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.
|
