
In this month’s blog, we’ll shift our focus from the fish to the microbes.
As we discussed last month, your fish have eaten, and pooped. You’ve collected this poop as a slurry using something like a settling tank for smaller systems, or a rotary drum filter for larger systems. The poop is holding the phosphorus your plants need hostage, so what do you do?
You use it to build a Poo Zoo.
As discussed in my first blog, the solid slurry (fish poop, uneaten fish feed, and water) you’ve collected contains everything needed for micro-organisms to grow. It has nutrients which can be used as the building blocks to grow new tissues and repair damaged ones. It also has energy, which provides the fuel needed for growth, repair, and reproduction.
If you take your waste, and seal it in an environment without any oxygen, you will get what is called “anaerobic digestion.” A group of organisms that can survive in oxygen-free environments will break down the waste, and make use of the nutrients for growth, releasing some back into the slurry. As they break down the carbon chains in the waste for energy, they will create methane. This methane can be collected and used as natural gas (e.g. for heating, electricity production). Downsides of anaerobic processing include:
(1) the pH of the resulting solution is high, which causes essential nutrients like calcium and phosphorus to come out of solution and not be available for your plants,
(2) the process can create some molecules that can limit plant growth, and
(3) it smells terrible.
If you take your solid waste and process it with oxygen available, you get what is called “aerobic digestion.” This process is frequently used in human waste water treatment. By getting the energy out of human poop with controlled microbial growth, we avoid uncontrolled microbial growth which could damage downstream habitats. An ecosystem of micro-organisms (the Poo Zoo) is nurtured to optimally process the waste. In human waste water treatment, this is called an “activated sludge process.” The same process can be done with fish solid waste, using the same micro-organisms.
At the start of the process, the solid waste slurry has a lot of energy, a lot of nutrients, and not a lot of microbes (a high food-to-microbe-ratio). The first wave of zoo inhabitants, like free-swimming ciliates (left-most picture) and flagellates, use that energy to buzz around the solids slurry gobbling up waste, using up energy, and releasing some of those nutrients back into solution. As the process goes on, the number of microbes increases, and the energy gets used up (low food-to-microbe-ratio). This causes the ecosystem to shift. The filter feeders like rotifers (middle-left) and stalked ciliates (middle-right) start to become more predominant. These microbes don’t use a lot of energy to get their food, giving them an advantage as the energy in the waste decreases. As the energy gets depleted further, the predators come in. Nematodes like bristle worms (right-most picture) start to show up as the clean-up crew. They eat the remaining microbes, releasing the last nutrients into solution.
The aerobic digestion process, done right, creates a solution that:
(1) has a good pH for nutrient availability for your plants,
(2) is filled with plant-available nutrients, without plant-limiting molecules, and
(3) it smells … pretty good actually? Like healthy, fresh soil.
But we’re not done with the microbes yet. Once the solid-waste processing is done and all the carbon-based energy is used up, there is still a little energy left. As the Poo Zoo creatures broke down the protein in the solids, they released ammonia as a waste product (just like the fish do!). For certain organisms, with specific molecular machinery, this ammonia can be a source of energy. We’ll talk about these nitrifiers in my next blog.
PS: Keep an eye on my Twitter page, @AquaponicsOT, where I’ll be posting some microscope videos of these microbes in action #PooZoo.
