Articles - SEPTIC SYSTEM FUNDAMENTALS

The disposal of sewage in Malibu is primarily through the use of private septic systems commonly referred to as on-site wastewater treatment systems (OWTS), most of which serve single-family residences. A number of so-called "package" treatment plants are used for high-density facilities such as mobile home parks or tracts, but the disposal of the treated effluent is a problem common to all such systems. The manner in which OWTS function is basic, but somewhat different concerns apply to existing systems than to any now proposed. The state of affairs regarding the controlling standards is somewhat fluid (no pun intended) and for the most current information, specialists and City officials should be consulted.

SEPTIC SYSTEM OPERATION

The operation of a septic system depends on two independent processes. One is the manner in which the septic tank functions; the other is the manner in which the spreading facility functions.

Design and Function of the Septic Tank

The purpose of the septic tank is to retain waste water to allow for anaerobic decomposition, the end products of which are scum and sludge. In some tanks this digestion occurs in a single chamber having a design volume, commonly 750-, 1000- or 1,500-gallons where anaerobic digestion takes place. According to common code standards, the volume is established by the number of bedrooms of the house the system is to serve, or the number of household waste water outlets which are assigned various flow rates, whichever is greater.

More advanced designs have two or more chambers with those upstream larger than those downstream. Digestion in the tank results in "scum," a somewhat foamy mass that floats, and "sludge," a mass of solid particles that settles to the tank bottom. In multiple-chamber tanks, the chambers are separated by a partition which extends up from the bottom and down from the top to an opening into the adjacent chamber at about mid depth. This arrangement prevents the scum and sludge from entering the downstream chamber. In all septic tanks the inlet and outlet elevations are the same. Some tanks are fitted with a baffle in the upstream chamber suspended from the top of the tank so as to block direct flow from the inlet to the partition opening into the downstream chamber. The purpose of this is to impede the flow thus reducing its ability to carry suspended solids and hence increase the rate of sludge settlement.

It is estimated that as much as a third of residential waste water nationally is disposed by septic systems. However, relatively few understand how the system functions. Imhoff (1989, pp. 211 - 213), although referring generally to industrial treatment facilities, describes the fundamental process of anaerobic digestion that occurs in the ordinary septic tank through acid and methane fermentation. His comments regarding overloading are particularly instructive (ibid., p. 212):

Although the methane fermentation sets in spontaneously, there may be conditions when acid fermentation cannot be overcome. As shown in section 7.6, both acid and methane fermentations proceed simultaneously., however, in a broken-in digester the acids that are prodicued are rapidly decomposed. When the second reaction becomes slower, acid fermentaion may prevail. This happens when the solids load is too large or when the temperature inside the digester drops. When during methane fermentation, the methane bacteria cannot keep pace with the produced acids, acids build up, the pH drops, the carbon dioxide content in the gas will increase, and the digester may fail. The remedy is to reduce the solids load and/or to raise the temperature.

The relevance for the typical septic system in Malibu is clear. Don't overload. To avoid overloading, the use of the automatic garbage disposal, beloved by housewives everywhere, is to be avoided. It is further clear that the efficiency of the septic system is increased either by fewer persons, or longer detention time. This suggests that a larger septic tank than the Health Code requires is an good idea.

Spreading Facility

The purpose of the spreading facility is to transfer the effluent from the septic tank, ideally free of sludge and scum, to the subsurface. To some, this is arbitrarily bad, because it sounds like pollution. In fact, the only type of problem where this might arise in that where insufficiently treated effluent carries pathogens to an aquifer or to a body of water used for recreational purposes involving bodily contact. However, as Imhof notes (ibid., p. 275):

The treatment efficiency of the septic tank alone is very low and most of the BOD and bacteria removal occurs in the soil.

This fact emphasizes the importance of the spreading facility. Assembly Bill 885 proposes to monitor septic systems throughout California. The purpose of this is to determine whether effluent that passes through the spreading facility results in pollution. The foregoing provides a basis for considering what is required when either deciding to purchase a property or attempting to determine what septic system design is appropriate when applying for the building permit.

SEPTIC SYSTEM DESIGN

However, actual testing is seldom if ever done by a prospective purchaser who is well advised to consider this aspect of property development base on initial research. There are essentially three issues to be addressed: [i] what type of spreading facility is to be employed; [ii] where should that facility be located; [iii] and how should it be tested? The geological character of the site should dictate determinations for both [i] and [ii]. As to [iii], City Division of Environmental and Building Safety Services has established certain guidelines. Essentially, the applicant must produce two documents. One concerns the proposed septic system design; the other is a "supporting geology requirement" which is a special limited report by the project geologist of record.

The City of Malibu has adopted the Los Angeles County Plumbing Code (Title 28). Design requirements for a septic system are contained in Appendix K of the Plumbing Code. It is incorporated as extensively amended in the City's Municipal Code, Title 15, Chapter 15.12, as amended by Ordinance No. 194. The fundamental design criterion is the minimum septic tank volume which is 1,500 gallons for residences of up to 6 bedrooms or as determined by the number of waste/sewage design flow rates, whichever is greater (Sec. K2). It is to be noted that the location of the septic tank can be almost anywhere so long as local set-back restrictions are observed.

Dimensions of a drain field or a seepage pit are based on an absorption capacity (percolation rate) and a sidewall area sufficient to dispose of the septic tank volume in 24 hours. The square footage must have a total absorption capacity great enough to dispose of 1,500 gallons in a period of 24 hours, except that in the case of a drain field none may have a size predicated on an absorption capacity (also percolation rate) of less than 0.83 gallons per square foot of side wall area (Subsection K4c). Absorption rates may be estimated by on the type of material according to Table K-4 for "five typical soils" or alternatively by a test procedure "... acceptable to the Administrative Authority" (Sec. K4).

In practice, the absorption rate for a drain field is determined by a Ryon type test which measures the rate of static decline in one or more pre-soaked one cubic foot test holes dug in the bottom of the proposed field or invert of the proposed leach line. For a seepage pit, the loss in 24 hours of a measured volume of water from a pre-soaked 1- or 2-foot diameter test hole extrapolated to the design diameter of the seepage pit. In the case of a seepage pit, the test depth is recommended by the geologist at the time the test hole is logged.

Supporting Geology Requirement

A geology report is required supporting any plan to dispose of sewage effluent by means of a drainfield, leach line or seepage pit. To be included in the report is a map showing the location of the test for the proposed facility, a geologist's log of the materials in the excavation or test boring, a detailed statement regarding the occurrence and migration of any ground water encountered, and in the case of a proposed seepage pit a recommendation of the capping depth, all of which is reasonable. However, for either a drain field or a seepage pit, the supporting geology requirement also specifies that a "stability statement" be included requiring the geologist to "... unequivocally state that the disposal (at the proposed facility location) will not cause any instability either for the subject property or for any neighboring property." That is unreasonable. Its intention and effect is to make the geologist an insurer of the City. The most a geologist can offer is an opinion. No professional involved in construction of any kind can be unequivocal about this work product, whether an architect regarding earthquake damage to a house he has designed or a geotechnical engineer regarding the stability of a slope for which he has estimated a safety factor.

EXISTING SYSTEM CONSIDERATIONS

The septic system serving an existing property can be OK, sort of OK, or not OK. An pre-purchase inspection by a specialist, preferably one approved by the City as an "Onsite Wastewater Treatment Inspector," is advisable. However, such an inspection can provide only limited information. The best test of the manner in which a septic system will perform in the future is the manner in which it has performed in the past. In this regard, the record of pumping for the system, the immediately previous period of occupancy, and the number of inhabitants served during that period is especially of interest.

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