Inspection and Approval of Purification (Depuration) Systems Guidance Notes for Local Enforcement Authorities

Transcription

1 Inspection and Approval of Purification (Depuration) Systems Guidance Notes for Local Enforcement Authorities Version 3 Page 1 of 31

2 This protocol describes the work undertaken by Cefas on behalf of the Food Standards Agency. Authorised by : Andy Younger Date: 26 May 2011 Page 2 of 31

3 CONTENTS 1 Introduction Shellfish Intended for Depuration Principles of Depuration Inspection by the Local Enforcement Authority Inspection Criteria Modifications to the existing system System Operation Shellfish Seawater Quality Measurements Record keeping Cefas Attended Inspection and Re-inspection Procedures Reasons for Inspection Guide to Cefas inspection procedures References Appendices...16 Appendix 1. The Mechanics of Depuration...16 Appendix 2. System Operation Criteria...23 Appendix 3. Guidance notes for cockles Appendix 4. Guidance notes for Ensis spp Appendix 5. Distinction between the legal requirements (stipulated in the legislation and/or Conditions of Approval) of operators and recommendations often made for good working practice Appendix 6. Depuration Record Sheet...31 Page 3 of 31

4 1 Introduction Under Regulation (EU) No. 853/2004 of the European Parliament and of the Council, the approval for shellfish purification plants to operate in England and Wales must be given by the Local Enforcement Authority (LEA) subject to the conditions set out in the Regulations and any additional conditions notified by FSA. In England and Wales, Cefas has the delegated responsibility for setting the specific conditions under which the system should operate. Cefas is UKAS accredited against ISO for the purification system inspection service. For new systems Conditions of Approval are set following a technical inspection. Subsequent to the approval process, responsibility for ensuring that the system is maintained and operated correctly is passed to the LEA. This is currently supported by periodic technical inspections by Cefas. In addition LEA s can contact Cefas for technical advice and can request that an officer from Cefas accompany them on an inspection. As part of their responsibilities LEA officers are required to make periodic inspections of the premises. Further inspections of existing purification systems may be undertaken by Cefas at the request of the LEA, where changes to the system have taken place; where outbreaks have occurred; or where microbiological end-product failures have occurred. The FSA also requires Cefas to carry out technical re-inspections for all purification plants in England and Wales at a frequency based on an assessment of risk. Cefas only has the responsibility for ensuring the system is of acceptable design and construction and that purification practices employed by the operator are appropriate. This includes procedures for the collection and treatment of water and shellfish prior to depuration. General operation of the centre such as packaging and dispatch; provision of the washing facilities and cleanliness of the centre are the responsibility of the LEA. This document outlines parameters that are critical for successful depuration and give guidance to LEA officers on aspects of system design and operation that require attention during inspection. 1.1 Shellfish Intended for Depuration Live bivalve molluscs that are to undergo depuration effectively must be in good condition. They are sensitive animals that are susceptible to temperature extremes, salinity extremes and physical shock. It is therefore vital to ensure that good harvesting and general handling practices are followed so that the animals are not unduly stressed Post-harvesting, the re-immersion of live bivalve molluscs (other than during depuration) should be avoided. Before shellfish are loaded into the depuration tank, they must be thoroughly cleaned. Any batch of shellfish undergoing purification must be of the same species and from the same class/grade of production area. Page 4 of 31

5 Whilst most shellfish can be harvested by mechanical means, cockles (Cerastoderma edule) have been shown to exhibit high levels of mortality under depuration conditions due to the damage and general stress caused by such practices, in particular by suction dredges. Similarly the effect of such harvesting on razor clams (Ensis spp.) is unclear. Consequently, it is currently a requirement that cockles and razor clams are only handgathered if they are to be depurated (Guidance of approval for cockles and razor clams (Appendix 3 & Appendix 4). 1.2 Principles of Depuration Depuration or purification is one of the major treatment processes in controlling the public health risks associated with sewage-contaminated shellfish. Depuration is the process by which shellfish naturally relinquish any microbiological component bio-accumulated in the environment. Only shellfish harvested from class B (and where deemed necessary class A ) harvesting areas are permitted to be depurated. Whilst current procedures are unable to guarantee the production of shellfish free from all microbiological contaminants, depuration plays a significant role in eliminating bacteria and reducing the viral content of shellfish. This has undoubted public health benefits. Given the relative inefficiency of viral reduction during depuration and the significance of this control step it is critical that system design and operation is followed to recognised standards to gain the maximum benefit from the process. There are five types of standard design depuration system ranging in capacity. The range of standard design purification systems, developed by Seafish, are built to specified designs that meet the technical requirements following extensive assessment and testing over a wide range of conditions. Being proven designs, bacteriological testing is less stringent and consequently they have a more predictable, simplified and potentially less time consuming and less expensive approval process. The operating manuals for all standard designs and non standard designs are found at the Seafish website and a brief overview of all the standard systems is included Appendix 1. The depuration process involves placing trays of shellfish into a purpose made tank, filled with clean seawater. In England and Wales, seawater is currently treated by ultraviolet (UV) disinfection prior to purification to prevent possible contamination of shellfish during the process. Water is then recycled through the system through the UV system and returned to the system via a cascade or spray bar that aerates the returning water to ensure the good activity of shellfish. Dissolved oxygen levels will directly affect the clearance efficiency. Given the correct physiological conditions, shellfish will resume normal filter-feeding activity and excrete contaminants in their faeces. The faecal material so produced should be allowed to settle to the bottom of the tank and then be removed at the end of the cycle. There are a number of requirements that are detailed in the Regulations for the construction and general running of purification centres. These cover elements such as the operation of batch systems, non-mixing of species during purification and the requirement that purified shellfish must meet an end product standard of 230 /100g E.coli of shellfish flesh and intravalvular fluid and absence of Salmonella. As far as the purification process itself is concerned, three essential features are identified: that shellfish should rapidly resume filter-feeding activity; remove sewage contamination; and to Page 5 of 31

