What is septic tank? Design of septic tank

 what is septic tank

Septic tanks are commonly used around the world as the first stage of on-site wastewater management systems As wastewater flows through a septic tank, solids (such as faeces) settle to the bottom to form sludge, and floating scums (e.g., oils and fats) are retained in the top of the tank allowing the liquid component to exit through the outlet This effluent is still highly contaminated and contain high concentrations of faecal microorganisms (such as bacteria, viruses, parasitic worms and protozoa) that can be dangerous to human health. The septic tank should, therefore, be followed by a suitable land application system to provide further treatment and soil infiltration of the effluent. The sludge and scum that accumulates in the septic tank slowly breaks down, but a portion accumulates in the tank and must be periodically removed. This accumulated sludge and scum, known as septage, is normally sucked-out by a septic tank cleaner truck and then disposed of to a proper treatment and disposal facility. This is generally required every 3–5 years, once the sludge and scum occupies half the depth or more of the septic tank.

Typical septic tank

 Purpose

A safe, sustainable, effective and affordable sanitation system is an essential service for the health and well being of all people. This guideline is aimed at rural dwellings, clusters of dwellings, schools or community buildings in Fiji that are not serviced by a reticulated sewer service. Septic tanks are widely used for initial on-site treatment of human waste waters before further treatment and/or soil infiltration.This guideline provides basic information for septic tank construction using reinforced concrete blocks. It is based on the joint Australian and New Zealand standards (AS/NZS 1546:2008 Pt 1 and AS/NZS1547:2008). Prefabricated septic tanks (concrete and plastic) are also available and may be a preferred option providing they are of sufficient quality and suitable size (as outlined in this guideline).

 History

The first known installation of a septic tank in the United States was in 1876, although Louis Mouras of Vesoul, France was given a patent in 1881 and credited with the invention.Baffles, which regulate the flow, were added in 1905 to make the septic tank more efficient. The first baffles were made of oak boards.At the turn of the century, there were some very large community septic tanks. In 1903, four community tanks were constructed in Saratoga,New York, with a total capacity of one million gallons.By 1920, septic tanks began to be a common feature. After World War 11, septic tanks became important to housing developments in unsewered areas.

water supply for septic tank

Flushed toilet systems increase the demand on the village water supply system. An adequate and reliable water supply is an essential requirement before installing flush toilets. For example, the daily volume of water needed for a flush toilet will be about 30 to 40 litres for every person living in the house.Flushing toilets soon become smelly and a serious health risk when water is not available. Other toilet options are available that do not rely on water-flushing.

 Location of septic tank

Careful thought must be given to choosing a suitable site for a septic tank. Septic tanks are normally completely or partially buried in the ground. It is preferable to locate the septic tank close to toilet and completely or partially buried in the ground. It is preferable to locate the septic tank close to toilet that must also be considered are:

1. The septic tank should not be closer than 1.5 m from any building foundation, 3 m from any window opening or tree, and 2 m from any land boundary. Where there are sandy soils that could collapse during construction and undermine building foundations a 3m distance is recommended.

2. The septic tank is to be located to provide the required minimum fall for the drain or drains leading in to the tank. The reason for a minimum fall is to avoid deposition of waste solids in the pipe, risking clogging. Typically, the minimum fall for a 100mm drain is 1:80 (125 mm drop for each 1 m length of pipe), however for details on the required fall refer to Section DF6 in the National Building Code, Fiji Islands, 1990.

3. Do not locate the septic tank in flood prone areas, or where the groundwater is at less than 1 m below the bottom of the septic tank. Avoid areas where water storage tanks overflow or storm water discharges from roofs.

4. Do not install septic tanks where vehicles (cars, trucks) travel or park as they may damage the tank.

5. Ensure the tank is located for good access for periodic sludge removal.

6. Odours from septic tanks are minimized by installing a terminal vent However, it is also sensible to locate the septic tank where the effect of the odours will be minimized.Note that if a septic and outflow pipes are well constructed, are water tight and do not leak, there is minimal risk to groundwater and wells from the septic tank itself. A substandard septic tank that may leak, is a substantial risk if sited too close to wells and other sensitive public facilities.Such tanks should be decommissioned and replaced with a water tight septic tank.

Design of septic tank

Septic tanks can be used for just wastewater from flush toilets (black water) or for the combined wastewater from toilets plus sinks, washing machines, tubs, wash stands, showers and baths (combined black and grey-water). Because of the greater volume generated, larger septic tanks are required for combined black and grey water, and the size must also be adjusted for the number of people in the household using the facilities. Typical daily volumes are 30–40 L/person for black water and 80 to 180 L/person for combined black and grey-water. If taps are leaking or toilet cistern valves broken, the above daily volumes may be significantly higher. Care should, therefore, be taken to fix any leakages to avoid excess burden on septic tanks and subsequent land treatment systems.The recommended size of a septic tank is a function of the volume of wastewater it receives each day.This volume will depend on the number of people occupying the dwelling and whether it receives black water only, or both black and grey water. Well-constructed septic tanks last for a long time and are difficult to alter once in the ground. Therefore, they should be sized for the potential number of people that could live in the house (based on size and number of rooms) rather than just the number of people currently occupying the house.The tanks are dimensioned to provide working capacities based on international best practice codes and standards, as adopted in New Zealand and Australia (e.g., AS/NZS 1547:2012).Table 1 is for a two-chamber septic tank receiving black water only from a domestic dwelling (assuming a daily black water volume of 40 L/house occupant).Table 2 is for a two-chamber septic tank receiving both black water and grey-water from a domestic dwelling (assuming a daily wastewater volume of 180 L/house occupant).

Note that the tank capacities given in these tables refer to “working capacity”. This is the liquid volume within the tank below the outlet invert.For a two-chamber septic tank, the first chamber needs to about 2/3rds of the total tank volume and,the second chamber about 1/3rd of the total tank volume. For black water only, it is sometimes recommended that the first chamber is 75% of the total tank volume.

   Table-1 For Black water only

No of occupant	A(m)	B(m)	D(m)	C(m)	Working capacity in (Liters)
1-5	1.2	0.6	0.9	1.0	1620
6-8	1.6	0.8	1.0	1.0	2400
9-10	1.8	1.0	1.0	1.2	3360

   Table-2 For combined black and Grey water

No of occupant	A(m)	B(m)	D(m)	C(m)	Working capacity in (Liters)
1-5	1.4	0.8	1.2	1.2	3170
6-8	1.6	0.8	1.2	1.4	4030
9-10	1.8	1.0	1.2	1.4	4700