The glass is greener
Recycled glass in wastewater treatment shows promise as a sustainable alternative to sand. Dr Nigel Horan of the University of Leeds and Mark Lowe of Aqua Enviro reportThe pressure is rapidly mounting on companies in the domestic and industrial wastewater treatment sector to demonstrate commitment to sustainable practices.
Many major shareholders now demand that environmental reporting forms a key part of a company's annual statement.
In addition, the recent AMP4 proposals have required that the water companies' sustainability takes many forms and for most companies there are still no clear guidelines as to what constitutes the most sustainable practice.
However it is widely accepted that all companies should strive to minimise their use of non-renewable resources and wherever possible substitute these with renewable or recycled material.
One such recycled material that is showing a lot of promise for the water industry is recycled glass, which has the potential to replace non-renewable sand, gravel and aggregate in a wide range of applications. To evaluate the potential for recycled glass, the Waste Resources Action Plan (WRAP) has recently funded a large-scale pilot trial to investigate a range of glass as a replacement for sand to remove suspended solids in the tertiary treatment of wastewaters.
To ensure a representative range of effluents was evaluated, trials were undertaken at three sites: a domestic WwTW using biological filtration;
a food processor with sequencing batch reactor technology; and a paper mill employing conventional activated sludge. The study was done in two stages, with a pilot-scale first stage aimed at selecting the most efficient glass media, and the second stage full-scale trial comparing the performance of this glass medium with its more widely used sand counterpart.
The first stage utilised five filters at each site with a diameter of 0.2m and a height of 0.9m. The filters were packed with 20kg of media - fine, medium and coarse glass, Active Filter Media (AFM) and sand - and the properties of each of these media are summarised in Table 1.
The filters were operated continuously in an upflow mode with a retention time on the filter of between two and four minutes. Backwash was achieved by reversing the flow, in response to a specified headloss across the filters. A typical backwash employed 4% of the volume processed by the filter.
During the assessment of sites for undertaking the pilot studies, one of the main selection criteria was that the final effluent for treatment should have suspended solids in the range 30-80mg/l and analysis of historic data was undertaken to confirm this.
Needless to say, over the course of the study this criterion was breached on a number of occasions and the suspended solids applied to the filters actually ranged from 15-310mg/l.
Nevertheless, a clear picture did emerge with the glass media demonstrating better removal efficiencies than the sand in all cases, but with little difference in performance between the four glass types (Table 2). The fine glass produced a better effluent quality but it required frequent backwashing. The medium glass could process twice the flow with an effluent quality that was only slightly worse than fine glass. Consequently medium glass was selected for the full-scale trials.
The full-scale trials were undertaken at the same three sites but in filters with a height of 2.1m and an area for filtration of 0.44m2. This enabled a flow rate of 10m3/h to be treated. In addition to evaluating the performance of the glass medium in direct comparison with sand, the full-scale trials also aimed to optimise the back-flush regime and it was recognised that an air scour would be required to achieve this.
The optimised cycle comprised treatment for 2.5h at a flow rate of 10m3/h. This was followed by an air scour for two minutes at an airflow rate of 30m3/h, and concluding with a counterflow backwash with final effluent at 20m3/h for a period of five minutes. The total volume of backwash water thus represented about 4% of the final effluent generated by the filter.
When operated with the optimised backwash cycle at the domestic treatment plant, the influent suspended solids to the filters were in the range 26-84mg/l with an average of 46mg/l. The filters demonstrated an average removal efficiency of 68%, and achieved an effluent quality which averaged 15mg/l for the glass medium and 17mg/l for the sand medium.
The filters could consistently remove around 25mg/l of suspended solids from the influent and thus when there was a large peak in influent solids, although this was attenuated it still appeared as a peak in the filter effluent (Figure 1). Clearly there was little difference in performance between glass and sand under identical influent and backwash conditions.
The performance of the filters varied in a linear way in response to the applied solids loading to the filters (Figure 2) and clearly, in order to achieve an effluent suspended solids from the filters of less than 20mg/l, a solids loading of below 0.1 kg solids/m3 media h must be maintained.
Although the same relationship was evident at each of the three sites, the critical solid loadings were different and varied between 0.1 and 0.25kg solids/m3 media h for the same effluent quality.
This study has clearly demonstrated the potential of recycled glass as a replacement for sand as a filter medium, but there are many other potential applications in the industry that are now at the initial stages of investigation. For instance, reed beds - considered a sustainable, low-cost option - utilise large quantities of sand and aggregate in their construction.
A pilot-scale reed bed constructed using recycled glass in place of aggregate is currently being trialled at the Bernard Matthews Great Witchingham site, and early results are very promising. Considering the large number of applications in environmental engineering which utilise sand and gravel - such as sustainable urban drainage (SUDS), green roofs or living ponds - clearly the opportunities for recycled glass as a possible alternative are large.
In order to explore further the possibilities of recycled materials, a one-day exhibition and seminar, Sustainable Engineering Solutions for the Water Industry Using Recycled Materials, is being held on 26 January at the Royal Armouries, Leeds. Further details are available at www.aqua-enviro.net
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