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Living with Tanked Water

We have been living with rainfall collecting tanked water for 5 months now (Nov 2011) and I have not checked or done anything with the system. One tap in the kitchen has an in-line filter, but again I have not looked at it. A few days ago we both came down with a stomach upset and although I am pretty sure it was from undercooked frozen beef, the alarm bells started ringing about our water.

Amongst the house plans passed over to us, is a schematic for the rain collection system but no detail. Checking the outside of the 2 tanks, I could find no capacity label just AHI on the man hole cover. Out with the tape, Diameter 3.5 metres, height to top of dome 2.6 metres, height of maximum water level above outlet pipe to pump is 2.1 metres.

Update spring 2012: I changed the filter on the tap. The new one is 0.6 micron so very few nasties will get through. I advise visitors to use that tap for drinking water, but I use any tap in the house, and I am still here.

The usual quote about tanked water is “just think of all the nasties that land on your roof”, but just think of all the stuff that enters the local reservoir!!

Foot note: The UV in sunlight (when shining) will quickly kill any bacteria and viruses on your roof. The greater danger is water pooling in your gutters

This article is really only for tanked water from your roof, but borehole water does get a mention although it's is a different kettle of fish. Borehole water can contain all sorts of chemicals and bacteria from soil run-off

Calculating Tank Capacity

If you do not remember your school boy/girl maths,

Area of a circle is pie * diameter squared, divided by 4. (pie*D2/4) in metres

The volume is area times height, the answer is in cubic metres. (pie*D2/4 * h)

One cubic metre = 1000 litres.

So from my measurements the theoretical capacity of our tanks is 25000 litres each which ties in with standard tank capacity in NZ. The actual usable capacity is 20000 litres per tank (the outlet to the pump is 500mm from the base), giving us 40,000 litres when the tanks are full.

Water usage per person per day in New Zealand

Kiwi average water usage per day:

Outdoor 30 litres

Toilet 35

Shower and Bath 50

Laundry 29

Dishwashing 13

Total 157 litres of water per person per day

How much water will we collect when it rains

From the plans our roof area is 311 metres squared, so in theory from each millimetre of rainfall we should get 311 litres to our tanks, that's in theory!!

Drinking Water Quality

This gets heavy, So I will skirt over this but include a link to a MOH NZ document:$File/05-microbiologicalquality.pdf

Power Cuts

Power cuts for most tanked water users means no pump, no water (some farmers pump up to a hill top tank and use gravity), therefore contingency plans are required. The simplest is to have a large water container in the garage or just drink wine or beer for the duration. Water in a container will last at least a year if a few drops of bleach are added. The alternatives are a generator or a large UPS (uninterruptable power supply), or a battery with inverter on a solar trickle charger. At the moment I only have the water container, power interruptions have been only about 3 hours long in the evening. Update, I forgot that I have two outside taps below the tanks water level and they will supply water through the pump due to gravity

Maintaining water quality in Tanked Water

This is one area I do have some experience. One of my past projects was to supply potable water to a 60 acre site supplying 4 food factories and offices. The water was supplied from bore holes and because the water had been contaminated with sulphurous compounds (smelt like Rotorua) we had a lot of kit prior to final distribution. The final check on water quality was to add 0.3 parts per million (ppm) of sodium hydroxide, the main active ingredient in household bleach. Mains water is generally just dosed with sodium hydroxide.

There are many things you can do to improve water quality, but firstly I will elaborate on adding sodium hydroxide (bleach) to your tanks to kill of the nasties, because its simple and cheap.

Bleach is not a poison, it can be drunk and you would probably survive, but its not recommended

Using Bleach to disinfect water tanks

How much bleach to add to your water tanks?

