Foul flow calculations in a Foul Sewage Assessment
Population equivalent, peak factors and infiltration decide whether your foul connection is accepted. Here is how the calculation is built and what the undertaker does with it.
Work out the peak foul flow before you approach the sewerage undertaker. A foul sewage assessment flow calculation starts with a population equivalent, applies a peak factor and an infiltration allowance, and ends with a figure in litres per second that the undertaker can test against the receiving sewer. Get that number wrong and the pre-development enquiry answers a question you never asked, which on a nine-unit scheme can cost you six weeks you did not budget for.
The number has to exist before the enquiry
Sewerage undertakers do not size your drainage for you. They respond to a stated flow. If you send a pre-development enquiry that says "eight houses" without a flow figure, you will either get a generic reply or a request for the calculation, and the clock restarts.
So the calculation sits at the front of the Foul Sewage Assessment, not at the back. Everything after it, the connection point, the need for on-site treatment, the phasing condition, follows from that one figure.
Local validation lists usually ask for foul drainage details on any application creating new dwellings or new non-domestic floorspace. The planning officer is not checking your arithmetic. The undertaker is.
Population equivalent and litres per head
Population equivalent converts a mixed scheme into a single number of notional people. For housing, that means an occupancy assumption per dwelling multiplied by a daily flow per person.
The daily figure is not arbitrary. Part G of the Building Regulations sets a water efficiency requirement of 125 litres per person per day for new dwellings, with an optional tighter requirement of 110 litres per person per day that planning authorities can apply through a local plan policy. Foul flow is not identical to water consumption, because some supply goes to irrigation and evaporation, but the consumption figure is the honest starting point and the one an officer recognises.
Occupancy is where people quietly inflate or deflate the answer. Take the occupancy assumption from the undertaker's own design guidance or from the local housing mix evidence, state it in the report, and show the multiplication. An assumption you can name is defensible. An assumption you cannot is an invitation to argue.
Non-domestic uses work differently. A commercial kitchen, a nursery, a gym and a small workshop all have different discharge profiles, and some produce trade effluent that needs a separate consent from the undertaker under the Water Industry Act 1991 rather than a simple foul connection.
Peak factors and infiltration
Average dry weather flow tells you almost nothing about whether a 150 mm sewer will cope at eight in the morning. The peak is what matters.
Undertakers publish peaking factors in their design and construction guidance. For small populations the factor is high, commonly around six times average dry weather flow, because a handful of dwellings all shower and flush inside the same narrow window. As population rises the factor falls, since usage spreads out.
On top of the peak you add an allowance for infiltration, meaning groundwater that enters the pipework through joints and defects. It is usually expressed as a proportion of dry weather flow or as a rate per hectare of drained area.
| Component | What it captures | Where the value comes from |
|---|---|---|
| Population equivalent | Notional people served | Occupancy assumption times unit count |
| Average dry weather flow | Litres per head per day | Part G consumption figure or undertaker guidance |
| Peak factor | Morning and evening concentration | Undertaker design guidance, higher for small populations |
| Infiltration allowance | Groundwater entering the system | Proportion of dry weather flow, agreed with the undertaker |
| Trade effluent | Non-domestic discharge | Separate consent, quantified per process |
Show the four lines and the arithmetic between them. A reviewer who can follow the sum rarely challenges it.
One more point on infiltration. On a small infill plot with new pipework the allowance is modest, because the pipes are new and the drained area is tiny. On a conversion that reuses an existing private drain of unknown age, the honest position is that the allowance should be higher and the condition survey should say so.
Where the public sewer is not the answer
The foul drainage hierarchy runs in a fixed order, and the drainage hierarchy section of a drainage strategy follows the same logic for surface water. Connection to a public foul sewer comes first. A package treatment plant discharging to a watercourse comes next. A septic tank discharging to a drainage field comes last, and only where the ground and the groundwater position allow it.
