Category Archives: Liquids

Prototype Update, 17 Feb 2017

Liquid System Update

Last month we briefly introduced the idea of the “user-day equivalent”. You might be wondering what that means, exactly?

As we develop, deploy, and test our systems, we need a way to compare data we collect in different phases of testing and at different sites in a way that makes sense. In particular, we need a way to compare data that we collect in the laboratory — where we control how much urine and feces are flushed through our system, and measure it all very precisely – and data that we collect from a field site, where we simply log the number of times that users visit the toilet. We have to constantly evaluate (and re-evaluate!) whether the assumptions we make in the laboratory bear out in the real world.

A user-day equivalent is a number we calculate based on an estimate of how much urine and feces the average person produces in a day (roughly 1.5 liters and 130 grams, respectively, if you’re curious). So by adding up how much material we’ve flushed through our laboratory system, we can estimate what that is equivalent to in terms of users (and days) in a working toilet system. So 10 user-day equivalents is an amount of material roughly equal to 1 person using the toilet for 10 days, or 10 people using it for 1 day, or 5 people using it for 2 days… anyway, you get the idea.

When we look at our field site at CEPT University, the math is much easier: we estimate that a typical user visits the toilet 3 times a day. So we just divide the total number of uses logged by 3.

Here’s the fun part: once we do this we can look at data from the lab in North Carolina and the field site and India, and see how they compare. Here are a couple of examples:

Starting on the left, conductivity (you may remember from last time) is a measure of the salts in our process liquid. Most of the salts in our process liquid come from urine. As you can see, the early data points are in really good agreement between the sites, but over time, we see much less conductivity in the liquid from the field site than we do in the lab. The same trend emerges with COD (chemical oxygen demand, right graph). COD is a measure of all the chemical species that consume oxidants, which here are generally nitrogen compounds that again, primarily come from urine.

So what’s going on, here?

Well, the honest answer is that we don’t know – yet. But we do have some ideas. One possibility is that we’ve underestimated the amount of hand wash water that is going into the system at CEPT, which would dilute everything. Another possibility is that our usage assumptions aren’t panning out at this test site: specifically, that users are defecating in the toilet much more often than they are urinating in it. After all, we don’t follow users into the toilet—even we have limits about what we’re willing to do for science! It’s also possible that both of these things are true.

One of the challenges for us in the laboratory going forward is to re-visit our usage assumptions and see if we can reproduce what we see in the field in the lab. Once we are able to do that, we’ll have a better idea about how to improve our system for use in the “real world”.

Prototype Update, 6 Jan 2017

Liquid System Update:

North Carolina, United States

Throughout the month of December, the liquid team focused on testing out the new baffle tank design. The new baffle tank is a great success with data showing it improves effluent qualities surrounding social acceptability. We have also gained valuable information on potential system parameters that may be adjusted to further enhance the electrochemical process and effluent qualities. During the month of January the liquid team will focus on exploring and optimizing these parameters

The graphs presented below highlight the effluent differences between the older baffle design (alpha) and the new one (beta 2.0). From the top graph, we can see it takes nearly twice as much material (on a user-day equivalent basis) for the conductivity (which reflects the accumulation of salts in our recycled process liquid) to reach its plateau in the new system. In the bottom graph, the total suspended solids (TSS), a measurement related to the turbidity, or cloudiness of the effluent, are consistently lower with the new baffle design. Both of these improvements are attributable to the superior design of the new baffle system.

Prototype Update, 1 December 2016

Liquid System Update

North Carolina, United States

A new fully automated Beta 2.0 system was recently installed at the RTI testing laboratory in the US.

This biggest change for the liquid system was a redesign of the baffle tank. The new design helps to direct flow away from the outlet. This new system has been up and running for 65 days and has processed over 10 kg of feces and 100 L of urine.

We will continue to test in this new system to develop and refine processes that allow us to not only disinfect the effluent, but meet social expectations and discharge standards.

The video below shows a flush with green food coloring being followed by a flush with water in the new tank.

Liquid Waste Processing Update

It’s been a bit since we gave an update on the status of our Liquid Waste Processing Unit so we thought we would take some time and put up a quick post about the unit and the recent changes we are evaluating with it.

Electrochemical Treatment of Urine and Feces

First and foremost we’ve started evaluating the use and efficacy of a different type of electrochemical cell in the liquid disinfection system itself. This electrochemical cell is adapted from a commercially available consumer product, which is good for several reasons, chief amongst them: potential lower cost and energy requirements.

During this evaluation, we’ve found the initial test results to be very encouraging! (see below)

The photo on the left shows a tank of urine and feces prior to electrochemical treatment. The photo on the right shows the same after treatment.

Urine and Feces disinfection test results

Plot of E. coli concentration and energy consumption over the course of a treatment test.

The 90 L tank test

Photograph of 90 L processing tank filled with 60 L of fecal contaminated urine.
The 90 L processing tank filled with 60 L of fecal contaminated urine.

We are also in the process of testing a larger 90 L processing tank.

During field trials of the RTI toilet, liquid waste processing will be carried out once a day. This process needs approximately 90 L of waste to complete.

Due to the larger size of this processing tank, tests are currently being carried out to understand how this level of  scaled up processing would impact the overall function our toilet.

Those results will be posted here as soon as they are available!

Disinfection of Fecal-Contaminated Urine (update)

Plated Specimens from Before and After Processing

Plated specimens from before and after electrochemical treatment of fecal-contaminated urine (human)

The first series of tests used 16 liters of human urine mixed with 0.5 %wt of human feces. This represents a significant milestone in demonstrating the liquid disinfection capabilities of the system using actual human-derived specimens. Future tests will focus on evaluating energy consumed during disinfection and further process optimization.

Liquid Processing Team testing with human urine contaminated with human feces

The RTI Liquid Processing Team prepares the system for disinfection tests with human urine and feces.

Collection tank showing fecal-contaminated urine before processing

Collection tanks holding the fecal-contaminated urine before processing.

The baffled collection tanks allow solids to settle and dissolve prior to entering the process module, eliminating the need for prefiltering the liquid before entering the electrochemical cell. Based on preliminary observations, the 0.5% fecal load represents a worst case for fecal contamination in the urine. Future experiments will evaluate nominal and peak-loading scenarios.