Concrete septic tank is one of the simplest form of on-site treatment technologies, which allows to undergo anaerobic digestion of organic materials of wastewater coming into the system. Effluent from septic tank is infiltrated into ground using soak pit/leach field or is sent to sewer line for further treatment. A properly designed, constructed and operated septic tank can treat wastewater to produce an effluent suitable for discharging without violating environmental health. Septic tank can be built using brick, concrete or stone. These days readymade/precast septic tank made up of fiberglass, plastic, cement concrete is also available in the market.
Table of Contents
Key Features of Concrete Septic Tank
A concrete septic tank should have following three key characteristics:
- It should be watertight
- It should consist of at least two chambers separated by a baffle wall
- It should be assembled with soak pit or provided with outlet
Working Principle of Concrete Septic Tank
Septic tank is a primary treatment unit, which is installed as a stand-alone structure to treat household wastewater and is prior to the secondary treatment then it passes out to sewage. During the storage, the heavy solids settle down at the bottom of the tank and the lighter solids float above liquid. During the storage period, anaerobic digestion takes place inside it and biodegradable solids are converted into sludge and gases. In this way, significant amount of pollution is reduced and deposited in the form of sludge. Liquid part is then conveyed to soak pit or sewerage network or secondary treatment plant.
The removal efficiency of septic tank depends, mainly, upon the hydraulic retention time and temperature. Normally, septic tank removes 30 – 50 per cent of BOD (Biological Oxygen Demand) and 40 – 60 per cent of TSS (Total Suspended Solids).
Key Components of a Septic Tank
A septic tank consists of seven main components:
- Inlet
- Outlet
- Manhole
- Liquid retention tank
- Baffle wall
- Soak pit
- Vent pipe
The soak pit is also an integral part of the standard septic tank though is a separate structure. Let’s discuss these parts in detail.
Inlet Chamber
Inlet is simply a chamber with cover, which allows wastewater in. Generally, it is located at center along the breadth of septic tank and it should be at the level of 100 -150 mm above the outlet level in order to avoid clogging the diameter of the Inlet should not be less than 100 mm. It consists of down pipe inside the septic tank.
Outlet Chamber
Outlet is similar to inlet chamber, which is provided at the end of the septic tank. Generally, it also is located at center along the breadth of septic tank. The level of outlet chamber should be minimum 1.2 m from bottom of septic tank. Normally the diameter of the Inlet should not be less than the inlet pipe. A down pipe is also provided at starting point of outlet inside the septic tank.
Manhole
Manhole is provided for easy access during desludging and to inspect inside the septic tank. Manhole should be well covered and the weight of the lid should be lighter enough to be opened easily. At the same time, it should be strong enough to bear a weight of an adult person.
Hydraulic Retention Tank
This is a major component of septic tank. A baffle wall separates Hydraulic Retention Tank (HRT) into two compartments. LRTs collect and store sewage till the hydraulic retention time. During the storage of sewage, heavy particles settle down and anaerobic digestion of organic part takes place inside the liquid retention tank. The collected sewage stores in three layers, namely sludge, liquid and scum.
Sludge Layer:
Heavy solids (heavier than water) settle down at the bottom of the septic tank and forms a layer. Over the time, the biodegradable solids accumulated at bottom undergoes into anaerobic digestion. Sludge is generated during digestion and deposits at bottom and forms a sludge layer.
Scum Layer:
The light-weight solids (lighter than water) such as grease, oils, soap films and other solid waste float above the liquid and forms a layer which is called scum layer.
Liquid Layer:
The layer in between the sludge layer and scum layer is termed as liquid layer. It holds liquid portion of sewage, which contains microscopic biodegradable and non-biodegradable solids suspended in the liquid. Anaerobic digestion process takes place in this layer as well. During the process, biodegradable suspended solids are digested and settle down at bottom. Thus, treated/clearer liquid is conveyed to the soak pit or sewerage network.
Baffle Wall
Baffle wall is a component that divides liquid retention tank into two compartments. It is provided at the 2/3rd of length from inlet side. Series of holes are provided in this wall and size of holes is minimum dimension of 100 cm diameter or square. Generally, the holes are provided at the height above the half of the liquid depth.
Vent Pipe
Anaerobic digestion process produces poisonous, inflammable, and foul gases. A vent pipe inserted in the septic tank allows safe exit to gases. Vent pipe also prevents smell entering to the toilet via conveyance system. The vent pipe should be erected with the minimum height greater than an average height of a person.
Advantages and Limitations of Concrete Septic Tank
Though there are several on-site sanitation technologies, concrete septic tank is commonly used because of its several advantages compared to other on-site technologies. However, it has few limitations as well. Some of the most common advantages and limitations are listed below:
Advantages of Septic Tank:
- Since septic tank is a water sealed treatment technology and effluent discharged out through soak pit or leach field resulting less environmental or ground water pollution.