6 ensure that shellfish are not re-contaminated. A more detailed overview of the system operation criteria detailing the physical and chemical parameters of depuration systems is given in Appendix 2. 2 Inspection by the Local Enforcement Authority 2.1 Inspection Criteria The following section details the major criteria that should be looked for when carrying out an inspection of a purification system. The list is not exhaustive but outlines the most often occurring problems that are reasonably easy to identify. It should be remembered that Cefas can give technical advice on any of the following issues if any uncertainty exists, but many of the non compliances are deviations from what is specified on the CoA document. Importance of Conditions of Approval Document Conditions of Approval documents should be displayed or readily available in the depuration centre. The CoA document should act as a reference and aid in depuration plant operation and inspection. Each individual depuration system will have been approved for a specific set of conditions as detailed on each current CoA. If there is any deviation from these specifications, the system may well not be working efficiently. Different tank systems and different bivalve species require distinctly differing operating conditions and it is important to ensure that these specifications can be referenced easily. A distinction between the requirements (stipulated in the legislation and/or Conditions of Approval) and recommendations for good working practice is given in Appendix Modifications to the existing system The Conditions of Approval only relate to the configuration of the system as observed at the time of approval. Any deviation from this configuration without consent from Cefas will invalidate these Conditions of Approval. Operators via the LEA or LEA s should notify proposals for any technical modifications of the system to Cefas Weymouth, so that possible implications of such changes can be assessed. Even slight changes to the plumbing arrangement, UV system etc., can have a significant effect on the depuration process and such changes should be assessed for their suitability before they are made. However, operators have on occasions proceeded with such modifications without notifying their LEA or Cefas. During an inspection the system should be checked for any modifications since the last inspection. It can be difficult to identify where changes have occurred but a good source of Page 6 of 31

7 reference is the Conditions of Approval and any previous photographs that may have been taken of the system. The operators will also often volunteer information on changes if they are asked. Any modifications identified should be reported to Cefas. It may be worth pointing out to operators at this stage that any modifications made without prior agreement may need to be changed again if they are considered inappropriate. 2.3 System The system should be of sound construction with no leaks and all associated plumbing should be of a sturdy construction. The system must be self-contained with no sources of external contamination. For example water from near by vivier tanks should not splash into the tank. Potential contamination from animal sources should not occur. Circulation of water in the system should be even throughout the entire system with no apparent possible deadspots. Shellfish trays should be stacked evenly in the system. If a flow meter is present a reading can be taken and compared with the stated minimum requirement in the Conditions of Approval. If there is no flow meter, i.e. the system was approved before this became a requirement, then there should be a method in place to check the flow rate is not below the minimum stipulated on the CoA. This can be undertaken by timing how long it takes to fill the tanks via UV and recirculation system or time taken to fill a known volume to calculate the flow rate. Cascades must not fall directly on to shellfish or water immediately above shellfish. Cascades are often changed by operators to spray across the surface of the tank to increase dissolved oxygen levels in the warmer summer months. This is not acceptable if the water falls directly onto the shellfish. If any supplementary aeration is present this should be supplied away from shellfish as bubbles in the water can interfere with filtration activity. Whilst the presence of flow meters is intended to ensure that there is a means of checking that dissolved oxygen levels are sufficient to promote acceptable levels of shellfish filtration activity, operators may wish to consider the use of dissolved oxygen meters to provide more specific confirmation. This may be particularly relevant where follow meters are not present. Trays and baskets used in the system must be of a suitable design. (e.g. open sides to allow even flow through the shellfish). The trays and baskets should be stacked in a stable configuration and raised off the base of the tank to prevent re-contamination by settled sediment. Operators may not change baskets from those that were approved at the time of the initial inspection. Cefas will specify the type of baskets/trays to be used in the Conditions of Approval. Page 7 of 31

