Find how to choose a sewage treatment plant. Its principles for treatment and costs. Independent advice. Includes calculators!

Sewage Treatment Plant Ultimate Guide (2021)

This is the ultimate guide to Sewage Treatment Plant.

So if you want to know:

  • How sewage treatment plants work
  • Different types of sewage treatment plant
  • Pros & cons
  • Size of sewage treatment plants
And much more!

You’ll love this new guide.

Let’s get started.

Contents

CHAPTER 1

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.

CHAPTER 2

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.

Example:

Klargester Biodisc Sewage Treatment System | Kingspan | Great Britain

Pros:

This system has been in use since many years
Most builders and installers prefer this system

Cons:

Maintenance cost is high

biodisc

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.

SBR Wastewater Treatment System I Septic Tank Upgrade | Biocell Water

Pros:

High performance
Low maintenance cost

Cons:

System costs more

Non-electric filters

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.

Non Electric Domestic Sewage Treatment Plant | ClearFox Nature | Biocell (biocellwater.com)

BIOROCK, the best Domestic, Residential Sewage Treatment Plant without electricity, zero energy. 25 years warranty! – BIOROCK Sewage Treatment Ltd

Pros:

No electricity required

Cons:

In areas where groundwater is high, non-electric filter cannot be an option

clearfox-nature

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.

Vortex Eco System Sewage Treatment Plant (crystaltanks.com)

Pros:

Easy maintenance
Low-cost

Cons:

High installation cost

Activated Sludge Treatment Plant Vortex

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.

Submerged aerated filter | Sewage treatment plant (premiertechaqua.com)

Pros:

Great for larger populations

Cons:

Constant power supply required

Septic Tank

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

  1. 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.
  2. 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.
  3. Emptying and servicing are required for Sewage Treatment Plants whereas servicing is not required for Septic Tanks.
  4. Frequent emptying is required for Septic Tanks while this is not the case with Sewage Treatment Plants.
  5. 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.
  6. Sewage Treatment Plants are suitable for large developments, commercial establishments and single houses.
  7. 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.
  8. Sewage Treatment Plants have many moving parts and therefore are subjected to wear and tear.
  9. 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.
CHAPTER 3

Legislation

The first step to installing a sewage treatment plant is to understand the legislation behind it.

Let’s understand it in more detail!

Permits: 

The Septic Tank and Sewage Treatment Plants are controlled by the general biding rules. The general binding rules are in place since 2015 and are being currently applied in England. 

You may require a permit from the Environment Agency if the treatment plant is outside of the general binging rules. 

Regulations:

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.

CHAPTER 4

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
  • Appearance

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.

 

Appearance

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.

CHAPTER 5

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. 

CHAPTER 6

Costs

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.

Sequential Test – District

Sequential Test City District - Urban Water

Our Benefits:

  • Immediate communication with council to determine study area
  • Report in accordance with the local guidelines
  • Assess multiple resources of planning information as per agreed with the council
  • Give preliminary advice on potential sites
  • Recommend appropriate site for assessment
  • Allows for one set of consolidated comments/amendments following the first issue of the report 

What we need from you:

  • Site Address
  • Development Plans
  • Local Planner Contact Details

Single House

Sustainable Drainage Systems Single House

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Detailed Hydraulic Modelling
  • Clear Planning Conditions
  • Produce Construction Drawings
  • Providing drawings in .pdf

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Minor Development

Flood Risk Assessment Single House

Key Benefits:

  • First draft in 48 hours with the final report in 20 days (dependent on ea information)
  • Report in accordance with the environment agency and with national policy planning framework
  • Allows for one set of consolidated comments/amendments following the first issue of the report

What we need from you:

  • Site address
  • Development plans
  • Topographical ground levels

Major Development

Flood-Risk-assessment-Major-Development

Key Benefits:

  • First draft in 48 hours with the final report in 20 days (dependent on ea information)
  • Report in accordance with the environment agency and with national policy planning framework
  • Allows for one set of consolidated comments/amendments following the first issue of the report

What we need from you:

  • Site address
  • Development plans
  • Topographical ground levels

Strategic Development

Flood Risk Assessment Strategic Development

Key Benefits:

