Every day, Austinites flush nearly 70 million gallons of water down the toilet. After passing through a maze of pressurized and gravity-fed sewer pipes, and one of three sewage treatment plants, the water re-emerges five miles downstream of Congress Avenue Bridge in a frothy, pungent stream.
But disposing of human waste doesn't necessarily require huge amounts of water or complex systems that transport the waste miles away. Recent innovations have improved the process of decomposing and dispersing effluent. Some of the new on-site systems even recycle wastewater by distributing it to trees and landscape gardens. Green builders often prefer to manage wastewater on-site because they are freed of the requirements and costs of a centralized sewer system.
To work efficiently, on-site systems depend on a complex series of factors, including soil depth, bacterial richness, slope, vegetation and development density. In the Austin area, septic tanks are by far the most common on-site system. They collect solids while dispersing effluent into the nearby soil. Septic tanks generally work well in areas with sufficient soil. The soil disperses, absorbs and decomposes the pollutants in the effluent. However, in the thin, rocky soils of the Hill Country, the wholesale implementation of conventional septic systems is a questionable practice. "The thin, caliche-type soils in the Edwards Aquifer region are generally low in organic matter." says Susan Parten of Community Environmental Services, Inc. "This can pose a problem over a subsurface of fractured or karstic [cave forming] limestone because adequate treatment might not be achieved before the effluent reaches underlying aquifers or nearby streams."
Problems with septic systems have drawn increased attention to alternative on-site systems. For example, the "aerobic" class of on-site systems introduces oxygen into the tank to begin the decomposition process. "Our systems work with basic parts and principals," says Michael Kelly of Clearwater Systems, an Austin company specializing in aerobic wastewater disposal. Kelly's system pumps compressed air bubbles through the effluent before it moves to two adjacent tanks for disinfection and storage. "Rather than a true alternative, our tanks are more like a mini-wastewater treatment plant. Human waste is already loaded with all the bacteria needed to break itself down. The only thing we have to add is oxygen," Kelly said, "lots of it." These aerobic systems have been in use for many years and can be permitted within the city limits.
Landscape architect Lucia Athens, of Earth Sky Designs, is working on new ways to disperse treated effluent using a combination of soil absorption, evaporation and plant transpiration. "One of the designs I'm currently working on directs wastewater toward a small orchard and some shade vines that we've incorporated into the landscape. An added rainwater collection tank helps drive the system, offering the owner increased independence and efficiency," says Athens.
However, localized sewage disposal can also present problems. Assessing the responsibility for pollution caused by on-site systems, as well as servicing the systems, remains a sticky question. These systems require homeowners to adopt a minor maintenance schedule, including changing filters and/or adding occasional activants. On-site systems may cost more than centralized sewer service. A septic system or aerobic on-site treatment system in the Austin area can cost up to $15,000. A composting toilet can cost up to $5,000. In addition, these on-site systems may complicate the task of assessing the resale value of a home.
The city's Environmental and Conservation Services Department (ECSD) is currently reviewing a range of on-site systems appropriate for residential use over the Edwards Aquifer. ECSD has also commissioned a study by the local engineering firm Loomis & Associates to obtain more comprehensive data on all potential sources of pollution in the Barton Creek watershed. The city hopes to determine what degree of pollution now appearing in the Barton Springs segment of the Edwards Aquifer stems from septic systems, as compared to private treatment plants or urban and ranchland stormwater runoff. This effort is intended to allow the most appropriate, cost-effective controls to be used in the problem areas.
Sherman Hart of the Texas Natural Resources Conservation Commission, says the new breed of on-site wastewater systems are attracting interest from many different areas. He points out that the Lower Colorado River Authority has developed a wastewater system that utilizes a system of canna lilies and horsetails to absorb and treat effluent. This type of "constructed wetland" may be applicable in agricultural areas along the border and in colonias that currently have no wastewater treatment facilities.