A case for glass
Recycled glass represents a growing, and as yet largely untapped, market in the UK. Mark Lowe of Aqua Enviro and Dr Nigel Horan of the University of Leeds look at its applications in the water industryEvery year in England and Wales, around 2.5M tonnes of glass is sent to landfill. In principle, substituting this used glass for virgin raw material should be straightforward as its structure does not deteriorate when reprocessed. And thus, in principle, it can be recycled indefinitely.
Although this route would provide the optimal environmental benefit (Butler and Hooper, 2005), there are practical reasons why it does not happen. UK manufacturing industry uses predominately clear and amber glass for packaging perishable products in jars and for bottling spirits. Consumers appear reluctant to return clear jars to bottle banks and much of the UK spirit production is exported. By contrast the UK imports twice as much green glass as it manufactures, mainly in the form of wine bottles. And recovery of this is very good. Consequently, there is a colour imbalance between glass imports and exports, which leads to a surplus of green glass that is presently destined for landfill. The EU Packaging and Waste Directive requires the UK to achieve a glass recycling rate of 60% by 2008 (EC, 2004), and, if this is to be met, alternative markets must be found for this resource.
Marketing recycled glass
Glass can be crushed and sieved to produce a range of products with specified particle size distributions and these find a range of diverse applications.
Coarse glass is used as an aggregate substitute or backfill material and a product known as Glassphalt is used for road surfacing, with around 14 million bottles crushed and used to resurface the M6 motorway. Finer glass has been used as a top dressing for golf courses or as a fluxing agent in brick manufacture. This fine glass has a particle size distribution that is very similar to that of the sand used in traditional sand filters, and it has many of the other properties associated with filter sand (described by Crittenden et al. (2005)). And so it is natural that a lot of attention has been focused on its potential applications in the treatment of water and the tertiary treatment of wastewater.
Recycled glass has already made inroads into the water treatment market, in particular in the form of the engineered glass product AFM, which is employed in applications ranging from pressure filters to swimming pools. Its application in wastewater treatment represents a growing, and as yet largely untapped, market in the UK.
Tertiary sand filtration is a traditional engineered solution to tackle the problem of high-effluent suspended solids. And it has found widespread application at both municipal and industrial treatment plants in the UK (Rundle 2005). The early generation sand filters were gravity fed with a counter-current backwash and air scour (IWPC, 1974). And, although the technology proved reasonably successful, many sites experienced problems. These generally resulted from failures in sand backwash, which led to the sand blinding and a consequent reduction in the flow rate through the filter. In addition, there have been examples of channelling in the sand bed, which has led to short circuiting and a reduction in performance.
Problems with backwash have been overcome by the development of continuous, moving bed backwash systems and deep bed filters. The former utilises a bed of mineral media based on sand but with larger diameters and a consequently higher cost. It is continuously removed from the bottom of the filter vessel by an air-lift or screw pump. This is then washed, the solids separated and the sand returned to the top of the vessel. The latter employs a deep bed of 1.2-2m depth with a rounded sand of around 2mm diameter that is backwashed daily with a powerful air and water wash (Rundle, 2005).
Recycled glass media has been shown to require lower backwash flow rates when used in potable water applications (Evans et al, 2002). In view of the problems that have been encountered with traditional sand filters, it was thought that a similar effect might be demonstrated in wastewater applications. And thus, it was thought, simple replacement of sand with recycled glass might provide a simpler and more robust solution.
Evidence from the US supports this concept as recycled glass can be crushed to meet the ASTM C-33 specification for sand, and is used widely as a tertiary filter media following residential septic tank systems (Aqua Test, 1995). It has been found to have a permeability more than nine times that of sand, attributed to the fact that glass particles are largely cuboid. This gives greater interstitial spaces than the rounder sand particles. As a result, the glass filters were much less prone to clogging. Indeed, this application is so successful that in San Juan County, Washington, US, this market is able to handle all of the counties recycled glass and at a price three times higher than its application as an aggregate replacement.
