Water & Rainwater Tanks

   

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General Overview

Tank Constuction Materials:

Roof surfaces:

The following list of roofing materials most suitable for collecting potable (drinkable) water was suggested in 'Sustainable Water - How to do it & where to get it', (© 1999 Rod Wade, Environmental Conservation Planning & Consultancy Pty Ltd, Oxenford, Qld):

  1. Colorbond® steel sheets or tiles
  2. Zincalume® steel sheets or tiles
  3. Glazed tiles well fired
  4. Concrete/cement tiles
  5. Clay tiles
  6. Correctly treated Fibro - new type only

Calculating the amount of water which can be collected from a roofed area:

  1. Calculate the area of the roof in square metres. Don't take the slope (pitch) of the roof into account.
  2. Multiply this figure by the mm of rainfall fallen
  3. Parramatta tank works suggest that you multiply this number by 0.8 (80%) to take into account losses, like rain that bounces off the roof.
  4. This is the figure in litres.

Links:

 

My Experiences

At present I have 2 tanks situated in the backyard - A 2000 Litre tank which is used for non-potable uses such as gardening, and a 350 litre tank which is used for drinking water. Both tanks were purchased at the same time from City Rainwater Tanks in St Marys. Below is an overview of my experiences with them:

Larger Rainwater Tank:

After some pondering, I decided to purchase an aquaplated steel tank, as this could be manufactured to a diameter to suit the limited space I wanted to fit the tank into, while still providing the maximum possible storage volume. The tank stand was an additional cost, purchased from the same company.

Trap #1 : The stand that was provided was for the larger standard size tank, not my custom size. After installation, I had to call the installers back to cut the stand down to the appropriate size.

The tank has a coarse filter covering the inlet. There is no other filtering provided. Water runoff for this tank comes from 3 roof surfaces: Terra-cotta roof tiles on the house roof, concrete tiles on the garage roof and a fibreglass roof over the pergola. The usable capacity of this tank is around 2000 Litres, while its rainfall collection area is around 77 square metres. See the Water Catchment page for further info.

 

Drinking Water Tank:

The 'Complete Rainwater Collection System' which I purchased is manufactured by Australian Rotomoulding Industries (ARI) located in Pakenham, Victoria. The capacity of the tank is 400 litres, but it has a 50 litre internal 'first flush' diverter installed, so the usable storage is around 350 litres. The purpose of the diverter is to let the rain fall on the roof for a few minutes to flush away any potential 'contaminates' which may have accumulated since the last rainfall.

The unit can be purchased with either wall mount brackets or a tank stand kit. It is supplied with an external carbon filter¹ and tap. For further details see the diagram below.

 

Basic operation (in theory)

Leaves and other large debris are caught in the mesh strainer at the top of the unit. Initially the foam ball is down the bottom of the diverter tube as shown. As the internal diverter tube fills, the foam ball rises until it gets to the top lip where it effectively seals off the potentially 'contaminated' water in the tube. Any further water is diverted into the storage tank through small holes in the side of the inlet pipe. The diverter has a small hole in the bottom of it which allows water to trickle out. Several hours after the rain has stopped, the foam ball should have dropped to the bottom of the diverter, ready to divert the first 50 litres from the next rain event.

This is the theory. Now for the practical:

Problems :

  1. The foam ball in my unit 'sticks' to the top of diverter after the rain has stopped. After speaking to the supplier and manufacturer, I've made a couple of attempts to file down the jagged edges around the lip of the hole that the foam ball is wedged into. This has improved things to a small degree - The ball has fallen of its own accord once or twice, but if not I can now usually dislodge it by thumping the tank. This wouldn't work before. It does require that manual intervention though.
  2. This is partly my fault for not asking enough questions, but when I got the original quote, it wasn't pointed out to me that my roof area was too small for the system to work effectively. I found out afterwards that the minimum recommended roof collection area was 40 square metres. Mine is around 25. As a result, the tank won't fill with light rain. Quite heavy rain is required². As discussed below, 5mm of rain over 1 hour will result in 15 litres of drinking water, whereas 10 mm of rain over the hour will result in 45 litres.On the upside, I can be sure that the roof area is well flushed by the time the tank does start to fill.

    Calculations: Water trickles from the diverter at a rate of about 1 litre per minute. With a catchment area of 25 sq metres, this means that 0.04mm of rain is required to fall every minute before the diverter's foam ball starts rising. This equates to a rainfall rate of 2.4mm per hour. In order to fill the 50 litre diverter, an additional 2mm of rain is required before any goes into the storage tank. When calculated out, 5mm of rain over 1 hour will result in 15 litres of drinking water (2.4mm for water trickling out of diverter, 2mm to fill diverter, and 0.6mm * 25 sq metres = 15 litres diverted into drinking water tank).
  3. The mesh strainer at the top of the unit is not designed to be removed for cleaning. It's screwed down with 3 screws, which are hard to re-align when refitting the strainer. You are meant to scoop any debris caught in the strainer by hand.
  4. According to the instructions, there is meant to be a brass shut-off valve between the water tank and the filter, so that the water can be turned off when changing the filter. This is not fitted on my unit. Without this valve fitted, it is necessary to empty the tank before fitting a new filter cartridge.

Recomendations:

  1. Do the maths before hand.
  2. Check at what rate the diverter empties. Mine is around 1 litre per minute, or 60 litres per hour. I've seen an add for an external diverter with an emptying rate of 4 litres per hour. Depending on the rain catchment area and rain intensity, this could have a large impact on the amount of drinking water obtained.
  3. If I could do things again, I'd probably choose an external diverter with an adjustable emptying rate.

Notes:

  1. The advertising pamphlet from ARI states that the carbon filter is 5 Micron nominal x 9.25" silver impregnated activated carbon. The replacement cartridge I bought from City Rainwater tanks is a Purtex brand, model PX 30-97/8 and rated at 30 Micron.
  2. Rainfall definitions: The Bureau of Meteorology's definitions for constant rain are: light rain < 2mm/hour, moderate rain 2-6 mm/hour and heavy rain as > 6mm/hour. The rainfall rates for showers are different from these.

Last Updated: 27/04/03