The concrete industries of Australia and New Zealand are booming due to strong construction markets. In Australia alone, the value of the concrete market is tipped to reach $2 billion in turnover this year. Every year, in excess of 19 million tonnes of concrete by-product and slurry waste is produced by the Australian concrete industry. Many companies still use slurry pits to store this waste until it can be dried and removed from the pits.
Environmental compliance surrounding slurry disposal is increasingly stringent and specific slurry disposal measures must be taken depending on the type of waste that has been produced and returned to site.
Four core concrete by-product types have been identified by Cement Concrete & Aggregates Australia (CCAA). These are hardened returned concrete, wash water, liquid washout/slurry and solid wash out. The CCAA developed guidelines to assist companies in the proper disposal of their concrete by-products.
Concrete producers are taking steps to find more effective and responsible means of disposing and managing resources and by-product. This paper seeks to highlight options for management and disposal.
In Australia, state and local bodies have regulations in place relating to waste disposal and what constitutes solids waste, liquid waste and contaminated waste. In NSW for example, there are two areas of legislation covering concrete waste; The Protection of the Environment Operations Act (1997) and the Protection of the Environment (Waste) Regulation (2005).
In New Zealand, regulatory legislation also includes the Resource Management Act (1991) which seeks to set out policies and enforcement regulations for the proper disposal of concrete and concrete by-products. The NZCMA, an industry education and advocacy body has released a ‘best practice’ guide to outline the suitable management and disposal techniques for fresh concrete and slurry.
Primary components of waste concrete and options for management
Returned concrete in slurry form – (ready mix)
Returning full or partial loads in slurry form to the concrete batching plant presents significant problems. Many operators seek to find alternative disposal options for this material but the exercise can be time consuming, inconsistent and costly. Full and partial loads are often dumped out on the floor of the plant and allowed to dry. Time, space and truck disposal costs increase. In times of high rainfall, this practice can expose the site to excessive runoff which affects neighbouring properties and waterways.
Dumping full or partial loads into settling pits can block pumps and transfer pipes as well as reducing the settling capacity of the pit itself.
Due to the inflexibility of its composition and the high alkalinity, liquid concrete slurry is most often classed as a ‘liquid waste material’. When this classification is attributed it must be handled in the same way as any other form of liquid or hazardous waste, with proper disposal at an approved waste disposal facility.
Special requirements exist for the transportation of liquid waste. Licensed contractors and specific vehicles are required for the extraction of the liquid waste from site. It may also attract an environmental levy upon arrival at the waste disposal facility. It is important to note that some facilities cannot accept liquid waste which may pose disposal issues at certain locations.
Liquid or slurry waste may require a tracking number (unlike hardened returned concrete) that will be issued by the transporter. The vehicle transporting the waste will also need to be approved to cart liquid waste.
Concrete slurry returned as full or half loads to the plant will attract lower disposal costs if the concrete is dried out which is often done as part of the washout settling process or simply pouring the material into moulds. Alternative options include reclaiming the sand and aggregates, directly the concrete slurry through a concrete filter press and recovering the water and concrete as a solid cake.
Concrete Wash Water (truck washout)
Concrete trucks are encouraged to washout residual concrete slurry at regular intervals during the operating period. This is particularly important for the business where colouring powder has been used or special aggregates have been used on previous loads. Truck washout dilutes the material significantly. This has a benefit in lowering pH levels but adds to the over-all volume of waste and increases the time over which the suspended solids will settle out and harden to a consistency where the solids can be captured and moved to a drying pit. Rain events and wash down water will also disturb washout settling pits and add to the settle time and solids percentage. Using this method of settling and separation of solids and liquids it will be important to calculate the number of pits required based on the maximum volume of returned residual concrete slurry as well as washout water, water added through rain, runoff and general wash down water.
All truck wash water has a high alkalinity, with values ranging from 3pH to 12pH. The concentration levels of chloride, sulphate, and chromium should also be monitored cautiously and will be considered when assessing runoff from the site to neighbouring facilities.
Adjusting the pH level can be a complex process with many factors to take into consideration. Such concerns include the additional water volumes added, concentration and build up, environmental issues that reagents produce, the risk of overdosing, and even under-dosing.
The pH level of concrete wash water is usually somewhere between 11pH and 13pH which indicates a high level of alkalinity (due to the presence of lime in the wash water). In many plants, the dosing of water is done in a central location where all runoff and captured water is pumped to a large number of tanks and treated using one or several reagents commonly used to lower the pH. Such reagents may include mineral acids, citric acid, and carbon dioxide.
Mineral acids are typically the most-used reagent for levelling pH, however there are several drawbacks including:
- Safe handling methods and secured storage are required for the use of sulphuric acid and hydrochloric acid.
- While sulphuric acid and hydrochloric acid are considered solutions for diluting pH, they also raise the level of sulphates and chloride in the water, which are pollutants in high concentration on their own.
- Costs associated with disposing of excess acids.
After water is “normalised” using these reagents, it may be reused in the batch plant or disposed to storm water if approved by the local authority.
More recent plant designs include the use of a concrete filter press system that delivers the slurry to the filtration machine removing the solids particles and separating the filtrate in the same process. The filtrate is then treated for pH “in-line” on its way back to the batching plant.
Managing solids in returned concrete
Solids consist of sand, aggregates and cementitious particles which are normally suspended in the slurry. The traditional method of managing the solids would be either pour the concrete slurry into moulds if the volume is high enough or, dump the concrete slurry into a wedge pit and allow the solid particles to precipitate (settle) out over time.
Issues relating to this method are many and various. They require a number of pits that allow water to flow over one pit and into another. Concrete can be hindered from settling whilst additional loads are dumped in to fill the pit. Semi-cured concrete is often removed from the pit and stored in bunker for long periods of time in order to harden the waste solids enough for transport. Of course hardened concrete is easier to dispose of than liquid waste - and is less expensive. Overtime, there will be more truck and loader movements required and take up more real estate. Another advantage of hardened concrete waste is the minimal chance of runoff or water washing contaminants into the waste water system or across boundaries.
Specifications exist for the re-use of hardened concrete waste. Some instances where the material cannot be re-used include;
- Rehabilitation works for mines, quarries or for sand dredging
- Reshaping the landscape of agricultural land
Typical reuse applications for cured concrete waste include;
- For road base after consent for the development has been received by the relevant environmental planning authority
- For road base to provide access (whether it be temporary or permanent) to a development that has been approved by local council
In order to use the material in private land applications, it first needs to be sampled and tested to ensure that it will not pose a major risk to, or damage, the environment.
To completely dispose of hardened returned concrete, it often needs to be transported to a licensed facility. Whilst the vehicle transporting the waste does not require a licence nor does the shipment need to be tracked, there will be a fee-per-tonne to dispose of the hardened returned concrete at a licensed facility.
More progressive operators are reclaiming both solids and liquids in an attempt to reduce costs, space and valuable resources for re-use. This often includes using reclaimers to recover sand and gravel as a re-usable commodity and a filter press system to capture clear water whilst presenting concrete particles as a solid cake for disposal.
While slurry separation using settling ponds is still the most common method of concrete by-product management and disposal, environmental variables determine how companies can safely store, reuse, transport and dispose of concrete by-product materials.
Market demands for more concrete and faster turnaround times are the primary driver for plant operators to seek more efficient processes. Environmental restrictions and obligations are driving up the cost of waste storage, separation, transport and disposal for suppliers which ultimately increases the cost of construction.