Alternate: Advancements in Wastewater Solids Management for the City of Cuenca, Ecuador

The City of Cuenca is a large metropolitan City located in the high lands of Ecuador at about 2,560 meters above seal level. The wastewater collection and treatment system for the City consists of approximately 1,300 kms of sewage networks and 80 kms of interceptor networks that convey wastewater to various treatment plants, the Ucubamba WWTP being the largest of all treating approximately 95% of wastewater produced by the City. The rated design capacity of the Ucumamba WWTP is 1.8 cubic meter per second (m3/sec). The treatment process at the Ucumamba WWTP consists of preliminary treatment consisting of screens, followed by a 3-step biological process consisting of primary aerated lagoons, secondary facultative lagoons and tertiary maturation lagoons. Exhibit 1 is a process flow diagram of the Ucumamba WWTP liquid treatment process. The Ucumamba WWTP is challenged with accumulation of solids in the lagoons. In 2013 the City installed sludge removal and dewatering system to remove and process approximately 3,000 cubic meter of sludge per year (m3/year). However, the sludge removal and dewatering is capacity limited to remove and process sludge accumulated in the facultative lagoons. As a result, approximately 50% of the lagoon volume is occupied by sludge, thereby reducing the active hydraulic retention time, and leading to compromised treatment efficiency. To address challenges with reduced treatment capacity of the Ucumamba WWTP, reduce dependence on the Ucumamba WWTP to meet the City's treatment needs, and diversify treatment options to meet future growth, the City completed the design of the new Guangarcucho WWTP having a rated design capacity of 1.8 m3/sec. With the new Guangarcucho WWTP being operational, the City will be able to effeciently treat wastewater produced by the City and remove portions of the Ucumamba WWTP out of service for maintenance and sludge removal to restore capacity. Exhibit 2 summarizes the future wastewater treatment strategy for the City of Cuenca after the Guangargucho WWTP becomes operational. Wastewater biosolids are considered a valuable resource in Ecuador due to the high nutrient content and its benefit to the agricultural industry. Evaluation completed by the City indicate that the sludge removed from the Ucumamba WWTP is low in nutrients (nitrogen) due to the prolonged stabilization and meets EPA Part 503 Class B biosolids standards. As a comparison, the wastewater biosolids to be produced by the advanced Guangarcucho WWTP is assumed to have a high nutrient content and potential to meet EPA Part 503 Class A biosolids standards. The liquid treatment process for the Guangargucho WWTP consists of preliminary treatment (screening and grit removal), primary clarification, biological treatment, secondary clarification and disinfection. Primary sludge produced by the primary clarifiers will be thickened in gravity thickeners and waste activated sludge produced by the secondary treatment process will be thickened with gravity belt thickeners (GBTs). Thickened primary and waste activated sludge will be anaerobic digested and dewatered with centrifuges. Exhibit 3 provides the process flow diagram of the solids treatment process at the Guangarcucho WWTP. Limited quantity of biosolids currently produced by the Ucumamba WWTP is currently conveyed to the Pichacay Landfill for disposal. However, the Pichacay Landfill is nearing the end of its service life by 2031, and thus does not have the capacity to accept additional biosolids to be produced by the City's Ucumamba and Guangarcucho WWTP in the future. The City is currently challenged to find disposal or beneficial use options for biosolids produced by the two WWTPs. In addition, there are no regulations to regulate the general conditions and obligations to be fulfilled for beneficial use of biosolids e.g. land application. To address its biosolids management challenges, the City has initiated a Feasibility Study to investigate treatment and beneficial use options to manage biosolids that will be produced by the two WWTPs in the future. The treatment approach will focus on maximizing reduction in mass and volume of biosolids that will require outside the fence management, and produce high quality biosolids that can be beneficially used for agriculture and/or mine reclamation. Treatment solutions that will be investigated include thermal drying, solar drying, incineration and innovative treatment solutions such as pyrolysis and gasification to produce value derived products such as biochar. Beneficial use opportunities for biogas to be produced by anaerobic digestion at the Guangarcucho WWTP will also be investigated e.g. cogeneration, biosolids drying, etc. The paper will provide an overview of the current and future wastewater treatment for the City of Cuenca, and the challenges the City faces to sustainably manage biosolids that will be produced by the Ucumamba and Guangarcucho WWTPs in the near future. An overview of treatment options investigated to achieve a reduction in mass and volume of biosolids requiring end use management, advantages and disadvantages of the various alternatives and a triple bottom line evaluation to identify the most cost-beneficial alternative will be presented. Consideration of other factors such as operational and maintenance know how, energy requirements and costs, availability of manufacturer services and spare parts in Ecuador will also be considered in selecting the preferred treatment approach. The presentation will be useful to Utility Managers, Consultants and Engineers who also face biosolids processing and management challenges similar to the City of Cuenca. The paper will provide the attendees an understanding of a Citywide approach and initiatives to implement a sustainable biosolids management strategy in a resource constrained environment. The paper will also provide an overview of the City's existing and planned wastewater collection and treatment infrastructure.