Aggregate Washing Waste Treatment Plant - Dartford

The Client’s Recycling Aggregate Works washes crushed aggregate and building materials received from various sites in the London area which would otherwise have been disposed to landfill. The washed and graded material is used in a concrete batching plant and recycled for use at the Client’s local construction sites. The complete Works comprises Crushing, Washing, Waste Treatment and Concrete Batching plants on the same site. Production quantities of each plant must be carefully matched and co-ordinated if bottlenecks are not to occur. The feedstock is controlled to minimise the amount of clay and tarmac received into the system. The Waste Treatment plant was originally built to process wash water from the Aggregate Washing plant, but after 3 years of operation was only processing an average of 35 tonnes/hr instead of the planned 65 tonnes/hr. This had a detrimental effect upon the economic viability of the entire Works.

The Waste Treatment Plant also incorporates a Drainage Treatment Plant for road gully waste recycling. This provides the make-up water for the entire system including the associated Concrete Batching plant. The waste treatment systems were extensively integrated with various recycle flows in operation which added to system complexity.

Oil Pollution Services were asked to undertake an initial design review of the waste treatment system as a result of previous experience of the separation processes involved.

A basic process review on site resulted in minor modifications to the works which produced an immediate increase of 5-10 tonnes an hour in the process throughput. Subsequently OPS reviewed the process in more detail and redesigned and implemented appropriate changes to enable the Client to exceed the original design flows.

Process Description

The aggregate is crushed, washed and separated into 3 sizes. The washings are passed through a cyclone to be de-sanded to less than 63 micron then pumped across a road to the Waste Treatment system.

The revised Waste Treatment plant has the following unit operations:

Polymer Dosing
Lamella Separation and Thickening
Buffer Storage
Polymer Dosing
Belt Press
Design flow rate 50m³per hour containing 5 to 20% solids with varying particle size distribution due to feed stock variance

The Drainage Treatment plant which provides the water make-up for the system remains in place. However, several of the recycle flows were eliminated and simplified in the redesign.

The mass balance of the plant was altered to ensure adequate flows to all unit operations.

The new Lamella Separator.

The new 160m³ Lamella Separator, shown above, was designed for a greater particle size distribution. This was sized to accommodate further expansion of the aggregate plant.

Challenges Encountered

Integrating significant changes to an existing works, including piping, electrical, instrumentation and control
The re-utilisation of existing pumps, tanks and valves where possible to save costs
Changes to the overall plant mass balance were required to provide ullage and reasonable times for process and operator response
Separation of high pH and low pH water streams to facilitate polymer dosing performance
Elimination of significant solids bypassing the original lamella system
Select and location of correct polymer dosing systems and injection points

Results

Process throughput for the Waste Treatment plant rose from 35t/hr to 65t/hr, working a 10 hour working day, 5 days per week, enabling the Client to substantially improve the overall economics for the whole Works
Plant reliability was significantly increased through the introduction of both duty and standby equipment
The Drainage Treatment Plant was better integrated to enable both processes to run efficiently in parallel
Improved mass balance of the Works allows easier operation with buffer times and better performance of equipment previously starved of wash water
Provision of spare capacity within the new Lamella Separator will permit further production increases when expansion plans for the Aggregate Washing Plant are implemented