Backyard Aquaponics
A place to discuss aquaponics

My new AP system is running on rainwater coming from a straight galv tank (not aquaplate). The tank is 45,000 litres (10,000 gallons) and was installed less than a year ago.

Having seen my Dad's system continually fail to keep fish alive (despite non-toxic ammonia and nitrite levels), I have grown concerned about Zinc toxicity. His system ran on rainwater out of a tank that is exactly the same construction as ours.

Today I got a friend at the university (where I work) to help me analyse some samples of my water using an Atomic Absorption Spectrophotometer (AAS). The results are approximate due to an imperfect calibration, but we can be confident that the true value lies within the following ranges:

Straight tank water: ~6 ppm Zinc (range 1.9 to 9.0 ppm)
AP pond water: ~2.2 ppm Zinc (range 0.7 to 3.2 ppm)

pH of the water was around 7.8 when I first tested, prior to putting any fish in the system -- no idea why it would be so high, it should naturally be below 7.

I also have no idea why the AP system shows a reduction in Zn relative to tank water, but it is something of a moot point as aquaculture limits are specified at <0.005 ppm Zn [1] (other sources say <0.01 [2]) so even in the absolute best case of 0.7ppm, we're still running at seventy to one-hundred-fortytimes the concentration guidelines for aquaculture.

[1] http://www.fish.wa.gov.au/docs/pub/WaterQualityDisease/index.php?0408
[2] http://www.rirdc.gov.au/reports/Ras/03-012.pdf

I think there is a very strong possibility that Zinc toxicity is responsible for the failure of Dad's system, and I am understandably rather nervous about the likely success of my system given these results.

For anybody still planning their system, I thoroughly recommend paying the extra money and purchasing a rainwater tank that is either aquaplate, straight poly, or anything other than straight galvanised steel!

However, for those of us who have already made this mistake and are unable to change it, we have to evaluate the options for fixing the problem.

Possible mitigation strategies
1. Line the rainwater tank with a poly liner (~$1000)
2. Line the rainwater tank with bitumen (~$1000)
3. Install sacrificial Magnesium anodes in the tank (~$900, between 4 - 16 years until anode replacement)
4. Install a RO (reverse osmosis) system to filter the water (~$500, plus $100-$200/yr filter replacements)
5. Install a 5000L poly tank upstream of the galv tank, to be used exclusively for AP (~$1300 including pump).
6. Run the system on straight tap water
7. Solar distillation ???

Options 1, 2 and 3 all have the extra benefit of increasing the life of the tank, which must be corroding at a fantastic rate if it's already built up to 6 ppm. However, with options 1 and 2 we would really need to empty the tank and dry it before proceeding.

Option 4 is the most fool-proof in terms of providing perfect water to the system, and has the benefit that it can run on either rainwater or tap water --- but it does nothing for the longevity of the rainwater tank.

Option 5 requires us to find a home for a large-ish tank at a particular elevation.

Option 6 is the cheapest and simplest but I'm not sure whether we can just rely on tapwater conditioner. In any case this is a step away from self-sufficiency.

Option 7, while appealing, is almost certainly unfeasible due to very low flow rates out of such systems (e.g. 3L/day), and high installation costs.

In my case, I will also have to line my growbeds (which are also galv), however I am focussing above on the rainwater tank as this will be the most hassle. In comparison, the cost and inconvenience of lining the growbeds will be trivial.

Thanks for the info Jim, sorry about the results
Is it worth testing tap water for comparison?

I would expect that the corrosion rate (and the zinc concentration in the water) will rapidly decrease as a protective layer oxide forms. I still won't be playing with galv tank water for AP any more (I have used some in the past). I've read that the oxide layer is quite easily disturbed too...

Can probably get tap water composition straight from the water authority if I ask nicely enough.

If I go with tap water for now, it will be interesting to re-check the tank water at a later stage to see whether the concentration has subsided. Still, I can't see it getting down to <0.01 ppm for a long while.

