Admirable goals, but I agree that it seems overly complicated. That said, it always grabs my interest when someone's venturing outside the norm. If you want to try it, more power to you. I'm all for someone doing it however they want. It's your tank, not mine after all. It'll be interesting to see how your setup evolves over the course of a year or more.
I'm
far from an expert, but presumably you've posted your build to get feedback, thoughts, ideas, criticisms, encouragement, etc... here are mine:
1. I agree, if you are going to go natural and sustainable, go natural and sustainable. The tower, chemical, and mechanical filtration seem problematic and unnecessary to me. It has been said that perfection of design is when there is nothing left to take away. I would spend some time thinking on that. What is the absolute simplest system to achieve your design goals?
2. I have heard claims that extra-deep sand beds (~25cm) can work fine under the right circumstances, but I'd advise caution. You're running a high risk of generating hydrogen sulfide and poisoning your inhabitants. If you smell rotten eggs, there's your culprit. Personally I'd stick with established DSB principles regarding grain size, bed thickness, flow, and no plenum.
3. It is my understanding that the plenum is characteristic of the Jaubert method. That method works, but part of the reason it fell out of favor was because it requires very specific design characteristics and deviating from those at best leaves you with a plenum that does nothing but take up space, at worst it becomes a nutrient sink, sulfide factory, and leads to a tank crash. If there has been more recent research with plenums then forgive my ignorance. I've been away from the hobby a bit myself.
4. If the goal is sterilization, that little UV unit is severely undersized. Emperor Aquatics has some good reading on their website that you may find helpful regarding dosage, contact time, etc. If low cost is one of your overall system design goals, true UV sterilization to the point of effectively neutralizing protozoa is probably beyond the scope of your system. Regardless of size, UV is not a good substitute for quarantine and it is just a matter of time before that tactic comes back to bite you anyway.
5. As a testament to simplicity, Inland Aquatics runs 40,000 gal off algae turf scrubbers and 5% annual water changes.
http://www.youtube.com/watch?v=wsVEVUfVtcM. Most home aquarists seem to be using a waterfall design, but the principle is the same. There's a lot on these boards about ATS and fans even have a dedicated site [edit: apparently I can't add the link, but if you do an Internet search for "Algae Scrubber" it is probably the first hit in the list].
6. I have to admit I've never given mangroves more than a passing look, but my instincts tell me they wouldn't be growing fast enough to meet your nutrient export needs. Part of the reason ATS are popular is ease of harvest. It also reproduces like crazy and every part of it is photosynthetic. Give it light and flow, scrape off the growth once a week or so, and that's all the nutrient export you need. Algae seems more in keeping with your design goal of sustainability also, since eventually mangroves will outgrow their seedling tray and need to be replaced, right?
7. The only reason not to quarantine properly is impatience--this is not the hobby for the impatient.
8. I admire redundancy in high risk areas. It is prudent to give thought to things like "What would happen if my return pump burned out at 10am on a workday?" Or "What would happen if my chiller died?" That said, I don't think the redundant filtration methods you've outlined add anything but more maintenance.
9. Speaking of maintenance, I'm a firm believer that the easier you make maintenance tasks, the more likely they'll reliably get done. You have
a lot of canisters and chemicals and media and buckets that all need cleaned and renewed, you have a lot of plants that need pruned, a DE filter that needs flushed (how are you going to supply make-up water to replace the flush water?)
10. I don't know what you pay for water, but by just spot checking the prices for some of the consumables you're planning, it looks to me like you're going to be spending more money than you would if you did 10% monthly water changes. If you managed to achieve Inland Aquatics rate of 5% annual, a single bucket of salt would last you ~10 years. There is point of diminished returns and if low cost is the overriding goal, the optimal target is likely something less than absolute zero water changes.
11. Finally in terms of costs, it is important to consider opportunity cost. For example, how much money could you earn working some overtime or a side job versus how much money you save spending that time cleaning and maintaining your extra system components. You have designed so much maintenance into that system that you must not place much value in your time.
(I don't mean that as an insult; it is meant to spark you into thinking of your time as a finite and valuable resource and consider how much of your time you're going to be spending on maintenance tasks.) One of your design goals is low cost. Your time is the most valuable thing you have and should be part of that cost analysis as well.
Well there you have it... my thoughts. Worth about what you paid for them, but I hope they spark some ideas.
Good luck with your build!