Part 3 Installation

Although my floor level being about a meter above ground level, meant that the tank could be above ground – I decided to bury the tank so it was out of the way and less of an eye sore.

Tank Installtion - backfilling

Most plastic tanks are not designed to be buried and will collapse, but if you fill them with water and then backfill the surrounds with a sand/cement mix this hardens into a nice compressive ring that doesn’t collapse (I use a similar method on my rain tanks which are much bigger and deeper and it is working a treat). A word of warning though if you have a high water table and then tanks are empty they do tend to want to float.

The picture shows the backfilling process with the 40mm(3/4”) in and overflow not yet connected. The overflow is connected to the sewer through a standard u trap to keep smelly gases in check.

The pump was then installed with a non-return valve right at the bottom which stops the pump becoming de-primed.

Sump View showing float switch

Also in the photo is the float switch which allows the tank to fill about halfway before it turns the pump on. On reaching empty it then turns the pump off again. It is a simple mechanical switch and is simply wired to the pump circuit in series.

Finished product :

Total cost of project : R 2000 ~ $200 give or take

Finished product

A note on the sprinkler – I decided to keep it simple and I have an outlet in one of the beds with a long length of hose so it can be moved around. The sprinkler is a simple one with a large nozzle which won’t block easily.

Part 2 – Design

Having decided to construct my own grey water irrigation system, I figured he first step was to do some flow measurements of the shower and bath outlets. It was easy enough and it involved a stop watch and a 20 l bucket. The results were as follows:

Bath was on average about 75 litres in total and it emptied at a rate of about 0.71 l/sec

Shower was less at about 0.3 l/sec giving about 60 l for a 3 min shower

Other design considerations were that the system needed to be fully automatic and pretty maintenance free. I also discovered that it is not a good idea to store your grey water as it promotes the growth of pathogens – so ideally the system should empty the sump at least once a day and the water spread over a large area so nature (mostly the sun) can kill the nasties.

Grey Water Design Sketch

My early idea was a 100 l sump which could buffer a bath been emptied with a submersible pump and a filter box of some sort (see sketch).

After a bit of investigation I discovered that submersibles are expensive – tend to clog easily and have seals that detergents and soaps love to attack. So short of spending loads of money on a good quality submersible which could handle the above, I decided an external pump was the way to go.

The smallest one I could find was a Pedrollo PKm 60 which has a nice little brass impeller was simply chops up any hairs (ie ditch the filter). The rated pressure of max 38 m and flow capacity of max 40 l/min (0.66 l/sec) also meant that I should have a nice sprinkler pressure. (Price R1500 ~ $150)

Obviously in practice you will probably operate some in the middle of the figures quoted above, namely as the flow increases the pressure will drop (each pump has its own operating curve but I wont go into that now). I found in practice my system flow matches the shower flow of about 0.3 l/sec , but as a result has an impressive pressure (probably a bit on the high side so I might add another sprinkler head to pull it down on the curve a bit).

Pedrollo PKm 60

To make the system operate automatically I bought a level simple floating level control switch which is very commonly used in pump stations. It allows you to have the pump turn on when the level gets to a chosen height – it also then turns of the pump when the sump is empty (Price R250 ~ $25).

So Armed with a 90 l plastic tank (Price R 150~ $15 ) , pump, a level control switch and a whole bunch of fittings (Price R 100~ $10) – it was time for installation.

Part 1 – The opposition

Part of our renovations philosophy was to try and do some “Green” upgrades where practical and affordable. One of our earliest ideas was to do a grey water recycling system where the waste bath and shower water could be used for watering the garden.

Given that we live in a water scarce country and we do have every few years, quite severe water restrictions, I started wondering why more people didn’t have one already. Although a quick google did turn up some hits in Cape Town there was nothing I could find that was local (I wasn’t overly surprised). So I tried looking at some of the local hardware type suppliers but this proved fruitless, the only luck I had was at a rainwater tank supplier who sold a complete system which looked quite promising.

The system seemed at first glance quite cleaver as all it required was a standard 40mm (3/4”) in and out (overflow) and a plug point for power. The supply from the pump was a standard garden hose fitting. On closer inspection however I found it had the following weak points:

1. The pump was a standard submersible water feature pump – normally pretty reliable but with very poor flow and head (pressure) ratings. Can’t remember the flow but the head was only about 5m max. Also the impellors are normally plastic and prone to blocking.
2. To solve the blockage problem they had incorporated an oversized bottlebrush arrangement filter to keep hair and other nasties out – I read this as “needs regular maintenance” – not good!
3. The unit did have a kind of a sump, but its capacity was only about 2 litres – this means that if the flow exceeds the pumps capacity it simply goes to waste (kind of negates the whole point don’t you think)

Anyway I guess it will be okay for someone in a small townhouse who doesn’t mind a bit of hassle cleaning it out, but I decided I could do better.