The Sol-Char Toilet uses concentrated solar energy to transform feces and urine into disinfected, commercially viable end products (e.g. solid fuel, heat, and fertilizer). Within the Sol-Char system, parabolic dishes concentrate sunlight; fiber optic bundles then deliver that light energy to heat the reaction chamber which transforms the fecal material into char. The char can be used as a solid fuel or soil amendment. Urine, once it has been thermally treated by the system for safe handling, can be used as a nitrogen rich fertilizer. The Sol-Char process can be designed for any number of users by scaling the solar power input accordingly.
Here are a few of the questions that we have received about the Sol-Char Toilet. If you have a question please submit it to:email@example.com
Our first prototype is designed for a family of four. However, the Sol-Char Toilet can be designed to accommodate any number of users with the solar input scaled accordingly. Economic and marketing factors will determine the practical operational scale.
Drying waste is the most energy intensive part of the Sol-Char waste treatment process. By diverting urine from the process up front, less energy is needed to disinfect, dry, and char solid fecal waste. Urine can be disinfected by passive solar heat treatment and later reused as a nitrogen rich fertilizer.
Energy from the sun is abundant, readily available and clean. Concentrated solar power technology uses mirrors with tracking systems to focus the sun’s energy onto a small area. By using a concentrating solar system, we capture this energy in a form that achieves pyrolysis temperatures for waste treatment.
Parabolic dishes focus the sunlight to a small focal point. This concentrated light is then transmitted to the reactor via flexible fiber optic cables. The use of fiber optics is a novel application that greatly simplifies our system compared to other solar concentrators. The flexible fiber optic cables allow the reactor to remain stationary and separate from the tracking components of the parabolic dish system, greatly simplifying the operations compared to other solar concentrator systems (e.g., solar furnaces).
The Sol-Char Phase I prototype is constructed with industrial-grade materials: aluminum for the metal and optical components, stainless steel for the reactor components, and industrial steel and plastic for the user compartment. However, a functional prototype could be constructed more economically. The fiber optic cables are the most expensive component. The team has explored outsourcing the production of these cables to India or China. We are working with companies interested in developing this market.
The majority of materials could be constructed in a local machine shop using more affordable materials such as concrete, spun aluminum, and injection-molded plastic. For instance, the carousel drive that rotates the reactors could be constructed out of recycled bicycle parts and scrap metal.
Design of innovative and affordable sanitation technologies is required to address the sanitation challenge. The Sol-Char toilet is one such technology in that it is a low cost, waterless, off-grid toilet that rapidly converts human waste to valuable end products.
Toilet Operation and Maintenance
No. The system is self-contained and the treated waste is rendered safe. Water is only required for hand washing.
The system is self-contained and off the grid. The necessary electricity to power any automated components such as the solar tracker or the automated flush mechanism is provided by a photovoltaic panel mounted on the user enclosure.
Any solar concentrator system needs a relatively clear sky to operate efficiently. Pollution and very cloudy weather will negatively impact its performance. These systems need to be located in areas with high levels of direct solar irradiance. Dirt accumulated on the reflective surfaces will slightly degrade the solar transmission, but periodic cleaning can restore the original performance.
As with any other toilet, the user compartment surfaces can be scrubbed with soap and water. The feces treatment containers are self-cleaned during the pyrolysis process via high heat.
Most odor escapes in the exhaust during the waste drying phase in the form of sulfide containing compounds. Odor treatment options have been tested and a combination of water vapor condensation and char adsorption eliminates the fecal odor. Biosorption is also being investigated. One benefit of pyrolysis is that the end product, char, no longer has a fecal odor since the volatile odor causing compounds are driven off due to the high temperatures achieved in the reactor.
The exhaust contains a mixture of carbon monoxide, carbon dioxide, volatile organic compounds, sulfides, and hydrocarbons. A combination of dilution and adsorption is used to minimize human exposure to these compounds. The exhaust pipe is over 2.5 meters above the ground to facilitate dilution and diffusion. The exhaust passes through an adsorption filter before exiting the Sol-Char system, removing harmful compounds.
Simple mechanical systems are used to move the waste between collection, treatment, and storage. The mechanical system can be built from easily machined and locally sourced materials, making the system easy to operate and repair. The system can be run automatically by the motors and software, or it can be run with a hand crank.
