Sunday, March 25, 2018

Design - Rain Harvesting for Toilet Flushing

We looked for almost five years for a suitable location for our energy-neutral passive solar home. Our search was confined to rural areas because it never dawned on us that we would find the perfect site eight blocks from Main Street in sight of the St Louis Gateway Arch (that is, if there were no trees or houses blocking our sight-line). In those early days, I researched water conservation, including recycling graywater and harvesting rainwater, for the day when we would have well water and a septic tank.  (I am still the proud owner of  the book, The Septic System Owner's Manual, which wasn't exactly on the New York Times Bestseller List).   I even researched composting toilets.

Once we found the Collinsville site with its access to cheap "city" water and sewer, I lost interest in all but the usual water conservation practices such as low-flow faucets, shower heads and toilets, a front-loading washing machine and landscaping with native plants that do not need watering.  After all, there is not a lot of incentive for water conservation here in the Mississippi River bluffs, eight miles from the mighty river and only a few miles below its confluence with two other major rivers -- the Illinois and the Missouri.  As a matter of fact, our regional problem is too much water from major floods, never too little. 

An Awakening
Michael Webber's recent book, Thirst For Power -- Energy, Water, and Human Survival, has radically changed my thinking.  He makes the point, for example, that the amount of energy required to provide water and sewer service to the average household exceeds the energy the household uses from the grid for lighting, heating and operating stuff plugged into receptacles.  Here's why.  

It takes energy to pump our water from a deep well on the Mississippi River floodplain to the purification plant.  It takes energy to purify the water and lift it to our homes on the hills above the floodplain. This says nothing about the energy embodied in the manufacture of the materials that go into building a purification plant, the chemicals used for purification, and the maintenance of and upgrades to the treatment plant and the aging water pipes.

Energy inputs do not stop with potable water.  After it goes down the drain, it takes energy to treat it at the waste treatment plant before it is released back into the environment and for infrastructure maintenance and upgrades to our century-old sewer system.

So, thanks to Webber, I  now equate water with energy.  Flushing our toilets with rainwater would be on a par with having a photo-voltaic array. Unfortunately, the Collinsville Public Works Director didn't see it that way; when our design was sketched to him, he disallowed it but said he would kick it upstairs to Illinois State officials and we have heard nothing since.  Rather than pursuing the issue, we are keeping it on hold as an easy future add-on if circumstances permit.  

Meanwhile, I would like to share our design in case there are viewers who are researching  rain harvesting or haven't yet considered it.  Perhaps there is something in it that might be helpful.

System Design
Rainwater Catchment
Fortunately, the 1,400 sq ft roof area of the second story section of the house tilts 
Side view of the second story roof tilted northward

northward towards the concrete back wall that has a short stick-built wall on top of it. I had planned to locate an indoor cistern high inside the storage area (that we sometimes call our "vertical basement") located at the back of the house adjacent to the wall.  The run-off from the roof would be collected by the gutter and directed through the stick-built top portion of the wall into the cistern. A couple of websites provided what I needed to be sure that such a catchment system would yield enough water to flush two toilets year-round. 

The first helpful website was the Texas A & M site, which not only provides generic information on rainwater harvesting but also links to a handy calculator.  The calculator uses the average monthly rainfall, the size of the catchment area and the average monthly water consumption to calculate the size of the storage tank (cistern) necessary to meet demand.  All I needed then was the average monthly water consumption for flushing.

The second website provided a handy water use calculator for estimating the amount of water a household consumes per function -- such as showering, dishwashing and lawn irrigating.  With it, I was able to determine that the amount of water we would need for flushing, as a percentage of our total water consumption, would be 27% or 10,000 gal/year or +/-800 gal/mo.  Plugging this figure into the A & M calculator told me that, because our rainfall is fairly consistent month-to-month (varying only between 2.14" in January to 4.11" May), a +/-500 gal cistern should meet demand well enough that supplementation by city water would be necessary only under special circumstances such as the drought we had in 2012. 

Click on the image for better visualization of the storage area
against the back (north) wall of the house
The cistern would comprise a heavy gauge translucent plastic tank that can be found online and at local farm and home stores for around $500.  We would need a rectangular model that will be narrow enough to pass through our 36" doors.  It would need to have fittings for 4" intake and overflow pipes.  Mounting the cistern high (+/- 12') in the storage area would offer the option of directing any overflow back outside to a swale rather than dumping it into the sewer.

In order to avoid any possibility of cross-contamination between potable water and the water in the tank when city water was added, I would use a dedicated cold water pipe that did not attach directly to the tank -- it would be suspended a safe distance above the tank.  When turned on back at the cold water manifold, the water would drop through a large funnel DIYed into the tank top.  Alternatively, I could wait until the need for supplementation occurred then send water through the gutter using an exterior sillcock and garden hose.  An upgrade to the interior approach could follow if necessary.

The cistern would be plenty high to gravity-feed the toilets between flushes but undoubtedly slower than would a connection to pressurized potable water, which for us would be only a minor inconvenience if any.