  • First draft in 48 hours with the final report in 20 days (dependent on ea information)
  • Report in accordance with the environment agency and with national policy planning framework
  • Allows for one set of consolidated comments/amendments following the first issue of the report

What we need from you:

  • Site address
  • Development plans
  • Topographical ground levels

Single House

Drainage Strategy Single House

Key Benefits:

  • The drainage strategy will inform the flood risk assessment
  • Assessment of available geological records or site investigation if available
  • Obtain and review sewer plans
  • Evaluate potential use of suds applicable to site
  • Determine climate change allowances
  • Undertake basic hydraulic modelling of foul and surface water
  • Calculate storage volumes to comply with suds guidance
  • Indicate discharge points and rates for foul water
  • Indicate applicable suds and volume distribution at a principle level
  • Provide single sketch concept of the drainage strategy
  • Allows for one set of consolidated comments / amendments following first issue of report

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Minor Development

Drainage Strategy Minor Development

Key Benefits:

  • The drainage strategy will inform the flood risk assessment
  • Assessment of available geological records or site investigation if available
  • Obtain and review sewer plans
  • Evaluate potential use of suds applicable to site
  • Determine climate change allowances
  • Undertake basic hydraulic modelling of foul and surface water
  • Calculate storage volumes to comply with suds guidance
  • Indicate discharge points and rates for foul water
  • Indicate applicable suds and volume distribution at a principle level
  • Provide single sketch concept of the drainage strategy
  • Allows for one set of consolidated comments / amendments following first issue of report

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Major Development

Drainage Strategy Major Development

Key Benefits:

  • The drainage strategy will inform the flood risk assessment
  • Assessment of available geological records or site investigation if available
  • Obtain and review sewer plans
  • Evaluate potential use of suds applicable to site
  • Determine climate change allowances
  • Undertake basic hydraulic modelling of foul and surface water
  • Calculate storage volumes to comply with suds guidance
  • Indicate discharge points and rates for foul water
  • Indicate applicable suds and volume distribution at a principle level
  • Provide single sketch concept of the drainage strategy
  • Allows for one set of consolidated comments / amendments following first issue of report

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Single House

Foul Drainage Design Single House

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Hydraulic Modelling
  • Treatment Plant Sizing
  • Drainage Field Calculations
  • Providing design in .pdf
  • Clear Planning Conditions
  • Produce Construction Drawings

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Major Development

Foul Drainage Design Major Development

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Hydraulic Modelling
  • Pumping Station Calculations
  • Rising Main Design
  • Provide design in .pdf and .dwg
  • Clear Planning Conditions
  • Produce Construction Drawings

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Minor Development

Foul Drainage Design Minor Development

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Hydraulic Modelling
  • Pumping Station Calculations
  • Rising Main Design
  • Providing design in .pdf and .dwg
  • Clear Planning Conditions
  • Produce Construction Drawings

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Minor Development

Sustainable Drainage Minor Development

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Detailed Hydraulic Modelling
  • Clear Planning Conditions
  • Produce Construction Drawings
  • Providing drawings in .pdf and .dwg

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Sequential Test – City

Sequential Test City Borough - Urban Water

Our Benefits:

  • Immediate communication with council to determine study area
  • Report in accordance with the local guidelines
  • Assess multiple resources of planning information as per agreed with the council
  • Give preliminary advice on potential sites
  • Recommend appropriate site for assessment
  • Allows for one set of consolidated comments/amendments following the first issue of the report 

What we need from you:

  • Site Address
  • Development Plans
  • Local Planner Contact Details

Major Development

Sustainable Drainage Major Development

Key Benefits:

  • Design compliant with Building Regs and local water company
  • Scope of site investigations
  • Capacity Assessments from Water Companies
  • Undertake Detailed Hydraulic Modelling
  • Clear Planning Conditions
  • Produce Construction Drawings
  • Providing drawings in .pdf and .dwg

What we need from you:

  • Site address
  • Site Layout
  • Topographical survey

Zero Carbon Design

Zero Carbon - Climate Change Adaptation

Key Benefits:

