The perils of rain water

[Kobus Jooste]

I have not read the basic aquaponic guidelines that I collected during my first research efforts in 2008 for some time, but I do recall the list of water sources ideal for AP systems as ranking rain water rather low.  When you have options, it could therefore be avoided, but when your options gets eroded, well then you have to give it a bash and see what happens.  We are still in the grip of a severe drought, with all indications that if it does not rain significantly soon, our dams will be dry by August at the latest.  Before the rain stopped being reliable, and even before our municipal management thought that restricting water use MAY actually be a good idea, our tap water was dodgy at best.  Chlorine application is hit and miss, while poor infrastructure maintenance lead to dubious standards. 

 

Well (borehole over here) water would have been nice, but I live right on top of an enormous protected aquifer and am not allowed to drill into it.  Our clever municipal managers, however, do not consider putting a cemetery on top of it too much of an issue.  Before we became integrated into a larger water grid, the aquifer fed Uitenhage and we had fantastic water.  Supplying fantastic water is optional here now.  So in this drought, I have little choice but to turn to rain water.  It does still rain in Uitenhage, but as the water grid’s dams are not located here, it only helps nature and people with rain tanks.  The rain water has caused much pandemonium and anger in our little town.  Being far removed from a beach, most of the middle class houses have pools.  We may not fill them with tap water, thus most people have turned to rain.  It has had devastating results for most people that do not dose their pools with paint-stripping levels of chemicals.  A clear pool turns to a mucky mess in days after adding rain water, and most people just don’t get it.  Rain water is supposed to be perfect, right? Right!!

 

 

To recap, I added to water column diagram from a previous blog.  As you can see, you need to have a large amount of alkalinity in your set-up to deal with the effects of nitrification, respiration and plant activity.  Without this alkalinity, your pH will be vulnerable to shifts – most likely down in Aquaponics.  Pool water need to have an alkalinity of around 100 - 120 ppm.  I do not have specific references to minimum AP standards, but believe that it can be much higher than the pool levels.  In my case, stored rain water has a pH trending slightly towards the acidic range, and with a alkalinity below 20 ppm.  That combination can have disastrous effects on an AP system over time.  In the past, Alkalinity did not concern me much because most water sources that spent some time travelling through or over geological formations around here would have a decent alkalinity.  I’m at the point where I am going to go to the local quarry to fetch a chunk of limestone to toss into the system to try and deal with the prolonged effects of using rain water in my home AP system.  The alkalinity from the original fill, which was tap water, is long gone, and lime as I may, I have a depressed pH.  It used to hover around 6.5, but those days are gone.  Even if I lime my rain water tanks and add potassium in advance (topping up slowly to avoid pH shock) I still have to keep a keen eye on the system.

 

So for all of you out there eyeing off the grid rainwater systems, a word of advice.  They work beautifully, but you cannot sit back and assume that your system is mature, which can allow you to slack off on water quality monitoring.  A rainwater AP system is high maintenance, and require management tactics that is not typical of mature AP systems I have operated or studied literature of in the past.  With nutrient availability and bacterial action highly dependant on pH, you have to be vigilant and test your pH and alkalinity weekly.

[Harold Sukhbir]

I have buffer everyday with lime to keep the PH up in the last 2 weeks. This started happening abt 1 month ago(system is now 6 mths old). Until Kobus posted on the PH problems involved with rain water i was clueless to the fact. I always lime directly to the beds to lessen the clogging issue. I switching to 24 hour, bubbled nutrient rich, PH 7 tap water!

[Kobus Jooste]

I use the garden lime variety but it gunks up the sump.  I basically want to get my system back where it is supposed to be, then treat my rain water and get some long-acting buffer in the sump in stead of trying to lime. Two pounds of lime has done nothing.

