Rainwater Harvesting - traps to avoid and options to consider
SELECTING THE RIGHT TANK
Before purchasing a tank, visit the web site of a dealer who is an agent for several manufacturers and who lists the capacity and dimensions of each tank sold. You can then best select the most appropriate tank for your selected site and compare prices.
The rule with tanks is: THE BIGGER, THE BETTER!!!
OPTIONING TANK FITTINGS
Syphonic fed inlets should be located a minimum 100mm above the bottom of the tank.
The pump’s draw outlet should be fitted a minimum 100mm above the base ON ALL TANKS, regardless of the harvesting system used. Many pump manufacturers also stipulate this in the pump’s installation instructions.
When not using SUPADIVERTA or a suitable pre tank filtering system, sediment build up can be rapid and a pump connected to a low outlet WILL draw sludge/grit. Huge numbers of pump draw valves are wrongly fitted very close to the bottom of tanks!
A low fitted dedicated drain outlet should be optioned.
The pump’s draw outlet should be fitted a minimum 100mm above the base ON ALL TANKS, regardless of the harvesting system used. Many pump manufacturers also stipulate this in the pump’s installation instructions.
When not using SUPADIVERTA or a suitable pre tank filtering system, sediment build up can be rapid and a pump connected to a low outlet WILL draw sludge/grit. Huge numbers of pump draw valves are wrongly fitted very close to the bottom of tanks!
A low fitted dedicated drain outlet should be optioned.
DO NOT BUY RUBBISH
Buy quality! A cheap tank can quickly turn into an expensive mistake.
COUNCIL REGULATIONS
Check local regulations for possible height, colour and heritage restrictions, a requirement for a building permit to install a large tank or erect a tank stand, set back distances, pump noise restrictions as well as tank construction material requirements in fire prone areas.
Check local regulations for possible height, colour and heritage restrictions, a requirement for a building permit to install a large tank or erect a tank stand, set back distances, pump noise restrictions as well as tank construction material requirements in fire prone areas.
TANK LOCATION
Many homeowners select the tank location but forget that pipe work may form an archway between the house and the tank. This has been particularly true with slimline tanks purchased to install `unobtrusively’ next to a fence. SUPADIVERTA allows ugly pipework or undesirable wet systems to be replaced at low cost plus provide the option of increasing rainwater collection by harvesting additional downpipes.
Full water tanks are heavy and should never be located immediately behind a retaining wall! Large poly tanks can also swell; NEVER site a large tank against a house or other structures!
Many homeowners select the tank location but forget that pipe work may form an archway between the house and the tank. This has been particularly true with slimline tanks purchased to install `unobtrusively’ next to a fence. SUPADIVERTA allows ugly pipework or undesirable wet systems to be replaced at low cost plus provide the option of increasing rainwater collection by harvesting additional downpipes.
Full water tanks are heavy and should never be located immediately behind a retaining wall! Large poly tanks can also swell; NEVER site a large tank against a house or other structures!
PLASTIC TANKS ARE LIGHT
And water is heavy! If the tank is exposed to wind gusts, try not to drain the tank. Many tanks have tie down lugs on their upper surface.
PLASTIC OR STEEL
Well built steel tanks will last much longer than plastic tanks and plastic tanks should never be used in fire prone areas. Tanks mounted on wooden stands are also not a good idea in such areas!
http://www.bushfirecrc.com/sites/default/files/managed/resource/tanks_on_trial.pdf
SLUDGE
Sludge removal is time consuming and unpleasant. SUPADIVERTA'S continuous sediment extraction diverts much of the finer heavier debris that passes through the 750 micron (0.75mm) filter, leaving minimal lighter material passing to the tank. A lot of this debris can be removed before it reaches the tank by fitting a simple DIY sediment trap to the horizontal pipe work.
COVER UP
For longevity; try not to expose plastic tanks to strong sunlight. To restrict algal growth, choose dark colours and place weighted shade cloth over the inlet mesh.
PUMPS
A pump’s start up energy demand is 3-4 times its run demand. Frequent starts promote excessive wear and are a large component of total energy consumption.
Many pumps are fitted with small pressure tanks that negate the pump’s need to start whenever a small amount of water is drawn but the ‘draw down’ is only 35-40% of the tank’s capacity. Aquatrek recommends the use of a large ‘stand alone’ pressure tank fed from the pump. A typical 100 litre tank can:
- Reduce pump start ups by 90% when connected to a toilet.
- Eliminate water hammer when connected to a washing machine.
- Reduce outdoor noise.
- Allow the use of a smaller and cheaper pump.
- Reduce energy costs.
