This study was run using both synthetic and field wastewater. The conventional sand or anthracite medium has a reverse porosity gradient because the fine grains accumulate on the top of the bed and the large grains accumulate at the bottom of the bed during stratification after backwashing. As a compressible material, crumb rubber media has an ideal porosity gradient because the top layer of the media is the least compressed and the bottom layer of the media is the most compressed. In comparison to the sand or anthracite filter, the crumb rubber filter favours in-depth filtration and allows longer filtration times and higher filtration rates. For turbidity and total suspended solids removal, the crumb rubber filter performed similarly to the sand/anthracite filter. Because of its longer filtration times and higher filtration rates the use of crumb rubber as a filter media could significantly increase filtration efficiency.
Granular media filtration is an effective tertiary wastewater treatment process to remove suspended solids from the effluent of a conventional biological treatment plant.1 The filtration process also further reduces the organics, nitrogen, and phosphorus levels in wastewater. By removing suspended solids that can harbor and protect pathogenic bacteria and viruses from disinfectants, wastewater filtration also assures effective disinfection for reuse of tertiary effluent. Filtration is also an effective process to remove helminth eggs and protozoal cysts.
Generally speaking, an effective filter medium should be (1) coarse enough to retain large quantities of suspended solids, (2) fine enough to prevent passage of suspended solids, (3) deep enough to allow relatively long filter runs. An ideal filter bed for downward flow filtration consists of a coarse medium in the top layer, a medium size medium in the middle, and a fine medium at the bottom, . However, during stratification after filter backwashing, the large grains settle first and the fine grains settle last and accumulate on the top of the bed. This confines effective filtering in the upper few inches of media. This results in significant surface plugging of filters and head loss development, and then significantly reduces filter run time. A dual-media filter with a heavier sand layer topped with a bed of lighter anthracite medium provides more effective filtration. However, the undesirable medium configuration still exits in each layer of the media. Therefore, the development of a filtration technology with an ideal medium configuration is needed for water and wastewater filtration.
Over 230 million of scrap tyres are generated each year in the United States.2 These tyres are a greater disposal problem than most wastes because they will not stay buried in landfills. Scrap tyre piles are a fire hazard and a breeding ground for vectors. Scrap tyre reuse or recycling is an effective solution for the growing scrap tyre problem. Crumb rubber, a tyre-derived material, is currently being used in highway pavement, athletic track, playground surfaces, landfill liners, compost bulking agents, energy recovery and artificial reefs for aquatic life.
The use of waste tyre chips in aerobic and anaerobic biofilm reactors has been reported.3 and 4 Tyre chips performed well in these biological wastewater treatment processes. Because of its elasticity, crumb rubber is an excellent filter medium for water and wastewater treatment.5 The crumb rubber bed has the smallest pore size at the bottom and the largest pore size on the top. This filter porosity configuration allows a higher filtration rate, lower head loss, longer filter run time, and better effluent quality. Using crumb rubber for wastewater filtration reduces scrap tyre problems, and more importantly, it provides a new filtration technology which is far more effective than the current dual-media sand-anthracite filters.
The objective of the project was to investigate the effectiveness of crumb rubber media for wastewater filtration. The filter performance was evaluated by head loss, suspended solids removal, and turbidity removal. The comparison study was conducted using both crumb rubber and sand/anthracite media. A laboratory study was conducted using synthetic wastewater samples and a field study was conducted using the effluent from a local wastewater treatment plant.
The results
Clean bed head loss
The head loss through a clean sand, anthracite or sand/anthracite media wass generally less than 0.9 m. The total head loss through the clean sand/anthracite media was 37.6 cm at a filtration rate of 12.2 m/hr and 89.4 cm at 24.4 m/hr. The head loss increase for the upper layer of the media was not as great as the increase for the bottom layer of the media. This was due to the grain size for the bottom media, sand, being smaller than that for anthracite. Increasing the filtration rate increases the head loss through the media.
At the same filtration rate, the head loss through the crumb rubber media was much less than that through the sand/anthracite media. At filtration rates of 12.2 m/hr and 24.4 m/hr, the head losses were 8.1 and 19.3 cm, respectively. At a much higher filtration rate of 61.1 m/hr, the head loss was only 62.0 cm.
In comparison with the sand/anthracite media, the crumb rubber media resulted in a smaller head loss. This indicated that crumb rubber filters could be operated at a much higher filtration rate or for a much longer period.
The head loss through each section of the crumb rubber media was similar. This indicated that the porosity of the crumb rubber media was not significantly changed along the filter. However, the compression of the crumb rubber media was indicated by decreasing the total height of crumb rubber media in the column. Further laboratory studies are underway to investigate the compression and porosity distribution inside the crumb rubber media.
Laboratory study
With a synthetic wastewater sample, a laboratory pilot study was conducted with sand/anthracite and crumb rubber filters. Both filters were operated at a filtration rate of 24.4 m/hr. The influent suspended solids concentration varied between 4–18 mg/L and the influent turbidity varied between 3–6 NTU.
