Orthophosphate removal from domestic wastewater using limestone and granular activated carbon
Research Highlights
► Limestone has ability to adsorb 98% PO4-P from aqueous solution at optimum conditions. ► Freundlich and Langmuir isotherms both provided comparable correlation coefficient values. ► Adsorption kinetics shows good compliance with the pseudo-second-order kinetics model. ► It indicates that the adsorption is chemisorption. ► Higher flow rate and pH yielded shorter column saturation time in upflow column study.
Introduction
Fresh water for human needs is a precious natural resource that in many parts of the world is becoming more scarce due to climate change impacts and increased human demands. Its sources and security of supply has become a key concern for communities and more attention is being made on water conservation and use efficiency, new water sources and advanced treatment technologies and water reuse. In recent years water recycling has become an important resource for agricultural and domestic use, including for potable supply in some countries. Since the early 1970s, the presence of phosphorous in domestic wastewaters has received increased attention due to the realization of negative impacts it can have on receiving waters in the environment. In wastewater treatment processing, phosphorus is a vital nutrient for bacteria needed to degrade and biologically stabilize the organic wastes. Phosphorus is also an essential nutrient for plant growth in lakes and streams. Phosphorus is generally present in the form of orthophosphate (condensed phosphate or polyphosphate) in natural and waste waters [1]. It commonly originates from human and animal wastes, agricultural runoff and household detergents. The discharge of excessive amount of phosphate ions from wastewater treatment plants may adversely affect the water quality of a receiving body. Domestic wastewater is an important source of inorganic nutrients such as ammoniacal-nitrogen (NH4-N) and orthophosphate (PO4-P). Phosphorus was found between the levels of 3–15 mg/L in domestic wastewater; merely about 3 mg/L was formed by the breakdown of protein wastes while the majority came through the usage of detergents [2].
Many countries around the world including the European Union allow 1–2 mg/L as the limit of total phosphorus (TP) for effluent discharge in wastewater treatment plants. However, some regions followed more strict measures of around 0.5–0.8 mg P/L to control eutrophication [3]. Algal blooms can occur if the concentration of PO4-P exceeds 0.1–0.5 mg/L which cause “eutrophication” in the receiving water, thus phosphate removal is an essential part of domestic wastewater treatment [4], [5].
Phosphorus removal can be brought about by several available wastewater technologies such as biological, coagulation-flocculation, physico-chemical and electrolytic. These include adsorption, ion exchange, chemical precipitation and membrane filtration/reverse osmosis [6]. Some of them are relatively expensive to run due to high operational and maintenance cost. Biological process is a low-cost operating technology compared to chemical process. But the process could not achieve the required level of phosphorus removal compared to a well-run physico-chemical process [7], [8]. In chemical precipitation, alum, lime and iron salts are widely used as effective coagulants for the removal of phosphate and ammonium ions from wastewaters. However, handling of high volumes of sludge, its disposal and neutralization of the effluent are the major disadvantages of this technology [6]. Orthophosphate removal has been accomplished by chemical precipitation as a wastewater treatment technology. However, as a new approach, its removal and recycling technologies have not been widely implemented due to scarcity of advanced scientific knowledge [9].
Pure MgO and low-grade MgO (LG-MgO) were used as a source of magnesium for the removal of ammonium and phosphate ions from wastewater by precipitating in the form of ammonium magnesium phosphate (MAP) MgNH4PO4 compound, also known as struvite. Pure MgO performed better than LG-MgO but it has substantial cost-effective disadvantages [10]. The recovery of MAP, though, would be beneficial to use it as a source of agricultural fertilizer [9]. Aguilar et al. [11] reported about 100% orthophosphate removal using different coagulants such as aluminium sulphate, ferric sulphate and polyaluminium chloride with or without coagulant aids such as powdered activated carbon, precipitated calcium carbonate, cationic polyacrylamide, polyacrylic acid, polyvinyl alcohol and anionic polyacrylamide. The addition of these coagulant aids caused reduction of sludge volume of up to 41.6%. Still the high cost of chemicals usage is the main disadvantage.
In order to solve these problems, it is desirable to develop a low-cost and simple treatment alternative. Domestic or municipal wastewater treatment could be achieved by adsorption using various adsorbents. Activated carbons, in granular or powdered forms, are widely used adsorbents due to the availability of their high surface area which enhanced adsorption rate. However, its expensive regeneration and disposal problems are the major disadvantages of this material [6].