- Allow the pump to operate at its optimum efficiency. Operating a pump at low efficiency can cause damage.
CAVITATION can damage a pump.
Cavitation occurs when low pressure bubbles form and nucleate against a surface, resulting in shock waves that can cause damage that includes impeller and casing corrosion. Cavitation typically emits a noisy rumbling sound which is often mistaken for worn bearings. Sometimes it can be both.Water flowing through a 90 degree elbow fitted too close to a pump’s inlet can generate low pressure, causing cavitation.
A 90 DEGREE ELBOW SHOULD NOT BE FITTED TO A PUMP AT A DISTANCE THAT IS LESS THAN TEN TIMES THE INFEED PIPE’S DIAMETER.
Cavitation is an interesting subject that is well explained in this link.
PLUMB THE HOT WATER SERVICE WITH RAINWATER
FACT: Most water tanks are under utilised.
It is easy to also connect a dual water system to most hot water services.
Filters must be fitted to avoid sediment transfer. Having the pump draw off valve a suitable height above the bottom of the tank and a good pre tank filtering system will give the filter an easy life. Electric hot water services installed in ‘hard water’ areas may be fitted with a sacrificial anode that is not suitable for use with rainwater. Always check the anode’s suitability for use and consider if the hot water service will alternate between mains and rainwater feed for extended periods.
This application is more suited to large tanks and householders must use a qualified plumber.
It is easy to also connect a dual water system to most hot water services.
Filters must be fitted to avoid sediment transfer. Having the pump draw off valve a suitable height above the bottom of the tank and a good pre tank filtering system will give the filter an easy life. Electric hot water services installed in ‘hard water’ areas may be fitted with a sacrificial anode that is not suitable for use with rainwater. Always check the anode’s suitability for use and consider if the hot water service will alternate between mains and rainwater feed for extended periods.
This application is more suited to large tanks and householders must use a qualified plumber.
LOW PRESSURE
A pump delivering low pressure to a hose is a common frustration. Standard 12mm garden hoses have high friction losses and the longer the hose; the greater the loss.
To reduce pressure loss, join/connect an 18mm hose to a shorter length of 12mm garden hose to allow the use of 12mm hose fittings.
An 18mm garden hose flowing at 20 litres per second has 8 times less friction loss than a 12mm garden hose.
It is recommended that the pipe used to deliver water from the pump to a garden tap has a minimum internal diameter of 20mm. Note that 25mm Blue Line poly pipe is measured as an outside diameter.
A pump delivering low pressure to a hose is a common frustration. Standard 12mm garden hoses have high friction losses and the longer the hose; the greater the loss.
To reduce pressure loss, join/connect an 18mm hose to a shorter length of 12mm garden hose to allow the use of 12mm hose fittings.
An 18mm garden hose flowing at 20 litres per second has 8 times less friction loss than a 12mm garden hose.
It is recommended that the pipe used to deliver water from the pump to a garden tap has a minimum internal diameter of 20mm. Note that 25mm Blue Line poly pipe is measured as an outside diameter.
LEAKING FITTINGS
Google the internet for the yearly water loss from a tap leaking at one drip per second and you will find answers ranging from 1,000 to over 30,000 litres per year with most sites quoting either 7,000 or 12,000 litres per year!
AQUATREK has conducted its own experiments and arrived at a figure of 4,800 litres per year.
Annual loss is easily calculated, simply count 600 drips (equal to 10 minutes) falling into a container, measure or weigh the volume and then use simple arithmetic to arrive at an annual total (1 litre = 1 kg).
Any loss should be avoided and most plastic 12mm and 18mm hose connectors leak when fitted to 13mm and 19mm poly pipe!
Note that using Apothecaries’ measure does not give a true to life measure. The websites that quote low losses have without doubt mistakenly used this measure.
Google the internet for the yearly water loss from a tap leaking at one drip per second and you will find answers ranging from 1,000 to over 30,000 litres per year with most sites quoting either 7,000 or 12,000 litres per year!
AQUATREK has conducted its own experiments and arrived at a figure of 4,800 litres per year.
Annual loss is easily calculated, simply count 600 drips (equal to 10 minutes) falling into a container, measure or weigh the volume and then use simple arithmetic to arrive at an annual total (1 litre = 1 kg).
Any loss should be avoided and most plastic 12mm and 18mm hose connectors leak when fitted to 13mm and 19mm poly pipe!
Note that using Apothecaries’ measure does not give a true to life measure. The websites that quote low losses have without doubt mistakenly used this measure.
EVAPORATIVE AIR CONDITIONERS
Older domestic units generally purge between 6 and 12 litres per hour but newer models can purge as little as 3-4 litres per hour. Some householders are tempted to collect the discharge and use it to water their garden but due prudence is advised as PURGING IS NECESSARY TO RID THE UNIT OF BUILT UP SALTS!
