Treatment Plants the basics
In this chapter, we’re going to cover the basics of sewage treatment plant.
(Including what it is, how it works and the different stages of sewage treatment plant)
First, what is a Sewage Treatment Plant?
The waste produced from residential properties, factories and industrial firms and other commercial establishments is called Sewage.
This sewage has to be treated before it mixes with any water body to remove contaminants, which otherwise potentially pollutes the entire water body. This is the purpose of a Sewage Treatment Plant.
A Sewage Treatment Plant treats the sewage from the establishment so that it is suitable to be released into the soil or to any water body.
So, how exactly does a Sewage Treatment Plant work?
There are three stages in a Sewage Treatment Plant.
- Primary Treatment
- Secondary Treatment
- Final Treatment
- Primary Treatment
The wastewater and sewage from residential properties and industrial establishments enter the Primary Chamber of the Sewage Treatment Plant.
In this phase, the solid substances from the wastewater and sewage settle down at the bottom of the Primary Tank while all the liquid stays at the top. The scum, grease and oil from the water moves to the surface.
This settled solid is known as sludge.
The liquid from the primary chamber moves the biozone chamber for further treatment.
- Secondary Treatment
Once the solids settle down, the liquid from the primary chamber moves to the secondary chamber where an air pump is used. This pump is used to circulate air around the entire chamber resulting in the growth of aerobic bacteria which helps to get rid of the pollutants or impurities in the water.
Some treatment plants use an air blower to mechanically add oxygen to the water. The air bubbles are blown up through the water using the blower which helps the bacteria to feed and hence speed up the treatment process.
- Final Treatment
The liquid from the second chamber moves to a settlement tank in this phase. This final chamber is known as the humus chamber.
The final solids or impurities that are present in the liquid after the above treatments settle down at the bottom of the tank in this stage, making the water fit to be discharged to a water body subject to the consent of the Environment Agency.
Types of Sewage Treatment Plants
There are different types of Sewage Treatment Plants available. They can be classified into the following.
- Rotating disc system
- Sequence Batch Reactor
- Non-electric filter
- Activated Sludge Process
- Submerged Aerated Filter
- Septic Tanks and drainage fields
Let’s understand them in more detail.
Rotating Disc System
Homes where an off-mains system is required will make use of the Rotating Disc System or the Rotating Biological Contractor(RBC).
These systems consist of one big media disc inside the tank called the ‘biodisc’ where the bacteria lives.
Similar to the process we saw above, the wastewater and sewage from industrial establishments and residential properties enter the primary settlement tank where the solids settle down at the bottom.
The remaining liquid flows to the ‘biodisc’ which is where the bacteria lives and helps in getting rid of the contaminants.
The solids move back to the primary chamber while the liquid flows into another ‘biodisc’ for the next round of treatment.
After this round of treatment, the liquid is discharged from the tank.
This system has been in use since many years
Most builders and installers prefer this system
Maintenance cost is high
Sequence Batch Reactor
Sequence Batch Reactors involve treatment using natural bacteria and oxygen which are fed into the tank using air diffusers and these are well-known for their high quality effluent level.
The wastewater from the establishments move to a primary chamber where solid contaminants settle down. The water then moves to the secondary chamber for further treatment and depending on the system type installed, it is mostly done via an airlift.
The air diffusers aerate the wastewater by supplying oxygen to the bacteria that will help get rid of the pollutants and also help in breaking down solids. This aeration phase is followed by a resting phase, where air diffusers stop and the solids start to settle down at the bottom of the tank.
After the resting phase the effluent is discharged out of the tank and based on the system type, the sludge or solids that have settled down at the bottom of the secondary chamber moves back to the primary chamber using another airlift.
A control panel and a compressor controls the operation of this system. These need to be situated near the tank.
Low maintenance cost
System costs more
As from the name, this filter does not require electricity and works using gravity.
This system also has two chambers – a primary settlement chamber and a treatment chamber.
As with the other systems seen above, the wastewater from the establishments enters the primary tank where solid contaminants settle down at the bottom while the effluent moves to the second chamber through media.
The media is where the natural bacteria lives and helps in cleaning the effluent that passes through it.
No electricity required
In areas where groundwater is high, non-electric filter cannot be an option
Activated Sludge Process
Activated Sludge Process is used in large sewage plants and domestic systems.
