Conceptual Designs Of Outfall Systems For Desalination Plants

Abstract

ABSTRACT A coastal, distillation desalination plant Will discharge a mixed effluent of brine blowdown and cooling water having a higher temperature, salinity and copper concentration than the receiving environment. The mixed, effluent, discharged offshore. will have a negative buoyancy and will tend to sink to the ocean floor. The initial dilution of the mixed effluent with seawater produced by turbulence through thee'; discharge, ports of a diffuser can be controlled by the design of the outfall to meet a predetermined. Water quality criteria. Experimental and numerical analyses of dense jets (1) directed upward, (2) inclined at an angle to the Horizontal, and (3) reaching the surface of the receiving water will be presented. For the same densimetric Froude?s Number, a jet directed at a 60-degree angle was found to have the longer trajectory and, therefore, a maximum dilution. Design concept and. conceptual designs of outfall systems for desalination plants of different sizes will be discussed and the economics analyzed. INTRODUCTION At coastal locations, an obvious means of disposal of desalination plant effluents is through an outfall properly designed and located offshore. The mixture of brine blowdown and cooling water discharge from a desalination plant, utilizing a distillation process, would have a higher temperature and salinity than the receiving environment and would sink to the ocean floor. The copper concentration in the mixed effluent will be several times higher than the recommended marine water quality standard of 0.02 mg/l, (1). When the dense effluent is discharged from one or many ports of a diffuser, it is immediately subjected to a negative buoyancy force proportional to the difference in density between'; the effluent and the lighter receiving water. The kinetic energy, due to thevelocity through the port, is dissipated in the turbulent mixing of the jet while the negative buoyancy force drives the effluent toward the floor of the receiving body. This initial jet mixing causes a field of diluted effluent to be formed near the bottom of the receiving body which then moves with the ocean currents or, is flushed away by tidal currents in the case of an enclosed bay or an estuary. The proper design of an outfall system is based on the degree of the initial jet dilution of the effluent to me-et. a predetermined water quality criteria. This study was carried- 611£ to develop conceptual designs of outfall systems for desalination plants of various sizes and to estimate the cost of each design and its effects on the cost of product water. DISPERSION OF DENSE EFFLUENTS The mechanism of dilution for the discharge of a single jet into a homogene of medium of higher density has been subject to the most theoretical and experimental studies (2), (3), (4). For design purposes, formulas have been developed to predict with accuracy the initial dilution in a rising Jet at the surface of, the receiving water (5).