8 2.4 Operation All shellfish must be totally covered with seawater throughout the period of purification. This may sound obvious but on occasions shellfish can be overloaded so that those in the top layer are not totally immersed. In particular mussels will expand in volume when they open and/or actively move upwards in the trays during depuration which can lead to some shellfish not being immersed (see section Loading). Shellfish should be thoroughly cleaned before being placed in the tanks. Minimal sediment should be present on the shellfish during depuration. Shellfish should be placed into approved trays to standard depths. Depths observed should be checked against the Conditions of Approval. Baskets and trays should only be loaded to the number of layers specified in the Conditions of Approval. The purification cycle should be for a minimum continuous period of 42 hours without disturbance to the shellfish. No shellfish should be added to purification tank during the cycle. The 42 hour period of depuration should be checked against the operators records. Following completion of the cycle, seawater should be drained from the tank to below the level of the shellfish in the bottom layer without disturbing the shellfish. Subsequently shellfish should be removed from the tank and thoroughly cleaned by rinsing with potable or clean seawater. It is important at this stage not to re-immerse the shellfish in any water that may re-contaminate the shellfish. This drain down and cleaning operation can obviously only be seen at the end of a cycle but it is worth checking with the operator that this is done. The tank should be thoroughly cleaned with potable or clean seawater before a new cycle is started. Evidence of a failure to do this can be seen from excessive sediment on the base of the tank. 2.5 Shellfish All shellfish should be clean, alive and healthy and appear to be actively filtering. Shellfish should be slightly open but not gaping during depuration. A reasonable amount of foam collecting on the surface of the water is indicative of actively purifying shellfish and is not a problem. Excessive foaming, however, may indicate a problem with the shellfish and should warrant further investigation. Shellfish should be harvested by an appropriate method (e.g. currently razor clams and cockles may only be depurated if hand gathered). Species of shellfish should not be mixed in the system. Page 8 of 31

9 Shellfish from different classification categories (i.e. Class A and B) should not be purified in the same system. 2.6 Seawater Quality Depuration systems will be approved for use with artificial or natural seawater or both. The type of seawater used will affect the permissible water reuse and any pre-treatment required. Only seawater for which the system has been approved should be used. This can be checked against the Conditions of Approval. Only clean seawater should be used in the depuration system. If the abstraction point for natural seawater used has changed since previous inspections then investigations should be undertaken to establish whether the current source of water is acceptable. Although no explicit definition of clean seawater exists it should be ensured as far as possible that a change of the source of the water should carry no significantly greater risk of contaminating the shellfish. Cefas will be able to provide advice on this and some microbiological testing of the source water may be required. Water should be visibly clear both during the depuration cycle and prior to entering the system. Pre-treatment of seawater (settlement or filtration) should be undertaken where source water is excessively turbid. The system should either be filled through an operational UV or recirculated through the system with the UV operational for a minimum of 12 hours before shellfish are added. If seawater is re-used for more than one purification cycle, there should be an adequate seawater storage facility to hold seawater between cycles. This will have been checked at the time of approval but it should be noted whether the storage tank has been changed. The storage facilities must be cleanable and allow drainage. Drainage of the water from the depuration tank at the end of the cycle must allow water to be transferred to the storage tank without sediment also being transferred. If there is more than one depuration tank at the premises empty clean depuration tanks can be used as a means of storage to hold seawater between cycles. Regulation (EU) No. 853/2004, Annex III, Section VII, Chap IV, A. 5 states that food business operators purifying live bivalve molluscs must ensure compliance with clean seawater ; this is not easy to determine in practice. The current Cefas advice on seawater suitable for use in depuration is as follows: Microbiological: Turbidity <1 E.coli/100ml seawater <15NTU 2.7 Measurements During an inspection the following parameters should be measured; Page 9 of 31

10 Seawater temperature. Compare with acceptable minimum temperatures on CoA. Seawater salinity. Salinity can be measured by a salinity meter, refractometer or hydrometer following the manufacturers instructions. Compare with acceptable minimum salinities on CoA. If a dissolved oxygen meter, turbidity meter or flow meter is available these parameters can also be measured. 2.8 Record keeping Purification centres must keep records of the permanent management procedures for the control and elimination of hazards to the safety of live bivalve shellfish as stipulated in EU regulation 852/2004 Article 5 (g) & Annex I Alll 7. Minimum legal requirements for keeping records at depuration centres are as follows; Time, quantity and species of shellfish in and out of the tanks. Microbiological results before and after depuration on a number of purification cycles. Microbiological testing by the operator should be carried out in accordance with the frequency determined in their own risk analysis according to HACCP principles (as required under EU Regulation 852/2004). UV usage. The hours of UV usage should be recorded and the maximum permissible hours of use stipulated in the Conditions of Approval should not be exceeded. Water reuse in exceptional circumstances. Where water is reused more than the normally permissible number of times due to exceptional circumstances this must be recorded along with the exceptional circumstances causing that extended reuse. Operators should also be encouraged to keep the following records for good practice and due diligence purposes. Seawater temperature Seawater salinity Flow rates Tank cleaning Water re-use at all times A checklist of the above parameters is given in Appendix 6, this is intended to serve as an aide during an inspection and notes can be made as required. Page 10 of 31

