Water Problem 2

[friartuck]

Hello fellow engineers

I have a problem that needs some serious thought.

We are involved with a new holiday facility in Antigua.

It comprises of about 25 holiday homes and restaurant (each home will probably contain 4 or 5 people)

The island appears to have a low rainfall and water is at a premium. We have yet to contact the Antiguan utilities company but I am getting ready to go out within the next month to see how they do things over there.

I have been told that the 'homes' will each have cisterns built underground (presumably these will be rain water recovery tanks sized for the occupancy and rainfall(whatever that rainfall may be))

I also understand that the islands utility companies water supply might be a little erratic and the Architect has suggested installing a desalination plant. Our site is next to the sea about 2m above sea level. We also have two swimming pools and a sports complex.

Question1.

Does anyone have a simple rule of thumb for rainwater storage cistern sizings. In the UK, modern housing sites are now being fitted with 3000 litre underground tanks. Would this be enough for Antigua??

Question 2.

We understand that the 'skill levels' available on the island may be a little poor and so anything that we install will either need to be really simple or available locally. Does anyone have the name of a local water specialist company that can provide a desalination plant design and cost??

Question 3.

Rain water --Potable or not??

Again in the UK we have rain water recovery systems that can be provided with simple coarse filtration and fine filtration systems. The water can then be UV treated etc to provide drinkable/potable water. Does anyone have a company name for a specialist capable of doing this type of work. (Turnkey for all of the water process would be nice)


Any suggestions and help would be greatly appreciated

Tucky



Friar Tuck of Sherwood

 

[AbbyNormal]

Here for sewage treatment you have to allow for 75 gallons per day per person. So for the homes, master bed is two occupants, all other beds count as a single occupant.

Don't bother trying to build something that can hold enough rain water to last the summer, won't happen. Figure out how often you want the truck to come out and fill it up. I posted a picture before, of some apartments of mine, it was originally design for a cistern for each 2 bedroom apartment, but we only put in one for emergency water source. 5000 US gallons for a two bedroom place.

I am using the water right now do get my landscaping established. I would not drink it myself, but you could filter and UV treat it like well water I suppose, but there is a general movement here against open to atmosphere water storage. Drinking water that is. I also have a well on my property.

Do your research and see if there is a municipal supply before searching out an RO plant supplier. I know an HVAC tech who was there and for a missile base they were collecting rainwater off of the runways. Search sotu florida for contractors.

I can only give you names of sewage treatment guys.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

I bet a 300 room resort easily goes through 100,000 US gallons a day, with swimming pools, landscaping , restaurants, rooms


Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

First post should not have said summer, as summer is when it rains, it should have referred to lasting a winter otherwise known as the high season.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[lilliput1]

Forget rainwater collection. It will never pay, more so if the rain is infrequent. You would need lots of roof area to collect water. It would not be potable water. It will be gray water to be used for toilet flushing or cooling tower makeup. You would have to filter the water and pump it. You would have to provide about minimum 48" water head on the pump suction so whirlpools don't form and cavitate the pump. You would have to provide a cushion of say 15 minutes usage for domestic water makeup. Active storage would then be above the submergence and makeup level.
If you have a daily rainfall data of a typical year, you can write a program estimating the rainfall that can be collected. Allow about 10% for evaporation loss. Estimate the daily water usage and do a daily balance of the tank, the volume of which you can set in increasing increments. The spreadsheet should calculate what water is stored, what water spills from overflow, and what is used.

Use google to find desalination plant designers. You need a reliable source of water!

 

[AbbyNormal]

Rainfall will be low in the winter. Maybe a private RO plant could make water for about 1.25 cents per US gallon plus the operators' wage.

The old days here, municipal supply was non existent. Homes all had cisterns, collected rain water in the summer,then water trucked in in the winter.

Cisterns are the most valuable now, as an emergency water source. Get hit by a hurricane, and you are instantly amish. Tourists would not be at a resort in the aftermath of a storm though.

Municipal water here is pushing 3 cents a US gallon, I am trying to get half an acre of grass to grow from seed right now, so emptying my cistern now.

The cisterns fill up pretty fast when it rains at 5 inches an hour as a tropical wave goes by, don't need a lot of roof space



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[friartuck]

Hi Folks

Thanks for the assistance

I am interested in how water is delivered by road?? In the UK, this concept is unheard of. Presumable a truck arrives with a long hose pipe and each property has a special fill valve located outside with a fill alarm to prevent over filling the tank (cistern)??

The cisterns that have been shown on the Architects drgs look like simple concrete boxes and are underground below the actual property. Are they best located outside the property or to the edge of the building.(thinking about maintenance/cleaning, chlorination etc)

We (in UK)normally use pre-fab plastic tanks with built in overflows, water intake connections,controls etc. I would prefer to buy a pre-fab tank and drop it in place (i.e drop the PVC tank into the concrete cistern as a kind of liner) rather than use a plain concrete tank (which might not be the cleanest of things and difficult to keep clean with its rough surface)

As mentioned, we don't yet know even if the local water utilities can give us water, but we will find out hopefully next month when we fly out.

I have my Bermuda shorts ready (Or do I need Antiguan shorts)

Thanks for the info

Keep it coming

Friar Tuck of Sherwood

 

[AbbyNormal]

Bermuda has degree days and a need for space heat. Make sure you go all out with those Bermuda shorts in your quest to look like a tourist. Wearing sandals with knee high socks will suffice.

Here it is common for the cistern to be a concrete vault below grade. Also see plastic tanks sitting on grade. An above grade tank will not suit the aesthetics of a resort.

Trucks aka "Lorries" have hoses. I think in a resort people would use the cistern water to bath, flush their toilets, wash dishes. Most likely drink bottled water, rum.

