Concrete ring septic tank: types, pros and cons

Recommendation points

• Treatment plant device
• Number and volume of chambers
• Hydrogeological constraints
• Active and passive septic tanks
• Maintenance and care

The problem of wastewater treatment in households can be considered almost solved. Local cleaning stations do an excellent job with this, the most effective and inexpensive solution is a septic tank made of concrete rings, the installation of which can be carried out by third-party contractors and on a turnkey basis..

Treatment plant device

Unlike a cesspool, in a septic tank, wastewater treatment is carried out not only according to the principle of filtration, but also by biochemical mechanisms. In the most advanced systems, cleaning takes place in several stages, each with different types of bacteria that process different types of contaminants. This requires that the septic tank from the rings be multi-chamber, while the drains must move from one container to another either naturally or forcibly..

Thus, we have a structure formed by two or three vertical wells of concrete rings. The tanks can have a different volume, which is done for the optimal course of the cleaning cycle, depending on the nature of the wastewater. The tanks are connected to each other by drain pipes located at different levels, each subsequent one is located below the previous one in the direction of movement of the treated wastewater.

Scheme of a septic tank made of concrete rings: 1 – sewage inlet pipe; 2 – tee; 3 – primary settling tank; 4 – aerotank; 5 – drainage well; 6 – HDPE pipe O110; 7 – HDPE pipe O70; 8 – crushed stone (gravel)

Removal of most of the contaminants takes place in the initial chamber, called the primary clarifier. Here, the solid fractions settle, forming a layer of waste sludge at the bottom. Lighter particles participate in the decomposition reaction, forming the bulk of the activated sludge. Bacteria working at this stage are predominantly anaerobic, therefore, aeration is not performed in the primary sedimentation tank..

When the liquid level in the sump rises to the overflow pipe, the activated sludge flows into a second chamber, sometimes called an aeration tank. To prevent the suspension of fatty masses on the surface from entering the further cleaning cycle, in the sump the pipe outlet is turned down and lowered approximately to the middle of the well height. The activated sludge at the outlet of the primary clarifier can be processed in different ways. If the second chamber is an aeration tank, the sludge is saturated with air in it, due to which colonies of aerobic bacteria develop successfully, which process the remains of solid particles. Also, downstream chambers can work on the principle of secondary clarifiers: even in the case of anaerobic processing, stepwise cleaning gives excellent results. The only difference is that clarified, odorless water comes out of the aeration tank, which can be discharged into open ground, while wastewater from the sump can only be discharged into the filtration field or into a drainage well..

Number and volume of chambers

The main difference between septic tanks made of concrete rings is the number of chambers and their capacity. The primary factor determining the volume of wells is the predicted load on the treatment plant. When designing, the performance of a septic tank is determined primarily based on the number of people living and the average water consumption. The norm is considered to be 150-200 liters of water per person per day, more accurate indicators can be established according to the meter readings.

It is believed that the total capacity of all chambers should not be less than the volume of effluent intake for three days. It is even better if the possibility of a salvo discharge is provided for the treatment plant: this way the water will have time to effectively clarify, while the septic tank will not be afraid of a one-time flow of a large amount of wastewater. The volume reserve in such cases should be at least double.

The number of chambers depends on the daily intake of wastewater and the method of their treatment. For anaerobic processing, the number of chambers increases in proportion to the load: up to 1 m³ one well is enough, if the flow exceeds 5 m³, then at least three are required. If the drains pass through the aeration tank, then another one is added to the total number of chambers – for aerobic cleaning, and its volume should not be less than that of each subsequent.

Traditionally, all septic tanks have the same capacity, but there are exceptions. For example, with different heights of wells, a septic tank can be placed on a slope or bypass violations of the horizons of sedimentary rocks. Also, with a low density of effluents, the primary clarifier can be made smaller for the sake of a larger capacity of the second and third chambers, where the waste is more and vice versa: with a high density of waste, it is better to give priority to the capacity of the primary clarifier.

