YOUR SEPTIC SYSTEM
Sand and Gravel Filters
All properties in Stinson Beach use some type of onsite wastewater disposal system for
the treatment and disposal of domestic wastewater. Systems range from water-tight
holding tanks to single or dual gravity leachfield systems, to systems that have an aerobic
unit (sand filter or gravel filter)
that pre-treats the wastewater prior to disposal in a leachfield.
Common features of the majority of systems are a septic tank
Wastewater flows by gravity from the various plumbing fixtures in a home into the
The septic tank provides a place to hold the waste and begin the treatment process.
Septic tanks are designed to partially digest the waste in a domestic wastewater. A typical
tank has a volume of 1000 to 1500 gallons, has a baffle separating the tank into two
chambers (inlet and outlet), has an inlet tee
and outlet tee (or effluent screen) to facilitate
the separation of liquid effluent from solids and has two manholes for inspecting and
servicing the tank. To function property, a tank must be water tight. Waste cannot drain
directly from the tank into the ground (cesspool) and groundwater must not seep in. Septic
tanks for new installations are made of a durable, non-corrosive materials such as
concrete, fiberglass or polyethylene. Some cinderblock and redwood tanks predate the
OSWMP and are replaced when they are found to have failed (leak).
For an onsite system to function effectively, the septic tank must retain the majority of the
solid material contained in wastewater. Septic tank size is generally at least three times
the daily design wastewater flow rate. This means that the average cup of water
that goes down the drain will spend three days in the septic tank before it flows or is pumped
out to the leachfields. During this three day retention time
the following happens: solid material
settles out as sludge, floatable material forms a scum layer, and in the middle a clear liquid
zone develops. This liquid zone of partially clarified sewage still contains significant
amounts of organic constituents and harmful bacteria and viruses. While in the tank, the
liquid and solid material undergo anaerobic (without oxygen) biological digestion. This
digestion process reduces the volume of sludge and scum (up to 50% of the solids are
digested), produces gases (methane, carbon dioxide, hydrogen sulfide) and effects the
metabolism of organic constituents from the clarified liquid zone. The tees, baffles and
screens are important to prevent short-circuiting and promote the retention of solids
(septage) in the tank. The relatively clear water between the scum and sludge zones is
the treated wastewater that is then disposed in the leachfields. Septic tank treatment can
remove up to 50% of the organic material and up to 30% of the nitrogen-rich compounds
from domestic wastewater.
For a septic tank system to succeed in separating solids from liquid waste, the
accumulated solids must be pumped periodically by a licensed septic tank pumper. The
frequency of pumping varies by use and practices of each household. Most full-time
residences can expect to pump their system every three to five years. Failure to pump will
result in the accumulation of sludge which will start to impinge on the volume in the septic
tank which in turn allows less settling time and therefore the effluent will carry more solids
into the leachfield. This will not cause immediate leachfield failure, but such neglect could
significantly shorten the effective life of the leachfield.
When the tank is pumped, it is important to ensure that all of the solids are removed.
(Please note, if there is a heavy sand layer in your tank, the pumper may be unable to
remove all of the sand-laden material without damaging his equipment.) If your system
has a fiberglass tank, ensure that the pumping contractor removes the septage evenly from
both compartments to minimize the risk of damage to the internal baffle. If your system
includes an effluent screen, the pumping contractor should clean the screen, inspect it for
damage and reinstall it.
Inefficient separation of solid material and liquid in a septic tank can results from: broken
or missing tees, broken baffles, excessive accumulation of solids, infrequent pumping and
hydraulic overuse of the system. Many of the
"Septic System Practices" guidelines are
aimed at ensuring maximum solids retention in the septic tank. If the solids are not
removed by the septic tank portion of the treatment process, they will be included with the
liquid stream that flows to the leachfield. Excessive solids in the leachfield clog pipes,
gravel and soil surfaces that then lead to premature failure of the leachfield portion of the
From the septic tank or pump vault, the wastewater is distributed via a
leachfield trenches that
further treat and then dispose of the wastewater. In the past, the
main objective of the leachfield was to dispose of the water --
keeping all the sewage (septic
tank effluent) underground with little regard to the possible degradation in groundwater
quality was the goal. Because of State and Federal mandates, it is now critical to consider
the ultimate fate of the sewage. This shift in emphasis to treatment has resulted in the
- More rigorous consideration of groundwater depth and movement,
- Increased scrutiny of soil texture and more rigorous percolation testing,
- The consideration of aerobic treatment afforded by the trenches has become
increasingly important, and
- The inclusion of aerobic (with oxygen) pre-treatment units that further purify the
clarified effluent prior to disposal in the leachfield trenches.
