Water Recycling, Desalination, and OTEC (Ocean Thermal Energy Conversion)
Length: 2044 words (5.8 double-spaced pages)
There are many technological solutions that could potentially help to alleviate the current water shortage on Oahu. For the purpose of this paper, our group has chosen to discuss what we believe to be the top three most feasible technological solutions. In order of current usability they are;
1. Water recycling
3. OTEC (Ocean Thermal Energy Conversion)
Water recycling here on Oahu is an important part of ensuring a sustainable water supply for future generations. Through the natural water cycle, the earth has recycled and reused water for millions of years. Water recycling generally refers to projects that use technology to speed up these natural processes. Recycled water can reduce much of Oahu’s aquifer water demands, as long as it is adequately treated to ensure water quality appropriate for the use.
The recycling of water has many benefits. The main benefit is that by recycling water, we are saving ground water for other uses. Another benefit is that recycled water is safe and has been used for over 40 years, with no negative human or environmental side effects (HWRF). Because the amount of precipitation on Oahu changes varies, sometimes the island goes through drought. With water recycling, the amount of secondary effluent available for recycling is always greater than the demand of it, therefore making it virtually drought proof. Also, recycled water can be produced at a price that is cost-effective, when compared with other methods.
Water recycling facilities on Oahu have been in place since 2000 (HBWS). The Honouliuli Water Recycling Facility in Eva, is the largest recycling plant on Oahu. The plant was built next to the City and County of Honolulu’s waster water treatment plant, which provides the recycling plant with secondary effluent as the base for the recycling process. Currently, the facility has the production capacity of 12 million gallons per day and produces two grades of recycled water. R-1 water is used for irrigational uses and Reverse Osmosis (RO) for industrial uses.
The facility is currently capable of producing up to 10 mgd of R-1 water, which is the highest level of treatment as designated by the Hawaii DOH. R-1 water is used throughout the state of Hawaii for golf course irrigation, landscaping, and agriculture. The RO water is used for industrial uses such as boiler feed water for producing high-grade steam, cooling tower water, and process water for refineries. The facility currently has an RO capacity of 2 mgd. Both types of recycled water begin with the same secondary treated effluent from the Honouliuli WWTP.
According to the Honouliuli Wastewater Recycling Facility website, the basic process of producing R-1 grade water is as follows:
1. Secondary wastewater from the Honouliuli waste water plant is piped over to the recycling facility.
2. Then polyaluminum chloride is added and rapidly mixed in one of two mixing tanks.
3. Next, the effluent then flows into one of three flocculation tanks to facilitate the coagulation of suspended and dissolved particles to form larger and/or denser particles.
4. Then it the water goes through sand filters that trap all of the small particles.
5. The water moves over to the U.V. disinfection system, where it is treated with powerful U.V. lights.
6. From there the water goes through pumps into one of two, 2.5 million gallon storage tanks.
7. Finally, the R-1 water moves through pumps that ship it to customers.
The R-1 water that is produced through this process has many agricultural benefits over water that is drawn from Oahu’s aquifers. The main positive attribute is that retains many of the beneficial nutrients of secondary effluent without the harmful risks to the environment. The nutrients that stay in the R-1 water allow farmers and golf courses to not fertilize as much, or at all. One of the best examples of the benefits of using R-1 water is the case of the Hawaii Prince Golf Course. Since the groundskeepers at the Hawaii Prince Golf Course switched over to R-1 water in 2002, they have stopped fertilizing their courses. They found that the nutrients that were available in the R-1 water were sufficient enough to allow them to have a healthy course, without having to buy the expensive fertilizers. The end result of no having to purchase fertilizer is an $80,000 per year savings for Hawaii Prince Golf Course, as well as a significant amount of water that was not removed from Oahu’s aquifers (HWRF).
Reverse Osmosis is the process in which water is passed through a semi-permeable membrane, under high pressure. The result is that the membrane allows only water molecules to pass through, while filtering out particles that are larger than water molecules. First, secondary effluent from the Honouliuli passes through microfiltration. According to GE water & process technologies, microfiltration consists of thousands of strands with pores that are 5,000 times smaller than a pinhole. The water then passes through reverse osmosis, which consists of membranes with pores that are 5 million times smaller than a pinhole.
The reverse osmosis process at the Honouliuli Water Recycling facility has a four step process for treating secondary effluent. The process is as follows;
1. Secondary effluent from the wastewater treatment plant is passed through microfiltration.
2. Next, the water is passed through the reverse osmosis filters.
3. The purified water is placed into storage tanks.
4. Then the RO water is pumped out of the storage tanks and on to customers.
The majority of the customers that use the RO water are factories in the Campbell Industrial Park. The businesses in this area prefer RO water over water from Oahu’s aquifers. The reasoning is because RO water is free of minerals. This allows the factories to produce what they term as “high grade steam”. High grade steam is better in factories because it boils at a slightly lower temperature, and does not leave any mineral residue that could potentially wear turbine parts down (GE water & process technologies).
Desalination is the process of converting saline water into fresh, drinkable water. Considering the geographic location of the island of Oahu and the amount of salt water that surrounds it, desalination seems like an obvious choice for solving part of the water shortage problems. Currently, there are two methods of desalination; reverse osmosis and multi-stage flash distillation. Reverse osmosis water is produced much in the same way as in the recycling process, but instead of waste being removed, salts and other minerals are removed.
