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How to Make a Rainwater Tank from Recycled Plastic Drums

My rainwater tank array made from recycled plastic drums, 1300 litres capacity

Rainwater is a valuable resource that is largely under-utilised in urban environments. It’s one of nature’s free resources that we can use to grow food, and considering that Australia is the world’s driest continent, rainwater is a very valuable resource, far to valuable to let it run down the drain!

Why Small Rainwater Tanks Are Worth the Effort

You’ll hear many opinions about what size rainwater tanks are worthwhile and that “anything under whatever size is not worth the effort”, and so on.

Most of this rhetoric is based on the assumption that people have a nice big house (which they own!), with a perfect roof, plenty of free space in the back yard to put in some really big tanks and finally, money to pay for it all.

Let me say, if you’re growing a garden, you’ll need all the water you can get, and every little bit makes a difference. If you can get it for free, it beats paying for it. With water restrictions in Australia, rainwater collection can really make the difference for a garden surviving through a hot summer.

Ultimately, we can choose to utilise the resources that nature provides us for free, as limited as they may be, or we can simply let perfectly good pure rainwater run off into the storm water drain… The choice is ours.

From experience, there is a place for small capacity water tanks in urban environments. Small capacity water tanks work in places such as rental properties, small courtyard gardens, and even big back yards on a budget.

In these situations, the considerations for choosing a water tank are completely different to the big house/big budget scenario.

In a rental property, you may want water tanks that are small enough to transport from place to place, can be dismantled, and can even possibly fit in a regular car for transportation (or a bike trailer for that matter!).

In a small house or unit, the water tanks must be able to fit through any doorways, gates and access paths to reach their final destination in the backyard. Additionally there is only limited space in most small backyards or courtyards anyway.

If you’re on a budget, the main criterion is cost. Just because a person doesn’t have loads of money doesn’t mean they can’t do their part in living sustainably!

What is the Most Cost Effective Number of Plastic Barrels to Use

You can purchase small tanks and install them yourself, or you can make them yourself out of recycled materials. Cost is the deciding factor here. Even when using recycled materials, the tank fittings usually must brought new, and they can quickly add up cost-wise.

A good friend of mine, David is a water-expert, and has done extensive research and costing on setting up small tanks to harvest rainwater, and he has come up with some significant findings. In his own words:

After working on the possibilities I came to the conclusion that going beyond 3 barrels was not really economical. I feel there are better commercial options – or look into buying something second-hand. Five barrels interlocked with stands, overflow and a tap is going to cost approx. $300 in materials. You can buy 1 kilolitre tanks for $200 or less.”

He is suggesting that a setup utilising three barrels is the biggest you would go in terms of cost effectiveness.

David has come up with some great designs for constructing rainwater tanks from interconnected plastic barrels, and with his permission, I’ll present some of the designs in the future.

For now, I’ll share my own designs of the setup with the blue drums pictured in this article.

How to Build a Modular Rain Water Tank System

Here are the step-by-step instructions for building a modular water tank system which works either as a single tank or as multiple tanks connected together for greater water storage capacity.

It’s all constructed out of a recycled plastic drum and a handful of common irrigation fittings that you can get from most hardware of garden outlets.

First, here’s the basic design:

DIY rainwater tank diagram

The following instructions are comprised of three parts:

  1. The construction of a single tank setup
  2. Additional steps required for the construction of a multiple tank setup
  3. Connecting to a pump and further enhancements

Step 1 – Prepare the Plastic Drum

Obtain a plastic drum with a lid for use as a water tank. Clean and rinse it out if necessary.

The plastic drum used in these instructions has a capacity of 220 litres (55 gallons). The lid is not shown in picture below, as it was left off to air-dry after washing it.

Recycled plastic drum with 220 litre (55 gallon) capacity

Step 2 – Gather the Tank Fittings

Some tank fittings are required to construct a rainwater tank. In this section we will list all the parts needed, and explained what they do.

For a basic single tanks design, you will need:

1.   20mm Threaded Tank Outlet (Bulkhead Fitting)

These are used to attach connections to the tank. You will need two of these, one for the outlet tap at the bottom of the tank, and another for overflow pipe at the top of the tank if you choose to use one.

