MOLLUSCAN SHELL GRIT AS A pH STABILIZER IN AQUAPONICS

Aquaponics is mishmash of hydroponics and aquaculture following recirculating aquaculture concept. In this system nitrifying bacteria plays crucial role like a bridge for nutrient circulation from fishes to plants. In aquaponics, pH is one of the important factors as plants need slight acidic pH for uptake of nutrient from the water by their roots. In contrast, fishes and bacteria prefers slightly alkaline for their optimal metabolic activity. Due to nitrification process the nitric acids produced in the aquaponic system, reduces pH below the neutral level (pH < 6.8) causing huge loss of energy by fish resulting lower production. Therefore, the optimal range of pH for aquaponic narrows from 6.8 to 7.2 for its sustenance. Hence, a 20 days experiment was conducted to investigate the effect of shell grit on pH stabilization in aquaponic system. The aquaponic model used for experiment was media bed based aquaponic system. A half cutted plastic barrel was filled with LECA (media bed) and a circular FRP tank with 300 litres of water. The tank was stocked with 30 nos. of fishes (tilapia) weighed averagely 30-40 g. The fishes were fed with pelleted feed at 3-4% of average body weight twice in a day. The plants cultivated were mint (Mentha sp.) and Mexican mint (Plectranthus amboinicus). Water circulation was maintained at 5 L min^-1 with the help of peristaltic pump. Normal filter bed with the charcoal and sand (lined by white cloth for both sides) below the LECA layer, resulted to form acidic pH (6.5) within 20 days. On the other hand, after addition of coarsely gritted molluscan shell layer (lined by white cloth for both sides) below the charcoal layer, reduced pH and was maintained to its neutral (7.1). Results of the experiment concluded that the pH of the system was increased from acidic level (10^th day) to neutral (7.0-7.2) and remained stagnant on 20^th day. In the experiment, calcium carbonate, the major compound in mollucan shell, played important role by slowly leaching out into water during acidic condition resulted in increase and stagnancy of pH by buffering action. To conclude, the mollucan shell grit is one of the cheaper and organic means of pH stabilizer in aquaponic system and can be utilized by small farmers to reduce loss during culture.