In the previous section of this cycle of articles dedicated to on-site Sewage Disposal Systems, we addressed issues related to Septic Tanks as individual components of such systems. You will find descriptions of their functionality, typical construction materials, crucial elements to meet environmental requirements, preferred location, issues related to maintenance and service, etc….
Now, it’s time to see how septic tanks fit into the operation of the on-site, individual household Sewage Disposal System. The truth is that the septic tank is just one of the elements of the system and depending on the properties of the drain-flied, climate zone, and local environmental protection restrictions (to name only a few) it may be used as a “Lego block” to accomplish required tasks!
Type 1 Septic Tank
The septic tank is the first stage of the sewage treatment system. Here, heavier solids sink to the bottom forming (after the biological decomposition) the layer of “sludge”, while lighter substances (grease, fat, oils, detergents, food, etc…) float to the surface forming the layer of “scum”. In between sludge and scum forms the layer of “greywater”-like liquid called effluent.
The concept of the Septic Tank. Source: “What is a Septic Tank? What is a Cesspool?”, ABC Cesspool and Septic Pumping)
During the sewage “retention” in the tank – the bottom layer of solids, floating scum, and particles of organic matter suspended in the effluent will slowly decay. This process of decomposition is carried out continuously thanks to an army of bacteria, digesting the organic matter. The end products of this process are correspondingly:
A) Less complex compounds of solid matter (non-biodegradable in their final form). Mixed with inorganic matter, they will settle on the bottom of the tank compacted into smaller volume (typically about half of the volume of original raw solid waste). With time, the accumulated sludge will have to be removed by a septic pumper truck.
B) Biogases (methane, carbon monoxide (CO) and dioxide (CO2), hydrogen sulfide, sulfur dioxide, nitrogen oxides, etc. In the case of small, on-site Septic Systems, biogases are released from the tank by a vent, contributing to the contamination of the atmosphere (note that so far, the treatment of biogases generated by small, on-site Septic Systems is not regulated by Environmental Protection Laws).
C) “Water” – (in general liquids) containing mineralized chemical substances that may have acidic and/or alkaline properties. Diluted with the affluent, they will be sent down the system for further treatment either to the next wastewater tank or if the effluent is clean enough – directly into the drain-field.
Septic (Anaerobic) Decomposition
Due to the lack of oxygen, the 1st stage pre-treatment process is mostly done by anaerobic bacteria (note that the upper layer of scum and stagnancy of the wastewater prevent infiltration of air and oxygenation). The most typical anaerobic break-down process of the organic matter consists of fermentation. Its byproducts are methane, hydrogen sulfide, and heat (fermentation is an exothermic process). The released heat is actually particularly useful in cold climate zones because it helps to keep the bacterial activities and biological decomposition process alive. Note that at temperatures below freezing point, bacterial activity slows down or even totally stops what disables the critical operation of the septic system – decomposition of organic solids.
If the decomposition process is not fully finished (in fact it may take several days for that to happen) then there is a high risk that not fully decomposed organic matter will flow down to the distribution system network (drain-field trenches).
As a result:
- The distribution system may be clogged making it inefficient or even completely out of service
- Not fully treated water may get to the groundwater sources causing environmental damage and health risks.
Note that the leaching bed is specified and designed for the disposal and post-treatment of the well-defined maximum volume of effluent and the concentration of organic matter. Otherwise, given the local geological conditions (limited depth of the soil, percolation rate, etc…) the drain-field may not be able to fully manage the post-treatment process.
Unfortunately, in the 1-compartment septic tank, the movement of wastewater is rather unrestricted. Turbulences caused by inflowing raw sewage, as well as drifts due to gradients of interior temperature and bubbling biogases may move effluent with plenty of suspended solids towards the outlet. This effect will be strongly magnified when the tank is not emptied on time and the accumulated volume of scum and sludge reaches the proximity of the outlet.
To prevent such a scenario, in many countries the main septic tank Must Have two separate compartments divided by a baffle wall. The well-designed baffle wall should only pass selected, higher purity layers of liquids (effluent) to the 2nd compartment. Thanks to this simple structural arrangement, solids (sludge) and scum are quite effectively held in the first compartment of the septic tank.