The process of Multi-stage flash distillation includes;
1. Heating salt water up to 100°C in a high pressure chamber.
2. Then the water enters a cooler flash chamber with low pressure, where the hot water is immediately turned into water vapor
3. The vapor collects of cool pipes, and flows down into a tank where it is collected as distillate.
4. The result is that the salts and other minerals are left on the behind in the process.
The process of desalination has been around for hundreds of years. In 1791 Thomas Jefferson published a technical report describing the process (McKinney, 2003). At that time, solar energy was used to separate water from minerals. Today, technology has perfected the process of desalination. There are currently more than 7500 desalination plants worldwide, 60 percent of which are located in the Middle East. The world’s largest plant is located in Saudi Arabia and produces 128 mgd of desalted water (McKinny, 2003). The cost for construction for the facility was 1.06 billion dollars (Water-technology.net). Production capabilities of plants similar to these here on Oahu, could virtually eliminate the current water shortage.
Currently, a $40 million desalination plant is being constructed on Barber’s Point. At the present time the construction is approximately 5% finished, and is expected to be completed in 2006. About 11 million gallons of raw seawater will be needed to produce 5 million gallons of potable water a day. The raw source of water is deep basal wells that will draw high quality, low mineral content seawater from basalt rock that underlies the island.
The advantages of desalination plant on Oahu are:
1. Geographically, salt water resources are virtually unlimited.
2. Desalination softens the water by removing minerals.
3. Competitively priced water production, as compared to other technologies.
4. Build facilities in areas that are more susceptible to water shortages.
5. Drought proof.
6. Capable of producing water for high-grade steam, due to low mineral levels
Ocean Thermal Energy Conversion (OTEC) is a relatively new concept. OTEC makes use of the difference in temperature between the warm surface water of the ocean and the cold water in depths below 2,000 feet to generate electricity. As long as a sufficient temperature difference (about 40 degrees Fahrenheit) exists between the warm upper layer of water and the cold deep water, net power can be generated. Along with the generation of power, freshwater is also produced in a condenser.
The natural ocean thermal gradient necessary for OTEC operation is generally found between latitudes 20 deg N and 20 deg S (National Renewable Energy Laboratory). The Hawaiian Islands are in this area, and there is sufficient temperature difference within 3 miles of shore in some locations of Oahu. According to Ocean Engineering & Energy Systems, each mega-watt of electricity produced has the ability to produce .75 mgd, if all of the energy is used in the flash distillation process.
Some of the many advantages that OTEC has to offer are;
§ OTEC uses clean, abundant, renewable resource
§ OTEC plants will produce little or no carbon dioxide or other polluting chemicals which contribute to acid rain or global warming.
§ OTEC systems can produce fresh water as well as electricity.
§ There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs.
§ The use of OTEC as a source of electricity will help reduce the state's almost complete dependence on imported fossil fuels while producing fresh water.
§ The cold sea water from the OTEC process has many other uses, including air-conditioning buildings, assisting agriculture, growing fish, shellfish, kelp and other sea plants which thrive in the cold, and nutrient-rich, pathogen-free water.
Overall, OTEC has the most promising application out of the three options for solving Oahu’s water shortage. The problem is at the current time OTEC is still relatively new and there have not been any large-scale commercial facilities constructed as of yet. In the next 20 years, the cost of OTEC technology should drop, and the technology will be far more advanced, making it a promising alternative to recycling and desalination for the Hawaiian Islands.
Cost Benefit Analysis
Water recycling, desalination, and OTEC could all potentially benefit Oahu’s water shortage. Figure 1.1 represents the cost of building facilities to utilize fresh water technologies, in relationship with the amount of production of fresh water.
Technology Cost Production Cost gallon per day
Recycling $48.1 million 10 mgd R-12 mgd RO12 mgd total $4.00
Desalination $40 million 5 mgd $8.00
OTEC $40 million 7.5 mgd $5.33
After deterring the cost in relation to the amount of water produced by each technology, our recommendations for solving Oahu’s fresh water crisis are as follows;
1. Expand recycled reverse osmosis capabilities from 2mgd to 5mgd at the Honouliuli Water Recycling Facility in the next five years. This will provide sufficient water for the growing factories in the Campbell Industrial Park.
2. Construct 5 mgd R-1 plant at Kahuku Waste Water Treatment Plant in less than 10 years. This will provide nutrient rich water for agriculture in this area of the island.
3. Construct 5 mgd R-1 plant at Wahiawa Waste Water Treatment Plant in less than 10 years. This will provide nutrient rich water for agriculture in this area
4. Construct 20 mega-watt OTEC facility within next 25 years capable of producing 15mgd. This should provide enough fresh water to balance the increase in demand due to increase in population.
If all of the recommended technologies are implemented, there will be a groundwater savings of 35mgd. We are confident that this amount of water is sufficient enough to counteract an increases in population as well as increases factory production in Oahu. We believe that these recommendations will alleviate part of Oahu’s fresh water crisis.
http://www.water-technology.net/projects/shuaiba/ Water-technology.net, Accessed on 12/7/04
Daene C. McKinney, Accessed on 12/6/2004
http://www.ocees.com/mainpages/Freshwater.html, Ocean Engineering & Energy Systems, Accessed on 11/28/2004
http://www.nrel.gov/otec/design_location.html, National Renewable Energy Laboratory, Accessed on 11/28/2004
http://www.hwea.org/watreuse/wrusfbws1.htm, Honouliuli Water Recycling Facility, Accessed on 12/7/2004