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Biochar Enhanced Infiltration Basin in Central Pennsylvania

This case study reviews the use of a bioenergy product for use in stormwater remediation.

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Updated:

September 23, 2024

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Introduction

One of the byproducts of some bioenergy facilities is biochar, which is a material that can be used in a variety of applications. Among those applications is the use of biochar in stormwater control systems. This case study provides a description of a biochar-enhanced infiltration basin, installed in central Pennsylvania in 2020. The case study begins with a background highlighting important information on biochar, stormwater management and how they are related. Following the background, a review of each process and component of the biochar-enhanced infiltration basin is written out and/or displayed. This starts with a case overview and site description that leads into a detailed description of the basin design and the construction process. After reading the case study, we hope you will have a greater understanding of the potential applications of biochar in stormwater treatment, and insight as to how you could leverage biochar in future stormwater management projects. The primary audience this case study was written for is landowners and construction companies looking for new and effective ways to help mitigate stormwater runoff problems on their sites.

Background

Biochar is a material produced from biomass by heating it in an oxygen starved environment until nearly all oxygen has been driven off, leaving behind primarily carbon. In recent years, biochar has come under the spotlight as a promising soil amendment used to improve soil health and retain pollutants. These properties could allow us to use biochar to mitigate problems associated with stormwater runoff. More specifically, biochar-enhanced infiltration basins could be a new option to meet stormwater management requirements in Pennsylvania.

Stormwater runoff and water pollution have become an important issue in Pennsylvania. The state has an estimated 19,000 miles of impaired streams (Johnstonbaugh et al., 2020). A big contributor to impairment of streams is increased land use from development and construction, as well as ineffective management practices associated with that development (DEP, 2006). Addition of impervious surface with new development creates problems because it greatly alters the natural hydrologic processes in the landscape. For example, by decreasing the area of land over which runoff can infiltrate into the soil and recharge the groundwater, streams are deprived of their base supply of water and instead increased runoff during storms damages stream beds (DEP, 2006). These activities also contribute to increased pollutants collected along the way from human activities (DEP, 2006).

Recommended volume control guidelines from the Pennsylvania Stormwater Best Management Practices (BMP) manual give a detailed summary of what can be done to mitigate increased stormwater runoff flows. The main guideline for volume control is that the development shouldn’t increase the total runoff volume for storms equal to or less than the 2-year/24-hour event.

Recommended quality control guidelines from the PA BMP manual are intended to be used to ensure that post-development flows aren’t contributing to increased pollutants in streams and rivers. Often, volume control can help with this as well, but additional measures may be needed to retain pollutants and meet the recommended goals of 85% reduction in particulate pollution load, 85% reduction in total phosphorus load, and 50% reduction in nitrogen solute load (DEP, 2006). Vegetated infiltration basins are a popular tool for pollutant reduction.

Biochar provides a new opportunity to enhance performance of current stormwater BMP options, due to its high surface area, cation exchange capacity and prolonged life span in soil (Johnstonbaugh et al., 2020). When added to soil, biochar’s porous structure allows for water to infiltrate at a favorable rate, while its chemical properties help to filter out contaminants. There’s also evidence that biochar's physical properties may improve soil structure by helping to bind soil organic matter, thus making the soil more resistant to disturbances from water and physical alteration (Wang, et al. 2017). That means, if used correctly, biochar can have profound effects on stormwater management practices. The prolonged lifespan also makes biochar extremely attractive because it can last in soils for centuries. This means that once implemented, biochar in the ground can act as a carbon sink, making it another possible way to help combat climate change.

