Precipitation and Sedimentation - Research Paper Example

UO27 PRECIPITATION AND FLOCCULATION Table of contents Table of contents ii Abstract iii 1.0 Introduction 1 1.1 Aims 1 2.0 Methodology 1 2.1 Apparatus 1 2.2 Operating procedure 2 2.3 Experimental operation 3 3.0 RESULTS AND DISCUSSION 5 3.1 Calibration curve for the Humic Acid concentration 5 3.2 Effect of coagulant dosage on humic acid removal 6 3.3 Comparison of raw water in and clean water out 7 3.3 Comparison of raw water in and clean water out 8 3.3 Comparison of raw water in and clean water out 8 3.4 Effect of coagulant dosage on time to coagulation 8 3.3 Comparison of raw water in and clean water out 9 3.4 Effect of coagulant dosage on time to coagulation 9 3.5 Relationship between humic acid removal and coagulant dosage 9 3.6 Viscosity gradient 11 4.0 Discussion and conclusion 13 4.1 Discussion 13 4.2 Conclusion 16 References 17 Abstract The main aim of this experiment was to address the issue of precipitation, flocculation and sedimentation. The apparatus are given and schematic diagram of their set given. The experiment set up is then given step by step. The first part in the result section gives the calibration curve for the humic acid concentration where a strain line is drawn passing close to most of the points and through some. It was found that increasing coagulant dosage resulted to an increase in the level hic acid removal. Coagulant dosage was found to have effect on time it takes before the commencement of macro-flocculation. The effect was much more prominent to a dosage of 20 to 40 but beyond 40 increasing coagulant dosage did not translate to a significant reduction of time of macro-floc formation commencement. In the mass balance analysis it was found that although increase in coagulant dosage resulted to an increase in level of humic acid extraction, the variation was not significant. The gradient velocity was calculated and was found to be 2861.75 s-1. One of the conclusions was that, while increasing dosage could increase the level of purification of the water, the cost implication need to be put into consideration 1.0 Introduction The experiment set has its main function as precipitation, flocculation and sedimentation. The major components of the system is the raw water feed tank, a precipitation tank which is equipped wit5h one agitator, the we have the flocculation tank , next to it is lamella vessel which discharges its water content to the treated water tank. The flocculation tank has tow chambers that are arranged in series, the first chamber being the coagulant part that has one agitator fitted. Next to coagulant chamber is the flocculation part that has one agitator and then we have chamber 3 which is the floc maturing vessel that comes with tow agitators. The system also has three variable speed metering pumps serving the precipitant, coagulant and the flocculant. 1.1 Aims Understanding the operating principles of precipitation, coagulation and sedimentation To determine the effect on Ph value on precipitation To create a stable operating state To determine the required metering quantities To determine the function principle for a lamella separator 2.0 Methodology 2.1 Apparatus Main components 1. B1 Raw water tank 2. B2 Precipitation 3. B3 Flocculation tank 4. B4 Treated water tank 5. B5 Sludge tank 6. B6 Precipitant tank 7. B7 coagulant tank 8. B8 Flocculant tank 9. F Lamella separator 10. P1 Raw water pump 11. P2 Metering pump (precipitant) 12. P3 Metering pump (coagulant) 13. P4 Metering pump (flocculant) 14. R1…R5 Starring machines Instrumentation and control 1. F1 Flow rate sensor (raw water) 2. L1 Level 3. T1 Temperature (precipitant tank) 4. Q1-01 pH value (raw water) 5. Q1-02 pH value (precipitant tank) 6. Q1-03 ph value (flocculation tank) 7. QIC Controller for pH value Figure 1 : Schematic diagram of experiment set up 2.2 Operating procedure First step it was ensured that all the valves were in closed positions after which valve V3 was opened fully. The way tap was opened to enable it discharge into clean water tank B4. It was important to ensure that precipitant, coagulant and flocculent solutions were at adequate levels. The raw water tank used had a capacity of 250l and having water with at 3g/L of humic acid concentration, Sodium Hydroxide at 80g/L concentration. There was also use of I l of ferric sulphate pentahydrate with concentration of 11.45g/L and 5 L of Hydrochloric acid at 0.27M. The mains electricity was switched on and there was measurement of pH value for the raw water with the target Ph value being 9-10. For PH going beyond 11 there was to bew addition more dilute hydrochloric acid and stirring after which the PH was supposed to be rechecked With the pH having adjusted to the desired level there was adjustment of V3 so that a flow rate of 60liters/hr (16.67ml/s) was attained. R1 was started at a point when the level in B2 covered the PH probe. Metering pump was started in manual mode and there was to adjustment of this pump either through adjustment of frequency or stroke length so as to ensure pH was maintained at 9. Tank B2 was filled with raw water to over flow level and then pump P1 was stopped only to be opened when the pH as indicated on Q1O2 reached 9. The flocculation tank B3 was partially filled up to the point when the chambers 1 and 2 were filled. 