Well and Pump vs Tank 1

[jmgray]

What are some of the considerations for a water distribition system for 4-13 homes in a rural area. An onsite spring has been measured to provide clean potable water at 60GPM. The two intial designs are (1) pump water from the spring 150 feet up to a possible tank location or (2) use wells do provide water on demand (elev of homes would be approx 75' above spring -exact depth to ground water is unknown at this time). We are a geotech firm and would be subing out the design, however I would like to know a little more about the general design critera and rough costs of the two alternatives.
Thanks in advance

 

[francesca]

It's kind of an anecdote and not professional experience, however I'd like to caution you that ground water doesn't necessarily have the same quality as the spring water.

My father-in-law used to get his water from a spring and the quality was impeccable. Unfortunately the source was directly across the street from him, and the State decided he couldn't cross the road to get his water. He had to tap his well and now the water is very sulphuric and he had to install a sophisticated (expensive) multi-stage filtration system to get the water to shower/toilet level. He does not drink the water, but buys bottled water instead. The spring and well are at most 500 ft apart.

 

[jmgray]

Thanks for the reply Francesca, I enjoy anecdotes as they often teach a lesson that to that can be difficult to learn first hand. I appreciate the caveat that the sources may differ in quality and possible recharge rates, ect.

It may be that we are going to have to use a water tank and common water system due to local fire regs. For the academic sense of the problem however, what are the issues you water guys look at when comparing the costs of different supply systems.

For example the tank alternative may require a disinfectant system. Do smaller tanks have a short enough holding time to not require this?

How does the cost of running and maintaining an on demand well system compare with a water tank system?

How many wells and pumps must be installed before the cost of a single larger pump and tank break even?

I understand that costs vary greatly with location, but I value the wide range of engineers that post here. I plan to talk with civils local to my area, and a little more knowlege before hand certainly cant hurt.

 

[cvg]

that's a broad question and without more info, difficult to give costs. Francesca hit on one issue that will need to be considered - water quality. Samples should be taken and analyzed and depending upon the quality, filtration or other treatment may be necessary.

Fireflow and emergency water is another issue to consider. It may be desirable - or required - to provide the following: emergency storage, emergency generator for the pump, standby pump and last but not least, additional storage for fire fighting.

Also, have you performed an analysis of the drawdown on the spring? If you begin pulling 60 gpm to serve 13 homes, will the water level in the well go down? Will this require additional pump head to supply the needed water? will the drawdown affect other nearby wells?

You indicate that you can provide 60 gpm. Will this be enough to provide for peak demand from the future homes? When everyone waters the lawn, turns on the shower and flushes the toilet at 7:00 am each morning, will the total demand exceed 60 gpm? If so, you need to provide sufficient storage in a tank and the pumping system to provide for that.

 

[RWF7437]

First, gravity is free and is never shut off for non-payment. If you have to provide fire flows it is those flows which will govern the design. This suggests that a tank above the service area( large enough to store the fire reserve and to provide aervice during the peak demand period ) will be the most reliable system. It is unlikely you would want to buy a very expensive, large fire service pump which would only be used occasionally. 60 gpm is not a fire flow. Even for residential areas fire flows are typically 500 gpm to 1000 gpm. The quality of the water source is not very important for fire flows so if you do have to drill a well to get the required water for fighting fires you might want to consider seperate systems.

Sounds like you're at the proposal stage. This would be a good time to select your Civil consultant(s) and ask them to help you put together a good proposal. Also, get more information on both water quantity and quality. The more you know now, the fewer unpleasant surprises you'll need to deal with later.

Good luck

 

[jmgray]

Thank you guys for the additional replies. We most certainly will be using a local civil to hash out the cruel details. I do find the topic interesing even though it is out of my scope, and would like to know more about what the board has had to consider on your similar projects. I will try to provide some additional info:

CCG-
Initial water quaility has been tested by others, and has been found to be beautiful!

The spring is currently flowing at a natural rate of no less than 60GPM year round. Becuase the spring will flow at this rate regardless if we use it as a source drawdrown/recharge will not need to be considered at this stage. If the spring was to be used as the sole water source over individual wells per dwelling, a tank is proposed insure the common water supply can meet the max daily demand (and fire (. The water tank will be locatated at an elev 150 above the spring so a pump will be required to provide flow.

RWF7437-
The project is indeed in a proposal stage, however we are not considering costs for the purpose of future billing, rather trying to estimate the ballpark extent one would need to involve themselves financially. The project consists of 4 single family dwellings. The additional 9 may or may not be built on a neighboring property. For the sake of simplicity lets assume its only the 4 to be built. I believe in the end we will have to provide to fire suppression, however the project is small and in a rural area so we may be below the regulatory threshold. For the sake of simplicity lets assume fire can be neglected from costs.

What are the methodologies you use to estimate costs? For example it is probably too simple to assume # of bores x depth of bores x linear cost to drill. What are some of the additional costs that need to be considered for a home to use a well? A UV system, water softener...

What are some of the costs to consider for a common water supply from a tank. the cost of the tank, pump, pipe...