  • Complete Cycle Assessment
  • Early Design Optimization
  • Embodied Carbon Calculation
  • Circularity Assessment
  • Carbon Benchmarking
  • Our calculations are based on the LCA standards (CEN/TC350, ISO 218930)
  • It follows the RICS building stages
  • All CO2 values taken from the verified EPD database
  • Compliant with BREEAM UK and CEEQUAL Standards

What we need from you:

  • External Work Layout
  • Initial Drainage Design
  • Site Layouts

Zero Carbon Design

Nature-Based-min.jpg

Key Benefits:

  • Tackle Climate and Nature Crises in Master Planning
  • Example of Using Nature-Based Solutions around the world
  • Utilize the Government’s Research to identify area in which Local and Regional Nature-Based Solutions can be implemented.

Urban Areas

Hydraulic River Modelling - Urban Water

Our Benefits:

  • Models fully approved by the Environment Agency
  • Obtain LIDAR data
  • Produce Hydrological and hydraulic assessment
  • Model Climate Change
  • Site Visit
  • Scope Topographical and measurement survey

What we need from you:

  • Site Location Plans
  • Topographical survey of the site
  • All correspondence with the Environment Agency

Rural Areas

Hydraulic Modelling of River Flow

Our Benefits:

  • Models fully approved by the Environment Agency
  • Obtain LIDAR data
  • Produce Hydrological and hydraulic assessment
  • Model Climate Change
  • Site Visit
  • Scope Topographical and measurement survey

What we need from you:

  • Site Location Plans
  • Topographical survey of the site
  • All correspondence with the Environment Agency

Exception Registration

Flood Risk Activity Permit Exception Application - Urban Water

Our Benefits:

  • Fast assessment on the required exception application
  • Follow up on the Environment Agency
  • Review of your design
  • Review of the contractor’s method statement
  • Assess  the environmental risk for the site

What we need from you:

  • All detailed drawings
  • RAMS
  • Flood Risk Assessment
  • Construction Phase Plan

Standard Rules Permit

Flood Risk Activity Permit Standard Application - Urban Water

Our Benefits:

  • Fast assessment on the required exception application
  • Follow up on the Environment Agency
  • Review of your design
  • Review of the contractor’s method statement
  • Assess  the environmental risk for the site

What we need from you:

  • All detailed drawings
  • RAMS
  • Flood Risk Assessment
  • Construction Phase Plan

Bespoke Permit

Flood Risk Activity Permit Bespoke Application - Urban Water

Our Benefits:

  • Fast assessment on the required exception application
  • Follow up on the Environment Agency
  • Review of your design
  • Review of the contractor’s method statement
  • Assess  the environmental risk for the site

What we need from you:

  • All detailed drawings
  • RAMS
  • Flood Risk Assessment
  • Construction Phase Plan

Sequential Test – Village

Sequential Test Village- Urban Water

Our Benefits:

  • Immediate communication with council to determine study area
  • Report in accordance with the local guidelines
  • Assess multiple resources of planning information as per agreed with the council
  • Give preliminary advice on potential sites
  • Recommend appropriate site for assessment
  • Allows for one set of consolidated comments/amendments following the first issue of the report 

What we need from you:

  • Site Address
  • Development Plans
  • Local Planner Contact Details

Sequential Test – Town

Sequential Test Town - Urban Water

Our Benefits:

  • Immediate communication with council to determine study area
  • Report in accordance with the local guidelines
  • Assess multiple resources of planning information as per agreed with the council
  • Give preliminary advice on potential sites
  • Recommend appropriate site for assessment
  • Allows for one set of consolidated comments/amendments following the first issue of the report 

What we need from you:

  • Site Address
  • Development Plans
  • Local Planner Contact Details

Extensions

Flood Risk Assessment Single House

Key Benefits:

  • First draft in 48 hours with the final report in 20 days (dependent on ea information)
  • Report in accordance with the environment agency and with national policy planning framework
  • Allows for one set of consolidated comments/amendments following the first issue of the report

What we need from you:

  • Site address
  • Development plans
  • Topographical ground levels