The sewer capacity enquiry in a foul sewage assessment

The sewer capacity enquiry is the part of a foul sewage assessment that cannot be written from a desk. Everything else in the section, the flow calculation, the connection point, the phasing, is your own work. The capacity position is the sewerage undertaker's, and the only way to get it is to make a pre-development enquiry and wait. A sewer capacity enquiry for a foul sewage assessment takes several weeks to answer, which is why it belongs at the start of a project rather than in the week before submission.

The legal backdrop is section 106 of the Water Industry Act 1991, which gives the owner or occupier of premises the right to connect drains and private sewers to a public sewer. That right is the reason most connections are straightforward. Its exceptions are the reason some are not.

The statutory right to connect and its limits

The right is exercised by giving notice to the sewerage undertaker, which then has a period in which to refuse. Refusal is only available on limited grounds: that the mode of construction or condition of the drain or sewer does not satisfy the undertaker's standards, or that connecting would be prejudicial to the sewerage system.

That second ground does real work. An undertaker faced with a network that already surcharges in storm conditions can and does say that an additional flow would be prejudicial, and the answer is usually reinforcement rather than refusal.

Note what the right does not cover. It is a right to connect foul drainage. It is not a right to discharge surface water to a foul or combined sewer, and undertakers routinely refuse surface water connections where any alternative exists. Keep the two flows separate in the assessment and in the drawings.

Making a pre-development enquiry

Every undertaker runs a pre-development enquiry service for exactly this purpose, and the quality of the answer depends on the quality of what you send.

Include the site address with a red line plan and a grid reference, the number and type of units or the nature of the non-residential use, the calculated peak foul flow, the proposed point of connection with its manhole reference from the sewer records, the surface water strategy in outline, the anticipated start on site and the build programme.

Ask specific questions rather than requesting general comments. Whether there is capacity at the proposed connection point for the stated flow, whether an alternative connection point is preferred, whether any reinforcement is required and over what timescale, and whether the undertaker would object to the application as submitted.

Buy the sewer records first. A connection point picked from a published map without the record plan is often wrong about depth, size and whether the pipe is public.

Calculating foul flows by population equivalent

The flow figure drives the whole exercise, so show the method rather than the answer.

For housing, the conventional approach is either a per-dwelling design allowance, commonly taken as 4,000 litres per dwelling per day at peak for sewer design purposes, or a per capita figure of around 150 litres per head per day with an occupancy assumption and a peaking factor applied. Say which you have used and why, and give the occupancy source.

For non-residential development, use a unit-based rate appropriate to the use: per bed for a hotel or care home, per pupil for a school, per cover for a restaurant, per employee for an office. State the rate and its source.

Then convert to a peak flow in litres per second, since that is the unit the undertaker models in. Include any trade effluent separately, because it is consented under a different regime and an undertaker will want it identified rather than buried in a domestic total.

A Foul Sewage Assessment that gives a peak flow with no working behind it invites the undertaker to substitute its own assumption, which will be the conservative one.

Pumped connections and adoption

Where the site cannot drain by gravity, the scheme needs a pumping station, and that changes the conversation from capacity to adoption.

An adoptable pumping station has to meet the undertaker's design and construction standards. Expect requirements for duty and standby pumps, emergency storage sized for a stated period of failure, telemetry, access for a tanker, an odour control position and a clear boundary from the nearest habitable room.

That last requirement catches layouts. Undertakers apply a separation distance between an adoptable pumping station and dwellings, and a station tucked into a corner of the site next to the plot boundary usually has to move. Resolve it before the layout is fixed.

Where the station will remain private, say so in the assessment and explain the management arrangement, since a council will want to know who maintains it in 20 years.

Combined systems, surcharge and the undertaker response

Much of the older network is combined, carrying foul and surface water in the same pipe. On those networks the undertaker's response often turns on storm performance rather than on dry weather flow.

Read the reply carefully and quote it accurately in the assessment. Undertakers usually write in one of three registers. Capacity is available. Capacity is available subject to a condition or a phasing restriction. Or reinforcement is required and will be programmed.

Where the answer is the third, ask for the mechanism and the timescale in writing. Undertakers have duties to provide capacity, and works can be requisitioned, but the programme is measured in months and sometimes years, and a scheme that depends on it needs to say so honestly in the planning application.

Do not paraphrase the response in the report. Reproduce it in an appendix with the enquiry reference and the date, because the case officer and the drainage consultee will both want to read the original.

Grampian conditions where reinforcement is needed

Where capacity is not available now but will be, the standard planning solution is a condition preventing occupation, or sometimes development, until the necessary works have been completed.

Offer the wording. A condition tied to occupation of a stated number of units is more workable than one tied to any development, and a condition tied to a drainage scheme approved in writing by the local planning authority in consultation with the undertaker gives everyone a route through.

The condition has to have a reasonable prospect of being satisfied within the life of the permission. Where the reinforcement is not programmed at all, a condition is not the answer and the honest position is that the application is premature until the undertaker commits.

Phasing conditions work well on larger sites. Linking occupation thresholds to network upgrade milestones lets a scheme start while the works proceed.

Assembling the section so the response is usable

Write the section so a reader can follow the chain from flow to connection to capacity without leaving the document.

1. State the existing drainage arrangement on the site, with the source of that information.
2. Give the foul flow calculation with the method, the rates and the peaking factor.
3. Identify the proposed connection point by manhole reference, with the depth and size from the record plan.
4. Summarise the pre-development enquiry, with its reference and date.
5. Set out the undertaker's response, quoted, with the full reply in an appendix.
6. State the conclusion and any condition offered.

Keep the Drainage Strategy consistent with it. Foul and surface water are assessed separately, by different consultees in many cases, and two documents that disagree about the connection point or the impermeable area will both be queried.

And check whether the site also needs a Water Supply Assessment, since the supply enquiry goes to the same company and can be made at the same time. The report index covers the rest of the drainage evidence base.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.