Project Description: Bryn Posteg Landfill Site - Surface Water Management Dosing System to control Water Quality

Project Description: To reduce the risk of fluvial sediment entering a tributary of the River Severn by introducing site control measures.

Introduction

The eastern catchment of the landfill site sheds surface runoff to the River Bradnant, which joins the River Severn some 3km downstream. As the Severn is a salmon river, the Environment Agency General Quality Assessment (GQA) scheme scoring system dictates it should meet a rating of Class B or above, which is typical for a natural river ecosystem.

Project Brief

Egniol were commissioned to design a surface water management system that would both attenuate runoff and provide solids settlement.

The site, some 13km south of Newtown in mid Wales experiences average annual rainfall of 1200mm and consists of thin glacial till overlying mudstone.

This mudstone is used extensively as capping, engineering fill and road construction. Unfortunately, it responds to rainfall by producing high volumes of runoff with a colloidal suspension. Residence times for gravity settlement can be several days if the water body lies undisturbed, so long duration rainfall events produce problems at the regulated site discharge point.


Project Delivery

Some experimentation to achieve solids settlement had been carried out by the operator using anionic floc blocks with limited success. The use of more effective cationic coagulant would interact with fish gills if allowed to enter a salmonid river such as the River Severn.

Our approach was to liaise with Environment Agency Wales (EAW) and suggest utilising a vacant, lined Cell for experimentation with site flows rather than laboratory scale samples. This way, supernatant would be trapped within the Cell and site flows could be added as required. Good results were obtained with a blend of polyacrylamides and the observed settling velocities were incorporated into a design for flow proportional dosing.

The construction of the system proceeded on this basis and a reception facility for dosing the turbulent flow at the eastern outfall was built. A quiescent area for settlement of dosed flow was provided, based on residence times advised from the site testing.

As a result, supernatant passed to a flow diversion structure where it was directed either to an existing lagoon or to a further purpose-built one used on a duty / standby basis to control the volume of runoff. Control measures were incorporated on the outlets of the lagoons consisting of dual siphonic overflows so draw off of clarified supernatant was undramatic and did not entrain settled solids.

Additional freeboard was built into the lagoon sizing to diminish the risk of overtopping and overland routing.

To predict surface runoff behaviour, a hydraulic model was built using Micro Drainage software and summer and winter rainfall events of 1% probability simulated. Long duration events were considered to identify worst-case dosing and settlement situations. The design was approved on submission to EAW.