These properties have shown in various studies to help increase soil water content and plant available water, increase nutrient storage in soil, and/or increase pollutant removal under different circumstances. For instance, biochar was found to significantly increase water holding capacity for soil, increase plant available water and even delay permanent wilting point in plants (Cao et al., 2014). Biochar also decreases bulk density by an average of 9% (Razzaghi et al., 2019). This is important because denser soils tend to infiltrate less water. Another analysis that conducted a review of other studies found that biochar generally increases removal of microbial pollutants and promotes the degradation of contaminants (Boehm et al., 2020). There is still much more research to be conducted on this product, but many studies have shown biochar to have a positive impact on soils and stormwater treatment. The methods used for biochar manufacture and feedstocks used can often have an impact on the properties and characteristics that biochar displays. Processing can be conducted at low temperatures (greater than 350 deg C), medium temps (350-500 deg C) high temps (500-800 deg C), or via gasification (900 deg C). In general, higher production temperature biochar's have higher surface area and increased hydrophobicity, leading to increased removal of trace organic compounds (Boehm et al. 2020). Lower production temperatures tend to increase polar surface functional groups and electrostatic attraction which leads to greater removal of inorganic contaminants like nutrients or metals (Boehm et al., 2020).

Along with the desirable physical and chemical properties, biochar can also be a low-cost addition to management practices as it’s typically made from waste organic products. The low cost of biochar is important because there needs to be economic viability for the product to be readily used. This is important to look at when evaluating methods to improve upon stormwater management practices because there is potential to save hundreds of thousands of dollars over time as the number of infiltration basins required could be decreased (Boehm et al., 2020). By increasing the functionality of stormwater BMPs through biochar, there could be reductions in the overall cost of stormwater treatment.

Case Overview

Metzler Forest Products, located in Reedsville, PA, has built an infiltration basin to manage stormwater runoff associated with construction of a new facility on their property. By installing the infiltration basin, about one-third of which is enhanced with biochar, Metzler hopes to not only have a more positive impact on the environment around them, but also learn more about how biochar performs in these settings.

Once completed and the success of the project is studied and reviewed, Metzler hopes to have further evidence for the benefits of biochar in stormwater treatment. They are interested in expanding their own markets beyond more traditional timber products and hope to eventually begin producing and distributing biochar themselves. As Pat Sherren, a head of this project, put it, "I believe in the power of innovation to overcome challenges." With stormwater runoff becoming a more pressing issue, Metzler believes biochar is a product that could be a solution. After all, they are a forest products company that could potentially produce large quantities of biochar in a cost-effective way. They also already produce their own compost and soil compost mixes that could be premixed with biochar as well.

Site Description

The area of the entire infiltration basin is approximately 35,000 square feet, or about 0.8 acres. Behind and uphill of the basin is the new construction site. The infiltration basin is designed to receive runoff from a total land area of 11 acres. Of those 11 acres, the total impervious surface area that will feed water to the basin will be 6 acres. As runoff flows downhill from the construction site, the infiltration basin will collect the water and infiltrate much of it back into the groundwater. The natural slope of the land on the site is approximately 2.5%.

According to the US Geological Survey, the bedrock that this plot of land sits on is limestone. Thus, care was taken to avoid any sinkholes. There were also response plans laid out in case a sinkhole does form. These planned steps include installing silt socks and construction fences around the sinkhole area and contacting the project’s geotechnical and civil engineers to determine a final course of action for repair.

Design of the Basin

The basin design consists of a nearly rectangular basin approximately 100 feet by 160 feet in size, with a built-up berm on three sides, and a “forebay” on the fourth side. Before the flow enters the basin, it is directed into a forebay area with four rock filters. These are designed to slow down and control the initial flow into the basin, as well as to remove some sediment as a pretreatment method. Slowing down the initial flow of runoff is necessary because otherwise the velocity of runoff could be erosive to the filter. A diagram of the basin and water flow can be seen in Figure 1.

$A rough design of what the infiltration basin looks like from a bird\'s eye view. Water flows down the 2.5% grade slope, through the forebay rock filters and into the infiltration basin. The perimeter dimensions of the diagram are estimated from design blueprints and not exactly measured on site.$

The basin is designed to have a 24” deep layer of specially prepared soil across its surface. The bottom 18” of material is a soil and compost mix made by Metzler. The top 6” consists of one of four mixes that serve as the main treatments being compared at the site. A cross-section of the infiltration basin can be seen in Figure 2 below.

