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Composting Aquatic Plants


  1. aquatic compost revisited
    by "Roger S. Miller" <rgrmill/> (Sun, 23 Aug 1998)

aquatic compost revisited

by "Roger S. Miller" <rgrmill/>
Date: Sun, 23 Aug 1998

A few weeks ago (four, actually) I floated the idea of recycling plant 
trimmings into fertilizer in some way analogous to composting for regular 
gardens.  I got some interesting feedback, particularly from Steve Pushak 
and Neil Frank, and decided to experiment with it a bit.

In short, the method may have worked, but its hard to say for sure.  At 
any rate, nothing died, nothing looks worse than it did (some things look 
better) and I saved myself some fertilizer.


The basic technique I had in mind was to take the weekly cuttings and puree 
them into a sort of green soup that I'd hold, continuously aerated, until 
it was substantially broken down to a relatively inert sludge and a 
(hopefully) nutrient-rich solution.  The process was supposed to be 
complete within a week, and restarted each week after the trimming and 
cleaning.  The solution would be used for liquid fertilizer and the 
remaining solids incorporated into the substrate.

What I did...

First, I stopped my normal fertilizign routine.

Then I started out just as described above, placing the pureed plant
material into an unused DIY yeast generator and bubbling air through the
mix for a couple days, then shutting off the air and letting the mix
"age".  The mix foamed a lot when it was being aerated and emitted an odor
that was reminiscent of some green leafy thing that rotted in the bottom
of the refrigerator.  When the bottle was reopened after aging the smell
was considerably more offensive.  But reaerating that eliminated the
smell.  After allowing it to settle I was able to decant a cloudy brown
fluid that I expected to use as liquid fertilizer, and I had some black
sludge left in the bottom of the bottle. 

I tested the liquid for ammonia, nitrite and nitrate and found none, so I
added it to the tank that most of the cuttings came from.  It was so dense
that it sank immediately to the bottom and the fish avoided the area until
it dispersed.

All in all, results from the first week were pretty gross.

In each of the following three weeks I pureed the plant cuttings and added
them to the sludge from the previous week (building up sludge in the
bottle), aerated for three days, decanted the fluid with just a little
sludge into a second jar and let that "cook" anaerobically for three more
days.  The remaining sludge was kept continuously aerated.  The
anaerobically processed liquid was again settled and decanted, tested for
nitrogen species (I never found a measurable amount of any - though there
was a show from ammonium in one batch), and the final liquid was kept
in the refrigerator for use the next day. 

Over this period, the initial foaming in the aerated bottle stopped.  The 
"something rotting" smell gave way to a heady bouquet that reminded me of 
old sneakers.  The fluid after the anaerobic aging had a strong 
barnyard odor that disappeared when it was aerated.  The final fluid 
remained brown but not cloudy.  The fish didn't avoid the brown fluid 
when it was added, instead they darted into it, as if looking for food.

I tried testing the final liquid for phosphorus, but I didn't get results 
I could really use.  I assume there was substantial phosphorus in the 
final liquid, so (per Neil's suggestion) I added a few grains of ferrous 
sulfate to the liquid to sequester the phosphate.  Initially I did this 
in the final liquid, after the anaerobic phase.  Later I added it before 
the anaerobic phase. 

After four weeks I had enough sludge left in the reactor that I needed to
find a way to use it.  I dug a shallow depression in my yard where the soil
is an inorganic, fine sandy loam, and emptied most of the sludge into the
depression.  I kept some sludge to continue the culture.  After the liquid
was mostly drained out of the sludge, I scooped it and some of the dirt up
into a bowl and mixed it well.  Then I added the sludge-soil mix to the
substrate below some unhealthy echinodorus osiris and around a few more
plants that I thought might respond well. 

The principle...

The method uses bacteria to (hopefully) convert the plant trimmings back
into plant available nutrients.  It works in two phases, an initial
aerobic digestion phase and an optional anaerobic denitrifying phase. 

In the aerated step, bacteria digest the chopped up plants aerobically. 
That reduces the amount of oxygen that would be consumed if the plant
"soup" were returned back to the aquarium directly or held in a filter. 
The aerobic step should also convert the nitrogen in the plant trimmings
from organic forms to ammonia, then nitrify it to nitrite and nitrate. 
The changing smell and the change in the way the mix foamed while it was
aerated show that the bacterial culture that I needed to process the plant
trimmings developed over time.  It was pretty well in place after the
second week.  Continued success with the method requires that the
equipment is maintained to breed a bacteria culture that's adapted to the

The unaerated step is intended to allow facultative anaerobic bacteria to
denitrify the mix.  The (temporarily smelly) output from the anaerobic
step should contain little or no plant-available nitrogen and from my test
that would seem to work.  If you wanted to add nitrogen back into the
tank, then you could skip the anaerobic step. 

I can't confirm that my attempt to sequester phosphorus by adding ferrous 
sulfate to the mix actually worked.  But it should.  Even if it doesn't 
its' almost a win-win procedure, because if the iron doesn't get locked 
up with the phosphorus then it will be available to plants.

The results...

Over the last month I used no fertilizer in the tank except the output
from the "aquatic compost".  I can't make great claims for the method
based on the plant's response.  That's partly because I changed the lights
on the tank a couple weeks before starting this experiment and the plants
were still responding to that change and partly because I didn't have a
control that would tell me what the plants would do without any

I can make a few observations:

1)  The fish avoided the first batch of liquid fertilizer when it was 
added, but subsequently they showed no avoidance behavior.  They also 
showed no ill effects of any kind.  So the fish seem to think it was 
all right.

2) I had no increase in algae problems.  I had some phytoplankton evident
in my polishing filter after the third week, but no noticable growth in
the aquarium, and no phytoplankton at all observed in the fouth week. 
There are SAE's in that tank, so an increase in attached algae growth
might not be evident. 

3)  None of the plants look worse than they did prior to the test period; 
in fact most of them show some improvement.  Unfortunately its hard to say 
what improvement was due to the lighting and what was due to the fertilizer.

4) Two of the plants are growing like I've never seen before.  Rotala
indica is growing very fast (well, duh), has developed larger leaves and
maintains a node spacing of about 3/8 inch.  It has a nice reddish color
on most of the length of the plant.  Barclaya longifolia is beautiful. 
It's putting on two or three new, large and robust, deep red and olive
green leaves each week. 

5)  I saved some money on fertilizer, entertained myself a little and 
even grossed-out my 12-year old a couple times! 

The jury is still out on using the sludge as a substrate amendment.  I
suppose that if there is an improvement from this addition that it's more
likely to be from adding fine sandy loam to the substrate than it is from
adding sludge to the substrate. 

I think the method is promising, but there's still a little work needed.  
In particular its difficult to keep the mixture well-aerated in the 
aerobic phase of the process and to keep the culture continuously aerated 
after most of the liquid is decanted and the sludge is held over for the 
next batch.  Odors are obviously a problem, mostly with the relatively 
mild odor from the aerobic phase.  Odor from the anaerobic phase is 
offensive, but that only lasts for a few minutes after the batch is 
opened and while its being reaerated.

Roger Miller

PS.  I did all that stuff in the garage - not in the house.

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