A combination media for dealing with rainwater alkalinity problems

[Kobus Jooste]

Having been experimenting with rain water as the dominant water source in my AP unit has highlighted the need for improved management of the alkalinity of the system.  In addition, some new members and posts have renewed my interested in sand as a media.  I have been tinkering with a thought that I think could address my concerns around the need for a carbonate source in the system, but will be a permanent feature thus I want to run it past more people before I go ahead.

 

I have beds with a volume of around 220 liters each, and filling one up entirely with shell grit may result in too much alkaline intervention (I need some because my pH tends to nosedive with the use of rain water).  Now my thoughts have gone to a 50/50 mix (or thereabouts) of fine shell grit and course silica sand.  We live close to Jeffreys Bay, where a family member has a beach house overlooking Supertubes.  The beach there is pretty much fine shell grit (nothing much bigger than 6 mm) around the rocks, and I thought about going over for a collection trip.  50% of this, together with the rest in slightly acidic sand (the stuff we use in swimming pool filters over here) should, in my thoughts, give me the alkalinity remedy I'm looking for without stuffing too much with my pH.  Our geology here is mostly sandstone, thus even if I go to a farmer and beg for some river sand, I'll still have a slightly acidic media. 

 

This will also give me a chance to experiment with a sand filter bed as a grow bed.

 

Thoughts?

[TCLynx]

Well, my big system started with most of the beds being filled with a mix of 40% shells and 60% river pebbles.

 

I do not recommend growing this way unless your plan is to grow just water cress (which might only do well in winter.)

 

My system pH stayed really high and it was only after about two years that I started seeing a pH below 7.6 and even when feeding was full tilt and all the top up water was coming from rain, I've still never seen a pH below about 7.1 in that system.  Now eventually the buffer capacity of those shells is going to be used up and the pH will suddenly drop and I'll be left with needing to add buffer regularly again.

 

I think I would probably recommend adding a bag of buffer material to your rain water collection tank which might help to more closely replicate what people get when topping up with well water out of limestone aquifers as well as providing the added bonus of helping extend the life of the rain water tanks if they are metal or concrete.

 

As to the sand.  I will be interested to see how it goes.  I know my tests with sand created way too dense a media which didn't let air in very well and I don't think it would work well for any constant inflow beds and may only be appropriate to some add on beds that only get inflow once or twice a day.

[Kobus Jooste]

TC Lynx - thanks for the feedback. I do not wish to change all my beds - just one out of four.  Are you still concerned if around 15% of the media is shell grit?  Also, my pH goes below 6.4 right now - what was your reference pH inthe system that went alkaline?

 

[TCLynx]

My Situation is different here with my high pH well buffered well water.  However if you mix the media into a grow bed, there is no easy way to adjust the amount of buffer as time goes on.

 

Then again if you are just doing one grow bed as an experiment, then I suppose if you later decide it doesn't work well, you can remove the media later and start fresh.

 

I find it easier to hang a bag of limestone chips or shells or whatever in the fish tank or sump tank and if I find the pH climbing too high I pull the bag out for a while and if the pH starts dropping, I know I need to refresh the bag or add some powdered limestone.

 

Even with my high pH well water, I found that with the brown river rock the pH of a system would still eventually dive as the bacteria are working hard and to bring it back up I would need to add more buffer or do a water change to top up extra with the well water.  I have seen variations during our hot and rainy season my system pH will tend to drop as the feed rates are up and I'm getting natural top ups with rain water so I'm not adding well water but during the dry season my pH will tend to be higher since part of the dry season is also cool and as things warm up without rain I'll be topping up with more of the well water.

 

The only reason having shell grit mixed into the media concerns me is that you can't regulate it.  It might be worth running some experiments on your shell grit to see what pH it buffers to since some types of shells tend to stop at 7.6 but other types will buffer higher up over 8 which would be more problematic.

 

If you are willing to take the chance of having to re-do the bed later, your results from trying it out will provide useful information for others.

[Kobus Jooste]

As per usual, I'm quite willing to give it a bash in the spirit of experimentation and discovery.  The shells are unidentifiable as they lie there on the beach, thus I cannot even hazard a geuss what the effect of them on the pH will be.  What I can do is use the experimental bed for short rotation crops, and start with a mix that leans a bit more towards sand than shells, and then monitor from there.  Adding grit will be much easier than removing!  My rain water has 0 alkalinity and I want to be at 100 thus I think I may need a decent dose of small shell fragments.  Will only discover through trials.  Going over to J-Bay next week-end to get the grit.

[TCLynx]

You can probably run a little test using some of the grit soaking in your water in a jar and see what effect the grit has on the pH over say a week or so.  If it were to bring the pH up fast and over 8 then you will know to be careful and use it like lime while if it has only minimal effect on the pH you will know it's already been leached quite a bit an you could probably use more of it.  Only testing will tell.

 

Good Luck.

[Burton]

couldn't you run a shell grit test by just using a bag and putting the bag in the FT to see just how much the system needs to be stable? I imaging 15% would be a bit much but to maintain the water ph level ...

 

At least with the bag test you can start really really small and mark changes then calculate based on the effect. Might want to pre-weigh the shell going into the system and use this as the measurement versus quantity ...

And as TC stated the more surface area the shell / limestone has the faster and higher a pH will likely raise.

I planned on using the bag method when I got my system up and running as it seemed the most adjustable over placing the pH adjusting material into the beds :) Which you could still do with the bag method just bury the bag in a corner if you like.

Good luck and I can't wait to see your results :)

[Kobus Jooste]

I'll go easy on the grit component.  There could be a lot of inert stuff in there too, but what I will do is first take about a kg of the stuff and check its influence on a container of water from my rain tanks.  I had a terrible time trying to lift the pH with lime though, which may have influenced my original thoughts of hitting the bed hard with the grit.

[TCLynx]

Biggest problem with shell grit or lime is they are both relatively slow acting compared to hydrated lime (calcium hydroxide) but over time once the carbonate hardness of the water has been brought up to an acceptable level, they are far more stable being a slow acting buffer.

 

Seeing as you have had to use large amounts of lime to budge your rain water pH in the past, you may need large amounts of shell grit too.  I think you will actually have better luck using the shell grit to adjust your rain water before using it in the system.  Topping up a system with water that already has a nice buffer to it (provided the shell grit only buffers to 7.6) will probably give you a nicer way to adjust system pH.

[Kobus Jooste]

I want to do that too, but what I found in summer here was that the water went out of my system so fast that often, the rain water did not have a lot of "residence time" in the tanks I was using.  That got me thinking towards experimenting with in-system alkalinity buffers as well.

TCLynx said:

Biggest problem with shell grit or lime is they are both relatively slow acting compared to hydrated lime (calcium hydroxide) but over time once the carbonate hardness of the water has been brought up to an acceptable level, they are far more stable being a slow acting buffer.

 

Seeing as you have had to use large amounts of lime to budge your rain water pH in the past, you may need large amounts of shell grit too.  I think you will actually have better luck using the shell grit to adjust your rain water before using it in the system.  Topping up a system with water that already has a nice buffer to it (provided the shell grit only buffers to 7.6) will probably give you a nicer way to adjust system pH.

[TCLynx]

Good luck with it.  Hum.  For a perhaps over wet and alkaline bed might want to choose plants that would appreciate it best.  In winter I know water cress would be a good choice since it definitely likes alakine waters.  For summer though I'm not sure but perhaps water chestnuts would do ok though I'm not certain of their preference of pH.  I have mine in some shallow flooded bins where the watercress took over for the winter but now it's warm and the cress suffers but the water chestnuts have come back up.