In aquaponic systems, fish are used to produce nutrients utilized by plants. While the majority of these nutrients are dissolved in the water, a significant portion of fish waste is solid and the nutrients within are unable to be utilized by plants. Mineralization is a process where microbes break down solid waste, releasing nutrients in a recoverable form for plants. This approach not only supports sustainable aquaponic practices but could also reduce costs on external nutrient inputs. While research on nutrient recovery using mineralization has been conducted, there is little information on low cost, low input mineralization methods. This study aimed to compare the effects of pH levels 5, 7, and 9 on the mineralization process in aerobic conditions using simple aeration and fish effluent from Nile tilapia (Oreochromis niloticus).
This study utilized 12 replicated aerobic systems, with four systems allocated to each of the three pH treatments (5, 7, and 9). The experiment was conducted over a 21-day period, during which the systems remained closed except for daily water parameter checks, pH adjustments, and comprehensive water quality tests. The parameters of interest were TAN, NO₂, NO₃, alkalinity, iron, potassium, magnesium, and phosphorus.
There were few differences in recovery between treatments that were not due to amendments to control pH. The large differences in K and Alkalinity between treatments were due to the addition of potassium hydroxide to maintain pH levels. When these additions were controlled for, it was calculated that there was K loss at pH 9. Phosphate loss was greatest at pH 9, possibly binding with calcium and potassium and falling out of suspension. Nitrate, Fe, and Mg recovery occurred at all pH levels, and no significant difference was detected between treatments.
While there were no significant differences found, mineralizing at a pH around 7 offers several advantages. First, constant additions were required to keep pH at 5 or 9. Second, many nutrients precipitate and fall out of suspension at pH 9, leading them to be unavailable for plants. Further research will examine mineralization at different oxygen levels and over longer periods of time.