Money Down The Drain?
John Powell of Associated Concrete Solutions Ltd explains the implications of the environmental pressures for dealing with waste concrete and washout water
Environmental pressures and water restrictions, together with increased financial implications, should encourage concrete producers to re-evaluate their treatment of waste concrete and washout water.
Waste is generated by spillage, returned concrete, bad mixes and the washing down of truckmixers and batching plants. The water used in the wash-down process increases that volume considerably; and it is usually fresh water. The amount of waste produced varies from plant to plant. The number of trucks operating from a plant and its location are all factors, with city-based plants usually receiving higher quantities of returned concrete. Concrete producers, both ready-mixed and precast, generally underestimate the quantity of waste they generate by as much as 50%.
The traditional hole in the ground or the corner of the yard where waste is dumped and allowed to go hard with surplus water soaking into the ground or finding its way off site to a ditch or drain is under increasing environmental pressure. With the Building Regulations requiring the principles of Sustainable Urban Drainage to be taken into account, together with the pending European Water Framework Directive to consider, there are now more restrictions on the quality of water being allowed to drain off site from both new and existing plants.
Drying bays, where mixer trucks discharge into a walled bay that allows the water to seep out, contain the waste in a more confined space and give the impression of control. Once full and dry the material is transferred to a second bay to complete its drying process or stockpiled for shipment off site at a later date. Bays holding over 250 tonnes of concrete waste are quite common — some of these being filled within just a couple of weeks.
A 250-tonne quantity of hard concrete waste has already incurred aggregate levy at £1.60/tonne as well as the material cost and transport to site. It is correct to say that, in the case of ready-mixed concrete, the material costs would have been included in the price of the concrete sold, but the cost implications of moving the waste around site are often ignored as a wheel loader is probably on site anyway and material is moved either on a Saturday morning or during quiet times. Transporting 250 tonnes off site in trucks, whether it is to a transfer site or landfill further incurs costs. Landfill would incur tipping tax as well as the tipping costs, and added together over a year these can be considerable.
Water that drains from the drying bays usually flows across a series of settling pools allowing fine material to settle out and relatively clear water, albeit with a high pH level, to go to waste. The pools are cleaned out on a regular basis, either by wheel loader, with the sludge returning to a drying bay, or by a ‘gulper’ for disposal to landfill. The outfall from these pools will be affected by increasing regulation of water flowing off site.
Although chemicals can be added to a mixer truck to reduce washing out, allowing the residue to be lost in the following day’s concrete, waste concrete arising from cleaning a batching plant, together with any returned concrete, still has to be disposed of. The volumes of waste may be reduced but not eliminated.
The washing process of a truckmixer or concrete mixer using water results in a wet liquid waste. Concrete producers in mainland Europe have, over the last 20 years, concentrated on processing the waste at this wet stage, washing it further to remove any solid material over 0.25mm and leaving it in a condition for re-use. Encouraged by environmental controls, recycling systems have become the norm for most plants and are accepted as part of the production process by operators and drivers.
Recovering the aggregate reduces 250 tonnes of waste to less than 20 tonnes. There is also an immediate cost saving of the aggregates levy, material costs and transport on to site as 200 tonnes is re-used. Eliminating the need for drying bays provides more space on site or can result in the need for a smaller site.
Including a recycler in the plant design from the beginning provides the benefit of allowing mixer wash and waste to be chuted directly into the recycler. The spillage from conveyor belts, under trays and the washout from travelling buckets in precast plants can also be fed in directly. Concrete producers in the US have traditionally integrated a recycling system together with the collection of rainwater as part of a controlled water-management system; a concept that Tarmac Northern’s engineering department has recently included in new plants at Barrow-in-Furness and Brunswick.
With the aggregate removed, the –0.25mm cement fines remain in the water. While this can be allowed to flow across the settlement pools, to re-use it in the subsequent production of concrete means that the fines need to be disposed of. To achieve this, the fines have to be kept in suspension and suitable agitation is required to prevent settling out. BS-EN 1008 2002 limits the amount of solids that can be included in the water to 1% of the aggregate total, which equates to a density of around 1.20. Automatic measuring systems are available to input the density figure into the batching plant’s control systems to allow automatic correction of the recipes. A well designed system should keep the water density in the 1.04–1.06 range, well below a level to cause any concern. Within these limits all the fines can be disposed of through normal production. By using the cementitious water, surface water off the yard is not so contencious and collecting this and introducing it into a recycling system can virtually eliminate the water flowing off site while drastically reducing the amount of fresh water used.
With the current water shortages in the South-East and the increasing cost of mains water, the controlled management of water on site should be a consideration for all concrete producers. Even with a density of only 1.04 the solid material is extremely abrasive and any pumps and pipework used on the systems need to be designed for the purpose if they are to be reliable. A water weigh hopper on a batching plant makes it easy to integrate the cementitious water although many plants only have a flowmeter; a non-contact flowmeter is the sensible option to handle recycled water.
The most important development of concrete reclaimers has been carried out in Germany with some specialist companies having more than 20 years’ experience, resulting in a variety of principles including a simple screw, trough washer or drum washer. The effectiveness of these systems depends on the application, the number of trucks being washed out and the volumes and product being recycled.
Mortar presents its own difficulties due to the high quantity of fine sand and not all systems can handle it effectively. A Bibko reclaimer for mortar, while looking identical to that for ready-mixed concrete, has subtle internal differences. On joint ready-mixed concrete and mortar sites there can be concerns over the effect of the retarding admixtures used in mortar on the concrete. A duplex machine with two separate compartments allows the water and the recovered aggregate to be kept totally separate while maintaining a smaller footprint and lower investment than two separate systems.
High peak volumes of washout and returned concrete are a common situation at inner-city sites and those operating a large fleet of trucks. A dosing buffer that allows more than eight trucks to be washed out at the same time without overloading the system and affecting the quality of the recovered aggregates is a common solution. Increasing the height of discharge of the recovered aggregates means that the pile does not have to be removed so often and can even allow a screen to be incorporated for separating the sand and gravel while maintaining a reasonable height of material.
Precast concrete, while very similar to ready-mixed concrete, usually produces drier concrete and uses less water. When washing casting machines more cementitious water is generated than can be used in the production of the concrete. This calls for additional treatment of some or all of the water to allow it to be disposed of.
Filter presses can be used to clean surplus water for all types of fines, which are discharged as a hard cake ready for disposal. Alternatively, clarification systems, often with the use of flocculants, will remove fines for discharge as a thick sludge that can be disposed of or, as is common in Europe, pumped back into the mixing process. Both options leave the water clean but with a high pH level that can be adjusted either by acid dosing or oxygenating before being returned for re-use or disposal.
Whatever the plant or the products produced, recycling systems offer an effective solution to the environmental challenges facing the concrete industry. Compared to a hole in the ground they need to be controlled and monitored more closely but also offer major cost savings. One of the main criteria for a successful recycling system is commitment from a company’s senior management.
Associated Concrete Solutions Ltd, PO Box 304, Flyford Flavell, Worcester WR7 4YS; tel: (01386) 793710; fax: (01386) 793789; email: [email protected]
