Novel reactor design for enhanced removal of fluoride using a modified Nalgonda methodEPA Grant Number: SV836950
Title: Novel reactor design for enhanced removal of fluoride using a modified Nalgonda method
Investigators: Weber-Shirk, Monroe , Lion, Leonard William
Institution: Cornell University
EPA Project Officer: Shapiro, Paul
Project Period: February 1, 2017 through January 31, 2019
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2016) Recipients Lists
Research Category: Sustainability , P3 Awards , P3 Challenge Area - Water
Groundwater is a feasible source of drinking water for many communities. In India, more than 80 percent of water is obtained from groundwater sources (The World Bank, 2012). Dependence on groundwater becomes a health concern when the aquifer’s composition contains minerals that cause fluoride contamination. Fluoride ions are highly soluble in water, which makes treatment extremely difficult. In developing countries and rural villages, fluoride removal becomes an even greater challenge because technology and funding are limited. AguaClara seeks to overcome obstacles by creating innovative solutions focused on promoting environmental, social, and economic sustainability.
Few fluoride remediation methods are available, and most of them are expensive or are not effective at high fluoride concentrations (Singh, 1999). Fluoride removal by adsorption to aluminum sulfate precipitate is known as the Nalgonda method (Venkobachar, 1997). In this method, aluminum sulfate is added to a batch of fluoridated water and the solution is mixed and allowed to settle. However, high aluminum sulfate concentrations are necessary and result in a large sludge volume and elevated sulfate concentration in the treated water (Fawell, 2006). The proposed research uses Polyaluminum chloride (PACl) and a continuous flow reactor to dramatically reduce the required chemical concentration. Previous research demonstrated that arsenic readily adsorbs to PACl (Zhi, 2015). Given that adsorption techniques are also an effective fluoride removal method, the proposed research tested PACl as the coagulant in the continuous flow reactor.
Key elements of the Phase I fluoride reactor design:
● Substitute polyaluminum chloride (PACl) in place of aluminum sulfate. PACl may be more efficient at fluoride removal than alum and will not add sulfate to the water.
● Use sand filtration to obtain better removal of fluoride at a lower coagulant dose and thus lower operating cost.
● Replace batch processes with continuous flow processes.
● Use high energy rapid mixing to obtain a more uniform distribution of aluminum hydroxide precipitate.
Phase I demonstrated that fluoride, just like arsenic, can adsorb to PACl precipitate and be removed through sand filtration and that fluoride removal by a fluidized bed floc blanket is also possible.
A disadvantage of sand filtration evaluated in Phase I is that head loss accumulation is directly proportional to the mass of coagulant introduced into the filter. When PACl concentrations as high as 50 mg/L as Al are added, filter head loss accumulates 1 m in about 10 minutes (Zhi, 2015). Short filter run times are not sustainable in a water treatment system because large volumes of water are required for cleaning. In the past decade, the AguaClara program has developed a high efficiency settling (sedimentation) tank with a fluidized suspension of conglomerated particles (or floc blanket), sludge consolidation, and plate settlers. This sedimentation tank has the potential to efficiently remove arsenic and fluoride as well as particles and pathogens.
One main advantage of the sedimentation tank is that it can operate continuously and produce a very small amount of concentrated waste. The AguaClara sedimentation tank produces a much lower volume of waste than rapid sand filters. The AguaClara sedimentation tank will be added to the treatment train in Phase II to reduce the solids loading on the rapid sand filter to extend the filter runtime and reduce the volume of waste. The new reactor features a floc blanket system to create a small continuous flow of highly concentrated sludge.
Over the course of Phase II research, a series of parameters will be varied to develop an efficient reactor design. Critical parameters already known to affect system performance are PACl dose, clay concentration, velocity at which water flows through the floc blanket (upflow velocity), and reactor configuration. Optimization strategies will focus around modifying the PACl and clay concentrations, since both significantly affect floc structure and floc blanket density. AguaClara researchers hypothesize that adequate clay concentrations are required to produce settleable flocs and concentrated residual sludge. Since groundwater has a relatively low clay concentration compared to surface water, more clay may need to be added to enhance floc blanket formation. Reactor configuration will be varied and will include a test with multiple floc blankets in series with countercurrent floc flow to reduce the required coagulant dose.
A direct filtration reactor system for the removal of fluoride was tested with an influent fluoride concentration of 10 mg/L (Figure 1). PACl was added at 20, 40, and 50 mg/L as Al. As expected, fluoride removal increased with PACl dose. Results showed that a PACl dose of 50 mg/L as aluminum (Al) effectively reduced fluoride concentrations from 10 mg/L (Dao et al., 2015) to meet the World Health Organization (WHO) fluoride standard of 1.5 mg/L (WHO, 2011). The 50 mg/L Al as PACl is much lower in contrast with the 1200 mg/L Al as aluminum sulfate required to treat similar fluoride concentrations (Dahi et al., 1996). These results suggest that a continuous flow PACl based fluoride reactor system provided effective removal and significant improvement in performance over the Nalgonda method.
Groundwater fluoride contamination poses a health threat to more than 66 million people in India (Arlappa, 2013). According to the WHO, dental damage can occur at a fluoride concentration above 1.5 mg/L and crippling skeletal damage can arise at a concentration above 10 mg/L (WHO, n.d.). Therefore, access to safe drinking water is critical in maintaining the health and productivity of a community. In Phase I research, the student team demonstrated that fluoride contamination of 10 mg/L can be successfully treated to safe drinking standards using PACl in combination with sand filtration.
Although the highly soluble fluoride ions are difficult to remove from water, results from Phase I demonstrated the potential for efficient removal using PACl and direct filtration. Experimentation proved that PACl and continuous flow direct filtration removed enough fluoride to reach WHO standards of 1.5 mg/L (WHO, 2011). Compared to previously documented values (Dahi et al., 1996), the apparatus used in Phase I required 24 times less mass of aluminum per mass of fluoride removed.
AguaClara’s improved fluoride removal technique incorporates the elements of People, Prosperity, and the Planet into a sustainable water treatment system (Rivas, 2014). The technologies are designed to use non-proprietary materials available in the national supply chain, and leverage community engagement and municipal resource management. Advanced technologies that deliver operational simplicity can decrease failure modes and improve economic efficiency. The long-term goal of AguaClara is to create and implement reliable and intuitive designs so communities can independently maintain their own water treatment system. The improved PACl fluoride treatment system embodies the idea of optimization by accomplishing fluoride removal with 24 times less mass as aluminum than the Nalgonda method. As a result, less residual sludge is generated. Since less coagulant and physical labor is necessary, the improved fluoride treatment technique has the potential to improve access to safe drinking water in villages with contaminated groundwater.