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Foul Drainage Design

From Planning to Construction

Experts in foul water drainage design from pipework to treatment systems and all Statutory Permits

Our foul water drainage design is cost effective for our developers. We use the latest software to design 3D sewer network and MICRO drainage to provide the hydraulic. Our service includes foul water drainage design, pumping stations and sustainable wastewater treatment systems.

Our foul water drainage designs are compliant with building regulations and sewer for adoption. We help developers to get the section 106 sewer connection and section 104 sewer for adoption.

Foul Sewer Drainage Design

See below for our easy pricing scale.

Foul Drainage Design Single House
From £650.00
Single House or Small Warehouse
Less than 250 sqm.
Foul Drainage Design Minor Development
From £850.00
Developments of fewer than 9 dwellings or of industrial units smaller than 3000 sqm.
Foul Drainage Design Major Development
From £1,500.00
More than 10 dwellings or industrial/office units bigger than 0.5 H.
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Our Benefits


Chartered engineers at the heart of the design.


Our designs obtain the relevant approvals from the sewer companies.


We create designs which have the lowest carbon footprint.

No Extra Fees

Fixed price for all of our projects.

Our Foul Water Drainage Design Methodology

We are a team of proactive engineers who work with developers and contractors to create foul water sewer systems that are sustainable and compliant with the latest regulations. We create long standing partnerships with architects, contractors and developers.

urban water location flood risk
Obtain project Details

We utilise mAOD to obtain drawings and levels for your development.

Flood Risk Assessment
Consult Authorities

We consult the Sewerage Undertaker and the Environment Agency to obtain their agreement on the capacity of their network and the specific requirements for approval.

Urban Water Submit Report Flood Risk Drainage
Scope Investigations

We scope CCTV surveys, trial holes, topographical surveys, connectivity surveys and soakaway tests to confirm the basic information necessary to complete our foul water drainage design.

Flood Risk Assessment
Design and Model

We use hydraulic models to design our networks and produce detailed construction drawings. We work together to achieve a technically sound design.

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Other Services

Sustainable Drainage System
We create fully adoptable SuDS designs that are modelled to safely manage surface water run-off.
Drainage Strategy
Drainage report for planning applications fully compliant with national and local regulation.
Flood Risk Assessment
Your planning application can rely on our flood risk assessments. 
Climate Change Adaptation
In order to protect communities, we incorporate green infrastructure and nature-based solutions to Zero Carbon into our developments.
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Frequently Asked Questions

What is the minimum fall for foul drainage?

The minimum permitted slope measurements for foul drainage are:

  • Houses: Slope1:40 with a 100mm pipe with a flow of less than <1l/s (1No.WC, sink and shower). If the flow is greater the fall can be 1:80.
  • 2 to 9 Houses: Slope 1:100 pipe diameter 100mm
  • 10 houses or more: 1:150 pipe diameter 150 mm 
These dimensions are based on building regulations Waste and disposal Approved document H and Sewers for Adoption
A Design and Construction Guide for Developers.
Foul Sewer Design Levels

What is a foul drainage system?

A foul drainage system is the combination of pipes manholes, pumping stations and treatment plants that safely move and treat foul water. Our foul water drainage design includes for all these elements.

What is the minimum depth for a drainage pipe?

The minimum depths for drains without protection should be as follows:

  • 0.35m for domestic gardens or pathways without car loading.
  • 0.50m for domestic driveways and areas where the gross vehicle weight is less than 7.5 tonnes.
  • 0.9m for domestic driveways, parking areas and narrow roads with limited access to gross weight higher than 7.5 Tonnes. 0.9m for agricultural land and public space.
  • 1.2m for other highways and parking areas with unrestricted vehicle access.

How are the foul flows calculated?