The threaded tank outlet has rubber washers on either side of the tank wall, the one inside the tank is the most important!

2.   Brass Tap or Ball Valve

For the tank outlet where the water comes out of, any type of tap can be fitted to turn the flow of water on and off. Either a common brass garden taps or a 1/4 turn lever handled tap (ball valve) are good choices.

When buying a tap, select one with the correct sized threaded outlet (where the water comes out, and a hose connects), as they come in two sizes, a 3/4″ outlet and 1” outlet.

The standard brass garden tap size in all states in Australia is a 3/4″ outlet, except in NSW, which uses a 1″ outlet tap.

Brass taps can be purchased from hardware stores and plumbing supplies stores

The threads at the back of the tap should screw straight into the hole at the end of the tank outlet. If the threaded section at the back of tap is too small to screw into the tank outlet, use a reducing bush, a threaded plastic adapter that allows you to screw a smaller fitting into a larger sized hole.

A threaded male-to-female irrigation reducing bush can be used to fit an undersized tap thread into a larger tank outlet

Instead of a brass tap, we can use a ball valve, which is much more durable, doesn’t need replaceable tap washers, and only requires a quarter of a turn to go from fully closed to fully open. Ball valves come in different sizes, and in this application we will use a 20mm ball valve.

Ball valves can be used in place of a tap, these are more durable but also more expensive

Ball valves cost more than brass taps, and since these have female threads on both sides, they require an additional fittings to connect them to the tank.

To connect a ball valve with a female end to a threaded tank outlet, we will need a threaded nipple fitting, specifically a Hex Nipple 20mmx20mm BSP Irrigation fitting.

hex nipple 20mmx20mm irrigation fitting has threaded male fittings on either side

Step 3 – Select Tools for Drilling the Holes

There are two tools that I would recommend for drilling large diameter hole into the plastic drum to fit the 20mm threaded tank outlet. Either a hole saw or a step drill will perform the task quickly and easily.

1.   25mm Hole Saw

25mm hole saw with arbor

A hole saw is the recommended option, as it’s impossible to make a mistake with this one. Used with a cordless drill or regular electric drill, it leaves a slightly rougher hole, which can easily be smoothed off with a piece of rolled up sandpaper.

To cut a hole that will be a snug fit for a 20mm threaded tank outlet (which has an actual outer diameter of 25mm), you will need a 25mm hole saw.

2.   Step Drill

A step drill can make various sized holes, this one is a 10-30mm size

A step drill is also used with a cordless drill or regular electric drill, and leaves a very clean-cut hole with a smooth finish.

Since this drill is conical (cone-shaped) and the plastic of the drum is quite thick, it’s necessary to drill the hole from both sides to get a straight through cut, otherwise the hole will be bigger on the outer side.

Step drills are slightly more difficult to use as there is the ever-present risk of pushing the drill one step too far further and making the hole overly large. Only use a step drill if you have experience using these on thick plastic.

The hole for a threaded tank outlet is approximately 25mm wide, so the step drill will need to go larger than this at its widest point. The step drill I have used here is a 10-30mm step drill.

Step 4 – Mark the Position of the Holes for Drilling

Next, mark the location of the hole on the plastic drum. Pick a smooth spot on the side of the tank where there are no seams or ridges, to ensure a watertight seal when you fit the threaded tank outlet.

In this case I have chosen to drill the hole about 10cm (4″) from the bottom. The reason being that if I have a tap fitted to the tank, the tap is high enough so that I can place a bucket underneath it, without having to raise the tank too high off the ground.

Keep in mind that the higher the tap is mounted above the bottom of the tank, the more water will be left at the bottom that wont drain out!

Marking the location of the hole on the plastic drum

If you are fitting an overflow pipe to the tank, mark a location on the side, near the top of the plastic drum. Give some consideration as to where you place the overflow, as this will be the level when water will drain out of the tank, so the higher the better.

If the sides of the plastic drum slope in towards the top, take a moment to see how the Threaded Tank Inlet will best fit.

Step 5 – Drill Holes in the Plastic Drum

Drill the hole on the spot marked on the plastic drum.

