Tank and Disposal Field Systems
> Risks and Impacts > Best
tank is a chamber used for the retention and partial treatment
of domestic wastewater. Effluent from the tank is discharged to
the land through perforated pipes beneath the ground surface.
A septic tank can be used wherever soil characteristics are suitable
and where there is sufficient soil depth above the ground water
table. The population being served must be sufficiently sparse
to allow adequate area for the distribution field required. A
septic tank is not normally used to serve a population exceeding
150 people due to the disposal field considerations (MELP, 1978).
themselves do not achieve a high degree of treatment. Their primary
purpose is to condition sewage so that it will cause less clogging
of the disposal field. Conditioning of the sewage is accomplished
by floating or settling out some of the suspended solids which
in turn anaerobically decompose in the tank (MELP [now WLAP],
1978). Solids and partially decomposed sludge settle to the bottom
of the tank and accumulate. Lightweight materials, such as grease
and scum, rise to the top. The partially clarified liquid flows
through an outlet structure just below the floating scum layer
to a disposal system (e.g., disposal field). Baffles (or tees)
at the inlet and outlet ensure proper flow movement. In addition,
baffles at the outlet prevent the scum from flowing out of the
tank to the disposal field (US EPA, 1980).
onsite sewage system (adapted from the Soap
and Detergent Association)
A single compartment septic tank will provide the minimum acceptable
treatment to household wastewater, whereas multiple compartment
tanks or two single tanks in series will perform better. Multiple
compartments will improve biochemical oxygen demand (BOD) and
total suspended solids (TSS) removal, as additional compartments
provide better protection against the carry-over of solids into
the discharge pipes (US EPA, 1980).
are sized according to the expected flow from the household with
sufficient volume for between 24 and 48 hours retention time.
Generally speaking, the longer the retention time or the bigger
the tank, the better. Typical recommended septic tank capacities
- 1 or 2
bedrooms — 800 gallons (3.6 m3)
- 3 bedrooms
— 1,000 gallons (4.6 m3)
- 4 bedrooms
— 1,200 gallons (5.5 m3)
- 5 bedrooms
— 1,400 gallons (6.4 m3)
- 6 bedrooms
— 1,600 gallons (7.3 m3)
As the sludge
depth in the tank increases, the tank's capacity and detention
time decreases, thereby decreasing the efficiency of the treatment.
Typically, pumping out of tanks is required approximately every
three to five years, however, inspections can determine the rate
of sludge and scum accumulation. Septic tanks have no mechanical
parts, yet routine inspections will determine if any repairs or
maintenance to the system is necessary (US EPA, 1980). The sludge
and scum must be handled and disposed of in a manner which will
protect public health and the environment. Disposal of these materials
should be undertaken by qualified individuals who are familiar
with the necessary precautions to prevent ground water contamination,
odors and aesthetic and health problems (MELP, 1978).
of the septic tank effluent is provided through sub-surface disposal
to land. Varying degrees of removal of pathogens, nutrients and
solids are achieved as the effluent percolates through the soils
of the disposal field. The actual degree of treatment obtained
is a function of the climate, the nature of the vegetation and
the amount of effluent uptake by the plants, the chemical, physical
and biological characteristics of the soil, and the depth of areated
soil above a water table (MELP [now WLAP], 1978). Treatment is
also influenced by the actual flow rate compared to the design
flow rate. The size of the disposal field is designed in proportion
to the wastewater flow anticipated and the characteristics of
the soil. Trench depth is determined from soil type, the position
of the water table and climate. A thorough site evaluation should
be conducted before the septic system is installed. Septic system
failures are often caused by poorly sited drainfields (NSFC, 1995a).
require the use of a distribution system to ensure an evenly distributed
flow from the tank to the disposal field. Uneven distribution
can overload certain areas of the disposal field, causing it to
fail. The most common types of distribution methods include:
Box — a tank-like box with as many outlets as
pipes in the disposal field. The effluent flows into the box
and through the different outlets to the disposal field. It
is important that the distribution box be level because this
system relies on gravity to ensure even flow distribution to
all outlets. Distribution boxes are easy to inspect and some
outlets can be blocked to give certain trenches a rest.
Box — a simple tank-like box used for distributing
wastewater to disposal fields on sloped sites using only gravity.
Inside the drop box, the pipe inlet is higher than the outlets,
allowing the wastewater to flow downward to the disposal field
trenches. A series of drop boxes can be arranged on a sloped
disposal field so that after the highest disposal field is saturated,
the flow of wastewater continues to the next drop box and trench
below. Drop box outlets can also be capped to control the direction
of flow and give the saturated upper trenches a rest.
and Pumps — are used to distribute wastewater in systems
where soil and site conditions do not allow distribution by
gravity. Siphons are used in systems that require even distribution
over a large area (i.e. more than 150 m of pipe). The effluent
flows from a dosing tank, to the siphon, then to the disposal
field in pressurized doses, making uniform distribution easier
to achieve. Siphons work using only air, water pressure and
pumps also deliver controlled amounts or doses of effluent to
the disposal field. Dosing can improve the performance of any
disposal field by producing more uniform distribution, but it
is especially advantageous for disposal fields with shallow or
poor soil conditions. However, electric pumps are more expensive
to operate than other distribution systems, require regular maintenance
(NSFC, 1995b), and are subject to failure with power outages.