The removal of phosphate by adsorption is quite simple and convenient. Successful results were reported by various researchers using different adsorbents such as fly and bottom ashes [12], [13], [14], [15], [16], [17], opoka and calcinated opoka [18], ZnCl2-activated coir pith [19], natural indigenous rocks and waste materials [20], [21], [22], red mud and sand [23], [24] and calcium hydroxide, iron oxide, mesoporous alumina [25], [26], [27] from wastewater. Limestone and zeolite were used as filter media to adsorb anions and cations from rainfall runoff [22]. Phosphate ion can be removed by ion exchange or physical adsorption [20]. Tanik and Comakoglu [28] evaluated effective phosphorus removal efficiency from domestic wastewater by rapid infiltration system through porous media (sand and gravel from crushed stone). The range of phosphorus removal achieved in the system was about 46–93%. The removal efficiency increased with decreasing effective size of soil media. The system was considered to be suitable because of low-cost media and energy saving.
Rahman et al. [21] investigated wastewater treatment with multilayer media consisting of natural indigenous rocks (andesite, granite, limestone and nitrolite) and waste materials (refuse concrete, waste paper and charcoal). In the multilayer media system, the removal of phosphate ion from wastewater, the combinations of andesite and nitrolite with charcoal and refuse concrete showed relatively higher efficiencies. In the single medium treatment, the removal efficiencies using andesite, nitrolite, refuse concrete and charcoal were 18%, 36–66%, 55–91% and 11–17%, respectively. Therefore, the addition of refuse concrete in the multilayer system resulted in higher potential to remove phosphate ion.
Kietlinska [18] stated that treatment of landfill leachate is difficult because of its complex composition. He performed a short-term column experiment to study the effect of opoka adsorbent for the removal of orthophosphate from landfill leachate. Columns I, II, III and IV were filled with opoka (Polonite) (< 1 mm grain size), sand (2 mm), calcinated opoka (< 2 mm) mixed with zeolite (2 mm) in the ratio 1:1 and peat mixed with calcinated opoka, respectively. Columns I and II removed about 42% and 39% ortho-P, respectively while columns III and IV removed more than 90% of ortho-P. Kirk et al. [16] investigated the removal of soluble phosphorus from wastewater using coal bottom ashes. The lignite bottom ashes had higher adsorption capacity (80–600 mg P/kgash) as compared to bituminous (14–1000.002 for flowrate = 10 mL/min mg P/kgash). The bituminous and lignite coal bottom ashes removed about 73% and 82% phosphorus, respectively.
Namasivayam and Sangeetha [19] studied the sorption capacity of phosphate from aqueous solution using ZnCl2-activated coir pith. The results suggested the possible monolayer coverage of phosphate on the surface of ZnCl2-activated coir pith. The adsorption kinetics showed a clear idea about pseudo-first-order and pseudo-second-order kinetics. It was concluded that the calculated qe values of second-order-kinetic were in agreement with the experimental values compared to the pseudo-first-order kinetics. The correlation coefficient R2 values of the second-order kinetics were greater than 0.99. Therefore, the study was represented by the second-order-kinetic model. The percentage removal increased from 20% to 95% with increased adsorbent dosage from 25 to 600 mg, respectively.
Limestone is a low-priced material “US$ 12/ton” compared to the cost of activated carbon “US$ 2.7/kg” [29]. It has shown good adsorption potential in several studies [22], [29], [30], [31], [32]. The main purpose of the present study was to investigate the effect and adsorption behavior of limestone, activated carbon and mixture of both materials as filtering media for PO4-P removal from domestic wastewater. This study was focused on the establishment of essential parameters for the design of limestone or mixture of limestone and activated carbon filter as a post-treatment of domestic wastewater before releasing it into the receiving water.
Section snippets
Wastewater analysis and adsorbents
Wastewater samples were collected from the influent point of the oxidation pond at Engineering Campus, Universiti Sains Malaysia, Penang, Malaysia. The oxidation pond receives a mixture of domestic wastewater from hostels, administrative blocks, schools and cafeterias. Characterization of the wastewater involved the determination of NH4-N, PO4-P, COD, BOD, turbidity and suspended solids (SS) using HACH DR2500 spectrophotometer. The pH of wastewater samples was analyzed onsite, immediately after
Results and discussion
Characterization of domestic wastewater: The composition of raw domestic wastewater collected from the influent point of oxidation pond is shown in Table 2. The measured average density of limestone and granular activated carbon were 2598 kg/m3 and 1265 kg/m3, respectively.
Conclusions
The present study aimed to investigate the adsorption of PO4-P using two media; one being the commonly studied granular activated carbon (GAC) and the other being limestone (LS) as low-cost material. In the batch study, optimum contact and settling times were obtained as 150 min and 120 min, respectively. Limestone alone has the potential to adsorb about 98% PO4-P from aqueous solution at optimum experimental conditions; the initial concentration of 20 mg/L dropped to the final level of 0.5 mg/L.
Acknowledgements
The authors would like to acknowledge the financial support provided by the Ministry of Science, Technology and Innovation, Malaysia for the IRPA research grant that has resulted in this article.
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