Establishing Failure Indicators for Conventional On-site Wastewater Treatment Systems

Abstract

Conventional On-site Wastewater Treatment Systems (COWTS) are systems used for the treatment of domestic wastewater. These systems comprise of a septic tank that provides primary and secondary treatment in which solids are settled and broken down by biological processes, and a soil absorption trench or field that provides advance treatment for the discharge of effluent, mainly through filtration and adsorption. These systems are used primarily in regions where there is no reticulated wastewater disposal; however, significant increases in population, and poor design and management of these facilities have led to a large number of failing systems throughout the world. Owing to the constituents present in wastewaters and discharged effluent, failure of these systems is a public and environmental concern, as they have the potential to contaminate both surface water and groundwater resources, primarily through the release of pathogenic microorganism and nutrients. This thesis identifies modes of failure for COWTS and establishes indicators that can signal irregularities in their performance before complete failure occurs. It also demonstrates how some parameters can intensify failure.

Design, technical, management and compliance are presented as the four categories of failure modes, and these are further divided into several sub–categories. The ratio of occupancy size, to septic tank volume, and the frequency of use contribute significantly to a system failure during the primary stages of treatment, while poor siting, user inexperience and soil properties within the drainage area largely contribute to failure during the secondary treatment stage. Parameters such as the proximity to a school, surface waterways and nearby dwellings are used to show how failures can be intensified.

A methodology in the form of a monitoring model has proven to be very useful in increasing user awareness of a system’s performance, and can aid in preventing complete failure. Success with this methodology came by combining the failure indicators and intensifying parameters to generate a numerical risk score. This score is compared with examples of the likely occurrences of failures at that particular score. Darfield Township on the South Island of New Zealand is used as the case-study area to demonstrate how the monitoring methodology developed can be applied.