Post date: Oct 6, 2014 12:31:06 AM
Background
Water in the Southwest of the United States is a scarce and valuable resource. The town, the pueblo, and many acequias (irrigation authorities) recently (but over a period of many years) hammered out an agreement on managing surface and ground water.
The site of Valverde Commons was clearly a river bottom. The topography and soil confirm this. However, now the water table is 5-6 feet below the surface. Successful local weeds have tap roots! No wonder that grass does not out-compete them.
Water price $0.0044/gallon in Taos in 2015 for use of 5-6,000 gallons/month. To this we must add the cost of sewage disposal: Sewage rates are $3.87/1000 gallons plus $20/month (includes the first 1000 gallons). This raises the total cost of city water to $0.00827/gallon (more for the first 1000 gallons). My estimated hot water use is about 560 gallons per month figured below, plus cold water use for toilet flushing, hand washing, and irrigation.
City water use assumptions, per person:
So far we have not exceeded the monthly base use, so conserving more won’t cost less, but will provide water for other people.
So conserving water is high on my list of design criteria, but not financially. Let’s see how far this goes.
Water Reuse
I was thinking of storing rainwater, but it seldom rains more than 1/4 inch a day, although it may rain 1/4” for two or three days in a row. Collect the rainwater in a 1000 storage tank underground or in a corner of the garage, costing about $1,000.
The collected rainwater and the greywater could drain slowly into a managed wetland. The greywater would be delivered to the soil as it is generated, but the rainwater would be stored so that it was delivered over a period of a month - until the next rain. Taos requires that greywater be delivered by underground drip system. (Every locale seems to have different constraints on greywater.)
The wetland would use the native soil, minus cobbles, underlain by a rubber sheet as for a pond. this would be shallow and slope very gently so that it flowed slowly and eventually became groundwater. I understand there are a number of species that filter greywater and clean it. However, the law requires that the discharge not be open to pets. Earthship was a local pioneer of greywater reuse.
Water reuse would allow me to have a more lush and verdant garden than without irrigation. And the water attracts birds.
Sewage
Separate greywater from blackwater. Plumb the greywater drains to a single point outside the foundation. For starters, let’s connect the greywater to the sanitary sewer, which connection will be required for overflow eventually. The soil is very limited for septic tank absorption fields.
A good greywater economic engineering overview is provided by Lowe’s. Use gravity if possible. Lowe's would connect to drip irrigation, either at the same point as the rainwater, or separate. Initially I’m considering a wetland rather than drip irrigation
Greywater production has been calculated under “Hot Water” below.
Blackwater comes from toilet and dishwasher. Toilet use might be 4 flush/day X 1.28 gallon/flush x 30 day/month or 153 gallon/month. I will still have to pay for treatment of all the city water that I use, but not the rainwater.
Hot water
Analyzing the effectiveness of solar systems requires reams of hourly data from the Taos airport, available for about $150. On the other hand, local vendors will have both the data and the expertise to use it effectively. So I’ll defer to them at this point!
Taos receives sufficient sunlight to heat hot water. But since it freezes often, a solar hot water system either requires drain back or recirculating anti-freeze with a heat exchanger. These add about $2,000 to the cost of solar hot water, and are not passive, meaning they can break and require electricity. Besides, natural gas is cheap now.
Assume use of hot water as follows and natural gas price of $0.50/therm (100,000 BTU) and a gas water heater efficiency of 75%:
5460 gallons x 8 pounds/gallon = 43,680 pounds of water. Assuming the tap water arrives at 50°F and can be raised to 70°F in the solar slab, 43,680# x 20°F = 873,600 BTU/year or 8.7 therms. Preheating the water to room temperature would save $5.82 /year. Any effort to do this must cost less than $100. A coil of tubing under the floor slab might allow pre-heating of water (indirectly by the sun in winter) on its way to the water heater. The tubing alone will cost this much, plus the labor to install it. Forget it.
To raise the water temperature from 70 to 120°F (another 50°F) my cost would be 43,680 pounds of water per year X 50°F X $0.50/100,000 BTU/75% efficiency = $14.56/year for gas to heat water above room temperature. Total cost to heat water with gas, about $21/year. No savings available there over 40 gallon gas water heater with 12-year warrantee and glass-lined tank. Don’t need sealed-combustion if it can be located in the utility room. About $1,000.
Solar hot water seems too expensive, given night-time freezing temperatures and (current) low price for natural gas. But if we could leave stubs for hot and cold water, just in case a future solar hot water solution emerges, I’d be happy. Heat pump hot water? Probably not, as my use of hot water is low and the winter air temperature is even lower.
If I am to have a tankless water heater I must estimate the maximum flow of hot water. I have measured the flow rate of my shower, bath sink faucet, and kitchen faucets. The faucets all run 1.6 gallon per minute (gpm). The shower 2.0 gpm. The Energy Star program estimates a dishwasher at 1.3 gpm. A clothes washer they estimate at 3.3 gpm - but I know that it simply takes longer to fill at lower rate. However, if the laundry (which I don't plan to install immediately) draws too much water, and the flow is restricted by the water heater, the other taps will suffer.
The biggest pain is the shower - if the hot water flow drops, the shower cools. My maximum flow would be about 5 gpm plus laundry. So the smaller 6.4 gpm Rheem RTG-64 EcoSense "mid-efficiency" heater at $630 would work. Their "high-efficiency" heater may be too big for me. The Takagi T-KJr2-IN-NG for $550 is well rated. Since this is about the price of a tank water heater, and the push is towards tankless, I'll go with that.
I may wish to install a separate water heater in the garage for the kitchen use, as the kitchen and bath are separated by 35'. The extra expense of the second water heater might be offset by the faster response in the kitchen and less wasted hot water in the pipe. It would mean another hole in the roof (or wall) unless the water heater vent could somehow be combined with the kitchen sewer vent (at least visibly) into one stack.