PE (Population Equivalent) and Drumbeg WwTw Capacity Concerns

Wastewater Management Issues in Drumbeg

The recent discussions on the Drumbeg Wastewater Treatment Works (WwTw) capacity have highlighted significant concerns about the proposed new developments in the area. Based on detailed reports from NI Water and the Departmental Solicitor’s Office (DSO), we delve into the critical data and implications for Drumbeg’s wastewater management.

Source: NI Water

The 2021 Drumbeg WwTw Review outlined the following:

“NI Water has developed an Asset Standard that defines the procedure for determining flow, population equivalent (PE), and loadings for both wastewater treatment facilities and networks. The current Asset Standard is Wastewater Flow & Population Determination v1.6, issued January 2019. This Asset Standard formed the basis of the Drumbeg WwTw PE 2021 review. The results of PE updates are fed into the Headroom Register, which holds information on PEs / Capacity for all WwTws.”

From Annex A of the 2021 Review Report:

  • The population served by Drumbeg WwTw in 2021 was 1833.1 (residential and non-residential).
  • The design PE of Drumbeg WwTw was 1919, with an actual PE of 1833, leaving a theoretical headroom of 86 PE.
  • There was a pending planning application for 17 detached dwellings at Quarterlands Road, expected to add a further loading of 35 PE.

Despite the theoretical headroom, the 17 dwellings will utilize the remaining hydraulic capacity stated by the Northern Ireland Environment Agency (NIEA). Consequently, approving these 17 dwellings will close Drumbeg WwTw to future development.

Source: Departmental Solicitor’s Office (DSO)

According to DSO correspondence dated 20 May 2024, regarding Planning Application LA05/2022/0033/F:

“Drumbeg WwTw currently serves a PE of 1877 (based on 2024 data). The WwTw has a design PE of 1919. The PE of 1877 does not include the 17 houses approved for connection as part of the planning application. This WwTw is now deemed to be at capacity.”

Comment on PE Regarding the Proposed Development

When considering the proposed onsite development:

  • For a standard 3-bedroom house, the PE is 2.4 per household (7.2 PE for this planning application).
  • For 5-bedroom houses, the PE is 4 per household (totaling 56 PE).

Thus, the total PE for the 17 proposed houses would be 63.2, which is 81% higher than the 35 PE estimated by NI Water. With the addition of these 17 houses, Drumbeg WwTw will be seriously over capacity. These calculations do not include the hydraulic capacity assessed in the 2021 Drumbeg WwTw Review.

Appendix 1 includes a table informing the design of sewage plants, attributing 5 PE per house. Using this model, the proposed 17 dwellings would account for 85 PE, nearly two and a half times the PE calculated by NI Water. The basis for NI Water’s estimate of 35 PE for the proposed 17 houses was not explained in Annex A.

Conclusion Based on Information from NI Water & DSO

From these calculations, Drumbeg WwTw, already deemed at capacity (DSO letter of 20 May 2024), would become seriously over capacity with the addition of the proposed 17 dwellings, even without considering the additional hydraulic capacity.

The figures provided by NI Water in its 2021 Review do not account for the shift towards more people working from home due to COVID-19 and post-pandemic changes. With fewer commuters, more foul sewage likely migrates back to residential properties from workplaces.

The discrepancies between the data from NI Water and DSO confirm our concern that Drumbeg WwTw cannot accommodate the additional load from the proposed 17 dwellings in planning application LA05/2022/0033/F.

Quarterlands Group