- Though septic tank needs regular desludging but it requires less other maintenance.
- Depending upon the material used to install and usage, it lasts for more than 20 years.
- It is an excellent onsite options especially for the areas with no or/and unfeasible sewerage networks.
- Septic tank can be constructed in situ using locally available materials (brick, concrete or stone) and human resource. These days there is a provision of buying readymade (made up of concrete, plastic, and fiberglass). Comparatively readymade septic tanks are cost effective and faster to install.
Limitations of Septic Tank:
- Since a septic tank is biological treatment technology, traces of chemicals like toilet cleaner, acid can be harmful to microorganisms (which helps to maintain the eco-system to treat wastewater) those cause the failure of septic tank.
- Since the septic tank is a structure with fixed capacity and piping systems, non-degradable or solid, that takes time to degrade, materials such as tampons, sanitary wipes, too much toilet paper may cause clogging of the septic tank.
- If the septic tank is not properly built, operated and maintained, it may cause bad odor, overflow, frequent clogging and untimely desludging.
- In case of flood prone or water logging area, especial attention should be paid while designing so that unwanted water does not enter from outside the tank.
- If the underground water table of the location is high, the soak pit cannot absorb the effluent.
Design of Concrete Septic Tank
Design of concrete septic tank is a process that determines internal dimensions of septic tank based on the prescribed design parameter. The dimensions should be large enough to retain sewage produced from particular number of users for anticipated time (i.e. Hydraulic Retention Time). It should also provide sufficient opportunity to settle and float solids. Design defines the treatment efficiency of septic tank.
The objective of concrete septic tank design is to determine the volume and hence dimension of septic tank.
Design Principles
Septic tank design principles are:
- Provide sufficient hydraulic retention time to settle down the heavy solids of sewage and stabilization of liquid.
- Ensure stable quiescent hydraulic conditions for efficient settling and floatation.
- Provide adequate volume of septic tank for accumulation of sludge and scum for desludging period.
Design Parameters
Design parameters involves qualitative and quantitative factors that decide a design. It determines cost, size, working efficiency and risk related to the septic tank. Following are the basic design parameters of a concrete septic tank:
- Volume of wastewater
- Hydraulic retention time
- Desludging period
- Temperature
These parameters impacts design of septic tank in following ways:
Incoming volume of wastewater ~ higher incoming volume increases size of the septic tank.
Hydraulic retention time ~ longer retention time increases size of septic tank and treatment efficiency.
Temperature ~ Anaerobic digestion is more effective in higher temperature. Thus, treatment efficiency of septic tank increases with rise in temperature.
Desludging Frequency ~ Longer desludging period demands bigger size of septic tank.
Design Steps
Step 1. Determination of Wastewater Volume
Volume of wastewater can be determined using two methods i.e. theoretical calculation and actual measurement. In actual measurement, volume of wastewater is determined by measuring existing flow of wastewater at outfall. Generally, the accumulated volume of wastewater of 24 hours is considered as the total volume of wastewater production per day. In theoretical calculation, volume of wastewater is calculated based on the numbers of users (n) and specific water consumption (q).
In general, certain amount of total water consumption is considered to be converted into wastewater, which is known as conversion factor (fc). The conversion factor is expressed in percentage and the consideration may vary country to country. The volume of wastewater flow is calculated using the following formula.
Volume of Wastewater = Q = n x q x fc (m3/d)
Step 2. Determination of Peak Flow
Flow of wastewater does not remain same all the time and is fluctuated based on the water consumption. Water consumption is generally higher in morning, midday, and evening due to various activities such use of toilet, food preparation, bathing, face washing, brushing, utensil cleaning etc. Therefore, these periods are known as peak hour (tPEAK) and the flow that takes place within this period is called peak flow (Qp). Peak flow is determined based on the volume of wastewater and duration of peak flow (tPEAK) as follows:
Peak Flow = Qp = Q/tPEAK
Step 3. Determination of Liquid Volume
As wastewater enters into septic tank, separation of heavier and lighter particles takes place forming sludge, scum and liquid layer. The volume required for the liquid layer can be determined using following formula:
Liquid volume (VL) = Qp x tHRT
Step 4. Determination of Sludge and Scum Volume
Volume of sludge and scum depends upon the number of users (n), sizing factor (fs) and sludge and scum accumulation rate (Vsa). Value for Vsa varies depending upon the type of wastewater coming into the concrete septic tank whether it is black water only or black water and grey water combined. Similarly sizing factor depends on temperature (T), desludging period (t). The sizing factor decreases with increase in desludging period and reduction in the temperature.
If we look at table, the minimum value of the sizing factor is 1 for the desludging period 2 or more than 2 years regardless of temperature differences. The sizing factor decreases with increment of desludging period and. However, it remains unchanged for more than 6 years. (See table). Formula suggested by Pickford (1980) for calculation of sludge and scum volume is as follows.
