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The Vault Evaporator Toilet System: An Innovative Alternative for Waste Disposal

Biological Mediation Systems' technology for toilets was profiled in the national publication of the USDA Forest Service, Engineering Field Notes. The article won an award for author Marina Connors as one of the top three articles in 2001.

By Marina Connors, Civil Engineer

Photo: toilet building
The vault evaporator toilet building

Due to rapid development in many states, the cost of disposing of vault waste is increasing significantly. In addition, as wastewater treatment plants are nearing capacity, finding locations that will accept vault waste is becoming more difficult.

One solution to these problems is to construct composting toilets instead of vault toilets. Composting systems, however, require significantly more maintenance and a building with a basement. Biological Mediation Systems' patented Vault Evaporator is an innovative alternative for waste disposal that avoids many of the drawbacks of both standard vault toilets and composting toilets.

This system uses mechanical ventilation to effectively control odors and substantially reduce waste volume and, therefore, pumping frequency. In addition, one vault with a vault evaporator system can accommodate up to eight toilets, drinking fountains and sinks. The Colorado Department of Transportation at highway rest areas and the National Park Service have successfully used Vault Evaporators for several years.

Project Description

Fish Creek Falls Recreation Area is a very popular year-round day use site in Steamboat Springs, Colorado on the Routt National Forest. The site has an existing vault toilet that is inaccessible and undersized. Development at this site included designing and building an accessible toilet to handle 750 visitors per day on weekends and 300 visitors per day on weekdays during high use season.

photo of vault under toilets
Constructing the vault for the vault evaporator system

Potential Waste Disposal Solutions

The alternatives evaluated for this site included standard vault toilets, a vault toilet with a vault evaporator (BMS Vault Evaporator), composting toilets (Clivus Multrum) and evaporation/dehydration toilets (BMS Devap2000 system and Bio-Sun System). Cost, the visual impact of the building with or without solar arrays, fitting the building to the site topography, and minimizing maintenance were factors weighed in the selection of the preferred alternative.

The composting system was immediately eliminated as a viable alternative because the visitation patterns would require many composting units. Vault toilets using SST design principles would have required two separate buildings some distance apart, which could not be accommodated on the site.

A life cycle cost comparison was done on feasible alternatives that included a comparison of waste systems and grid vs. solar power. Capital construction costs included the waste system, the building cost, earthwork and site work. Annual costs included replacement parts, maintenance and pumping the vault.

Vault Evaporator: the Most Advantageous Alternative

The preferred alternative was the vault evaporator using grid power. This system was the most effective waste reduction system with the least amount of maintenance. It further met the needs of the project in that it permitted all the toilets to be located in a single building that did not require a full basement and could therefore be sited more conveniently for maintenance and visitor access.

As a bonus, the building could be customized to be consistent with other structures and the site and the architectural guidelines in the Forest Service's Built Environment Image Guide. It fell in the middle of the total present worth cost range, but was clearly the most advantageous solution for the site.

System Description

The vault evaporator waste system has a three-foot deep concrete vault beneath the building floor slab and an 18-inch diameter pipe around the inside perimeter of the vault. The pipe is slotted in one section to allow liquids to enter and has two vents fitted with fans that force air down the toilet risers and through the pipe. The pipe provides a large surface area for the liquids to evaporate, reducing the amount of waste that collects in the vault. The evaporation process is estimated to reduce the volume of vault waste by two-thirds.

Although the forced air system is designed for evaporation, an added benefit is the waste remains aerobic, which further eliminates odor problems. The toilet building built at Fish Creek Falls Recreation Area is a 234 square foot rustic building containing two women's toilets, one men's toilet, one urinal and a hose bibb for cleaning. It has one 4,000-gallon vault under the building. The two vent fans connected to the evaporator pipe each blow 640 cubic feet of air per minute.

It is estimated that the vault will have to be pumped only once each year, compared to three times per year for two double standard vault toilets. The vault system can be purchased by itself from BMS or BMS can build a custom building on top of the vault. The Fish Creek Falls toilet building has a rustic theme with a log exterior and a three-foot stone wainscot. It was constructed by BMS at their plant with input from the Forest Service district and regional offices. The total cost of the building was $88,716 delivered to the site from Ft. Collins, Colorado. This cost did not include the stone wainscot or site work. The building was completed in September 2001.

Feedback thus far has been very positive. After one season of operation the operator has said "The system is pretty much maintenance free. A biological agent is added down each toilet chute once per week. The best part is there is no odor." The vault did not need pumping at all the first season. The arguments in favor of the vault evaporator system have proven to be so compelling that construction of several others in this region is underway.

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