Top down and bottom up: two ideas often discussed in biology. Specifically, the idea that an organisms population is controlled by both bottom up controls (things like the amount of food around, temperature, freshwater availability) and top down controls (i.e., predation). But today, I’ll be thinking about ‘top down’ and ‘bottom up’ in a widely different capacity. Mainly, as the emerging pressures on wastewater treatment facilities. I realize this is a bit of a jump, but hear me out…

Let’s think about the water cycle and the water table for a little bit and the pressures the two largest reservoirs of water pose on the land surface. 1. Precipitation from the atmosphere (aka: top down) and 2. Groundwater and saltwater intrusion from below (aka: bottom up). So when I say ‘top down’ and ‘bottom up’ for this post, I literally mean from the top (rain) and from the bottom (groundwater).

The various ‘top down’ (precipitation) and ‘bottom up’ (rising groundwater levels and rising sea level) controls on wastewater treatment plants.

Got it? Okay, good. Now let’s put a wastewater treatment facility on our land. We all use the bathroom, right, and that waste is flushed down the toilet (or down the sink or shower drain) and has to go somewhere in order to be treated and released back into the environment – which is where wastewater treatment facilities come in. And its important to remember these facilities are part of the larger hydrologic system and vulnerable to changes in the water cycle.

What do you think happens to wastewater treatment facilities when it rains? Not only does water from homes and businesses makes it’s way to these facilities, but all that runoff from the street (re: storm drains) goes to wastewater treatment facilities too. Which means when it rains, it pours for wastewater treatment facilities. And normally, these facilities are well designed to handle storm events meaning the occasional summer thunderstorm or dreary, winter weeks pose absolutely no problem to these systems. But what happens to these systems when it rains and rains (and rains and rains)? Think events like Hurricane Matthew that hit South Carolina and North Carolina in October 2016 or even more recently Hurricane Harvey and the Houston area. When something like these events occurs, all bets are off, and typically, due to either too much volume coming through the pipes or rising flood waters, wastewater treatment plant facilities become overwhelmed and raw sewage is accidentally released to the environment.

Not a great scenario, and a scenario that is expected to become more and more common in the predicted warmer and more extreme world. Two recent studies indicated that the rainfall from Hurricane Harvey was intensified by changing climate and numerous reports and studies have indicated a recent increase in extreme precipitation events associated with climate change. And a rise in extreme precipitation events means an increase in accidental raw sewage discharge in response to these  events. In other words, the top down controls (i.e., precipitation) on wastewater treatment plants are only expected to intensify in coming years.

Flooding in Port Arthur, TX following Hurricane Harvey. You can bet if these houses are flooded, any wastewater treatment plants in the area are also flooded. Image from here.

Now let’s think about the bottom-up controls on wastewater treatment plants, mainly a rising groundwater table and encroaching sea level and saltwater intrusion. These risks are especially pertinent to systems located in low-lying areas near large water bodies. And where do you think wastewater treatment plants are typically located (hint: what’s the easiest way to get water to move to a central location)? Yup, you probably guessed it: wastewater treatment plants are largely located in downhill locations (where the least amount of energy is needed to move all that wastewater) and near large bodies of water (where the wastewater treatment plant effluent can be easily discharged). And, unfortunately, these are the prime locations for issues associated with a rising groundwater table, encroaching sea level, and saltwater intrusion. Meaning that as water levels rise from below, wastewater treatment plants may be easily overwhelmed, resulting in a literal, bottom up control.

As you can probably see, in a stormier, higher sea level world, the current wastewater infrastructure is highly vulnerable. A recent study found 27.8 million people would have their wastewater system impacted with a 6 foot rise in sea level (and this doesn’t even come close to including the millions of people who would be impacted by extreme precipitation events that also overwhelm these systems). In other words, it’s not looking so good for our current system, meaning that changes are needed as both an increase in capacity to handle the increase in extreme precipitation events as well as moving systems to higher ground in response to sea level rise. It’s a tall order, but these changes will be necessary to ensure the viability and sustainability of our wastewater treatment systems in the future (because, nobody wants raw sewage in our streams and waterways…).

Inspiration drawn from the following articles:

Sea Level Rise May Swamp Many Coastal U.S. Sewage Plants

Scientists Link Hurricane Harvey’s Record Rainfall to Climate Change