2.3 Experimental operation Agitators R2 and R3 were started followed by starting of metering pump P3. There was adjustment of P3 by altering of frequency or stroke as directed by staff so as to achieve desired dosage. The contents of tank B3 were observed with agitators R4 A and R5 being switched on when chamber 3 level reached a 40% mark. With fluids in tank B3 being keenly watched P4 was started at a point when micro-flocs appeared. There was observation of B3 so as to note the point at which macro-flocs would start forming and the agitator speeds and pump settings were continuously monitored. With the coagulant being strongly acidic there will be a fall in PH with increased flow and thus it was important to ensure that this did not fall up to a point where there would be re-solution of the precipitation product. The lamella separator (F) was to be observed and the time of over flow of treated water occurrence was noted. The experiment is supposed to end when flocculant, precipitate, or raw water is used up. In order to extract sludge valve V8 is opened so as to empty into tank B5. Stopping the operation involves stopping all pumps and agitators with levels of B1, B6, B7 and B8 being recorded for use in subsequent calculation 3.0 RESULTS AND DISCUSSION 3.1 Calibration curve for the Humic Acid concentration In order to come up with a calibration curve various concentration of humic acid were used. The concentration of the humic acid used are as summarized in table 3. 1. From the table it can be seen that we have 7 samples of 30ml humic acid solution with level of humic acid in mg varying from 0 to 30mg. absorbance test was done on the 7 samples so as to obtain absorbance value. Table 3.1 : Humic acid concentration samples Concentration Humic acid (mg) Volume of humic acid 9(mg) Volume of water (ml) Absorbance 0 0 30 0.009 5 5 25 0.199 10 10 20 0.307 15 15 15 0.349 20 20 10 0.478 25 25 5 0.596 30 30 0 0.663 Plotting the concentration against absorbance result was as shown in figure 2 below. From the figure it can be seen that there is a linear relationship between absorbance and concentration of humic acid where reducing concentration results to reduction in absorbance with absorbance level reducing to also most 0 at the point when the humic acid concentration is zero. Figure 2: 3.2 Effect of coagulant dosage on humic acid removal In order to find out the relationship between coagulant dosage , the dosage of the coagulant was varied through increasing variation of stroke of the coagulant metering pump, and noting the corresponding absorbance at various times. The results are as shown in figure 3.2 and the table 3.1 gives the table with the results where the values used in generating the graph. It can be seen from the figure that as increasing stroke results to reduced absorbance meaning that increase dosage of coagulant results to more humic acid being coagulated and thus having lower absorbance. It can also be seen that the graph flattens as the stroke increases towards 100%. Table 3.1 : Relationship between coagulant dosage and absorbance Stroke length Time (min) Sample Absorbance Mg/L 20 15 1 0.317 14mg/L 40 5 1 0.294 13/mg/L 40 15 2 0.125 5mg/L 40 25 3 0.245 10mg/L 60 5 1 0.122 5mg/L 60 15 2 0.071 2mg/L 60 25 3 0.170 7mg/L Figure 3 : 3.3 Comparison of raw water in and clean water out The relationship between feed water rate and the product water rate are as shown in Table 3.2 figure 4. From the figure it cab be seen that at 20% stroke of coagulant metering pump the feed water rate is just slightly below the product output rate. The product water out put increases with increase in the stroke while the feed water rate remains constant at 16.67ml/s. Table 3.2: Raw water in and clean water out Stroke length Flow rate (in) Qin Flow rate (out) Qout Qout- Qin Time (s) 20 15 1 -0.703 57.58 40 5 1 -2.45 54.29 60 15 2 -3.39 49.84 Figure 4 : Comparison of raw water in and clean water out 3.4 Effect of coagulant dosage on time to coagulation In this experiment the dosage of coagulant was varied and this had effect on the time that lapsed before there was formation of macro-flocs. The dosage was increased by increasing stroke length. Table 3.4 and figure 3.4 shows the relationship between coagulant dosage and time to commencement of macro-flocculation. From the