Team members: Chen, A., de-Graft Johnson, E., Gitter, A., Kozak, S., Niquice, C., Zimmer-Faust, A.
Problem Statement
Treated wastewater is a potential resource, especially in regions facing water shortages. Furthermore, when compared with alternative water resources such as rainwater and stormwater, wastewater offers a more constant and dependable volume of supply. It is used as an alternative water source for aquaculture cultivation in developing countries because of the high nutrient and biosolid availability for shellfish, fin fish and aquatic vegetation, according to FAO. As the ultimate goal of reusing treated wastewater is ensuring that it is fit-for-purpose, the comprehensive assessment, management and communication of potential health risks are necessary. Although not mandated by policy, QMRA is recognized as a potentially useful methodology for estimating potential levels of health risk associated with exposures to microbial pathogens.
With the increase in global population and seafood consumption, aquaculture practices are essential for meeting global food demands. Aquaculture currently supplies 43% of all seafood to consumers, according to Cole et al.. The United States continues to be a primary consumer of aquaculture, with around 91% of total seafood consumed originating abroad, causing a seafood trade deficit of over $11.2 billion per year, according to NOAA. With increasing population growth and the need for sustainable aquaculture practices, the human health concerns for aquaculture grown in treated wastewater should be reviewed as a possible alternative for aquaculture production. Two successful wastewater-fed aquaculture systems exist in Munich, Germany and Calcutta, India and serve as representations of the economical benefits of these operations. The system in Calcutta has been in operation since the 1930s and has optimized their batch-fed pond systems with low organic loading rates of sewage (similar to the maturation pond stage), according to Strauss. Strauss also stated that the Munich fishponds system is used as a step of secondary wastewater treatment and to yield fish for consumers. In both wastewater systems, fecal coliform concentrations are measured around 101 to 103/100 mL.
Penaeid shrimps are the most common shrimp species grown in an aquaculture system and account for 80% of total farmed shrimp production, according to FAO. Penaeus vannamei, Whiteleg shrimp, have been effectively grown in multiple states in the US, including Alabama, Arizona, Florida, Georgia, Hawaii, Louisiana, Mississippi, South Carolina and Texas, according to Rosenberry. NOAA states that imports of aquaculture raised seafood to the United States is dominated by shrimp, followed by Atlantic salmon, tilapia, and shellfish with Asian countries and Ecuador supplying the majority of shrimp to the U.S.. Several different pathogens play a potential role in gastrointestinal infections and illness from aquaculture produced in wastewater ponds. For the potential application of wastewater-fed aquaculture on a commercialized scale, especially for US consumers, the primary health risk concerns stem from bacteria and viruses, according to Strauss. Salmonellosis has been linked to consumption of shrimp grown in aquaculture ponds and is of great concern since shrimp is commonly consumed undercooked or even raw, according to Koonse et al.. While the Food and Drug Administration (FDA) requires inspection of the safety of shrimp products, only 0.7% of shrimp imports were reported to be inspected and over 58% of seafood products identified as being contaminated for Salmonella are shrimps and prawns (Consumer Report). Shrimp have been identified as the most important traded seafood commodity, according to Amagliani et al.. Sapkota et al. states that aquaculture production has rapidly increased in the last several years, contributing over 40% of the total global seafood production. The use of wastewater or treated wastewater has been used in the developing world to increase aquaculture production and meet the increasing demand of population growth and limited water resources. Concerns of gastrointestinal infections from fecal waste present in treated wastewater does exist, and therefore requires assessment of the consumer risk for a GI illness.
While numerous experiments have been conducted to assess the application of wastewater stabilization ponds for aquaculture production, especially for non-bivalve species, there exists the potential for use of this practice at the commercial scale, according to Edwards. A microbial risk assessment for crustaceans, specifically shrimp, needs to be conducted to determine if the potential for widespread application of the growing commodity of aquaculture can continue to grow given less resources and land available for traditional agriculture practices, according to Strauss. This study aims to conduct a QMRA to estimate the relative risk for a GI illness, represented by Salmonella, for human consumers of wastewater-fed aquaculture products in USA. The products of interest includes crustaceans, specifically shrimp.
Hazard Identification
Salmonella has been pathogen linked several different food outbreaks due to human/animal contamination or cross-contamination from other sources. In a study conducted by the FDA, aquacultured seafood products were found to more likely contain Salmonella than wild-caught seafood products, according to Koonse et al. Source water has been previously identified as a primary contributor of Salmonella and other fecal bacterial organisms to shrimp aquaculture ponds.
The genus of Salmonella are facultative anaerobic gram-negative rods which are non-spore forming and motile by flagella. Salmonella spp., of which at least 2,500 different serotypes have been identified, can be pathogenic to both humans and animals and primarily transported by the fecal-oral route, according to Mufty and Iwamoto et al.. The two most common serovars of Salmonella include serovar Enteritidis and serovar Typhimurium, according to OIE, cited in Mufty.Iwamoto states that Salmonella is one of the leading causes of foodborne illnesses in the United States, contributing to at least 1.4 million cases annually.