Basically all leachfields consist of perforated pipe and gravel that distributes the septic
tank effluent into the surrounding soil. The leachfields are sized based on the soil
absorptive capacity and the anticipated wastewater design flow rate. The effluent either
flows by gravity or is pumped into the pipes, flows from the pipes through perforations,
drains into the gravel and is absorbed into the surrounding soil. As opposed to the
treatment in the septic tank, the trenches rely on aerobic bacteria to decompose the
majority of the pollutants in the effluent. Again, many of the practices enumerated in the
"Septic System Practices" guidelines are aimed at ensuring aerobic, unsaturated
conditions in the leachfields which will prevent clogging of the soil.
Engineered systems installed in Stinson Beach have two complete leachfields with a
This manual or automated valve diverts the effluent to one leachfield
allowing the other to rest. As the waste is applied to the soil in a leachfield, the aerobic
bacteria utilize the oxygen in the surrounding soil. Organic compounds are taken up by
the bacteria, a biomat is formed, pathogenic bacteria and viruses are trapped and
deactivated and the oxygen in the surrounding soil is depleted. When the leachfield
diversion valve is switched, the oxygen in the resting field is replenished and the biomat
breaks down -- effecting a rejuvenation of the leachfield capacity. Some systems are
designed to be switched semi-annually; the District generally recommends a schedule that
rotates the field that is used during the wet winter months.
Even distribution of wastewater is critical for effective utilization of the leachfield capacity
and to prevent localized overloading and clogging. While gravity systems are attractive
because of their simplicity, they do not provide even distribution of the effluent. Many of
these leachfield systems are in fact 'designed' to fail serially -- one leachline clogs and
only then will the effluent flow into the next leachline, etc. until the last line clogs and the
entire system has failed. Pressure distribution systems provide for efficient, even
utilization of the entire leachfield and allow for a dosing-resting cycle so that re-
oxygenation and therefore revitalization of the leachfield can occur.
Finally, it is critical to maintain unsaturated conditions within and beneath the leachfields.
Oxygen transport is inhibited in saturated soils, therefore if the leachfield is saturated it
is more likely to turn anaerobic. Secondly, saturated conditions under the leachfield leads
to the transport of pathogens (bacteria and viruses) into the groundwater. In fact, three
feet of unsaturated soil is required between the bottom of the leachfield trench and
seasonally high groundwater for this reason.
Sand & Gravel Filters
Many regions of Stinson Beach do not contain adequate depth to groundwater and/or are
comprised of soils or sands that are too coarse, percolate too rapidly and therefore do not
adequately purify the effluent of pathogens nor adequately protect the groundwater. To
mitigate for these conditions, a sand filter
pre-treatment unit is required between the septic
tank and leachfields. Sand filters are a mixed media, self-contained aerobic bed of sand
and gravel that removes the majority of the pathogens, organic compounds and
oxygenates the effluent. To ensure the continued ability of a sand filter to pre-treat septic
tank effluent prior to disposal, the District is requiring property owners to have a contractor
ensure that all distribution laterals
are clear and free of debris and that all perforations
are open to ensure even distribution within the bed. Routine pump readings and a check
of proper pump operation will also be required.
As with leachfields, once a sand filter starts to become anaerobic, a series of reactions
hastens the process until the filter is a clogged, failed unit that is not effectively treating
the wastewater. The addition of air into the bottom of the filter appears to be a promising
repair strategy for reversing the process and re-oxygenating the units. In addition,
changing to time-dosing (instead of on-demand dosing) allows the unit to be dosed evenly
and prevents saturation of the filter. The new sand filter units are required to incorporate
both of these design changes.
If you had a brand new Mercedes in your driveway, would you drive it 100,000 miles
without changing the oil? Would you be careless about the quality of the fuel? Probably
not. Like that Mercedes, your septic system is worth a pretty penny, and it will give you
years of trouble free service if you maintain it.
Maintenance is the single most important consideration in making sure a septic system will
work well over a long period of time. Too often homeowners forget that whatever goes
down the drain or toilet ultimately either finds its way into the soil or remains in the septic
tank until it is pumped out. Use common sense and you should have few problems with
your septic system.
The threat of disease is a key problem with treating human wastewater. The epidemics
that killed millions of people in the Middle Ages were caused by mixing of human waste
with drinking water supplies. Domestic wastewater may contain bacteria and viruses that
cause dysentery, hepatitis, and typhoid fever. To protect your health, it is important to
exclude these organisms from both surface and groundwater. That is why sewage
treatment plans use chlorine and other biocides (substances destructive to many
organisms). Fortunately, soil and soil bacteria can effectively remove pathogenic
(disease-causing) micro-organisms from wastewater treated in a properly functioning
Nutrients such as nitrogen and phosphorus, contained in domestic wastewater, can cause
both health and nuisance problems if allowed to reach surface or groundwater supplies.
Nitrogen in its nitrate form poses the most significant threat to our health. When ingested
by infants, nitrate can interfere with the blood's ability to carry oxygen, causing 'blue
baby' syndrome. Nitrogen carried in septic tank wastewater is usually in the form of
ammonia. This ammonia is readily transformed into nitrate, which can easily become part
of ground and surface water supplies.