Connecting to a public sewer will likely always be the preferred choice for any housing or commercial development, or any existing dwelling or premises. If there’s a public sewer nearby, it’s the most economical option, and will result in the lowest levels of ongoing maintenance. However, a mains sewer connection might not always be possible (or economical) to undertake, if there are no public sewers in the vicinity.

Some public sewers are marked on sewer maps, which are viewable at your local authority’s offices, or you can find out more via an asset location search (many authorities offer this service online through Digdat). This will give you details including the location of known water and sewage pipes in the vicinity of your site, including pipe sizes and direction of flow, plus pipe depths and cover levels. The charge for an asset location search varies between water authorities. We work with Thames Water, Southern Water and Anglian Water, and their current fees are £68.72, £51.76, and £41.83 respectively.

Not all public sewers are marked on the drainage map

However, since October 2011, when 40,000km of privately owned sewers became the responsibility of the local water authorities, the sewer maps have been less accurate. It means some public sewers simply aren’t marked on the records.

If there are unrecorded public sewers close to your site, then a proper investigation is advised, to determine the actual drainage arrangements. It will also be necessary to confirm that the nearby sewers have been adopted by the water authority in question.

No public sewer? Install a sewage treatment plant

In rural areas, it’s not unusual to find that there are no public sewers nearby. In this sort of situation, a sewage treatment plant is the most common alternative arrangement to a sewer connection.

A sewage treatment plant processes the effluent from a property or properties, breaking down the solids, and enabling bacteria to digest the organic matter, as the sewage moves through the chambers. The effluent that is produced by a sewage treatment plant is typically 95% clean, and can be discharged into a watercourse, ditch, or soakaway, with consent from the Environment Agency.

Specifying the right size of sewage treatment plant

Correctly sizing the sewage treatment plant is important – there are some quick rule-of-thumb calculations that can help enable the specification of the right size installation. Sizing is based on ‘population’ (P); the number of people living in the property or properties:

• Single domestic dwelling:
  • Minimum 5P for any dwelling, regardless of bedrooms
  • Add 1P for each additional bedroom over 3 bedrooms

(e.g. a 4-bedroom house would be a minimum 6P treatment plant)

• Groups of smaller 1/2-bedroom houses or flats:
  • Total P load needs to be calculated specifically for each group (taking into consideration the maximum possible occupancy and flow)
• Groups of larger houses:
  • Add the P values for each house to determine the overall system size
    (e.g. two houses, with 3 and 4 bedrooms respectively, would need an 11P plant)

With bigger developments (>12P), reductions can be made to adjust for daily fluctuations:

• Where P load total is 13-25:
  • Multiply total P load by 0.9 to provide adjusted P value

(e.g. Four 4-bedroom houses, total P value 4 x 6 = 24
24 x 0.9 = 21.6, rounded up = adjusted P value of 22)

• Where P load total is 26-50:
  • Multiply total P load by 0.8 to provide adjusted P value
    (e.g. Four 3-bedroom houses and three 4-bedroom houses, total P value = 38
    38 x 0.8 = 30.4 rounded up = adjusted P value of 31)

What factors can affect P load?

The above are of course very basic guides – it’s worth bearing in mind that specific circumstances should also be taken into account. For example, larger, more luxurious properties will likely have increased water consumption (especially if they have a spa bath or hot tub), while holiday homes often have higher occupancies as living areas are sometimes used as bedrooms (though they might only be used intermittently, depending on booking frequency).

It’s worth factoring in householder hobbies and lifestyles, too, where possible. Home brewing and photo developing will increase the amount of effluent being produced, and the use of chemicals could affect the plant’s effectiveness, while keen cooks whose kitchens are equipped with waste disposal units will create effluent with an increased biological load.

The final, golden rule is to always round up, rather than down.

At Premier Drainage Solutions, we’ve got more than 40 years’ expertise in sewer connections, sewer diversions, and sewage treatment plants. For free advice on your works, including the sizing of a sewage treatment plant, call 01268 950050 or send us an email to [email protected].