11 3 Cefas Attended Inspection and Re-inspection Procedures 3.1 Reasons for Inspection A number of situations arise where inspections of a purification system(s) accompanied by a competent Cefas Inspector is required. These are listed below: New system (either at an existing centre or at a new centre) Modification to an existing system including movement to new premises By invitation from the responsible Enforcement Authority usually following endproduct failures, outbreaks of illness associated with shellfish from the system or change of ownership As part of ongoing re-inspection programme Relocation of existing approved plants (generally treated as a re-inspection to ensure plant installed correctly) 3.2 Guide to Cefas inspection procedures New purification (depuration) plant approval procedure. Completion of the accompanying Request to Inspect data sheet needed. Please also include photographs of the system and any design schematics available. Below is a guide to the Cefas approval process. Enforcement Authority notification to Cefas of approval request by industry. Request to Inspect data sheet ed to Local Enforcement Agency Officer recipient for completion. On receipt of completed Request to Inspect data sheet, an evaluation will be conducted of the information to determine the system type (Standard design or otherwise) and its suitability to task. A preliminary inspection may be required. Local Enforcement Agency Officer recipient will be informed of any remedial action required prior to the continuation of the Approval process. or A preliminary date for inspection shall be arranged. Confirmation letter will be sent to the relevant Local Enforcement Agency Officer recipient detailing the following: Cefas s requirements for the inspection and microbiological challenge test (normally undertaken at the same time as the inspection - Responsibility of Local Enforcement Agency Officer to organise). Page 11 of 31

12 Inspections will be undertaken during the microbiological challenge preferably to coincide with the termination of the cycle. Any further remedial action required prior to approval will be identified at this time. On receipt of the pre and post depuration results, an evaluation will be made by Cefas to establish if: The system has failed to demonstrate a satisfactory clearance. (A successful challenge analysis is required prior to approval). or The system has successfully demonstrated sufficient clearance and to end product standards. Following completion of remedial actions, and the demonstration of a satisfactory clearance, the Conditions of Approval will be drawn up and issued to the Relevant Local Enforcement Agency Officer recipient. Specific advice concerning the design and running of systems should be sought from the Sea Fish Industry Authority or other suitable consultancy. Inspection Cefas will need to inspect the fully loaded tank whilst it is running. It is preferable to make the inspection at the end of the cycle so that the procedures for draining the system down can be observed. If this is not possible the systems must be at least 24 hours into the cycle at the time of the inspection. Depuration inspection criteria System type and specifications mode of action (standard Seafish design or novel ) Species depurated method of harvesting (suitable for species to be depurated e.g. cockles and razor clams) Maximum loading capacities loading arrangements (must facilitate free and even flow of water) tray type (as above) UV specifications dose and utilisation (must exceed minimum dose prescribed for comparable standard system of interest or in any event >10mJ/cm 2 ) pre-treatment of seawater (to achieve clean seawater ) Flow rate: flow meter must be present on all new systems dissolved oxygen levels (should be >5mg/L) Page 12 of 31

13 clarification, turbidity assessment UV treatment prior to use (fill via UV or recirculate for 12 hrs) Minimum period of Depuration (42hrs) Drain down procedures (to avoid resuspension of sediment) Record checks end product Testing UV Usage purification times temperature salinity seawater re-use loading density Microbiological test In addition to an inspection, a microbiological challenge test will need to be carried out using a full load of shellfish. This is normally timed to coincide with the inspection visit. Shellfish with mid-range class B E.coli levels (i.e. at least 2000 E.coli/100g) should be used to enable adequate demonstration of depuration efficiency. A repeat test may be necessary if predepuration E.coli levels prove to be too low. Three post-depuration samples should be taken after 42 hours from the following points: top tray near the cascade middle tray in the middle of the tank bottom tray near the suction bar Shellfish used for this test should preferably originate from a class B site. If, however, the local class B shellfish are not sufficiently contaminated a number of options are available: 1.Use a full load of class C shellfish In this situation it is up to the local authority to ensure that the shellfish do not go for human consumption, but are re-laid for at least 2 months in a designated class A/B relay area or are replaced in the class C area. Page 13 of 31

14 2.Part load with class C shellfish for sampling only The class C shellfish should be clearly marked (preferably in sealed net bags) and used for sampling purposes only and spare shellfish should not be placed back on category A or B beds. The rest of the load may be class B (or class A) but at the end of the cycle, all of the shellfish must be placed back in the harvesting area or in a designated class A/B relay area for at least 2 months. 3.Artificially contaminated shellfish The only realistic way of artificially contaminating shellfish is with sewage e.g. by placing shellfish near an outfall. Shellfish contaminated in this way will have unknown virus levels and must be assumed to be of prohibited status. As such, at the end of the cycle, the shellfish must be re-laid for 6 months before they can be harvested for human consumption. Specific advice concerning the design and running of systems should be sought from the Sea Fish Industry Authority or other suitable consultancy. If you have any queries concerning the approval process itself then please contact Cefas Re-inspection procedure The procedure for re-inspection is similar to that for new inspections except that a microbiological challenge test is not normally required. At least one type of each approved system will need to be fully loaded and operating at the time of inspection. At re-inspection there is a greater emphasis on paperwork (system operation records and traceability). Following the re-inspection a letter will be sent by Cefas to the local authority detailing the findings and any necessary remedial action. Local authority assistance is essential in ensuring any identified problems are rectified. The local authority is asked to confirm when the necessary changes have been made. If this confirmation is not received (a reminder letter will be sent by Cefas after 3 months) then the matter will be referred to the FSA. For further details of Cefas depuration inspection requirements and criteria see: Cefas Protocol for Inspection and Approval of Purification (Depuration) Systems England and Wales The latest version can be found at: Page 14 of 31