 

[RWF7437]

At this stage, we know far too little to be estimating costs. In any event, without knowing where the project is any cost information I could give you would probably be useless in your geographic area. Without at least a preliminary design costs cannot be estimated. If you're trying to estimate your design fees you need some kind of work plan and some experienced people to put that together. The geotech part of the job will probably be the least of it, in my experience. And most important, who regulates small water sytems in your part of the world ? Is this considered a public water system subject to both State and Federal regualtion (Safe Drinking Water Act) ? Who provides fire protection services and what are their requirements?

Good luck

 

[cvg]

I agree that a gravity tank is technically probably the best alternative, it would require a longer transmission main, larger capital cost to construct and would require purchase of land to site the reservoir.

Pumping directly to the homes will also require a tank (or tanks) which are pressurized with air and provide a steady supply of water at a relatively constant pressure without surges and without constant stop and start pumping. For instance, you could install a large hydropneumatic tank at the spring or pump directly to each house and each house would have a small hydro tank in the basement or garage. This may be a cheaper initial cost due to the shorter transmission main and smaller reservoir, plus no need for additional site. But the cost of O&M for the pump system will probably be higher and will have less redundancy.

 

[bimr]

jmgray,

It is probably not practical to consider a water distribution system for only 4-13 homes.

For such a small system, you probably need storage of about 50 gals. per capita, so the total storage would be only about 600 gals to 2000 gals.

Plus you have the situation regarding fire flows. The fire demands are so much larger than the normal daily flows, the water distribution system would never work properly. You also will have high ratio for peak flows to average daily flows with such a small system.

You will also have all of the headaches that come with managing a water company. Sampling, billing, etc.

Save yourself a headache. You would be better off recommending individual wells for each house. Unless of course, the geology of the site preclude well construction. I think wells go for $4-5K each.

 

[jmgray]

Thanks bimr,

I had not considered that a new water district would likly be formed with a common water supply. That is the type of information I'm kinda poking for.

RWF7437,
I understand that costs vary by location, the dollar amount in your area may not correspond to those here, but would still be interesting to consider. What I'm trying to ask for are what are the componets of which the costs must be considered.

 

[RWF7437]

I still don't understand what costs you are seeking ?

If it is construction costs these must include, at least:
Land cost
Well drilling if necessary
Piping costs for the entire distribution system
Pump costs
Reservoir costs
Controls
Design, survey, inspection, and testing
Interest on borrowed funds
contingencies which at this very early time must be at least 25% of the total

Then there will be the continuing operating costs including:
salaries of the operator and others
chemicals
pumping power
planned replacement ( a sinking fund is often used )
billing
meter reading ?
routine required periodic testing


For this small system these costs will be relatively high per customer.

Are you sure you want to undertake all this without the help of experienced people ?

 

[semo]

I don't know where this is located; but, providing drinking water for multiple homes from a spring without treatment is a scary thought.

The water quality may be good; but, what sort of bacteria is or could easily get in the water?

A lot of states don't regulate below 25 connections; however, as an engineer I wouldn't want families to get giardia or crypto because I didn't filter and/or disinfect the spring (ie. surface) water. What keeps the beavers, rats, squirrels, cats, dogs, mice, surface runoff, etc. away from the spring?

This water could possibly work fine for fire flow; but, you will need some form of storage (at the spring or elevated) to provide enough volume. 60 gpm isn't a high enough flow and you need to store enough water to meet your fire flow demands.

If this is rural, a large pond without distribution system may meet the needs. In my area, it is common for the rural fire department to pump water from local ponds to their water trucks or directly to the fire if it is close.

 

[rconner]

This is probably a stupid question as I don't deal in this specific field, but if the "spring" is now feeding some sort of stream or other watercourse I am curious are there additionally in any areas any sort of impact statements to fill out with regard to how downstream might be impacted?

 

[francesca]

What are some of the additional costs that need to be considered for a home to use a well? A UV system, water softener...

My father-in-law had to install a sand filter, UV filter, chlorine addition and salt softener system to meet VA mortgage requirements. It was very expensive. I would budget for the whole shebang because if the homes can't meet VA requirements, it could dent their value, particularly if you're near a military town.

Check with the local fire dept. for their fire flow requirements. Where I worked, the rural fire departments were all volunteer fire departments and they had trucks that they'd fill and pump from the truck. We weren't required to meet fire flow rates or residual pressures on the water mains. If this is a non-municipal water service area, you may not have any fire flow requirements at all. (Which doesn't necessarily mean you shouldn't provide any.)

if the "spring" is now feeding some sort of stream or other watercourse I am curious are there additionally in any areas any sort of impact statements to fill out with regard to how downstream might be impacted?

Wells and perennial springs are waters of the state, so without state approval, it's not possible to tap into either source. In TN, TDEC's Division of Water Supply provides well permits. I'm not sure how using a spring would be regulated (water intake?), but you're right, it is important to consider the downstream impacts of reducing the flow. Wetlands or fish ponds could be severely impacted, particularly if flow is intermittently interrupted during dry spells.

 

[cvg]

depending on the location, a spring may also be claimed as a private water right, not waters of the state. However, you may affect downstream users of the water from the stream if you divert it not to mention the environmental impacts.