1. What is the blue pipe for sticking on the side of the bucket? Is it to drain the res or check the water level?
** Both. The tube is flexible, and may be tugged out of the loop that holds it upright to allow draining via the tube, if you wish.
2. I understand that you need to keep the res level below the roots so you don't damage the roots, but what happens if your roots are long? Is there a minimum level of water to have in your res? I was thinking about a 5g bucket.
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4. I don't understand the airstones aswell. Do they go into a pipe?
5. Is DWC and waterfarm the same thing?
The Waterfarm, as it comes out of the box, is a drip system powered by air bubbles. Air is pumped down a small tube that is contained within a larger tube which sits into the water. As the air bubbles rise up inside the larger tube they push water up with them, which drips through small holes in the drip ring and down onto the clay pebbles.
As it comes out of the box the top part of the system has a few small drainage holes that let the drip water drain back into the reservoir at the bottom. If you don't modify the original design, you'll get a few roots growing down into the reservoir but not many. In this mode, water level is maintained below root level, and the clay pebbles are watered by the drips alone.
Many modify the original Waterfarm to convert it into a DWC system, by drilling lots of extra drainage holes in the top section and adding an airstone to the reservoir. When the roots reach the bottom of the clay pebbles they grow through the extra holes and down into the water below. If you do this you need to make sure the water is very well oxygenated so that the roots won't rot. This is what the airstone is for - you connect an air tube to it and it sits below the water making lots of tiny air bubbles which allow plenty of oxygen to dissolve in the water.
Some people disconnect the air tube from the drip system once the roots reach the water and connect it to the airstone instead. If you do this, the Waterfarm switches from being a drip system to a DWC system. Others connect a second air tube to the airstone and leaves both the drips and the airstone running throughout the grow - a kind of DWC/drip hybrid system. If you do this you will probably need a better air pump than the one that ships with the waterfarm.
If you do not modify the original Waterfarm in any way it is purely a drip system - it does not come with an airstone and is not suitable for DWC. Modification to DWC is desirable because it allows a much larger root mass and more efficient nutrient delivery to the plant, which allows larger plants to be grown than is possible with no modification, and faster.
General Hydroponics also make a larger version of the Waterfarm called the Aquafarm. With it's larger reservoir it does not need to be refilled as often once the plant gets big, or alternatively you could grow upto four plants in it. It's also possible to link several waterfarms together in different ways to get various combinations of multi-bucket drip system and RDWC (Recirculating Deep Water Culture) system attributes. Google 'Waterfarm modification' and you'll find all kinds of info on different ways the Waterfarm can be used. I hope this helps.