An innovative sludge removing system for intensive aquaculture ponds

Abstract

The accumulation of sludge during fish culture has long been regarded as a serious threat to the water quality of intensive aquaculture ponds hence limiting the culture density. Frequent removal of the sludge is beneficial but hard to achieve due to a lack of practical methods that can quickly remove the sludge over a large area of the pond while minimizing the release of toxic substances into water. In this paper, an innovative sludge removing system was developed and tested against the well-known center-drain method. The sludge removing system used a series of water jets instead of suction ports to quickly remove the sludge while rotating on a circular center platform. Based on the Bernoulli's principle and the wall-jet effect, the water jets could form a concentrated flow to create a negative pressure to suck in the surrounding water thereby preventing the spread of the sludge. Numerical analyses were made to verify the flow pattern within the sludge removing system and to determine effects of critical design parameters. Performance tests were conducted for 10 weeks in four 15 m * 10 m shrimp ponds each having a 6 m diameter concrete center platform with a center drainpipe. The results show that the sludge removing system could remove 82.3-99.7% of the sludge on the platform in just 8 min using 3.2 ton of water and left less than 0.33 cm of the sludge on the platform. In contrast, the center-drain method used the same amount of water but could only remove up to 5% of the sludge and allowed the sludge to accumulate to a maximum height of 21.1 cm at the end of the experiment. Most of the sludge removal by the center-drain method occurred in the first minute, but the sludge removing system could continuously remove the sludge during its entire course of rotation around the platform. The effective radius of sludge removal by the center-drain method was less than 0.3 m, whereas the effective radius of the sludge removing system was its length, i.e., 2.8 m. The sludge removing system was modularized so that its length could be increased by connecting more sludge removing modules to have a quadratic effect on its effective area. As such, it is estimated that the sludge removing system could be effective in ponds up to 1.6 ha based on comparison with a biofloc system where proper water circulation around a center sump was accomplished. The sludge removing system could provide automatic sludge removals as frequently as wanted to help maintain the water quality in intensive aquaculture ponds thus may contribute to increasing the culture density and production. Compared to human labor, the sludge removing system may make a saving of nearly 90% of its investment in just one season.