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The potentials of using the hydraulic technique in combined unit for municipal wastewater treatment were studied. A combined unit in which processes of coagulation, flocculation and sedimentation, has been designed utilizing hydraulic mixing instead of mechanical mixing. A jar test treatability study has been conducted to locate the optimum dose of the coagulants to be used. Alum, ferrous sulfate, ferric sulfate, a mixture of ferric and ferrous sulfates, and mixture of lime and ferrous sulfate were all tested. A pilot unit was constructed in the existing wastewater treatment plant at El Mansoura governorate located in north Egypt. The optimum dose of coagulants used in the combined unit gives removal efficiencies for COD, BOD, and total phosphorous as 65%, 55%, and 83%, respectively.
Journal of Advanced Research (2012) 3, 331–336 Cairo University Journal of Advanced Research ORIGINAL ARTICLE Combined coagulation flocculation pre treatment unit for municipal wastewater Ibrahim M Ismail a,*, Ahmed S Fawzy a, Nabil M Abdel-Monem a, Mahmoud H Mahmoud b, Mohamed A El-Halwany b a b Chemical Engineering Department, Cairo University, Giza, Egypt Mathematical and Physics Department, Faculty of Engineering, Mansoura University, Egypt Received June 2011; revised 26 October 2011; accepted 28 October 2011 Available online March 2012 KEYWORDS Combined unit; Coagulants; Flocculation; Hydraulic mixing; Municipal waste water Abstract The potentials of using the hydraulic technique in combined unit for municipal wastewater treatment were studied A combined unit in which processes of coagulation, flocculation and sedimentation, has been designed utilizing hydraulic mixing instead of mechanical mixing A jar test treatability study has been conducted to locate the optimum dose of the coagulants to be used Alum, ferrous sulfate, ferric sulfate, a mixture of ferric and ferrous sulfates, and mixture of lime and ferrous sulfate were all tested A pilot unit was constructed in the existing wastewater treatment plant at El Mansoura governorate located in north Egypt The optimum dose of coagulants used in the combined unit gives removal efficiencies for COD, BOD, and total phosphorous as 65%, 55%, and 83%, respectively ª 2011 Cairo University Production and hosting by Elsevier B.V All rights reserved Introduction Since the first half of the 20th century, pollution in the Nation’s urban waterways resulted in frequent occurrences of * Corresponding author Tel.: +20 1000 401077; fax: +20 25266166 E-mail address: dr_ismail@instruchem.org (I.M Ismail) 2090-1232 ª 2011 Cairo University Production and hosting by Elsevier B.V All rights reserved Peer review under responsibility of Cairo University doi:10.1016/j.jare.2011.10.004 Production and hosting by Elsevier low dissolved oxygen, which represents a hazardous impact on the aquatic life It kills fish, blooms algal and increases the eutrophication and bacterial contamination [1] Municipal waste-water is a combination of different types of waste waters originating from the sanitary system of commercial housing, industrial facilities and institutions, in addition to any groundwater, surface water and storm water that may be present [2] Untreated wastewater generally contains high levels of organic material, numerous pathogenic microorganisms, heavy metals as well as nutrients and toxic compounds These waste waters entail environmental and health hazards and, consequently, must immediately be conveyed away from its generation sources and treated appropriately before final disposal The ultimate goal of wastewater management is the protection of the environment with public health and socio-economic concerns [2] Many different wastewater treatment technologies are used worldwide Each one has its advantages and 332 disadvantages in terms of construction costs, operational costs, energy consumption, operational complexity, effluent quality, reliability, land requirements, and environmental impact Recently some modern technologies were reported for waste water treatment like up flow anaerobic sludge blanket (USAB) [3–5], multi stage bubble column reactor [6] sequential batch reactor (SBR) [7], fixed film anaerobic filter (AF) [8], expanded granular sludge bed (EGSB), which is a modification to UASB [9], up flow septic tank/baffled reactor (USBR) [10], submerged membrane hybrid system [11], anaerobic-anoxic-aerobic bioreactor [12] In more than 38% of the wastewater treatment plants in Egypt, the levels of BOD and TSS in their effluents exceed the 60 mg/l and 50 mg/l allowable limits for disposal into drains, respectively Usually 65–90% of the organic matter in wastewater is colloidal or particulate matter, which can be reduced by chemical pre treatment of raw wastewater Therefore, chemically enhanced processes can be utilized to improve the efficiency of primary