While the loss concerns many householders, the amount pales into insignificance when compared to the average 60 litres of water used per hour when operating the average domestic evaporative unit!
PURCHASING A TANK - sometimes it's what they DON'T tell you!!
ADDITIONAL COSTS
- Tank delivery fees can be expensive if the plumber (and assistant) picks up and delivers! Most tank suppliers have reasonable delivery fees and some are free!
- Choose a pump appropriate for your intended use.
- Do not fit an elbow within 10 pipe diameters to the pump’s suction side.
- A pump cover is a necessary and wise investment.
- An electrician will need to install an outside power point for the pump. Do you have other jobs that need doing while he is there?
- What tank base is required? Remember, full water tanks are very heavy! Get quotes or do you know someone who is competent to do the job? If concreting, don’t forget the Reo!
- Avoid costly time wastage by having the tank in place before the plumber turns up.
- Many installers offer packages that include tanks, accessories, plumbing, concreting and electrical connections but packages are not always good value. Do your homework!
- Have you considered a garden tap? Where do you want it? What depth does the supply pipe need to be and can you dig the trench yourself? Don’t forget the rainwater tap signage!
- The tank’s outlet that supplies the pump is best located at least 100mm above the bottom of the tank.
- Have the fittings needed to connect the SUPADIVERTA to the tank ready or DIY.
- Have SUPADIVERTAS installed on other downpipes to double or triple rainwater harvesting.
DON’T BELIEVE EVERYTHING YOU READ
- A favourite advertising ruse is to exaggerate the average square metre area of house roofing and claim a tank or a diverter can harvest the entire roof every time it rains! One diverter can only harvest one downpipe and it is the actual area of roof harvested that matters!
- Does that glossy photo of an installed tank with accessories show the overflow pipe? Many photos don’t! Typical examples will show accessories extending from the downpipe that leave nowhere to plumb the overflow while others will show wet systems plumbed to a tank away from the house but without the overflow pipework connected. What may look neat and tidy in a photo is not always practical in real life!
- Standard rainwater harvesting systems that divert additional downpipes to tanks will often exceed the overflow pipe’s capacity, causing the tank to over top when the tank fills. Despite this, many ads and sites show questionable installations. Many consumers get caught!
- Downpipes do not drain equal roof areas and householders often unwittingly site tanks next to low yielding downpipes. When using SUPADIVERTA to harvest additional downpipes, this is not an issue.
- Our advice is to do your homework, talk to tank owners and seek advice from reputable companies.
QUESTION If the pump is connected to the toilet, does the pump always need to be turned on?
ANSWER Not always.
The average person flushes a toilet six times a day which, for the average household, is nearly six thousand flushes a year. That is a lot of wear and tear on the pump!
A typical cistern has a float level approximately 750 mm above floor level and most rainwater tank levels are higher than this when full.
Most cisterns have an inlet at each bottom end to fit a cistern valve. Most people are unaware of this but most are familiar with valves that have a long arm with a float at the end that rests on the side opposite the valve.
The FLUIDMASTER cistern valve (400UK063) is compact, inexpensive (about $20 in Bunnings) AND has the option of a very low pressure (red) gravity fed seal (242LP071) which can be ordered at the Bunnings Special Orders Desk. This seal allows a toilet to be gravity fed from a tank that provides a minimum one metre static head to the cistern valve seal.
FITTING:
- Buy two Fluidmaster 400UK063 cistern valves and one 242LP071 low pressure seal.
- Remove the plastic ‘pigs tail’ flow restrictor from the valve that will supply the rainwater.
- Replace the standard seal in the above valve with the low pressure seal. Make sure you keep the arm lifted as per the instructions when turning the top cap.
- To fit, simply remove the current valve (if it has a long float arm that would foul a second valve) and fit the two Fluidmaster valves.
- Reconnect the standard pressure cistern valve to the mains water supply and connect the low pressure valve to a supply line fed from the tank.
- Fit a stop cock to the rainwater supply line.
- Fluidmaster cistern valves have an air gap at the top that prevents cross contamination but do not have each valve open at the same time.
- The pump must be turned off and the fill rate will vary with each application. Filters fitted between the pump and the cistern can restrict the flow and pressure needed to activate the seal.
- Auspex pipe is easier to use than copper pipe and push on fittings are available. Do not attempt to drill through wall tiles unless you know how. Also check the wall cavity for pipes and cables before drilling all the way through.
- If the float does not shut the flow off after the cistern fills, adjust the float lower into the water.