Dorchester

Dorchester Drainage and Wastewater Strategy 

This Drainage and Wastewater Strategy covers the area served by Dorchester Water Recycling Centre (WRC), also known as Sewage Treatment Works, including Charminster, Frampton, Martinstown, Crossways and Owermoigne. This area is a part of the Dorset Management Catchment and Wessex Water’s Drainage and Wastewater Management Plan.  

Catchment background 

The area 

Dorchester is the county town of Dorset, on the hilly southern bank of the river Frome. The town is of Roman origin and there are several Roman remains and other areas of archaeology in the town. The area is predominantly chalk geology which supports the unique ecology of the nationally protected river Frome.  

Sewer network 

The Dorchester area has both combined and separate sewer systems to convey wastewater, sewage from homes and businesses, and storm water, rainwater collected from roofs and yards. Under heavy storm conditions, where the sewer conveys both wastewater and storm water, 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 Dorchester WRC are treated under normal flow conditions and flows are further treated through phosphorus removal to reduce the nutrient load discharged to the river Frome. 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 the river as a storm overflow, as permitted by the Environment Agency, having benefitted from screening and a degree of settlement within the storm tank. 

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 or other inflows such as surface water entering the sewer system. 

The Dorchester area has a high risk for sewer incapacity and high groundwater levels from prolonged rainfall periods affect areas of this catchment. The catchment has experienced sewer flooding due to hydraulic incapacity in the past three years but currently no frequent spilling storm overflows have been identified 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 Dorchester area has a high risk for blockages and has experienced sewer flooding due to blockages in the past three years. 

Asset health 

Sewers are inspected to assess the 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 medium risk for 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 surface water sewers, which convey rainwater from roofs and yards to the river. Where heavy rainfall occurs, overland flow collects runoff into rivers or low points on the ground. See the Environment Agency flood maps or the Dorset Council’s website for more information. 

Water recycling centre 

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. 

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

The Dorchester catchment has not experienced any significant pollution event in the past three years.  

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 are not permitted. 

The unitary authority of Dorset Council replaced the former West Dorset, Weymouth and Portland Council in 2019 and will prepare and publish a new Local Plan during 2023. The adopted Local Plan 2015 currently remains in place and we will continue to work with Dorset Council over this period. The village of Crossways is proposed as a focus of development through the allocation of sites in West Dorset and Purbeck Districts. Frampton and Owermoigne are identified as settlements suitable to cater for local need.  

To support the recent and ongoing development, and to reduce the risk of flooding, local improvement will be required to the network. 

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 and create impermeable area for use such as 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. 

As a part of the Drainage and Wastewater Management Plan process, we will produce models to understand how these challenges may impact the area. 

Strategy 

Partnership working 

We are working in partnership with Dorset 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  

  • Investigate and trial catchment management to reduce nutrient loading, in specific nitrate, to the river Frome as a more sustainable option to constructing a traditional engineering solution at the WRC. 
  • Reline priority sewers to reduce risk of infiltration and collapse. 
  • Deliver localised improvements to accommodate developments.  

Medium term 

  • Increase phosphorus removal at the WRC to reduce nutrient loading to the River Frome further.  
  • Investigate the potential for seasonal permitting of phosphorus removal at the WRC to improve cost benefit.  
  • Deliver enhancements required to accommodate developments at Warmwell and Crossways. 

Long term 

  • Review capacity at the WRC, considering growth in the catchment and climate change, and identify mitigation measures and construct if required. 

We are developing 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. This could include the use of sustainable drainage systems, new technologies and working with partners more to align long term plans and schemes.