summary

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.

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).

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.

Least expensive / removes 40 - 60% of the organic and suspended matter

Inexpensive / improves performance of septic tank to approximately 60 - 80% removal

Least Expensive / substantially smaller than a sand filter / up to 95% removal

Most expensive / largest treatment system in size / up to 905 removal

New technology / recirculating sand filter, some proprietary systems

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.

Least Expensive / typically used with septic tanks

Perforated pipes are placed in shallow or standard trenches

Perforated pipes are placed on 12" of sand, covered w/ soil

Perforated pipes are placed on undisturbed earth and covered w/ soil

New Technology / least expensive of all drain fields / must use advanced treatment

Adequate depth of soil and depth to seasonal groundwater

High seasonal groundwater

Low permeable soils - Level site

Low permeable soils - Sloped site

Repair (with poor soils) - Larger parcel

Repair (with poor soils) - Smaller parcel

Septic tank and standard trench system

Sand- filled mound, At- grade mound with advanced treatment ,

Shallow PD system with /without advanced treatment,

Drip irrigation with aerobic treatment

Sand- filled mound, At- grade mound with advanced treatment,

Shallow PD system with /without advanced treatment,

Drip irrigation with aerobic treatment

PD system with /without advanced treatment,

At- grade mound with /without advanced treatment,

Drip irrigation with aerobic treatment

PD system with /without advanced treatment,

At- grade mound with /without advanced treatment,

Sand- filled mound, Drip irrigation with aerobic treatment

PD system with advanced treatment unit,

At- grade mound with advanced treatment,

Drip irrigation with aerobic treatment

PREPARED BY: Storm C. Goranson, P.E., R.E.H.S., R.E.A.

Former Staff Engineer - State Regional Water Quality Control Board

Former Public Health Engineer - County Health Department