Graduation Year


Document Type




Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department


Major Professor

Jeffrey Cunningham, Ph.D.

Committee Member

James Mihelcic, Ph.D.

Committee Member

Maya Trotz, Ph.D.


Wastewater, Mesoamerican Barrier Reef, Septic Tank, Biodigester, Onsite Wastewater, Coastal Contamination


Three wastewater treatment systems (WWTS) situated on Cayes in the Belize Barrier Reef System were assessed in terms of the unique public health and environmental circumstances of being a tourist destination surrounded by fragile coral reef. Laughing Bird Caye, Silk Caye, and Little Water Caye are three small cayes that are the staging points for local diving, fishing, and other recreational tourism. All three systems are based upon pour-flush toilets, semi-anaerobic biodigesters and drainage fields. Limitations in cost, available resources, useable area, high infiltration rates of the sand, and salinity of the water have played a major factor in the construction and performance of the WWTS on the Cayes. This thesis aims to form an understanding of treatment efficiency of the WWTS, investigate the effectiveness of decentralized saltwater-based WWTS in comparison to freshwater-based WWTS, and provide recommendations to improve the performance and resource recovery in a manner appropriate for the context in which the systems are deployed.

A mathematical model was developed to predict the performance of the WWTS based on available operational and water-quality input data. The model is based on the mass balances of six species: inert solids, fecal solids, bacterial biomass, soluble substrate (i.e. dissolved organic carbon), ammonium and nitrate. Effects of salinity were estimated for the two saltwater-based WWTS. The model predicted the effluent concentrations of fecal solids, soluble biological oxygen demand (BOD), ammonium, and nitrate. A sensitivity analysis was also performed on the predicted effluent treatment efficiency based upon influent load, oxygen concentration and system salinity.

Results from Silk Caye and Laughing Bird Caye indicate that varying the number of visitors from seasonal lows to highs has a moderate impact on the effluent fecal solids and soluble BOD in the effluent. Due to the relatively large volume of the WWTS at Little Water Caye, and thus high HRT, varying the number of visitors did not have a significant effect. The model predicted a reduction of nitrogen from the effluent due to settled solids and the assimilation of the nitrogen into bacteria. However the model consistently projected an effluent nitrate concentration (as mg/L as N) between 60 and 63 across the three WWTS. The oxygen concentration within the WWTS had the greatest effect on effluent BOD of the three parameters tested in the sensitivity analysis. Results from the sensitivity analysis indicate that a minimum concentration of 0.95 mg/L of oxygen is required before the model can accurately predict the effluent BOD concentration. The concentration of effluent fecal solids did not significantly change with changes in oxygen concentration. Salinity had a significant effect on the predicted fecal solids and soluble BOD in the effluent. Predicted fecal solids in the effluent wastewater increased approximately 60 percent from freshwater conditions to 4 percent salinity. Similarly, effluent BOD concentration increased strongly with increasing salinity. The increase in concentration is due to the major reduction of substrate-consuming bacteria by cell-die-off. The model predicts that a significant increase in cell die-off begins to occur at 2.4 percent salinity.

The predicted effluent of the freshwater-based WWTS on Little Water Caye was compared to 166 wastewater treatment plants operating in Brazil. Comparison between the WWTS on the Caye and the decentralized WWTS in Brazil indicate that the predicted removal efficiencies of total suspended solids and soluble BOD are higher than the measured efficiencies of the WWTS. However, the total nitrogen removal efficiency for the WWTS on the Caye was the least effective; most-likely because the model does not account for denitrification within the biodigester. The comparison between the WWTS illustrates that the predicted removal efficiency of BOD and TSS solids is most likely less in the actual measurement than predicted value from the model.

The WWTS on the Cayes were constructed to mitigate the impacts of the wastewater produced by visitors on the general health of the pubic and the environment. Considering the reports of the eutrophication affecting the coral reefs surrounding the Cayes, the WWTS have largely failed in at least one aspect of their purpose. The effluent water quality predicted by the model also suggests that significant concentrations of nitrogen are entering the surrounding ocean habitat as ammonia and nitrate. Recommendations to improve the effluent wastewater quality were separated into three categories based upon the required level of input to realize the recommendation. The input includes the capital cost and labor of the change, the level of buy-in from the users of the system, and the resulting maintenance requirements. The implementation of a urine separation toilet system was proposed as a method to reduce effluent nitrogen entering the environment and to create a resource recovery system (RR) from the already constructed WWTS.

sensitivity analysis_gamma.xlsm (318 kB)
Sensitivity Analysis Gamma

Included in

Engineering Commons