Concept of a two-compartment septic tank. Source: “What is a Septic Tank? What is a Cesspool?”, ABC Cesspool and Septic Pumping)
Graphical visualization of the impact of the baffle-wall in the septic tank. Source: “Overview of The Conventional Septic System”, JT’s Septic (AZ, USA)
As can be seen in the picture, thanks to the baffle-controlled flow of wastewater, the probability of overflow of solids to the 2nd compartment is low. In practice, (provided that sludge and scum are periodically removed from the tanks) the amount of scum and sludge accumulated in the 2nd compartment will be very low as it comes only from suspended in effluent minuscule particles of solids. Note, that the baffle hole(s) is located “strategically” in the area far from the inlet, scum, and sludge corresponding to the most “stable” (settled) layers of effluent. Usually, the length of the inlet compartment measures about 2/3 of the tank’s overall length, while the outlet compartment only 1/3rd.
To further limit the possibility that larger particles of solid waste will reach the leaching bed, it is suggested to install an Effluent Septic Filter at the tank’s outlet. A sieve filter is a low-cost solution preventing the soil in the drain-field from clogging. The only inconvenience – the filter must be periodically cleaned (at least once a year) and frankly, it is not a dream job ☹.
Under normal circumstances, the Type 1 Septic System is passive. The movement of the wastewater down the system is achieved thanks to the forces of gravitation. The pump(s) may be needed only if the container house has a basement (very rare case), or when the drain-field soil is inadequate and an above-the-ground post-treatment leaching bed (mounds) is necessary. In such cases, the wastewater will have to be pumped up (lifted) and so the system will need the energy to run it.
Type 2 Septic Tank
Often, the drain-field has limited capacities to further purify the effluent – for example a shallow groundwater table, not sufficient soil depth, poor quality soil, too low (clay) or too high (sand) percolation rate, etc…). In such a case, before being released into nature, the effluent must be purified to a much higher level than the anaerobic treatment can achieve. In practice, it means that the Septic System must be equipped with the 2nd stage of wastewater treatment – usually, an additional tank, where an army of microorganisms can further decompose the solid waste suspended in the effluent.
Unlike in the Type 1 Septic System (mandatory 1st stage of wastewater treatment) where the decomposition process is dominated by anaerobic bacteria, the decomposition in the 2nd stage is usually carried out by aerobic bacteria. These bacteria decompose the organic matter into gaseous forms, liquids, and mineralized components (sludge).
2nd stage of the Type 2 Septic System for Aerobic Decomposition. Source: “Types of Septic Systems” – (EPA, US Gov)
Such an approach has important consequences. Aerobic bacteria need an oxygen-rich environment, so the tank must be continuously fed with oxygen. And there is only one way to effectively accomplish this task – an electrical pump!
Note special design requirements for the aerated tank (interior dividing walls, the distance between the aerating pipes and the outlet, etc). It’s because the aeration process causes turbulences in the wastewater and if this effect is not taken into account, suspended particles of solids may be discharged to the drain-field before their full decomposition.
If properly designed and operated, the Type 2 Septic system offers several benefits, like for example:
1. Extended lifespan of the drain-field (lower the volume of discharged solids leads to a lower probability of soil’s saturation and/or clogging).
Or – looking at it from a different point of view:
2. Smaller-size drain-field can process the effluent from the Type 2 Septic System. In other words – drain-fields connected to Type 2 Septic Systems can support higher Hydraulic Loading Rates (HLR). This may be the solution for small lots or these with numerous “exterior” limitations. Note that drain field’s borders must be separated from neighbors’ lots, home(s) water wells, veggie gardens, swimming pool(s), etc… by distances determined by local laws!
Minimum separation distances imposed by Canadian legislation. Source: “Septic Smart – Understanding your home’s septic system” – RVCA.ca, Ontario, Canada
Obviously, all these potential benefits come at a price. Type 2 Septic System is an active structure. It needs a continuous supply of electrical energy to aerate (oxygenize) the wastewater in the tank. This may be inconvenient in an off-grid “environment” or for seasonal houses.
Warning: Unfortunately, “cleaner” effluent does not mean that it contains fewer pathogens. Their “neutralization” may still require a sufficiently deep layer of drain-field soil!
Comparing the Anaerobic and Aerobic Decomposition Processes
It may be worth shortly underline here the pros, cons, and differences between both processes to answer a legitimate question: Why we make the system more complex – can just one process do the job?
From the scientific point of view, the anaerobic treatment is a process of biosynthesis. transforming organic matter into dense biomass.