15 4 References Cefas Protocol for Inspection and Approval of Purification (Depuration) Systems England and Wales Regulation (EU) No. 853/2004 of the European Parliament and of the Council Seafish Operation manual, advice and good practise guidelines for bivalve molluscs. SEAFISH Generalised Operating Manual for Purification Systems of Non- Standard Design. March 1995 SEAFISH Operating Manual for the Small Scale Shallow Tank Purification System. March 1995 SEAFISH Operating Manual for the Medium Scale Multi-layer Purification System. March 1995 SEAFISH Operating Manual for the Large Scale multi-layer Tank Purification System. March 1995 SEAFISH Operating Manual for the Bulk Bin System for Mussels.March 1995 SEAFISH Operating Manual for the Vertical Stack System. March 1995 Page 15 of 31

16 5 Appendices Appendix 1. The Mechanics of Depuration 5.1 System general design and function Currently, all depuration systems in England and Wales use re-circulation of either artificial or natural seawater. Essentially, molluscs are placed in mesh type trays and immersed in clean re-circulating seawater for a minimum period of 42h. Seawater enters via a spray bar above one end of the tank and in doing so oxygenates the water. Water flows through the containers of molluscs to a suction pipe or weir across the other end. The tanks are typically constructed of food grade plastic or stainless steal. The water flow rate in this simple design is relatively low and hence the capacity to supply sufficient dissolved oxygen to the molluscs is also low and the permissible loading (i.e. the stacking) of the system is limited. The seawater is re-circulated via a pump and the flow rate controlled by a valve and flow meter. The seawater receives microbiological treatment by passing through an enclosed ultra-violet light (UV) sterilisation unit. Resumption and maintenance of normal filter feeding activity is essential during which time the molluscs naturally purge themselves of any bacterial/viral contamination. Trays are raised off the base of the tank to facilitate settlement of faecal strands (containing any relinquished bacteria or virus) and detritus. The removal of waste products and detritus via sedimentation is the key principle employed by all Standard Design Systems. 5.2 Standard Design Systems and Non Standard Systems There are five types of standard design system ranging in capacity. The range of standard design purification systems, developed by Seafish, are built to specified designs that meet the technical requirements following extensive assessment and testing over a wide range of conditions. Being proven designs, bacteriological testing is less stringent and consequently they have a more predictable, simplified and potentially less time consuming and less expensive approval process. The operating manuals for all standard designs and non standard designs are found at the Seafish website search depuration The Standard Design Shallow Tank System (Small Scale) Figure 1 This uses a 650 litre Allibert Type plastic pallet box or similar as the tank into which six mesh type plastic trays may be stacked in two tiers, each three trays high. All trays are raised off of the floor of the tank by a minimum of 25mm. To facilitate this trays are normally stacked on top of lengths of plastic pipe battens, which keep them clear of the tank floor. Seawater recirculation equipment consists of a suction pipe, waterproof pump capable of delivering a minimum flow rate of 20L/min (1.2m 3 hr.), flow control valve and in- Page 16 of 31

17 line flow meter, a minimum 25 watt UV water steriliser, perforated spray bar and interconnecting pipe work. T pieces either side of the pump provide for seawater filling and emptying. The suction pipe must be clear of the tank base, and a drain is fitted in the tank floor for final drainage and cleaning. When fully loaded the system has a maximum capacity of 90 kg of mussels and requires a minimum operational seawater volume of 550 litres. Figure 1. Small Scale Standard design (Shallow Tank) schematic detailing tray arrangements and flow dynamics. Adapted from Seafish Operating Manual for the Small Scale Shallow Tank Purification System, March The Standard Design Multi Layer Systems (Medium and Large Scale) Figure 2&3 Currently there are both medium and large-scale standard design multi-layer systems in use in the UK, with maximum capacities of 750 kg and 1500 kg of mussels capacity per tank and seawater requirements of 2,600 litres and 9,200 litres per tank respectively. Page 17 of 31

