Aquaponics is, of course, a closed system of food production whereby fish are raised intensively in tanks; while the water from the tanks is circulated through grow beds by a pump where vegetables remove the fish’s waste products from the water, using it as fertiliser to grow. The vegetables grow well on the fish waste and the fish grow well in the water cleaned by the plants, giving harvest of fish protein and vegetables for the table from a comparatively small area. You will note from other articles on the site that I have been in the process of setting up an aquaponics system. One of the stumbling blocks preventing me from completing this project has been getting 240 volt power to the area to run the pump.
This process of providing fish and veggies in abundance form a small area with minimum work does seem too good to be true, but I have been struggling with, among other things, the energy cost of the system. On the weekend just gone (Apr 2014) I took a long hard look at the project which has been unfinished for a couple of years. I worked out that if my reservations were serious enough to make me procrastinate that long I had not thought the process through well enough. I have now thought through the design and decided not to go ahead with it. During my ruminations, following is some of the stuff I came up with, which I would like to share with you.
Energy
The energy used by this system is mostly consumed by the pump. The pump serves a number of functions but mainly keeps up a steady supply of water to the veggies and ensures the water is agitated and oxygenated. This second function is not a small one. Due to the fact that the fish are raised intensively and their numbers considerably exceed the stocking rate of fish which can survive on oxygen dissolving in the water naturally, artificial agitation and oxygenation are vital. In fact, depending on the stocking rate of the particular system, if there is a power outage and the pump stops fish can begin to die in less than an hour. So the pump is a critical part of the system and a backup in the event of failure is vital.
The pump which I bought was designed to be submersible and sized for the task. It uses only a small amount of electricity, ie 75 watts (or a bit more than a third of an amp at 240 volts). However, the need to run the pump 24/7 means that even the small consumption of the pump will add up after a while, such that it will consume 1.8 kWHr of electricity each and every day. To put this in context, we have worked hard to reduce our electrical energy requirements to the point where we now consume a bit less than 5 kWHr per day. This means that using the pump in this way will result in an increase in our electricity consumption of almost 40%. That in itself is scary!
I know what you’re thinking; there is always the option of going solar! But the cost of infrastructure to set that up is quite high. You must have enough generating capacity and battery storage to keep the pump running even through dull days in the middle of winter (low light levels + short day length = lousy charging) and anything that runs 24/7 is going to suck quite a bit of 12 volt power. If I were to use the same pump, but through an inverter it would draw over 6 amps at 12 volts and this continuous drain would take the smile off the face of any battery bank. To ensure continuous electricity supply I would need a 240v back up and a black box that could check the volts in the batteries and switch the pump over to the reticulated 240 volt supply once the charge stored in the batteries got below a safe level. To my knowledge no such box exists and I’m not an electrical engineer, I don’t know enough to design and build one from scratch.
So you see why the energy requirements of the system have me concerned enough to scratch the project. Yes I could design a smaller system but I wanted something that could make a meaningful contribution to our protein requirements.
Ethics
There is also another issue that I have been wrestling with which you really don’t see discussed much in the aquaponics literature. Is it ethical to keep 100 fish in the space occupied by 2000 litres of water? Does this bring up the same sort of issues that battery hens do? I don’t have an answer for that one but I think it should be considered by anyone thinking of setting up an aquaponics system.
What’s Next?
So what do I do with the grow beds (baths) and fish tank (cattle water trough) etc? I’m going to remove the gravel from the grow beds and store them away hopefully to be part of a vertical wetland to be used to clean greywater some day. The gravel itself will be used in a couple of wicking beds I am installing in the back yard to take advantage of some space left by the removal of the potato cages.
The fish pond itself will remain and I will be turning it into a place to grow edible water plants similar to my other productive water garden but on a larger scale and with a larger variety of edible plants, so it will still be productive but in a different way. I may even get hold of a couple of silver perch and chuck them in and grow them up to plate size, but without the hassle of trying to rear them intensively.
As far as the wooden platform on which the grow beds currently sit? I am unsure about that yet. I may find a use for it as is or it may be deconstructed and recycled into other projects around the backyard. We shall see!