Step drill used to drill hole in plastic drum

Step 6 – Fit Threaded Outlet to Tank

Screw the threaded tank outlet into the plastic drum.

I have chosen to put the shaft of the threaded tank outlet on the inside of the plastic drum, so the fitting can be as short as possible on the outside, to save space in the walkway where I’ve located the tanks.

With the short side on the inside the tank, it will be easier to get more water out of the tank, and pour out the very last bit by leaning the tank a bit sideways. With the short side on the outside, like I’ve done here, it wont be possible to do that.

Threaded tank outlet fitted into plastic drum (inside view)
Threaded tank outlet fitted into plastic drum (outside view)

Step 7 – Tighten Tank Outlet

Tighten the threaded tank outlet by using two adjustable spanners, one inside the tank and one on the outside.

Hold the one inside the tank in place with the handle pointing up to make it easy to reach it, and turn the outside one to tighten it. Tighten reasonably firmly but do not over-tighten!

NOTE: A fairly BIG adjustable spanner is required to fit around the hexagonal part of the 20mm threaded tank outlet!

Use an adjustable spanner to hold the threaded tank outlet inside the tank
The other adjustable spanner used outside the tank to turn the threaded tank outlet to tighten it

Step 8 – Fit the Tank Overflow Pipe

Even though it’s optional, it’s best to fit an overflow outlet, so when the tank fills with rainwater, the excess can be directed where needed rather than overflowing and running on the ground.

A length of flexible pipe can be connected to the overflow outlet to direct the water into the garden, a pond, water garden, or another tank.

Tank with water inlet in lid and overflow pipe on the left side

With the tap facing forward, locate the overflow pipe on either the left or right side of the tank, close to the top, on a smooth section away from any seams or ridges for a good watertight seal.

Completed overflow outlet on tank, with 22mm washing machine style flexible hose connected, running out to the garden

Here is a closer look at the overflow outlet, it’s identical to the lower tap outlet fitting, the only difference is that there is an 20mm elbow connected to the outside to keep the hose close to the side of the tank.

Another benefit of the elbow is that by turning it upwards, it will allow the water to reach a higher level in the tank before it flows out of the overflow pipe.

Overflow pipe mounted on the side of the tank near the top. In this picture, the end has a cap screwed on to shut it off because the overflow is not being used here, and this prevents entry of mosquitos

If you are connecting multiple tanks together, only one tank needs an overflow pipe connected, as all the tanks fill and drain simultaneously.

The whole setup behaves as if it were one large tank, with a single inlet where the rainwater comes in, and a single overflow to let out excess water.

Step 9 – Fit Rainwater Inlet to Tank Lid

With a single tank setup, you will need some kind of inlet where the captured rainwater enters. The style of inlet you choose will partly depend on the kind of lid your plastic drum comes with, as you’ll need to adapt it for your purpose.

You must use a lid in order to prevent mosquitos breeding in the water tank. Any holes in the lid need to be screened with a fine mesh (such as shade cloth) for the same reason. The fine mesh will also work as a coarse filter that prevents debris and dirt from getting into the tank.

NOTE: The lid cannot be airtight otherwise it will create a vacuum in the tank when water is drawn from it, and the resulting “vacuum lock” will stop the water flowing, so some kind of vent is necessary.

The lid on my tank came with a small screw top as pictured below:

Plastic tank lid with screw cap in the centre

Looking at the underside of the lid, I have regular clear silicone sealer to glue a 90mm PVC female gutter outlet over the hole.

Sitting to the right of it is a 90mm stainless steel mesh PVC fitting that can be plugged into the 90mm PVC female gutter outlet to act as a removable filter for the rainwater tank inlet.

90mm female gutter outlet siliconed to underside of lid, to hold removable filter, pictured right

Different tank lids will require different solutions. Don’t be afraid to innovate! Instead of the lid, a piece of shade cloth, flyscreen or similar material can be stretched over the top of the tank and tied in place it around the rim of the tank.

Step 10 – Assemble the Tank Stand

Construct a tank stand out of concrete blocks (Besser blocks). Place two side by side so they are wide enough apart to support as much of the tank base as possible.

