Development of lahar filtration system for recirculating aquaculture system

The study aimed to develop a low cost and efficient lahar filtration system for recirculating aquaculture system. The filtration system was composed of the settling tanks, the mechanical filter and the biological filter with lahar as filter media. The system was attached to a culture tank with Tilapia (Oreochromis niloticus) as fish culture and the efficiency and optimum conditions were determined using the Box and Behnken three level-incomplete factorial design. Fifteen experimental runs were conducted to determine the effect of feeding rate in percent of fish body weight, percent water recirculation and depth of the mechanical filter media on the change of temperature, total dissolved solids (TDS), pH, dissolved oxygen (DO), un-ionized ammonia (NH3), nitrite (NO2) and nitrate (NO3) in the recirculating water that affect the growth of Tilapia. Results showed that as the wastewater moved from the settling tanks to the biological filter, decrease in TDS (1.82 percent), TSS (79.17 percent), NH3 (71.68 percent) and NO2 (61.08 percent) were observed. On the other hand, temperature (0.44 percent), pH (3 percent), DO (44.35 percent) and nitrate concentration (34.46 percent) were increased. Except for NH3 which was significantly affected by percent feeding rate, the recirculated water to the culture tank met the permissible water quality limits for culturing tilapia. The response surface regression of Statistical Analysis System (SAS) was used in analyzing the responses at 90 percent and 95 percent confidence level. Contour and surface plots were generated using STATISTICA program. This was also used to generate the predictor equation for the different significant response variables in relation to the independent variables and the optimum conditions of the filtration system. The overall performance of the system in reducing or increasing the concentration level of the different response variables were significantly affected by the percent feeding rate, percent water recirculation and the depth of the mechanical filter media. The optimum conditions obtained to achieve the concentration of the response variables within the permissible limits were 4 percent feeding rate, 100 percent water recirculation and 360 mm depth of the mechanical filter media. Financial analysis was performed to determine the cost effectiveness of the system. The system has a payback period of 2.33 years, breakeven point of 1.31 cropping per year, benefit cost ratio of 1.06 and internal rate of return of 39.02 percent which showed its financial viability. The developed filtration system can be applied to other systems that generate wastewater with high physical and chemical concentrations.