Potential for Biosolids Use on Solar Sites

W. Lee Daniels, Emeritus Professor of Soil Environmental Science, Virginia Tech, wdaniels@vt.edu

Over the next several decades, conversion of existing agricultural and forested landscapes into utility scale solar (USS) facilities will likely be the dominant land use change across rural landscapes in Virginia and the Mid-Atlantic. Total area affected by USS in Virginia, according to information provided by VA Department of Environmental Quality (DEQ), is expected to range from 150,000 to > 300,000 acres by 2045. Many community-scale systems will affect smaller footprints (approximately tens of acres) in more urbanized areas. Many of these sites would benefit from utilization of biosolids during their site development/stabilization and operational management phases but will present unique and different challenges for land application vs. current typical practices.

The rate of USS development in Virginia increased dramatically over the past three years due to the Virginia Clean Economy Act (VCEA: 2020) and related federal “green energy infrastructure” incentive programs. Typical USS sites under review and development range from ~25 to >100 megawatts (MW) in projected output. It generally takes approximately 7 to 10 acres of land per MW to fully encompass panel arrays, perimeter buffers, stormwater BMPs, access and service roads, and other infrastructure for internal transmission and connection to the grid. However, panel arrays usually cover < 50% of the total site footprint. All solar development sites must meet Virginia DEQ and/or local stormwater (SW) and erosion & sediment control (ESC) requirements.

Most sites <150 MW are permitted by DEQ under their Permit by Rule (PBR) program, while larger sites and certain multi-site proposals are permitted by the State Corporation Commission (SCC). Under the PBR program, future site disturbance (after 2024) of more than 10 acres of prime farmland or 50 acres of contiguous forest lands will require mitigation as mandated by Virginia House Bill 206. DEQ is currently drafting those regulations. 

Depending on the location and pre-existing soil/landscape conditions, USS developments disturb from 10% to 25% of gently sloping sites while almost complete soil disturbance occurs on many strongly sloping sites. Following (1) initial development and revegetation/stabilization, most sites are (2) maintained in a conventional forage cover for 25+ years, and then (3) will undergo another round of extensive soil disturbance when or if the infrastructure is removed. Each of these three management phases offer distinct but differing opportunities to utilize biosolids to enhance soil quality, improve initial revegetation success, enhance long-term forage yields, and reduce overall stormwater runoff and sediment losses.

During the site development phase, extensive areas of acidic clayey subsoils are frequently exposed that are low in pH, plant-available N and P, and organic matter. This is particularly true of steeply sloping sites in the Piedmont and Valley and Ridge locations where deeper cuts and fills along with topsoil removal can lead to more extensive disturbance. Root-limiting compaction of the final regraded surface and subsoil is also common, leading to enhanced runoff and sediment loss risks. Fortunately, at least in Virginia, we have a long history of dealing with this combination of soil reconstruction problems through the beneficial utilization of biosolids coupled with appropriate soil testing, liming, subsoil ripping, and rapid incorporation of amendments.

However, the application of biosolids to USS sites in their initial development and stabilization phase may be constrained. First, unless the land-applied areas are revegetated before active panel installation begins, large areas (e.g. > 5 to 10 acres) may not be available for application at any one time. Current ESC regulations require permanent revegetation within 7 days following final grading and/or temporary seeding within 14 days after rough grading. Secondly, current conventional application and incorporation methods may not be feasible within panel array areas due to low elevation of panels, narrow spacing between panel rows, and other infrastructure. However, broadcast of pelletized products with smaller equipment could be effective followed by immediate soil incorporation and seeding/stabilization. Third, steep slopes on many sites will limit application areas.

Once a new USS site has been successfully revegetated and stabilized, most are managed in conventional grass/legume forage stands with varying degrees of management intensity. Interest is growing in the potential for agrivoltaic management practices, particularly grazing by sheep. Once these sites are released from initial SW and ESC requirements and move into a routine operational phase, there is potential for routine surface applications under conventional hayland/forage nutrient management and/or grazing restrictions. Biosolids could be particularly useful for areas within panel arrays where carryover soil disturbance effects from trenching and grading/cut/fill operations interact with differences in solar insolation/shading and panel “drip lines” to generate a distinct pattern of differing vegetation under and between panel rows.  While this avenue for biosolids application will take several years to fully develop, it does have significant potential for beneficial reuse and improvement of soil conditions and productivity for literally tens of thousands of acres.  

Since we are still decades away from the final decommissioning of newly established sites, projection of final soil remediation measures is speculative. However, we have recent experience in reclamation of over 2,000 acres of prime farmlands in Dinwiddie, Greensville and Sussex Counties after mineral sands mining.  This experience indicates that one-time application of biosolids at 35 DT/acre coupled with extensive deep ripping and surface tillage was a superior treatment which equaled or exceeded topsoil salvage for returning those lands to forage, row crop and forest productivity.

It is important to note that a number of proposed USS sites may elect to emphasize utilization of pollinator-friendly and/or native plant species, particularly in buffer areas and drainage setbacks, which may not be compatible with the biosolids management practices reviewed here.

In summary, the development and management of USS sites in Virginia and the Mid-Atlantic region could be a promising option for the beneficial utilization of biosolids, but will need to be carefully coordinated with the unique nature of the industry and its emerging regulatory and approval structure.