The on-site Septic System must prevent the contamination of the environment (especially groundwater) by waste products. The raw sewage is composed of water (about 99%), organic matter (human feces, toilet paper, food leftovers, etc..), chemicals (detergents, soaps, cleaners, etc..) as well as invisible, but potentially harmful toxins, microorganisms, pathogens, parasites…. Due to environmental and health hazards, in most countries, releasing untreated sewage into nature is strongly prohibited by legislation. In larger agglomerations, the raw sewage is collected by the municipal sewage system and send to the Wastewater Treatment Plant(s). At off-grid locations (often preferred for construction of container-based houses) the raw sewage must be directed and processed by the on-site Septic System.
The two most important elements of an on-site Septic System are Septic Tank(s) and Drain-field (Leaching Bed).
On-site, individual household Septic System includes Interior installation w/plumbing vent, Line Pipe w/service access, Septic Tank, and Drain-field (Leaching bed). Source: Overview of The Conventional Septic System, JT’s Septic (AZ, USA)
Septic Tank Functions
The main purpose of Septic Tanks is the protection of the drain-field from the saturation by solid waste and by that, protection of the environment from hazardous contamination. By storing the hazardous waste, Septic Tanks keep the drain field’s absorption as well as its filtering and processing abilities within the required range over a long period of time (possibly forever). Drain-field acts as the final wastewater purification stage, so its failure will lead to environmental disaster. To achieve these goals, the Septic Tank must carry out the following tasks:
- Separate solid waste (sludge and scum) from the liquid to prevent it from flowing down the system to the drain-field.
- Support at least partial biological decomposition of the solid waste during the “Retention Time”. This natural process is carried-out mostly by anaerobic bacteria.
- Store the solid waste (in raw and/or decomposed forms) and when necessary, allow for its removal by specialized services.
- Prevent retained wastewater from leaking into the surrounding ground (for this reason, modern septic tanks must be watertight!)
- Discharge the partly purified effluent into drain-field for further treatment by the soil.
The required watertight design makes a big difference compared to Cesspools (old, rural, and these days rather illegal “tankless” septic systems). Cesspools were large pits (usually with perforated concrete walls and gravel-filled bottoms) designed for direct discharge of the raw sewage. While they were able to temporarily collect (detain) some volume of sewage, mainly, they served as wastewater leaches. Due to the lack of separation of solid waste, their performance was decreasing with time in an irreversible process. While the solid waste could be periodically pumped out of the cesspool, the surrounding soil was getting saturated with solids (an effect known as bio-mat formation). As a result, the surrounding soil (leaching-field) was gradually losing its crucial function: the ability to absorb and purify wastewater.
The concept of Cesspool. Source: GroundStone – Septic System Services
Modern septic tanks are designed to collect the sewage generated by an individual household and detain it for a limited retention time. In the first approximation, their storage capacity is determined by the maximum daily volume of generated sewage (in gallons) and the minimum retention time (in days) necessary for microorganisms to decompose the organic waste. Usually, estimation of these numbers is provided by the local authority based on the “size of the household”, expected activities (if other than “living space”), and the climate zone. To give you some idea – in moderate climate zones the “residential” sewage should be detained in the septic tank for at least 3 days!
Note, that usually in summers, due to more frequent showers and laundry, the daily volume of sewage will be higher than in winters. It means, that for a given retention time (in days), the required capacity of the septic tank must consider the worst-case scenario reflecting seasonal variations of the generated wastewater volume.
On the other side of the “equation” – the biological decomposition process of the solid waste evolves faster at higher temperatures and slows-down at lower ones. In other words, in the summertime, sewage retention time can be shorter, and these two requirements (maximum capacity and minimum retention time) may compensate each other (at least to some extent). However, keep in mind, that the final words in that matter belong to the Local Authority!
See details at:
Septic System – Impact of the Climate Zone
Generic Septic Tank Design
Basic Structure of the 2-chamber Septic Tank. Source: “Septic System Design” by Luis Goncalves (GroundStone, BC, Canada)
The size of the septic tank, its functional structure, and construction material depends on many factors including not only the amount of generated sewage and the minimum retention time but also the drain-field, type of soil, geological conditions, etc… However, regardless of all these factors, the tank must meet some basic requirements (often reinforced by local laws). And these are:
A. Inlet Baffle
The inlet should be designed as a “quite inlet”, minimizing disturbance in two already settled layers of sewage: Scum (upper layer) and Sludge (bottom layer). Any turbulences will lead to the mixing of solid waste with effluent so it may flow out of the tank into the drain field. To minimize this effect, the inlet discharge should take place in the middle of the effluent’s layer.