Two concrete blocks used to elevate the tank

Place two standard rectangular pavers across the top of the concrete block to complete the tank base.

Water tank base made from two concrete blocks with two concrete pavers laid across them

Step 11 – Position Tank on the Stand

Position the tank on top of the concrete block stand.

Important – make sure that the bottom edge of the plastic drum is sitting on top of the pavers. The simplest way to do this is to position the base and the pavers so that the edges of the pavers extend past the tank bottom as much as possible. The edges of the plastic drum support the weight, and a full tank of water weighs 220kg!

Tank seated on concrete block base, with the paver edges extending past the tank base

Step 12 – Install the Tap and Overflow Pipe

If setting up a single tank, fit the brass tap or ball valve to the tank.

To fit the brass tap, wind Teflon plumbers tape around the threads to create a watertight seal (see instructions below), and then screw it by hand into the threaded tank outlet. Hand-tighten only.

Install any fittings required to connect a pipe to the overflow outlet, and use Teflon tape on any threaded (screwed in) joins.

The water tank should now be complete and ready to use!

How to Use Teflon Tape on Threaded Plumbing Joins

Here are a few tips on how to use Teflon plumbers tape on threaded (screw) joints of irrigation fittings to create watertight fittings that won’t leak: 

  1. Begin by holding the pipe or fitting in the left hand with the end facing you (as shown in the picture)
  2. Hold the roll of tape so it looks like a snail, with the tape unrolling off the bottom edge (as shown in the picture)
  3. Lay the tape across the bottom edge of the pipe or fitting, then wind it on clockwise, moving upwards, in the direction of the threads, with a little bit of tension, for around 7 turns.

Wrapping in the direction of the threads is important, as wrapping the wrong way may result in the tape coming unwound as the fittings are tightened. Also be careful not to wrap over the hole.

You wind the tape clockwise so it stays on when you screw the fitting in, if you wind it the other way it will unwind when you screw it in. You hold it “snail-wise” with the tape feeding from the bottom so you can tension the tape as you wind it. If you hold it the other way the tape loops out faster than you can wind it and it’s impossible to tighten it as you wind it!

How to Connect Multiple Rainwater Tanks Together

In this set of instructions five tanks have been connected together across the bottom via the threaded tank outlet where the tap or ball valve is normally connected.

Instead of fitting a tap, a 20mm x 20mm threaded nipple is fitted to the tanks (just like when were attaching a ball valve tap) which allows us connect the tanks to a 19mm polypipe main supply line that connects to a pump.

Step 1 – Wrap Threads with Teflon Tape

On one end of the 20mm x 20mm threaded hex nipple fitting, wind around the threads with Teflon plumbers tape to create a watertight seal.

20mm x 20mm threaded hex nipple fitting

Step 2 – Attach Fitting to Rainwater Tank

Screw the 20mm x 20mm threaded hex nipple fitting into the threaded tank outlet on the rainwater water tank, and gently tighten it with an adjustable spanner.

There’s no need to over-tighten the fitting because the Teflon plumber’s tape will create a tight seal with only a moderate amount of tightening.

Fitting wrapped with Teflon plumber’s tape screwed into threaded tank outlet for a watertight seal

To connect a 20mm threaded tank Inlet to a 19mm irrigation pipe, we can use a 20mm nut and 19mm tail, pictured below:

20mm nut and 19mm tail irrigation fitting can be unscrewed by hand

A nut and tail can be turned by hand easily to screw and unscrew it from the tank. It has a rubber washer to create a nice tight seal with minimum pressure. This is rather handy as you don’t need any tools to remove a tank from the outlet pipe.

Step 3 – Construct Adapter to Connect Tank to Other Tanks

The adapter can be made from a 19mm barbed T-piece, or T-joiner, a short length of 19mm irrigation poly pipe and a 20mm BSP Nut x 19mm tail, as shown below.     

Fit the Nut and Tail to the T-piece, the assembled adapter shown below:

Step 4 – Fit Adapter to Tank and Balance Pipe

Screw the adapter onto the tank, hand tighten, and push fit the balance pipe (which connects all tanks together) into one end of the T-joiner.

