Part 1: Understanding what I am trying to achieve.
In a number of blogs and posts on the mixed design forum I have often made statements regarding why I am pursuing a mixed system as my choice for a compact home unit. In order to take a break from running out to the unit and trying to make small alterations in the limited time I have available at present, I thought it sound to first review everything I am working through and towards in order to make sure that I achieve the desired outcome. In stead of just standing in my system staring into empty space (soon to be occupied by something), I decided to pen it into a series of blogs.
A lot of the reasoning behind my thoughts are also used by individuals with “mono” systems to describe their advantages. Most home systems have been adapted from larger designs that either focus on pre-filtering water or in having short rotation crops. Another feature of most systems are that they are expansive, utilizing the low power consumption benefits of having little elevation difference between the main components of the unit. Building a mixed system is not driven by the idea that none of the other units out there are worth having, just that I want to have something that can cater for my “whole basket” philosophy and space use. I firmly believe that each different system has advantages, and by carefully selecting these characteristics and incorporating them into a mixed system can therefore result in a good design for my particular wants and needs.
Thus, having decided that my unit has to do unconventional things, I have been reading, experimenting and theorizing over the course of the last year and a half or so. The following short list are the main criteria of my set-up:
Designing a “cube” in stead of a “slab” has its drawbacks. Light availability and the pump heads you are faced with immediately comes to mind. For the rest, I have found standard solutions for most of the practical issues that appear. Different “layers” can cascade from one level to the next, and flows by gravity back to the fish tank / sump array. A whole multitude of combination components have also come to mind. Before I get too far into what I am building, I want to spend the rest of this blog on the theory behind my components. Not everything I’m going to say here is supported by all other people designing systems, and they are welcome to state their point of view on the matter if they wish. For me, the following aspects of aquaponic functioning are foremost in my mind when I design:
A whole array of items can be employed here, but I do not want anything that need to be cleaned anytime soon, thus I believe in structures that can act as filtration (nitrification and mineralization) sites on a permanent basis. For this purpose I have media beds that by themselves account for sufficient nitrification and bio-filtration sites to satisfy the rules of thumb and calculators out there. I also believe that planting beds to the hilt influences their ability to act as filters. Especially if you have a passion fruit in there somewhere. Thus I believe in adding sumps filled with floating media or gravel wherever I can. As these are not planted, they ensure that the core water quality management of my unit should remain sound no matter how much I plant in the beds.
This is achieved with raised beds cascading back into the fish tank, which is the lowest point of the unit. I took 37 tilapia (full grown) in 1000 litres of fish tank through a South African summer like this and they were all happy. With flood and drain beds and cascades, the only place left to consider mechanical aeration is in the horizontal 4 inch pipes I have been adding.
Some plants like rafts and some do better in media. Some media with water retention capacity is great for towers and root crops want something they can push out of the way. I cater for all of these with a sand bed (still experimenting), horizontal 4 inch pipes, hanging towers filled with pvc granules and vermiculite and gravel beds. I will look at all of these in detail in follow-up blogs.
Mixed system design ratios:
I am a big fan of the calculator Dr Lennard published. In turn, I believe that it is based on UVI research as it uses exactly the same fish food input rates per unit plant area. I picture systems better when filtration is related to feed ratios. In mixed systems, I believe in having all the criteria related to filtration met with media filled structures, no matter what rules you follow. It can be artificial or rock or clay, but I take filtration seriously. I also believe that the ratios were designed on a “minimum to function” philosophy, in other words, “less than this and you will be in trouble”. I therefore have set myself the goal of having more than enough filtration in what I deem the “core” of my unit – the fish tank, sumps and media beds. The “periphery” is then treated as surplus growing space, not contributing to water quality at all. My 4 inch lines (eventually about 70 plants) and my towers (eventually around 130 plants) will play the role of eating up all available nutrients. I have a 1000 litre fish tank and the fish stocking density will be experimented with until I find the minimum that works for all the plants in the system. I’m starting with a low density stocking rate and will lift it if I see nutrient issues, up to the point that I find unused nutrients building up again.
In the next edition (read here when I catch up with my graphics) I will start looking at all the components that make the system what it is.