DAIRY WASTEWATER TREATMENT BY PHYSICOCHEMICAL METHODS AND SEQUENTIAL BATCH REACTOR

Abstract

Dairy industries are involved in the manufacturing of various types of milk products. It is one of the most polluting industries not only in terms of volume of effluent generated as well as in terms of its characteristics too. Dairy wastewaters are treated using biological and physico-chemical methods. Most attention has been paid to anaerobic treatment process followed by aerobic and membrane methods. High energy requirement by aerobic biological treatment methods is the primary drawback of these processes, whereas, anaerobic treatment of dairy wastewater reflects very poor nutrient removal. Therefore, further treatment of anaerobically treated effluent is required. Among physico-chemical methods: adsorption, coagulation-flocculation and electro-chemical treatment (ECT) are reported for the treatment of dairy wastewater. Coagulation with inorganic coagulants reported in the literature lack studies on mechanism of treatment process. Moreover, coagulation studies using natural coagulants except for chitosan are not reported in the literature. Further studies have to be concentrated on these aspects of coagulation treatment. ECT using various types of electrodes also requires immediate attention. Disposal aspect of residues (scum and sludge), which are generated during the ECT have not been dealt in previous studies. Among physico-chemical methods, adsorption is an important treatment method for treatment of various types of wastewaters. However, only few studies are reported for the treatment of dairy wastewater by adsorption. Moreover, in these studies, mechanism of adsorption is lacking; kinetic, isotherm, and thermodynamics aspects have not been dealt with which are important for the design of any adsorption unit. Disposal aspects of the spent adsorbents were also not reported in these studies. Disposal of sludge during biological methods is also very important; however, only few investigators have dealt this aspect of wastewater treatment. Present study has been undertaken with endeavor to study the treatment of dairy wastewater by following methods: 1. Treatment by coagulation process using inorganic coagulants such as poly aluminum chloride (PAC), FeS04 and potash alum; and organic coagulants such as guar gum, sodium alginate and sodium salt of carboxy methyl cellulose (Na-CMC). 2. Treatment by electro-chemical method. 3. Treatment by adsorption process using commercial activated carbon and bagasse fly ash. 4. Treatment by aerobic sequential batch reactor. To avoid the change in COD and nutrients during storage, simulated dairy wastewater (SDW) was used in the present study. SDWwas generated in the laboratory by dissolving 4 g of milk powder (Amulya brand, manufactured by Banaskantha District Cooperative Milk producer's Union Ltd., Palanpur, Uttarakhand, India) per litre of distilled water. Several investigators used same method for making SDW (Ramasamy et al.. 2004b; Balannec et al, 2005; Leal etal.. 2006). Inorganic coagulants like poly aluminum chloride (PAC), ferrous sulphate (FeS04) and potash alum (KA1(S04)2.12H20) were used for the treatment of SDW. Batch coagulation experiments were conducted to evaluate the influence of initial pH (pHHn: 5-10) and coagulant dosage (min: 100-5000 mg/1) on COD removal from SDW. Residual COD and system pH were observed as function of time. Optimum pH,.m (pHj.m,op) was found to be 8.0 for all the three coagulants. Optimum min (min.op) was found to be 300, 800 and 500 mg/1 for PAC, FeS04 and KA1(S04)2.12H20, respectively, giving 69.2, 66.5 and 63.8% COD removal efficiency in 30 min at pH,.in<o/7~8.0. COD removal mechanism by the coagulants was mainly due to charge neutralization and adsorption (Kushwaha et al., 2010b). Slurry produced by PAC and KA1(S04)2.12H20 showed good settling characteristics. Organic coagulants such as guar gum, sodium salt of carboxy methyl cellulose (Na-CMC) and sodium alginate (Naalginate) were also tested for their efficiency of COD removal at pH, (pH,.na) range of 3-10 and dosage range (mna) of 10-200 mg/1. Maximum COD removal efficiency of 66%, 67% and 74% for gar gum, Na-CMC and Na-alginate respectively, was obtained at pH,_na=4 and mna=\00 mg/1. Response surface methodology with four-factor and five-level full factorial central composite (CC) design has been used to design the experiments for batchECTof SDW using aluminum (Al) and iron (Fe) electrode. Cuboid shape batch reactor of dimension 108 mm><108 mm*130 mm having working volume 1.5 litre of Perspex sheet was used to conduct the ECTexperiments. Magnetic stirrer was used to agitate the SDW. Two pairs of Al and Fe electrodes of thickness 1 mm and 1.5 mm, respectively, each having dimensions of 10 cm x 8.5 cm with inter-electrode spacing of 1 cm were used. Four operational parameters, namely J: 61.7-308.6 A/m2; m(weight ofNaCl): 0-2 g/1; /: 10-90 min and pH,\ 5-11, were taken as input parameters and percentage COD removal (7/) and specific