[TCLynx]

Hum.  I think Jon and Cat Billings had an issue when they first switched to rain water.  I think he told me that by using a mix of different lime stone he got his system back under control.  I think a mix of dolemitic lime and calcitic lime (sorry I probably don't have the right terms, still need more coffee) might be far better than simply using hydrated lime or some other really fast acting pH buffer.  I often get a bag of garden lime to use in the soil gardens and it is usually a mix so I get not only the buffer and calcium but also magnesium.  Around here a bag of garden lime is pretty darn cheap.  I still have to be careful not to use too much but it's definitely slower acting than the hydrated lime.

 

My 300 gallon system has seemed more stable for me lately as I've had a small bag of limestone chips hanging in the fish tank.  However, I'm not filling with rain water, my top ups are still nicely buffered well water.

[Kobus Jooste]

Any one have clear guidelines for alkalinity in aquaponics? I'm working with 80 - 140 ppm from koi ponds.  Tragically for me, I just checked the tap water and the pH and alkalinity is almost perfect.  At some point, I may just have to swipe some of it to add periodically to deal with alkalinity issues if I do not find a safe way to get the alkalinity stable.  the pond guys use sodium bicarb for rapidly increasing the alkalinity, but I do not want too much sodium in my system.

[Harold Sukhbir]

Please don't forget that movement of PH should be done incrementally and over a period of time, sudden swings affect our fish and bacteria and upset the delicate balance in our systems.

[TCLynx]

Yea, just check your pH regularly.  If it is down around or below 6 you might want to add something to buffer it up but as kobus says, go slow.

 

Many people find that a bag of chicken grit (crushed oyster shells or limestone chips) stuck in a stocking hanging into the fish tank is an easy way to buffer and if the pH starts getting too high, it's easy to take out.  The buffer bag is only good for helping maintain a pH as it would be way slow to say bring the pH up form somewhere in the 5 range. 

[Kobus Jooste]

Rachel - I'm not sure if my and your scenarios will be the same.  My rain water comes off a corrugated iron roof and gets stored in platic tanks.  After a few weeks, they have a pH of 6.8 and alkalinity of less than 20.  The low alkalinity over a period of 9 months have eroded my system's alkalinity to the point that the plants' pH lowering effect was not buffered any more.  If you have a well buffered system, the rain water should not be too detrimental.  See if you can measure alkalinity (cheap pool test kits if you do not have one) in your AP system and rain water.  If the rain water and the AP system both have low alkalinity, you are likely to have the risk of "jumpy" pH.  If it gets too low, you will see all kinds of nutrient stress.  That happens in my AP system, but for some reason (maybe the lack of plants :)) my pool goes the other way and jumps to over 8.5 even with a decent alkalinity level.

 

Any relatively pure form of lime (CACO3) or lime stone (shells, coral sand) will act as a source of alkalinity introduction in your system, but if you do decide to add, do it gradually if you go for powdered lime as it can make a mess of a tank!  If you do not have signs of pH problems (such as the inhibition of nitrification) or nutrient stess I would not go about stressing too much though. 

[Raychel A Watkins]

Interesting info Kobus.  I have been experiencing a lot of rain lately and all my tanks are out in the open mostly.  They get all the rainwater in them and I never even thought about pH changing.  I try to do mine once a week but ai concentrate more on ammonia.  Let me get this straight the pH goes down with the rainwater. At what pH should I consider adding something to the water.  What kind of lime is safe and how much do you add. Mahalo for the info.

[Kobus Jooste]

I forgot to mention the sterile nature of rainwater in terms of nutrients, as I sarted on tap water and was cycled before I turned to rain water.  You would have to dose up with trace elements if you start on rain water, but since I have started getting a grip on how my system behaves, I have not had to add any trace elements.

[Harold Sukhbir]

Now you tell me!. Using 100% rain water also meant i had to add missing nutrient that are present as a normal consequence in the pipe borne counterpart and reaching a point of adjusting PH very regularly(each week). I agree, it may be safer but requires higher maintenance.