Water quality data for SA Water mains supply:

http://www.awqc.com.au/NR/rdonlyres/674F778A-7129-4E0F-9F5C-AD2A5EF28BB3/0/Copyof5yeardata03_08.xls

Quite comprehensive. The only problem is their maps don't actually tell me which supply I belong to! They are all reasonably good in terms of Zinc. Next thing to do is check their min/max/average values against the recommended levels of each component.

And remember that they can lie about whats in the water. In this area we have leeched chemicals from all the cotton farms but it dosn't show up in any of the water quality reports for the area.

out of interest have you had any quotes for tank liners or are you thinking of using pond liner and shaping it yourself?

I hadn't actually considered lining the tank myself - it's a possibility.

I once attempted to build a small (1000L) rainwater tank and line it with big sheet of pond liner. Talk about "square peg in a round hole"!!! Very hard to do as you have to bunch the sheet up around the edges (like the bed sheet bunched up under the corner of the mattress).

If I was doing it myself on this scale, I'd make a circle for the base and then run a long vertical sheet all around the sides, and then use silicon sealant to join it up. Pond liner is not cheap - roughly $8/square metre. So that'd be something like $600+ for the liner on a tank my size. Given that cost, and given that you'd be doing all this inside the tank, I think it might be easier (and similar cost) to just climb in there and slap on a couple of coats of bitumen. I reckon less can go wrong with that plan.

with an RO you also should factor in the water waste issue, as roughly 1 3rd of the water will be rejected by the membrane, whilst on a domestic RO this is probably still useable for things like flushing the loo and on gardens etc if you have a limited scource it can be a big issue

another factor with RO water is that it can be very aggressive (the universal solvent )though i doubt a domestic grade RO would produce water of a quality that it would be a huge issue

as to what i would do in your situation, its a tough call but i would probably go for a new tank up stream as at least you are increasing your total storage as well, the old 2 birds with one stone

CHeers
pete

It depends which two birds you're aiming for. Anything that can be done now to prevent corrosion of zinc from the inside of the existing tank will serve to improve the lifespan of that tank - perhaps in the long run this would be better than increasing the storage?

I'm really warming to the idea of painting the inside of the tank with bitumen (e.g. Sikatite)

considering the Zinc is there as a sacrificial anode to the iron beneath, preventing corrosion is going to be hard. Preventing corroded Zinc getting into the water would be easier.

If one adds another sacrificial anode, such as magnesium, then we have to deal with high levels of magnesium in the water, as well as Zinc and Iron (sacrificial anodes aren't perfect, just almost)

true enough Jimmy i see your point on the coating the tank too

KP if high levels of magnesium are a problem then you can use a water softener to remove them, though then you will have high sodium levels, which in most cases isnt a problem, seems there is always a trade off though

CHeers
pete

Thank you so much for getting your tank and system water tested. Now we know with some actual test results to back us up that using galvanized tanks without liners or coating is a bad idea.

Yes, thanks for this info. I am currently running my zinc coated sump without its liner installed. Looks like I have better get busy and put that liner back in asap.

Go for the Sikatie BE , Jimmy

it worked a treat for me and one barrel goes a hell of a long way

Yes, so far this sounds like the winning idea. Big job though!




No worries.

Given that the literature out there is indicating that significantly smaller levels of zinc can lead to death of fish and other aquatic life, I suspect that zinc toxicity explains the ongoing fish deaths in my Dad's system. The only thing he ended up being able to keep alive were goldfish, and even with those one would inexplicably turn up dead every so often.

It's easy to convince oneself that it'll all be okay, and that some other, more easily controllable element (e.g. ammonia, pH) is responsible for killing the fish - therefore it's good to have some hard results in front of us confirming that we MUST act on our zinc problem.

I'd be keen to know how many other people on the forum are running systems on rainwater out of straight galv tanks.