GEOTEXTILE bags (geobags) for use in on-site desludging wastewater projects, have advantages over conventional methods, which are often unaffordable to authorities and contractors with limited budgets.
That’s according to Meera Govender, Technical Product Manager, Hammarsdale-based Fibertex SA, who said benefits of using geobags for sludge removal include ease of operation, efficient retention of solids, compliance with environmental regulations and cost optimisation. A further advantage is this dewatering system is passive and does not require constant monitoring and maintenance of equipment.
She added that the filtration properties of geotextiles – which are effective in retaining the fine-grained materials found in sewage sludge, while allowing the water to filter through – make these materials suitable in desludging wastewater ponds. The effluent that passes through the woven fabric can then be transferred to a designated safe disposal site, given that it meets the regulations set by the Department of Water Affairs (Policy and Strategy for Groundwater Quality Management in South Africa).
“When wastewater treatment ponds reach their capacity, the options available are to either build new facilities or to empty the existing ones to make additional space available for waste. Traditional methods of waste containment, which usually require large amounts of space, stringent environmental permits and frequent dredging, are normally unaffordable to municipalities,” Govender said.
“A feasible solution needed to be found to dispose of extracted material through the desludging process, which is not restricted by budget constraints and the limited space available at existing wastewater facilities.
“The use of geotextile dewatering bags is recognised globally as the most efficient and economical method to achieve the required desludging process, to improve the functionality and safety of wastewater treatment works.”
Fibertex geobags, manufactured from UV-stabilised woven Polypropylene (PP) geotextile materials, are incorporated in the desludging process in wastewater treatment – the process used to convert wastewater into an effluent that can be returned to the water cycle with minimal environmental impact.
Solid particles (sludge) are then separated from the effluent and transported to a designated safe disposal site or stored in a tailings storage facility at the wastewater treatment plant.
Fibertex geotextile dewatering bags, with high strength seams for a strong and durable dewatering system, are used to capture dredged sludge, whilst minimising water loss. The thread used to stitch the bags has a higher breaking strength than the geotextile itself, providing sufficient tensile strength to the geobags, to withstand the stresses associated with pumping the material at high pressures.
Dewatering bags allow water to flow out through the porous geotextile fabric of the bags, while filtering any solids. The concern of the woven fabric opening size appearing to be slightly larger than the particle size of the dredged sludge material is countered by the formation of a filter cake on the inside of the fabric, with a resultant effective retention of the solids.
This creates an equivalent two-stage filter, with filtration efficiencies above 90% for fine-grained material filtered through the high-strength woven geotextile bags. Certain geotextile designs are able to retain all particles larger than 10 microns.
In recent local projects, where wastewater treatment plants had reached their maximum storage capacities, the Fibertex solution was to remove sludge from the existing tailings facilities by using geotextile dewatering bags on-site. Fibertex also provided the geosynthetic products required to carry out repairs to existing lining systems that were damaged, and for installation of new lining systems in facilities which previously had none.
To accommodate pipe connections, two inlets per bag of standard size 200mm were selected for pipe sizes up to 200mm diameter. The pipe was inserted approximately two thirds of the way into the injection port and secured with tension strapping. At the end of the filling process in all installations, once the pipe is removed, inlets can be tied off easily.
The through-flow property of the geotextile determines the rate at which the effluent flows out of the geobag and these pumping rates are constantly monitored. A moderate rate ensures these bags keep their structural integrity throughout the operation.
Selection of the final disposal site for the dewatering geotextile bags needs to be as close to the generation area as possible to minimise transportation costs. In addition, Fibertex recommends the following selection procedure for the suitable area: ensure that the disposal site is not located in a sensitive area where disposal is not permissible; ensure that the sludge disposal site is located as far as possible from the area where the final effluent is discharged to limit possible contamination of the final effluent and to limit possible contribution of contaminants to the water resource.
It is important to allow for the maximum buffer zones, greater than 400m from surface water; to consider the slope of the disposal site to minimize run-off, erosion and ponding and to ensure the disposal site is not within the 1:100-year flood line.
In these recent projects, prior to placement and filling of the geobags, Fibertex lined the dewatering area with a 1mm-thick geomembrane layer, according to SANS 1526 specifications for thermoplastic sheeting. This material prevents local erosion and collects all effluent released from the geobags, thus preventing any seepage of the wastewater into the ground. The effluent is then channelled back into the dam or taken for further treatment.
Flocculants may sometimes not be necessary in this process, as there is sufficient time available for the material to dewater by gravity, resulting in further cost reductions. To ease the removal of the waste, water is pumped into the dams to agitate the sludge. Transfer of the sludge from the tailings dam into the bags is through pumps fitted with an impeller on the inlet to reduce bigger particles as the pumping is conducted.
The Fibertex team assists customers with detailed studies to determine the volume of waste that could be extracted from tailings dams per bag.
The duration of the dewatering and consolidation period varies depending on the type of geotextile, the bag sizes, fill material and site conditions. On average it takes about a week for the bags to drain.
The solids are retained in the geobags and can then be safely disposed of or used as fertilizer. These geobags are also disposed of, as they are not reusable.
The selection of the geobag size is dependent on the volume that needs to be removed from the facility and the space available on site where the geobags will be placed.