More practically – it’s the familiar fermentation process leading to the separation of solids and their sedimentation into a compacted form ready for removal. Anaerobic bacteria digest organic solids reducing them to a much smaller volume of stable, compacted substances! It is a simple, natural, self-sustaining process (it does not need any extra energy). The removed sludge can be easily dewatered and used as a sort of humus for gardening. On the negative side – byproducts of the anaerobic process include highly odorous and potentially toxic gases like methane (CH4), ammonia (NH3), and hydrogen sulfide (H2S) – the latter responsible for the “rotten-egg” smell.
While anaerobic bacteria can process a large amount of organic load contained in the sewage, it’s a slow and incomplete process that cannot fully purify the effluent. That is why it must be followed by the post-treatment stage. Usually, it is accomplished by percolation across deep layers of soil over a large area of a drain-field. In a favorable geological environment combining the septic (anaerobic) process with the drain-field is the lowest-cost solution for the on-site sewage treatment system. Anaerobic decomposition is temperature-sensitive and drastically slows down at low temperatures.
When applied on a larger, industrial scale, the generated by anaerobic bacteria methane gas can be captured and used as the source of renewable energy.
Using the scientific language, it’s the process of biological oxidation transforming organic matter into simple end-products like water, water-soluble minerals, and gasses.
The aerobic decomposition is efficient, but only if applied to wastewater with low content of organic matter. Overloading the aerobic process with a large amount of solids will result in the generation of an excessive volume of not-fully decomposed sludge known as “activated sludge” (much larger than anaerobic treatment will produce).
That is why, typically, it is applied at the 2nd stage of wastewater treatment to already partially purified effluent. On the positive side – aerobic bacteria, thanks to the abundance of oxygen generate odorless gases (oxides of carbon and nitrogen – CO2, NO3). Theoretically, after the aerobic decomposition, highly purified effluent may be directly recycled into nature (OK, we may still have to take care of potential pathogens). Unfortunately, oxygenation requires energy and a pump, which largely increases the system’s cost and maintenance requirements.
Note that the chemical process of oxidation of carbon (characteristic for aerobic treatment of wastewater), generates energy promoting the growth of beneficial microorganisms (bacteria, fungi…).
Princip of the Aerobic Treatment. Source: “Biological Wastewater Treatment” by Arun Mittal (Aquatech), WaterToday.org)
Both, anaerobic and aerobic decomposition processes are complementary to each other and applied alone cannot purify the sewage to the form acceptable for release to nature. In simple words, one can say that the anaerobic process represents a crude stage of the wastewater treatment, while the aerobic one – the fine (polishing) step. The decision to use one or both types of wastewater treatment depends on the quantity and type of raw sewage, local geological conditions, and local environmental laws.
Biodegradation is sensitive to temperatures, microorganisms thrive in the temperature range from 68 °F to 104 °F (20 °C to 40 °C), but unfortunately, become dormant when the wastewater temperature falls below the freezing point 32 °F (0 °C). Fortunately, thanks to the fact that the biodegradation process itself is exothermic (heat-releasing), a substantial amount of water used by households is hot and tanks are buried, the interior tanks’ temperatures are always higher compared to ambient ones.
Temperature is not the only limiting factor – the biodegradation process is also sensitive to wastewater’s pH level (acidity or alkalinity – the latter increases in concrete tanks) and toxins!
Type 3 Septic System
Basically, the on-site Type 3 Septic System is a sort of small Water Treatment Plant including three wastewater treatment stages: Anaerobic, Aerobic, and Disinfection. It is the most complex and expensive septic system, rather rarely used for the needs of individual households.
Adding the Disinfection Stage to the Type 2 septic system allows for a much higher level of purification of the effluent (to such extent that it can be odorless). However, the main purpose of the chemical disinfection is the total neutralization of pathogens contained in the effluent before it is discharged into nature. The disinfection process can be accomplished chemically (for example by chlorination), or by exposing the wastewater to ozone or UV light. Out of them, chlorination is the most popular and relatively inexpensive, but environmentally not fully neutral. The ozone and UV-treatment are environmentally neutral (if we ignore the need for electrical energy), but unfortunately more expensive and possibly less reliable.
Type 3 Septic System: AQUA AIRE Aerobic Treatment System – model AA500-4075. Source: “Aerobic Systems”, Victoria Precision Products, Inc. (TX)
A good example of the Type 3 wastewater treatment system may be the model AA500-4075 manufactured by Aqua Aire® (TX, USA). It’s a sort of a mini version of typical municipal systems. It includes the classical Anaerobic Pre-treatment Tank (Stage1), Aerobic Processing Tank (Stage 2), Chlorination Chamber (normally the disinfection will be carried out in a dedicated 3rd tank, here for the practical reasons the chamber it co-located in the 2nd tank). The last tank serves as a “final clarifier” housing also the effluent pump to disperse highly the purified effluent.
