Water is an important resource for people’s quality of life and is indispensable for several uses, including desedentation. However, this resource is finite and available in small quantities in the quality of fresh water, at liquid state and on the surface. Therefore, rivers and lakes, providers of surface water resources, must be treated and kept clean, and protected from pollution.
Even with people’s knowledge of the importance of water, the scenario is just the opposite, with surface water resources being treated unsustainably, receiving loads of effluents, such as domestic and industrial sewage, and being constantly degraded. Due to this scenario, Ghisi (2006) states that several countries, including Brazil, will suffer from problems and insufficiencies in the distribution of potable water to their populations.
Seeking to collaborate with the maintenance of water resources, regulating agencies, fiscally responsible for the quality of the water and the effluents that it receives, seek to maintain the current state or improve the quality of the water bodies. In Brazil, the regulatory agency is the Conselho Nacional do Meio Ambiente (CONAMA) (National Environment Council) and the supervisory agent, for example in the state of São Paulo, is the Companhia Ambiental do Estado de São Paulo (CETESB) (Environmental Co. of the State of São Paulo). But despite a resolution and enforcement, pollution and negligence still occur in rivers and lakes across the country.
The water made available to the population as potable is treated in water treatment plants (WTP) and, after being used, is then classified as sewage and again treated, now in sewage treatment plants (STP). If water is used for industrial purposes, additional treatment steps may be required, and, in such cases, wastewater treatment should be performed at industrial wastewater treatment plants (IWTP) (Di Bernardo and Dantas 2005). The treatment plants seek to comply with Resolution 430 of CONAMA (2011) and then return the treated effluent to water bodies (Marçal and Silva 2017).
However, in the cleaning processes of the treatment plants, significant amounts of wet waste (sludge) are generated, which have low solid content. The waste generated then needs proper transportation and final disposal, but the transporting and disposing of these untreated wastes can be impracticable, resulting in the transport of large wet volumes, being charged (disposal cost) for this volume, and allowing the formation of larger amounts of slurry in the landfill, which represent a risk for the environment in case of leakage. Therefore, techniques and processes that collaborate with the dewatering of these sludges are fundamental, allowing the transport of a smaller volume, with lower tariffs and implying more safety for disposal in landfills.
In view of the problems caused by the generation of wet waste, this article examines the use of the dewatering technique in geotextile tubes (Figure 1 on pages 32–33), providing the elevation of the solid content of the dewatered material, decreasing its volume, and facilitating its handling as solid or semisolid (Müller 2019; Vertematti 2015).
The geotextile tube dewatering technique is a mixed dewatering solution consisting of a geosynthetic tube that receives hydraulically filling cycles with sludge (Castro 2005; Koerner 2005; Müller 2019; Pilarczyk 2000; Tominaga 2010; Vertematti 2015). In the filling step, corresponding to the mechanical dewatering, hydraulic pressure causes liquid to be expelled through the geosynthetic (forced filtration), while the solids are retained by it. After the filling and relieving of pressure, the sludge is naturally dewatered, where the evaporation process becomes increasingly relevant as unsaturated zones begin to form.
For the correct use of this dewatering technique, the geotextile tubes must be positioned over a draining cradle; the cradle is responsible for collecting percolated water from the tubes, allowing their recirculation in the treatment system or their correct return to the relevant water body.
In this article, then, a dewatering project is presented and discussed. The project consists of the dewatering of the material contained in three sedimentation lagoons of the STP Campos de Boituva in São Paulo. Using the existing drying beds in the STP for the correct positioning of geotextile tubes, the STP sought lower intervention costs and increased dewatering capacity in relation to existing beds.
STP Campos de Boituva is operated by Companhia de Saneamento Básico do Estado de São Paulo S.A. (SABESP) (Basic Sanitation Co. of the State of São Paulo) and serves a large portion of the city.
The treatment plant has three aeration ponds for its processes, three decantation ponds and 20 covered drying beds, each 807 square feet (75 m²) in size. Layout of the station can be seen in Figure 2.
The three settling ponds of the Campos de Boituva STP were heavily loaded with solid organic material in their interiors, compromising the receipt of more effluent and impairing its operating processes. Thus, cleaning of the three ponds was necessary.
Through tests performed by the treatment plant, 423,776 cubic feet (12,000 m³) of densified material with 5% solid content per mass was identified. This material was predominantly composed of organic matter.
Given the need to clean the three settling ponds, solutions were evaluated. Initially, it was considered to pump the dense material with hydrovacuum tank trucks, but soon it was realized that the wet volume to be transported was too great, making this option unfeasible. Then, dewatering techniques were considered, aiming to separate the liquid part from the solid part, providing a smaller volume to be treated.
At the top of the list of the various dewatering techniques available in Brazil, the mechanical options were soon discarded, representing high acquisition and maintenance costs. Therefore, as the existing drying bed dewatering technique was not able to deal with the required volume in pertinent time, the geotextile tube dewatering technique was chosen.
Implemented dewatering solution
The selected dewatering technique corresponds with the use of containers made of geosynthetics, assuming a linear tubular shape upon filling, which is designed in a variety of perimeters and lengths to result in a geotextile tube (Müller 2019). Geotextile tubes have, then, the function of filtering the sludge, retaining the solid particulate inside and allowing the water to percolate (Castro 2005; Koerner 2005; Moo-young et al. 2002; Müller 2019; Pilarczyk 2000; Tominaga 2010; Vertematti 2015).