B. Outlet Baffle
Outlet’s intake must be located in the center of the effluent’s layer to minimize the probability that either scum or sludge will be carried out of the tank. For proper operation, the outlet should be at a level a few inches lower than the inlet and should extend deeper into the effluent.
For the mentioned above reasons, both inlet and outlet should be equipped with baffles (usually in a form of a T-shaped pipe known as Tee. The lower vertical leg of the tee should be reaching down to the effluent’s midpoint to prevent clogging the inlet and outlet by the floating scum (typically consisting of easy adhering grease, fats, oils …. ). The upper leg of the tee should be extended well above the scum level and open, to allow for the evacuation of biogases and equalization of the pressure via the line pipe and house’s Stench Pipe.
Configuration of inlet and outlet PVC baffles. Source: “EADE Organization vision in designing of the septic tanks” by Sati M. Al-Rawi, MSc (EADEorg, Iraq)
Some precast concrete septic tanks may come with built-in concrete baffles. While they will certainly do their job, they will erode over time what may affect their functionality. It’s not the end of the world; much longer-lasting PVC baffles can replace them although as can be expected, it’s not a dream job.
Cross-section of the precast concrete septic tank with built-in inlet and outlet baffles. Source: Overview of The Conventional Septic System, JT’s Septic (AZ, USA)
C. Effluent Filter
An Effluent Filter is one more built-in level of security preventing solid waste from leaving the tank. The filter is removable and requires periodic cleaning (not a pleasurable job) so it should be easily accessible from the outside. Typically, it is housed by the outlet baffle pipe (maintenance access provided via the tank’s manhole).
Septic tank effluent filter w/outlet baffle: model TUF-TITE 6″
D. Manhole(s) & Riser(s)
The septic tank must have a large access point(s) (manhole(s)) necessary to pump-out the content of the tank but also for its cleaning and visual inspection (if necessary). Manholes are not watertight, so an attempt should be made to prevent (or at least limit) the infiltration of water (and soil) to the tank. Given the fact that tanks are usually buried under the soil, you will need riser(s) to prevent the surrounding soil from falling into the tank when lids are open for service.
Septic raisers: left – model Aero Stream AR-S19, right – septic tank riser installed in the ground above the tank. Source: Waters Vacuum Truck Service (NV, USA).
E. Dividing wall w/opening
While you may still find old, one-chamber septic tanks, these days they are banned as inefficient. New standards call for 2-chamber tanks with compartments separated by a so-called baffle wall. The main purpose of the baffle wall is to physically separate large amounts of solids hold by the first (and usually much larger) inlet compartment from the smaller, outlet compartment. The opening in the baffle wall should be located in the middle of the effluent’s layer so at least theoretically, the relatively clean effluent flows to the outlet compartment from where (after the next phase of decomposition) it is sent out into the drain-field.
The baffle wall creates two separated wastewater zones inside of the tank, but it must keep open an area above both compartments to keep the same pressure inside of the tank and facilitate the outflow of generated biogases. Typically, the baffle wall divides the tank in a proportion of 2/3 (inlet compartment) and 1/3 (outlet compartment).
Impact of the dividing wall on the distribution of solid waste. Source: Overview Of The Conventional Septic System, JT’s Septic (AZ, USA)
F. Pressure-Equalizing Vent
The vent is necessary to equalize the tank’s interior pressure due to fluctuation of the level of sewage and changes in the atmospheric pressure. Also, it must provide an evacuation route for gases produced by the biological decomposition (decay) of organic matter that takes place in the tank.
Note, that sewer gases include methane, but also in smaller quantities hydrogen sulfide, ammonia, carbon monoxide, sulfur dioxide, nitrogen oxides. They may cause health hazards and due to their strong smell cause unpleasant sensations (stench).
The simple vent installed above the tank will be good enough, provided that the tank is far from the home so escaping gases do not cause any health hazards (and let’s face it – unfriendly sensations characteristic to the smell of “rotten eggs”).
To be effective, the septic tank vent must be located no more than 30ft away from the tank and high enough to discharge gases well above the level of human heads. Another (but not necessarily suggested) scenario is to use inlet, main sewer pipe, and house “Vent Pipe” to equalize pressures and evacuate biogases from the tank. Note, that the house’s interior sewer installation MUST have the vent pipe (often called “Stench Pipe”) ending with the roof-mounted vent. It will serve the purpose of dispersing (usually warmer than ambient temperatures) gasses up to the air, without causing unpleasant sensations. Note, however, that in some states/countries such a solution may be prohibited by local laws.
