Abstract

ENVIRONMENTAL ENGINEERING

Modelling and automation of the process of phosphate ion removal from waste waters

L. Lupa^* * To whow correspondence should be addressed ; P. Negrea; A. Negrea; A. Iovi; L. Cocheci; G. Mosoarca

Politehnica University of Timisoara, School of Industrial Chemistry and Environmental Engineering Timisoara, Phone/Fax: 004 0256404192, Victoriei Square No. 2, 30006, Timisoara, Romania, E-mail: lavinia.lupa@chim.upt.ro

ABSTRACT

Phosphate removal from waste waters has become an environmental necessity, since these phosphates stimulate the growth of aquatic plants and planktons and contribute to the eutrophication process in general. The physicochemical methods of phosphate ion removal are the most effective and reliable. This paper presents studies on the process of phosphate ion removal from waste waters resulting from the fertiliser industry’s use of the method of co-precipitation with iron salts and with calcium hydroxide as the neutralizing agent. The optimal process conditions were established as those that allow achievement of a maximum degree of separation of the phosphate ions. The precipitate resulting from the co-precipitation process was analysed for chemical composition and establishment of thermal and structural stability, and the aim was also to establish in which form the phosphate ions in the formed precipitate can be found. Based on these considerations, the experimental data obtained in the process of phosphate ion removal from waste waters were analysed mathematically and the equations for the dependence of the degree of phosphate separation and residual concentration versus the main parameters of the process were formulated. In this paper an automated scheme for the phosphate ion removal from waste waters by co-precipitation is presented.

Keywords: Eutrophication; Phosphate removal; Fertilisers; Waste waters; Process modelling; Automation.

INTRODUCTION

The importance of purifying waste waters has increased at the international level, the main reason being the limit the flow need to impurities in waters and of ensure a quality suitable for all their uses. Phosphorus compounds are of great importance to some branches of the economy (Hodge and Popovici, 1994). From technological processes for obtaining these compounds as well as other processes in which they are used, waste waters with a phosphorus content, either inorganic (especially as phosphates) or organic, result. The residual waters released into effluents cause this pollution; phosphorus compounds stimulate the growth of aquatic planktons and contribute to the eutrophication process. For this reason the detection and removal of phosphorus compounds from waste water are of great importance to the environment (Burtica, et. al., 2000; Hammer, 1986). This phenomenon (eutrophication) is determined not only by nutritive substances, but also by other physical and chemical factors (the water flow speed, depth, temperature, light, etc). The eutrophication phenomenon can especially be observed in slow flow waters (canals) or in still waters (ponds, lakes, dams, etc). To prevent eutrophication there are some methods which can be applied, among which is to decrease the quantity of nutrients, i.e., the phosphor compounds (Roques, 1990; Tehobanoglous et al., 1997). From the production of phosphoric fertilisers waste waters containing phosphates are obtained. (Corbitt, 1990; Bower and Stensel, 1990).

The physicochemical methods of phosphate removal are the most effective and reliable (Lupu et al., 1986). Taking these considerations into account, studies were conducted to establish the optimal conditions for the process of phosphate ion removal from waste waters resulting from the manufacture of complex fertilizers by co-precipitation using as co-precipitation agents FeCl₃ and FeSO₄ and as neutralising agent calcium hydroxide. The precipitate resulting from the co-precipitation process was analysed for chemical composition and establishment of thermal and structural stability. Based on the studies, an automatic scheme for the process of phosphate ions removal from waste waters by co-precipitation is proposed.

EXPERIMENTAL

The waste waters of the fertiliser industry were analysed in accordance with Law no. 107/1996 (Official Monitor, 1997), which establishes the quality conditions of waste waters before they are evacuated in to the water sources. To remove the phosphate ions from the waste waters the Jar-Test methods were used. Water samples with a given content of phosphate ions were treated with an appropriate quantity of ferric chloride and ferrous sulphate (10 g Meⁿ⁺/l) and a well-determined quantity of neutraliser (a saturated solution of calcium hydroxide), under intense stirring (n=100-120 rot/min) for 2 minutes. Then the samples were stirred slowly (n=40 rot/min) for 10 minutes and left to rest for 30 minutes for decantation of the precipitate formed. To establish the optimal conditions for phosphate ion removal, the dependence of the degree of separation of P₂O₅versus different parameters (coagulant agent / P₂O₅ mass ratio, the quantity of coagulant, pH and the nature of the precipitation agent) were studied. The pH mass reaction was measured with a DENVER 250 pH-meter and the residual content of phosphate ions was determined by the spectrophotometer method, using a UV-VIS JASCO-V-530 spectrophotometer (Lungu and Duda, 1999).

