildup in my pipes and lines; will installing a digester help/hurt with this issue?

These salts are the result of ammonium, phosphate, and magnesium fusing and forming salts (struvite). While adding a digester does not increase the amount of nitrogen and phosphorus in the slurry, concentration of ammonium and phosphate increases. The formation of salt buildup (in particular struvite) may stay the same or increase but we do not have enough data support this claim. Salt buildup provides an opportunity to install nutrient recovery systems to harvest ammonium and phosphorus as struvite and distribute off-farm as a byproduct.

Where does the phosphorus in the manure go; does it stay in the digester or pass along to the lagoon?

Phosphorus generally follows the solids. After adding a digester, solids will start to build up in the digester until the sludge is removed. So, when digester routine cleanout is performed, the sludge will contain that phosphorus.

Where should I dispose of the rainwater that collects on the lagoon cover?

In the digester designs we receive, we expect to see details of the rainwater removal system. This includes where rainfall will collect on the cover, the pumping system, and an outlet that will convey water without risk of erosion or structural damage. The pumped water could be discharged in a grassed area. A critical requirement is that the operator ensures the pumped rainwater does not contain waste leakage from the digester. In cases of one or more inches of rainfall, the operator is required to inspect the facility (the same requirement existing in the state general permits for animal operations without digesters). As part of that inspection, they need to ensure the rainwater pumping system is operated and that it is not mixed with manure or digester contents. We have not seen any cases where the digester contents were mixed with the rainwater pumped off the cover. We expect as the covers age or sustain damages that they may become a contamination risk to rainwater and should therefore be inspected before decanting the rainwater from the cover. This rainwater system can be manual or automated. We are trying to move towards automation, so we are designing these systems to kick in at a certain rainfall depth to pump rainwater out through the outlet to a grassed area on site. Operators still need to observe and look for any signs of waste mixing with rainwater.

What happens if a very heavy rain (22 inches or more) falls on top of the cover?

We learned from past experiences that with proper design, and a seal weld between cover and liner, we can isolate rainwater and swine effluent under heavy rainfall conditions. Some digesters are built without a weld between cover and liner; these digesters could present a risk of digester content leakage under heavy rainfall. In cases of extreme events, the rainfall pump system is unlikely to keep up with the high rainfall rate. While there is some capacity to store some excess rainwater on the cover, this capacity could soon fill to the point of overflowing the top of the digester embankment. To prevent erosion or structural damage, a second line of defense could be a spillway, to allow this excess rainwater to get off the cover quickly and safely.

What if a power outage takes place when rainwater needs to be removed?

We have not seen situations with new digesters where power failures were an issue on site. Once power is run to the site, the system works well. We are not aware of issues of not being able to pump rainwater off cover. Most farms have a backup generator on site to ensure continuous feed, and water delivery to the pigs. It is advised to address this during the planning and design stages to add the rainwater pumping to the backup generator capacity. Another point to keep in mind is that farmers also keep non-electric pumps on site which could be utilized to get rainfall off the cover under power failure. Keep in mind that during hurricane conditions, you would want to first lower the digester cover then keep some water standing on the cover. This will help protect it from wind damage. But leaving too much water standing on the cover may compromise the structural integrity of the digester.

Questions RE: Safety

What is the safe distance from a digester when a fire happens?

Project insurance requires a clear idea on this distance around the site. For fire to spread, methane needs to be at least 5% of the air volume. When we did our analysis we found that 20 feet from the source of the leak is enough to dilute the methane and eliminate spread. Most methane leakage we see are pinhole discharges of methane. As a result, we require operators to wear gas detectors and monitors if they are within 100 feet. Ditto Christine’s comments about signage for farms. NRCS standard requires flares be a minimum of 50 feet from source. 100 feet is a conservative approach. Wearing monitoring devices is important and posting signage regarding the risks on farms is very important to reduce the fire risk. Signs need to be posted at the entrance of the facility and in proximity of the structure.

How close can a residence be located to a digester without having safety concerns?

From a regulatory perspective, the Swine Farm Siting Act applies to digesters. In it, digesters, much like barns or lagoons, must be at least 1,500 feet from residences not owned by the farm owner. For a swine farmer that owns a home on the farm, while they are exempt from this regulatory requirement, using this distance as a guideline is advised to ensure safety to persons and property.