This system consists of two chambers – a biozone chamber and a settlement chamber.
As seen earlier, the water from the residential and industrial establishments flows into the biozone chamber where an air diffuser provides the oxygen for the bacteria and keeps the bacteria alive.
These bacteria help to get rid of the contaminants by breaking down the solid particles.
After the effluent is treated, it flows to the settlement zone where suspended solids settle at the bottom before it moves back to the biozone.
The water at the top of the settlement zone gets discharged from the tank.
High installation cost
Submerged Aerated Filter
Submerged Aerated Filter has three chambers – a primary settlement chamber, a secondary treatment chamber and a third settlement chamber.
It uses media and aeration for wastewater treatment.
As with all the other treatment plants, wastewater from residential and commercial establishments enters the primary settlement chamber where solid contaminants settle down at the bottom.
Water from this chamber flows into the biozone – the secondary chamber and this chamber consists of both media and aeration.
The media consists of bacteria which are supplied oxygen via the air diffuser. This media floats in water.
The water flows from the secondary chamber to the third settlement chamber where suspended solid particles settle down at the bottom.
These solid particles move back to the primary settlement chamber though a sludge return.
Great for larger populations
Constant power supply required
The solid contaminants settle down at the bottom in a Septic Tank while the effluent is discharged into a drainage field.
It is important to note that the effluent from the tank still contains a percentage of the original pollutants.
The difference between a Septic Tank and a Sewage Treatment Plant can be seen in detail below.
Septic Tanks vs Sewage Treatment Plants
- There are no mechanical components in a Septic Tank. Hence, it doesn’t require electricity. This is usually not the case with Sewage Treatment Plants wherein it requires permanent electricity to operate.
- A Sewage Treatment Plant has three zones while a Septic Tank has only two zones. The treated water that comes out of the Sewage Treatment Plant is cleaner when compared to the water that comes out of the Septic Tank as Septic Tank only separates the waste while the Sewage Treatment Plant treats the wastewater.
- Emptying and servicing are required for Sewage Treatment Plants whereas servicing is not required for Septic Tanks.
- Frequent emptying is required for Septic Tanks while this is not the case with Sewage Treatment Plants.
- Septic Tanks are suitable for small developments (up to 3 houses max) and single houses where there is a porous texture in the ground for a soakaway.
- Sewage Treatment Plants are suitable for large developments, commercial establishments and single houses.
- Installation, running and maintenance costs are relatively low in Septic Tanks and they require emptying once or twice in a year. Whereas, Sewage Treatment Plants require regular maintenance and power supply.
- Sewage Treatment Plants have many moving parts and therefore are subjected to wear and tear.
- The Septic Tank consists of natural bacteria and these could be potentially killed by the harsh chemicals entering the system. For this purpose, the Septic Tanks must be treated regularly for healthy bacteria to be active in the system and for effective maintenance.
The first step to installing a sewage treatment plant is to understand the legislation behind it.
Let’s understand it in more detail!
You may require a permit from the Environment Agency if the treatment plant is outside of the general binging rules.
The location of the treatment plant and safe distances is guided by Building regulations section H.
Treatment Plant & Drainage Field Guidance
If you answer yes to any of these questions, the treatment plant and drainage field needs to be moved or require a permit.
Choosing a Treatment Plant
Now it’s time to choose your treatment plant
When it comes to choosing the best Sewage Treatment Plant for your establishment, or house there are four important aspects to consider. They are
- Quality of the effluent
- Ease and cost of maintenance and operation
- Ease and cost of installation
Let’s get right into the details.
Quality of the effluent
For a Sewage Treatment Plant to be legally compliant, it must pass the BS EN 12566-3 standard.
The discharge quality has to be maintained by this standard which states that the effluent produced by the Sewage Treatment Plant must meet the following criteria.
BOD5 = not greater than 20 mg/litre
SS = not greater than 30 mg / litre
NH4-N = no greater than 20 mg / litre
Look for the treatment plant to be approved to the BS EN 12566-3 at the number of population equivalent of you development.
Ease and cost of maintenance and operation
The power consumption of the Sewage Treatment is mostly specified in kWh per year. But some may also be specified in kWh per person per year wherein the figure must be multiplied by the number of people the unit is going to service.