Hydrogeological constraints

There is a reason why septic tanks have the form factor of underground structures. In addition to the fact that this option does not require forced pumping of wastewater, guaranteed absence of exposure to sunlight and low temperatures in winter. However, underground placement of VOCs presents a number of challenges..

A septic tank is a series of sealed containers that are acted upon by Archimedean force. If the viscosity of the environment changes over time, which can happen when raising the groundwater level or installing a septic tank from rings in unstable soils, the containers can be squeezed out only for the reason that their specific bulk density is less than that of the soil. The situation is aggravated in regions with prolonged exposure to frost: in order to maintain an acceptable temperature in sedimentation tanks, the VOC has to be deepened more and more, while its upper zone remains free of liquid and the mass-to-volume ratio decreases.

In fairness, it should be noted that concrete structures in this regard are less vulnerable to conventional septic tanks due to the greater weight of the structure. Nevertheless, specialists in the design of underground structures strongly recommend anchoring the tanks, increasing their weight or rigidly attaching them to a common base, represented by a reinforced concrete slab of a large section..

Another problem associated with groundwater is the likelihood of a septic tank flooding or, conversely, uncontrolled seepage of unclarified wastewater into the soil. This problem can be solved only if comprehensive waterproofing is performed. Unlike body plastic containers, which are guaranteed to be sealed, precast concrete structures require high-quality sealing of joints between rings, the use of reinforced concrete products with locks, as well as the use of modern systems of penetration and coating waterproofing.

Active and passive septic tanks

When using methods of aerobic biochemical purification, not only the design of the septic tank itself becomes more complicated, but also the degree of its technical equipment. Active septic tanks are those treatment facilities in which the movement of waste masses is performed by fecal or sludge pumps. Such a mode of operation is extremely necessary for aerobic cleaning, because due to the caking of activated sludge at the bottom, the bacteria simply stop doing their work, and sometimes even poison the entire volume of the chamber with rotting products and destroy the entire colony..

On average, active septic tanks consume about 50-70 W for each m2³ working volume. In addition to transfer pumps, the VOC technical equipment set also includes a compressor with a duct system and an automation unit. In general, the rise in price due to electrical equipment can be about 20-25% of the cost of building the chambers themselves, however, due to the intensification of the cleaning cycle, the septic tank can have a smaller working volume, so that additional investments are often fully justified.

Active concrete septic tank: 1 – compressor; 2 – membrane aerator; 3 – cable; 4 – float; 5 – drainage pump; 6 – tubular diffuser (aerator)

Of course, with the forced method of moving wastewater, there is no need to arrange the septic tank chambers in a cascade to ensure the gravitational overflow of the liquid. Thanks to this, the irrational use of the volume of the secondary clarifiers is eliminated and the likelihood of reverse movement of wastewater is excluded. However, electrified VOCs require careful monitoring and maintenance..

Maintenance and care

It would be unreasonable to consider the septic tank cleaning process as completely wasteless. As a result of water clarification in a biochemical way, a certain amount of solid particles is formed that cannot be processed by bacteria. For active anaerobic septic tanks, the share of waste can be up to 3-5% of the total volume of effluent passed through the VOC. For passive septic tanks, this figure is slightly higher and can reach 7-8%.

The disposal of processed sludge is carried out by cesspool machines, but this has to be done less often than when operating a cesspool. At the same time, in active septic tanks, it is possible to use a pump for pumping waste, for example, into a collecting well or compost pit. The suitability of processed sludge for self-disposal or use as fertilizer is determined by the nature of the wastewater and the type of septic tank.

During operation, the septic tank almost never needs to be washed or repaired; structures made of concrete rings have high durability and resistance to seismic vibrations. The only thing that falls on the shoulders of VOC users is the concern for maintaining a diverse and developing microflora, which ensures such a deep wastewater treatment. For these purposes, special preparations have been developed that are introduced according to the instructions either after a long downtime of the septic tank, or regularly for preventive purposes.