Harvest draining associated mass discharged from intensive shrimp culture pond: a case study of commercial shrimp farms in Chachaengsao, Province of Thailand

This study was conducted to evaluate effluent and solid waste management practices used by shrimp farmers in Thailand and to assess nutrients and solid waste loading to the external environment associated with harvest draining. In this study, 24 shrimp farms were selected in the Chachaengsao province, Thailand. One pond from each farm was randomly selected for effluent and sediment analysis. Effluent samples during harvest draining were collected at three depths namely, when pond water level was between 100-50 cm (surface), 50-30 cm (middle), and 30-0 cm (bottom) and were assessed for effluent quality. After harvest draining, sediment samples (10 cm depth) were collected from each pond for nutrients and organic carbon analysis. Effluent quality at harvest varied largely among the 24 shrimp farms surveyed. Total ammonia nitrogen in effluent ranged from 0.0 to 1.1 mg l-¹, but was not significantly different among the three pond depths. Total nitrogen and total phosphorus ranged from 2.3 to 9.7 mg l-¹ and from 0.2-1.3 mg l-¹ , respectively; and the values were significantly higher at the bottom effluent than at the surface effluent. Organic matter and total solids content in effluent ranged from 0.7 to 1.0 g l-¹ and 3.3 to 3.8 gl-¹ , respectively, and the values were not significantly different among the three pond depths. Total settleable matter, total volatile solids and total suspended solids in bottom effluent were significantly higher than effluent from the surface and middle of the pond. The results showed that pond sediment contained about 74% nitrogen and 78% phosphorus of the total outputs compared to only 9% nitrogen and phosphorus contained in the effluent water. The study suggested that retaining bottom effluent (30 cm) and sediment (10 cm) at harvest can significantly reduce nutrient loading to the external environment and thus, minimize the negative impacts of intensive shrimp farming.