The process of reverse osmosis is a relative latecomer to the desalination scene. It is perhaps the most “high tech” means of deriving freshwater from seawater. It is also perhaps the most energy inefficient and expensive of all the desalination processes currently in use. It is also the process that Biwater intends to use to produce freshwater to satisfy its contract with the Virgin Islands government.
From recent developments, it is apparent that the feedwater source on which the original proposal (and contract) was based was brackish water from some sort of aquifer in the Paraquita Bay area. Unfortunately, this source now appears to be inadequate to supply adequate quantities of water for the contract plant output of 2.3 million gallons per day. Biwater has drilled looking for alternate sources of brackish water at other locations in the Paraquita Bay area, but that search apparently has not resulted in the location of a viable feedwater supply.
Recently, I wrote a commentary on the environmental impact assessment processes in use locally. I mentioned Biwater and the reverse osmosis plant, and I touched on the need for an alternate feedwater source. Within the last week, I received a communication from a person (whose opinions I deeply respect) who is involved with the production of EIA documents for various local projects of a major nature. He is clearly an expert in his field, and his comments are germane. He has been involved with the Biwater project from its inception as a no-bid contract awarded by the Virgin Islands Party government some years ago.
Biwater design
According to my source, the technology that Biwater used in its design allowed for an extremely high recovery rate based on feedwater rates. This may have been as high as 50 percent. But this was based on brackish water from a subterranean aquifer. I have some small experience with such aquifers in the Pockwood Pond area as related to the design of the original incinerator. And, while the areas are very different, the results of our studies were equally disappointing.
This type of water source was inadequate to supply even 2,000 imperial gallons per day of water that was of adequate quality for use at the incinerator. We did not look at water purification based on desalination since that required even more water to serve as feedwater and allow for some quantity to be discarded after purification.
Osmosis is a naturally occurring process that has existed as long as life has existed on this planet. The natural process involves the transfer of material (usually water) across a semi-permeable membrane. The transfer goes from a low solute concentration area to a high concentration area, thereby resulting in a reduction of the high concentration. The transfer of material results in an increase of the solvent (commonly water) in the high concentration area and also results in an increase in pressure in the high concentration area. This pressure increase is called osmotic pressure, and the pressure changes can be substantial.
The operation of the membrane results from the fact that some membranes will pass some molecules while blocking others that are larger. Under osmosis, water will flow across the membrane from a freshwater source to a saltwater source. But the membrane will not pass the salt molecule in the opposite direction.
The reverse osmosis process depends on a reversal of this natural process. If, in a saltwater/freshwater system, a semi-permeable membrane is interposed between the fluids and if sufficient pressure is applied to the saltwater section, water molecules can selectively be forced to pass from the saltwater side to the freshwater side. The salt molecules are blocked by the semi-permeable nature of the membrane.
Reverse osmosis
The design of the RO unit for the incinerator eventually used seawater as a feedwater source. MM Dillon Limited was not expert in the production of RO units but purchased a commercial version. Dillon learned a lot from that experience! We never really had a successful plant, and we never tried it again!
The capacity of any RO plant depends on the characteristics of the membranes, the saline concentration of the feedwater, and the pressure differential across the membrane. You can increase the capacity of a plant by increasing the membrane area, improving the transfer characteristics of the membrane used, increasing the pressure differential (to the point where the membrane ruptures), or by reducing the salt concentration of the feedwater source. But increasing the pressure generally reduces the quality of the desalinated product.
In the case of the incinerator RO unit, the ratio of feedwater to product was 10 to one. For every gallon of “freshwater” (four to six parts per million salt) produced, we fed 10 gallons of seawater. It is the latter ratio that attracted the notice of my correspondent. Biwater proposes to produce 2.3 million United States gallons per day of potable water. The latter means probably less than six PPM salt. At concentrations higher than that, “potability” becomes questionable. In the original proposal, this would apparently have required 4.6 million USGPD of feedwater. But the switch in feedwater sources to seawater may mean that the incinerator ratio is closer to the truth. And this is where my contact commented that the increase in feedwater rates, possibly to 23 million USGPD, seemed excessive. But it may not be!
Indeed this may be correct. But, as usual, the level of factual information from either Biwater or the government is minimal. If there is an application in place with Town and Country Planning from either the government or Biwater for a new seawater pumping station to be located in Paraquita Bay, then the capacity of this station would tell a lot. It is almost absolutely certain that this new capacity will be substantially higher than the old predicted draw from the aquifer.
An interesting question may arise as to where the water discarded from the plant will be dumped. If the quantities are as high as seems possible, then the added concentration of salt in Paraquita Bay may have an adverse effect on the currently healthy mangroves in that area. How far out into the ocean does Biwater intend to take its effluent?
Legal questions
The switch from the aquifer to the seawater as a supply has another problem associated with it: Both sand and algae are fouling agents for the RO membranes. Note the major filter system at the Sea Cows Bay plant. These are undoubtedly there only to protect the membranes of the RO units.
There are some interesting legal questions as well. Biwater got the information on the capacity of the well fields in the Paraquita Bay area from somewhere. If Biwater developed the area itself, then the work represents a terrible engineering effort. But if the government gave it to Biwater and if the original design was based on information supplied in error by the government, all bets may be off as to the enforceability of any contract. And then who pays for a plant that either cannot work at all or produces a minimal volume of water?
Moreover, where is the water to be measured? If it is measured at the receiving reservoir, the leaks between the plant and the reservoir are Biwater’s problem. But if the volume is measured at the outlet of the plant, then the same leaks are the government’s problem
We have the potential for a horrendous mess on our hands!