septic systems are more likely to fail than systems which are
inspected regularly and pumped out as required. Sludge and scum
can plug the tile field causing system failure, which typically
results in ponding of effluent above the field. Failing septic
systems are expensive to repair or replace and poor maintenance
is a common cause of early system failures. The cost of maintenance
is very little in comparison to repair or replacement. For example,
replacing a system can cost as much as $15,000 or more while inspections
can cost as little as $75 and pump-outs as low as $200 (NSFC,
systems fail, inadequately treated household wastewater is released
into the environment. Any contact with untreated human waste can
pose significant health risks and untreated wastewater from failing
septic systems can contaminate nearby wells, ground water and drinking
water sources (NSFC, 1995a).
disposed of in a septic system can also pollute local water sources
and contribute to system failures. It is important for homeowners
to educate themselves on what should and should not be disposed
of through a septic system (NSFC, 1995a). Septic tank additives
or "starters" should never be used; they are unnecessary,
expensive and may cause pollution.
maintained septic systems can impact the economic health of the
community. Failed septic systems can reduce property values and
contribute to the pollution of local waterbodies used for commercial
and recreational activities (NSFC, 1995a).
signs can be an indication of a failing system:
draining sinks and toilets;
sounds in the plumbing;
odours in the house or yard;
- wet or
mushy ground in the disposal field;
growing faster or greener in one particular area of the yard;
showing the presence of bacteria in nearby wells.
None of these
warning signs can be considered a sure indication that a system
has failed, but the appearance of one or more should prompt homeowners
to have their system inspected. Septic system failures can also
occur without any of these warning signs. For this reason, a yearly
inspection of systems is recommended (NSFC, 1995a).
BMPs are designed for homeowners and will help ensure that their
systems function properly and maximize the lifetime of the system:
- Make sure
that your system meets legal requirements before installing,
repairing or upgrading an onsite sewage system. In BC, the Ministry
of Health Planning is responsible for septic systems and installation
is permitted under the Sewage Disposal Regulation of
the Health Act. Contact your local public health authority for
permits for repairs, improvements, installations and further
a map of your septic system showing the location of all components
and keep it with your maintenance and repair records. This will
make maintenance easier and be useful to future owners.
- Keep your
septic tank cover accessible for inspections and pumpings. Install
risers if necessary.
- Have your
system inspected annually to ensure that it is working properly
and to determine when it should be pumped out. By inspecting
and pumping your system regularly you can prevent high repair
or replacement costs. A professional can do a thorough inspection
of the entire system including the disposal field and individual
components of the system.
- Pump out
the tank regularly to prevent accumulating solids and clogging
the disposal field. The frequency of pump outs will depend on
the size of your system, the number of people in the house and
the habits of those individuals. A general rule is once every
three to five years.
your system when you upgrade your home (i.e. when you add a
bedroom or suite).
water conservation. Continual saturation of the disposal field
can affect the quality of the soil and its ability to naturally
remove contaminants. The following points will help you to use
any leaking faucets or running toilets;
dishwashers and washing machines only when full;
letting the water run when washing or brushing your teeth;
taking long showers and install water-saving features in
faucets and shower heads;
low-flush toilets or put a toilet dam (e.g., brick) in the
tank to reduce the capacity of the tank;
out activities requiring heavy water use, like laundry,
over several days;
roof drains, surface water and sump pumps away from the disposal
field. Don't saturate your disposal field with automatic sprinklers.
using garburators — this will reduce the amount of solids
and grease you put into the system.
use toilets as trash cans — excess solids can clog your
drainfield which will cost you money for more frequent pumping.
Items that should not be flushed include:
grease or oil
towels and facial tissue
put toxic chemicals (paints, varnishes, thinners, waste oils,
photographic solutions, or pesticides) down the drain because
they can kill the bacteria at work in your system and can contaminate
- Use biodegradable
household cleaners instead of bleach or other hazardous products
(which will kill the good bacteria in your system).
- Do not
use toilet cleaners that are placed in the tank.
flushing pet wastes into the system — bury them
away from the drainfield instead.
drive, pave or put heavy objects or machinery over the septic
system and disposal field. Don't cover the disposal field with
a hard surface such as concrete or asphalt since evaporation
will be prevented. This area should only have a grass cover
which will prevent erosion and help remove excess water.
plant trees or shrubs near the drainfield because their roots
can damage or plug the pipes. Plant grass instead.
use septic tank ‘starters’, additives or similar
products. These products usually do not help and can sometimes
harm your system. Allow bacteria to act on their own.
- Use low-phosphate
or phosphate free detergents.
go down into a septic tank. Toxic gases are produced by the
natural treatment processes in septic tanks and can kill in
minutes. Extreme care should be taken when inspecting a septic
tank, even when just looking in.
information, contact the Ministry of Health Planning's
Health Protection Branch. General information is available
from the National
Small Flows Clearinghouse.