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Cesspool type septic systems: this document explains what a cesspool is, gives important safety and maintenance advice for cesspool systems, and defines the criteria for cesspool failure. We also provide critical safety warnings concerning cesspool systems as with some older and especially site-built cesspools there is a risk of dangerous collapse or cave-ins.
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Guide to Using a Cesspool for Onsite Sewage Disposal - properties of cesspools used for onsite wastewater disposal
This material is a chapter of our Septic Systems Online Book that explains septic system inspection procedures, defects in onsite waste disposal systems, septic tank problems, septic drainfield problems, checklists of system components and things to ask. Septic system maintenance and pumping schedules. Home buyers who want less technical advice should see the Home Buyer's Guide to Septic Systems. Also see The Septic Systems Home Page.
What is a cesspool? A cesspool combines the septic treatment tank and absorption system into a single component. A cesspool is not a septic tank and does not work like one.
In its most basic and traditional form a cesspool is a hole in the ground to receive sewage: the walls of the "hole" are lined with stone or concrete block or (safer) pre-cast concrete (photo) to form a masonry-lined pit into which sewage is discharged. Solids (sewage from the building) remain in the pit, effluent is absorbed into soil below and at the sides of the cesspool. Solids settle to the bottom, floating grease and scum collect at the top, and liquid seeps into the ground, initially through the bottom and most of the time through the side of the cesspool.
Cesspools as a means to dispose of sewage have been around since the late 1400's at the beginning of the Renaissance. (Before that people simply tossed their night soil into the street.) While cesspools have been used for a long time, since the development of the more modern septic system (tank and drain field) cesspools have often used where there is limited physical space (no room for a leach field), and perhaps where the soil absorption rate was high such as areas of gravel or sandy soils.
For most buildings and onsite waste disposal installations which use a "cesspool", you should consider the use of a cesspool as an obsolete, limited capacity system likely to need replacement, and involving significant cost. Some immediate concerns are stated next.
Cesspool Failure Criteria (MA)
Failure Criteria for Cesspools: If the waste level is within 12" of the inlet pipe near the top of a cesspool the system is at end of life and needs to be replaced. Some municipalities and experts will state other distances. In Massachusetts according to the Massachusetts Title 5 Septic Law the following are considered a failed or unacceptable cesspool installation:
A cesspool needs replacement if it meets any of these failure criteria. While a septic company may offer to pump, partially pump out, or agitate or aerate the bottom sludge in the cesspool in an effort to extend its life, these procedures are potentially very dangerous (see Cesspool safety Warnings and at best will give only temporary relief.
Fortunately there are alternative onsite wastewater disposal alternative designs which can handle limited or even zero-space sites, so a simple cesspool as a destination for blackwater is no longer the only choice for limited-space sites.
Are Cesspools Even Legal?
Watch out: cesspools are not legal for use as wastewater treatment and disposal systems. In some areas existing cesspools used for wastewater handling [no conventional cesspool adequately treats wastewater] may be "grandfathered" in and permitted to continue in use.
But home buyers purchasing a property served by a cesspool should keep in mind not only the limited function of such systems but the significant cost that will be faced when the system is no longer functional - because at that time the local authorities may require that a different, more effective wastewater treatment system be installed, such as a septic tank and drainfield. And space limitations on a property can cause real trouble when the cesspool has to be abandoned.
Cesspools are flatly declared "not legal for use as wastewater treatment and disposal systems" in other jursidictions such as in the Alaska state sanitary code. 
Cesspool Safety Warnings and Cesspool Capacity Limitations
Very Important Cesspool Safety Warnings
In summary about cesspool hazards: watch out for cave-ins, keep away: cesspools, particularly older site-built cesspools present a very high risk of collapse from an unsafe cover or following some types of service involving pumping, aeration, or hydro-jetting.
Adults or children should not walk over or even near cesspools because of the risk of falling-in followed by collapse, a virtually certain cause of death. If the presence of a cesspool is known or suspected at a property its location should be roped off to prevent access and it should be investigated by a professional.
Cesspool Capacity and Testing Limitations
Cesspools can obscure or prevent effective septic loading and dye testing: If a cesspool is in use at a property a septic dye test would not be a reliable indication of good system condition since the test could simply be trying to fill a large near-empty hole in the ground; yet such a system may fail soon after being returned to active or heavier regular use.
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Frequently Asked Questions (FAQs) About Cesspools: installation, inspection, troubleshooting, maintenance & repair
Question: my friend said dump a 50 pound bag of lime into the cesspool
Does lime help to break down the walls in a cespool. My friend said to dump a 50 pound bag in. He said his friend did it and it ate the walls and his pit is working great. He also said he saw cracking in the side walls and after adding the lime the sides of the walls basicly fell in. This was concerning to me. - Lance 5/11/12 (originally asked at SEPTIC TREATMENTS & CHEMICALS)
Reply: do not add lime to a cesspool; watch out for fatal cesspool collapse hazards
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
Design Manuals for Septic Systems
Onsite Wastewater Disposal Books