A Water Treatment Plant (WTP) was used to remove contaminants from water during the remediation. After treatment, the water was discharged to the sewer under a trade waste agreement with Sydney Water.
Contaminated water sources
Water that required treatment was generated when excavation activities intercepted contaminated groundwater and through decontamination activities, such as the washing of plant and equipment. Contaminated water was pumped to holding tanks before passing through the water treatment plant.
Groundwater was encountered during the remediation works. Water was pumped out of the excavation during the remediation works in order to allow the excavation of contaminated material to continue.
Decontamination of plant and equipment
All plant, equipment and vehicles required decontamination before being removed from the Environmental Control Enclosure (ECE) where the main remediation works took place. Contaminated water was generated when high pressure water was used to remove soil from plant and equipment, including washing the wheels of trucks.
Decontamination of project personnel
All project personnel involved in contaminated works followed a decontamination procedure before entering a non-contaminated area. Contaminated water was generated from decontamination activities such as showering, washing clothes and personal protective equipment, and from cleaning the decontamination unit.
The water treatment plant
All contaminated water was temporarily stored in tanks onsite before being pumped to the Water Treatment Plant (WTP) where a combination of processes took place: coagulation, flocculation, sedimentation and Granular Activated Carbon (GAC) adsorption.
The first treatment process in the WTP was coagulation. A chemical was added to the water to make suspended fine particles clump together so they could be more easily settled out of the water at a later stage. Solids, including particulate-bound contaminants, were also removed from the water by flocculation and sedimentation.
The water containing the coagulated solids was then flocculated, which involved gentle mixing of the water, causing the suspended particles to further clump together, or form a floc.
The flocculated water flowed into a tank known as a clarifier where the particles settled. The settling occurred due to gravity and contact with the surfaces of the clarifier.
Granular activated carbon adsorption
After the solids were removed from the water, dissolved contaminants were removed by passing the water through granular activated carbon (GAC) filters in a process known as adsorption.
Offsite disposal of treated water
Following the GAC treatment, the water was pumped to a clean water tank before being pumped to the Sydney Water sewer under a trade waste agreement. The agreement required monitoring of flow rates, volumes and the residual contaminants.
As a result of the treatment activities, solids removed from the contaminated water during the treatment process were thickened and dewatered. The resulting cake was discharged into a bin and remediated along with the other material from the site.
Over time, gasworks waste in the soil had caused groundwater beneath the site to become polluted. One of the main reasons for undertaking the soil remediation was to protect groundwater beneath the site into the future.
A programme of groundwater monitoring has been implemented to monitor the success of the remediation and assess changes in the groundwater condition over time. The current condition of the groundwater was established through a sampling event in early 2015 known as a baseline groundwater monitoring event (GME). The baseline GME involved the collection of water samples from 25 onsite and offsite wells installed to depths of between 4m and 15m.
Many of the onsite wells were destroyed during the excavation of the site and some of the wells located on Burren St, Erskineville, which were old or unused, needed to be decommissioned. As a result, replacement wells were installed after completion of the remediation excavations to allow the future monitoring of the groundwater aquifer.
Stormwater was managed during the project to prevent erosion of soil and the washing of sediment into stormwater drains and waterways. Silt fencing was established along the site’s western boundary so that run-off did not enter the stormwater drain or Burren St residences. Sandbags or silt socks were placed around stormwater drains to prevent them being affected by run-off. During rainfall, regular checks were made to confirm the sediment controls were intact and operating effectively.
Throughout the remediation works, soil topography was altered and the stormwater and project’s sediment erosion control map was updated as required. Water discharge from the site was conducted in strict accordance with the conditions of the Project Approval and water pollution legislation.