Treatment wetlands offer a solution to improving treated effluent discharge from wastewater treatment works, but they are not without risk.

Current targets for waste water treatement works

The 2021 Environmental Act (EA) sets new targets for water companies and drives monitoring requirements and asset investments in Asset Management Period 8 (AMP8) and beyond.

Treated effluent from wastewater treatment works currently contains unacceptably high concentrations of phosphorus.

The key points for waste water in the 2021 Act are:

• a legally binding target of reducing the phosphorus in treated waste water by 80% by 2038 against a 2020 baseline
• the Storm Overflows Discharge Reduction Plan, including a report on the feasibility of eliminating discharges from storm overflowstherequirementto publish data from event duration monitors (EDMs) in near real time, for dissolved oxygen, pH, temperature, turbidity and ammonia (and new substances can be added by the Secretary of State at any time)
• the requirement to monitor the quality of the receiving water upstream and downstream of assets, including storm flows and the outlets of wastewater treatment works (WwTW).

75% of assets discharging to high priority sites must be improved by 2035 in terms of number of spills, so there is no local adverse ecological impact, and they spill only in heavy rainfall.

Watercourses are classified in high(H)and low(L)priority sites:

• rivers (H & L)
• other inland watercourses (canals, lakes, and groundwater) (L)
• estuaries (H)
• coastal (H & L).

Water companies are also required to upgrade 160 of their wastewater treatment works to meet the strictest phosphorus limits by 2028.

A further 400 wastewater treatment works will need to be upgraded by 2038, to reduce harmful nutrient pollution from treated wastewater.

Treatment wetlands

UK water companies have started to look at treatment wetlands as one way to improve treated secondary and tertiary effluent discharge from wastewater treatment works.

Treatment wetlands situated above aquifers or near public water abstractions however, put protected groundwater resources at risk. Aquifers are particularly vulnerable to pollution from near-surface water infiltration where groundwater levels are high and overlaying ground conditions are permeable.

Show stoppers are where:

• the treatment wetland is in hydraulic continuity with the local groundwater
• the outflow water quality causes a deterioration in the quality of the receiving surface water course or groundwater

Potential risks to groundwater

A detailed quantitative hydrogeological risk assessment is required to assess the potential risk to groundwater from infiltrating pollutions from treatment wetlands.

The pollution risk to groundwater is not just associated with elevated nutrients detected within treated wastewater effluent. Other potential contaminants of concern (CoC) that might be present within the effluent include:

• persistent organic pollutants (POP)
• per-and polyfluorinated substances (PFAS)
• polycyclic aromatic hydrocarbons (PAHs)
• metals
• pharmaceutical compounds
• pesticides.

Wetland liners can reduce risk

Regulatory Position Statement (RPS) 260 requires the installation of either a soil or synthetic liner that meets the low-permeability criterion of lower than 1×10-7m/s to prevent unacceptable effluent infiltration rates downwards into underlying groundwater.

At locations where natural clay-rich superficial deposits can be used for a treatment wetlands base layer, cost can be reduced significantly.

Pre-feasibility assessment is needed to estimate soil permeability based on published soil type for the site. This will indicate whether the local soil is likely to meet the criterion for base-soil liner construction.

Ground investigation in-situ and soil laboratory test data is also required to confirm soil suitability. Permeability testing must meet the standards in BS1377-6:1990 or to BS EN1997-2:2007 and confirm the requirements.

Detailed quantitative hydrogeological risk assessment of the fate and transport of contaminants through the soil liner should also support the base layer design (e.g. clay layer thickness) and provide further confidence to stakeholders that the infiltration rate is reduced to a level that protects groundwater resources.

The last word

WHS provides environmental consultancy services for the full water cycle.

WHS Water Services support our clients in obtaining bespoke environmental permits for discharges and groundwater activities. This includes:

• designing drainage
• quantifying flows and loads
• assessing qualitative and quantitative hydrogeological risks for controlled waters.

We have detailed knowledge and understanding of catchment hydrology, hydrogeology,environmental impact assessment.Most importantly, we know how theregulators work.