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Optimising Concrete Washout Storage Options

Disposal of ready mix concrete in slurry form is always necessary. Even with all the best planning, communication and coordination, concrete waste disposal is a factor in any concrete construction business. In most cases, storage and handling of concrete slurry waste is difficult and costly to do on-site and is more often than not returned to the batching plant. It is here that cementitious wash water and its solids are separated into aggregate solids, cement solids and water.

The need to explore new concrete washout storage options stems from local and federal government demand to improve the environmental footprint that businesses and individuals leave behind. New storage options bring an array of possibilities; this article will focus on current storage options and how to solve some of the common washout problems for concrete batching plants.


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Washout Facility Design

When planning or designing a facility, the details of the washout facility should be documented to include the physical location of the washout facility at the plant, and what the storage plans for the plant are.  Typically, the waste management plan should cover how the waste will be stored, where it will be stored, how the waste is collected from the storage area, and when and how the waste treated, transported and disposed?

Considerations for location and management of washout facilities

  • Keep the storage area well away from any drains, storm water drains or natural waterbodies
  • The location should be convenient near washout equipment
  • The washout area is for wash water only – concrete remnants should be removed prior to wash down
  • Save water through using high-pressure, low-volume water nozzles
  • Regulations apply for water discharge, refer to the Water Discharge and Reuse policy for more information
  • Weekly monitoring to ensure storage area is functioning as expected (this is a guideline for dry weather)
  • Monitoring before forecasted rain events, and during them

Site-based Storage Options

Particularly in New South Wales, the POEO Act governs environmental protection, as set out by the EPA. Acts of wilful or neglectful harm to the environment attract penalties and in severe cases, imprisonment. For larger sites it is required the site owner has a defined environmental plan that outlines how waste concrete in slurry form will be managed.

Storage options for site based liquid waste management may include:

  • Sealed skip containers: sealed and transportable with no means of leakage, but also no means of drying or water recovery.
  • Double-lined slurry traps: use a geotextile liner to trap solids and allow liquid to seep up for tapping or evaporation. This method is mobile and re-usable but takes time to deliver a good outcome.
  • Berm trap: this is the least favoured option and simply offers a geo textile floor layer and an earth bund to prevent leaching of the slurry. It is not relocatable and requires a long time for solids to dewater. It potentially offers a hazard if not fenced off.

Concrete Batching Plant-Based Storage Options

Chute Washout

Some trucks use a washout box mounted to the truck rear to wash the waste concrete from the truck body, chutes and ramps. This material is then kept and returned to the batch plant for disposal.

Truck Washout

This is the most common method of dealing with waste concrete slurry and is performed by a high pressure water spray gun (either auto or driver operated). Truck washout can be carried out either on-site or back at the batch plant. Site-based washout requires one of the aforementioned storage methods to cope with the waste slurry.

 

Storage and Treatment Options

In a batch plant, storage and treatment options vary, but generally, the most basic method is to wash the truck out and allow the solids to settle in a storage pit while the water overflows into another pit.

Alternatively, the washout concrete can be emptied into formwork bunds, and the liquid seeps out and into a second pit where again the cementitious particles settle out over time. Water then overflows to a third or fourth pit where it may be used for washout or treated to lower pH levels.

Many batch plants utilise a reclaimer to capture sand and aggregates while cementitious slurry flows under gravity to settling pits,

Regain Valuable Site Space

While the traditional washout storage methods have their benefits, the removal of onsite storage and its associated costs are goals that most plant owners and managers have.

There is a range of equipment designed specifically to meet the environmental and spatial challenges that concrete batching plants experience in day-to-day operations. When combined, the individual pieces of equipment form an entire concrete truck washout solution that includes a Filter Press, Concrete Reclaimers and Dump Hoppers/Feeders. The combine to separate the sand and rock, cementitious fines, and water from the truck washout stream.

Example of a complete Concrete Plant Solution

Dump Hoppers/Feeders

A dump hopper/ feeder serves two functions - namely to store truck washout for a short period of time and to deliver concrete waste to the washout plant in a controlled/consistent way. You don’t want your truck operators to be waiting for the washout system to do its job, they need to be washing and dumping so they can deliver more loads. However, a successful and efficient washout recovery plant would either need to be oversized if it was to handle a large surge volume, or operate inefficiently if it relied on trucks to dump directly into the wash plant. The dump hopper/ feeder solve both of these problems enabling you to get the most out of your plant.

Concrete Reclaimers

Concrete Reclaimers separate sand from rock utilising a screw washer, which is well known and understood by the industry. Truck washout is fed to the wash box of the screw washer, where gravity enables sand and rock (aggregates) to settle in the wash box. A screw auger provides gentle agitation to clean the aggregates and also conveys them up an incline, out of the water bath. This enables the water to drain away naturally and delivering washed, relatively dry aggregates. The water and cementitious fines contained in the wash box, overflow into the weirs of the Concrete Reclaimer, ready for further downstream processing to recover the water, and separate the cementitious fines out. Concrete Reclaimers are generally limited by the area of the wash box, which dictates how fast the wash water will overflow into the weir. A high flow rate will carry sand, and can potentially result in lost product, inefficient operation and surging problems. It is important to select a provider with extensive knowledge in this area that enables you to maximise the efficiency and recovery potential of the Concrete Reclaimer.

Concrete Filter Press

Lastly comes the Concrete Filter Press. A proven design, the standard Filter Press can dewater difficult-to-handle slurries and mine waste to form solid cakes suitable for dry stacking in applications such as coal, mineral and aggregate tailings, and has recently been successfully used to dewater cementitious water from concrete truck washout.

Filter Press technology typically dewaters fine slurries below 75µ in particle size. The press produces a ‘filter cake’ that can contain anywhere between 65-85 percent solids based on standard operation variables. The cake is then immediately able to be loaded, stacked or otherwise taken away for disposal. 

The Filter Press uses pressure to force slurry into the plate chambers of the Filter Press where filter cloth separates the recovered water from the solids. During operation, a pump feeds the slurry into the press to filter the incoming slurry stream. Water passes through the filter cloth while the solids are retained in the chamber. When the chamber is completely full of solids, filtration ends and the plates separate. This allows the filter cake to fall into the collection area below.

This method allows for slurry water that would normally be lost to waste, to be recycled and reused elsewhere on site, such as washing out trucks, or in normal batching operations.

Before and after liquid/solid separation

Problems Concrete Washout Systems Can Help You Solve:

  • Free up valuable plant space by eliminating settling ponds altogether
  • Reduce employee intervention
  • Reduce disposal costs
  • Save wear and tear on vehicles
  • Automatic wash down for filter plates and cloths prevents sedimentary build-up
  • Customisation available to meet specific plant requirements

Conclusion

Migration away from traditional washout storage options is inevitable with the mounting pressure from local and federal government to commit to a long-term environmental sustainability plan. While several options exist for traditional washout storage, the applications for those options is becoming less and less. By treating truck washout efficiently, it is possible to recover water and aggregates that would typically be lost, while simultaneously eliminating traditional washout pits, and reducing waste volumes that immediately add value to your bottom line.

Image courtesy of: http://www.ccgov.org/government/public-works/public-works-divisions/stormwater-branch/construction-site-bmps