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Sanctuary sink to toilet greywater system

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This page is a project page in progress by students in Engr215. Please refrain from making edits unless you are a member of the project team, but feel free to make comments using the discussion tab. Check back for the finished version on 5/9/2016.

Humboldt State University's Engineering 215 Introduction to Design, introduces the Toilet to Sink Freshwater System created by Team Greenlight at the Sanctuary in Arcata, California. The image below is the completed system in the men's bathroom.

Toilet to Sink Freshwater System in the men's bathroom located in the Sanctuary


The toilet to sink freshwater system's objective is to provide The Sanctuary with a more sustainable and inspiring bathroom. Fresh water is redirected from the toilet tank to the sink to use for hand washing, this reduces the total water output used in the restroom. The greywater that flows down the sink drain after hand washing, flows to the toilet bowl used for flushing waste down the toilet to the sewer. The sink and toilet are placed next to each other, making the design unique compared to the more common sink-toilet systems, where the sink replaces the lid on the back of the toilet. This toilet to sink freshwater system was implemented by Team Greenlight from Humboldt State University’s Engineering 215 Introduction to Design class, taught by Lonny Grafman, and through funding provided by The Sanctuary and HSU student’s working on the design. The background and next steps will also be discussed on this page.


The toilet to sink freshwater system is a system implemented by Team Greenlight in the men's bathroom for their client, the Sanctuary located in Arcata, California. Team Greenlight includes Chris Alvaro, Jocelyn Barber, Margaret Koval, and Gilbert Ordaz. The Sanctuary is a non-profit organization that promotes green technology, art, community involvement and sustainable living by the reuse of every day resources and items. The Sanctuary's objective was to remodel a bathroom that would reduce freshwater usage while at the same time inspiring the user to continue reducing their own personal freshwater usage outside of the Sanctuary. The toilet to sink freshwater system is a circular system as the fresh water flows from the toilet to the sink, and then greywater flows from the sink to the toilet.

The start of this circular system is implemented by flushing the toilet.  The flush lifts the flapper, draining the tank water into the toilet bowl.  The water supply line from the wall to the toilet, which is Arcata’s city water, then fills the fill valve located in the toilet tank.  The fill valve then sends water to the inlet tube, where it then flows into the PVC vinyl tube, into the check valve connection, and out of the faucet.   The water from the faucet then flows down the sink drain into the PVC pipe.  The water flows through the PVC pipe and out of the hooked pipe connection into the overflow tube.  The water flows through the overflow tube and through the rim holes on the toilet bowl.  The water then flows out of the rim holes into the toilet bowl where it will stay for the next person to use with the next flush.  

There will always be grey water in the toilet bowl, while freshwater stays in the back of the toilet tank.   The tank water is filled with the city water through the water supply line from the wall. The tank consistently has the clean city water in the back of the tank. The sink will consistently get the clean city water either through the inlet valve, which comes from the tank’s water supply, or the hot water supply line that is connected to the sink. The toilet bowl is always going to have greywater in it which comes from the water that flowed out of the sink drain into the overflow tube. When the toilet is flushed, the greywater in the toilet bowl goes down the toilet drain into the sewer with the human waste. The toilet tank’s water also is rinsing the toilet bowl during the flush as that water goes down the flapper drain in the back of the tank, out into the bowl through the rim holes, and down the toilet drain to the sewer. When the flapper closes again and the city water refills in the back of the toilet tank as the float ball rises with the city water, the greywater from the overflow tube then flows back into the bowl to remain there until the next person uses it.

Problem statement and criteria

The Sanctuary wanted to improve the previous bathroom by reducing freshwater usage and at the same time inspire the users to reduce freshwater usage outside the restroom.


The criteria listed below in table 1 includes constraints for the final design criteria. This helped in deciding solutions for the design by eliminating the ones that didn't fit the criteria.

Table-1: Criteria and Constraints
Criteria Importance
Safety The design must work without posing any safety hazards to the user
Inspiration The design must inspire users to reduce clean water usage through the use of signage and art.
Cost The team’s maximum budget for the project is $100
Environmental Impact The new toilet and design should cause a reduction in the water bill.
Ease of Access The design must be operable by the average nine-year-old child which is the minimum age group the client expects to have.
Aesthetics The design must look better than the previously existing toilet.
Ease of Plumbing Maintenance Any clogs or leakages in the design must be repairable by the average person with limited plumbing knowledge in less than 20 minutes per year.

Description of final project

Projects gallery
Figure 1: Design before stall wall is removed  
Figure 2: Full design view with visible PVC pipes  
Figure 3: polyvinyl chloride (PVC) piping  
Figure 4: Vinyl tubing attached to Overflow fill tube in the toilet tank  
Figure 5: Vinyl tubing attached to sinks water supply  

Figure 1 shows what the restroom looked like before the remodeling process began. The toilet to sink freshwater system shown in figure 2 was designed to save the amount of water used when washing your hands after using the restroom. The design uses polyvinyl chloride (PVC) piping, shown in figure 2 and figure 3, to reroute the water's drainage onto the overflow tube so that the used soapy water flows directly into the toilet bowl. The design works by attaching the toilet tank's overflow fill tube shown in figure 4 with the sink's hot water supply line using vinyl tubing shown in figure 5. The sink's water runs approximately 40 seconds when the toilet is flushed giving double the amount of time than the recommended 20 seconds according to the Centers for Disease Control and Prevention (CDC). What makes this unique to the standard toilet to sink combinations is that the sink and toilet are placed like any ordinary bathroom would have them rather than replacing the top of the tank with a sink.

