Wednesday, June 16, 2010

85 Year Payback for Cistern

We have been working on the One Planet Reno water principle in order to ready the non-potable (meaning not drinking water) water system for the building permit.  The Building Code allows us to use rainwater for toilet flushing, but also has provisions to ensure we don’t contaminate the city's drinking water from our cistern. We also need to identify the pipes as carrying non-potable water used only for sewage conveyance.

The easist way to avoid back contamination is to not connect the systems.  However, if the cistern is dry – we still want to flush the toilets (the odour is already often a serious issue in the can, not being able to flush could be a marriage breaker), so we need a way to have potable water as back up.  You could do this by filling the cistern part way with potable water if it gets too low, but that would take capacity away when the rain does come.  In Ottawa, most of our rain comes in the summer and it can come in short monsoon type events.  Therefore we really want all the capacity available.  The other alternative is a series of valves that protects the potable connection from any back contamination from the non-potable water.  See below for a schematic borrowed from Rainwater Management Solutions that is basically the system we propose to use.  We will post the final design schematic and specs later.

These important items add cost.  The current estimate for this non-potable water system is around $7,500.  Considering the price of water in Ottawa is $2.76 per cubic meter (including the sewer surcharge) we would need to save 2,717 cubic meters of water for this system to pay itself back.  Our toilet flushing will require somewhere less than 32 cubic meters per year  - so after 85 years of toilet flushing we will break even (either that or we need to start using the can a lot more to justify this investment).  The problem in this equation is not the cost of the non-potable system, but how incredibly cheap potable water is in Canada relative to its global scarcity.

We have also posted the calculations to show you how we arrived at the cistern size and tested to see if we could capture enough water in the summer to carry us through the winter.

We have been involved with several commercial and multi-unit residential projects that have done non-potable water, but the city of Ottawa official I spoke with did not think any residences had such a system.  We will see what they think of the design in the coming weeks.  


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