Section 106 of the Water Industry Act 1991 gives an owner a right to connect foul drainage to a public sewer, subject to the undertaker's conditions about the mode and point of connection. That right is why "the sewer is a long way away" is a cost argument rather than a planning one. Distance alone does not push you down the hierarchy.
Where you do propose on-site treatment, the general binding rules for small sewage discharges apply, and above their limits you need an environmental permit from the Environment Agency. Councils refuse schemes that assume a package plant without showing why a sewer connection was rejected, and they are right to.
The pre-development enquiry and what comes back
Every water and sewerage undertaker runs a pre-development enquiry service. You submit a site plan, the flow figure and the proposed connection point. The reply normally confirms the sewer records, whether capacity exists, and whether any works are needed.
Three answers come back regularly:
- Capacity exists at the stated point. Attach the letter and move on.
- Capacity exists after off-site improvement, funded through the undertaker's own programme or by the developer.
- No capacity at that point, and the connection has to move, or the scheme has to wait.
Answer three is the one that changes your application. It does not usually stop the scheme, but it does change what the council can lawfully condition.
Allow real time for this. Enquiry responses take weeks, not days, and undertakers will not compress that for a determination deadline.
The reply is also evidence. A validation officer looking for foul drainage details will usually accept a dated enquiry response alongside the calculation, whereas a statement that says "connection to the public sewer" with nothing behind it invites a request for further information.
Trade effluent is worth flagging early on any commercial element. A laundrette, a bakery or a food unit discharges something the undertaker regulates separately, and the consent that governs it is not a planning consent at all. Officers often miss this. Your report should not.
Capacity constraints, phasing and what gets conditioned
Where the network needs reinforcement, the usual planning answer is a condition preventing occupation until the works are complete or a connection agreement is in place. That is a Grampian condition, and it works because the developer can reasonably secure the outcome through the undertaker's own statutory process.
Two other routes matter. Section 98 of the Water Industry Act 1991 lets an owner requisition a new public sewer. Section 104 lets a developer build sewers to the undertaker's standard and have them adopted. Both carry their own timescales and both belong in the programme section of your Drainage Strategy rather than being discovered after permission.
Phasing is the practical fix on larger sites. Fifteen units connecting now and twenty-five after reinforcement is a normal, conditionable outcome. Write the phasing into the flow calculation from the start so the figures in the report match the figures in the condition.
And keep the assumptions visible. If the occupancy, the per capita figure, the peak factor and the infiltration allowance are each stated with a source, the calculation survives a change of case officer.
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Frequently asked questions
What is population equivalent in a foul sewage assessment?
Population equivalent converts a scheme into a notional number of people so that domestic and non-domestic discharges can be added together. For housing it is the occupancy assumption per dwelling multiplied by the number of units. Non-domestic uses are converted using a discharge rate for the process involved. State the assumption and its source in the report.
Do I need a foul sewage assessment for a single house?
Not always. Many councils ask for foul drainage details on any application creating a new dwelling, and the national application form includes a foul sewage question. A short assessment is worth doing where you propose on-site treatment, where the sewer is distant, or where the local validation list asks for one. Check the list before you submit.
How long does a pre-development enquiry take?
Water and sewerage undertakers publish their own response times and these run to weeks rather than days. Submit the enquiry as soon as the unit count and the likely connection point are settled, because the answer can change the scheme. Undertakers will not shorten the process to suit a council determination deadline.
Can the council refuse permission on sewer capacity alone?
Capacity is a material consideration, but the usual outcome is a condition rather than a refusal. A condition preventing occupation until reinforcement works or a connection agreement are in place is common, because the developer can secure that outcome through the undertaker's statutory process. Refusal is more likely where no workable connection exists at all.
When is a package treatment plant acceptable?
Only where a connection to a public foul sewer has been properly ruled out. The foul drainage hierarchy places a public sewer first, then a package treatment plant, then a septic tank with a drainage field. On-site treatment also engages the general binding rules for small sewage discharges, and an environmental permit above their limits.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.