Protection of Groundwater Below On-site Systems – Determination of Pathogen Removal Efficiencies by Soil Absorption Systems

Lead Researcher: Doug Joy –
Partners: MOE Best in Science Research Grant
Soil absorption systems have long been used as an effective means of wastewater treatment and disposal in rural areas that do no have access to municipal sewers. By the nature of their design, soil absorption systems return wastewater to the groundwater. Soil absorption systems are usually preceded by some form of treatment which can range from a simple septic tank to more sophisticated treatment units. Almost no on-site systems utilize any form of disinfection prior to discharge to the soil absorption system.

It has been generally accepted that the soil absorption systems play an important role in removing bacteria and viruses to acceptable levels prior to entering the groundwater. In order to ensure that there is enough soil available to remove the bacteria before it is discharged to the groundwater, minimum vertical separation distances were introduced. These standards have remained unchanged for the past 30 years and are generally set to a minimum distance from the bottom of a soil absorption system to the seasonally high groundwater table is 0.9 metres for nearly all systems.

The effectiveness of soil absorption systems in removing bacteria depends on a number of factors including soil type, distance, initial concentration, loading rate and other, highly variable, factors. Current design standards lead to highly variable factors of safety since in many cases improved technologies are not recognized (e.g. treatment units) or excess loading rates are not accounted for (e.g. smaller area for chamber systems). In addition, the combinations of effects such as higher loading rates and improved technologies are largely unknown and unaccounted for. The following two objective questions have been developed for this project.

1. Are the separation distances currently legislated below SAS’s sufficient to reduce bacterial levels to acceptable levels prior to re-entry into the groundwater?

2. What levels of bacteria are being recorded at present at the existing minimum separation distances?

In order to investigate the effective of soil absorption systems in removing bacteria an extensive field investigation will be conducted. Wastewater samples will be collected from a variety of soil absorption systems (SAS) using pan lysimeters. Four pan lysimeters will be installed per SAS. The pan lysimeters will be placed at a depth of 0.9 metres below the bottom of the point of discharge. For those systems with reduced vertical separation requirements (certain area bed systems) the lysimeters will be placed at the regulation depth. For ease of installation, the investigation will be conducted only on new systems being installed at the time of project commencement. Five of each of the following types of systems will be investigated: conventional trench, filter bed, Shallow Buried Trench (SBT), and area beds.