treatment processes and to reduce the cost of secondary treatment stage either by eliminating biological treatment, where it is possible, or by reducing the load of secondary treatment units [13] Generally, the chemical treatment process involves a series of three unit operations; rapid mixing, flocculation and settling At first, the chemicals are added and completely dispersed throughout the wastewater by rapid mixing Coagulated particles are then brought together via flocculation by mechanically inducing velocity gradients Finally, the solid materials are separated in clarification unit by gravity [14,15] Two basic types of flocculation systems are able to induce slow movement of the fluid; static hydraulic flocculators and mechanical flocculators In static flocculation systems, hydraulic mixers, slow mixing of the coagulant with water is achieved by hydraulic means through sudden directional changes by baffled channels, which could be either horizontal or vertical [16] This method is simple and free from moving parts, therefore it needs minimal operation and maintenance It also exerts minimal head loss across the flocculation tank Disadvantages of this type of flocculators include excessive velocity gradients at the bends of the baffled channels and the dependence of the velocity gradients on the flow rate within the basin; therefore, they offer lower degree for control [17] Usually it is feasible to use chemically enhanced treatment for small and medium size plants For small plants (less than 1000 m3 per day), chemically enhanced treatment only could be feasible For medium scale plants (less than 10,000 m3 per day), combined enhanced primary treatment with reduced secondary treatment may be feasible For larger plants, the combined unit is expected to be feasible only, if the discharge limits are strict so that tertiary treatment may be necessary in case of using conventional primary treatment The important factors that should be studied in pilot-scale flocculation facilities are the appropriate chemical dose, the effect of mixing energy and the effect of mixing time, which are achieved experimentally using the jar test [18,19] Chemical coagulants that are commonly used in wastewater treatment include alum (A12(SO4)3Ỉ18H2O), ferric chloride (FeCl3Ỉ6H2O), ferric sulfate (Fe2(SO4)3), ferrous sulfate (FeSO4Ỉ7H2O) and lime (Ca(OH)2) Recently some natural based materials like chitosan and chitosan derivative were utilized in coagulation/ flocculation processes [20–22] Synthetic organic polyelectrolyte’s are also sometimes used as flocculation aids [19,23–25] I.M Ismail et al The aim of this research is to conduct a jar test treatability study to locate the optimum doses of the used coagulants and to study the effect of different variables affecting the treatment efficiency Based on the results of the treatability study, a pilot plant for the treatment of sewage combining rapid hydraulic mixing coagulation, flocculation and settling in a single unit is to be erected and utilized for the treatment of a real municipal wastewater The final objective of this study is to find a reasonable method to treat sewage wastewater in a touristic village far from governmental treatment stations Experimental Materials Raw sewage The experimental study was carried out using raw sewage of the El Mansoura governorate wastewater treatment plant, Egypt Due to the variation in the composition of wastewater produced by Mansoura governorate, composite samples from the effluent of the existing physical sedimentation tank were collected using a continuous flow peristaltic dosing pump The samples were collected during 16.0 h daily Characterization of the wastewater was carried out for almost months to cover the variations in the composition of the effluents as they change by daily operation Table shows the average characteristics of the wastewater used in this investigation Coagulants The selected coagulants for the chemical treatment are alum [Al2(SO4)3Ỉ18H2O], 99% purity, ferrous sulfate [Fe(SO4)Ỉ 7H2O], 97% purity, ferric sulfate [Fe2(SO4)3], 97% purity and lime [Ca(OH)2], 99% purity These coagulants are all of technical grade They have been used since they are produced locally and available in the market with relatively low prices Methods Jar test set-up Jar tests were conducted in a set up with six stirred beakers of 2.0 l capacity The beakers were filled with 1.5 l of wastewater The procedure of the jar tests and the values of different parameters were obtained from literature with some tuning [6,26] as follows: The alum and ferrous sulfate solutions were prepared at concentration of 1.0 g/l Flash mixing is started at 350 rpm and continued for one minutes during addition of coagulant with dosage of 30, 60, 80, 100, and 120 mg/l These dosage values were selected based on the value of suspended solids and Table Average characteristics of wastewater