You can have elaborated float aswtiches to sound an alarm as a tank fills or just have a port with removable cap and when you see water coming out, shut the water off.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Even a concrete tank underground can float, an emopty plastic tank will be even more prone to floating. They will be underground empty never doubt that.

Paying for the freight on a lot of air shipping in plastic tanks.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

The mnajority of the cistern will be below ground. Maybe 0.5 meters of it will be above ground. Perhaps the top of the cistern makes a verandah or a porch.

There are capped pipes, above grade on the side of the cistern, the caps are removable.

Also possible to have a remote fill port similar to a fire department conncetion.


Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

My own cistern is below grade, my 4 unit apartment complex is on city water.

I also have a well, but rain water is better quality water.

I plan on using the cistern to help with irrigation for the landscaping and as an emergency source of water, given that we were hit by a Category 5 storm in 2004 and were without running water and power for an extended period of time. One to three months of being Amish, depending on what part of the island you were on.

My cistern has the lid flush with the surface and looks like a concrete patio.

I have a lockable hatch for access. I could fill the cistern through this hatch by a truck if I wanted to. The main reason for the hatch is to be able to get inside the cistern to clean, perhaps repair. It is nice to have a sump pit in the bottom. You could clean and 'squeege' the wash water to the sump and then pump it out.

I have a small pump house with a well pump/pressure tank. This pump is capable of drawing water either from the cistern or from the well. The municipal lines run from the meters through this pumphouse as well.

In the aftermath of a storm, I could switch the entire building over to cistern water, provided I have fuel for my generator and everyone has running water. By the time city water is restored,it is going to be boil only advisory anyways. I could chemically sanitize the water lines once the public supply was deemed potable again.

Water to drink is the least of your worries. Flushing toilets and to be able to wash clothes, wash your self, clean your homes is a luxury of the past century.





Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[friartuck]

Abbynormal

It's interesting that you mention having the tank (cistern) outside with a raised section (0.5 metre above ground)

In the UK our water regs are quite strict about water tanks and backflow prevention. (i.e. stopping water contamination getting from the users end back into the utility suppliers clean mains.

We have type 'A' air gaps for instance which requires a fill point (i.e. Ball float operated valve) to be positioned so that the inlet is always above the flood plain of the tank. This is achieved by positioning the overflow so that there can never be a situation where stored cistern water can rise to a position where it can flow back into the main. This could be achieved by having a raised ball valve enclosure for instance)

We have various grades of contamination requiring different methods of backflow prevention, for instance:

If we were to be storing water to feed a hot water cylinder (for domestic washing etc) then this would be classed as risk level 1 since the water would be clean but affected by perhaps colour or temperature.(i.e. a low risk or health hazard)

A risk level 4 or 5 system i.e.where human waste, bio-hazards etc are encountered, would require much more sophisticated backflow prevention devices such as RPZ Reduced pressure zone valves or 'break tanks with water bosster sets)
Domestic hoses (for gardening watering etc. ) require double non-return valves (Double check valves) etc.

If we put a cistern totally underground (i.e. level with the ground), that would make it more difficult to fit a suitable backflow device and so I would think that some portion of the tank might need to be above ground (as you described earlier)to provide a type 'A' air gap for backflow prevention.

OUTSIDE TANK-(Below Ground Configuration)

If we did fit a tank outside though,(to let the 'turret' be raised and give us our safe air gap) would the water get warm??

This would contravene legionella regs in the UK. (Note that the below ground temps in the UK are about 8-10C. and so installing a tank outside below ground is not a problem)

What would the below ground temps be in the Bahamas area. (This would affect the decision to put a tank outdoors)


PS If we DID use a PVC type tank, we would concrete it in situ so that it would not 'Float' when empty.

Thanks for your continued interest in this problem



Friar Tuck of Sherwood

 

[AbbyNormal]

All our hurricane shelters have three sources of water, muncipal, cistern and well. City water can supply everything, cistern water everything but drinking fountains, food prep and medical areas, and a well could be used to supply toilets only.

I did a large hurricane shelter here and cursed the well and the three sources of water as there was miles of 50 mm pipe going all over the place.

I ate my words when there was no water for months. The well supplying the toilets was a godsend, they were trucking in about 32,000 litres per day for the cistern, and they would have needed far more if it was not for that well.

So with a float valve fill of piped in city water, the professor and the taught us the backflow preventer becuase it was a little too technical for Ginger and Maryanne
Sorry couldn't resist.

The cistern such as mine is not filled by city water. You would have a well pump pressurizing the same lines and a small 19 to 25 mm backflow preventer on city water if it was available somewhere just inside the tie in in the individual units. Mine is simply valved, the building will never be on it unless it was another emergency.

If you had city water, you would not really need the cistern for domestic use only for an emergency and tourists are the first off any island as a storm approaches or in the aftermath of a hit.

Backflow prevention is also required here when you boost city pressure for multistory buildings, or when you have it piped directly to irrigation pumps. There is no concern that they will freeze so you could see a 100 to 200 mm backflow preventer above grade in some landscaping near the street in the case that they were boosting pressure.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Bet the ground temp be 25 to 27C, if you are not careful you could have a legion of frogs.

Again, maybe the tourists would be drinking more havana club or perrier compared to tap water.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

In Canada if you let the taps run, the water can be refreshingly cold. The water starts on the warm side, influenced by the room air temperature, then as you let it runs can be quite cold, especially in the winter.

In Antigua, I will presume that the coldest water is that which first comes out of the taps, as the piping is influenced by the air conditioned room temperature. As you let it run, it will warm up to about 27C or so I would imagine.

One benefit of this, water heaters do not work as hard as in Canada.

Take the "V" out of HVAC and you are left with a HAC(k) job.