Provided that it is authorized by the law, this scenario will work ONLY if the septic tank is located close to the house (less than 30ft away).
Interior installation w/plumbing vent (stench pipe) and line pipe provide ventilation path for the septic tank. Source: The Natural Home
The main problem may be caused by the sudden discharge of larger amounts of water down the drain (flushing toilet). If the water fills the pipe, it may temporarily prevent the evacuation of air (and gases) from the tank via the stench pipe. Fortunately, in typical households, the volume of instantaneously discharged water is relatively small so it will flow down using the effect of the vortex, living the central part of the plumbing open for air.
Sudden discharge of the water into the house’s sewer system (here the problem is solved by the dedicated tank vent). Source: WTE Ltd (UK), courtesy of Humm-Busters Ltd.
Another solution is the location of the vent somewhere down the distribution pipe(s) near the Distribution Box (if it is located within the 30 ft distance from the tank). It wouldn’t change much in terms of toxicity and potentially noxious smell, but being located farther from the house, it may be less annoying.
Note that the perforated distribution pipes dispersing the effluent over the drain-field are “breathing” so some may assume that they can provide the path for the exchange of air and evacuation of biogases from the tank serving as a “distributed vent”. This action however and its efficiency (volume of air/sec) is not specified nor guaranteed. And frankly, such “distributed ventilation” cannot equalize fast changes of pressure caused by large inflows of wastewater to the tank(s). In other words, it may help, but it is not good enough to provide efficient ventilation of the septic tank!
If technically possible, it may be wise to install a larger capacity tank compared to the one determined by the local authority. Potentially, this brings several benefits:
A. Larger capacity tank allows for the accumulation of a larger quantity of sludge, increasing the length of time between consecutive pumping (lower cost of operation).
B. Larger the septic tank, the longer the sewage retention time and so higher the level of biological decomposition. This in turn results in “cleaner” effluent getting out of the tank to the drain field and its longer lifespan.
C. Oversized tank offers an extra margin of retention for operation at low temperatures.
D. Increased volume of stored effluent helps to maintain its laminar (smooth) flow out of the tank. It’s due to the lower impact of mixing of the inlet wastewater (“influent”) and associated turbulences with already settled layers of the effluent.
E. Capacity “margin” for eventual future extension of the household (number of inhabitants, the addition of extra bedroom(s), etc.
Most modern septic tanks are made either from concrete or reinforced plastics.
a. Concrete septic tanks
They come in two versions – factory precast and on-site poured. Their main advantage of concrete tanks seems to be a low cost (unfortunately, it’s mostly perception). Precast versions are made from reinforced concrete, so they are strong and if carefully installed (robust, compact bed able to sustain the weight of the tank filled with sewage), they will serve for long years.
Unfortunately, precast versions increase the overall cost due to the need for heavy-duty transport and necessary crane track (note that a typical 1,500 gallons concrete tank (approximately 6’ x 11’ x 5’) will weigh on average about 12,000 lbs!).
On-site-built concrete septic tanks usually come at the lowest overall cost, however, these days they are not necessarily preferred solutions. And it’s not only due to numerous construction “headaches” but also usually, their lower quality compared to precast models. Nevertheless, poured concrete tanks are valid options at locations inaccessible for tank(s) delivery trucks.
Note: A well-designed, properly manufactured, and installed as well as regularly serviced concrete tank will last at least 40 years!
The main causes of shortening its lifespan are:
- Cracks are caused by unstable soil that cannot sustain the weight of the tank and its content or by expanding the roots of nearby trees. Unfortunately, concrete structures are prone to cracks!
- Degradation of concrete caused by acid water.
Concrete can be “eaten’ not only from the outside by acid surface water (exterior seal coating may help to lower this risk). Unfortunately, it’s not the only source of acidity. During the biological decomposition process, anaerobic bacteria produce hydrogen sulfide gas, which in combination with the water (effluent) turns into sulfuric acid! This is an inherent outcome of the decay of the organic matter inside of the tank, so it cannot be eliminated, although its destructive impact can be minimized if tanks have proper and functional vents!
