Hydrodemolition is the process of using ultra high-pressure waterjets to remove damaged concrete without affecting other parts of the structure or the steel reinforcement contained within. Once the damaged concrete is removed and the rebar exposed, the structure can be recast in-situ. This saves significant money overcomplete replacement or repair after breaking/cutting using mechanical means. However, when cutting concrete with water, the freshly exposed lime reacts with the water to produce a highly alkaline (high pH)wastewater. Because pH is logarithmic it is not possible to treat via dilution and even a small wastewater flow can kill aquatic life. A solution is required as the wastewater cannot, by environmental laws, be allowed to drain away. It is important to collect/contain the wastewater, but transporting the water for offsite treatment can be prohibitively expensive. Whether flow is generated from a single hand lance or a larger flow robotic system, there are a range of system sizes to suit. Multiple treatment stages are used to correct the pH and remove solids, producing a treated water that is safe to discharge.
Case Study
About 17,000 vehicles a day use the New Elvet Bridge over the River Wear in central Durham. It was opened in 1975 but closed in July 2020 for essential maintenance, which included:
• Lifting the entire 490tonne centre span
• Removing and replacing the southern span
• Removing, repairing and recasting the bridge’s joints
• Installing a concrete protection system
• Improving waterproofing and drainage
A bespoke truss system lifted the entire bridge deck about 300mm to expose the concrete joints. One of the project’s most crucial events was the hydrodemolition of defective concrete to expose the concealed reinforcement bar and allow the bridge joints to be reconstructed. The specialist hydrodemolition contractor generated around 10m3 per hour of blast water which was highly alkaline and laden with tiny particles of concrete. If this blast water had escaped the bridge and entered the River Wear, it would have caused a major pollution incident. Siltbuster was approached to provide a robust mobile water treatment solution to allow the treatment and disposal of excess water safely to the River Wear, without transportation to an offsite treatment facility. Over the course of the project, the Siltbuster equipment removed the need for almost 600 vehicle movements, providing significant cost and environmental savings. Siltbuster visited the project in August 2020 and started treatment for the contractor in October 2020 using a Siltbuster PMPU20 integrated treatment unit. It comprises two dedicated reaction tanks and a lamella clarifier for the multistage treatment of alkaline conditions and retention of settling suspended solids. This sequential system reduces the amount of reagent required per cubic metre of water (in this case carbon dioxide (CO2), and also reduces the hardness/alkalinity.
The PMPU20 has a typical continuous operating range of up to 20m3/hr and its own peristaltic slurry pump controlled via an asymmetric timer. The operator optimises the timing system to both dispose of captured slurry and recirculate it to a High-Density Sludge (HDS) which assists the nucleation of calcium carbonate precipitation, minimises slurry removal costs and minimises overall CO2 consumption. Laboratory analysis (see Table 1, below) of water samples before and after treatment showed a significant reduction in suspended solids loading and a near-neutral pH value. The water disposal activity was regulated by an Environmental Permit granted by the Environment Agency. In England, the typical quality criteria for safe disposal of water to controlled waters would be:
• TSS less than 60mg/l (i.e. visually clear
water)
• pH6 to pH9
Table 1 – water samples
| Sample ID | pH | Total Suspended Solids (mg/l) |
| Untreated (Inlet) | 10.77 | 4,536 |
| Treated (Outlet – to river) | 7.54 | 43 |
After a successful treatment campaign, the Siltbuster PMPU20 was collected from the site after a six-month hire period. During this time the plant operated predominantly automatically and without a single reported discharge failure.