pH Temp (°C) TDS (mg/l) Turbidity [NTU] COD (mg/l) BOD (mg/l) TSS (mg/l) PO3À ðmg=lÞ NHỵ mg=lị 7.44 25 570 60 360 140 86 27.3 Combined coagulation flocculation pre treatment unit 333 50 cm R1 10 cm cm 20 cm 50 cm Annulus R2 25 cm Sewage from presedimentation basin resulting from the jar test treatability study, through a pipe line connected to the suction line of the sewage centrifugal pump The rapid mixing of the sewage and the coagulant is carried out by the centrifugal action in the feed pump Slow mixing occurs in the annulus between the inner cylinder and the outer body of the basin (the outer zone), where the flocculation takes place The inner zone is the space enclosed within the inner cylinder, where sedimentation occurs The sludge settles into the central hopper at the base of the treatment basin Valves are provided to redirect the settled sludge to the sludge holding tank The clarified effluent from the settling tank passes over adjustable ‘‘V’’ notch weirs in the peripheral launder and then an outlet pipe carries the treated effluent stream Analysis The physical and chemical properties of the supernatant separated after flocculation and settling was analyzed according to the well-known Standard Methods [1] Treated Sewage Sludge Fig The compact treatment unit our previous experience A flocculation process was conducted for 30 by gentle stirring, and then sedimentation for 30 is carried out The over flow samples have been drawn and analyzed The possibility to utilize a mixture of more than one coagulant was also tested The effect of the addition of different doses of lime to the optimum dose of ferrous sulfate, 60 mg/l, and different doses of ferrous sulfate to the optimum dose of ferric sulfate, 60 mg/l, was also studied Pilot unit The pilot unit consists of A compact treatment unit, which is the core of the proposed system, is shown in Fig It consists of a cylindrical tank provided with a conical bottom and inner vessel Two designs for the inner vessel were tested; cylindrical shape and three conical shapes with three different cone angles The advantage of the conical shape is that the velocity gradient in the system was created due to the tapering of cross sectional area in the path of water descending downward in the annular space This gives the chance for efficient flocculation The main dimensions of the inner vessel designs are given in Table 2 Coagulant feed tank, 20 l capacity connected with a variable speed dosing pump to control the dose of the coagulant Centrifugal sewage feed pump Methodology The sewage water is pumped into the compact treatment basin after the injection of the coagulants with the optimum dose, Table Results and discussion Jar test treatability study A jar test treatability study was carried out to select the optimum coagulants to be used in the pilot test and to determine the optimum coagulant dosages The use of coagulants such as alum or ferrous sulfate is a common practice to coagulate the suspended solids present in sewage wastewater, these coagulants are also cheap and safe and the produced sludge can be easily handled Four sets of experiments were carried out to cover the corresponding variables and the used coagulants The effect of coagulants dosages on the removal efficiency of Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and total phosphorous ðPO3À Þ are illustrated in Fig As indicated in Fig 2, the removal efficiencies of COD, BOD, TSS and PO3À increase with the increase of alum dose till they reaches the maximum value at about 60 mg/l It is worth noting that alum addition increased the particle size of suspended material This in turn enhances the settling of suspended matter due to coagulation Consequently, this will affect the removal of some biodegradable organics belong to the suspended solids, so the values of BOD and COD will decrease Moreover alum addition will increase the possibility of precipitation of insoluble phosphate, but the orthophosphate remains as soluble material No need to adjust pH in case of using alum since the pH was around 6–6.5, which is the optimum condition After the maximum dose value of 60 mg/l, no appreciable improvement in the removal efficiency is observed by increasing the coagulant dose The removal efficiencies of COD, BOD, TSS and PO3À at the optimum dose of Geometrical shape and dimensions of inner vessel Type Upper radius, R1 (cm) Lower radius, R2 (cm) Height, H (cm) Slope h (°) Cylinder Cone Cone Cone 20 22.5 20 17.5 20 12.5 12.5 12.5 50 50 50 50 90 75 80 85 334 I.M Ismail et al 90 (a) 80 %Removal 70 60 50 PO4 [ Ci = 4.5 mg / lit ] 40 TSS [ Ci = 86 mg / lit ] 30 COD [ Ci = 400 mg / lit ] 20 BOD [ Ci = 150 mg / lit ] 10 20 40 60 80 100 120 Alum Dose (mg/lit) 90 (b) 80 %Removal 70 60 50 40 PO4-3 [ Ci= mg/lit ] 30 TSS [ Ci = 86 mg / lit ] COD [Ci=320 mg / lit ] 20 BOD [ Ci = 140 mg / lit ] 10 20 40 60 80 100 120 Ferrous sulfate