For routine monthly dosing 1 ppm (part per million) is required. For this calculation we need the tank capacity

So 1 ppm is 1 ml (millilitre) of sodium hydroxide per 1000 litres of water, that's easy, but household bleach only contains 4% of sodium hypochlorite (look on the label at active ingredients) so we need 25 times the dose (100/4). Therefore for 1 ppm, 25 ml of bleach is required for every 1000 litres of water, so my tanks are 25000 litres if full, (adjust your dosing to the level of water in your tanks,) so 25ml times 25=625ml of bleach per tank.

Do not use scented bleach, plain Janola is fine, but I used Woolworths brand from Countdown at about half the price, it still has 4.2% sodium hypochlorite

“Budget” brand bleach from the supermarket seems to be half strength, 2.1%


If your tanks have never been dosed before it my be worth shock dosing with a level of 3 to 4ppm, anything above 4 is wasted.

At 3ppm you may smell and taste bleach, this can be boiled away for drinking, although it will do you no harm

So for 3 ppm the dosing is now 75 ml of bleach per 1000 litres

Give the bleach time to circulate in your tank then run through your pipes, assuming you can afford the water.

Nov. 19 2011: dosed 3 ppm

Dec 22 2011: 3 ppm, Jan 24 2012 3ppm (Still using 3ppm has bleach is so cheap and we have no taste in the water) Feb 24 2012 1.5 ppm (strong breeze caught me out, I now have a spotty tee shirt). Still dosing every month.

There are other branded water treatments available in NZ. One of the most popular is based on Hydrogen peroxide (hair Bleach). This is a very effective chemical and works well. The normal dosing rate is 2.5 litres per 25000 litres and costs about $40 for 2 litres. this would work out at $90 per monthly dose on my tanks compared to $2 for bleach

What else can you do or be persuaded to buy at A&P shows to keep your tanks clean

OK so you wander round the shows or scan the internet and there are lots of things you can spend your money on. One attraction of tanked water is not having water rates, but if you get an annual tank clean your savings have gone. Whilst dosing our tanks, I took along a torch to have a look at the tank bottom, the slave tank was almost clear (as you would expect as the crap should drop out in the main inlet tank), the main tank had some sediment on the bottom, but I could still see the bottom of the tank. I do not know if the tank was cleaned recently but I doubt it. I must add though we are on a ridge and do not suffer from a lot of falling leaves. My conclusion was, the tank does not need cleaning or a crap filter added to the inlet pipes (almost bought one at Clevedon A&P).

UV filter lamps

Again I do have experience of UV filters, another project from my past was to replace the water system for renal dyalisis. Town water was passed through reverse osmosis and then (as belt and braces) passed through a UV filter.

The reason for mentioning this, is that the water for dialysis has to be extremely pure, no bacteria what so ever, so my opinion and it is my opinion only, UV filtration for drinking water is a bit expensive and OTT. There is a thriving industry in selling UV, good profits and on going service charges, so it is pushed an awful lot!!

UV filters will only work correctly on very clear water as contamination will block the UV light, therefore a series of filters are required prior to the lamp. The final filter if 1 micron will remove 99.9% bacteria and cysts anyway, so what's the point!

I was charged $80 for a 0.6 micron filter at the Auckland home show. My next two were bought from, they cost $35 each with postage. I found Mitre Ten Mega selling quality filters for $28!!!

Filters to remove bacteria

I borrowed this from another website, so its non specific, for chlorine think sodium hydroxide, although there are other ways of producing it, and yes it does mention well water

Bacteria range in size from 0.2 to 2 microns in width or diameter and from 1 to 10 microns in length for the nonspherical specie, so a 1-micron filter will remove most bacteria and cysts. If you are on municipal water, the water has already been treated with chlorine, chloramines and/or fluoride to remove bacteria, so chances are you do not have a bacteria problem unless there is a problem with the water pipes between the water reclamation plant and your home. If you are on well water and have a bacteria problem, installing a 1-micron absolute water filter at the point of use will remove most of the bacteria (99.9%), however, if your water has tested positive for bacteria, it is safer to install a chlorine injection water sanitizing tank to purify the water before it enters the house, followed by a whole-house filter to remove the chlorine.

