Sorry that it’s been so long since we’ve sent an update!
Things have been quite busy at work. I was going to write an update a couple of
weeks ago, but then our water tanks started exploding… let me explain.
One of our main projects over the past few months has been
quantifying the impact that first flush systems have on water quality for
rainwater collection systems.
The concept of first flush systems is simple: they flush the first amount of rain during a storm, essentially washing the roof and gutters before water starts collecting in the storage tank. There’s lots of research that compares the dirtiness of the first flush water compared to the tank water, but very little research showing the long-term impact to the quality of the tank water. We’re trying to answer the question: are first flush systems worth the time, effort, and cost? EMI installs them on all of our rainwater harvesting systems, but do they really make a difference?
I’ll talk about more of our process in a later update, but a key part of our project is installing first flush systems in existing systems, paired with systems that don’t have first flush. By directly comparing those pairs, we hope to understand the impact that the first flush is having on water quality. So, we’ve been retrofitting a bunch of systems with the simplest, cheapest system we could come up with:
When rain starts to fall, it will initially just fill the first flush tank. When that gets full, it fills the standpipe, and then water can start to enter the main collection tank, as shown below.
Anyone with basic hydraulics knowledge might already be seeing the potential issue with this design. As the water fills that standpipe, it results in significant pressure on the tank. This is a principle called Pascal’s paradox – the pressure in an open-air hydraulic system, like this one, is set by the height of the fluid. It doesn’t matter that the standpipe is a fraction of the size of the tank – that small amount of water in the pipe creates significant pressure in the tank. In the small tank of the system in the picture above, it might be ok. But on our larger tanks, with 500mm diameter lids, a 2-meter standpipe results in almost 400kg (880lbs) of force pushing up on the lid of the tank. And, it turns out, our tanks were not designed to handle that force.
We discovered the issue after the first major storm, after we had installed most of the systems. Most of the tanks just leaked, but a couple of them built up enough pressure to burst the lids. Nothing too serious, just water spraying out of the large cracks in the lid. But enough to severely impact the systems and prevent most of the water from getting to the storage tanks
And so, the past 2+ weeks have been a bit of a mad scramble of testing new tank configurations, figuring out how to reinforce our existing systems, and applying those changes to the existing systems. Every tank configuration has needed different combinations of fixes, so it’s been a lot of testing, troubleshooting, and problem solving – basically just a normal few weeks in the R&D department of EMI.
But, even as we’ve worked through all of that, we’ve started collecting data from all of the working systems! I’ll be writing more about that soon. We are aiming to write weekly updates like this through the end of the year, with the goal of giving you a better idea of what our day to day and week to week life looks like here, so be on the lookout for those emails!
We are so grateful for each of you who partner with us financially — your generosity is what makes this work possible. If you’d like to join us in supporting EMI’s mission here in Uganda, we’d love to have you on the team!