Domestic septic tanks are the stage that takes the household wastewater of a single house and treats it on the plot, where there is no mains drainage. The catalogue line consists of biological treatment units in a single vertical body: the wastewater passes in turn through several zones inside it, the organics are oxidised by the microflora living in the unit, and the treated water is separated from the sludge and goes to discharge or to soil polishing.
Principle
The unit is a single vertical body set into the ground. Wastewater reaches it by gravity along the outfall from the house, and inside the body the zones are arranged one after another, each with its own job. No separate tanks are dug on the plot: a vertical design has a small footprint. The price for that is the depth at which it sits, and it is the depth rather than the body itself that decides the earthworks.
In the lower zone the oxygen content is low. Heavy matter settles here, and denitrification and primary biological reduction take place: some of the nitrate formed further along the path is converted into gaseous nitrogen and leaves the water altogether. The same zone serves as the receiving one — it is here that the surge arriving from the house within a few minutes is absorbed.
In the aeration zone fine-bubble aeration, biomedia and active microflora oxidise the organics and the ammonium. The air is supplied by a compressor, and that is the only part of the unit which runs continuously. A direct consequence follows: the biological part of the treatment exists exactly as long as there is electricity and a steady inflow of wastewater for the microflora to feed on.
In the clarifier the treated water is separated from the activated sludge — the flocs of biomass in which the treatment actually happens. The sludge stays in the unit, the water goes to the outlet. The separation works on the difference in settling rates and is therefore sensitive to the flow rate: the faster the water passes the clarifier, the less time the sludge has to settle and the more of it is carried out.
The governing quantity for the whole chain is the time the wastewater spends inside the body. Capacity in litres per day describes that only indirectly: at a steady inflow the time is sufficient, at a surge it is not. Not a single part fails in the process, and outwardly the unit runs as usual; what diverges is only the result at the outlet.
Principle
The word «septic tank» is used in everyday speech for two different structures, and which of them stands on the plot decides the whole of the rest of the scheme. A classic septic tank is a settling chamber in which the wastewater separates and the organics ferment without access to oxygen. It removes the bulk of the suspended solids and part of the organics, but the water leaving it is not sent straight into the ground: soil polishing in a drainage field or a soakaway is obligatory, and without it the structure does not solve the problem at all. The catalogue line is arranged differently — it is biological treatment with aeration and activated sludge. For the owner the conclusion is the same in both cases: the question of where the water from the outlet will go is settled on the plot before the purchase, not by the name of the structure.
The governing quantity in sizing is not the number of occupants in itself but the time the wastewater spends inside the body. A surge is therefore harder on the unit than an even daily flow of the same size: a bath, a washing machine and a dishwasher emptied at once push the water through the zones faster than the sludge can treat it, and turbid water with sludge carry-over appears at the outlet. The daily capacity is formally not exceeded — that is exactly what the typical sizing mistake looks like.
Spare capacity here does not work the way it does in other equipment. A unit taken with twice the margin «to be on the safe side» turns out to be permanently underloaded: the activated sludge is short of food, the biomass does not build up to the design figure, and the quality of treatment becomes unstable. A sensible margin is one step of the line, not two, and it is taken on a specific indication: the calculation has landed on the boundary, the house is fitted out for the upper limit of water use, an addition to the family is expected.
Practice
The problem is solved on the owner's own plot: no mains connection is needed, and there is no need to cart away the whole volume of wastewater. The difference in orders of magnitude shows in the arithmetic — a house with four occupants at 200 l per person produces some 290 m³ of wastewater a year, while the vacuum tanker takes only the settled part accumulated in the body.
No reagents are dosed and none are stored on the plot: the treatment is biological, and nothing but air is supplied to the unit. What remains among the running items is the electricity for the compressor and the periodic emptying of sludge.
The footprint is small — the design is vertical, in a single body. The price of that compactness lies underground: depth is required, and with it a check of the groundwater level at its seasonal maximum.
The dependence on electricity here is a matter of principle rather than of engineering. Aeration runs continuously, and a long outage puts the unit into an oxygen-free regime; once the power is back the quality of treatment does not return at once — the biomass needs time, not just a switch.
The SOLVIA Bio line carries our own brand, but the subject of our work is not the design of the body: it is the load calculation from occupants and surge, the scheme for taking the treated water away on that particular plot, the choice of model, supply, installation, commissioning and service in Georgia.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
A septic tank is never a stage on its own. Ahead of it stands the internal drainage of the house: an outfall laid to a fall, a soil vent pipe, rodding points. The working of the unit depends on that no less than on the model chosen — the wastewater has to arrive by gravity and evenly, otherwise the unevenness of the inflow is created inside the house already.