The precipitates formed during the co-precipitation process were separated under optimum conditions, dried at a temperature of 105ºC until they had a constant mass and then were dissolved with HCl for a complete leaching. The precipitates formed were analysed chemical composition and establishment of thermal and structural stability. Thermal-gravimetric and thermal-differential studies were carried out using a Perking Elmer TGA 7 thermo balance. To estimate the residual concentrations of P₂O₅ for the automation process, the dependence equations of the residual concentration of P₂O₅ versus pH for different types of coagulant agents were determined using as neutralising agent a saturated solution of calcium hydroxide. Based on the studies conducted, an automatic scheme for the process of phosphate ion removal from waste waters by co-precipitation is proposed.

RESULTS AND DISCUSSION

Waste Water Analysis

The experimental data on the composition of the waste waters of the fertiliser industry and the maximum of the parameters permitted by the legislation are presented in Table 1.

From the experimental data it can be observed that the water had a complex chemical composition with a high level of nitrogen and phosphor content that create conditions appropriate for eutrophication. In the current legislation the permissible limits for nitrogen and phosphate compounds are far too high.

Studies the Establishment of Optimal Conditions for Phosphate Ion Removal

a) The Influence of pH on the Degree of Separation

Ferric Chloride

The experimental data on the dependence of the degree of separation of phosphate ions on pH at 25º C, using as neutralisation agent a saturated solution of calcium hydroxide and as co-precipitation agent solutions of ferric chloride of different concentrations, are presented in Figures 1-3. The experimental data show that degree of separation of P₂O₅ increase with a pH. At pH~10, the degree of separation of P₂O₅ is ~10 %.

Ferrous Sulphate

The experimental data the dependence of the degree of separation of the phosphate ions on pH at 25º C using as neutralisation agent a saturated solution of calcium hydroxide and as co-precipitation agent solutions of ferrous sulphate of different concentrations (Negrea, et al., 1998) are presented in Figures 4-6. From the experimental data can be observed that degree of separation of P₂O₅ increases with pH, achieving values over 90%.

b) The Influence of the Dose of Co-Precipitation Agent on the Degree of Separation

Ferric Chloride

The experimental data on the dependence of the residual concentration of P₂O₅ on the degree of separation waste waters and the quantity of FeCl₃ at 25º C using as neutralising agent a saturated solution of calcium hydroxide (Negrea, et al., 2000) is presented in Table 2. From the experimental data can be observed that the residual concentration of phosphate ions decreases with the increases in quantity of FeCl₃. The degree of separation of P₂O₅ is unsatisfactory because of the high residual.

Ferrous Sulphate

The experimental data on the dependence of the residual concentration of P₂O₅ on the degree of separation from waste waters and the quantity of FeSO₄ at 25º C, using as neutralising agent a saturated solution of calcium hydroxide is presented in Table 3. From the experimental data it can be observed that the residual concentration of P₂O₅ decreases suddenly, tending to wards a constant value when the quantity of FeSO₄ increases. Thus, it can be said that for a quantity of 100 mg/L FeSO₄ the residual concentration of P₂O₅ is 10.8 mg/L. FeSO₄ is more efficient than FeCl₃.

c) The Influence of the Co-Precipitation Agent/P₂O₅ Mass Ratio on the Degree of Separation

Experimental data on the dependence of the degree of separation of phosphate ions on the residual concentration and the co-precipitation agent/P₂O₅ mass ratio at 25º using as neutralisation agent a solution of calcium hydroxide and as co-precipitation agent ferric chloride and ferrous sulphates shown in Table 4. From the experimental data can be observed that bigger degrees of separation of phosphate ions are obtained at a lower co-precipitation agent/P₂O₅ mass ratio when ferrous sulphate is used as co-precipitation agent than when ferric chlorides.

d) The Influence of the Nature of the Co-Precipitation Agent used on the Degree of Separation

The experimental data on the dependence of the degree of separation of phosphate ions on the nature of the co-precipitation used are presented in Figure 7. From the experimental data it can be observed that the degree of separation of the phosphate ions depends on the nature of the co-precipitation agent used for the same neutralising agent, calcium hydroxide. The iron (III) ions provide the highest degree of separation of the phosphate ions for quantities larger than 100 mg/L.

Characterisation of the Precipitate Formed During the Process of Separation of the Phosphate Ions

The chemical composition (% mass) of the precipitates separated from the waste waters under study is P₂O₅=19.7%; CaO=15.8% and Fe₂O₃=17.3%. This composition shows that the precipitates probably contain the following components: Ca₅(PO₄)₃OH, FePO₄ ·2.5H₂O and FeOOH. To characterise the separated precipitates, thermal-gravimetric and thermal-differential studies were carried out.

The thermal-gravimetric and thermal-differential curves are alike for all the products obtained under the optimal conditions and they are presented in Figures 8 and 9. From these data it can be observed that the process has two stage

up to 200º C a larger loss of mass (~23%), determined by loss of physical and absorbed water takes place;

between 200 and 500º C, the period characterised by a slower speed of decomposition with a small loss (~7%), the crystallisation water and the basic components of the precipitate are eliminated.