18 Figure 2. Medium Scale Standard design (Multi Layer System) schematic detailing tray arrangements and flow dynamics. Adapted from Seafish Operating Manual for the Medium Scale, Multi-Layer Purification System, March Figure 3Large Scale Standard design (Multi Layer System) schematic detailing tray arrangements and flow dynamics. Adapted from Seafish Operating Manual for the Large Scale Multi-Layer Purification System, March Medium Scale Mesh type plastic trays may be stacked in ten tiers, each five trays high (5 x 2 x 5). All trays are raised off of the floor of the tank by a minimum of 50mm. The medium-scale system is available both in glass reinforced plastic and marine grade stainless steel. Seawater recirculation equipment consists of a suction pipe situated behind a perforated screen, waterproof pump capable of delivering a minimum flow rate of 208.3L/min (12.5m 3 hr.), flow control valve and in-line flow meter, a minimum 4 x 30 watt UV water steriliser, perforated spray bar, perforated screens and interconnecting pipe work. T pieces either side of the pump provide for seawater filling. The suction pipe must be clear of the tank base, and a drain is fitted in the tank floor behind the perforated screen for final drainage and cleaning. When fully loaded the system has a maximum capacity of 750 kg of mussels and requires a minimum operational seawater volume of 2500 litres Large Scale Mesh type plastic trays may be stacked in twenty tiers, each five trays high (10 x 2 x 5). The Large-scale system is available marine grade stainless steel. Seawater recirculation equipment consists of a suction pipe situated behind a perforated screen, waterproof pump capable of delivering a minimum flow rate of 158.3L/min (9.5m 3 hr.), flow control valve and in-line flow meter, a minimum 6 x 30 watt UV water steriliser, perforated spray bar, delivery perforated screen and interconnecting pipe work. T pieces either side of the pump provide for seawater filling. The suction pipe must be clear of the tank base, and a drain is fitted in the tank floor behind the perforated screen for final drainage and cleaning. When fully loaded the system has a maximum capacity of 1500 kg of mussels and requires a minimum operational seawater volume of 9200 litres. Page 18 of 31

19 5.2.5 The Standard Design Bulk Bin This type of system has been developed for mussels only and is not suitable for other species, as they do not function naturally when held in deep layers. The mussels are not placed within trays but instead up 300kg are held in a deep layer in a number of specially modified plastic pallet bins. Each bin is transportable and can be connected to a common seawater supply and return system. This system consists of a sump from which seawater is pumped to an ultra violet light (UV) steriliser unit, where it receives microbiological treatment, and then to a main supply line. (A) (B) Figure 4. Standard design Bulk Bin System schematic detailing tray arrangements and flow dynamics within each bin (A) and the modular arrangment for a given system (B). Adapted from Seafish Operating Manual for the Bulk Bin Purification System, March From this, each bin receives a controlled seawater supply, which ultimately returns to the sump via a common drainage channel. The pallet bin is a GPG Dolav Type D12105 with a capacity of 650 litres and external dimensions of 1200mm x 1000mm x 740mm. It is fitted with a plastic false floor, fabricated from plastic sheet and drilled with holes to permit the flow of water and detritus. The false floor is a good fit to the sides of the box, and is Page 19 of 31

20 supported 80mm clear of the box floor that is removable for cleaning purposes. Mussels are loaded to a depth of 380mm above the false floor, which corresponds to approximately 300 kg of mussels per bin. UV treated water (2 x 30 watts per bin) is supplied at a minimum rate of 108.3L/min (6.5m 3 hr) per bin from the main supply line via a control valve and a flexible feed pipe such that it enters each bin above the mussels but below the water surface. Water outlet pipes are fitted, one below the false floor but clear of the base of the bin, and the other near the top rim of the box. The lower water outlet pipe is sized to permit a water flow of 6,500 litres/hour down through the mussels with the water level at the upper outlet pipe, which acts as an overflow. Each bin drains automatically when the water supply is turned off. Aeration takes place as the water cascades into the drainage channel and again as it cascades from the drainage channel into the sump. The down-welling flow carries detritus to the base of the bin where most of it settles out. Some is inevitably drawn to the sump, which is designed to enable further settlement. The sump tank should have a seawater capacity of at least 1m 3 per bin and be designed to enable further settlement of any detritus from the bins. This is often achieved by use of a baffle plate where the water returning to the sump is directed down to its base, and the suction pipe to the pump draws water from closer to the surface (Figure 4(B)). The specific pipe diameter on the bin outlet (to be specified by Seafish) provides a flow through of the water through the bin at 6500l/hr Flow meters are not required on each bin if the outlet pipe diameter is correct, but a flow meter for the for the whole system is required to ensure: There is no reduction in flow to the system as a whole over time (e.g. due to wear of pump) The confirmation of UV applied dose is above the minimum required (10mJ/cm 2 ) The Standard Stacking System ( Figure 5) The bivalve molluscs are held in purpose designed solid sided trays which are supported in a stainless steel frame over a plastic pallet box (650 litres) used as a sump tank. The framework supports a total of sixteen containers in two vertical columns. Seawater is drawn from the sump up to one end of the top container of each stack. It flows along the container and then cascades down from the other end into the container below, and so on until it returns eventually to the sump. The cascades provide re-oxygenation between containers. The mollusc containers used are a specially modified Allibert Type The modifications ensure a uniform flow of seawater through the molluscs and that the cascade is not directed onto the molluscs in the container below. This involves the partitioning of the recesses at one end of the container with small plastic flow screens to contain the initial turbulence of the water cascading into them. Page 20 of 31