As a general rule, the smaller micron rating for a filter is better, especially if you are on well water, but as with most everything, there is a trade-off. Flow capability usually drops off as the micron rating gets smaller, especially if the water has a lot of sediment, which well water often has. If the filter at the end of your kitchen sink spout needs to be cleaned frequently, it's a good bet that your water has a lot of sediment.

To overcome sediment-causing flow rate problems, low micron-rating filters must have larger elements to keep from sacrificing precious flow, like a sediment prefilter to remove larger particles that clog the 1-micron filter and reduce water flow. In a well water situation, it is common to see two or three sediment prefilters in the water flow, starting with the higher micron rating to remove the larger particulate, followed by lower micron rating filters. For example, first filter the water through a 20-micron sediment prefilter, then a 5-micron and finally through a 1-micron or a sub-micron filter. This process extends the life of all filters, including the carbon filters which do the work of removing chemicals, herbicides, pesticides and other water pollutants.

Carbon filters will not remove bacteria, may even be a breeding ground for them

Carbon Filters effectiveness

First Flush water diverters

First flush added to your tank

I decided to open the inspection cover on the 75mm inlet pipe to our tanks. I expected some water to come out, but I got a full bore flow and drenched. Panic set in, “I am emptying the tanks” and I struggled to get the cover back on against the flow. Once the cover was back on I realised that the water was just the content of the pipework from the house, which is at least 30 metres away.

The reason for mentioning this, is that looking at the websites for first flush diverters, the thought was, thats a great idea, but in my case the contents of the pipe would fill the diverter before the contents from the roof arrived and they would still enter the tank.

There are first flush diverters for the down pipes from the roof, but we have 7 downpipes!

Update Sept. 2013:I mentioned earlier about removing the inspection hatch on the pipe from the house to the tank and that an awful lot of water came out. Thinking out of the box, I have no gunk on the tank floor, but some crap must be coming off the roof, therefore the slow movement of water through the pipes must be letting the contamination drop out. So off came the cover, carefully, this time, the water gushed out and brought with it some leaves and twigs, plus I hope pollen etc. (I could not see the smaller stuff, the water was too fast). This is now going to be an annual event.

In-line pre tank Filters

If in the future a pre tank filter is required it's considering this product (no affiliation)

For an inline filter to the tank, but will hold off for a while to see if the tank gets dirty over the autumn period.


This seems a sensible addition if your outlet pipe is near the bottom of the tank, our outlet is about 500mm up and well away from the dregs. Fitting one could be fun, either empty the tank or snorkel down and fit, (their suggestion not mine).

If you still feel uneasy about drinking your tanked water, then boil the water, let it cool and store in the fridge. Coffee and tea should be fine, as you boil before drinking

Legionnaires Disease (another reason for regular dosing of your tanks)

My interest in Legionnaires Disease comes from working in a hospital, patients often have a suppressed immune system making them vulnerable to legionella bacteria. Healthy people also can get infected, so its strange there is not much mention of legionella in NZ water systems. You are not vulnerable from drinking contaminated water but from breathing the aerosel when in the shower etc. Keeping the hot water at 60 degrees will protect you on the hot side, but if your water tank has legionella bacteria and the tank water warms in summer, then you have an ideal breading ground. Regular chlorine dosing will mostly control it but keeping the water cool is an important check. Read on:


These first appear between 2 and 10 days after exposure to the bacteria. Legionnaires' initially produces a flu-like illness with tiredness, high fever (often 39.5?C or above), headache, muscle aches and a dry cough. As the pneumonia develops there may be chest pain, shortness of breath, abdominal pain, vomiting and diarrhoea, and hallucinations.

Legionnaires' is caused by a bacteria known as Legionella pneumophila. The disease and the bacterium were discovered following an outbreak at an American Legion convention in Philadelphia in 1976, hence the name.