Sludge and Scum Volume = Vs + Vsc = n x fs x Vsa x t
Step 5. Determination of volume of Septic Tank
Volume of concrete septic tank is calculated as the sum of liquid volume (VL), sludge volume (Vs) and scum volume (Vsc).
Volume of Septic Tank = V = VL + Vs + Vsc
Step 6. Sizing of Septic Tank
Sizing of septic tank is the process of determining its dimensions (length, breadth, and liquid depth). Liquid depth is generally considered between 1.2 to 2.2 m depending upon the land area and practicality. The minimum liquid depth of 1.2 is essential for anaerobic digestion process while water depth higher than 2.2 may create practical hassles during operation and maintenance. Ideally, length to breadth ratio of concrete septic tank should be 2-4. Once we have the liquid depth and length to breadth ratio, sizing can be done using following formula.
Volume of Septic Tank = V = L x B x H
Where, L = (2-4) B and H = 1.2 -2.2 m
Design of Concrete Septic Tank Example
Design a concrete septic tank to treat wastewater from an institute having number of users 50. Consider the specific water consumption is 100 lpcd (liters per capita per day) and desludging period is 2 years.
Given parameters:
Number of users (n) = 50
Specific water consumption (q) = 100 lpcd
Average temperature (T) = 20°C
Chosen parameters:
Hydraulic retention time (tHRT) = 24 h
Peak hour (tPEAK) = 12 h/d Conversion factor (fc) = 80%
Type of wastewater to be treated is combined.
Step 1: Determination of wastewater volume
Volume of Wastewater Q = n x q x fc
Q = (50 x 100 x 0.8) / 1000 = 4 m3/d
Step 2: Determination of Peak Flow
Peak flow Qp = Q/tPEAK
Qp = 4/12 = 0.33 m3/h
Step 3: Determination of Liquid Volume
Liquid volume VL = Qp x tHRT
VL = 0.33 x 24 = 8 m3
Step 4: Determination of Sludge and Scum Volume
Sludge and Scum Volume Vs + Vsc = n x fs x Vsa x t
From the matrix (See table 1), fs for temperature 20°C and desludging period 2 years is 1 and Vsa = 40 liters per person per year for combined (black water and grey water) wastewater.
Vs + Vsc = 50 x 1 x 40 x 2 = 4 m3
Step 5: Determination of Volume of Septic Tank
Volume of Septic Tank V = VL + Vs + Vsc
V = 8 + 4 = 12 m3
Step 6: Sizing of Septic Tank
Volume of Septic Tank V = L x B x H
Let’s assume liquid depth septic tank = H = 2 m and ratio of Length to Breadth is 2
12 = 2B x B x 2
B = 1.73 m
Take, B = 1.75 m
Length of septic tank L = 2B = 2 x 1.75 = 3.5 m
Since Length of first compartment of septic tank (L1) = 2/3 of total length (L)
Length of first compartment L1 = (2/3) x 3.5 = 2.33 m
Take, L1 = 2.3 m
Length of second compartment of septic tank (L2) = 1/3 of total length (L) or L – L1
Length of second compartment L2 = 3.5 – 2.3 = 1.2 m
Construction of Concrete Septic Tank
Most of the concrete septic tanks are not being properly constructed due to the lack of technical knowledge and the not considered to have a proper septic tank is an important issue. Proper construction including site selection not only increases the work capacity of the septic tank, but it also contributes on cost effectiveness.
Site Selection
Following factors should be considered while selecting the location for septic tank:
- Groundwater flows from the higher water level to the lower water level and the groundwater circulation is according to the contour of the ground surface. Therefore, the concrete septic tank should be located at the lower level of the water source to prevent the potential pollution chances.
- As septic tanks require regular desludging, it should be in an accessible location for desludging service. The most suitable place for desludging is the front yard or the side of the building.
- Water discharge from the concrete septic tank is sent to the soak pit for infiltration into the ground. Therefore, the soil texture of the site should be permeable.
Points to Ponder during Construction
The following points will help the mason and the owner in properly constructing the septic tank.
- Concrete septic tanks must be properly sized or designed based on the number of users avoiding over size and expensive.
- Excavations for the concrete septic tank must be large enough to work around the septic tank so that it becomes easy during bricklaying and shuttering for concreting. Generally, 50 cm of space should be provided around the structure.
- Compaction must be done properly.
- Minimum thickness of stone soling should be 150 mm and it should be on edge in case of brick soling.
- In the case of concrete structure, concrete with a ratio of 1: 1.5: 2 or 1: 2: 4 should be used.
- Clear height between liquid level and septic tank cover should be at least 30 cm.
- Inlet should be higher than outlet.
