Hydroponics in a polytunnel

I'm surprised its cost effective in any setup! With care and planning I expect the nutrients can be used for other purposes, but even for someone like me, with experience of hydroponics, it still seems wasteful??

 

Drain to waste has its advantages. The roots in the earlier pic could be fed by dunking them flood and drain style into a bucket of nutrient solution periodically.. Every time they are dunked, the balance of nutrients, EC and PH of the solution is slightly altered and by the end of the day it would need a top up and a ph adjustment due to water and various elements being taken up by the plant. The bucket will start day two with a different balance of nutrients to day one. The more days you run the bucket, the further it gets from the start point. Ultimately, you have to tip it on the garden and start over.

If you can run the contents of the bucket (drain to waste) for the same length of time it would be no less wasteful

With drain to waste nothing comes back to affect the balance, ph, or hygene of the nutrients. The last drop is the same as the first so you dont need to make any adjustments until the bucket is empty. Pathogen and nutrient issues are more common in recirculated (closed) systems than drain to waste (open system). The nutrient returning to the reservoir from the root zone can carry all kinds of nasty stuff back with it, including heat.
With commercial hydroponics, drain to waste is normally used with bag culture (perlite) and drip fed rockwool slabs.

 

Maybe I am misunderstanding "drain to waste". I was assuming Flood and Drain, but maybe you are only applying a measured amount, e.g. via drippers, to each pot such that the amount that "drains" is minimal, rather than gallons of nutrient solution?

 

Flood and drain is a closed system (recirculated) as you completely flood the medium for a while and then drain it back to the reservoir until the next flood. My chillies are in flood and drain, 10L buckets filled with hydroton, it takes around 4L of nutrient to flood each bucket.
Drain to waste is where the plants only see the nutrient once and it isnt recycled.
If you have a recirculated system that holds 50L and you add 50% plain water back over a period of 2 weeks before tipping it on the garden, you could run a 75L drain to waste reservoir over a 2 week period and "waste" exactly the same amount. The difference between the two is the plants will get fed a constant nutrient with the drain to waste. The recirculated system will be in constant flux as the run off mixes back into the reservoir.
Imagine putting 2 weeks worth of meals into a bin and then scraping all the leftovers back into the same bin. The 1st meal will be great, but the last one will be pretty yucky as you`ll have progressively more leftovers mixed in with your next meal

 

I have a small (6 unit) auto pot system that I was going to use. That's not quite a flood and drain system as I understand it. I was thinking of a rock wool / perlite mix for the substrate and then just use that system for the nutrient feed. As suggested I'll try and compare the results to a normal pot grown plant.

It will be next year though - I don't have a grow tent or anything like that

 

A grow tent isnt essential, outdoor/greenhouse hydro is more productive and the light is free.