Treatment 1, which covers nearly one third of the area of the basin, consists of 100% unamended local topsoil. Treatment 2, adjacent to the first treatment area, consists of a mix of 2/3 topsoil and 1/3 compost (with microbes). The third treatment is the area that contains biochar. This consists of 63% topsoil, 27% compost (with microbes) and 10% biochar by volume. There is also a fourth small treatment section that was extra land, and it is covered with 92% topsoil and 8% compost. This last treatment is located on the edge of the infiltration basin, next to the area that is 100% topsoil. The characteristics described here are summarized in Table 1.

Table 1. Ratios of components in each section of the infiltration basin.
	Treatment 1	Treatment 2	Treatment 3	Treatment 4 (smaller)
Topsoil (% volume)	100%	66.67%	63%	92%
Compost (% volume)	0%	33.33%	27%	8%
Biochar (% volume)	0%	0%	10%	0%

The soil used in the project was classified as a clay loam. The soil/compost mix with a pH of 7.3 and phosphorus concentration of 50 ppm. The organic matter percentage was 6.3% and the carbon to nitrogen ratio (C:N) was about 13:1. The infiltration rates for this blend ranged from 1.44 to 5.04 in/hr with an average infiltration rate of 2.64 in/hr.

The soil containing biochar (Treatment 3) had a bulk density of 14.0 lbs/cubic feet, a surface area correlation of 242 square meters/gram dry soil and a maximum particle size of 16-25 millimeters. The soil’s pH was 8.23.

The biochar used in this project was purchased from the company ARTi, based out of Iowa. The Metzler project utilized a biochar that was manufactured from hardwood as a feedstock and was designed specifically for infiltration purposes. The wood was pyrolyzed at 500+°C with a 20-minute residence time in the reactor. The biochar has a particle size of 1-3 mm.

The compost used in the mixes is made by Metzler Forest Products from feedstocks of wood fiber and leaves. Compost additions can give soils more desirable attributes for stormwater management including increased soil infiltration rate, water holding capacity, and cation exchange capacity (Minnesota Stormwater Manual 2020). Recommendations on the proportion of compost to use in stormwater BMP soils have varied between approximately 3-30% in various states, with some recommendations being revised downward due to concerns about nutrient leaching.

The infiltration basin is planted with a wildflower seed mix. This seed mix is designed for stormwater BMPs, but to also be aesthetically pleasing. For Metzler, appearance was a consideration because the field is in a highly visible location. As the basin operates in the coming years, Metzler will look for differences in growth and plant health between each section. The wildflower mix is also designed for minimal mowing and maintenance in order to avoid compacting the soil.

The basin itself has very little slope. This is required for infiltration basins in order to maintain uniform ponding and infiltration across the soil surface. More detailed design considerations and motivations pertaining to infiltration basins can be found in the Pennsylvania Stormwater BMP Manual.

By having multiple treatment areas with varying materials, Metzler hopes to get a more detailed understanding of the difference that biochar can make when used as an enhancement for stormwater management. One of the results they hope to see is the difference in plant growth and health by each treatment. If the biochar increases water holding capacity, plant available water, and removes nutrients from runoff, then it’s likely that the plants in the Treatment 3 area will have the healthiest growth.

Construction

The construction procedure for the biochar-enhanced basin was very similar to that for a traditional retention basin. Metzler used a combination of products they already owned and produced, along with biochar they had purchased. The biochar was mixed with the soil and compost blend by using a front loader. Under dry conditions, the two piles of materials at their respective volumes were combined until the new treatment was uniform throughout. After the treatments were prepared, an excavator was used to grade the site and remove the original layers of land and soil one row at a time. Once a row was cleared, front loaders and dump trucks were used to fill in the bottom layer, followed by the top layer (treatment) being used in that area. An excavator was used to smooth and level out the soils piled in. The method of completing one row at a time is necessary because it’s important to avoid compacting the soil blends used for the infiltration basin.

