On this page
- Understanding waterbody classification and effluent controls under Criterion 2.6
- Classifying Receiving Waterbodies and defining the Waterbody Unit of Management
- Implementing point-source TSS capture and settleable solids limits
- Compliance matrix, execution procedure, and practical application
- Example
- Before your audit
- Questions people ask
- What is the default classification for an enclosed freshwater lake under the ASC Farm Standard?
- How is the 65% Total Suspended Solids capture requirement calculated for point-source discharges?
- Who is exempt from defining a Waterbody Unit of Management under Criterion 2.6?
- What testing method is required to measure settleable solids in farm effluent?
- Reference documents
The short version
- Indicator 2.6.5 requires farms to classify receiving waterbodies as Type A, B, or C using retention time and flow.
- Type A waterbodies require defining a Waterbody Unit of Management and establishing a Water Quality Monitoring Network.
- Point source culture systems must capture at least 65% of Total Suspended Solids prior to effluent discharge under Indicator 2.6.16.
Understanding waterbody classification and effluent controls under Criterion 2.6
Criterion 2.6 of the ASC Farm Standard establishes rigorous water quality rules to prevent eutrophication and trophic status shifts in natural receiving waters. Aquaculture activities releasing nutrients, faeces, and uneaten feed can degrade aquatic ecosystems by triggering oxygen depletion and harmful algal blooms. To mitigate these cumulative environmental impacts, the standard differentiates between receiving waterbody sensitivity and enforces point-source effluent treatment controls.
The scope of Criterion 2.6 applies to all aquaculture facilities that use feed or fertilisers and release effluent into surrounding aquatic environments. Sites that operate closed systems and never release effluent are exempt from this criterion. Compliance leads must implement a dual-track approach: establishing spatial receiving waterbody classifications to manage cumulative watershed impacts, while installing physical effluent treatment systems to control point-source particulate releases.
Classifying Receiving Waterbodies and defining the Waterbody Unit of Management
Under Indicator 2.6.5, units of certification must classify their Receiving Waterbody as Type A, Type B, or Type C following Appendix 8.5. Type A waterbodies represent sensitive systems with a Hydraulic Retention Time of 5 days or greater, serving as the default classification for enclosed freshwater and marine systems. Type B applies to freshwater bodies with a Hydraulic Retention Time under 5 days. Type C covers exposed near-shore or offshore marine waterbodies, as well as freshwater bodies where low-flow velocity exceeds 1000 cubic metres per second or Total Suspended Solids exceed 20 mg/L.
For all Type A waterbodies, Indicator 2.6.6 mandates that the unit of certification define the Waterbody Unit of Management following Appendix 8.6. The Waterbody Unit of Management sets the spatial scope required to evaluate cumulative eutrophication risk, defaulting to Hydrodynamically Isolated Embayments where applicable. Quality managers must produce a spatial map illustrating WUM boundaries, all operational aquaculture facilities, and any upstream point-source discharges located within 1 km of a Type A inlet. Furthermore, Indicator 2.6.8 requires farms in Type A waterbodies to establish or participate in a collaborative Water Quality Monitoring Network.
Implementing point-source TSS capture and settleable solids limits
Land-based culture systems discharging point-source effluent, including flowthrough raceways, ponds, and recirculating aquaculture systems, must comply with Indicator 2.6.16. The standard mandates the installation and continuous operation of an effluent treatment system prior to final discharge. This treatment infrastructure must capture at least 65% of Total Suspended Solids and restrict the concentration of settleable solids in discharged effluent to less than 3.3 ml/L.
Demonstrating compliance requires systematic testing and standardized analytical methods set out in Appendix 8.12. Settleable solids concentrations are determined by measuring the volume of solids that settle in an Imhoff cone after 1 hour, or via calibrated spectrophotometer readings. Total Suspended Solids capture efficiency must be verified through laboratory analysis of unfiltered samples using glass-fibre filter filtration under ISO 11923:1997 or Standard Methods 2540 SOLIDS. For point-source discharges into Type B waterbodies where effluent flow contribution exceeds 10% under Indicator 2.6.17, Indicator 2.6.18 restricts the net increase between upstream and downstream Total Suspended Solids concentrations to under 25%.
