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Onsite wastewater systems are the sole means for the treatment and disposal of
wastewater generated from sites without the benefit of public sewerage.There are a number of
combinations of treatment and disposal systems which can be used for the treatment and disposal
of such wastes.
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The optimal type of treatment and disposal system is a function of both the regulatory
requirements and site conditions. Such conditions include the soil type (e.g., clay, rock, sand);
steepness of slope; depth to seasonal groundwater; setback to prominent site features (e.g.,
creeks and banks); size and shape; and whether it is already developed (e.g., a repair).
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There are several methods which can be utilized for the onsite treatment of sewage. The
choice of the treatment mode is based upon site conditions and availability of area for the treatment
system. In selecting the mode of treatment, the issue of paramount concern is the importance of
organic constituent removal. This is especially important in specifying systems for sites with lower
permeable soils (e.g., clay), and where the area available for leachfield is limited.
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Treatment Modes Selected Features
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Septic Tank
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Effluent Filter
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Least expensive / removes 40 - 60% of the organic and suspended matter
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Inexpensive / improves performance of septic tank to approximately 60 - 80% removal
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Advanced Treatment ("Pre-Treatment")
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Least Expensive / substantially smaller than a sand filter / up to 95% removal
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Most expensive / largest treatment system in size / up to 905 removal
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New technology / recirculating sand filter, some proprietary systems
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Aerobic Treatment Unit
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Sand Filter
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Nitrate Removal Systems
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There are many ways in which to properly dispose of onsite treated wastewater. In the
Greater San Francisco Bay Area, and much of Northern California, all onsite generated residential
sewage must be assimilated through some method of subsurface application. As in the mode of
treatment, the type of disposal system is predicated on site conditions.
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Disposal Systems Selected Features
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Some typical applications of treatment and disposal methodologies are presented below.
Remember, there are always exceptions.
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Site Conditions Types of Systems
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PREPARED BY: Storm C. Goranson, P.E., R.E.H.S., R.E.A.
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Former Staff Engineer - State Regional Water Quality Control Board
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Former Public Health Engineer - County Health Department
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