WATER TREATMENT PLANT DESIGN TO ADDRESS LARGE FLUCTUATIONS IN RAW WATER TURBIDITY AND ORGANIC CONTENT

This paper presents engineering planning and key design concepts for a new 2.5 million gallon per day (mgd) water treatment plant that must properly manage a water supply source exhibiting extreme variation in solids content and possibly organic content. The intent of this paper is to present an overview of how the plant was engineered to properly treat the highly variable raw water supply to consistently meet potable water standards. Since the treatment plant has not yet been constructed or operated, only a basis of design for the plant is presented herein.The water source for this project is the Rio Grande de Añasco located in the western portion of Puerto Rico in the municipality of Las Marias. Turbidity levels range from less than 10 NTUs during dry weather conditions upwards to greater than 8,000 NTUs during heavy rain events. The organic content of the water supply can also vary due to organic solids being washed into the river during rain events, thereby potentially leading to the formation of disinfection byproducts as a consequence of the proposed final chlorine disinfection process for the plant.To properly treat the raw water supply that contains such a large variation of these contaminants, advanced and modified conventional treatment technologies were identified and engineered that would produce a finished water supply which would consistently be in compliance with applicable federal and state regulations. Specifically, a pre-treatment system consisting of hydrocyclones, a patented ballasted flocculation/clarification system, a polishing filtration system, and a chlorine disinfection system was specified to properly treat the raw water supply. In addition, a design for an ozone oxidation system was developed which can be used if determined necessary now or in the future to manage organic constituents, which could form disinfection byproducts.Another unique aspect of the treatment plant is the system that would receive, treat, and store sludge wastes from the various unit water treatment processes and operations. During the dry season of a typical year, solid wastes generated by the plant would be approximately 1,300 pounds per day (ppd) on the average. However, during the rainy season, solids production would increase to an average of 15,900 ppd with a peak solids generation rate of up to 82,500 ppd during a single, strong rain event. Due to this extreme variation in solids production, a Solids Handling System was developed including an influent solids equalization basin, a conventional gravity thickener, a set of large belt filter presses to dewater sludge produced by the plant, and a dewatered sludge storage facility. The system, as designed, will be capable of processing and storing the full range of potential sludge quantities produced by the plant during the year.