graph it can be seen there is reduction in time with stroke length meaning that that as dosage is increased coagulants are formed faster. It can also be seen that the graph appears to become steeper with an increase in stroke length. Table 3.4: Raw water in and clean water out Stroke length Time (s) 60 19.52s 40 31.33s 20 78.57s Figure 5: Coagulant dosage vs time to coagulation 3.5 Relationship between humic acid removal and coagulant dosage There is a mass balance with regard to the amount of humic acid in feed water and the humic acid retained in product water and humic acid that is removed as sludge. Flow rate in x concentration in =Flow rate out x concentration out +humic acid accumulated We know that Flow rate in =16.ml/s =1L/min Concentration of humic acid in feed water =3g/L =3000mg/L Flow rate out = 1.042L/min For dosage rate of 20% stroke Corresponding absorbance =0.317 This gives a humic acid concentration of 14mg/L 1X3000mg=1.042x14mg+Humic acid accumulated  Humic acid = 3000mg-14.488mg Humic acid = 2985mg= 2.985g Similar solutions at give humic acid accumulation at 40% and 60% stroke. Plotting the accumulated humic acid versus stroke (dosage) for the coagulant gives graph figure 5. From the graph it can be seen that changing the dosage from 20% stroke to 60% results only in a few more grams being accumulated in the sludge. It can also be seen that as the dosage increases the graph starts flattening. Figure 6: 3.6 Viscosity gradient Viscosity gradient is calculated from (U.S. EPA ,1979) The volume V is the retention chamber which is the coagulation chamber in this case. V=46.5X33X26.5cm3 =40664.25cm3 Retention time =  The power to be used is dependant on the available motors and from table 6-7 appendix We choose motor power of 450w Assuming water power is 80% efficiency in comparison to motor power Taking water viscosity to be 1.081x10-3 4.0 Discussion and conclusion 4.1 Discussion One of the important steps in the analysis was coming up with calibration of humic acid. It was important that the water that was to be used in the preparation of the solution to be very pure (ultra pure). From the results of the calibration curve the it can be said that the curve was good enough for uses in getting reliable subsequent results. At zero humic concentration the value gotten was very close to zero the small value was an indication that the water used was of high purity but did not reach the 100% ultra pure mark. The desire curve by producing a straight line which was very close and passed through some of the points. The slight deviation of some of the points from the straight line could be attributed to there some error in measurement of the humic acids matter and also in measurement of the concentrated humic acid solution that was used to come up with the final diluted solution for each point on the curve. The main aim of the this experiment was to find out how to exract maximum level of of the himic acid from the raw water as possible so that to come up with water that if portable. Also as much as we would like to have pure water it is important to know at what point increase in coagulant was most sensitive to the extraction of humic acid. This experiment successfully demonstrated the how increase in dosage of coagulant was related to the extraction of humic acid. From 20% stroke to 40% it was seen that the absorbance reduced linearly with a very steep gradient. From 40 to 60% the curve started flattening which was an indication that in this region the coagulant was less effective in reducing the humic acid level. From literature it has also been found that increasing the coagulant further would be counter productive where by instead of having a further reduction in turbidity (increase in humic acid extraction) there would be increase in turbidity (Leffler, M. R. and P. Bizier,2009). This is because the coagulant is acidic and beyond a certain concentration in the mixture it will lower the pH significantly and this leads to re-solution of coagulant. It is also important to know the optimum point so as to avoid having un-reacted coagulant in what is supposed to be purified water. This could make water to have un pleasant taste of it could also the a health hazard apart from making the purification exercise more expensive because of use of high amount of chemicals (Steel, E. W. and T. J. McGhee ,1979). In this experiment the main source of the water in circulation was the raw water from the raw water tank. The other sources that could have been a source of any fluid in the system would be the diluted chemicals that were added at various stages. The feed water rate was compared with the product out put with the product output being the purified water. It was found that as the dosage of the coagulant increased through increasing stroke the product discharge went