Septic Tank

Overview

Septic tank for small domestic wastewater treatment plant

Category / type: mixed wastewater pre-treatment, decantation, primary sedimentation, clarification.
Application: residential users, restaurants, kennels, breeding farm, fruits and vegetable industries, etc.
Total inhabitants and population equivalents (PT): from 2 to 50.
Normative references and guarantees: EN 12566-1, d.o.p. According to Regulation (UE) n.305/2011.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).


The septic tank is the most common solution for mixed wastewater (grey and black) from civilian settlements. It is essentially a sedimentation basin parallelepiped-shaped or cylindrical.
They are used for water with a high content of settleable solids as kennels, breeding farm, fruits and vegetable industries.
Sedimentable solids, heavier than water, settle on the bottom, meanwhile lighter substances (mainly oil and grease) skim on top. Sediments are degraded by anaerobic digestion processes. Accumulated sludges and floating materials must be removed periodically by qualified companies and transferred into appropriate treatment systems.
The effluent can be dispersed into the ground by drainage trenches (depending by local law), delivered in sewage system or other wastewater treatment plats as activated sludge, constructed wetlands, trickling filters, etc.
Septic tank garanties also a good qualitative homogenisation of incoming wastewater.
Organic substances decomposition takes place by anaerobic bacterias, grown within the sludge digestion zone. They can be introduced by inoculation of biological activators or by pre-digested sludge.
Septic tank may have one or more compartments (usually up to three), where the division improves the efficiency of the separation processes.
Septic tank is an elementary process with no power supply.
In order to remove gas produced by digestion processes (biogas) and avoid odour problems a vent pipe must be provided/installated.

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How It Works

A septic tank may have one, two or three chambers, available in different materials depending on the technical and economic requirements. Usually in multi chambers septic tank, the first chamber is usually at least 50% of the total volume, and when there are only two chambers, it should be two-thirds of the total volume. Most of the solids settle in the first chamber.
Baffles, between the compartment, are aimed to prevent solids and light liquids drag to the effluent. A typical solution is a “T” shape discharge pipe immersed under the water surface.
When biological process are activated, anaerobic bacterias and microorganisms begin to digest sludge in anaerobic conditions, converting it into CO2, CH4 (biogas) and produceing heat.
Laminar and undisturbed flows are usually optimal condition for sedimentation, whereas anaerobic digestion processes are optimized by a fast and intense contact between wastewater and sludge, especially when the flow is turbulent. Treatment efficiency is conditioned by which of the two conditions is predominant depending on characteristics of inlet flow. Degradation of the suspended and dissolved solids begins more quickly with turbulent flow, but by contrast it has a greater dragging of the suspended solids discharged in the effluent.
Proper ventilation pipe must be installed; if buildings are equipped with ventilation columns to the roof, gas can leak by the sewage pipes, otherwise an additional pipe must be connected.
Oversizing does not cause problems: in fact long permanence periods increase efficiency and decrease sludge volume by liquefaction processes.
Septic tank sizing depend on the number of users, on the average consumption per person and the annual average temperature but also on the frequency of purges and on inlet flow characteristics.
Normally septic tanks are distinguished by number of equivalent inhabitant (PE) served equal to 60 gBOD5 per day and a water availability of 150/200 litres per capita per day.
Average treatment efficiency can be obtained with about 48 retention hours.
A septic tank can remove 30-50% of BOD (Biological Oxygen Demand), 40-60% of TSS (Total Suspended Solids), although efficiencies have a great variability, depending on pollutions concentrations input, climatic conditions and pattern flow.

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Operation and maintenance

A septic tank should be started up inserting sludge from similar plant or by adding biological activators.
Aggressive chemicals must not be conferred in septic tank in order to avoid any efficiency decreasing up to the complete stop.
It is important to monitor sludge accumulation in the tank; purging is required when 1/2 to 2/3 of the total depth between the water level and the bottom of the tank are occupied by sludge and scum. Excess of sludge reduces residence time up to a few minutes worsening performance.
The cleaning operation must be done by a licensed and specialised contractor; sludge must be dispose to suitable treatment plants.
Septic tanks should be checked regularly to ensure watertight. During the cleaning operation all light substances and coarse materials should be removed; it’s recommended to leave part of the digested sludge on the bottom in order to accelerate the recovery of biological reactions. When cleaning operation is finished, the tank must be filled with clean water up to the exercise level to supply the medium necessary for sedimentation/floating processes.

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Regulatory framework

The European standard EN 12566-1 “…specifies the requirements for prefabricated septic tanks and ancillary equipment used for the partial treatment of domestic wastewater for a population <=50 PT” (PT definition)
The identifier parameter of a septic tank is the nominal capacity (CN) that represents an integer in cubic meters (3.3); the classification is by preferential dimensions from a minimum of 2 m3 with steps of 1 m3.
The legislation specifies that requirements of hydraulic efficiency must be verified by test methods described in Appendix B (5.5).
Every product requires a declaration of performance (DoP), as indicated by the EU Regulation 305/2011.
In Europe it is widespread the use of septic tanks for small units wastewater treatment before the ground dispersion.
A septic tank installation is always subject to authorization by institutions.

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Features and benefits

Septic tanks are most commonly used in household. It is possible also use them for small users such as clusters of houses or residential buildings.
The septic tank can be used as a single treatment to discharge wastewater into public sewer or leakage (e.g. where there is no centralized plant or where, due slope problems, it is impossible to connect) or as a primary treatment upstream to a further treatment as activated sludge plant, constructed wetlands, etc.
Septic tank must be subjected to regular maintenance that, during building planning, must be considered to guarantee the accessibility for a vacuum truck for cleaning operation.
Septic tanks can be installed in every type of climate (in particular if underground), although efficiency decreases with temperature.

Advantages (benefits)

  • treatment simplicity;
  • no power supply;
  • small treatment volumes;
  • little space required (underground installation);
  • low operating costs;
  • long service life.

Disadvantages

  • low efficiency;
  • periodic maintenance depending on the use, by authorized companies;
  • suitable for small residential areas and with low water table.

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Products in category Septic Tank

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