Like many municipalities in the U.S., the City of Austin, Texas has for the past few decades been challenged with continuing to provide cost-effective and appropriate infrastructure and services to increasingly spread out suburban and semi-rural areas around the City. Austin is particularly challenged with this because the western half of the City is characterized by very rocky hills, with large portions of the City’s service areas overlying the Edwards Aquifer. Sensitive watersheds are scattered throughout the area, with the Lower Colorado River running through the middle of downtown Austin. The City has implemented a variety of land use protection measures and purchased conservation lands or development rights to help prevent adverse impacts to water quality of the Edwards Aquifer and surface waters in the area. The use of conventional gravity wastewater collection systems to the west of the City usually necessitates cutting through massive limestone rock and/or using multiple lift stations to convey sewage to one of the City’s municipal wastewater treatment facilities. Further, it was recognized by city planners and politicians that providing conventional wastewater service tends to lead to intense development in areas where there has been an interest in preserving water quality and the natural hill country setting.
In 1994, the City of Austin contracted with CES to assist the City’s Water & Wastewater Utility with the identification and evaluation of wastewater options that would be appropriate for use where traditional options were less feasible. The project work was carried out in several phases, due to its comprehensive nature and the need to coordinate the work with other on-going City planning efforts.
As a part of that work, research was conducted on the performance of existing “basic”/standard types of system operating in the rocky hill country conditions. Those systems consisted of only septic tank pretreatment followed by either gravity or low pressure subsurface dispersal.
Following that work, and based on those findings, several demonstration projects were designed, constructed and monitored to study and verify the performance of alternative systems that were thought to be most appropriate for addressing performance shortfalls observed for existing conventional systems in those rocky/hilly conditions. The four demonstration projects included a residential subsurface wetland using sand-lined low-pressure dosed trenches; a subsurface wetland in tandem with a Bioclere trickling filter unit, with recycle constructed at a City treatment facility; and two residential intermittent sand filters followed by low pressure dosing of the treated effluent.
Monitoring of Existing “Conventional” Onsite Wastewater Systems
Cost-effective decentralized wastewater service depends on using the land’s natural treatment capabilities to an optimal extent. For onsite wastewater treatment and disposal systems (OSSF’s) located on sites where the land cannot provide sufficient treatment, added treatment must be provided beyond primary treatment in septic tanks prior to final subsurface disposal.
Some land types are able to provide an excellent quality of effluent in just a few feet of soil with a properly designed and installed OSSF. A large amount of data has been collected on the performance of both large and small scale land treatment systems for soils such as sandy loams or clay loams which are relatively common, and have long been considered very suitable for the land application of wastewater. Other land types typical of hill country areas west of Austin are less common across the U.S., and very little data was previously available for the performance of OSSF’s in the rocky limestone conditions found in this area. Monitoring was therefore performed to better evaluate suitable options for OSSF’s in those land types.
The purpose of this monitoring effort was to determine what if any additional pretreatment of septic tank effluent is needed prior to subsurface disposal in specific geographic conditions common to western Travis County. Monitoring was conducted at five residences served by OSSF’s in western Travis County to evaluate the suitability of those systems for the limestone conditions at those sites. The five systems monitored all used septic tank pretreatment followed by either low pressure dosing or conventional gravity flow disposal.