Such systems are designed for on-site household septic systems with small lots and/or unfavorable soil conditions. These will be sandy (high percolation rate) or clay (low percolation rate) soils, narrow dept of soil (shallow bedrock or groundwater table, etc..). In such cases, the pump is necessary to allow for the disposal of clear and odorless effluent over the surface (gardening, lawns…) or created for that purpose – absorptive mounds.
In contrast to the biological nature of the anaerobic and aerobic treatments, the disinfection is rather a chemical or physical process. It’s designed to kill (or make inactive) all microorganisms contained in the effluent that are hazardous for human health (pathogens, microbes, viruses, etc), so they will not contaminate groundwater once realized to nature.
Chlorine (Cl2) is a very reactive, water-soluble gas. It kills microorganisms by breaking the chemical bonds in their enzymes. Chlorination, as relatively safe (if used under strict control), reliable (in terms of doing the job) and economic disinfection method, it is commonly used in municipal wastewater and freshwater plants to neutralize microorganisms. Typically, it is used in the liquid form of chlorine-based bleaches.
Unfortunately, chlorine, even in small quantities is toxic for humans (respiratory problems, irritation of eyes, and skin…). What makes it worst, it is heavier than air, so it does not disperse easily. It’s also highly corrosive and environmentally unfriendly, so sometimes, in environmentally sensitive areas (once pathogens are dead), a de-chlorination process is necessary to prevent the subsequent contamination of nature (it’s more complex process than chlorination).
For small, on-site Septic Systems (similarly as swimming pools), the health risk and environmental damage by use of chlorine are negligible and (so far) acceptable by law.
Ozone (O3) is a very strong oxidant agent. Released into wastewater, it adheres to microorganisms’ cells oxidizing their membranes, which leads to ruptures and immediate deaths of cells. It is a highly effective disinfectant (about 1000 times more potent than chlorine)!
Unfortunately, in larger concentrations, it can cause respiratory problems, so at “ground-level) it is considered harmful for human health and the environment (in contrast to its life-protecting function in the upper stratosphere).
Due to its atomic structure (loosely bonded extra atom of oxygen) it is relatively unstable, easily breaking down to “friendly” and life-supporting atoms of oxygen!
Ozonator, (known as “Corona”) is probably familiar for owners of garden spas (jacuzzi) where it is often used for sanitation of water.
Oxygen passing by the zone of high-voltage discharge (corona), is converted to ozone. Source: “Ozone production from Corona Discharge”, Oxidation Technologies LLC (IA, USA)
Ultraviolet radiation carries high-energy photons able to destroy RNA and DNA bonds, which makes them incapable to reproduce and infect. For that reason, UV light (wavelength range from 200 to 300 nm) is categorized as “germicidal”. UV-based disinfection is a physical process, and as such, it is environmentally friendly as it does not leave any chemical residues. In practical implementation – UV lamps are submerged in the effluent, instantaneously neutralizing everything “alive” that passes-by.
Unfortunately, to be fully effective, the wastewater must be clear (any organic or inorganic micro-sediments will either absorb UV radiation preventing it from deep penetration of the effluent.
Example of a UV-disinfection system designed for small wastewater treatment systems. Model Trojan-UV3000-Plus
The extra cost associated with UV lamps, electrical energy, and required maintenance may not justify UV-based disinfection systems for an individual on-site Septic System. They are however gaining popularity across municipal wastewater treatment plants because they lower the overall environmental costs associated with chlorination (health, contamination, corrosion…). And the fact that the UV radiation is also effective against chlorine-resistant microbes (protozoa) as well as chemical contaminants like pesticides, pharmaceuticals, (and the list is longer), is an extra argument for it!
Impact of toxins on the biodegradation process
The biological decomposition process entirely depends on an army of active bacteria. They break-down organic solids and eventually digest them.
Any toxins going “down-the-drain may harm bacteria, and, in their absence, the biological decomposition process will stop. Obviously, you can assume with certainty, that all antiseptics (disinfectants) will do exactly what they supposed to do (after all – they are designed to kill bacteria!). Another red line is the warning sign” Harmful if swallowed” on specific products – they will be deadly for bacteria as well.
Typically, wastewater with antiseptics (especially detergents and soaps) is coming from washing machines (dishwasher and laundry), bath sinks, and showers. Known as Greywater, it does not support the lives of microorganisms! The diluted chemical compounds carried by the greywater cannot be broken down in a biological decomposition process.