The Sewage Treatment Plant is most likely to run 24/7 for 365 days in a year. Hence, it is better to calculate the number of people the unit is designed for instead of the number of people living in the property.
To ensure your Sewage Treatment Plant is compliant with the guidelines of the Environment Agency and to maintain its efficiency, it is important to regularly service your Sewage Treatment Plant either annually or once in six months. Servicing contracts are provided by most manufacturers along with the tank.
Along with servicing, it is also important to desludge and empty your tank for optimum efficiency. The sludge accumulated over time could be a potential threat to the efficiency of the Sewage System if not cleaned in a timely manner.
The frequency of desludging could range from 3 months to 3 years based on the model and the manufacturer.
Depending on the service provider and the plan, costs for annual servicing and desludging could range between £100-£200 each.
Sewage Treatment Plants are prone to buzzing and vibration sounds produced by the air blower inside the Sewage Treatment Plant. Sometimes, when there is a maintenance issue, these plants could emit foul orders. So, it is important to consider all these aspects and warranty for the parts while investing in a Sewage Treatment Plant.
It is also worthwhile to consider the type of alarm used for your Sewage Treatment Plant.
Ease and cost of installation
The tank of the Sewage Treatment Plant can be made using a number of materials like PP, GRP, Concrete, HDPE.
It is also important to consider the excavation size required. If the size of the Sewage System is larger, more excavation space is required and cost incurred could be high.
When it comes to the excavation depth, a tank that requires deep digging can lead to an increase in cost but will consume less horizontal space below the property. A tank that requires shallow digging can lead to a decrease in cost but will consume more horizontal space below the property leading to an issue in case of minimum distance restrictions. In high water table areas, a shallow dig tank could be suitable.
Kiosk type Sewage Treatment Plants are either located in the tank or in a separate housing above the ground. The above ground housings are easy to maintain. Those that are in the tank are relatively difficult to maintain.
Covers range from large to small produced of different materials depending on the type of the loading.
Size of the Treatment Plant and Drainage Field
Now it’s time to show you how to calculate the size of the sewage treatment plant.
We will also see how to size a drainage field for your treatment plant or septic tank.
The size of the Sewage Treatment Plant depends on the size of your property and the number of people living there.
When deciding on the size of the treatment plant, one must decide based on the potential occupancy and not the current number of people living there. This could prove cost-efficient in the long run.
For a commercial Sewage Treatment Plant, one must be familiar with the PE number that stands for Population Equivalent. Population Equivalent represents the number of workers in the building.
Domestic Sewage Treatment Plant (upto 12 people)
The calculation for a single pair of houses is as follows
For a lone house with upto three bedrooms, the calculation should be based on a minimum populace of five people(5P).
For a single house with more than three bedrooms, 1P to be added for each extra bedroom.
To calculate the size of the treatment plant for two houses with three and four houses respectively, there should be a minimum of 11P.
Domestic Sewage Treatment Plant (12 to 50 people)
When there are more than 12 occupants and the Sewage Treatment Plant is required for a group of houses, there is a different calculation to be used.
To allow for the balancing effects on the daily flow of the group of the houses, some reductions could be made.
Multiply it by 0.9 when the total number of occupants in the property is 13 to 25. The P-value must always be rounded up and not rounded down.
Multiply it by 0.8 when the number of occupants is between 26 and 50.
How to size a drainage field for your treatment plant or septic tank?
The first step is to complete a percolation test to BS . We have created a video showing how to undertake it. Make sure the test in undertaken 3 times and the groundwater table is checked.
Now that you have the results of the test. You can input the information into our drainage field calculator and obtain the dimensions of the required drainage field.
Let’s face it:
Construction and maintenance of a sewage treatment plant is one the most important steps to protect the longevity of a sewage treatment plant.
In this chapter let’s look at the costs associated with the different types of sewage treatment plant.
Domestic sewage treatment plants can cost from £1500 to £5000.
For a 6-person treatment plant, prices can range from £1700-£3000.
For an 8-person treatment plant, prices can range from £1900-£3500.
For an average system, installation costs range from £7000-£9000.
A drainage field costs varies from £2000-£8000.
Yearly service cost can incur about £120.