Poole drainage and wastewater strategy

Poole drainage and wastewater strategy

This strategy covers the Poole area, including Broadstone, served by Poole water recycling centre (sewage treatment works). This area is a part of the Dorset management catchment and Wessex Water’s drainage and wastewater management plan.

Catchment background

The area

Poole is a large coastal town in Dorset and with nearby Bournemouth and Christchurch, it forms the south-east Dorset conurbation. It lies on the northern and eastern edges of Poole Harbour, a large natural harbour designated for its wildlife and habitats of national and international importance, and as a sensitive water body under the Urban Wastewater Treatment Directive. The area is characterised by wet, sandy soils, and naturally supports a heathland environment, with some areas to the north and west remaining, most notably Upton Heath and Canford Heath.

Sewer network

The Poole area has a predominantly separate sewer system, where wastewater, sewage from homes and businesses, is collected into the foul only sewer and is conveyed to the water recycling centre (WRC). Storm water, rainwater collected from roofs and yards, is collected into a separate surface water sewer which conveys the rainwater to the river. However, in some situations the surface water sewer discharges to the foul sewer. In these cases, under heavy storm conditions, sewer capacity can be exceeded and built in safety valves called storm overflows, permitted by the Environment Agency, can operate to prevent sewer flooding.

Water recycling centre

Wastewater received at Poole WRC is treated under normal flow conditions and are further treated through nitrate removal to reduce the nutrient load discharged to Holes Bay and Poole Harbour. The flows are also disinfected by UV treatment to reduce the bacteria load to the shellfish and bathing waters in Poole Harbour. Under heavy storm conditions, flows into the WRC can exceed its capacity. These excess flows will first overflow to storm storage tanks. If these tanks become full, they in turn spill to Holes Bay as a storm overflow, as permitted by the Environment Agency, having benefited from screening and a degree of settlement within the storm tank. The site also receives trade discharges for treatment through the WRC process.

Current performance

Sewer capacity

Hydraulic incapacity is when the drainage network cannot convey the runoff from heavy rainfall and can lead to sewer flooding. It can be exacerbated by groundwater entering the sewer systems. The Poole area has an average risk for sewer incapacity although there are no frequent spilling storm overflows in the catchment.

Sewer misuse

Sewer misuse includes flushing anything other than the three Ps (pee, poo and toilet paper) down toilets. Wet wipes, nappies and sanitary products should not be flushed regardless of their labelling. Fats, oils and grease should not be poured down sinks in the kitchen, as this creates 'fatbergs'. Sewer misuse can lead to blockages which can cause sewer flooding.  The Poole area has a high risk for blockages because of sewer misuse.

Asset health

Sewers are inspected to assess their condition using a risk-based approach, using the likelihood of it failing and consequence of failure. The sewers in the worse conditions are prioritised for more frequent inspection or rehabilitation. The risk profile for the condition of the sewers in this area is at high risk compared to the rest of the Wessex Water region.

Surface water flooding

Surface water flooding occurs when very heavy rainfall overwhelms drainage systems. Responsibility for surface water flooding is complex, but in summary Wessex Water is only responsible for sewer flooding. Where heavy rainfall occurs, overland flow collects runoff into rivers or low points on the ground. See the Environment Agency flood maps for more details.

Sewage treatment works

Each WRC has a permit, as agreed with the Environment Agency, for how much water is treated under different weather conditions and the quality of the water that is discharged to the environment. Poole WRC is approaching capacity to meet its permit.

Water quality

When untreated wastewater is discharged to a watercourse it can affect the downstream environment including the river and coastal areas. This could be from:

  • pollution
  • unauthorised wastewater spills or leaks
  • misconnections (when wastewater from household is incorrectly connected to the surface water sewer)
  • storm overflows.

Future challenges in the catchment

Growth

New developments can cause an increase in wastewater requiring conveyance and treatment. Improvements to the foul sewer system to support new development will be assessed by Wessex Water developers' group and infrastructure charges paid by new developments will fund required upgrades to ensure sewer flooding risk is not increased. 

Developments can also increase the area contributing to rainwater runoff to the urban drainage networks, whether it is a surface water or combined sewer, causing an increase risk in surface water and potential sewer flooding. Best practice is to utilise sustainable drainage systems (SuDS). We have a policy that surface water connections to the foul sewer system is not permitted. 

Poole site specific allocations and development management policies development plan document (2012) allocates a number of major and medium sized brownfield sites for housing development (including Poole Power Station in the town centre area and the Poole and Bournemouth College site at Parkstone) and a major greenfield site at Wallisdown Road. Employment provision includes proposals at Fleets Corner and Sopers Lane, and Sterte Avenue West. 

The current version of the new Poole local plan (modified version 2018) indicates: "To meet Poole's housing needs it is evident that a step change in delivery is required" (6.19). The new local plan seeks to maximise brownfield development and allocate new urban extensions at Merley and Bearwood. These allocations are close to neighbouring sewage treatment catchments of Wimborne and Kinson. Wessex Water assessment will consider a strategic solution to these allocations and others emerging close by in neighbouring planning authorities. 

To support this growth, strategic improvements will be needed to ensure the WRC can accommodate this increase in flow. Local improvements to the network will also be needed to support this growth and reduce flood risk.

Climate change and urban creep

Climate change is likely to increase the intensity of rainfall leading to higher risk of flooding in the future; however, the magnitude and timing of this change is highly uncertain. 

Urban creep can also pose a challenge for managing our drainage and wastewater networks. This is when existing households extend or build over gardens for car parking. This type of growth increases the area contributing to fast runoff to the urban drainage system and can increase the risk of flooding.

Strategy

Partnership working

We are working in partnership with the lead local flood authority, borough of Poole council, to develop surface water management plans. These plans set out how surface water will be managed in the long term to prevent flooding from sewers, drains, groundwater, land runoff and small watercourses.

Short term

  • Upgrade of East Quay pumping station and the rising main.
  • Investigate and identify options to mitigate for development in the area.
  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures if required.

Medium term

  • Increase capacity at the WRC to accommodate development in the catchment.
  • Investigate and identify options to reduce the nutrient loading from the WRC to Poole Harbour.
  • Reduce spill frequency from storm overflows to reduce impact on bathing waters.
  • Investigate impact of Wessex Water's operation on Holes Bay and the shellfish waters of Poole Harbour.

Long term

  • Investigate options to reduce the impact of the WRC on Poole Harbour such as the possibility of a new long sea outfall.
  • Complete improvements to the WRC and install any additional nutrient removal required to reduce the nutrient loading to Poole Harbour.

We are developing further long-term options that address and mitigate for climate change, development, urban creep and other future challenges as a part of the drainage and wastewater management plan process.