Through filtration and retention of solid particulate, this dewatering technique increases the solid content of the sludge. The operation reduces the moisture content so the generated waste can be handled as a semisolid/solid, thus providing much easier and more efficient handling and transportation than would happen with the previous wet material (Castro 2005; Koerner 2005; Müller 2019; Pilarczyk 2000; Tominaga 2010; Vertematti 2015).
As mentioned previously, for the correct operation of the technique, the geotextile tubes must be arranged over a draining cradle, ensuring the collection and concentration of the percolate, gauging its quality and then promoting its correct destination (Müller 2019; Vertematti 2015). In the application scenario, the drying beds were used as drainage cradles for the geotextile tubes, the percolate being redirected to the settling ponds. This avoided the need to build a cradle, saving costs and speeding up the beginning of the necessary dewatering process.
The geotextile tubes were then designed and quantified, taking into account the length and width constraints of the existing drying beds, which were 16-feet (5-m) wide and 49-feet (15-m) long. The software GepCoPS 3.0 (Leshchinsky and Leshchinsky 1996) was used and, by geometric and stress analysis, 14 geotextile tubes were defined for cleaning the lagoons. Figure 3 shows the geotextile tubes designed, with a perfect fit in the existing drying beds.
Through cone testing (Vertematti 2015), high-strength polypropylene woven geotextile was defined as the best option for the sludge filtration. This geotextile was then used to make the geotextile tubes for the pond-cleaning service. The geotextile had the following characteristics: Filtration aperture (NBR 12.956 2013) of 0.008 inches (0.2 mm), speed of normal flow through the plane of the geotextile (NBR 11.058 2013) of 20.10-3 m/s and resistance to tensile strength (NBR 10.319 2013), both longitudinal and transversal, of 600 pounds-force per inch (105 kN/m).
The sludge was then pumped through a dredger, removing the densified material from the ponds and filling the geotextile tubes located in the drying beds (Figure 4). The geotextile tubes received several filling cycles until their maximum admitted volume was reached. After this process, they rested for a period of three weeks.
Through the dewatering process, the sludge increased from 5% solid content by mass to approximately 25% inside the geotextile tubes. This represents a reduction from the initial 423,776 cubic feet (12,000 m³) to 49,441 cubic feet (1,400 m³) at the end. The process provides 8.5 times less volume than the original, resulting in lower transportation and final disposal costs.
Considerations about the dewatering process
The maximum height that a geotextile dewatering tube can reach must be correctly and clearly indicated by marking on the geotextile surface in an easily viewable location. The indicated height must be strictly adhered to, as it represents the means of the tensile forces acting on the tube, the maximum allowable height for safety, and the correct functionality of the dewatering system.
If the maximum height is not adhered to and is exceeded for any reason, there is a high risk of tube rupture. This risk is increased by the gain of height with its filling, due to the increase in internal pressure, and it should be avoided.
As the geotextile tube is made by a filtering element, over time the water will percolate, and the tube will lose height due to consolidation. In this way, new filling cycles are possible, treating and dewatering larger volumes of sludge. Thus, a geotextile tube can receive a sludge volume many times larger than its nominal volume.
The filling of a geotextile tube with predominantly organic sludge provides biological clogging of the geotextile pores. This clogging comes from the formation of bacterial and algal colonies that blind the geotextile, preventing water percolation and, thus, retaining moisture inside the tube. This clogging is easily avoided, however; for example, in the case studied, frequent washes of the pipe surface were performed with a pressure hose.
Through the application case presented, it is possible to realize that dewatering in drying beds would not provide the effective cleaning and dewatering of the sludge from the consolidation ponds in time. Furthermore, mechanical dewatering techniques, which would be effective for the purpose of cleaning the ponds over time, were not viable, representing high investment and maintenance costs for the service.
Thus, the mixed dewatering technique with geotextile tubes is interesting, representing low investment and maintenance costs, combined with the effectiveness in gaining solid content over time through its dewatering technique (Figure 5).
Therefore, the use of geotextile tubes offers advantages over conventional dewatering techniques, such as the need for reduced implementation area, low cost of implementation of the technique, good increase in the sludge solid content, great reduction of volume, ease of transport and disposal of waste generated, rain tolerance, use of existing facilities (sediment pond/drying bed) and recirculation of percolated water at the treatment plant.
M. Müller is CEO at SALUS Dewatering Engineering in São José dos Campos, São Paulo, Brazil.
E. A. Guanaes is product manager at HUESKER in São José dos Campos, São Paulo, Brazil.
D. Vida is professor at the Instituto Tecnológico de Aeronáutica (ITA) in São José dos Campos, São Paulo, Brazil.
M. R. Freitas is professor at the Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP) in Guaratinguetá, São Paulo, Brazil.
All figures courtesy of HUESKER.
To HUESKER for its support in the real case analyses.
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This article was part of a white paper technical presentation at GeoAmericas 2020.
Geotextile tubes for a Brazilian sewage treatment plant
OWNER: CIA de Saneamento do Estado de São Paulo (SABESP)
LOCATION: Boituva, São Paulo, Brazil
CONTRACTOR: Freitas Guimarães Construções LTDA
DESIGN ENGINEER: Saneflora
GEOSYNTHETICS PRODUCT: SoilTain PP 105/105 DW P 100 L150 geotextile tubes
GEOSYNTHETICS MANUFACTURER : HUESKER