But, in order to convince UK water companies of the efficacy of recycled glass, the technology has a number of important requirements it must fulfil, namely that it:
- Provides a reliable solution to ensuring that a treatment plant always remains in consent
n Is a cheaper whole-life solution (typically over ten to 15 years) than alternatives such as moving bed systems, membranes or increasing secondary clarification capacity
- Is technically simple and robust without the need for skilled staff to operate it
- Can be safely used at remote sites which receive no operator attention for periods of several days
- Does not require the use of chemicals
- Does not produce a sludge that is difficult to handle
- Demonstrates a commitment to sustainability that will be recognised by Ofwat as part of the PRO9 process
bench, pilot- and full-scale in order to test the above requirements (Horan and Lowe, 2006). These trials have investigated both municipal wastewaters and industrial wastewaters with influent solids concentrations ranging from 30mg/l to 150mg/l, and a clear picture has emerged with respect to its performance and practical applications.
Recycled glass is commercially available in a range of particle sizes (although bespoke glass can be prepared to any specification). Fine glass has a particle size of 0.2-1mm; medium glass 0.5-1.45mm, coarse glass 1.5-2.5mm (all supplied by AllGlass) and AFM (Dryden Aqua) has a particle size of 0.5-1.1mm. When used in
a conventional upward flow filter in place of sand, fine glass is able to
produce a very high-quality effluent with solids typically less than 10mg/l. But it clogs rapidly and is difficult to backwash.
Medium glass by contrast provides a balance between good solids removal, high flow rate and ease of backwashing. Speciality glasses such as AFM offer additional benefits to suspended solids removal such as the removal of toxins (for instance tri-butyl tin) as a result of the engineered reactive glass surface.
When trialled against the sand media typically used in conventional sand filters, it is clear that glass performs at least as well and indeed slightly better for suspended solids removal (Figure 1). But the biggest advantage of the recycled glass media is that it is able to maintain performance longer and can treat an additional 10% of the flow before a backwash is required.
In addition, it is easier to backwash the recycled glass filter with again around a 10% improvement in backwash efficiency.
Calculating the volume of glass required for a tertiary filter is quite straightforward and depends on the required effluent quality.
In the example below for an industrial wastewater with an effluent suspended solids up 40mg/l, a suspended solids consent of 20mg/l, with a 95 percentile compliance, can be achieved with a solids loading of 0.1kgm3/h (Figure 1).
But this value is indicative only as it has been shown to be extremely sensitive to the nature of the wastewater and has varied from 0.1-0.25kgm3/h over four wastewaters evaluated. By contrast, the upward flow velocity has been much more consistent with a value of <15m/h required for a 20mg/l consent.
Full-scale comparison trials are currently taking place at Yorkshire Water's Malton WwTW where two identical filters have been used in the trial (Figure 2). One has conventional high-grade silica sand, and the other filter has been replaced with medium-grade glass. The results to date have shown that the glass removes about 10% more suspended solids than sand and is able to process about 23% more flow.
The air lift pump has also been reduced by 5%, which has saved around £600 a year in electricity costs.
Crushed and graded recycled glass is able to remove suspended solids from the both domestic and industrial effluents when used as a medium for tertiary filtration.
Its performance is generally better than the sand medium traditionally employed in tertiary filters. And up to 70% removal of suspended solids can be achieved and a consent of less than 20mg/l met, providing that the influent does not exceed 70mg/l suspended solids. For a typical domestic WwTW where the suspended solids in the effluent would not be expected to exceed 50mg/l, an effluent suspended solids of less than 20mg/l can be achieved with a solids loading rate of less than 0.25kg/m3/h and an upflow velocity less than 15m/h. With an air scour using 3% of the treated effluent glass filter medium can treat 10% more flow than the sand.
The market for tertiary filters is likely to continue to grow as UK water companies continue to invest in a range of technologies to ensure that discharge consents are met at all times. At the same time the UK Water Act 2003 has extended the duties of Ofwat, the water industry regulator, to include a statutory duty to contribute to the achievement of sustainable development (Ofwat, 2006). Recycled glass offers the water industry the opportunity not only to safeguard consent compliance, but also to do so in a more sustainable manner.