Geotextile dewatering bags increase tailings dam longevity

23rd November 2018

Geotextile dewatering bags increase tailings  dam longevity

FUNCTIONAL FILTRATION The filtration properties of geobags are effective in retaining the fine-grained materials found in sewage sludge, while allowing the water to filter through

Manufacturer and supplier of geosynthetic products Fibertex SA’s geotextile bags for use in on-site desludging wastewater projects have advantages over conventional methods, which are often in addition unaffordable to authorities and contractors with limited budgets.

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.

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 regulations such as the Policy and Strategy for Groundwater Quality Management set by the Department of Water and Sanitation.

“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,” explains Fibertex SA geotechnical engineer Johnny Oriokot. “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 ultraviolet-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 are separated from the effluent and transported to a designated safe disposal site, or stored in a tailings storage facility at the wastewater treatment plant.

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.

Fabric Concerns

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 98% for fine-grained material filtered through the high-strength woven geotextile bags. Certain geotextile designs are able to retain all particles larger than ten microns.

Fibertex also provides the geosynthetic products required to carry out repairs to existing lining systems that have been damaged, and for installation of new lining systems in facilities which previously had none.

In order to accommodate pipe connections, two inlets per bag of standard size 200 mm are selected for pipe sizes up to 200 mm diameter. The pipe is 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 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.

Preventive Measures

It is important to allow for the maximum buffer zones, greater than 400 m from surface water; to consider the slope of the disposal site to minimise runoff, erosion and ponding; and to ensure the disposal site is not within the 1:100 year flood line.

Fibertex lines the dewatering area with a 1-mm-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.

No flocculants are 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 solids are retained in the geobags and can then be safely disposed of or used as fertiliser. 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 tailings facility and the space available on site where the geobags will be placed.

“Geobags are an environment-friendly solution and are suitable for use in aquaculture, industrial lagoons, sedimentation ponds and wastewater plants,” concludes Oriokot.