The composting toilet system at Flagstaff Hut was manufactured by Clivus New England. The manufacturer’s web site has a good link to the the technology. Key components of the system include toilets that flush with a 3 oz water/foam mixture, waterless urinals, and a central composting camber in the basement of the hut. The compost pile is kept warm, oxygenated, and moist to facilitate aerobic oxidation of human dejecta. Human waste is mostly water (Composition of Human Dejecta) and organic carbon so the oxidation of waste produces carbon dioxide and water vapor that leave the compost chamber as gas. The residual solids are mostly inorganic phosphorus, calcium, and potassium salts that represent less than 10% of the initial waste volume. An interesting calculation would be the expected annual mass of composted material produced by 500 hut visitors per month.
Interesting technical challenges are the physical limitations of component placement since gravity and very low water volumes are used to carry waste from the bathrooms to the compost facility. The Clivus system does require regular maintenance and, therefore, requires easy access.
Urine is high in nitrogen and the composting process converts most of the nitrogen, urea, to soluble nitrate or nitrite that collects in the liquid sump in the bottom of the composter. This liquid is pumped out of the system and added to the gray water of the hut. It would be interesting to know more about the gray water system and regulations on gray water system placement in Maine. Would it “pay” to install this system in your own home? What is the market for Clivus systems. The hut crew sold the benefits of the system based on water savings? Is this the real environmental and economic cost?
The heating system was manufactured by Tarm Biomass. The HS Tarm Solo Innova 50 boiler is about 80% efficient, has a normal output of 171,000 BTU/hr, and the hut burns about 20 cords of dry wood per year to heat the buildings and produce hot water. The plumbing schematics from Tarm are pretty close to the installed system.
Interesting calculations would be the total energy produced by 20 cords of wood in BTUs and joules and equivalent amount of oil or natural gas required to produce this amount of heat. The Flagstaff Hut installation uses an 850 gallon water tank to store heat from the Tarm boiler. Assuming the water used for storage has a temperature swing of 100 oF (80–> 180), what is the total heat stored in the tank and how long would it take the boiler to produce this much heat. Why was my shower cold on Saturday mourning? The hut uses a backup propane heater which they use in the summer. Why is the backup heater important? Would it “pay” to install this system in your own home?
The hut runs on standard 120/220 volt electrical service. The hut is not connected to electrical grid and must generate all of the electrical power using solar panels and a propane generator. The solar panels produce 5.6 kW of DC electrical power at peak performance and store this energy in twenty four, two volt, lead/acid batteries. The total capacity of the batteries is 24 kW-hr. As a good assumption, a solar system rated at 1 kW of power at full sum will produce 1000 kW-hr of total electrical energy in one year (stay tuned for this calculation).
The solar panels can not meet the entire electrical demand of the hut so a propane generator is used to supply additional power. The generator uses about 400 gallons of propane a year. Key to operation of the system is the Outback inverter/control system that can charge the batteries from AC or DC power inputs and deliver power to the hut from AC (generator) or DC outputs (battery).
A few basics on propane generators:
- It requires 2 horsepower to produce 1000 watts of energy per hour under load.
- Under load, each horsepower consumes 10,000 BTU per hour.
- Propane contains 92,000 BTU per gallon.
- Propane weighs 4.2 pounds per gallon.
The hut crew mentioned that the generator always came on when the guests were using a lot of water and suspected that the water pump was using a lot of power. Pumping water requires very little energy (work = mgh) once the pump is started. However, the starting load on the electrical system to start the pump is very large. One solution may be to install a smaller water pump. It would pump longer, but require a reduced starting load. If the Outback control system logs power usage it would be informative to look at the electrical load profile of the hut to determine if additional energy efficiency is possible.
Interesting calculations would be the total electrical energy produced by the solar and generator systems over the entire year. How does the fuel-only cost of these systems compare to residential electrical costs of 0.14 $/kWh? If an installed propane generator costs $5000 and the hut’s solar system costs $35,000 what is the simple 25 year cost of running both systems?