21 Figure 5. Standard design Vertical Stack System schematic detailing tray arrangements and flow dynamics between and within each tray. Adapted from Seafish Operating Manual for the Vertical Stack Purification System for Mussels, March At the other end of the container plastic overflow pipes are fitted to maintain the required water level and direct the water into the recesses of the tray below. Each container is fitted with removal mesh plastic mat. This holds molluscs above the bottom of the container and accumulated sediment. Seawater recirculation equipment consists of a suction pipe, waterproof pump capable of delivering a minimum flow rate of 15L/min (0.9m 3 hr.), flow control valve and in-line flow meter, a minimum 25 watt UV water steriliser. T pieces either side of the pump provide for seawater filling and emptying. The suction pipe must be clear of the tank base, and a drain is fitted in the tank floor for final drainage and cleaning. Mussels often move around during the depuration process by attachment of their byssal threads to the side of trays. It has been observed that mussels may climb out of the Allibert Type trays, and it is therefore generally recommended that the standard Vertical Stack System is not used for the purification of mussels. However, if used, any mussels found to be out of the water during the 42 hour cycle should be discarded Non-Standard design There is no obligation on a purification centre operator to install standard design systems. The operator may wish instead to install a system to his own specification. Such a purification system must still meet the technical requirements of Government Departments and help and advice on achieving this can be obtained from the Seafish Guidelines and the Page 21 of 31

22 Seafish Advisory and Consultancy Service. Although the initial capital cost of such a system using components already available to the operator may be less than that of a manufactured standard design, additional costs may accrue from consultancy fees and extensive bacteriological testing. There may also be risks, delays and costs associated with remedial work following inspection and testing. Non-standard designs currently in use in the UK are built as variations on the shallow tank, multi-layer or vertical stack themes. For novel systems, which are distinctly removed in terms of design from standard design systems, a different approval approach is warranted. On receipt of the Request to Inspect data sheet, a preliminary assessment based on this information, as to whether an approval visit is at all worthwhile must be conducted. Such systems will generally require a preinspection visit and assessment in the presence of both operator and Enforcement Authority Officer to be followed by rigorous assessment prior to any approval. Page 22 of 31

23 Appendix 2. System Operation Criteria Resumption and maintenance of normal filter feeding activity is essential if depuration for the prescribed minimum period of 42h is to be effective. The operation of the entire process must: Allow rapid resumption of filter feeding Facilitate the removal of contaminants from the shellfish Avoid re-contamination of the shellfish 5.3 Rapid Resumption and Maintenance of Filter Feeding Activity Dissolved oxygen To facilitate normal shellfish activity, sufficient oxygen must be available in the water. Minimum dissolved oxygen levels of 5mg/L are recommended for purification systems, however a well designed and operated system will maintain levels that are much higher. In recirculating systems, the dissolved oxygen content of the water can be affected by a number of factors such as: water surface area to volume ratios; flow rates; shellfish to water ratios; seawater temperature; the metabolic rate of shellfish under purification (which may be environmentally and/or genetically determined); seawater salinity and the method of aeration used in the system. All these factors must therefore be carefully controlled during the purification process. Primary aeration is normally by means of a cascade but supplementary aeration may be added by using air diffusers placed in the bottom of the tank or sump provided such aeration does not disturb the molluscs or the settling of faecal material Loading Shellfish must be loaded in the trays at a density that allows them the room to be able to function normally. They should be able to open as they would in the natural marine environment and carry out their normal filter-feeding activity. This loading arrangement will vary according to the species of shellfish being depurated. The level of water above the shellfish should also be sufficient to ensure that the shellfish remain immersed throughout the entire period of depuration. Mussels often move upwards in the trays during the process by attachment of their byssal threads to the side of the trays. A greater depth of water above the uppermost tray of mussels is therefore required (80mm is normally specified in the Condition of Approval). Other species such as cockles are more sessile and consequently do not need to be immersed to such a depth (30mm is normally specified). The trays of shellfish within a system need to be arranged in such a way as to ensure that water cannot short circuit around them. Therefore trays are normally orientated so as to provide a complete barrier to flow. In this way, water must pass through the trays of shellfish (providing oxygen and dispersing metabolic by-products as it does so) before it can be recirculated back through the system. Page 23 of 31

