Thoughts on individual residential desalination plants vice cisterns

Your property is ocean front?

A very small percentage of homes are ocean front accessible - you would have to get a CZM Tier One approval as you mentioned.

The initial cost, the continuous maintenance and the fairly limited output of an affordable R/O plant  might be worth it. But since the rain is free, the trucked cost is not crazy as well, would the whole thing be worth it.

i have worked with R/O systems on boats for quite a while but the water tanks on a vessel are very small compared to a 10,000 -25,000 Gal home cistern so it is more of a needed replenishment system on a boat than a storage plan like a large roof area and big cistern gives you.

A gray water cistern for irrigation or even toilet flushing is a great addition, A driveway/parking area can provide quite a bit of grey water if it is collected, stored and plumbed to appropriate uses.

The CZM permits for putting a line into the water are the most painful part of the process.  As an alternate, we've done a few residential ROs on STJ where both the intake and outflow are via brackish wells on the property, which has worked well and avoided the CZM Major Water permits.  Once it's set up, it does make a great deal of consistent water. 

I like the guys at Elemental, and their focus on keeping the RO off-grid, whether via solar and batteries or pumped and gravity fed - do you have the elevation to be able to do their storage tank RO system?  

As EZ noted, setting up the RO system may never be "worth it" in terms of cost.  A little more water collection, a little more cistern to provide a bigger buffer, and collecting and using grey water will probably be more cost effective, unless it's a large house with consistent, high water use.

Thanks for the responses. To answer the first question, yes, my property is waterfront, but I figured, as most of you also mentioned, the CZM permit for a pipe into the sea would be prohibitively difficult. I really like the idea of using a well, but until now I wasn't even aware it was possible, so the responses here have already been a great help. The only reason I have been considering the R/O system to begin with is because of the fairly steep grade of my lot. Due to this factor, the combined costs of both excavation and the amount of concrete needed to construct footings, slabs, and retaining walls is quite high. This led me to consider options that would allow a pier based foundation, the elemental system seemed a natural fit given the circumstances. 

It looks like the gravity system requires 290 ft of elevation. I unfortunately only have 110 ft.

It would be a very unlikely scenario that you would have an accessible water table on a lot that steep on the shoreline to provide well water.

There is no doubt that construction costs here are high. Exploring the CZM permitting requirements for a R/O plant may well be worthwhile, The concerns may be about the salinity of the outflow and the effects on the water. You will still have the storage to solve so a cistern of some size is certain, even if it is tank based. Storage that small will easily fill and overflow with  some rainfall most of the time [which would drive me crazy]  so deciding on storage capacity will only determine how often you would need to run the R/O plant. The black water will also involve a 100% containment in a tier one lot. It is not likely you will save much on excavation costs even with a pier construction. Retaining wall construction can be minimized with planned good excavation techniques.

I wonder if the lack of a proper sized cistern would severely impact the sale price of the house later and am skeptical that the  building cost of a cistern outweighs the R/O plant installation,maintenance, initial and running costs. Are you located in an arid part of the island?

My lot is on peterborg; not sure how that compares in rainfall to the rest of the island. The lack of cistern potentially affecting the resale down the line concerned me as well. Although one would hope that, should the system prove itself over the years, that wouldn't be the case. But I realize that in reality buyers and banks are often fickle and don't like any deviation from the usual methods. What concerns me more at this point is the possibility that the up front costs of the R/O system would approach that of the cistern anyway. If that is the case then it wouldn't make much sense for me to go that direction. I am curious as to what the reasoning was for the project in the BVI using the system? 

Regarding the excavation, I know I'm pretty much going to eat it on the price of that no matter what, but using piers could, at least in theory, save me quite a bit of concrete by allowing me to avoid building several 40' retaining walls. Do you know if many/any people utilize pier based systems on island? 

No chance of a water table for a well on Peterborg. Yes it is a very dry peninsula, just look at the native vegetation - cactus, succulents and no big trees with broad leaf plants. The road is heavily traversed by water trucks every day, primarily for the rental villas that have vastly different water needs than the residential homes there, it was not even developed as a residential area until the late 1970's after you could reliably BUY water. Your concerns are well founded. Lots were a hard sell there for quite a while after Jim Armour put the road in despite the spectacular setting and were a good value. Sustainability from rainfall was a big factor.

My uneducated guess on the BVI installation is that it is a supplemental supply system for a large cistern.

Partial use of pier construction incorporated into the design is not uncommon here with the steep terrain. Many retaining walls are constructed with boulders especially in areas that are not susceptible to erosion from heavy rainfall. Your earth change, and even more so,earth change from building lots above you alter how the natural water flow has already sculpted the stability of the current landscape. Building into the land, [rather than onto the land], with extensive excavation usually results into erosion problems that have to be solved instead of avoided from the beginning.

Ah, more info is helpful.  Whether you do RO or standard roof catchment, your storage capacity (cisterns) wouldn't change at all.  The only thing the RO system will offset, cost-wise, is your need to purchase water.

There are very few locations on Peterborg where the on-land well system would work, given steep rocky cliffs down to the ocean, so that would guide you to the Major Water Permit process to get your pipes into the water, which can be costly and time intensive.

40 foot retaining walls?  Multiple??  No matter how steep your site is, there should be no 40 foot retaining walls.  My guess is that the footprint of the designed house is twice as deep as it should be, and the design does not work with the topography at all.  If you're throwing away hundreds of thousands of dollars on 40' walls that don't benefit the living spaces in the house, it may be time to rethink the design of the structure!

Most of the RO systems we've seen have been installed for self-sufficiency - some of the smaller BVI locations don't have the ability to truck in water whenever needed, so the RO is added on to the generator, solar panels, batteries, for full off-grid ability.  One in STJ was done for a large rental house, which could easily go through a half dozen water trucks a week with full occupancy and no rain.  Another was done for a private house that wanted clean, non-rusty, non-silty, endless water for their needs and their gardening.

Hopefully he meant 40' long retaining walls NOT 40' high!!

Lots of useful info here. I'm afraid you are probably right about the plans. I wanted to stick with them as I like the design, but I don't think they are ideal for the lot. The double 40'  (long) retaining walls are actually only part retaining walls and part below-grade structural walls resulting from the house being built back into the hillside in steps. So, I'm not sure if calling them retaining walls was technically correct, as only about 25' is strictly not part of the dwelling. It sounds like what I'm facing here is more of a design issue than necessarily needing an alternative technology for water processing. I read an article concerning a project on STJ wishing to run a pipe to the ocean for the same type of R/O I was looking into and it was denied. Not sure if that is the default or not, but regardless, I'm sure the CZM process is unpleasant, even if it is possible. 

I was thinking that by raising the structure up a bit and using piers, I could avoid the long segments of retaining walls needed to back the patio area between buildings. Will have to get back with my architect I suppose. 

You touched on the subject of black water.  What is the situation on STT concerning septic systems/sewers?  We were considering a waterless toilet system ( like Sunmar for example).  Are those allowed on the island?  I would like to hear everyone's thought on this or do I need to take this question to a different forum?

Some advice,

Keep it simple, remain practical.