Case Study: Yosemite National Park

Case Study: Yosemite National Park

Pasteurization is a heat treatment process traditionally used to make milk safe for human consumption. By heating a substance to a specific temperature for a specific amount of time, one kills all harmful bacteria and creates a safe product.

California’s Yosemite National Park and the U.S. National Forest Service experimented by using the technique of pasteurization on the product of composting toilets. The goal of the experiment was to find whether or not pasteurizing human waste would result in pathogen free compost. By eliminating pathogens in the compost, it is possible to use human waste to aid in the growing of safe food gardens (Lachapelle, 1997).

Yosemite's Solar Pasteurizer (Lachapelle, 1997)

Yosemite sees a volume of about 70,000 users per year between two composting toilets, which is about the expected use volume in Monwabisi Park. These composting toilets create a product comparable to sewage sludge in both consistency and the number of pathogens. The sludge they tested consisted of feces, urine, and wood chips, which is similar to what will be expected from the toilet systems in the Water Centre.

The pasteurizer created for the experiment was a solar hot box with a tray inside on which to place the compost. The top was made of the plastic material used to encase greenhouses and the sides were made out of welded aluminum. The testers placed temperature probes inside the hot box to monitor the temperature of the compost heap during the pasteurization process.

For pasteurization to be considered effective, the U.S. Environmental Protection Agency (EPA) requires a minimum temperature to time relationship of 62 °C for six or more hours. During testing, the Yosemite project required their hot boxes to hold a minimum temperature of 65 °C for the six hours. They tested their hot boxes for the variables of  moisture content within the sludge, and volume of waste to see which would have an effect on the time it took to pasteurize completely.

The experiment proved that it is possible to pasteurize human compost thoroughly enough for it to be considered pathogen free by the EPA. The testers found that the lower the moisture content and the smaller the volume, the more efficient the pasteurizer. They found that the optimum amounts for pasteurization were below 40% moisture and less than 15 cm of depth in the compost pile.

The test results also showed that the system was easy to use and required very little maintenance or labor. These are both important considerations when proposing a pasteurization system for Monwabisi Park because the simpler the system, the more sustainable it will be.

When Yosemite analyzed the results of the pasteurization experiment, they noted that making the box shallower would have a positive effect on the efficiency of the hot box. They also suggested a primary drying stage before putting the compost into the pasteurizer which would reduce moisture levels and speed up the process.

The Equation

The EPA came up with a universal equation that allows for safe pasteurization of human compost:

D = (1.317*10^8) / [10^ (0.14*T)]

D is the number of days it takes to turn the sludge into grade A compost

T is the temperature in degrees C reached by the pasteurizer

Hot Box Dimensions:

• Box: 122 x 94 x 69 cm at highest point (46 cm at lowest)
• Tray: 109 x 81 x 38 cm

Materials

• 0.64 cm aluminum sheet (for outside walls and floor)
• 0.48 cm aluminum sheet (for removable tray)
• Heat Resistant flat back paint (for inside of tray)
• 122 x 94 x 0.64 cm transparent Lexan thermoclear polycarbonate sheet (for solar glazing)
• 5cm polyisocyanurate closed cell foam with black tar paper coating (to coat walls and floor… facing the inside of the box)
• 6 L-Shaped bars (to support tray above floor and secure glazing to top of hot box)
• Heat resistant silicone sealant (to seal air gaps between solar glazing and top of hot box)
• Temperature probes

Method

1. Outside walls & floor: cut and arc welded from .64 cm aluminum sheet (see dimensions)
2. Removable tray inside cut and arc welded from .48 cm aluminum sheet. (see dimensions)
3. Paint inside of tray with heat-resistant flat back paint.
4. A single 122 x 94 x 0.64 cm transparent Lexan thermoclear polycarbonate sheet was used for solar glazing.
5. Bolt glazing to the box and secure at 15 degree angle (specifically designed to maximize the angle of incidence during the summer solstice for the latitude of Yosemite (38 degrees North)).
6. Insulate inside walls and floor with 5cm polyisocyanurate closed cell foam with a black tar paper coating. R value 14.4 (The R value is a measure of the material’s resistance to heat flow)
7. 6 L-shaped bars support the tray above the floor and secure the glazing to the top of the hot box
8. Seal air gaps between the glazing and the top of the hot box with heat resistant silicone sealant.
9. Position a door at the back of the hot box in order to gain access to the tray.
10. Insert temperature probes to various sections of the compost pile through a small hole at the back of the hot box.

Return to Pasteurization

[09Gardens]