Desalination - Team D

Team D: Final Report

Authors: Thomas Aunins, Robert Cignoni, John Dombrowski, Iris Zhao

Instructors: Fengqi You, David Wegerer

March 11, 2016

Executive Summary

Water shortage is one of the foremost and most urgent issues facing the world today, as developing and developed countries alike have struggled with depletion of natural reservoirs and severe droughts. This issue has resulted in the recent rapid development of desalination technology and the construction of desalination facilities. Since the turn of the millennium, the United State alone has increased its desalination capacity from 600 million gallons per day to 1650 million gallons per day, with much more currently being planned. California, in particular, is the focus of a large amount of the United States’ desalination efforts, as its current drought has exposed a discrepancy in water supply contingency and demonstrated a need for non-natural freshwater sources.

This project aims to design a 10 million gallon per day seawater desalination plant on the Southern California Bight--near San Diego--to fill this need. A reverse osmosis system was chosen based on the fact that it is the most rapidly developing and innovating technology in the desalination field, as well as the fact that it has a lower theoretical energy production per gallon of water than the common multi-stage flash purification methods. Our plant will pressurize seawater from subterranean wells off the coast of the bight and send it to our pre-treatment system. There, it will go through a drum screen, multimedia filter, antiscalant addition, and finally ultrafiltration to remove varying size of suspended solids and contaminants, before entering our reverse osmosis system.

The RO system itself is a 2-stage, 6 element per stage process, using Dow SW30XHR-440i membranes and operating at 50% recovery with a feed of 20 million gallons per day. This allows the process to achieve a final dissolved solids concentration of 109 mg/L, far below the California drinking water recommendation of 500 mg/L. This freshwater can then be sent to post-treatment and merged with water of the San Diego County Water Authority’s distribution system. Waste concentrate from the process is sent back into the bay through a long diffuser pipe system that will dilute the brine to necessary levels to avoid environmental damage.

An economic analysis of the process found total capital costs to be slightly more than $600 million, with yearly revenues and operating costs at $25.4 million and $6.2 million, respectively. On a 25 year time scale, this results in a final net present value for the project at -$402.5 million, causing us to conclude that as a commercial venture the project is not viable. We do note, however, that increased demand and decreased supply may cause water prices to rise and create a motivation for government investment in the project in the future. For this reason, we believe that it is possible for this project to become an economically feasible and practically necessary venture in coming years.

Introduction

Background

Due to drought and the depletion of groundwater, desalination is becoming an increasingly viable source for drinking water in the San Diego, California area. A map of the plant location can be found in Appendix 1. Reverse osmosis appears to be the best route for desalination due to its lower energy costs and high volume of current research efforts. It is also capable of purifying California seawater to the levels recommended by the World Health Organization (WHO) and the state government. The process will separate solids from seawater before subjecting it to a two-stage reverse osmosis unit. Concentrated brine waste will be diluted with seawater before going back into the environment. Permeate streams will be remineralized and disinfected before leaving the facility.

Problem Statement

The objective of this process will be to produce fresh drinking-quality water according to standards recommended by the Water Research Foundation. This sets an upper limit for the total dissolved solid in our product at 1000 mg/L, with a non-mandatory guideline of approximately 500 mg/L as an appropriate target. This can be found from in Appendix 2. This encompasses the secondary maximum contaminant levels (MCL) set forth by the State Resources Water Control Board [1]. Additionally, there are guidelines set forth for primary MCLs, which encompasses more dangerous and/or toxic substances in the water. These are a smaller concern for our project because sea water does not naturally contain amounts of these contaminants above the MCLs [2].

Technical Approach

Site Location and Capacity

This project is planned for construction on the Southern California Bight, located just north of San Diego and nearby the San Diego County Water Authority’s (SDCWA) distribution system. This area is of particular interest for seawater desalination projects due to the projected discrepancy between water supply and demand in upcoming years. Statewide in California, the demand for water is expected to increase by 1.2 billion cubic meters per year by 2030, as projections show that population increase of 16% dramatically outstripping water conservation goals. [3] Southern California in particular has a great need for more freshwater sources, as the lower two-thirds of the state require 80% of California’s water, while the upper third of the state supplies 75% of it. [4]

Per the aforementioned water scarcity, California’s water demand has become a large part of this growth. There are several large scale desalination plants planned for the area, including large-scale projects at Carlsbad and Camp Pendleton. Each of these plants will be constructed to produce 50 MGD of freshwater to the San Diego area, with the latter expected to expand to 150 MGD within ten years of completion. The construction of these plants, along with other smaller scale plants in the area, indicates an urgent need for desalination capacity. Our plant is being designed to produce 10 million gallons per day (MGD) of fresh water for the San Diego area.

Feed Stream

Seawater will be fed from a submerged pipeline off the coast of the Southern California Bight. The subterranean feed inlet will allow for an initial pseudo-filter as the water is pulled through the porous ocean floor, preventing large debris and aquatic life from being pulled into the process intake. Worldwide, seawater salinity averages approximately 35,000 mg/L of total dissolved solids, with the primary salts present being chloride and sodium at 19,000 mg/L and 10,500 mg/L, respectively. [5] It should be noted that while data on average local seawater composition for Southern California was not available, this area is known to typically have lower total dissolved solids concentrations than average seawater, placing our calculations on the conservative side. Further breakdown of the dissolved ion concentration of our seawater input can be found in Appendix 3.

Product Stream

The objective of this process will be to produce fresh drinking-quality water according to standards set by the California state government and the World Health Organization. Regulations set an upper limit for the total dissolved solid in our product at 1000 mg/L, with a non-mandatory guideline of approximately 500 mg/L as an appropriate target. This encompasses the secondary maximum contaminant levels (MCL) set forth by the State Resources Water Control Board. [6] Additionally, there are guidelines set forth for primary MCLs, which encompasses more dangerous and/or toxic substances in the water. These are a smaller concern for our project because sea water does not naturally contain amounts of these contaminants above the MCLs. [7]

Further goals for the permeate composition and quality following post-treatment were taken from recommendations given by the Water Research Foundation on seawater reverse osmosis and from averages taken from San Diego water treatment plants. These can be found in Appendix 2.