energy consumed (KWh per kg of COD removed) (Y2) were taken as a responses of the system. Multi-response optimization technique was applied to find values of operational parameters which maximize the Yi and simultaneously minimize Y2. The optimum values of operational parameters were found to be J=123 A/m2, m=2.0 g/1, f=74 min andpH0=6.5 with Y, and Y2 were found to be 68% and 1.22 kWh/kg COD removed by Al electrode (Kushwaha et al., 2010c). Whereas, corresponding values for Fe electrode were 270 A/m2, 0.0, 50 min, and 7.0, respectively, with Yi =70% and F2=2.76 kWh/kg of COD removed (Kushwaha et al., 2010d). TS, TN and turbidity at optimum condition by Al electrode were found to be 54%, 919% and 99%, respectively. Fe electrode showed TS, TNand turbidity removal efficiency of 48%, 93% and 99%, respectively. Electro-coagulation (charge neutralization of anionic colloids present in the SDW by monomeric cationic aluminum species and sweep coagulation), electro-floatation and electro-oxidation by hypochlorite were found to be the real mechanism of SDW treatment. Adsorptive treatment of SDW was done by means of adsorption onto activated carbon-commercial grade (ACC) and bagasse fly ash (BFA). Dosage study was carried out by varying the dosages (mad) in the range of 0.5-25 g/1 for both the adsorbents at the optimum pHj (pHi-opl) and 303 K. The adsorption of SDW by the adsorbents was studied over a pHt range of3-10at 303 K. For SDW treatment by adsorption, optimum pH ipHi.opt) was found to be -4.8. Optimum adsorbent dose (mad.opt) were found to be 20 g/1 for ACC and 10 g/1 for BFA. Equilibrium contact time was found to be 8 h for both the adsorbents. Pseudo-second order kinetic model was found to fit the kinetic data and Redlich-Peterson isotherm model was generally found to best represent the equilibrium data for SDW treatment by ACC and BFA. The change in entropy and enthalpy for SDW adsorption onto ACC and BFA were estimated as 126 kJ/mol K and 92 kJ/mol; and 26 kJ/mol K and 18 kJ/mol, respectively. The negative values of change in Gibbs free energy indicate the feasibility and spontaneous nature of the adsorptive treatment (Kushwaha et al., 2010e). Aerobic sequential batch reactor (SBR) technology was used for the biological treatment of SDW. The experimental set-up consisted of a well-mixed cylindrical glass reactor having maximum volume of 10 litre and a working volume of 5 litre. A stirrer was used to keep reactor contents homogeneous. Temperature and SRT were maintained constant 28±2 °C and 20 d, respectively, during the study. The reactor was operated on a fill-and-draw basis, with a cycle time (tc) of 12 h, while, settle time (ts), decant time (tD) and idle time (t/) were 1, 0.5 and 0.5 h, respectively. React time (tR) were varied according to the fill and anoxic phases strategies to degrade the carbonaceous materials and nitrogen present in SDW. For this four phases study was carried out. In the phase-I of SBR operation, instantaneous filling strategy, tf=0, was implemented with varying volume exchange ratio (VER) and hydraulic retention time (HRT) from 0.40-0.80 and 0.625-1.25 d, respectively. Aeration was started at the start of the reaction time and closed when settle phase started. Phsae-II was initiated by varying tF in the range of 0.5-2 h at optimum VER (VER^,) and HRT (URTopl) obtained from phase-I study. The filling of SDW to SBR was carried out under aerated and mixed fill strategy. In phase-Ill and IV, an anoxic zone (tA) were introduced to enhance the nitrogen removal just after fill phase and react phase, respectively, at optimum tF (tF,opt), VER^, and HRT0/„. tA after fill (tA.F), varied in the range of 0.5-2 h, was introduced in phase-Ill. The effect of tA after reaction (tA-R), which was varied in the range of 1-3 h, was studied in phase- IV at optimum tA-F ((a.opi-f)- An increase in HRT from 0.625 to 1.25 d decreased the final liquid effluent COD. This was due to increased biological conversion at higher HRT. Optimum HRT(HRTop/) was found to be 1.0 d giving 97% COD removal efficiency and 65% TKN removal efficiency. tF,oPi=\-0 h was found to be sufficient to treat SDW of Co=3900 mg/1. With an increase of tF from 0.5-1.0 h, more and faster nitrification was observed. Optimum tA.F (tA,oPt-F) was found to be 1.0 h giving 77% TKN removal at the end of reaction, while introduction of tA-R showed no effect on TKN removal. Heating values of the sludge generated by the coagulants PAC, FeS04 and KA1(S04)2.12H20 were found to be 20.7, 29.6 and 17.3 MJ/kg. respectively. Sludge generated in other treatment methods also showed heating values in the range of 16.5-19.2 MJ/kg. Due to the high heating value of residues generated during various treatment processes, they can be dried and used as a fuel in the boilers/incinerators, or can be used for the production of fuel-briquettes. Comparison of various treatment methods shows that treatment in SBR removes highest amount of COD, however, it requires higher treatment time as compared to ECT or coagulation.