Wastewater coming from flushing toilets and kitchen sinks (especially those with installed garbage disposal devices) is known as Blackwater. Overwhelmingly, it contains only organic matter (assuming that quantities of toilet cleaners are negligible). It comes with an army of microorganisms so it can be fully treated in a biological decomposition process.
Household sewage is a mixture of both: grey and blackwater in proportions largely depending on the family’s lifestyle. Needless to say, that for the proper operation of the on-site septic system, it is important to minimize the use of detergents, soaps, cleaners, antiseptics (in general substances designed to kill bacteria). Killing all microorganisms (including pathogens) before those “good” (digestive) have a chance to do their job is like throwing the “baby with the bathwater”.
Sometimes, to overcome these contradictory conditions it is suggested to divert greywater (laundry, dishwasher, shower, bath sink(s)…) to a separate leaching bed. While such a dual system will guarantee the biological decomposition of the blackwater (bacteria will be thriving in septic tanks in absence of disinfectants), it will be quite a costly solution. The good news is that the household-generated greywater does not contain microorganisms. All deadly pathogens by the time the greywater reaches the leaching field will be already dead (after all, that’s what soaps and disinfectants are for), so the greywater does not represent similar health hazards as the raw sewage (toilets and organic waste). However, chemical substances contained in the greywater still must be treated before it can be released into “nature”.
- To keep the efficiency of the wastewater treatment process within the imposed by law environmental limits, the septic systems need a continuous operation. The most critical in this sense are Type 2 and 3 systems because their operation will be affected by any disruption (including seasonal turning off) of electrical energy. Unfortunately, also the natural biological treatment process (anaerobic) exclusively used in Type 1 Septic Systems may not be sustained under conditions of seasonal use, especially in cold climate zones.
- Do NOT use kitchen-sink grinders (Garburators) to dispose of organic food waste (it should be composted instead of being sent down the drain). Firstly, they increase the load of organic matter that must be processed by the Septic System. Secondly, the macerated solids will have a tendency to longer drift in the effluent (instead of quickly settling at the bottom of the tank) so they may be easily discharged into the drain-field before being fully decomposed.
It may be worth mentioning that popular in America and Australia Garburators (also known as SinkErators or simply Waste Disposal Units) in most countries are banned!
Bottom line: Use sink strainers to prevent food leftovers (and any other solids) from entering the sewer system. In fact, compost as much organic matter as possible instead of throwing it into the drain.
1. In cold climate zones, the operation of the Septic System will be affected by low temperatures. You can minimize the impact of below-the-freezing point temperatures by the following:
a.Install the tank(s) bedded in the thick layer of gravel to drain the water from the area surrounding them. Preventing the accumulation of water (and eventually ice) in the area around the tank(s) helps to retain more heat and so to sustain the biological decomposition process of the sewage.
b. Divert the rainwater or melting snow water away from the area of the tank. It can be done by slopping the ground on the top of the tank away towards its perimeters as well as by installing a waterproof tarp above the tank (for best visual effects covered by the layer of soil and lawn). The main goal is to re-direct the water a few feet away from the tank. Keeping surrounding soil (gravel) dry helps to retain higher temperatures in the tank.
c. In arctic zones, add a layer of thermal insulation on the exterior walls of the tank((s). Most effective will be a spray-foam type of insulation as it easily adheres to the tank’s surface preventing any thermal bridges that can be infiltrated by the water.
4. When the septic tank is pumped (and eventually cleaned), it must be immediately re-filled with water. An empty septic tank may not have enough structural strength to withstand the pressure of the soil around it (especially when the soil is saturated with water). The extra stress may be caused by the pumping truck if it is parked too close to the tank.
The best time for pumping the septic tank is late spring (warm ambient temperatures). Note that refilling water will not have any bacteria, and it will take time to “naturally” replenish them. The biological decomposition process of the sewage will gradually re-start, however, if the tank is pumped in the late fall, initially low bacterial activity in heavily diluted sewage will fail to generate much-needed heat. This in turn (given low ambient temperatures) will additionally slow down the decomposition process, which in extreme situations may fully stop.
The typical cycle of pumping for a properly sized and operated septic tank should be in the range from 2 to 5 years (more often in colder climate zones).
5. Do not flush such items as sanitary pads (they are not biodegradable), paper towels, paints, solvents, cigarettes, diapers, medicines, gasoline, oils, anti-freeze, etc….