Dose (mg/lit) 90 (c) 80 Pilot unit 70 %Removal the ferric sulfate dosage till it reaches the maximum at dosing value of 60 mg/l It can also be noticed that the value of the removal efficiencies obtained using ferric sulfate is slightly more than those values obtained using ferrous sulfate, which is attributed to the greater charges of the ferric ions The addition of lime slightly improves the removal efficiency of the organic and suspended solids loads The values of the measured parameters of the treated effluent are lower than the case of using ferrous sulfate alone as a coagulant This can be attributed to the higher pH of the solution after adding lime Increasing the dose of lime gives a slight increase in COD, PO3À removal After a lime dose of 20 mg/l, no further improvement in removal efficiency was observed The use of extra dosage of lime increases the alkalinity of the waste water and may result in a final pH greater the recommended range 6– In case of ferrous sulfate it was found that, by increasing the dose of ferrous sulfate, the removal efficiency increased gradually till it reaches the maximum removal efficiency at dose equal to 20 mg/l After that no further improvement in the removal efficiency was observed by increasing the coagulant dose Table shows a summary of all the jar test results Although the summarized results shown in Table shows that the highest removal was optioned upon using a mixture of ferrous and ferric sulfate, alum was selected for the pilot test as the difference in separation efficiency between alum and the mixture of ferrous and ferric sulfate was small and does not justify the use of mixture of two materials, which will need extra mixing unit Based on the results of the jar test study, the use of 60 mg/l of alum was selected as the optimum coagulant The effect of retention time and geometrical shape of the inner vessel on the removal efficiency of BOD, COD, TSS and total phosphorous for treated stream flowing from the combined treatment basin are discussed below using alum 60 50 PO4 [ Ci = mg / lit ] 40 TSS [ Ci = 86 mg / lit ] 30 COD [ Ci = 320 mg / lit ] 20 BOD [ Ci = 140 mg / lit ] 10 20 40 60 80 100 120 Ferric sulfate Dose (mg/lit) Fig Effect of different coagulants dose on % pollutants removal (a) Alum; (b) ferrous sulfate; (c) ferric sulfate Flash mixing time = 60 s, settling time = 30 min, flash mixing speed = 350 rpm, pH = 6–6.5 and temperature = 30 °C alum were found to be 61, 53, 77 and 73 respectively These removal efficiency values are slightly lower than the case of using the more expensive polyelectrolyte [19,23–25] Similarly, it is also clear that increasing the dose of ferrous sulfate increased the removal efficiency of pollutants gradually till it reaches its maximum value at dose equal to 80 mg/l Similar to the case of alum, the using of ferrous sulfate will increase the particle size of suspended materials, which enhances settling and coagulation of suspended materials After maximum dose value no appreciable improvement in the removal efficiency was observed by increasing the coagulant dose The removal efficiencies of COD, TSS, BOD, and PO3À are increased by increasing Effect of retention time on wastewater characteristics The effect of retention time on the percentage removal of TSS, BOD, COD and Phosphorous for sewage was investigated and displayed in Fig Results indicate that the optimum retention time is 150 min, which corresponds to a flow rate of 0.7 l/min This retention time is lower than the retention time of conventional chemical treatment unit, where it is usually more than 3.5 h After a retention time of 150 min, no further improvement in the removal efficiency is observed by increasing the retention time These observations may be explained by the fact that at flow rates higher than 0.7 l/min., the residence time is low and only a partial treatment for organic and suspended solids loads was obtained Effect of geometrical shape of the inner vessel on the percentage removal efficiency of pollutants The effect of geometrical shape of the inner vessel on the percentage removal efficiency of pollutants has been studied The different designs of the inner vessel were tested to check the effect of the velocity gradient in the system that was created due to the tapering of cross sectional area in the path of water descending downward in the annular space, on the efficiency of the treatment process The results of the study of the Combined coagulation flocculation pre treatment unit 335 Summary of the results of the jar test study.