APPENDIX G STATUS OF SHELLFISH DEPURATION IN MASSACHUSETTS By Mario M. Boschetti Massachusetts State Department Boston, Massachusetts Induced self-purification of shellfish, presently known as "depuration," has been practiced in Massachusetts since the early 1930's. The first shellfish treatment plant located in Newburyport is still in operation although the methods originally employed have been modified over the years (1). Shellfish treated at Newburyport are taken from moderately polluted areas in which the waters have a median coliform MPN of 700 or less and the shellfish a median coliform MPN of 24,000 or less. The only species of shellfish treated at Newburyport is the soft shell clam (Mya arenaria). Other depuration plants, which have not been in operation for 20 years or more, were treating quahaugs (Mercenaria mercenaria) for a short time with limited success. The cost of operation coupled with certain difficulties inherent in the process, such as water temperature regulation, forced these plants to close their doors. Prior to 1962, the method of depuration used in Massachusetts was a modification of the Dodgson (2) Mussel Purification Method. A full discussion of this method will not be dealt with here, but such information will be made available upon request. Briefly, chlorine was used as the water sterilizing agent with the concentration kept below 0.5 parts per million. The work of several investigators (3) has since demonstrated that chlorine in amounts approaching zero parts per million can hinder the self-purification process and even when neutralized with sodium thiosulfate. Results over the past 30 years at the Newburyport Shellfish Treatment Plant indicate that effective shellfish depuration, i.e., reduction of coliform content of shellfish to 2400 MPN or less, was not consistently obtained. However, the epidemiological "record" established over these years indicates that the purification process was sufficiently effective to reduce the bacterial and viral content of the soft shell clam below disease causing levels or that the soft shell clam, once cooked, rarely, if ever, has been identified as the responsible agent in the transmission of water-borne diseases. Let me hasten to add that additional possibilities may exist. One that comes to mind is that disease outbreaks due to the consumption of treated shellfish may have occurred undetected or at least not in sufficient numbers to cause public health concern. Impetus was given to the use of ultraviolet light in sterilizing seawater by research on shellfish depuration performed both overseas (4) and in this country (5). You have already heard of work done in this area today, and I am certain that you will hear more before this Workshop is adjourned. In Massachusetts, the initial research work on the use of ultraviolet light for shellfish depuration was carried out at the Newburyport Shellfish Treatment Plant during 1962 and 1963. This work indicated that ultraviolet light was effective in reducing the coliform content of seawater to virtually zero even when starting with a water containing several thousand coliform per hundred ml. The initial research further indicated that shellfish allowed to feed in ultraviolet treated water showed a marked coliform decrease over that achieved with the use of chlorinated water. Results of the ultraviolet treatment indicated that effective treatment could be obtained 95% of the time while the use of chlorinated water produced only 70% effective treatments. A large ultraviolet unit was constructed similar in design to the Purdy unit. The Newburyport unit contains 14 G.E. germicidal lamps (G30T8), and has the capacity to sterilize seawater at the rate of approximately 3,000 gallons per hour. Due to the hydraulic characteristics of the depuration system, sterilization of the water in one of the depuration tanks which holds about 3500 gallons of water takes approximately four hours. To facilitate further discussion relative to the treatment process at Newburyport, a rather brief stepwise description of the depuration process follows: 1. Shellfish arrive at the plant in 1/2 bushel boxes of wood and wire construction. 2. If time is available and the shipment is not too large, the shellfish 3. The shellfish are loaded into the concrete tanks which hold approximately 250 boxes. They are kept from direct contact with the bottom of the tank and so placed as to allow for circulation of the tank water. 4. The tanks are filled with seawater and chlorine is added at one end of the tank while the tank is aerated for a short time to assure mixing of the chlorine. The chlorine dissipates in 15-30 minutes. Pre-chlorination is necessary since the source of water does not meet the standard of 70 MPN. 5. The tank water is then continuously circulated through the ultraviolet unit at an approximate rate of 3000 gallons per hour. 6. Bacteriological samples are taken of the raw water, the untreated shellfish, the treated water, the treated shellfish, and the tank water before discharged to waste. 7. To indicate whether or not the shellfish receive the full specified 24hour treatment, a mechanically operated, manually energized recording thermometer is used. One of two leads is submersed in the tank water and the other is positioned directly in the flume of the ultraviolet effluent. In the event of a power failure, which temporarily inactivates the ultraviolet unit interrupting the flow of water, the lead in the ultraviolet effluent will register air temperature and the deviation from the water temperature in the tank will be easily noted. The treatment time lost can be determined and the shellfish will be treated an additional equivalent time. 8. The ultraviolet lamps are checked periodically with a G.E. ultraviolet meter which measures micro-watts per square centimeter of light intensity. The bulbs are cleaned daily and replaced when necessary. The treatment tanks are constructed of concrete and measure 15' x 5' x 5' in depth. They are divided by wooden baffles into two compartments of unequal size. The smaller compartment, barely 3' in length, serves as the aeration chamber. Air forced in at the bottom of this compartment causes water to rise over the dividing baffle and the resulting currents effect circulation of the water throughout the tank. The larger compartment contains the shellfish. The primary baffle extends from a short distance below the top of the tank walls to a short distance from the bottom of the tanks. A secondary smaller baffle situated near the bottom of the primary baffle prevents the trespass of debris, such as broken shells, pseudo-feces, sand, etc., into the aeration compartment. During the past ten months, samples of untreated and treated clams have been collected from the Newburyport plant for bacterial examination at the Lawrence Experiment Station. Analyses of the results (see graph 1) which include 70 pairs of samples having an MPN range of 2300 to 92,000 show that: 1. 2. Sixty-five percent show some reduction of the coliform density, 35% show no reduction or an increase in coliform density. Fifty percent of the samples show at least a 30% reduction in coliform 3. Twenty-five percent of the samples show at least 64% reduction in coliforms. REFERENCES 1. Fox, L., Shellfish Sanitation Program of Massachusetts. Sanitalk Vol. 5 2. Dodgson, R.W., Report on Mussel Purification, Fish Invest., Series 11, 10, 1, London. 3. Kelly, C.B.; Arcisz, W.; Presnell, M.W.,; and Harris, E.K., Bacterial Accumulation by the Oyster (Crassostrea virginica) on the Gulf Coast. R. A. Taft SEC, Cincinnati, Ohio, Technical Report F60-4, 1960. 4. Wood, P.C., The Utilization of Ultraviolet Light in the Purification of Oysters. International Council for the Exploration of the Sea. C.M. 1959 Shellfish Committee No. 41. 5. Kelly, C.B., Disinfection of Sea Water by Ultraviolet Radiation, A.J.P.H. Vol. 51, No. 11 November 1961. |