Precast concrete septic tank. Source: Del Zotto Products (Gladewater, TX)
Construction of a poured-concrete septic tank. Source: Gill Shiels – “Septic tank being built”.
b. Plastic tanks
Made as molded, mainly from the reinforced fiberglass, they have several advantages compared to concrete ones. Firstly, due to their lightweight, they can be installed in the DIY process without the use of heavy equipment what lowers the initial cost. No less important is their durability (especially in wet soils), resistance to cracks, and water tightness (compared to the porousness of concrete). On the negative side – they may be quite expensive.
Infiltrator Septic Tank, Model IM 1530. Source: Infiltrator Systems Inc.
Cross-section of the Infiltrator Septic Tank, Model IM 1530. Source: Infiltrator Systems Inc.
Cross-section of the Infiltrator Septic Tank, Model IM 1530. Source: Infiltrator Systems Inc.
Plastic Septic Tank. Source: Build with a Bang
c. Steel tanks
They were very often used in the past, however, due to their susceptibility to corrosion, leaks, and subsequent contamination of the environment, these days they are mostly forbidden for use as underground septic tanks.
Note that rounded structures (“hockey-rink shapes) have higher mechanical strength than pure rectangular ones. It is especially important for concrete tanks because most plastic ones by the nature of the molding process will have rounded shapes. Unfortunately, this problem is often underestimated.
Septic Tank: Wastewater Separation Process
The Septic Tank acts as a “separator” of components carried by the wastewater. It’s a natural process based on physical laws (“weight” or if you wish – gravitation) that only need time and a quiet environment (no turbulences). It leads to the separation of the wastewater into three distinct layers:
Scum: It contains all lighter than water components like fats, oils…. By floating on the surface of the effluent the scum is exposed to the air, so it is mostly decomposed by aerobic bacteria. Note, that due to its “oily” content, scum largely prevents the natural aeration of the wastewater in the tank, leaving its decomposition at the mercy of anaerobic bacteria.
Sludge: It contains heavier than water solid waste, decomposed parts of organic waste, food leftovers, dead microorganisms, inorganic matter, etc… that will sink to the bottom of the tank where they settle. In the absence of oxygen, sludge is undergoing decomposition by anaerobic bacteria. After some time, the compacted deeper layers of sludge become not farther biodegradable and have to be removed from the tank.
Note: Conditions required for the anaerobic breakdown of the organic matter are commonly referred to as” septic”, hence the name Septic Tank.
Effluent: It is “clarified” wastewater (sort of greywater) mostly free of suspended particles of solids. In this form, it can be released from the tank into the drain field for further treatment. The layer of effluent is sandwiched between the scum and sludge filling the central part of the tank.
Septic Tank Maintenance
a. Removing sludge
Over time, the septic tank will accumulate by-products of bacterial digestion mixed with non-biodegradable inorganic matter (if it made its way into the household’s sewage). They will form a compacted layer of sediments (sludge) at the bottom of the tank and if not removed, sludge may overflow. To prevent the contamination of the drain-field by escaping sediments, septic tanks must be periodically emptied. As a rule of thumb, it is assumed that tanks should be pumped out when the level of solids exceeds 1/3 of their capacity. In practice, you may expect that to happen every 2 to 5 years.
Source: “Septic Tank Pumper Trucks”, vcrma.org
Cleaning: If it is not necessary, you should avoid any serious cleaning of the septic tank when emptied. And if required – then only with water. In general, cleaning removes the “colonies” of anaerobic bacteria from the tank. They will have to be “naturally” replenished before the biological decomposition process will restart. However, using chemicals will kill bacteria and create a toxic “environment” inside of the tank so it will take much longer to re-grow colonies of bacteria necessary for restarting the biological decomposition process.
Note that fats and oils are not easily biodegradable, as such, they may accumulate and clog downstream pipes. Whenever possible, compost them rather than throwing them down the drainpipes.
Mechanical Stress: The structure of the septic tank is seriously “tested” when emptied for sludge removal. Typically, during this process, not only sludge but also affluent and scum are pumped out. An empty tank may not withstand the pressure of the surrounding soil (especially when wet). The stress is additionally increased by the weight of the septic pumper truck (if it parks too close to the tank).
Estimated Septic Tank Pumping Service in years vs household’s size (number of occupants) and tank’s size (gallons). Source: “How often is Service Needed?” – ABC Cesspool and Septic Pumping, Hawaii (USA)
b. Replenishing Bacteria
Bacteria make a critical element of the Septic Systems and under normal circumstances, they come for free. They decompose organic solid waste reducing its original volume, so they increase the period between consecutive pumping. To some extent, they also neutralize some microorganisms (pathogens) that can be harmful to human health if disposed into nature untreated.
