The aim of this paper is to prove the applicability of Auxipó as an auxiliary agent of coagulation for pulp and paper industry wastewater. Thus, it is possible to clarify the effluent more efficiently before launching it in waterbodies. To test the applicability of the auxiliary agent of coagulation, two coagulation and flocculation assay diagrams were developed: the first diagram only shows coagulant dosages, and the second diagram represents the coagulant and Auxipó dosage. Both tests demonstrated efficiency in turbidity removal; however, the results showed higher removal efficiency after the dosage of Auxipó.
Staying competitive and with high environmental efficiency is one of the objectives in the technological advance of the paper and pulp industries (Figueiredo 2014). The final destination of wastewater in sustainable and economically viable means is one of the challenges of the paper industries (Maeda 2013).
Brazil is an international reference in the pulp and paper sector, as well as one of the world's leading producers; have great sustainable practices (BRACELPA 2009).
Global consumers in the industry have already shown that the environmental factor directly influences their purchasing decisions. Besides, a survey conducted by Tetra Pak with 6044 consumers from 12 different countries resulted in a 78% assertion that the environmental performance of the packaging interferes in the decision to choose the brand (Tetra Pak 2015).
Large amounts of water are incorporated into the production process of the pulp and paper industries. The average consumption is between 23 and 49 m3/adt (dry air ton) (Meissner & Lima 2015); however, in the past years these amounts have been even more volumous, with a consumption of 91.3 m3 per ton in 2009 (Bachmann 2009). In 1972, water consumption reached 112 m3/odt (absolute dry ton) and in 1946 the consumption values registered 417.3 m3/odt (Foelkel 2011).
The applicability of water in the industry occurs in the most diverse forms, such as washing of machinery, incorporation to the product, production of steam, cooling systems, water applied in the process of product confection, among others. The use of water in the industrial processes and routines add contaminants to the liquid portion, thus increasing the quantity of wastewater generated (Giordano 2004).
According to Metcalf & Eddy (2013), before implementing a wastewater treatment plant it is necessary to characterize and assess the effluents generated in the operation unit of the process, as well as its variation in time.
The characteristics of a wastewater vary according to the raw material, process technology, production process, amount of water used in the process and reuse of wastewater (Pokhrel & Viraraghavan 2004).
The chemical-physical processes are widely used in wastewater treatment plants. One of the technologies applied is coagulation followed by flocculation, two stages used worldwide.
For flocculation to be effective, it is necessary that the colloids present in the wastewater are arranged to agglutinate therefore forming flocs (in the later stage – flocculation). Coagulation consists of the destabilization process of impurities present in the wastewater (Kim et al. 2001), so colloids can be aggregated forming the flakes and subsequently be removed in the sedimentation step.
According to Eckenfelder (1989), the colloids present negative surface electrical charges, which creates repelling and prevent agglomeration. The addition of the coagulant destabilize these negative charges, making the formation of the flakes possible. Often, the use of some flocculation aid is advantageous for it accelerates and potentiate the floc formation process.
There are natural and synthetic flocculation auxiliaries, each with its potential applicability. Auxipó (technology developed by SENAI) is a coagulation auxiliary from mining waste, which is considered an environmental liability. Its applicability as a flocculation aid is feasible, since its cost is negligible (Zawadzki et al. 2013).
The aim of this research is to apply Auxipó as an agent of coagulation in the wastewater of a multinational pulp and paper industry, in order to demonstrate its viability.
To perform the characterization of the wastewater, samples were collected shortly after the Parshall flume.
To evaluate the removal efficiency in the wastewater treatment plant, collections were carried out after the Parshall flume and after the Clarifier.
To carry out the stage of sampling and evaluation of the viability of the Auxipó, the collection was conducted after the Clarifier.
In the first step, only the characterization of the wastewater was performed. Along with its characterization, a monitoring of 7 months was done and daily samplings for different parameters were executed.
In the second step the same procedure was approached. After getting the monitoring data, it was possible to perform the removal analysis and evaluate the efficiency of the wastewater treatment plant for a period of 7 months, when several variations occurred in the process, culminating in different characteristics for the effluent.
The third step consists of a single wastewater sample. Since after the characterization, the monitored parameters presented a non-varying mean during the period of analysis, so this sample was validated to begin the test procedure in jar test, where it was possible to evaluate the coagulation/flocculation efficiency of the sample with coagulant and coagulant associated Auxipó.
In Figure 3 it is possible to visualize the two sampling lines that start in the third stage. It is important to mention that only the coagulant assay was performed in the first line; consequently, the second line analysis counts the addition of Auxipó.