In total, the construction process took approximately 3 months to complete. Despite there being four different treatments, they were all installed in the same manner. The only real time difference present to install each treatment is the time it took to mix them. Aside from that, using biochar in treatment 3 didn’t require additional time for installation.

This project did not have observation wells installed due to time constraints; however, infiltration basins do commonly have observation wells, and they could be retroactively installed if necessary. This can allow for future testing on how the basin is functioning and can also help with monitoring of current conditions. Observation wells can be checked for characteristics such as groundwater levels, drain down time and evidence of clogging (Santa Barbara BMP 2013). They can also be used to collect water samples to measure contaminant levels.

Lessons Learned

Throughout this project, Metzler has gained valuable knowledge on infiltration basins and the potential benefits of using biochar as an amendment. Pat Sherren described this project by saying, “We are happy to reprocess the co-product (wood waste) from one process, make valuable heat for the first process and have an additional co-product (biochar) to solve environmental problems as part of the Metzler circular economy." By taking on this project, Metzler has learned that there are many ways to reach its goals of being a more sustainable company. Using biochar as an enhancement in infiltration basins and taking the steps of each process into account can allow the company to decrease waste and possibly generate a future profit as well.

Another lesson Metzler learned from this project is that the quantity of soil and materials needed is not always easy to estimate. Since they have experience mixing blends of soils, this was not the issue, but mixing the right amount to fill the basin proved to be the unexpected problem to solve. Their original estimates didn’t account for the soil settling slightly as it’s placed in the basin. Despite taking the precautions to make sure their equipment never drove over the newly laid soil, the soil still slowly compacts on its own. This caused their original calculations of volume of soils needed to be slightly short of the amount they needed.

There are also some precautions that need to be taken before and during installation. Each of the components to the blends needs to be stored in dry areas. Metzler kept their soil and compost safe in one of their buildings to keep those dry and kept the biochar in another to keep that dry. The blends were mixed outside, but only when the weather was fair. They also only worked on installation when it wasn’t wet and rainy outside. There are two reasons they had for this. First, they didn’t want the blends to get wet during installation because that could throw off the amounts needed to fill. Second, they didn’t want to drag mud from varying places around the facility with the machinery.

Conclusions

Overall, it may be said that we still have a lot to learn about the use of biochar in stormwater management practices, but hopefully projects like this help show some of the benefits and obstacles. The potential benefits of biochar shown in various research studies include increased soil water content and plant available water, increased nutrient storage in soil, and/or increased pollutant removal under different circumstances. Thus, this product is a focus for helping to mitigate stormwater runoff issues. As the Metzler infiltration basin operates over the next few years, we hope to more clearly see the difference that biochar can make as an enhancement to stormwater BMPs.

This project shows that installing a biochar-enhanced infiltration basin is similar to installing a traditional infiltration basin. For the most part, Metzler was able to install the basin with their regular machinery and equipment. Adaptations to storage space for the biochar and super sacks to store the product in before mixing are new issues that arise. The experience of installing this biochar-enhanced infiltration basin has shown that using biochar in stormwater BMPs is feasible to achieve. However, it’s important to note that many types of biochar are available on the market, and it may be necessary to consult scientists and engineers with biochar experience before installing projects like this.

Acknowledgements

The author would like to thank Metzler Forest Products for allowing us the opportunity to observe and record details about their project, especially Pat Sherren for being extremely helpful throughout this project. Thank you also to the Graves Extension Fund and the Penn State Department of Agricultural and Biological Engineering for providing the opportunity to work on this project.

Disclaimer

This case study is meant to be illustrative in nature and should not be used as a basis for design for construction without first consulting trained design professionals. No endorsement is intended when a specific company or product is mentioned.