Reduction of phosphates in sewage using kiln ash as an adsorbent

Phosphate is a naturally occurring chemical found in large quantities on the Earth’s surface that causes eutrophication when deposited into rivers. A number of studies have been conducted to assess the ability of various treatments to remove phosphates from sewage. Lately, it has been discovered that phosphorus recovery may be accomplished by filtering the water. The cost of the filtering materials, on the other hand, is prohibitively expensive. As a result, current research has concentrated on utilising low-cost ones to minimise the expense of filtering. Steel production residues, such as kiln bottom ashes, are being utilised in this study to recover phosphates from contaminated wastewater. A variety of operational settings have been investigated in order to obtain the highest possible extraction efficiency at the lowest feasible cost. Bottom ashes were demonstrated to be an effective substitute for phosphorus extraction. The highest phosphorus extraction was 90.1 percent after 40 minutes, with a starting concentration of 5 mg/L and an ash dose of 530 mg/L. The findings were used to create a prediction model with a high degree of reliability.


Introduction
Although phosphate is available at high concentrations in soil and freshwaters, urban spread and industrial expansion drive huge global demand for drinkable water that exhausts the available freshwater rapidly, and in return, increasing the volumes of discharged wastewater that contains high concentrations of phosphate [1,2]. In addition, the urban spread and industrial expansion badly reflected in the availability of freshwater [3][4][5], global warming [6,7], and precipitations [8,9]. The growing population increases the rate of sewage released into waterways, which has severe effects on the health of consumers [10][11][12]. Furthermore, the increase in the population increased the dumping of solid wastes in landfills, which resulted from the expansion in the residential areas [13,14], that pollute groundwater and then surface water [15][16][17][18]. Toxins must not be dumped directly into waterways, and all effluents, whether household or manufacturing, must be treated adequately to eliminate or limit the negative impacts of the contaminants. One of the types of impact of pollutants is algae flourishing which is due to phosphorous penetration into the water, which is harmful to the environment [19,20]. All types of sewage water contain phosphorus [21,22] and nitrate [23,24] beside other pollutants such as fluoride [25], heavy metals [26,27], bacteria [28,29], coloring pollutants [30][31][32] and organic matter [33][34][35]. Additionally, industrial and domestic wastewater [36][37][38][39] and atmospheric pollutants [40][41][42] are playing a role in the increasing of such pollutants in water bodies. The effluents of the construction industry [43][44][45][46], production plants [47][48][49] and food industry [36,50]. Surface waters are depleted of oxygen due to algal blooms induced by phosphorus and nitrate contamination [12,51]. A large proportion of aquatic creatures and plants might die in a short period of time. As just a result of a substantial rise in organic matter content, the taste, smell, and structure of the water will be changed significantly [21,22]. A rise in algal development might have severe effects on health, industries, seafood, and agriculture, and farmers [24,52]. Many studies revealed that microalgae types can generate poisonous chemicals that cause a range of severe illnesses and, sometimes, fatality. In drinkable water, the allowable concentration of phosphate is specified at 0.1 mg/L, whereas in streams and ponds it is 0.05 mg/L [21,22]. As far as the upper limit pollutant concentration in water is concerning, the Environmental Protection Agency has strict standards. It is therefore important to remove phosphates from contaminated water, depending on what has been said previously. When phosphate is removed via traditional methods, it generally requires a long and complicated procedure. As an instance, the normal filtering process [25], with some modifications, provides an effective approach for phosphate removal, according to the authors. However, the usage of expensive screens makes this approach uneconomical [50,53]. It is possible to remove phosphate from the water bodies by converting it to a solid state, which would be subsequently removed from the water via flotation or some other means. Thermal processing, lakes, anaerobic digestion, absorption, and other techniques are used to transform phosphorus [22,54]. As just a result of the typical operations of species of bacteria and microalgae, phosphate could be biologically processed. Depends on the kind of microbe, anaerobic or aerobic techniques could be used in these situations. The pH, starting dosage, etc. are all factors that affect the efficacy of the treatments. In regards to employing biological intervention to reduce the existence of phosphate, several disadvantages are discovered [21]. Soluble carbon, fast development of microorganisms, and pH level of the fluid are all factors to be considered. Phosphate particles can be converted by adding metallic compounds to water polluted with Phosphorus particles. Usually, sewer water encompassing phosphate is treated using chemical techniques. Calcium ion is amongst the most frequently utilised chemical additions for phosphorus removal for its availability, affordability, and convenience of usage. Especially in an acidic medium, aluminum hydroxide is believed to be highly successful in eliminating phosphate from wastewater. As a consequence of the usage of chemical substances, chemical treatment methods frequently lead to toxic sludge [24]. Due to the utilisation use reagents, the technique is much more costly than many other procedures. Physically, phosphate extraction methods do not change the chemical composition of the phosphate, which is resulting in a change. After the removal of phosphate pollution using this approach, it is possible to separate these contaminants (phosphate contaminants) from the removal medium. For phosphate removal, filtering and screening technologies are frequently employed [21,22]. Substances sensitive to pollutants are used in filtering. Water is then separated from pollutants using a variety of processes such as coagulation [10,31], absorption [11,55], and straining hybrid methods [23,33]. To remove phosphates from water, several permeable mediums were employed. The general rule, the expense of filtering is determined based on the cost of the adsorbent materials employed. The phosphate removal capability of membranes has been demonstrated in previous studies. There are several methods to extract phosphates from water at low levels, but reverse osmosis is among the most popular. Polluted water is forced through a barrier with really limited porosity under high pressure. Assist in removing pollutants in water by means of high-pressured water. Membrane systems are rarely utilised due to their expensive running costs difficulties in discarding them, and the requirement for specific equipment and expertise. Recently, studies have centered on the use of waste products or natural resources as economical and useful replacements for traditional materials. Fly ash is a standard manufacturing byproduct that would be used to remove phosphate from wastewater, and also it widely used in construction industry [43,56]. Water with an acidity level of 7 can be effectively treated with this substance. Due to the above, our research focused primarily on the possibility of using furnaces bottom ashes as an absorbent to extract the phosphates from polluted effluent.