- Septic tank should be provided with ventilation pipe and the height of pipe should not be lower than 2 m.
Cleaning/Desludging of Septic Tank
Desludging is crucial activity that affects efficiency of septic tank and infiltration rate of soak pit as well. Technically, designer based on the design consideration provides desludging period. Higher the period increases, the volume of septic tank, and the annual desludging is practically annoying to owner and increases the operating cost. Therefore, desludging period is generally taken as 2-3 years.
Sludge accumulation must not exceed 1/3rd volume of septic tank. Because, sludge accumulation will be about 1/3rd of septic tank volume during the design period. However, it may vary depending upon the type of wastewater coming into it. So, it is required to inspect or check sludge depth before 6 months of design period and should be emptied if the volume of sludge is about 1/3rd of septic tank volume.
Sludge accumulation in the septic tank should be emptied timely because further accumulation of sludge beyond its designed capacity will be flushed out of it and enters into soak pit, which may create chocking of soak pit.
Soak Pit
Structurally soak pit is a separate entity. However, it is an integral part of the standard septic as the effluent from septic is safely discharges into ground via soak pit. A soak pit is a lined, generally circular in shape, pit with cover and porous or honeycomb wall. The pit is filled with brickbats or brick ballast or stone aggregates of size 50 – 100 mm as media up to the inlet level.
The unsealed surface and media facilitates to spread incoming flow into ground and provides support to protect from collapse of porous wall. The distance between soak pit and water sources should not be less than 4 m vertically and 30 m horizontally to minimize risk of biological contamination in water body.
Advantages and Limitations of Soak Pit
Advantages:
- It is a simple technology which can be built and repaired with locally available mason and materials.
- Small land area is required.
- It is cost effective technology i.e. low investment and low operating cost.
- It recharges groundwater bodies.
Limitations:
- At least primary treatment is required to prevent clogging the filling media.
- It may affect soil negatively and might contaminate groundwater depending upon the type and concentration of inflow.
- It is not feasible for cold climate as there is chance of freezing the pores.
- It is not feasible for location having impermeable soil layer.
Site Selection for Soak Pit
Following factors should be considered during the site selection for soak pit:
- It should be at least 1 m away from the outlet of septic tank and is recommended that it should not be too far. Longer distance cost high.
- The site should be at least 4 m away vertically and 30 m horizontally from water sources like dug well, spring etc.
- Technically sloppy, elevated terrain, sunny area is preferred for septic systems because these locations have no chance of water logging even during rain and it dries out in a short time.
Design of Soak Pit
Purpose of soak pit design is to determine the surface area of soak pit that can absorb particular volume of water coming into it. And absorption capacity of soak pit is determined also by the infiltration capacity of soil.
Design Principles of Soak Pit
Soak pit design principles are:
- To provide sufficient surface area to percolate water coming into it.
- To ensure uniform distribution of water over the surface for efficient percolation and to avoid choking due to concentrated application.
Design Parameters
The parameters of soak pit are:
Incoming volume of water:
Higher incoming volume increases size of the surface area of soak pit.
Percolation rate:
Higher percolation rate increases faster drying up of soak pit.
Design Steps of Soak Pit
Step 1: Determine Inflow
Volume of incoming effluent is considered as equal as the incoming volume of wastewater of septic tank.
Step 2: Determine Wall Surface Area of Soak Pit
Surface area (As) of soak pit is determined putting the value of incoming effluent volume (V) and the infiltration rate (IR) of soil in the following formula.
Surface area = As = V/IR
Infiltration capacity is the maximum rate at which soil can absorb fluid and it depends upon factors such as grain size. It tends to decrease as the saturation level of soil increases. Infiltration capacity varies from place to place due to dissimilarities of soil. Therefore, designer should have information on infiltration capacity of site for the design of soak pit. Following values can be used if the data on infiltration capacity is not available.
Step 3: Sizing of Soak Pit
Depth and diameter of soak pit is calculated using the surface area determined above.
Surface area of soak pit = As = 2л RH
From the equation, either depth or radius of soak pit is assumed and dimensions of pit are calculated.
Design of Soak Pit Example
Design a soak pit to dispose effluent from septic tank.
Given parameters:
Flow of effluent n = 4 m3/d
Chosen parameters:
Infiltration capacity of site IR = 120 ltrs/m2
Step 1 Determination of flow
Volume of effluent = V = 4 m3/d
Step 2 Determine wall surface area of soak pit
Surface area As = V/IR
As = 4X1000/120 = 33.33 m2
Step 3 Sizing of soak pit
Surface area of soak pit As = 2л RH
33.33 = 2 X 3.14 X R X H
Assuming, depth of pit H = 2 m.
Therefore, Radius of pit R = 1.9 m
Want to Read More?
Reinforced Concrete Pipes | A Comprehensive Guide