پیش تصفیه فاضلاب شهری با انعقاد شیمیایی پیشرفته Pretreatment of Municipal Wastewater by Enhanced Chemical Coagulation

INTRODUCTION Nowadays, the treatment and disposal of pollutants, as one of the most important issues in environmental engineering, becomes even more complex given in place limit in terms of disposal options. From historical point of view in previous centuries (by the end of 1900) sufficient attention was not paid to the issue of wastewater treatment due to readiness in releasing untreated wastewater into recipient sources. But in the early of 20th. century, failure to provide wide and sufficient areas for disposal of untreated wastewater, particularly in mega cities, culminated into applying more effective methods in wastewater treatment (Metcalf and Eddy, 1991). Former experiences especially in 1980s revealed that wastewater treatment projects in developing countries not just requires technological facilities but these facilities could be established exploiting human and financial resources. Therefore, these projects are often constructed in big urban areas enjoying great human and financial sources and this fact limits their development into poor areas (Sonune and Ghate, 2004). To solve the above mentioned problems, Chemically Enhanced Primary Treatment (CEPT) is tenable as an appropriate, executive and effective method (Kurniawan, et al., 2005). This technology not only brings proper and comparable results in terms of reducing the COD, turbidity and TSS in comparison with current systems, but also implies a very cost effective and productive method to upgrade the capacity of conventional plants (Olive, 2002). CEPT is a process in which the chemicals (generally metal salts or polymers) are added for pretreatment purpose. These chemicals conglomerate the suspending solid particles via coagulation and flocculation processes (Zhou, et al., 2004). The accumulated particles or flocs have high sedimentation velocity and consequently the treatment performance of parameters such as COD, Phosphorus, turbidity and TSS will be improved. This process can be performed in sedimentation tanks of conventional treatment plants and is cheaper, simpler with higher performance comparing with conventional systems (Harleman and Morrisey, 1992; Harleman and Murcott, 1992, 2001a, 2001b). Among other advantages over conventional practices, it requires half of the necessary volume of sedimentation ponds in comparison to conventional methods, shows more discharge rate instead of smaller needed space required for installation of necessary facilities, has appropriate elimination function for wide range of wastewaters with various specifications, appropriately complies if being added to the various treatment facilities (Olive, 2002) and also bears numerous economic advantages in terms of production and exploitation (Harleman, et al., 1997). Chemical Enhanced Primary Treatment includes coagulation followed by sedimentation and removal of flocs in a sedimentation unit. Dispersed solids inside wastewaters include non sedimentary suspending materials or particles with very negligible sedimentation velocity, in which the colloids are as constituents of major part of non sedimentary particles. Since most of natural colloids have negative charges that expel the similar charges, these particles grant stability to a suspension. When a coagulant is added to wastewater, disintegrates and via hydrolyzing the metal ion, metal hydroxide ionic complexes with high positive charge will be formed. Since these complexes have high positive charges, they absorb to the surface of colloids and by reduction of negative charge, they are being made to be neutralized and condensed via Vandervalce forces. This absorption is strengthened by water turbulence (flocculation) and particles with proper sedimentation capability will be formed (Reynolds and Richards, 2000). Regarding the modality of phosphorus removal by coagulants it can be said that adding these chemicals to wastewater causes the cations of these salts to combine with anionic insoluble phosphates inside the suspension and creates insoluble metal phosphate. Given the light weight, this particle has not enjoyed sedimentary virtue yet and will be precipitated using flocculation (Daniels and Parker, 2003). In fact, upon adding coagulation salts, although wastewater receives Fe2+, Fe3+ and AL3+, phosphorus removal in pH lower than 6.5 will be accompanied with formation of insoluble substances of AlPO4 , FePO4 and in pH higher than 6.5 with aluminum and Iron oxide and hydroxide (Irdemez, et al., 2006; Jiang and Lioyd, 2000). Numerous researches have already been conducted in various countries particularly in developing countries. In coastal resort city of Rivera in Brazil which is facing with four times increase in basic population in summer time the wastewater treatment plant always experienced problems. In 2000, Yu and Bourke implemented CEPT system using 50 mg/L of ferric chloride with 0.5 mg/L of anionic polymer as coagulant aid, which was able in 60 and 85% reduction of BOD and TSS contents, respectively. In another instance, in 2001, conducted study by Harleman and Morcutt on economic assessment of implementing CEPT system in Rio de janeiro treatment facilities revealed that this system not only dose not require ,major capital investments but also can increase the capacity of existing treatment facilities without any requirement to change the current plan of working systems (Olive, 2002). The objective of the implemented study in UK by Song, et al. in 2003 was to develop a treatment system that can effectively reduce the concentration of pollutants in tannery wastewater to environmentally acceptable levels and that can greatly reduce the cost of discharging the effluent. During coagulation process, in optimum pH 7.5, 30-37 % of COD and 38-46 % of TSS were removed by and ferric chloride as coagulants, respectively. Also, ferric chloride produced better results comparing with (Song, et al., 2003). In 2003, Delgado, et al., conducted an experimental laboratory scale study using aluminum sulfate, Ferric chloride, and Poly aluminum chloride to obtain the required water quality (3-5 NTU) in turbidity in the discharges of the secondary treatment from a conventional wastewater treatment plant in Spain. The Poly aluminum chloride showed the best performance in wastewater natural pH and 50 mg/L of coagulant dosage (Delgado, et al., 2003). In 2005 another research was conducted by Mahvi et al. in Iran to study the CEPT system performance on the waste of largest Iranian industrial complex into the municipal wastewater treatment plant. Applying of lime and FeO3 as chemical coagulants, reduced the BOD, COD and TSS levels 27-53, 25-59, 46- 94 percent, respectively and showed that the pretreated wastewater can be discharged into municipal wastewater treatment plant in terms of quality (Mahvi, et al., 2005). In 2005, Amuda et al. studied the performance of CEPT pretreatment process on the wastewater of an beverage industry in Nigeria, using Fe2 (SO4 )3 .3H2 O as coagulant and also in conjunction with neutral Poly Acrilamide Polyelectrolyte. The results revealed that using coagulant at 500 mg/L dosage removes 78, 75 and 74% of COD, Phosphorus and TSS contents, respectively, and applying coagulant aid at about 25 mg/L dosage enhances the removal of said parameters 93, 96 and 94% ,respectively. The results of this experiment were reported as completely appropriate for biological refining stage (Amuda, et al., 2005). On the basis of the above discussion, the main objective of this research was to investigate the feasibility of treating municipal wastewater by CEPT process. This was achieved by conducting laboratory settleability studies and jar tests using alum and ferric chloride. Furthermore, the optimum conditions under which the wastewater would be treated were investigated.