Understanding Sludge in Aquaculture: Norms and Guidelines

In aquaculture, the term "sludge concentration" is not a formally defined water quality parameter in most regulatory frameworks. Instead, it generally refers to two distinct aspects: suspended particulate matter in the water column and accumulated sediment on the pond bottom. 

Currently, no unified national standard exists for suspended solids within aquaculture water bodies in China; management requirements primarily focus on three areas: farming water quality, effluent discharge, and sediment quality.

For farming water quality, the national Fishery Water Quality Standard (GB 11607-89) does not specify a numeric concentration limit for suspended solids. Instead, it stipulates that human-induced increases shall not exceed 10 mg/L, and that any deposited material must not cause harmful effects to fish, shrimp, or shellfish. Effluent discharge standards, however, provide more concrete limits. 

The recommended industry standard Discharge Requirement for Freshwater Pond Aquaculture (SC/T 9101-2007) sets suspended solids limits at 50 mg/L for Class I discharge and 100 mg/L for Class II discharge. For marine aquaculture (SC/T 9103-2007), the limit is 40 mg/L. Some provinces have adopted even stricter local standards, with Hubei and Jiangxi setting Class I limits at 45 mg/L.

For accumulated bottom sludge, management focuses on sediment thickness and pollutant content. Technical guidelines generally recommend that pond sediment thickness should not exceed 0.2 meters (20 cm). Intensive culture ponds should undergo desilting every three to five years to maintain sludge thickness around 20 cm. Regarding pollutant limits, the Shanghai local standard Quality Requirement for Freshwater Aquaculture Pond Sediment (DB31/T 1590-2025) specifies limits for heavy metals. The pollution-free aquaculture standard (NY/T5361-2016) requires cadmium content below 0.5 mg/kg, lead below 30 mg/kg, and mercury below 0.3 mg/kg in sediment.

Ideal sediment organic matter content ranges from 1.5% to 2.5%. Below this range, the pond is considered too lean to support phytoplankton growth; above it, excessive organic matter depletes dissolved oxygen in the sediment and accelerates reduction processes. The ideal sediment pH falls between 7.5 and 8.5.

International studies provide reference values for settleable solids in culture water: 5–15 mL/L for shrimp, 5–20 mL/L for tilapia fingerlings, and 20–50 mL/L for juvenile and adult tilapia. These values serve as practical management references rather than statutory standards.

Effective sludge management requires integrated approaches: regular desilting, maintaining appropriate aeration to oxidize the bottom layer, and monitoring both water quality and sediment indicators throughout the production cycle.