Different sources for bacterial contamination of seafood products, such as shrimp, include sewage-discharge into harvesting areas and sewage runoff from inland point by flooding or heavy rains. Seafood products can also become contaminated through processing and handling, including storage and transportation without proper temperature control, contamination from an infected handler and cross-contamination from other products such as raw poultry and seafood, according to Iwamoto et al..
Routes of exposure include both waterborne and foodborne routes, as well as person-person contact and animal contact, according to Iwamoto et al.. Iwamoto also statest that seafood, while a significant part of a healthy diet, is considered to carry risk for a foodborne-illness due to the potential contamination of the environment seafood is harvested in, the mode of feeding for most aquatic species and the preparation of seafood, such as being eaten raw or undercooked.
The estimated risk of developing a GI infection and illness from Salmonella was measured for consumers of the wastewater-fed aquaculture product, shrimp. Gastrointestinal illnesses from seafood has been a public health concern due to the susceptibility of these organisms to several contaminants, including microbial organisms. Historically, organic and inorganic contaminants including, such as metals and pesticides, have been of human health concern for consumers, according to Cole et al.. The risk of illness from microbial contaminants, specifically Salmonella, is not as well documented or understood.
Exposure Assessment
The exposure route examined was for individuals consuming shrimp grown in wastewater-fed aquaculture facilities. Consumption scenarios were conducted including different preparation methods. The overall exposure scenario is illustrated in Figure 1.
Figure 1. Exposure pathway for individuals consuming shrimp which has been produced in treated wastewater aquaculture systems.
The exposure pathway begins with primary influent entering a wastewater treatment system and undergoing secondary treatment, including anaerobic and facultative processes. Salmonella concentrations in primary sewage and treatment reductions expected following a two-pond system were gathered from the literature to determine the bacterial water quality of wastewater entering a maturation pond used in this scenario for grow-out of shrimp aquaculture. Upon entering the maturation grow-out pond, the treated wastewater effluent is diluted with brackish water. Bioaccumulation of Salmonella in shrimp during the grow-out period was assumed based on concentrations reported in pond water and shrimp during aquaculture processes. Following maturation, the shrimp are then harvested and immediately frozen in a brine solution. Frozen shrimp are then gamma irradiated and transported to market and consumer.
It was assumed that the concentration of Salmonella in the shrimp did not change between processing techniques and during transport to the consumer. The dose was calculated from the concentration of Salmonella in the cooked product and the ingestion rate for adults (per serving size). Different processing scenarios evaluated in this study are presented in Figure 2.
Figure 2. Different Processing Scenarios Evaluated
The uncertainty in this model will be evaluated by a sensitivity analysis of the variables used in the dose calculations and the dose-response parameters of α and N50. The greatest uncertainty is expected to be contributed from the parameters used in the exposure scenario. While the ratio describing the accumulation of Salmonella from the surrounding water environment in the pond to shrimp is poorly understood and requires further data collection, different measures in the preparation of the shrimp will assist in managing the risk.
There is currently no international agreement for acceptable levels of Salmonella on food, but several countries including the United States and Australia have a zero tolerance policy for the presence of the pathogen on both raw and ready to eat/cooked shrimp, according to Wan Norhana et al.. Management methods that will be modelled in this assessment include the product undergoing gamma radiation, brine freezing and boiling until fully cooked (floating on top of water).
Dose Response
The dose response model selected for Salmonella follows a beta-Poisson distribution for the probability of illness. The parameters for the model have been published and widely used in the risk assessment literature and are based upon pooled data from Hornick et al. 1966 and 1970, according to QMRA. The model is based upon human feeding trials of oral exposure of the pathogen in milk.
Risk of illness (Dose-Response Model)
\begin{align*} P(illness) = 1 - [1 + Dose \frac{(2^{\frac{1}{\alpha}} -1)}{n_{50}}]^{-\alpha} \end{align*}
Probability of annual risk is calculated based on probability of illness per a single serving event of shrimp (P(illness)) and the number of exposure event in a year (n).
\begin{align*} P(annual) = 1 - (1-P)^n \end{align*}
The risk model comprises the final stage of the risk assessment. The risk of a gastrointestinal illness from different consumption scenarios was calculated using the dose-response formula of Salmonella. The recommended serving size for shrimp (as protein) is three ounces or approximately 84 grams, according to FDA. The average American consumes around four pounds of shrimp annually, equating to around 22 servings, according to NOAA and FDA. The risk of illness for adults was calculated for both consumption of cooked and undercooked shrimp. The dose was measured for a serving of cooked and undercooked shrimp.
Risk Characterization
The final concentration of Salmonella in a serving of shrimp was calculated for each of the five scenarios tested. Gamma irradiation of the shrimp following freezing led to significant reductions in Salmonella levels in the product.
Figure 3. Concentration of Salmonella in CFU/g for final product, depending on treatment scenario.
Acceptable risk levels were measured following freezing, gamma irradiation, and proper cooking. During scenario one, where there was no treatment of the shrimp the majority of the model uncertainty was related to the concentration of Salmonella in the influent as well as the log reduction occurring during primary and secondary treatment. This was a consistent trend noted for all five of the scenarios tested, where the log reduction occurring through the treatment train contributing the most to the model uncertainty, with decreased reductions associated with increased risk to the consumer. Concentration of Salmonella is the influent was also highly correlated to risk to the consumer with increased concentrations associated with increased risk (as expected).
