Biosolids and Green Infrastructure
Combined Sewer Overflows (CSOs) give wastewater treatment agencies a bad name and a big headache. Biosolids products, when used to create soils for bioretention systems, can be part of the relief. CSOs pose a very complicated and expensive set of challenges. Increasing regulations on permitted CSOs, increasing bad press and public perceptions associated with CSOs and increased frequency of intense storm events have combined to be big problems for wastewater districts. Sewer systems designed intentionally to drain rainfall swiftly to watercourses, albeit containing street dirt and collateral amounts of sewage, are now being “re-purposed” to contain and transport this water for pollutant removal before discharge. No small wonder that many of our urban centers are facing price tags in the billions of dollars for reconstructing sewers. A fierce effort is underway to accomplish this mission by altering urban soils to absorb rainfall and thereby reduce the amount of rainfall runoff to sewers. Traditional engineered, or grey, approaches to storm water treatment are one very costly direction for mitigating CSOs. Green infrastructure, including a range of technologies such as rain gardens and bioretention systems, permeable pavement and green roofs, is increasingly being seen as a sustainable, environmentally and economically friendly alternative to grey systems. In many cases, a combination of green and grey solutions is being developed. Green infrastructure poses hurdles for traditional wastewater systems. It is decentralized, relies on public participation and acceptance, and is based on natural systems. Natural systems are not so easily modeled by equations and will often show variability. A major effort by the wastewater profession is assembling performance results for elements of green infrastructure. This month’s library is a step by step guide to green infrastructure, including articles that show how biosolids can be a key component of these systems. The library starts with a pretty dense life cycle assessment of green infrastructure systems. The article notes that previous LCAs have focused on the greenhouse gas savings associated with green infrastructure. The authors note one study of green systems in the Bronx (that is part of NYC) that showed 75-95% life cycle GHG savings in comparison to grey systems. This article broadens the LCA to consider other factors. For me, the key is figure 4 which shows the cost- benefit ratios of different systems for kg P, climate impacts and energy use. By far, the most efficient of the different green alternatives modeled is the bioretention system. Living plants and soils are key to the success of bioretention systems. Permeable pavement and green roofs are much less efficient. The second article is about legal aspects of green infrastructure systems. The author is an attorney from Beveridge and Diamond, the law firm that has supported land application of biosolids in the person of Jimmy Slaughter. The article notes that jurisdiction of all green infrastructure projects falls under the Clean Water Act and that EPA has been encouraging this approach for many years. The authors provide examples of different municipalities that have worked with EPA to gain approval of these systems. This is critical, as the performance of these systems is, in many cases, a big unknown. From here we go to an article that presents two case studies on applying green infrastructure. Both cities used in this example are faced with declining populations and decreased revenues for implementing large scale projects. The challenges that are described may offer insight into how such systems might be implemented in your own community. The last two articles focus on soil amendments that are the key to the success of bioretention systems. The LCA article showed that bioretention systems give you the best bang for your buck for green infrastructure. The first of these two soils articles evaluates two types of compost in comparison to topsoil for use in bioretention systems. A yard waste and a food scrap based material were tested. Both types of compost were more effective than the topsoil, and the differences between the two composts were minimal. Although the nutrient characteristics of the two materials were not presented in the study, the food waste compost was likely similar to a biosolids-based compost. Metals data from the composts were included, with both materials having total Pb > 100 mg/kg , total Zn > 200 mg/kg and total Cu about 60 mg/kg. No significant leaching of metals was observed for either of the compost systems. The second soil article reports growth response of urban trees to a range of amendments including compost, compost tea, biochar and biosolids. Trees are a critical component of green infrastructure systems. For all measured variables, the biosolids amendment was the best of all amendments tested. In many cases, this difference was statistically significant while in others it was only a trend. The point here, and with this library, is that green infrastructure is coming into its own as a viable alternative to or partner with grey systems. Biosolids-based soil products have a critical role to play in these systems. It is time to start research and outreach to optimize their contribution.