Methods
According to the studies, furnace bottom ashes may be utilised as a convenient and efficient adsorbent for the removal of phosphate from the wastewater. Studying the grain size, permeability, porous structure and relative density of the bottom ashes is essential. The particle size is highly essential because it is directly related to the adsorption surface. When sieve review is performed, sieve sizes of 4, 6, 10, 16, 30 and 50 are utilised, and the quantity of mass kept on each sieve is calculated by analysing the distribution of particle size of the waste. This formula is used to determine the specific gravity test using the gas jar method: Where 1 : Weight of jar + weight of plates 2 : The total weight of the sample, plate, and jar, including the plate and the jar itself. 3 : Water, dry sample, plate, and jar weight together. 4 : Weight of the jar plus water Using an X-ray fluorescence analyser, the chemical test is used to determine the structure of the ashes of the furnaces. It is performed to assess the effectiveness of extracting phosphates from the bottom ashes of the furnace since materials contain calcium, iron, and aluminum can be utilised for phosphate removal. To extract the phosphate in contaminated water, the bottom ashes were employed as a filter. The Box-Behnken design was utilised to improve the technique in terms of burner bottom ash dosage, duration of treatment, and temp. A potassium diphosphate solution in deionised was used to modify the amounts of phosphate in synthetic water. An experiment was conducted in which polluted water was combined for various lengths of time with the furnace's bottom ashes to determine the effect of duration. In such tests, the retention duration ranging from 10 to 70 min, while the burner ash dosages varied from 100 to 1000 mg/L. Calculations of phosphate levels are made with the use of a Hach Lange spectrophotometer. As a result of this study, the Box-Behnken Design was utilised to create an estimation model in order to obtain the highest water treatment. The response variable was phosphorus removal in this research. The determinants were the phosphorus content level, bottom ash dose, and the duration of treatment. In order to construct the estimation matrix, Minitab 20 was chosen. The ranges of predictor factors considered are shown in Table 1. A powerful determinant of linearity and relevance is the α value, which may be calculated using the formula below: Where X is the number of variables included in the model.

Properties of ashes from furnace bottoms
Based on empirical evidence, the majority of the bottom ash grains are maintained on sieves of lower diameters of less than 16. The results of the sieving tests show that the majority of the

(3)
The gradation coefficient = 3 2 1 * 2 (4) When plotting the passing rate with particle diameter, x2, x3, and x1 show the percentages of particles that pass through each size. Analysis indicated that x2, x3, and x1 the quantity of the first term were 3.06, 1.36, and 0.48, correspondingly. Furthermore, the uniformity coefficient is 6.21 the gradation coefficient is 1.32. The specimens of bottom ash are judged to be of good quality. Because of this, it is unlikely that the bottom ashes would float, as their relative density is greater than those of the liquid. Permeability was 0.61 m 2 /g, while the total area was 6.81 m 2 /g. A wide surface, good permeability, and high relative density make the bottoms of the furnace excellent for phosphorus extraction. Chemists determined that the bottom ashes included aluminium, iron, and, calcium oxide, magnesium in roughly 25% of the furnace's ash. As a result, it may be argued that the bottom ash of a burner is suitable for removing phosphates.

Phosphate Removal
To investigate the effectiveness of burner ash in removing phosphorus compounds from wastewater, batch experiments were run. For each of the 15 tests, the experimental variables were modified in order to identify the optimal experimental set for achieving high removal efficiency at low costs. The findings are shown in Table 2 below. As can be seen in Table 2, the Box-Behnken Design and the removal efficiency obtained are provided.  As a result of this investigation, it was shown that the removal efficiencies are proportional to the dose of furnace bottom ashes and the length of time intervals. Phosphorus removal, on the other hand, has an adverse correlation with starting phosphorus concentrations. This is because the accessible surface area of the adsorption is insufficient to collect the phosphorus from the liquid. As a result, there is more surface area accessible for phosphate adsorption from the contaminated water in the furnace bottom ash. It also enhances the interaction of phosphate and the adsorbent. 90.1% of the phosphate concertation in the contaminated water could be removed by using 530 mg/L of bottom ashes for 40 minutes, as shown in Figures 1-3.
It can be seen from the presented studies about the adsorption method, the depletion of the active sites results in a serious reduction in the adsorption process, therefore; the application of successful sensors (in the literature) helps the operators of the treatment plant to monitor the depletion process, two good examples of the sensors that widely used in the civil and water engineering are the microwave [57][58][59] and electromagnetic [60][61][62][63] sensors.

Conclusion
To identify the optimum processing factors, the prospect of using furnace bottom ashes to treat contaminated water for phosphate was studied. The chemical and physical properties of furnace bottom ashes are essential to remove phosphorus from contaminated water, according to research. This is in line with earlier research findings of about 90 % elimination using the bottom ashes. The dose of the bottom ashes as well as the detention time is positively related to the percentage of phosphorus removal from the polluted wastewater. Relative to phosphorus adsorption capacity, on the other hand, it is inversely correlated with the initial concentration of the pollutant. On the basis of furnace bottom ash dosage, phosphate content, and treatment duration, Box-model Behnken's may also forecast how the removal efficiency will vary.