Thermal-gravimetric and thermal-differential studies show that the precipitates formed in the process of water purification which contain P₂O₅ have a complex composition and they are probably formed from amorphous precipitates of hydroxyl apatite, ferric hydroxide and other basic phosphates. We must stress that hydroxyl apatite as a colloid has a large absorption capacity.

Modelling of the Process of Phosphate Ion Separation

The experimental data for the mathematical equation for the dependence of the residual concentration of P₂O₅ (the equation term "z") on the pH (the equation term "x") and on the co-precipitation agent used (the equation term "y") in the cases where ferric chloride and ferrous sulphate where used as co-precipitation agent and calcium hydroxides as neutralising agent are presented in Table 5 and Figures 10 and 11. The modelling of the process of phosphate ion removal (Negrea et al., 2002) shows a significant decrease in the residual concentration of P₂O₅ with pH and doses of the co-precipitation agent Fe (III) have a less visible influence. This dependence shows that the phosphate ions are eliminated, especially in the form of hydroxyl apatite, with the variation in pH being ensured by the Ca(OH)₂. The co-precipitation agent ions precipitate in the form of hydroxides, the former acting rather as a coagulant for the colloidal hydroxyl apatite.

Automation of the Process of Phosphate Ion Separation

The proposed automated scheme, based on the studies of the process of removal of phosphate ions from waste waters resulting from the fertiliser industry by the co-precipitation method, (Perju et al., 2001) is presented in Figure 12. As the raw water enters the reaction chamber, the following parameters are measured: pH with the A₁T pH transducer, the initial concentration of P₂O₅with the A₂T transducer and the flow with the FT transducer. The responses of this transducer are measured by the central processing unit (computer) with the help of an analogue-digital converter (ADC). These responses are the input data for the program of the main response calculation with the soft being adjusted to the laboratory experimental data and turned for real installation versus the flow, pH and initial concentration of P₂O₅ in the raw water and of the assessed residual concentration of P₂O₅ in the treated water. The main responses from the digital-analogue converter are transmitted to the flow regulator for addition of the co-precipitation agent and calcium hydroxide solutions. From the reaction chamber the water enters the clarifying tank and after settling the water and sludge are treated. To reduce the volume of sludge, this is filtrated and forms a cake which contains specially calcium phosphates the water resulting from filtration is mixed with the treated water. The residual concentration of P₂O₅ in the treated water is determined by the A₂T transducer and sent to the central processing unit by the analogue-digital converter. This response can be stored to watch the installation and can be used as a process feedback.

The treated water having a high pH (pH= 9 - 10) is neutralised with sulphuric acid to adjust the pH to 7. This neutralisation is realised in an automation curl which does not pass through the central processing unit; because of its simplicity, the curl is made with an A₁T pH transducer and an A₁C pH regulator. Also, an automation curl was proposed for the level of solution in the reaction chamber with the level LT transducer and the LC level regulator.

CONCLUSIONS

The research made it possible to establish the optimal conditions for the process of phosphate ion removal from fertiliser industry waste waters by treating it with ferrous chloride and ferrous sulphate and neutralising it with calcium hydroxide, thus obtaining a minimum residual concentration of P₂O₅. The experimental data show that phosphate separation is a complex physicochemical process. Some authors (Roques, 1990) believe that the chemical processes play a fundamental role and that elimination of phosphorus is the result of FePO₄ precipitation, which can be complicated by the simultaneous precipitation of Fe(OH)₃. These hydroxides perform a flocculant function, facilitating the separation of the precipitated phosphates. Other authors (Roques, 1990) believe that the phenomena of flocculation and hydroxide formation are responsible for phosphate ion removal by absorption of the phosphate ions. Based on the studies conducted, we believe that the two types of phenomena work simultaneously.

The research on the removal of P₂O₅ from waste water by co-precipitation methods shows that

The pH of the system is the control parameter;

Ca(OH)₂, is the most efficient neutralising agent because it reacts with the phosphate ion, forming a precipitate that flocculates and clarifies more easily; it also acts as a coagulation adjuvant and is cheaper than NaOH;

Ferric salts have several advantages: the precipitates obtained are easy to clarify and ferric salts are cheap;

Under optimal conditions, a minimum residual concentration of P₂O₅ of 3 mg/L was obtained, which was within the permitted effluent discharge values.

The experimental data were processed mathematically, thus making it possible to establish the equations for the dependence of the degree of phosphate separation and the residual concentration on the pH and the dose of co-precipitation agent, so as to allow automation of the entire process. Based on the studies conducted, an automatic scheme for phosphate ion removal from waste waters by co-precipitation was proposed.

REFERENCES

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(Received: April 25, 2006 ; Accepted: September 12, 2007)

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