Bioreactors need monitoring and management08/20/2019 | Soil Health, Water Quality
By Carol Brown, ISA communications specialist
Bioreactors are a fairly new edge-of-field practice being installed on Iowa farms that can help improve water quality from tile drainage.
These underground trenches are filled with woodchips and covered with grass or pollinator-friendly perennial plants. Tile drainage from the adjacent field is diverted to enter the structure, allowing bacteria on the wood chips to convert the nitrate in the tile water to nitrogen gas to be released naturally into the atmosphere. This reduces the amount of nitrate in the tile water before it enters a stream or ditch.
Some of the first bioreactors installed in Iowa are now reaching 10-years-old. Research is being conducted to confirm their lifespan, which currently is considered 10 to 15 years before the woodchips need replacing.
In the meantime, these structures do require some monitoring and maintenance. Chris Hay, Iowa Soybean Association’s senior environmental scientist, offers some management tips and general guidelines for farmers and landowners who have a bioreactor, and provides insight on management for those who are thinking about installing one.
What are some basic maintenance guidelines that every bioreactor owner should follow?
Hay: Some general tips to follow include: avoid vehicle and equipment traffic going over the bioreactor to prevent compacting the wood chips. Monitor the drainage system that contributes to the bioreactor for blowouts and for damaged outlets, which could flush sediment or trash into the bioreactor.
Each spring and fall, the control structures and outlets should be inspected. It’s also a good idea to inspect control structures after major precipitation events. Any sediment or trash accumulated in the control structure should be removed so that flow is not impeded. Stop logs, or standpipes depending on the control structure, and gaskets should be inspected and repaired or replaced as needed. Grease the gaskets with lithium grease before replacing to minimize seepage and aid smooth operation. If there are other control structure components damaged, they should be repaired or replaced to make sure the control structure is functioning properly.
How does one know if their bioreactor is functioning?
Hay: Spring is a good time to observe whether water is flowing into and out of the bioreactor. Temporarily adjusting the stop logs (or standpipe) can help with this. If the drains are flowing but water is not flowing into or only very slowly through the bioreactor, further investigation may be needed to determine if the inlet or outlet is plugged.
Monitoring is the only way to know for sure how well the bioreactor is removing nitrate. Depending on the funding sources used to fund the bioreactor installation, some monitoring may be required. Regardless, occasional monitoring provides valuable information for making sure the bioreactor is functioning as intended and for making any adjustments to improve performance. Check the water depths in the control structure as well as the nitrate-nitrogen concentrations in the water flowing into and out of the bioreactor. Nitrate test strips are an easy and inexpensive way to quickly test for nitrate concentrations, but laboratory analysis will be more accurate.
Do the stop log settings need to be changed?
Hay: When there is enough slope from the drained field to the bioreactor and it is functioning well, the stop log settings can be maintained. However, if seasonal management of the inlet control structure is required to prevent potential crop damage, or if the bioreactor is being used in conjunction with drainage water management, the stop log settings will need to be adjusted seasonally. If seasonal management is required, that should be indicated in the bioreactor design documentation.
How does one manage the bioreactor stop logs?
Hay: Generally, there will be separate stop log settings for the nongrowing (or fallow) season, spring and fall field work, and growing season periods. The level of these stop log settings should be provided in the operation and maintenance guidance as part of the bioreactor design documents. Often the stop log settings are given as distance from the rim of the control structure to the top of the stop logs. It may be easier, however, to record how many and what size stop logs this translates to, which are usually in some combination of five- and seven-inch boards.
In the fall, as soon after harvest as practical, the stop logs should be set at the nongrowing season level, which will be the highest setting. As planting time approaches, lower the stop logs to the spring field work setting one to two weeks before the desired planting date to fully drain the soil and create trafficable conditions. Usually this means removing all the stop logs. Around mid-June, V6 stage for corn, the stop logs should be raised to the growing season setting which will allow for good crop development. Fall is often dry enough that the stop logs do not need to be changed for harvest, but if conditions are wet, lower the stop logs for additional drainage one week prior to harvest. Once harvest is completed, the stop logs should be reset to the nongrowing season setting.
With fall approaching, how should bioreactors be managed?
Hay: In the fall, or after drainage has stopped for the season, the outlet control structure should be inspected to make sure the bioreactor has fully drained. Most bioreactors now have a notch or hole drilled at the bottom stop log of the outlet to allow the bioreactor to drain out. If not, or if the bioreactor is not fully draining for another reason, stop logs in the outlet may need to be pulled to let the bioreactor drain out. If water is stagnant or remains in the bioreactor too long, it can result in sulfur reduction, creating a noticeable sulfur, or rotten egg, smell and leading to potentially undesired impacts like methyl mercury production.
Are there required maintenance issues that need to be addressed?
Hay: There may be monitoring and maintenance requirements depending on the funding sources that were used to pay for bioreactor installation. These should be spelled out in an operation and maintenance plan.
Are there other things to watch for beyond water flow?
Hay: Over time, the wood chips and soil on top of the bioreactor will settle. If the soil over the bioreactor settles lower than the surrounding ground or water ponding is observed, additional soil or wood chips may need to be added. Additionally, rodent activity as well as trees or other woody vegetation growth should be controlled near the bioreactor to prevent damage to the system.
Bioreactors are just one edge-of-field structure that can help improve water quality in Iowa to help reach the goals of the Iowa Nutrient Reduction Strategy. The strategy, in place since 2013, recommends practices including bioreactors and cover crops, to reduce nitrogen and phosphorus by 45 percent in Iowa’s rivers and streams before reaching the Gulf of Mexico.
For more information on bioreactors and other drainage practices, visit transformingdrainage.org.
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