Compliance matrix, execution procedure, and practical application
Quality leads and internal auditors must evaluate water quality controls against formal audit criteria and maintain structured verification records across all operational phases.
| Requirement | What the auditor checks | Evidence to have ready |
|---|---|---|
| Indicator 2.6.5 | Valid Receiving Waterbody classification based on retention time, flow, and TSS data | Calculation models, hydraulic retention data, and secondary hydrological reports |
| Indicator 2.6.6 | Complete spatial mapping of the Waterbody Unit of Management boundaries | GIS map detailing WUM boundaries, operational farm sites, and 1 km upstream inputs |
| Indicator 2.6.7 | Demonstrated competence of personnel conducting water monitoring and defining WUM | Staff training certificates, academic qualifications, and competency logs |
| Indicator 2.6.16 | Point-source effluent treatment achieving required particulate removal thresholds | Effluent treatment schematics, quarterly TSS capture logs, and Imhoff cone test records |
| Indicator 2.6.18 | Upstream and downstream water quality monitoring for significant Type B discharges | Quarterly sampling logs and differential calculations showing under 25% TSS increase |
Implementing an effective water quality and effluent management protocol requires internal auditors and compliance leads to execute a structured three-step process:
- Conduct hydrological characterisation to determine Hydraulic Retention Time, low-flow rate, and background Total Suspended Solids, assigning the official Receiving Waterbody classification under Indicator 2.6.5.
- Draft a comprehensive Waterbody Unit of Management spatial map for Type A classifications under Indicator 2.6.6, documenting all active aquaculture sites and executing formal outreach to establish a Water Quality Monitoring Network.
- Validate point-source effluent treatment performance under Indicator 2.6.16 by conducting quarterly inlet and outlet sampling, verifying at least 65% Total Suspended Solids capture and settleable solids below 3.3 ml/L.
Important: Settleable solids testing must be conducted using an Imhoff cone with a 1-hour settling period, and results must consistently demonstrate less than 3.3 ml/L prior to effluent release.
Common finding: Internal teams frequently fail to map upstream point-source discharges entering within 1 km of a Type A waterbody, resulting in non-conformities during WUM boundary audits under Indicator 2.6.6.
Tip: Schedule quarterly TSS and settleable solids sampling to coincide with peak feeding and maximum biomass periods to ensure treatment systems are tested under maximum operational loading.
Example
A quality manager at a land-based trout facility operating flowthrough raceways prepares for an ASC initial audit. The facility discharges effluent into a river classified as a Type B receiving waterbody. Hydrological calculations under Appendix 8.10 show that the farm's effluent flow contribution represents 14% of the receiving river's low flow. Because this contribution exceeds the 10% threshold in Indicator 2.6.17, the quality manager establishes paired upstream and downstream sampling stations. Quarterly analytical results confirm that the effluent settling basin captures 72% of incoming Total Suspended Solids, maintaining settleable solids at 1.2 ml/L. Furthermore, comparative downstream analysis demonstrates a 16% increase in Total Suspended Solids compared to upstream background levels, satisfying Indicator 2.6.18 and ensuring full audit compliance.
Before your audit
- Verify that the Receiving Waterbody classification report contains validated hydraulic retention time and flow calculations. ☐ Confirm that the Waterbody Unit of Management spatial map includes all operational aquaculture sites and upstream inputs within 1 km. ☐ Check that personnel conducting water sampling and defining WUM boundaries hold documented competency and training records. ☐ Review quarterly effluent testing records to verify at least 65% capture of Total Suspended Solids prior to discharge. ☐ Confirm that Imhoff cone testing records demonstrate settleable solids concentrations consistently below 3.3 ml/L. ☐ Ensure paired upstream and downstream monitoring records exist for Type B waterbody discharges exceeding 10% flow contribution. ☐ Verify that Total Suspended Solids increases between upstream and downstream Type B monitoring points remain below 25%.
Questions people ask
What is the default classification for an enclosed freshwater lake under the ASC Farm Standard?
An enclosed freshwater lake is classified as a Type A Receiving Waterbody by default under Indicator 2.6.5 if its Hydraulic Retention Time is 5 days or greater. If the retention time is under 5 days, it is classified as a Type B waterbody unless flow or suspended solids thresholds reclassify it as Type C.
How is the 65% Total Suspended Solids capture requirement calculated for point-source discharges?
Under Indicator 2.6.16 and Appendix 8.9, TSS capture efficiency is calculated by comparing Total Suspended Solids concentrations measured at the treatment system inflow against concentrations measured at the final discharge outflow. The treatment system must remove at least 65% of the influent particulate mass.
Who is exempt from defining a Waterbody Unit of Management under Criterion 2.6?
Facilities discharging into Type B or Type C Receiving Waterbodies are exempt from defining a Waterbody Unit of Management. Under Indicator 2.6.6, WUM definition applies exclusively to sites located in Type A waterbodies.
What testing method is required to measure settleable solids in farm effluent?
Under Appendix 8.12 and Indicator 2.6.16 guidance, settleable solids are measured by calculating the volume of solids that settle to the bottom of an Imhoff cone in 1 hour, expressed in ml/L, or via a calibrated spectrophotometer.
Reference documents
- ASC Farm Standard V1.0.1A2 - August 2026
- ASC Farm Standard Interpretation Manual V1.0.1 - September 2025
- ASC Farm and Feed CAR V1.0 - May 2025
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