The water coming off of the roof would need some filtration although less for our metal roof than for shingled roofs that shed the stone granules found in gutters. Debris from trees should not be a big problem because there will be no trees close by because the subsoil north of the house is largely glacial till (hardpan) which does not allow root penetration.  Presumably therefore a pool-type of inline filter between the gutter and the tank, perhaps preceded by a screen, should suffice. (I would have to flesh out the requirements for filtration as the time for installing the system approached.)  In drier climates with long intervals between rains, a "First Flush" approach is used whereby the first rain of the season or the first rain after a dry spell is used to clean the catchment surface before harvesting begins (as I learned from The Greywater Action site). Our rains are frequent enough that first flushing would probably be unnecessary.

Our toilets would have to be on dedicated supply lines or on one dedicated line supplying both toilets.  Otherwise, there would be no way to send rainwater to them without contaminating the potable water system.  As described in prior post, I installed all of the hot and cold water lines for the entire house as home runs, meaning all faucets, all appliances and both toilets have dedicated lines with individual manifold-level cut-offs on the storage room wall next to the water line from the street and the water heater. Without a home-run system, a rainwater system as a later add-on would be impractical.  In our case, it would be a simple matter to dedicate to toilet flushing one small manifold with two lines.  If future residents wished to deactivate the rainwater system, the toilet lines could be disconnected from the manifold, decontaminated and connected to a potable water manifold.  In a worst case scenario, the contaminated lines could be retrieved and replaced since they are encased in PVC conduits (prior post).

Return on Investment
A rainwater system would cost approximately $500 for the tank plus some change for PVC pipes, say, a total of $600.  If toilet flushing represents 27% of our water withdrawals or 27% of our yearly water bill, we would save approximately $54 per year. At that rate, the return on investment will take +/-10 years which we feel is no big deal one way or the other when compared to the golden opportunity to strike an easy blow for sustainability indefinitely.

Thermal Mass
The Annualized GeoSolar system for conditioning our house, that has been covered extensively in dozens of prior posts and quickly accessed by clicking on "Featured Post" in the left column above, depends on the thermal mass of the earth under and around the house and, to a lesser extent, the concrete floor and earth contact concrete walls. Since water is an even better heat sink than earth and concrete, a cistern full of water inside the house would somewhat augment the passive solar conditioning.

Potential Problems
If the water in the tank was much colder than room temperature, the tank would sweat. This might happen after cold rains, during snow-melts or in the unlikely event the cistern is supplemented with a large amount of ground temperature city water.  At this early stage of planning, I would be content to wait and see if the problem exists then install sheet plastic to catch the condensation and a plastic tube to carry it to a nearby floor drain.

Another potential problem might be algae buildup that is common for stagnant water exposed to the environment.  However, the tank would reside in a space without windows and without artificial lighting most of the time so photosynthesis may be minimal if not impossible.  In a worst case scenario, it might be necessary to add vinegar to the tank occasionally.  While a dark-colored tank controls algae outdoors, it would be redundant in the dark storage room; a translucent tank would be better at revealing any buildup.

Still another potential problem is mosquito breeding. The rain barrel on our current house has no openings to the outside similar to the one in the nearby photo; the water from the downspout goes directly into the tank and mosquitoes do not negotiate the downspout in order to breed in the tank.  Our rain harvesting gutter would have a downspout located in the middle of the roof to drain both halves equally then it would extend horizontally quite a ways to reach the cistern located in the storage space near the east wall of the house. The long horizontal pipe would be sloped excessively to eliminate any chance of any puddling so that its length and lack of standing water would discourage mosquitoes.

Zoning Issues
Plumbing codes for graywater, especially graywater for reuse indoors, are ambiguous and inconsistent.  According to Webber, the International Plumbing Code allows toilet flushing with graywater from showers and bathtubs but the Uniform Plumbing Code that is used more widely in the US does not.  Yet the The Greywater Action site shows the inconsistency of regulation between states and says the International code is even less helpful than the Uniform code.  So go figure.

When it comes to rainwater harvesting, regulation seems to be much more lax even with regard to using rainwater inside a dwelling.  The Greywater Action site on rainwater harvesting provides a link to Laws, Rules and Codes for each individual state.  Of the three items listed for Illinois, one leads to the full text of  HB1585 that merely defines rainwater harvesting and specifies that systems must be constructed in accordance with the Illinois Plumbing Code.   Our Director therefore seems to have a lot of discretion but remains cautious by not allowing for now toilet flushing with rainwater in Collinsville. 

An additional issue that came up in discourse with the Director was that the sewer fee the city collects is pegged to the water meter readings.  He was afraid that the reduction in water consumption due to rainwater flushing would automatically under-report our discharge into the sewer.  He did, however, seem open-minded about our continuing to pay the same sewer fee as we were before rainwater harvesting, or some other amount, that arbitrarily compensated for the discrepancy.

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