To calculate foul flows it is necessary to know which sanitary fittings are connected to a particular drain run. This can generally be calculated in 2 ways:

  • The first method uses discharge units that are applied to each type of sanitary fitting and added together and a calculation is used to determine the peak flow for each section of the drain. The discharge unit method is shown in BS 12056. The pipe size can then be selected that will carry this peak flow whilst maintaining at least the self-cleansing velocity. This method does tend to give relatively high peak flows on the final outfalls from some buildings where there is a high ratio of sanitary fittings to building population.
  • The second method calculates flows in relation to the building population and a daily volume is assigned to each person relevant to the type of building as per the British Water Code Of Practice – Flow And Loads. The usage figures for different types of buildings do tend to differ across various publications.

There are other methods of calculating flows for water authority capacity check applications and for sewers for adoption.

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When to use grease traps?

The initial consultations with the water authorities are:

  • Sewer Plans. These are obtained from the local water authority and will require a marked up street plan to indicate the area you need information for. The results are usually received within a week.
  • Capacity Checks. If it is considered possible that a proposed development will increase flows to a sewer, then it is a requirement to have a capacity check carried out by the local water authority. The process does vary across the different authorities and it usually needs a form to be filled out and returned. The capacity checks can incur a fee from the local water authority.
  • Obtain the baseline discharge flows and the potential for adoption of sections of sewerage systems.
Foul Sewer Design Calculations

What are the consultations with the Environment Agency?

The removal of fats, oils and grease (F.O.G.) should be considered when kitchen waste is being discharged to a drainage system. Careful consideration is needed to decide on the best method of controlling food waste and greases that may be contained within the waste water. The waste management strategy for the kitchen needs to be discussed with the client or kitchen specialist. A flood risk assessment may be required to consult the Environment Agency.

It is standard practice to provide a grease trap close to any appliances producing f.o.g. such as wash-up sinks, dishwashers and some types of ovens. Where possible the trap should be positioned outside the building.

Another method used to reduce grease deposits is enzyme dosing. This process involves injecting grease digesting bacteria into the above ground waste pipework. This solution may be used on its own or in tandem with a grease trap and is usually detailed by the above ground drainage designer.

Sewage Treatment Plant

What are the requirements for pumping stations?

Pumping stations can be expensive to install and can incur running and maintenance costs. Many sites will be suitable for the use of a packaged station and these usually take the form of a prefabricated GRP or plastic chamber encased in concrete. Some larger stations may require the use of large GRP tanks or for chambers to be purpose built in concrete rings.

The main requirements of a pumping station in a foul water drainage system are:

  • A twin pump arrangement should be used and this can be used in a duty and standby arrangement or a duty and assist arrangement.
  • The duty of the pump is based on the incoming flow and what is considered an appropriate outgoing flow to discharge into the receiving drain or sewer. The outgoing flow should be above that of the incoming peak design flow and sufficient to give less than a 15 start stop per hour.
  • When the flow has been decided, it is then necessary to decide on a suitable size for the pumping main. This will be based on maintaining a minimum velocity within the main of 0.9m/sec.
  • Polyethelyne (PE) is the most common material for pumping mains below ground with other options being ductile iron and PVC.
Foul Water Drainage Design
  • It is a requirement of the Building Regulations that the pumping station is able to store the inflow for a 24 hr period in the event of pump failure.
  • The controls to the pumping station will be provided within a control panel and this may be housed within a waterproof kiosk for external areas. Pump control panels should be positioned within close proximity of the pump chamber and within line of sight.
  • Pumping stations that are to be adopted by the local water authority will need to meet the requirements of Sewers for Adoption and will require the appropriate approval and adoption procedures to be conducted.
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Foul Water drainage design, what are Wastewater Treatment Systems?

When the development does not have a sewer connection. It is necessary to consider the requirement for storage or treatment of sewage on site. The usual wastewater treatment solutions to be considered are:

  • Septic Tank
  • Cess Pits
  • Treatment Plants

Septic Tank:

This is a tank that stores the sewage and allows the solids to settle out and the liquid to pass into the ground via a land drainage system. This can only be used where ground conditions are suitable and effluent percolation tests to BS 6297 should be carried out.