Bacteria naturally exist in the human digestive system; they are also present in leftovers of the raw food, so they are carried-out with sewage into the septic tank. In practice, however, the household sewage also carries disinfectants, soaps, detergents used exactly to kill bacteria. Usually, when typical household “chemicals” are used in moderate quantities they will not harm the septic system. Nevertheless, some agents may suggest that from time to time (especially in cold climate zones where the process of natural multiplication of bacteria slows down), it may be necessary to periodically replenish bacteria to keep the biodegradation process alive.
Well, there is no clear evidence that “additives” (whatever they are: enzymes, tank “rejuvenators” etc ) can improve the performance of the septic system. There is more evidence that they may rather harm the septic system and contaminate the environment. That is why, before taking any action, please consult the relevant authority.
Septic Tank: Location
Household sewage is carried by an underground pipe so obviously, septic tanks must be buried. Keeping tanks underground is not only a matter of aesthetics (architectonically, even modern septic tanks are not pieces of art to keep them exposed). Underground location has few more benefits
a. You do not have to use an ejector pump to transfer the sewage from the level of the lowest drain in your house to the tank.
Note that the highest projected level of sewage in your tank (determined by the location of an outlet), must be below the level of the lowest drain in your house. Given the fact, that majority of container houses do not have basements, the tank does not need to be buried deeply, just enough to be covered by the layer of soil and grass.
b. You do not lose precious space on your lot.
And the most important
c. In colder climate zones you protect the content of the tank and pipes from freezing.
Note that freezing also stops the bio-degradation process, so basically, it puts the whole septic system out of service!
Minimum separation distances imposed by Canadian legislation. Source: “Septic Smart – Understanding your home’s septic system” – RVCA.ca, Ontario, Canada
When it comes to the location of the Septic System, there is no room for “randomness”. The distance of the septic tank and drain-field from sensitive objects (house, wells, watercourse, neighbors properties, …. is closely regulated by local laws.
While preparing the plan of your Septic System to get the final “go ahead” from the Local Authority, keep in mind the following:
- Septic tank should be located at least about 7-to-8 ft away from the house (actually from the traditional house foundation, so it may slightly differ for container houses). However, it does not mean that a far-away location is better. Longer the distance, the longer the line pipe carrying the raw sewage. This may have few consequences of which the most important are:
a. Longer the line pipe, the higher the probability of its damage and costly effects of environmental contamination. Keep in mind that leaks from the tank’s outlet pipe carrying already “pre-treated” effluent have much less drastic consequences (if any).
b. Larger the distance of the septic tank from the house, the less likely you can use the house stench pipe to vent the tank. Most likely, any dedicated, near the tank vent will expose your house (open windows) to bad smells and toxic gases.
- Septic tank(s) must be also located at a reasonable distance from trees (10ft ?). Their expanding roots can exert huge force on the tank’s structure leading to cracks. This is turn will put the septic tank out of use and additionally, expose you to the potentially high cost of environmental cleaning (and associated legal costs) that may be imposed on you by local authorities.
- Keep the tank away from areas where downspouts discharge the rainwater. The water-saturated soil surrounding the tank increases the pressure on its structure and spreads away (sucks) interior heat much needed to keep bacterial activities alive.
- Plan tank’s location in an area accessible for the Pumping Truck (necessary for periodic sludge removal).
- Keep the tank away from the boundary of your lot, wells, veggie gardens, swimming pools, watercourse, etc.. (distances are subject to local laws).
- Do not plan to install the tank in the area where you want to park your cars, trucks… Heavy loads may damage the tank.
And when already digging the pit for the tank:
6. Do not forget to surround the tank with thick layers of gravel. The gravel thanks to its high percolation rate, will keep water away. Also, make sure that the base of the pit must be leveled, and, to guarantee stability – filled with gravel and compacted (especially in sandy soil). Filled with wastewater tank is very heavy (especially concrete tanks).
Sewer Line Pipe
Typically, a four inches diameter PVC pipe is used to connect house sewer installations with the tank. Its horizontal section should be straight (no bends) and have a slope in a range of ¼ of an inch per foot to avoid blockage by solids. Note that a bigger slope may NOT improve the transport of solids down the pipe, however, it will certainly cause more turbulence in the tank. This in turn will mix sediments (settled sludge and scum) with the effluent and that’s what should be avoided!
The optimal slope of the sewer line pipe. Source: “Septic Tank Design”, HPD (Nairobi, Kenya).
Note that the sewage pipe should have a service access point located near the home’s foundation for cleaning/unclogging when necessary.