For suspended solids and turbidity removal, both the sand/anthracite and the crumb rubber media performed similarly. Both filters produced an effluent which contained less than one mg/L of suspended solids. For wastewater filtration, the typical filter effluent concentration should be 10 mg/L or less. For turbidity removal, both filters produced an effluent containing a turbidity of one NTU or less. Again, this is well below the typical filter effluent concentration of 3 NTU or less.
A significant difference was observed for the head loss development between the crumb rubber filter and the sand/anthracite filter. After 135 minutes of operation, the head loss inside the sand/anthracite filter was 107 cm. However, the head loss inside the crumb rubber filter was only 45 cm. A slow development of head loss indicated that the crumb rubber filter could be run much longer before a filter backwash is required.
The synthetic wastewater sample contained a high percentage of settleable solids because no sedimentation was used for both synthetic wastewater samples or the mixed liquor samples. In practice, sedimentation effluent is used for filtration. A high level of settleable solids could cause a rapid development of head loss through the filter media. The significant difference in head loss development between the crumb rubber media and the sand/anthracite media could be amplified by non-settleable solids in the synthetic wastewater.
Field study
The effluent from the secondary sedimentation process at the Palmyra Wastewater Treatment Plant was used for the field filtration study. Both filters were operated at a filtration rate of 24.4 m/hr. The concentration of suspended solids in the influent varied from 0.4 to 3.6 mg/L and the concentration of turbidity varied from 1.5 to 3.5 NTU. The crumb rubber filter was operated for 13 hours and the sand/anthracite filter was operated for 5 hours.
During the operation period, the crumb rubber filter and the sand/anthracite filter performed similarly on suspended solids and turbidity removal. For the suspended solids measurement, water samples were taken at 1, 3, 5, 7, 9, and 11 hours. For the turbidity measurement, water samples were taken at 1, 2, 3, 4, 5, 9, and 13 hours. The filtered water contained less than 0.5 mg/L suspended solids and 1.0 NTU or less turbidity for both filters. There was no significant difference between the effluents of the two filters in terms of suspended solids and turbidity.
There was a significant difference between the two filters in head loss development. The head loss through the sand/anthracite filter was 206 cm after five hours of operation. The filtration was stopped because a backwash was required. However, the head loss through the crumb rubber filter was only 30 cm after five hours operation. After 13 hours of operation, the head loss was 38 cm. The filtration was stopped after 13 hours of operation due to the time constraint. It is estimated that the crumb rubber filter could be run for at least 24 hours before a backwash is needed. In general, filters are backwashed every 24 to 48 hours to prevent the filters from becoming septic.
The preliminary field study indicated that the crumb rubber filter outperformed the sand/anthracite filter in water production while maintaining the effluent water quality. The crumb rubber media can significantly reduce the filter size and its construction and land cost. In general, backwashing is conducted with filtered water. By minimizing the filter backwash times, a crumb rubber media can significantly reduce the water requirement for backwashing and increase the water production. It is expected that the crumb rubber filter will outperform the sand/anthracite filter for water containing high suspended solids. Further studies should be conducted with water containing high levels of suspended solids and turbidity.
Conclusions
Crumb rubber is an effective filter media for wastewater filtration. In both laboratory and field pilot studies, the crumb rubber filter and the sand/anthracite filter performed similarly for suspended solids and turbidity removal. However, the head loss in the crumb rubber filter was much less than that in the sand/anthracite filter. Because of the low head loss, the crumb rubber filter could be operated at a high filtration rate, for a longer operation time, and with a less frequent backwash. This could reduce the filter size or increase the water production. Further studies are needed to investigate the compression of the crumb rubber media during filtration and the performance of the crumb rubber filter using wastewater containing high levels of suspended solids.
References
1 Metcalf & Eddy, Inc, “Wastewater Engineering: Treatment, Disposal, and Reuse”, McGraw-Hill, Inc, New York, NY (1991).
2 H. Pillsbury, Resource Recycling 10 (6) (1991), p. 19.
3 K.-S. Yoo and J.K. Park, Water Environmental Research 71 (1999), p. 363.
4 E. Sanchez, N. Rovirosa, R. Borja, M. Cruz, M.F. Colmenarejo and R. Escobedo, Bioresource Technology 70 (1) (1999), p. 55.
5 C. Graf and Y.-F. Xie, Keystone Water Quality Manager 33 (6) (2000), p. 12.
Uncited reference
APHA, AWWA, and WEF, “Standard Methods for the Examination of Water and Wastewater”, 20th Ed, APHA, AWWA, and WEF, Washington, DC, 1998.
The authors are grateful to the Middletown Wastewater Treatment Plant for assisting with sample collection, the Palmyra Wastewater Treatment Plant for assisting with the field study, Drs. Samuel A. McClintock and Charles A. Cole for their technical assistance.
