Aquaculture sludge removal and stabilization within created wetlands

Abstract

The objective of this research was to investigate treatment of the concentrated solids discharge produced during clarifier backwash within an aquaculture facility. Solids removal and stabilization were investigated within two types of created wetlands where water flowed either: (1) vertically, down through a porous substrate; or (2) horizontally, over soil and through plant hedges. Six 3.7×1.2×0.8-m (L×W×H) wetland cells were used to provide three replicates for both types of wetland. Approximately equal numbers of vetiver grass (Vetiveria zizanioides) tillers were planted on both wetlands types in November of 1994. Sludge (7500 mg l−1 solids) was loaded onto both wetland types six times day−1, with no scheduled drying cycle, from 12 May 1995 until 28 February 1996. Sludge was applied at a rate of about 1.35 cm day−1, or about 30 kg dry solids m−2 year−1. Results from this short study indicated that the vertical flow and horizontal flow wetlands, respectively, removed 98 and 96% TSS, 91 and 72% total COD, and 81 and 30% dissolved COD. Both types of wetland cells removed most (82–93%) of the total kjeldahl nitrogen, phosphorus, and dissolved phosphate. Measurements of sludge depths and TVS at the end of the study indicated considerable mineralization occurred in the wetlands; stored sludge at the end of the study had 50% less TVS than untreated sludge.