Adapter connected to new tank and balance pipe

Step 5 – Optional – Fit Siphon Pipe to Tank

This step is optional, but it’s one innovation that I invented that’s worth it’s weight in gold if you are using a pump. If you’re not using a pump, you might want to skip this step, thought it still has some benefits even when you just fit a tap to the tank.

One of the problems with having the outlet on the side of the tank is that you always end up with water below the outlet that you can’t get to, so it just sits there. No big deal with one tank perhaps, it may be just 20 litres. But when you have multiple tanks, this unusable residual amount adds up. With a six-tank setup like mine, that adds up to around 120 litres, which is equivalent to a half of a single tank!

The simple solution is to connect a 20mm elbow to the threaded tank outlet inside the tank, then screw in a short 20mm riser tube and push the end of the tube down so it touches the bottom of the tank, as shown in the picture below.

A short riser tube is attached to the end of the threaded tank outlet with a 20mm elbow join

With a pump connected, it works just like drinking with a straw, extracting every last drop of water out of the bottom of the tank.

Step 6 – Connect Rainwater Tank to Pump

Connect the other side of the T-piece to a short length of 19mm irrigation poly pipe, and connect an inline 19mm tap , then run the other side of the tap to the pump.

Balance pipe connected to tank, terminated with an inline tap, and connected to pump inlet hose

Step 7 – Setting Up the Pump

This is my pump setup. The white pump inlet hose is connected to a filter (bottom left) to remove any particles from the water, in order to prevent wear on the pump, and to prevent clogging in the drip-line irrigation system.

The green hose is the pump outlet. This pump has a manometer switch, the big yellow cylinder at the top – it’s a pressure activated switch that switches the pump on when you try to draw water, and switches off automatically when you stop. It all sits on a raised concrete plinth to keep the pump out of dirt and water on the ground.

Step 8 – Connect Pump to Tap

This is the complete pump setup. The plastic pump cover protects the pump from rain and sunlight. The pump is connected to an outdoor waterproof power inlet. The green outlet hose is connected to a tap mounted on the wall.

As the pump has a pressure activated switch, when you turn on the tap, it turn off the pump, and water runs, when you turn off the tap, it switches the pump off.

This system allows me to use the tap in the same way as the taps on the mains supply, the only difference is that this tap supplies rainwater from the tanks. Just like the mains water taps, I can connect a hose to it, fill buckets or watering cans, it works identically.

A closer look at the tap setup. I’ve chosen to use regular garden hose “click-fittings” for convenience to connect the pump to the bottom of the tap, and to connect things to the tap. I’ve drilled into the brickwork to secure the tap firmly, and used plastic plugs with screws to fasten it in place.

Explaining the Design Rationale

At this point I will explain some of the design of the multiple tank setup and the rationale as to why I’ve used the fittings that I have. feel free to skip this section if you so choose to!

The pipe that connects the tanks together is cheap 19mm black irrigation poly pipe. This is adequate to join all the tanks together and works reasonably well as a “balance pipe” which allows all the tanks to fill, and drain, at the same time.

The smaller 13mm irrigation poly pipe is not large enough and the flow is drastically reduced, so in a heavy downpour, the main collector tank (where the rain water flows into) will overflow because the thin pipe cannot fill the other tanks fast enough.

The bigger the pipe diameter the better, but if you use 25mm irrigation poly pipe, you have to use larger size fitting, and the cost of the larger size fittings starts to become prohibitively expensive, especially with multiple tanks!

So, using a 19mm irrigation poly pipe is the optimum size between cost and performance. When plumbing any pipes to supply water, you ideally want to keep the size of the pipe the same size, as using any fitting that narrower than the pipe size restricts the flow of water.

So what does this mean? If you’re using a pump with your tanks, the pump won’t be able to pump as much water, so you won’t be able to pump it as far, or the lengths of irrigation pipe/number of irrigation points that your pumping to, will be reduced. So it’s critical to not restrict the supply pipes!

So what does this mean for our project? Basically, you want to keep the inner diameter (the “hole size”) of any fitting at around 19mm. We’re using 19mm irrigation poly pipe, and a 20mm threaded tank outlet. If we are going to connect anything to this we want to keep the internal pipe diameter the same.

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