Moving Beyond Chlorine in Puerto Rico: A Comprehensive Evaluation of Wastewater Disinfection Alternatives

Chlorination has historically been the dominant method of wastewater disinfection at municipal treatment facilities in Puerto Rico due to the relative simplicity of the process and favorable operating costs. However, concerns about operator health and safety and security issues surrounding the use of liquid chlorine containers, as well as requirements for dechlorination, have stimulated the consideration of alternative methods of disinfection. A comprehensive evaluation of disinfection alternatives was performed for the Barceloneta Regional Wastewater Treatment Plant (BRWWTP), a secondary treatment facility owned by the Puerto Rico Aqueduct and Sewer Authority (PRASA) and located on the north central coast of Puerto Rico in the city of Barceloneta. The average and peak design flows for the new disinfection system are 0.53 m3/s (12 MGD) and 0.96 m3/s (22 MGD), respectively. The BRWWTP currently does not perform effluent disinfection and there are no existing facilities or equipment for disinfection at the plant. With no existing facilities to weight the consideration of one disinfection technology over others, and no significant footprint restrictions, the opportunity was presented to evaluate a wide range of alternatives on the basis of predicted disinfection performance, technical advantages and disadvantages, capital and life-cycle costs, and health and safety considerations. Furthermore, due to the limited scope of available plant data and the significant industrial contribution to BRWWTP influent, the evaluation was supported by supplemental data collection and benchscale testing for critical disinfection-related parameters, including chlorine demand, disinfection by-products, ultraviolet light transmittance (UVT) testing, ultraviolet (UV) light disinfection performance (collimated beam testing), and post-disinfection effluent toxicity. Bench testing of secondary effluent samples collected under both low and high flow conditions was conducted. The design chlorine demand and dose were determined, and disinfection by-products and acute toxicity were assessed. A design UVT of 50% and design UV dose of 40 mJ/cm2 were also determined. Five disinfection technologies were screened in the preliminary evaluation: chlorine with sulfur dioxide dechlorination; sodium hypochlorite with sodium bisulfite dechlorination; chlorine dioxide with dechlorination; ozone; and UV light. Preliminary design criteria were established for each alternative. Conceptual construction and O&M cost ranges were also established for the purpose of relative comparison among the alternatives. In addition, operational advantages and disadvantages, construction feasibility and impact on plant operations during construction and health and safety considerations were evaluated. After evaluation of these criteria for each alternative, chlorine/sulfur dioxide, sodium hypochlorite/sodium bisulfite, and UV light were selected for the final evaluation. In addition, an equipment cost and life-cycle analysis of alternative UV light technologies – low pressure/high output (LPHO) and medium pressure (MP) – was performed to select the most appropriate UV option for the final evaluation. In the final evaluation, detailed design criteria were developed for each alternative based on the preliminary evaluation and subsequent input from PRASA. Based on these criteria, a conceptual design and layout was developed for each of the three alternatives. Site plans indicating overall footprint and location within the plant were also developed. A comparison of LPHO vertical and horizontal UV equipment and costs was also performed in order to select the design configuration for the UV alternative. In addition, onsite generation of hypochlorite (OSG) was also evaluated as a separate option. Based on these conceptual designs, construction and O&M cost opinions, as well as life cycle costs, were further developed for each alternative. Note that these costs included the provision of reliable backup power for each alternative. With the lowest costs and lowest health and safety risk, as well as a high potential for public acceptance, a vertical LPHO UV light disinfection system was recommended as the preferred alternative for design. The advantages of LPHO UV light over chlorine and sodium hypochlorite in terms of capital and life-cycle costs are significant for this 0.96 m3/s (22 MGD) (peak flow) wastewater disinfection system installation in Puerto Rico. Preliminary effluent characterization was effectively used to identify a lower-than-typical design UVT and to establish a design UV dose. The single largest factor in the significantly higher costs of chlorine and hypochlorite was the requirement for construction of new redundant contact basins sized for contact time at average flow. Also, the requirements for effluent dechlorination and scrubber equipment (for the gas chlorine/sulfur dioxide option) added significant cost to these alternatives. The unusually high O&M cost for hypochlorite was driven by the high cost in Puerto Rico of bulk sodium hypochlorite (15%) at 1.60/gallon. With competitive costs, operational benefits and health and safety advantages, disinfection with UV light is a legitimate alternative for wastewater disinfection in Puerto Rico.