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  Back of the Envelope

  Previous Issues

Water Train

Many states in the US are in a brutal drought.  Reservoirs, ponds and lakes, which supply drinking water to many cities, have been drying up.  Even water wells have been forced to go deeper to bring up the valuable water.
Some cities have resorted to draconian measures to reduce water consumption.  Cities have banned lawn irrigation.  Car washes have been shut down.  They have even suggested that people minimize the number of times toilets are flushed.  How could a city survive without water?  One ďout of the boxĒ suggestion I read about was for the city to buy water and have it shipped by rail.  Would that be possible?  How practical would it be to aid a small city caught in a terrible drought to have water delivered using conventional railroad tanker cars?  How much water could be delivered?  How long would the water last?
Tanker cars
A typical railroad tanker car, like the one above, can carry about 10,000 gallons of water.  A fully loaded train might haul 100 such cars.  So, 10,000 gallons times 100 is 1 million gallons of water.
A 1000 foot by 1000 foot reservoir (23 acres) would be filled to a depth of 1 foot with one train load.10 such train loads would bring the depth to 10 feet.  But, even if each household restricted their water consumption to just 50 gallons per day, a one million gallon train load, would keep only 55 homes going for a year.  Ten such million-gallon-loads would increase the total to just 550 homes for a year.  A small community of just 500 homes would need a fully loaded train every month to keep it going.  This does not sound like a practical solution.  But, letís see what it might cost each home for such water.
City Reservoir City Water Tower
Fifty gallons of gallons of water per day may sound like a lot but most homes consume a lot more than that.  As the chart below indicates, clothes washers, toilets and showers consume most of a homeís water.  Anything that can be done to reduce the flow to those devices would help a lot.  Low flow toilets and showers do help.  People can buy retrofit toilet valves which can turn a standard toilet into a dual flush toilet.  Flushing liquid waste requires only half as much water as solid waste.  Washing dishes only when there is a pile of dirty dishes helps too.  Grey water from showers and tubs can be reused to flush toilet. This allows some water to be used twice.  NASA has developed ways to recycle water aboard spacecrafts.  They use a reverse osmosis scheme together with UV light to turn waste water back into drinkable water.  But it may be a while before homes have such space-age water conservation measures installed.
Typical Home Water Consumption  Low Flow Shower Head Dual Flush Toilet
A deep water well might require a 2KW pump to bring water up to the surface at a rate of only 10 gallons of water per minute.  At the going rate of $0.1 per kilowatt-hour, such a well would produce water at a cost of about 3000 gallons per dollar.  Using this as reference, a 1 million gallon train would require about $333 in electricity.  That is pretty cheap. Iím guessing that water might be pumped into the tanker cars at a rate of about 1000 gallons per minute.  So a 10,000 gallon tanker car might be filled in 10 minutes and a 100 car train would take about 1000 minutes or 17 hours to fill.  I think this filling operation could be done with two people working 20 hours for 40 man-hours.  It might take the same 40 hours and $330 in electricity to off-load the water to the city reservoir.  Using a figure of $50 per hour, the total cost for labor and electricity to fill and empty the fully loaded train might cost about $4700.00.
I found some figures on line for shipping costs for coal trains.  The rough figure I got was $0.02 per ton-mile.  I think a drought stricken city might have to bring water from about 200 miles away, so the shipping charges might be about $4.00 per ton.  Water weighs about 8.35 pounds per gallon, so a single 10,000 gallon tank car would haul 83,500 pounds of water.  That is about 42 tons.  A 100 car train would therefore haul 4200 tons of water.  At $4.00 per ton for a 200 mile shipment, the shipping charges for a 100 car train would be roughly $17,000.00.
Letís total this up.  It might cost about $17,000 for the rail shipping charges.  Add the $4700 for material handling and that totals $21,700 for a million gallons of water delivered.  If this cost is divided up between 500 homes, the cost would be $43.40 per month to keep them going, assuming severe water restrictions would continue.  This figure was a lot less than I expected.  Could this scheme work to move water from those areas experiencing floods to other areas stricken by a drought?  Maybe.  In many areas the largest use of water is for irrigation.  A farmer would need a lot more than 50 gallons of water per month to save his crop.  I think this rail delivery scheme only makes sense for delivering potable water directly to a city water tower or reservoir.  It would be too expensive for farm irrigation.

First Quarter,  2012    


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