If the automated flush mechanism fails, manual hand cranks can be used to move the components between collection mode and treatment mode. When engaging the hand cranks, the user is not in contact with fecal matter.
Solar components should be cleaned whenever there is an accumulation of dirt or dust. Each mechanical component will have a specific recommended maintenance schedule that will vary depending on the component. As a rule of thumb, the solar surfaces will need to be cleaned on a monthly basis, mechanical components will need to be greased and cleaned on a yearly basis, and worn components such as bearings, pumps and valves will need to be examined and potentially replaced after some years. However, depending on the location, cleaning and maintenance schedules may need to be adjusted.
Theft is a concern and will be considered when designing the layout for a field system. Design options might include moving the solar components to the roof or insuring that sensitive components are encompassed in a “black box” type structure. Additionally, at the household shared, public shared or centralized treatment plant scale, an attendant would be on site to monitor the equipment and prevent theft. This person would be in charge of maintenance and cleaning as well.
In order to ensure safety, all moving parts are shrouded. Fire hazards from the concentrated solar light are mitigated with the system design which ensures that light exits fibers and diverges within 8”. Precaution is only necessary within the 8” range.
Typical waste we collected is 100 to 200 g. In developing countries it is expected to be higher because of vegetarian diet (usually between 300 and 400 g). In experiments conducted on the toilet, waste reduction was from 85 to 92% from the original mass (85% corresponds to pyrolyzing it at 300 degree C and 92% for pyrolysis at 750 degree C).
The carbon-enriched solid product of thermal biomass decomposition consisting largely of condensed aromatic (graphitic) zones that when applied as a soil amendment (1) imparts agronomic benefits and (2) is recalcitrant over a long timescale.
During the heating up and charring of the waste the majority of gases produced are water vapor and carbon dioxide. There will be some other trace gases which we are further evaluating for odor issues or possibly combustible gases.
When char is mixed and applied in an agricultural setting it increases the soil organic matter, water retention, nutrient retention. Often the pH can be improved with the application of biochar and cation exchange capacity has been shown to increase over time.
Disinfection of pathogens in urine will occur using passive solar heating to a target of 70° C for a minimum of 30 minutes. Urine contains nitrogen and phosphorus, both of which are valuable as a liquid fertilizer. The urine will be minimally diluted based on the local salt content in the soils in order to make it an effective fertilizer. Another option is the adsorption of treated urine into the fecal char, making the fertilizer easier to transport, and enhancing the value of the char for agricultural purposes.
Fecal char has a high energy content that is comparable to wood charcoal and bituminous coal. Briquettes that have been made out of char have almost the same amount of energy as a commercial charcoal briquette. Testing of how well fecal char briquettes perform in cooking and water boiling situations compared to other solid fuels is ongoing.
Fecal char is chemically identical to other charcoal, consisting of carbon and a few trace minerals. When fecal material is transformed into a char via high temperatures, the pathogens are killed, rendering it completely safe. Public health issues and environmental hazards due to handling or exposure of fecal material are eliminated. In addition, testing is currently underway to ensure that emissions produced from cooking with fecal char are comparable to or better than that of other types of charcoal. The same precautions should be taken when cooking with fecal char as would be taken with any type of solid fuel.
Instead of feces being left in the environment untreated, the char is a pathogen free and useful product. It can supplement the use of environmentally costly fuels such as wood and wood charcoal, thereby helping to decrease deforestation and other negative environmental effects. When used as a soil amendment, it decreases the need for synthetic fertilizers.
The cost of the Sol-Char unit will depend on the size, number of users, and desired end product. Using the Sol-char toilet for disinfection, but not for charring can be significantly cheaper, but does not offer as much business opportunity.
Low Temp. System
|High Temp. System (Charring)|
|Number of users per day||
Estimated Capital Costs
|System life expectancy||20 years||20 years|
|Estimated daily operation cost per user||
1 L of treated urine/user/day
Up to 80 g char/person/day and 1 L of treated urine/user/day
$0.05 to $0.09/user/day from usage fees and end products revenue
$0.05 to $0.09/user/day from usage fees and end products revenue
The Sol-Char toilet can be made more affordable by simplifying the structure, employing different manufacturing methods, scaling up production, automating fiber optic cable assembly, using more affordable and locally sourced materials, and outsourcing high-tech components. The fiber optic cables are the most expensive component. Production in countries such as India or China has been suggested by research advisors as a way to significantly reduce costs.