Bivalve depuration: fundamental and practical aspects

FAO Fisheries Technical Paper No. 511

Bivalve depuration:
fundamental and
practical aspects


Ronald Lee
FAO Consultant
Weymouth, United Kingdom of Great Britain and Northern Ireland

Alessandro Lovatelli
Aquaculture Management and Conservation Service
FAO Fisheries and Aquaculture Department
Rome, Italy


Lahsen Ababouch
Fish Utilization and Marketing Service
FAO Fisheries and Aquaculture Department
Rome, Italy

Rome, 2008

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ISBN 978-92-5-106006-3

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© FAO 2008

Lee, R.; Lovatelli, A.; Ababouch, L.
Bivalve depuration: fundamental and practical aspects.
FAO Fisheries Technical Paper. No. 511. Rome, FAO. 2008. 139p.


Bivalve molluscan shellfish concentrate contaminants from the water column in which they grow. These contaminants may then cause illness to humans when the bivalves are eaten. For microbial contaminants, the risk is enhanced by the fact that these shellfish are often eaten raw (e.g. oysters) or relatively lightly cooked (e.g. mussels). Limiting the risk of illness depends partly on sourcing the shellfish from areas in which such contaminants are at relatively low levels. The risk may be reduced further by appropriate treatment following harvest.

Depuration (purification) is a process by which shellfish are held in tanks of clean seawater under conditions which maximize the natural filtering activity which results in expulsion of intestinal contents, which enhances separation of the expelled contaminants from the bivalves, and which prevents their recontamination. Depuration was originally developed as one of a number of means to address the problem of a large number of shellfish-associated outbreaks of typhoid (caused by the bacterium Salmonella typhi), which caused illness and death in many European countries and in the United States of America at the end of the nineteenth century and beginning of the twentieth century.

Depuration is effective in removing many faecal bacterial contaminants from shellfish. As currently commercially practised, it is less effective at removing viral contaminants such as norovirus and hepatitis A. It is not consistently effective, or is ineffective, in removing other contaminants such as naturally occurring marine vibrios (e.g. Vibrio parahaemolyticus and Vibrio vulnificus), marine biotoxins (such as those causing paralytic shellfish poisoning PSP, diarrhetic shellfish poisoning DSP and amnesic shellfish poisoning ASP) or heavy metals or organic chemicals.

Effective depuration requires the shellfish to be properly handled during harvest and pre-depuration transport and storage. It also requires proper design and operation of the depuration systems to meet the requirements identified above for removal and separation of contaminants. Likewise the establishments in which the system or systems are located need to be operated to good levels of food hygiene in order to prevent crosscontamination between, or recontamination of, different batches of shellfish.

This document is intended to provide a basic introduction to the public health problems that can be associated with shellfish consumption and to provide guidance as to how a depuration centre, and the associated systems, should be planned and operated. It also includes guidance on the application of Hazard Analysis Critical Control Point (HACCP) plans and associated monitoring. The document is intended to be of use to members of the shellfish industry with no or limited experience in the area and to fishery and public heath officials who may be involved in providing advice to the industry. Supplementary material may be found in the publications given in the bibliography.

Keywords: marine aquaculture, bivalve depuration, pathogenic micro-organisms, faecal contamination, food hygiene, oysters, clams, scallops


Part 1 (633 Kb)

Preparation of this document
List of figures
List of tables

Part 2 (271 Kb)

Chapter 1 – Introduction
Chapter 2 – Why depurate?

2.1 Bivalve molusc-associated illnes
2.2 Which species need depuration ?
2.3 Legislative requirements
2.4 Biosecurity

Chapter 3 – General principles of depuration
3.1 Resumption of filtration activity
3.2 Removal of contaminants
3.3 Avoidance of recontamination
3.4 Maintenance of viability and quality
3.5 Limitations of depuration
3.6 Biotoxins
3.7 Chemical contaminants

Chapter 4 – Site requirements
4.1 General location
4.2 Seawater quality
4.2.1 Natural seawater
4.2.2 Artificial seawater
4.2.3 Saline borehole water
4.3 Access to utilities and labour

Part 3 (433 Kb)

Chapter 5 – Plant design and construction
5.1 General plant considerations
5.2 Depuration tank design and construction
5.3 Trays/baskets for depuration
5.4 Plumbing and water flow arangements
5.5 Discharge of used seawater

Chapter 6 – Water treatment methods
6.1 Setlement and filtration
6.2 Ultra violet Light
6.3 Chlorine and chlorine containing compounds
6.4 Ozone
6.5 Iodophors

Chapter 7 – Pre-depuration considerations
7.1 Harvest
7.2 Transport
7.3 General handling
7.4 Ozone
7.5 Washing, culling and debyssing

Part 4 (298 Kb)

Chapter 8 – System operation
8.1 Tray loading
8.2 Tank loading
8.3 Batch operation
8.4 Conditions for depuration
8.5 Depuration period
8.6 Drain down
8.7 Monitoring

Chapter 9 – Post-depuration handling
9.1 Unloading
9.2 Washing/debyssing
9.3 Packing
9.4 Storage
9.5 Transport

Chapter 10 – Microbiological monitoring
10.1 Process verification
10.2 Ongoing monitoring
10.2.1 Seawater
10.2.2 Shellfish
Chapter 11 – Hazard Analysis Critical Control Point (HACCP)
11.1 Basic principles of HACCP
11.2 Application of the HACCP principles to shelfish depuration
11.3 Traceability

Part 5 (87 Kb)

Chapter 12 – Problem solving
Chapter 13 – Selected reading

Part 6 (591 Kb)

Appendix 1 Proposed draft code of practice for fish and fishery products

Appendix 2 Proposed draft standard for live bivalve molluscs and for raw
bivalve molluscs processed for direct consumption or for further processing

Appendix 3 Example of a depuration cycle record sheet

Appendix 4 US national shellfish sanitation programme depuration criteria

Appendix 5 WHO guidelines on drinking water quality

Appendix 6 Lobster storage and shellfish purification

Appendix 7 Enumeration of Escherichia coli in molluscan bivalve shellfish