beyond the feed rate. This partly is because increasing the dosage resulted to more water from the coagulant solution being added to the feed water and all this combined to have increased amount of freed water. The fact that the product volume is more than the feed water is also an indicator that there is no leakage in the system. The amount of water that comes out of the system is expected to be 100% of what enters the system. This is because the sludge accumulates at the lower at the lower for relatively long time and the concentrated sludge is and it is only occasionally moved to sludge tank by opening valve V8. The amount taken before having coagulation is very important because a water system is designated to meet a certain demand of water. It is thus important to ensure that the water movement through the system is as fast as possible. This will ensure that the water system is meeting the required demand. It was observed that for retention time to be reduced then the coagulant dosage need to be increased. Increasing dosage from 20% stroke to 40% had significant improvement in reduction in the time required for coagulation to set in. while there was further improvement in reduction in time to coagulation by increasing the dosage from 40nstroke to 60 stroke this improvement was small. It can be expected that beyond 60% stroke improvement in the increase in dosage would be counter productive. It is important for the system to reach a dosage of 40% so as to ensure the design is utilized optimally. Beyond the 40% mark other factors need to be put into consideration including the increased cost of excess coagulant and the chances of having the coagulant retained in treated water. If the level of removal of humic acid is not satisfactory at 40% stroke there will need to increase the dosage up to the point where we have satisfactory purification. Also the demand nay so high that increasing to the maximum would be justifiable so as to have the system producing enough out put to meet the demand. The mass balance equation was used in giving further details on the level of purification that is achieved the concentration of humic acid is known and the concentration of the purified water was by use of the calibration curve to match with the absorbance values obtained from tests on samples of the purified water. From the result it is seen that a very big proportion of the humic acid is removed from a coagulant dosage of 20%. This shows that increasing the dosage of coagulant beyond 40% is should be more to the purpose of increasing the flow rate in the system by reducing the retention time. A lot has been said with regard to the role the coagulant and its dosage with regard to the performance of the water purification system. Just having the right dosage of coagulant is not enough if there is no enough level of stirring to ensure interaction of coagulated particles. Velocity gradient is the quantity that measures the level of stirring that is required. From the analysis it was established that for the retention time calculated the dimension of the coagulation chamber and the flow sweep coagulation was applicable. The calculation resulted to the velocity gradient being 2861.75 which was higher than the 1000 required for sweep coagulation. But this is expected to reduce if it can be considered that we have two stirring machines and thus we can half the volume used in calculation of velocity. In choosing the velocity gradient there is implication in the level of power consumption. The motor power will dictate the level of power consumed. The motor chosen shown be such that it suits the design and not beyond what is needed. 4.2 Conclusion From the experiment it has been established the coagulant dosage need to monitor closely in order to ensure optimal performance of the system. While increasing dosage could increase the level of purification of the water, the cost implication need to be put into consideration. It has also been seen that coagulant dosage level beyond 30% may be more important with regard to adjustment of the rate at which the water is purified and not the level of purification. In order for the coagulant to be utilized fully stirring need to be efficient by ensuring that velocity gradient requirement is met. References U.S. EPA (1979) Process Design Manual—Sludge Treatment and Disposal, U.S. Environmental Protection Agency, EPA Pub. No. 625/1-79-011, Washington, D.C., pp. 4-1–4-74, 6-2–6-127, 14-1–14-3, 9-1–9-70. Leffler, M. R. and P. Bizier (2009) “Refining Residuals,” Civil Engineering, American Society of Civil Engineers, vol. 79, January, pp. 56–63. Steel, E. W. and T. J. McGhee (1979) Water Supply and Sewerage, 5th ed., McGraw-Hill, New York,p. 466. Liptak, B. G. (1974) Environmental Engineers Handbook, Chilton Book Company, Radnor, Pennsylvania. Read More