Optimization Pulp and paper wastewater treatment by using advanced chemical oxidation Fenton method

Introduction
The complexity and diversity in production of paper and high volume of water consumption in each steps, has made the paper industry as one of the largest industrial consumer of water after the chemical and metal industry (Covinich,et al,2014). High water consumption is between 76 to 227 cubic meters per ton of product (Ginini et al, 2014). The wastewater, in addition to high flow, contains high concentrations of BOD, COD, pH, suspended solids, color and turbidity. Pollution of water bodies created by these industries is one of the major world concerns (Khansothong, 2009). More than 250 chemical substances have been identified in various stages of paper production. Apart from a large variety of air pollutants and their complex building, many compounds in wastewater are poorly water soluble, and their resistant to harsh treatment processes, particularly biological filtration and reddish-brown color of has created many problems for treatment processes (Birjandi, et al, 2014). The other part of these pollutants, consist of chemical compounds such as chlorinated compounds, phenols, absorbable organic compounds (AOX), and also additives that are used during the paper-making industry processes such as chemical pulping process. All of this factors have caused that wastewater of paper industry has one of the most difficult water treatment, which associated with the complexity of processes (Zahrim et al, 2007). The majority of industrial wastewater arises from pulp and paper pulping process Depending on the type of pulping process, wood and paper industry produce various pollutants. Raw materials for pulp of Old Corrugated Container (OCC) unit supplied from waste paper and cardboard waste. Raw material to produce Kraft pulp is hardwoods and softwoods (karmali 2014). Research studies show that a variety of methods have been used for wastewater treatment in factory of pulp and paper. It can be noted that in the following ways: physicochemical treatment, settling and filtration, coagulation and sedimentation, adsorption, chemical oxidation, membrane filtration, ozonation, biological treatment, activated sludge, aerated lagoons, aerobic biological reactor , anaerobic treatment method, fungal treatment, combined treatment process (Buyukkamaci and Koken 2010). Biological treatment methods are generally preferred due to environmental impacts and low cost and it is a Conventional method for treating wastewater containing organic materials. One method of biological treatment is Activated Sludge. This method produces variable sludge settling characteristics and Often is sensitive to shock loading and toxicity. Their capacity to remove toxic biodegradable substances is extremely low (Katal 2011). The barriers to biological systems require large surface area, difficult to control the population of microorganisms, control of pH, temperature and nutrients for creating bulking sludge (Tambosi et al 2006). Wastewater of Pulp & Paper has restricted the use of biological methods, due to the presence of toxic compounds with Low capable of biodegradation. For this reason, the uses of chemical methods have become more acceptable (Bianco 2011). Among chemical processes, advanced oxidation processes are used for organic load reduction or toxicity of waste water (Martinez et al 2003). So this method has become a competitive technology to remove organic contaminants that are refined by conventional methods. Chemical oxidation is based on the production of hydroxyl radicals oxidizing agent in order to mineralization as a complex chemical composition of wastewater. Hydroxyl radicals are a strong oxidizing with power of oxidizing 2.23 V in comparison to conventional oxidizers (Perez et al 2002). Fenton process (H2O2/ Fe+ 2) One of the most common advanced oxidation process that is used for wastewater treatment and it is possible to use it in different scales used because of the ease of implementation, low reaction time, process of coagulation and flocculation, non-toxic compounds and low economic cost in comparison with the other methods (Neyens 2003) (Barreto-Rodrigues et al 2009) (Badawy et al 2009).


Materials and methods
Studied Wastewater was prepared from pulp and paper mill in Iran. In the investigated factory, Depending on the used process, a Kraft pulp or OCC pulp generally produced two main wastewaters. One of them is Wastewater fiber and other is alkaline Wastewater. Considering the type and volume of produced waste water, a mixture of the two wastewaters (in the ratio 3: 1) was moved to laboratory to evaluate the treatability. In this study for optimizing variables, five factors pH, temperature, reaction time, concentration of iron and hydrogen peroxide concentration are considered, the response surface methods (RSM) was used in the application minitab17 for Optimizing of these factors. Response surface methodology is based on central composite design is one of the statistical methods for design of experiments, modelling, evaluating the effects of several factors, optimal conditions for favorable response and reduce the number of tests. 33 designed tests on the Fenton process with a batch Pilot scale was done in a Pyrex cylinder with capacity of 500 ml. After transferring the mixture of fiber and alkaline Wastewater, the Pyrex was placed on a magnetic stirrer equipped with a thermostat to adjust the temperature. Since the Fenton reaction occurs in an acid environment, Wastewater after acidification with sulphuric acid (H2SO4) to set a normal pH was subjected to specified concentrations of iron salt As a catalyst (FeSO4·7H2O, Merck) and finally, the concentration of hydrogen peroxide (H2O2) 30% w/w were added to it slowly and at a constant temperature until the hydroxyl radical OH° will be produced. When hydrogen peroxide was added to solution, it was considered as the start time of reaction and at the end of reaction time to neutralize, added to hydroxide sodium sewage compound to get PH=8 (NaOH,merck) and after the addition of 0.5 ml of polyelectrolyte as a coagulant enter into the tank of solid liquid separation. COD measurement, using reactor and close reflex method was performed according to methods described in standard methods.
Results and discussion:
The results showed to remove organic material and achieve maximum COD removal, optimum values of pH, reaction time, reaction temperature, concentration of Fe (II) and H2O2 concentrations, are respectively 3.5, 17.5min, 32.5° C, 6Mm and 27.5Mm. In these condition, if chemical oxygen demand percent of waste wood and water in the following diagram showed that more than 90% of COD will be removed. The results of analysis of variance (ANOVA), to test the utility of model are according to table 1.
Table1: results of Analysis of Variance response surface model in COD removal
p-value F-value Mean square Sum of squares Degrees of freedom source
0.001(significant) 50.46 38.8 3445.3 28 model
- 7.59 53.11 5 residual
0.878(insignificant) 1.4 0.91 23.6 2 Lack -of -fit
- - 6.68 29.51 3 Pure error
- - 46.39 3498.11 33 total
Press=515 R2adj=0.965 R2predic=0.83 R2=0.985

The high amount of R2 and its proximity to number one implies that the COD removal can be
well defined by the model and version of the utility is sufficient. Significantly of model for COD removal is expressed by the amount of F, which is equal to 50.46.also, no significantly of the test, confirming the lack of fitting data with selected model is good fit. This means that there isn't significant portion between lack of fit and amount of pure error. Survey results show that, RSM is an effective way to optimize variables of Fenton process. And there is good agreement, between the predicted values by the model and the results of test. Wastewater of wood and paper, to a large extent is purified by the Fenton reaction. This treatment will depend on many variables .pH should be adjusted to ensure the stability of the catalyst. Iron remains stable in an acid environment and reduction potential of response system will be reduced. In pH lower than 3.5, there is (Fe (II) (H2o)) +2 which is less reactive with hydrogen peroxide. Producing a smaller amount of hydroxyl radicals reduces efficiency of the organic matter decomposition. What is important at very low pH, aggregation effect of hydroxyl radicals is by