24 5.3.3 Shellfish to water ratio The loading of shellfish for a given volume of water needs to be controlled, both to maintain dissolved oxygen levels to ensure optimum shellfish activity and also to ensure that the build-up of metabolic by-products does not reach inhibitory levels. The maximum shellfish capacity is therefore specified in the approval conditions of each type of system. This will be dependent upon the type of system and the individual species concerned Water flow It is essential to provide a sufficient and even flow of water throughout the system to maintain adequate levels of oxygen in the water and prevent the build-up of metabolic byproducts, which may inhibit normal shellfish activity. The flow of water must not however, be so great as to prevent the settlement of faecal material or cause the disturbance of such material that has already reached the bottom of the tank Salinity It is also necessary to provide seawater of the correct salinity range for the shellfish being depurated as requirements vary according to species. Salinity requirements are specified on each systems Conditions of Approval (CoA) document and can also be found in the Cefas Protocol for Inspection and Approval of Purification (Depuration) Systems England and Wales (see References) Artificial seawater may be used where access to a ready supply of suitable natural seawater is not available. It is important that the correct artificial seawater formulation is used Temperature In general, the metabolism of shellfish is directly affected by the temperature of their environment. With decreasing temperature, shellfish become less active and contaminant removal is decreased. Therefore water temperatures are required to be kept above a minimum level during depuration. Minimum temperatures are specified on each depuration systems Conditions of Approval (CoA) document and can also be found in the Cefas Protocol for Inspection and Approval of Purification (Depuration) Systems England and Wales (see References). If during the cycle the temperature falls below these minimum values then the period of time that the system is below that temperature should be added to the purification time Turbidity Control of turbidity is important for two reasons. Firstly, because UV disinfection efficacy is considerably reduced by relatively low turbidity and so contamination or recontamination may occur if purged microbial contaminants are recirculated throughout the system. Secondly, if the turbidity is excessive this may have a detrimental effect on the filtration activity of the shellfish. As a consequence, it is recommended that turbidity should not exceed 15NTU (Nephelometric Turbidity Unit, the measurement of suspended solids in a sample). Page 24 of 31

25 5.3.8 No disturbance In addition to all of the above, it should be noted that shellfish are sensitive animals and if disturbed directly by the effects of cascades, aeration or operator handling during the purification cycle, will cease to function effectively. If shellfish are disturbed in this way the period of purification should begin again and a further 42 hours required. 5.4 Removal of sewage contaminants During depuration contaminants are excreted as part of the digestive process predominantly in the form of mucoid faecal strands, which must be allowed to settle to the bottom of the depuration tank. Once settled, re-suspension of this faecal matter must be avoided as this may lead to its re-ingestion by the filter-feeding shellfish. At the end of the cycle, seawater in the system should be drained down below the level of the shellfish before they are removed. This prevents turbulence caused by removal of trays of shellfish immersed in water leading to the possible re-suspension and re-ingestion of faecal material in neighbouring shellfish. At the end of each cycle, the remaining water must be discarded and the bottom of the tank thoroughly cleaned, as this is where the shellfish faecal material containing the contaminants will be concentrated. 5.5 Avoiding recontamination Tanks should be covered or housed in a building with a roof to prevent aerial contamination from birds. Vermin such as rodents should also be excluded from the area. In order to avoid re-contaminating the shellfish, it is vital that all steps should be taken to avoid the possibility of re-suspension and thus re-ingestion of shellfish faecal material. One of the most important practices in this regard is the operation of a batch system, i.e. once the tank has been appropriately loaded and the cycle has commenced, additional shellfish should not be added or any removed until the full cycle (currently 42 hours) has been completed and the tank drained down. If this practice is not followed then recontamination, either from added shellfish or by re-ingestion of re-suspended shellfish faecal material caused by trays being removed whilst still immersed, may occur Seawater quality The lack of any defined values has caused some practical problems with the interpretation and implementation of the requirement for clean seawater to be used. Incoming water, often collected in estuaries should be treated to reduce any turbidity. Settlement periods of 12 hours or filtration of turbid waters is required before filling tanks via UV to ensure UV efficacy. If treatment of the seawater is necessary, then the competent authority must verify the treatment method and authorise its use. Page 25 of 31

26 If the purification system is recirculating water then steps must be taken to ensure that the recirculating water is of adequate quality. A feature vital in this respect is in-line UV disinfection. In addition, the adequate provision for the settlement of shellfish faecal material for reasons previously discussed is required Seawater re-use If seawater is to be re-used from batch to batch then this too must be of a suitable quality. The re-use of natural seawater is permitted for certain species and types of system, provided that there is adequate provision for storage of the water drained from the system at the end of the cycle. The remaining water in the tank that is below the shellfish must be discarded, as this will contain the shellfish faecal material egested during the cycle. The main reasons for limiting the re-use of seawater are: The possible build-up of shellfish metabolic by-products e.g. ammonia leading to inhibition of shellfish activity. The possible build-up of contaminants from batch to batch. In the absence of an adequate storage vessel, reuse of seawater is not permitted and the Conditions of Approval must reflect this. The re-use of natural seawater is normally limited to 2 weeks with an allowance for an extra 2 weeks should exceptional weather conditions prevent the collection of water of suitable quality for small-scale systems. Records must be kept for such extended use. Where permitted artificial seawater may be used for 4 weeks providing at least 10% is replaced after each cycle and that the shellfish to water ratio is not excessive. For the high-density systems (generally shellfish to water ratio of 1:3 or less)), such as the medium scale system and some bulk bin systems, re-use is limited to 3 purification cycles for both artificial and natural seawater. For the depuration of cockles and razor clams the reuse of seawater is prohibited. Page 26 of 31