How to use

The sink to toilet design begins on the initial flushing of the toilet. Like any toilet, the flapper is lifted and the water travels through the flush valve and into the toilet bowl. In any ordinary flush, the overflow inlet tube is placed directly above the overflow tube but because it is redirected the sink’s water faucet, the freshwater is used to wash your hands for 40 seconds . Finally, the water travels through the PVC pipes which flows directly into the overflow tube, ending with a toilet bowl filled with grey water from hand washing. The video below shows how the design works.


The tools and materials used for the whole design is shown in Table 2.

Table-2: Materials and Costs
Quantity Material Location Full Price ($)
5 PVC Plumbing Ace Hardware 9.90
1 PVC Tubing Ace Hardware 2.00
1 Nuts/Bolts ACE Hardware 4.00
1 Masonry Sawzall blade ACE Hardware 30.00
1 Masonry Bosch blade ACE Hardware 10.00
1 Masonry angle grinder blade ACE Hardware 10.00
1 Sink Drain ACE Hardware 17.00
1 Angle Grinder Donated from Sanctuary 0.00
1 Hack Saw Donated from Sanctuary 0.00
1 Miscellaneous Tools Donated from Sanctuary 0.00
Total Cost $82.60

Testing Results

The results proved users had trouble knowing what was going on. The testing proved there had to be signage and other adjustments to explain how the design worked and make the use of it a little simpler. The problem was users would shut off the hot water valve which cut off the flow of water to the toilet bowl defeating the purpose of the design. To fix this the hot water valve was purposely stuck in the on position so it was not able to be turned off and signage was placed above the sink to inform the user. Also, because many people did not know how a toilet worked when flushed cutouts, shown in the how to build section, were made to help users better understand the process of how the water flows.

How to build

Shut off the toilet and sink from the water supply line located underneath and attached to the wall. Next replace the tiny fill tube leading to the overflow pipe located in the tank of the toilet with a vinyl tubing. This vinyl tubing is to be connected under the sink where the hot water line would normally be connected. Some clamps may be necessary to ensure the tubing is watertight. Once this is accomplished, part of the toilets fresh water should now flow through the sink when the toilet is flushed, meanwhile, the main fill valve fills the toilet tank as any toilet would. Finally, to reuse the water dispersed from the sink's faucet, a series of PVC pipes are arranged with a sufficient drop to allow the water to flow back towards the toilet tank and into the overflow tube as originally designed and end up in the toilet bowl.

Building the Sink to Toilet
Step 1: Turn off the toilet and sinks' water supply and test the water to ensure the water is no longer flowing. Next replace the overflow fill tube with a vinyl tubing that connects from the toilet onto the sinks water supply and use clamps to make it watertight.
Water supply  
vinyl tubing replacing the toilet tanks overflow fill tube.  
vinyl tubing attached to sink water supply  
Step 2: Using an angle grinder donated by the sanctuary, a square cutting is made in the back of the toilet tank. Safety masks are worn to avoid damaging the lungs from grinding the porcelain with the masonry saw blade. The square cutting is measured just enough so that the PVC piping fits without having a lifted toilet tank lid.
Grinding the toilet tank outside  
The finished square cutting  
Step 3: The next step is to rearrange the sink's drainage pipes, in this case PVC, in order to direct the sink waters' flow onto the overflow tube located in the toilet tank. This allows the used soapy grey water to flow directly into the toilet bowl.
PVC pipes rearranged  
PVC pipe in toilet tank's overflow tube  
Step 4: Finally, the aesthetics of the design may be applied. In this case, a mural of Humboldt bay and the local rivers are painted. Plywood is then primed, cut out, and painted to depict and give visual on how the design works
Close up of mural  
Entire mural  
Toilet and sink depictions of the design.  


The design may require maintenance like any other household appliance. In order to maintain this design any clogs or harmful chemicals must be accounted for. In order to get this message across, signs are hanging on the wall that say not to pour down any hazardous chemicals through the sink. Furthermore, there are two filters in case someone pours down food or any large particles that could potentially clog the pipes. Cleaning products such as vinegar should be poured down every week to prevent any cavities from building up inside the PVC piping. Finally, if a leak were to occur, the PVC pipes may easily be removed if there should ever be a clog and can either be cleaned or replaced at a relatively low price usually around $15.00.


Maintenance Schedule

  • Must make sure that no harmful chemicals are poured down the sink and that the water knob is turned on at all times
  • In order to prevent any clogging, vinegar is poured down every week to clean out any cavity formation in the PVC piping
  • Buy cleaning supplies keep the sink and toilet from looking old
Every _5_ years

replace the signage and art on the walls as paints may be fading at this time


This is only how to troubleshoot basic operation. For complex issues, research common restroom plumbing solutions online.

Problem Suggestion
No water flows through the overflow tube Check to see if the PVC pipes have any clogs.
Water doesn't run when flushed Ensure that the water handle is turned on.
Leaking pipes Replace pipes or make sure that the pipes are properly connected.

Discussion and next steps



“Centers for Disease Control and Prevention.” (2015). Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, <> (Dec. 8, 2016).

Klee, E., A., and C. (2014). “CCAT Wash n Flush .” Appropedia , Appropedia, <> (Sep. 28, 2016).

Knight, L. (n.d.). “2.972 How A Toilet Works.” 2.972 How A Toilet Works, <> (Sep. 28, 2016).

“Refinish a Sink .” (2013). Do-It-Yourself , , <> (Sep. 28, 2016) }}