Septic tanks are utilised in cases where daily discharge volumes are comparatively low. Septic tanks provide a cost effective solution to low volume sewage disposal and will only require emptying once or twice a year.

The tanks are usually constructed in GRP and will require suitable base and surround details to suit the site ground conditions and loadings.

Sizing of septic tanks in domestic situations is based upon population and the manufacturers literature will provide guidance on the size of tank to use. Sizing of septic tanks for non-domestic situations should be discussed with the manufacturer.

Cess Pits: 

Where ground conditions are not suitable to discharge to ground and daily flows are low it may be appropriate to use a cess pit. For example: places of worship.

These have a high maintenance cost as the waste has to be removed by tanker at regular intervals.

The sizing of cess pits is based upon storing all of the incoming flows from the population served for a suitable period of time, based on the size of tanker that is most economical to use.

Tanks are usually manufactured in GRP and will require appropriate base and surround details to suit site ground conditions and loadings.

Foul Water Drainage Design

Sewage Treatment Plants:

An STP will be able to treat sewage to a defined standard and allow discharge to ground or to a ditch or water course.

 The effluent standard must be agreed in accordance with the EA under a consent to discharge application. Small or medium sized units are usually packaged and fabricated with plastic or GRP tanks with the proprietary treatment system, sized to deal with the proposed flows and consent effluent standard.

There are a number of different methods of treatment, the most common two being Rotating Biological Contactors (RCD) and Aeration systems.

All types are designed to create natural biological action by increasing the flow of oxygen into the effluent. RCD plants have the advantage of low energy consumption and being almost silent in operation.

Aeration plants rely on compressors or blowers providing large amounts of air at the cost of higher energy use and noise generation. Their advantage is that on medium to larger size installations they have a smaller footprint and can more effectively deal with tight effluent standards.

STP’s are sized generally on site population and the biological loadings that this will produce. The size is based on the ‘British Water’ code of practice which stipulates flows, biological loadings and ammonia levels for different types of population usage.

A standard consent is 30 mg/litre suspended solids, 20mg/l BOD and 20mg/l ammonia, but this could be made tighter by the EA if it is considered that the receiving environment could be affected and this will be very much site specific. Detailed guidance on foul drainage and sewage treatment plants is given in BS EN 12566.

The effluent should be discharged into the ground and it will therefore be necessary to carry out ground investigations to determine the soil and sub-strata together with percolation tests. The information required by the EA will increase in line with the volumes that will be discharged from the site or if there are any particular environmental issues attached to the site.

Where sites are situated above aquifers, there may be strict requirements on the quality of effluent and the methods used to dispose of it. The EA may ask for further information during the consideration of the consent and this can lead to delays in the process. The EA agree to determine a consent application within a period of 4 months but this is often extended on difficult sites.

If ground conditions are considered unsuitable to accept the effluent it will be necessary to look at discharge to a water course. This can be in the form of a ditch, stream or river and again the quality of effluent will need to be suitable for the receiving environment and various investigations may be requested by the EA, such as confirming that the watercourse has water throughout the year. For a treatment plant you will need a soakaway test to BS6297.

What is a section S106 or sewer connection?

A Section 106 is a process that water companies have in place to allow a development to discharge into an existing public sewer asset.

S106 connections typically involve the alteration of an existing sewer to create a new manhole or branch. Connection work under S106 does not directly follow the same standards in ‘Sewers for Adoption’, 7th edition, as this standard relates to new build work, and is not appropriate for modification of existing sewers.

What is a section S104 or sewer adoption?

A section S104 is a sewer adoption process that allows a water company to take responsibility for new sewer constructed in accordance with ‘Sewers for Adoption’, 7th edition.

In general sewers are adopted from the demarcation chambers (located within the property boundary) to the discharge point S104.