* Table Coagulant % Removal Type Dose (mg/l) TSS BOD ðPO4 ÞÀ3 COD Alum sulfate Ferrous sulfate Ferric sulfate Ferric sulfate and ferrous sulfate Ferrous sulfate and lime 60 80 60 60 + 20 80 + 20 77 74 77 86 78 53 48 48 50 50 73 69 70 76 73 61 56 59 61 60 Flash mixing time = 60 s, settling time = 30 min, flash mixing speed = 350 rpm, pH = 6–6.5 and temperature = 30 °C G¼ 90 60 % Removal ð1Þ where x is the angular velocity (sÀ1) and Ri, Ro are the inner and outer vessels radii Fig shows a schematic for the pilot unit used with a cone as an inner vessel The angular velocity can be calculated from: 75 2ị x ẳ Vh =r 45 Tss [ Ci = 86 mg/lit] 30 COD [ Ci = 360 mg/lit] BOD [ Ci =150 mg/lit] 15 PO4-3 [ Ci = mg/lit] 30 60 90 120 150 180 210 Time (min) Fig Effect of the compact unit retention time on % removal of pollutant Inner vessel = cylindrical and coagulant = alum 60 mg/l geometrical shape effect for the inner vessel on the removal efficiency are illustrated in Fig It can be easily noticed that using of cone (1) results in the best treatment of the sewage Results indicated that the removal efficiencies for TSS, COD, BOD and total phosphorous are 83%, 65%, 55% and 76%, respectively The average velocity gradient of the waste water, G, is a key parameter in evaluating the performance of the coagulation and flocculation processes According to Leentvaar and Ywema [13], the maximum flocculation efficiency is obtained for a G value in the range 10–25, which can be calculated according to: where Vh is the velocity of liquid tangent to the inner vessel and r is the mean radius = (Ro + Ri)/2 Vh can be calculated from the empirical formula [6] Vh ¼ 3:98ðRo À Ri ÞÀ0:762 h0:379 Q0:121 80 70 ð3Þ The average velocity gradient of the waste water, G, has been calculated for each inner vessel case using Eq ()()()(1)–(3) as a function of the height h Fig illustrates the relationship between G and h for the four internal vessels It can be easily noticed that cone (1) has the highest G value up to 25 This explains the experimental finding shown in Fig that cone (1) has the best removal efficiency of BOD, COD, TSS and PO4 It worth noting that at higher flow rates, the activated sludge process may be more economic than the proposed method However, several problems arise using the activated sludge because of the toxicity of bacteria and the adverse effect on the biodegradation process The proposed combined unit will be mainly utilized for touristic villages, which are not connected to any sewage systems, as they are distributed along the sea side The discharge of this system in such case will be used to irrigate the gardens In case of utilizing this system to treat municipal waste water, where the effluent of this system will be fed to biological treatment unit, a by bass system could be used to adjust the COD of the influent to the biological treatment unit Ro (outer radius) % Removal (PO4) % Removal (TSS) % Removal (COD) % Removal (BOD) 90 R1 V(θ) 60 Ri (inner radius) 50 Vz 40 Annulus 30 h % Removal 2xRi R0 R2o À R2i H * 20 10 Vh Cylindrical Cone Cone Cone R2 θ Type of inner vessel Fig Effect of the type of inner vessel on % removal of pollutants Coagulant = 60 mg/l and feed flow rate = 0.7 l/min Fig Schematic of the inner vessel shape and dimensions 336 I.M Ismail et al Velocity gradient G (Sec-1) 40 35 Cylindrical Cone2 30 Cone Cone3 25 20 15 10 0 10 20 30 40 50 Height (Cm) Fig Variation of average velocity gradient (G) with height (h) Conclusions A jar test treatability study has been conducted to locate the optimum dose of the coagulants to be used in the treatment of the sewage from an existing wastewater treatment plant at El Mansoura governorate located in north Egypt Based on the results of this jar test study, the use of 60 mg/l of alum was selected as the optimum coagulant A combined unit in which process of coagulation, flocculation and sedimentation, has been designed and operated utilizing hydraulic mixing Optimum retention time in pilot unit is equal to 2.5 h compared to 3.5 h for conventional chemical treatment On using alum in the pilot unit, it gives 83%, 65%, 55% and 76% removal efficiencies for TSS, COD, BOD and total phosphorous, respectively The proposed combined unit can be integrated with existing sewage treatment plants to reduce the load on the biological stage It may be also used for the treatment of sewage effluents of remote small villages and camps Acknowledgements The 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processing wastewater: evaluation of settleability parameters and design of a clarifier–thickener unit J Hazard Mater 2007;148:6–14 ... enhanced treatment only could be feasible For medium scale plants (less than 10,000 m3 per day), combined enhanced primary treatment with reduced secondary treatment may be feasible For larger... settling in a single unit is to be erected and utilized for the treatment of a real municipal wastewater The final objective of this study is to find a reasonable method to treat sewage wastewater in... characteristics of wastewater pH Temp (°C) TDS (mg/l) Turbidity [NTU] COD (mg/l) BOD (mg/l) TSS (mg/l) PO3 mg=lị NHỵ ðmg=lÞ 7.44 25 570 60 360 140 86 27.3 Combined coagulation flocculation pre treatment unit