We temporarily shut off our demonstration system on Thursday 8.4.2016 due to high organics in the wash water as well as this water smelling of soap.
Shown in the photo the right most the the soap laden wash water, the center is concentrated greywater and the left is concentrated reusable blackwater. Clear water alone is not an acceptable indicator of safe water for reuse. At this point our treated wash water had an acceptable turbidity of less than 1.5 NTU, conductivity of 1 mS and an ultraviolet transmittance of greater than 90%. The unacceptable traits were that the water smelled very floral/soapy and had a dissolved organic carbon concentration of over 100 mg-C/L! The layer of bubbles on the wash water was only present when the water was vigorously agitated.
Root of the problem
The bubbles in the wash water were originating from soap that was able to pass through our nanofiltration membrane. To be treated the water has to pass through pores in our membrane than are less than 1000 Da in size (about 2 nanometers), soap when not aggregated with other soap molecules (in a micelle) are much smaller and will pass through the membrane. If your interested in learning more about soap check out this website where the photo below originates from. Below the figure illustrates soaps interaction with the air water interface, as separate molecules in the water and as aggregates (micelles).
Learning from YKHC
In April at the AWWMA conference, we heard YKHC discuss their issues with bubbles. As they were using treatment methods that utilized aeration they ended up having bubbles overflow their tanks! Yikes! While they found a way to retain the bubbles in the system, their issue provided us our treatment technique. We would create bubbles in a way that we could remove them and thus removing the excess soap physically from our system.
Photo provided by YKHC.
Finding our solution
Now the fun… I mean research began. How were we to make bubbles get rid of the soap? From here we looked to a concept in water treatment called dissolved air floatation but sadly this technique is not commonly applied at this scale. A similar concept of generating bubbles to remove dissolved constituents (primarily proteins) is common in maintaining salt water fish tanks. The utilized technology is called a protein skimmer that draws air into the impeller of a needlewheel pump, bashes the bubbles into smaller bubbles and then separates the bubbles from the water. The proteins stick to the bubbles and are removed from the water and the water is returned to the tank.
A quick trip down to Alaska Coral and Fish to buy a protein skimmer and a rubbermaid bin allowed us to test our idea. We took water from our concentrated greywater tank and subjected it to the protein skimmer. Check out the video!
It was amazing to watch the skimmer produce the amount of bubbles that it did. It took about 3 hours for the skimmer to create and remove all of the soap in the form of bubbles.
Implementing our solution
With successful demonstration we proceeded to implement our solution. Below is a photo of our implemented columns to capture created bubbles and remove them from the system. One bubble column is attached to our reusable black water tank and another to the greywater tank, the center column is the drop tube for the bubbles to fall into the keg below for disposal.
Testing this system out we were able to create a nice stream of bubbles that were easily collected. The stream of bubbles can be seen in the photo below falling out of the grey PVC fitting at the top of the column.
Restarting our system
We are looking forward to Monday August 22nd to restart our system. This restart will utilize rain water collected from the roof of our cabin and test a new set of membranes that are chlorine tolerant.
Stay tuned for updates and thanks for your interest in our project! -Aaron