Figure 4. Annual risk of illness for each exposure scenario.
Sensitivity Analysis for Alternative Processing Scenarios:
Figure 5. Sensitivity analysis for each processing scenario tested. Correlation coefficients reported for each parameter.
Risk Management & Communication
Salmonella contamination in shrimp and other seafood products does not only pose as a public health risk, but also an issues that demands extensive and costly resources. The FDA must sample and analyze products for contamination, as well investigating sources and causes for outbreaks in order to minimize consumer exposure to this pathogen, according to Koonse et al.. Shrimp are an important aquaculture product and potential management options can assist in mitigating the risk of Salmonella exposure, at least from wastewater being used in the aquaculture practices. A few different risk management strategies and recommendations could be implemented to reduce the potential exposure to the pathogen, reducing the risk of a GI illness.
- Monitoring the effluent and pond water quality to ensure the concentration of Salmonella does not exceed an established threshold.
- Protecting the aquaculture ponds from external pathogenic sources such as birds and other animals.
- Permitting sunlight to reach the ponds to assist in inactivation of potentially harmful pathogens.
- Educating and training workers how to properly handle the shrimp and minimize cross-contamination of the product when harvesting and transferring to freezing facilities.
- Ensure food handlers are properly trained in food safety and use clean tools and utensils when preparing the product to prevent cross contamination of Salmonella from other sources.
- Have shrimp cooked after irradiation and freezing before selling/providing to the consumer.
These recommendations can be integrated into developing suggested guidelines as part of the Hazard Analysis Critical Control Point (HACCP) plans for shrimp cultivation and distribution by the FDA. The HACCP is designed to promote guidelines that will prevent, eliminate or reduce food safety hazards within an acceptable level. While the HACCP is effective at minimizing hazards, it is not a zero risk system, therefore shrimp produced in a treated wastewater aquaculture system and properly processed and prepared by brine freezing, gamma irradiation and proper cookies.
Risk Perception
Perhaps the biggest issue regarding using wastewater for aquaculture is the negative perception associated with the water quality. Poor water quality for aquaculture productions is a concern in developing countries, but with minimal treatment and proper freezing, gamma radiation and cooking, the risk of illness from Salmonella is negligible for the consumer, according to Bunting. Regulating the treated effluent from into the ponds can assist in mitigating the risk of using treated wastewater for this commodity. The risks associated with handling and preparing is another route of exposure that is dependent on the individuals and sources involved, not the quality of the water used in the aquaculture production. Article 12 of the European Wastewater Directive informs officials that wastewater can be reused whenever appropriate, yet research concerning wastewater aquaculture systems in Egypt that met health standards was not viable due to consumers unwilling to accept the products, according to Bunting. Wastewater continues to be a driving force to meet agricultural and industrial needs. Using wastewater for agriculture has long been seen as beneficial practice to make use of the nutrients available in the water and to minimize the downstream environmental impacts of the wastewater into the environment. Wastewater is 99% water and using it can prevent the extra addition of feed and other compounds to meet aquaculture demands from being released into the environment, according to WHO. Reusing wastewater also promotes environmental sustainability and protecting the limited water resources available. Aquaculture, especially with the increasing demand for seafood products both in the United States and globally, has significant economic value and pursuing this practice provides job opportunities and other societal benefits.
Risk Communication Strategy
This risk assessment for Salmonella related GI illness acquired through wastewater fed aquaculture has multiple stakeholder groups including regulatory (federal and local government), commercial (aquaculture farmers and workers) and consumers. Since each stakeholder has different interests, different risk communication strategies must be employed to effectively communicate to each group the possible risk to consumers from wastewater fed aquaculture.
Regulatory government
- The cost-benefit of wastewater aquaculture compared traditional aquaculture practices, at least for individual farmers.
- Aquaculture industry provides potential for further job creation, especially if wastewater is used in areas that is water stricken.
- Environmental sustainability of reusing wastewater.
- Can be implemented by processes that minimize risk for human health.
Commercial
- Tabuchi states that in 2011, United States aquaculture farmers accounted for only 0.8% of global aquaculture production and fisheries officials estimate that doubling American aquaculture production could create 50,000 jobs and more than $1 billion in revenue for farmers.
- Implementation of wastewater fed aquaculture can help to create new jobs and reduce resource demand. In addition, compared to traditional aquaculture methods, wastewater fed aquaculture is more cost effective.
Consumers
- In order to effectively communicate to consumers about wastewater fed aquaculture, it is important to highlight that there is minimal risk of GI illness from shrimp after proper freezing, gamma irradiation and cooking.
- Implementation of wastewater fed aquaculture would lead to a cheaper product to the consumer, as money saved from the aquaculture farmers can be relayed to the consumer.
- Reuse of wastewater for aquaculture is a sustainable practice which can be a point of interest to consumers who want to be environmentally conscious.