The bacteria is found widely in low concentrations throughout natural water systems such as rivers and ponds but temperature and stagnancy are critical to its growth and it is in the tepid and warm static water of artificial water systems that it can really thrive, forming a biofilm or layer of living bacteria over artificial structures. Legionella will grow in water at temperatures from 20 to 50oC (68 to 122oF) with the optimum at 35oC. This temperature is readily found in above ground water tanks during the summer months. As the bacteria reproduce at the greatest rate in stagnant water, such as found in rainwater storage tanks, above ground tanks should be avoided if at all possible or at least sited in the shade out of direct sunlight.

How the infection is caught:

Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action, such a toilet flushing or garden sprinklers, breaks the surface of water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye but small enough to be inhaled into the lungs. This often occurs in poorly ventilated areas such as unventilated bathrooms and cloakrooms where the flushing action of the toilet can spread it throughout the entire room, infecting anyone not immune to the strain of bacteria. Potential sources of such contaminated water include central air conditioning systems, hot water systems, showers, whirlpool spas and poorly designed rainwater harvesting systems. Which Rainwater Harvesting Systems are Most at Risk?

In order to avoid the risk of Legionnaires Disease, you must avoid the two factors that contribute most to the breeding of the bacteria:

  Stagnant water
  Tepid or warm water

Rainwater harvesting systems that switch to mains water directly in the house when the water in the tank is low leave a small amount of water in the tank which stagnates, increasing the risks when the tank supply recommences. The best and safest systems keep the water in the tank fresh by adding more water to the tank, not by-passing it by feeding mains water straight to the appliances.

If the tank is too large for the rate of water use then stagnation can also occur due to the increased retention time.

Underground rainwater harvesting tanks keep the water much cooler than above ground tanks, particularly in summer. In a sunny position, it is not unusual to find water in an above ground tank with a temperature of 35 degrees C - the OPTIMUM breeding temperature for Legionella pneumophila. AVOID ABOVE GROUND TANKS if at all possible.

As the risks are so serious and oversized tanks were being sold by some 'Rainwater Harvesting' companies, the British Standards Institute brought out a new standard for rainwater harvesting, the BS 8515 2009. This stipulates the maximum size for the tank, depending on use, but many companies are ignoring it. Insist that the tank is sized according to BS 8515 2009.

This article is obviously aimed at the UK, but its recommendations are valid in NZ. I will log my tank water temps. throughout the summer when I dose them. (It is currently 22 degrees Dec 2011 at the top of the tank, I have no way of measuring the temp at the bottom)

From News Scientist, for people who suffer from dust mite allergen

Clean laundry on a 30°C cycle at your peril, says a team of Korean researchers. They looked at the health impacts of following the environmental wisdom of turning down the temperature knob on your washing machine to save energy.

It turns out that a 60°C wash kills 100% of the house dust mites that many are allergic to. Turn it down to 30 or 40°C and you have clean dust mites because 93.5% of the creepy crawlies survive.

What is the pink film in my shower and toilet

Probably Serratia marcesens, a bacteria which is found naturally in soil, food, and in animals. Serratia, which produce a characteristic red pigment, thrive on moisture, dust, and phosphates and need almost nothing to survive. These bacteria were thought to be harmless until recently, when it was discovered that, in some people, Serratia marcesens is a cause of urinary tract infections, wound infections, and pneumonia.

So now you know! I had never seen this film before moving to New Zealand from the UK. We lived originally in Auckland and it always seemed to be present in the shower, cleaning got rid of it especially as I used a spray bleach. My first thoughts was that it was a water borne thing and never investigated further. We have moved to the country and are now are on tanked water and yet the problem still exists, so a Google search was needed, and it threw up Serratia marcesens. The antidote is regular dosing with bleach, but this will bugger up the bacteria in our septic tank, so cleaning seems to be the only alternative.

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