Harvesting rain water for toilet flushing

I frequently pee in the garden, with the exception of really rainy days.  During one such downpour, as I expertly aimed into a pool of perfectly potable water in the indoor toilet, I became reflective, as one does at these moments, and stared through the window at the drenched garden. It seemed like such a folly to literally flush drinking water down the toilet.  How hard could it be to collect a little rainwater and to fill the toilet cistern with that?

Possibilities and questions arose in my mind. Could the rainwater be directed into a raised tank? Would gravity be sufficient to deliver it to the cistern? How would I get it through the wall, let alone into the cistern? One step at a time, I told myself.

The first step was to select and install a small water tank.  I chose a capacity of 260 litres, because its easy to carry and fitted neatly into a corner of the garden.  Nine narrow breeze blocks, two broader ones and four cement slabs made a sturdy raised platform.

The issue of how to direct rainwater into the tank (without flooding the garden in heavy rain) was the next thing to address.

Every system of rain water collection I had seen used a large diameter pipe from the overflow connection of the tank to the storm water drains. This system seemed cumbersome. My collection tank was happily nestled at the bottom of the garden, about 25m away from the house and the storm water drain, making it even more cumbersome, costly and unsightly to fit large pipes in both directions.

Could there be a way to fit some sort of device onto the existing down pipe to divert water to the tank until it fills up, and thereafter flow down the existing gutter system, without the need for an overflow pipe?

I bought some 20mm irrigation pipe and I designed a few bits for the 3d printer and printed them using yellow acrylonitrile butadiene styrene  (ABS).  I cut through a down pipe from the gutter, and found flanged collars from a hardware store which fit snugly into it.  Next I bought a small plastic box and cut some circular holes in the bottom of it.

When it rains, water falls out of the severed down pipe into the left of the container in this rendered image.  As it starts to fill, it drains down the 20mm pipe in the centre.  That is a metre or two long and has a removable block at the end.  The first flush of rain, with all the dust and pigeon poop, collects in this pipe, which can be drained by removing the block.

Once that pipe is full, water starts to fill the plastic box and fall into the strainer (the circular filtration device on the left in the diagram), and down the left 20mm pipe.  That goes about 25 metres to the end of the garden, and into the 260 litre tank.

I used a hole saw to make a new hole in the tank, about 150mm from the top, and added some 20mm sealable threaded fittings.  The idea was that as the tank filled up to the level of this inlet point, water would no longer flow along the long 20mm pipe because the level of the water – and therefore the pressure – would be equal.

When the tank is full and the water level equal, water will fill the plastic box. As is visible in the rendered diagram, this ‘overflow’ will flow down the remainder of the down pipe and directly into the normal storm water drainage.

All that was needed was a way to mount the plastic box on the wall at exactly the right height, a little higher than the hole in the tank.  The aim was to have the top of the large pipe at the level of the water in the rain water tank when the tank is completely full.

For that purpose I designed and printed two brackets which had several protrusions which fitted neatly into the lip of the plastic box.  Two of these could be screwed into the wall, and then the box slotted in at various heights.

A 45 degree bend on the top section of down pipe directs the water to the side of the plastic box, through a mesh which keeps large leaves and bits out of the plastic box.

The first time it rained I inspected the system.  I found that the box needed to be considerably higher than expected in order to get a satisfactory flow along the long 20mm pipe – perhaps I had misjudged the slight slope of the garden. I drilled new holes and moved the bracket a bit higher up the wall, cutting a bit off the down pipe to fit.

During the next rainstorm I rushed out and excitedly found that the system worked.  Whenever it rained enough, I had a full tank of strained rainwater. I was well on my way to saving drinking water for drinking and rain water for peeing into!

Coming next is to work out how to feed water from the tank into the toilet cistern…