Solving the Aquaponics Challenge

 At this point, most, if not all of us have heard of aquaponic plant and fish production. Though some confuse it with hydroponics, the method of growing plants in fertilizer solution without using soil, aquaponics generally means that the sole source of plant fertilizer is bio-degraded fish excrement. While fish waste is an excellent source of ammonia and, by conversion, nitrate, it is fairly poor in many of the essential elements that plants require to thrive and produce. The chemical make-up of aquaculture effluent and its ability to support the growth of some crops better than others is, in fact, just the tip of the iceberg that is this issue.

               Anyone who has tried their hand at amateur aquaponics knows you can get plants to grow reasonably well without too much effort and that some types of fish will survive longer than others in the conditions these simple recirculating systems create. However, these conditions are in fact less than optimal for the fish, the plants, and the nitrifying bacteria, leading to growth that is not up to par with the standards for commercial aquaculture or hydroponic plant production. The solution to this unique challenge is in better understanding each component of the system and trying to maintain ideal conditions for each, even at the expense of proximity and integration. Let’s take a look now at what these ideal conditions are in each case, as this will reveal the limitations of conventional aquaponic set-ups.

               Lettuces and herbs are the most common hydroponic crops and are generally considered to prefer their root zone pH in the range of pH 5.5 to 6.5 and a total dissolved solids (TDS) concentration of 400 to 800 parts per million (ppm). Of this TDS concentration, nitrate is typically between 150 and 300 ppm on its own. These values have been optimized over a long period of time and result in growth rates and qualities that are the current conventional standard. This standard needs to be matched or exceeded by new cultivation methods for these methods to be validated. In addition to many of the mineral nutrients indirectly supplied by the fish, other macro- and micro-nutrients are needed in quantities exceeding what the fish can provide. This often requires supplementation with water soluble calcium, iron, magnesium, and boron, for example, to avoid deficiencies that affect plant quality and growth rate. Not meeting the target pH and TDS ranges also results in less than optimal growth and quality compared with the standard. When it comes to dissolved oxygen, the more the better for root health and subsequently water and nutrient uptake. This is no different for bacteria or for fish, as they both thrive in high dissolved oxygen conditions.

               Nitrifying bacteria, generally grouped into ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), are the agents in an aquaponic system performing the conversion from ammonia to nitrite and finally to nitrate. In other words, they digest and convert fish waste to mineral plant nutrients. Species of nitrisomonas and nitrobacter are commonly used in aquaponic systems’ waste conversion component. Their ideal pH ranges for proper health and function are 7.8-8.0 and 7.3-7.5 respectively, meaning a compromise must be made at pH 7.65. Nitrisomonas bacteria convert ammonia to nitrite, and nitrobacter bacteria convert nitrite to nitrate, which means that nitrite and nitrate are both waste products of the bacteria’s metabolism. As the concentrations of nitrite and nitrate increase in the bioreactor component, the rate of conversion of ammonia by the bacteria decreases. Hence, nitrite and nitrate must be harvested and replaced with ammonia rich water from the aquaculture tank.

The most common fish species used in aquaponic cultivation is Tilapia (Oreochromis), and this is mainly because of its general hardiness in fresh water and its high ammonia output. Tilapia are fresh water fish that grow quickly and produce a significant amount of waste including ammonia. Males are larger than females in this species, and so are used alone in aquaponic cultivation, excluding females to avoid unwanted mating and ensure uniform fish crops. While generally hardy, Tilapia have thresholds for ammonia, nitrite, and nitrate toxicity. Water is considered toxic to Tilapia when ammonia exceeds 0.25 ppm, when nitrite exceeds 0.5 ppm, and nitrate exceeds 110 ppm. Their ideal pH range is 7.0-9.0, averaging out at pH 8.0.

As you can imagine, maintaining ideal conditions for all three components of the aquaponics system in one solution is a daunting task bordering on the impractical. Only by separating the three components can a grower achieve optimal health and performance of all organisms in the complete system. This means that the solution cannot recirculate freely between the aquaculture tank, the bioreactor, and the hydroponic system. Solution can and should recirculate within each component, of course, to save water. However, when ammonia rich water is harvested from the aquaculture component and moved to the bioreactor, it must be replaced with clean, fresh water for the fish to stay in good shape. Similarly, when the nitrate rich water in the bioreactor is harvested and transferred to the hydroponic reservoir, it must not return to the bioreactor and harm the bacteria with high salt content. When the nutrient solution being used to grow plants is spent and/or out of balance, it must be completely removed from the entire system, though luckily it can be used to irrigate garden beds and field crops. In this way, the solution in each component can be independently adjusted to meet the needs of the life within.

Even though this article barely scratched the surface of this issue, it is written with the intention of helping farmers, growers, and gardeners adjust their attitude so that they can operate extremely successful aquaponic cultivation systems full of healthy fish, bacteria, and plants. Without giving away any critical values or proprietary designs for systems, I want to encourage all to learn more about the plants, animals, and microorganisms they work with and experiment outside of the box using that insight that nature gives them. We are not reinventing nature, just learning to understand, simulate, and work within it in ways that benefit us. Cheers and happy growing!