Downstream of the unit comes what shapes the scheme more strongly than the body itself — the point at which the treated water is taken away. In permeable soil that is a soakaway or a drainage field; in heavy soil and at a high groundwater level, a filter cassette or a raised mound above the water; where there is neither, a holding tank emptied by tanker. The choice is made on the soil and on the seasonal maximum of the water, not on the owner's preference.
Gravity discharge requires the receiver to lie below the outlet. Where it does not, a pump is added to the scheme, and with it a second dependence on the power supply and one more item to service. This is settled by the levels of the plot rather than by the choice of model, and it is established before the purchase rather than after it.
Kitchen grease is a scheme question in its own right. It rises in the first zone and accumulates, and once it travels further it blinds the soil polishing: a clogged drainage field is restored by excavation, not by washing. A grease trap on the kitchen outlet deals with it ahead of the unit rather than after it.
Operation
Energy
There is one continuous consumer here — the aeration compressor. It runs without stopping, and the biological part of the treatment depends on it: without air the aeration zone ceases to be an aeration zone within hours. Neither the compressor rating nor the daily electricity consumption is given in the data of the line for any of the six models, so the figure is worked out from the data sheet of the particular compressor rather than derived from the number of users. A second consumer appears only in a scheme with pumped discharge: the outlet pump runs in short bursts and takes an incomparably smaller share of the daily balance. A long power cut is not the stopping of a mechanism but the stopping of biology: once the supply is restored the unit does not return to its previous quality instantly.
Consumables
What goes to drain
What is taken out of the water does not disappear; it accumulates inside the body as sludge and surplus activated sludge. It is removed by vacuum tanker, and that is the only waste stream leaving the plot. The frequency is decided by watching the unit rather than by the calendar: it depends on the actual load, on how much a particular model holds and on what goes into the drains. The second stream is the treated water at the outlet: it is not disinfected, and where it goes is decided by the scheme and the agreement, not by the data sheet of the unit. The third stream is not really waste at all, but it always appears: wet wipes, sanitary products and building debris do not break down biologically in the slightest, accumulate inside and are taken out only during servicing — together with the cost of that servicing.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
The answer depends on which structure is installed and on what the plot allows. A classic septic tank is a settling chamber, and soil polishing after it is obligatory: without a drainage field or a soakaway it does not solve the problem at all. The biological treatment units the catalogue line is made of produce water of a different quality, but even after them the question of where it goes remains: the receiver may be a soakaway, a drainage field, a filter cassette in heavy soil or an agreed discharge point. This is decided by the soil, the groundwater level and the agreement rather than by the type of structure, and it is settled before the purchase.
The compressor stops and the aeration zone stops receiving air. Mechanically the unit comes to no harm: the wastewater keeps arriving and separating — but the biological part of the treatment ceases, since microflora that lives on oxygen is suppressed within hours. Once the supply is back, quality does not return instantly: the biomass needs time, and the longer the break, the longer it takes. Short cuts of a few hours are harmless in that sense, cuts of several days are not, and on plots with an unreliable supply this is allowed for at the scheme stage.
Emptying is a routine operation rather than a sign of a fault: sludge and surplus biomass accumulate constantly and have to be carted off. The frequency depends on the actual load, on how much a particular model holds and on what goes into the drains: a unit that receives wet wipes fills up faster. What to go by is observation and the servicing routine, not a date in the calendar. The orders of magnitude bear no comparison with the volume of wastewater: a house with four occupants at 200 l per person per day passes some 290 m³ of water a year, and only the settled part is carted away.
Everyday detergents, yes — the unit copes with those. Two other things are harder on it: a surge of chlorine-based chemicals, a bucket of bleach poured away at once or the water after a disinfection, and pharmaceuticals. Both act on the microflora directly rather than being diluted: the unit is a domestic one, the zones are small, and the concentration inside turns out to be high. Recovery of the biomass after such a discharge is measured in weeks. The limit here is not a ban on chemicals but on the amount of them at one go.
For biological treatment that is a hard regime. The microflora lives on a continuous inflow of wastewater: five days a week without load it still tolerates, half a year of standing idle it does not, and after the winter the unit comes back into service from the beginning. If the house is used in bursts, it is more honest to consider a holding tank emptied by tanker: it depends neither on the pattern of occupation nor on electricity, and the cost moves from the equipment to the emptying. Choosing between them is choosing between a one-off and a recurring cost, and it is done on the actual regime rather than on plans for it.
No. There is no disinfection in the unit at all: biological treatment does not remove pathogenic micro-organisms, and the water at the outlet remains wastewater, treated though it is. Its purpose is to be taken away by the scheme, not to be reused. If reuse really is required, that is a separate task with a separate disinfection stage and a separate agreement, and it is not solved by the choice of septic tank model.
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