I’m trying out a new thing – writing summaries of interesting articles that I read while I’m doing my PhD. This is meant for an audience of engineers, scientists or architects – some knowledge of hydrology/chemistry/urban infrastructure is needed to understand this. I use scientific terminology, but I don’t structure my sentences in complicated ways.
Grebel et al. (2013) reviewed literature around infiltration systems and how those systems manage contaminants. The advantage of this literature review is that it pulls information from a variety of research fields, because research on contaminant removal (particularly the pathways involved) in engineered stormwater infiltration systems is still limited, or is lacking in sufficient detail to make conclusions about removal pathways.
First off – the context. Why do we care about contaminant removal? Well we are putting green infrastructure / low impact development practices throughout urbanizing areas. The goal is returning the hydrologic regime to a more natural state, meaning more evapotranspiration and infiltration, instead of runoff over hard surfaces. But our urban stormwater is full of some gross stuff – metals, oil & grease from wear and tear of vehicles and asphalt, bacteria and viruses from fecal matter, and nutrients from lawns and caught up in soil particles. The contaminants are various – and their impact on surface water and ground water quality is also variable. Are they removed on their own in our engineered systems, or do they require some kind of additional treatment?
The authors identify three design strategies for contaminant removal:
- Media amendments
- Controlling hydraulics and media saturation
- Manipulation of redox conditions
Media amendments receive the most attention – and are the focus of a lot of products currently being developed in the field. Media amendments can help with contaminant removal by enhancing sorption, transform the contaminants via chemical (abiotic) or biotic reactions. Selecting media with high cationic exchange capacity will enhance sorption. However this media tends to have lower hydraulic conductivities, and so more space will be required to meet storage requirements. Microbes can also be used to transform contaminants, such as using denitrifying bacteria to convert nitrate to nitrogen gas. The media and conditions need to be specific for the specific bacteria you want to grow – such as providing a carbon-rich biofilm and anoxic conditions.
Controlling hydraulics is an interesting one, and is being given more attention with technologies for continuous monitoring becoming cheaper. A great resource for designers interested in this approach is open-storm.org, which offers open source access to software, hardware design and algorithms for building continuous monitoring systems. This helps with the biological removal methods, but moisture content needs to remain above 50% to maintain the same level of biological activity between storms.
Lastly, redox conditions are important to control for either sorption or chemical transformation removal mechanisms. For example, denitrification occurs under reducing conditions. But you want to make sure it’s in the right zone, and not encourage sulfate-reducing conditions (unless you’re dealing with acid mine drainage!).
So what does this mean for designers implementing infiltration measures in urban environments?
Trying to treat a bunch of contaminants in a single system is not possible because the conditions needed to remove one contaminant might hinder the removal of a different contaminant. Similar to other engineering fields, it’s best to divide treatment methodologies into compartments with proper ordering. Systems engineering is needed to think through these processes.
We need to be creative about managing infiltration systems because they are distributed throughout the environment, and not centrally located and easily monitored. This is where remote monitoring technologies can really help.
We still don’t know much about how these systems perform over the long term and through a variety of dry/wet conditions. More research is needed in this area.
To read the specifics about a contaminant you’re concerned about and find literature related to it, I suggest you check out the article I
Janel E. Grebel, Sanjay K. Mohanty, Andrew A. Torkelson, Alexandria B. Boehm, Christopher P. Higgins, Reed M. Maxwell, Kara L. Nelson, and David L. Sedlak (2013). Engineered Infiltration Systems for Urban Stormwater Reclamation. Environmental Engineering Science.Volume: 30 Issue 8: August 14, 2013. https://www.liebertpub.com/doi/full/10.1089/ees.2012.0312