A variation in the dosages of the coagulant and Auxipó (along with the variation of the pH) was made in order to find the best point of coagulation/flocculation.
Before submitting the samples to the coagulation/flocculation tests, the characterization of this wastewater was performed so that the isolated efficiency of this removal step can be found.
The jar test used was Milan JT102, the turbidimeter was Hach 2100Q and pH meter was Qualxtron tx 1500 plus.
Description of sampling
Samples after Clarifier were conditioned in 60-liter flasks, requiring 5 vials for the total analyses. For the cooling, crushed ice was used on the flasks. A 1-liter sample of raw wastewater was also sent. For step 3, samples were received in two batches: Lot 1 on November 20, 2015 and Lot 2 on December 4, 2015. After receiving the effluent sample in the laboratory, first, the characterization analyses of the raw wastewater and Clarifier effluent were conducted. The obtained data was crossed with a historical series to verify if the values were consistent with the usual characteristics of the wastewater in the season.
After the validation, the samples from Lot 1 were submitted to the tests without using Auxipó (only Al2(SO4)3) and in the trials of Lot 2 dosages of Auxipó (Al2(SO4)3 + Auxipó) were applied.
The same gradient velocities and time routines were used for both samplings in the Jar test. The procedure applied to the Jar test corresponds to the dosage of the alkalizing or acidifying agent at the beginning of the test (0 seconds) followed by a 30 s dosage of the coagulant (aluminum sulfate) or coagulant and flocculation aid, allowing 10 s of stirring at 420 rpm (1,000 s−1). After this period the test enters the flocculation phase, maintaining a rotation of 55 rpm (45 s−1) for 20 minutes. Posteriorly, the jar test is switched off and the decantation process begins when the sample is maintained at rest (for 10 minutes) for flocks to deposit.
After the settling time has elapsed, the samples are collected and tested, thus obtaining pH and turbidity values.
For acidity and alkalinity correction, sulfuric acid (0.1 mol/L) and sodium hydroxide (0.1 mol/L) were used, respectively.
The coagulant used was aluminum sulfate with an Al2O3 content of 8.3%. To prepare the solution, 4 g of the aluminum sulfate was added to 1 L of ultrapure water type 1.
To prepare the coagulation aid solution, 30 mg of Auxipó was added to 1 L of ultrapure water type 1.
The wastewater treatment plant studied in this work shows a proven efficiency, since it has been operating for several years, discharging effluent into waterbodies in accordance with the current legislation. However, in order to corroborate this work, a monitoring of 7 months in the wastewater treatment plant in question was developed. With the mean of this period, it is possible to guide whether a single sampling is outside the usual characteristics. The mean results of the data collected are shown in Table 1.
|Characterization – Parshall gutter|
|Average||Sample 1||Sample 2|
|Characterization – Parshall gutter|
|Average||Sample 1||Sample 2|
In Table 1 it is also possible to visualize the data of Samples 1 and 2. Analyzing the data of Samples 1 and 2 it is possible to conclude that the values are close to the mean, that is, the values are acceptable as representative of the characteristics commonly observed in the wastewater treatment plant.
Observing the two overlapping curves in Figure 5, the lowest remaining turbidity values are visible when the coagulation aid is used, as expected. In some points, it is possible to visualize a worsening in the remaining turbidity with the use of the coagulation aid but in a smaller amount.
In order to choose the best coagulant and coagulant dosage option, it is not enough to analyze only the dosed amount of chemicals, but also the pH at which the sample obtained the best response. In other words, in order to choose the best chemical dosage option for turbidity removal, it is necessary to analyze the pairs of pH values and coagulant (and coagulation aid, if any) pairs simultaneously.
In Figure 6, some curves for the formation of values islands were created in order to visualize where the pairs of pH values and dosage had better responded to coagulation and flocculation assays.
Also in Figure 6, it is possible to visualize an island of values lower than 30 NTU, where the sample responded better to the pairs of dosage values and pH. It is perceptible that under more alkaline pH values the clarification response is not very efficient, which is also perceived at dosage values below 25 mg/L of coagulant.
The best point observed in Figure 6 is for the coagulant dosage of 60 mg/L and 10 ml of NaOH, since it should be taken into consideration that the higher the dosage of chemical required, the greater the chemical expenditure by the wastewater treatment plant.
The proximity of the 30 NTU and 130 NTU areas shows the potential risk if the dosing is applied incorrectly, whereas a lightly more basic pH (7.5) already presents poor floc formation.
For each point shown in the scatter plots, a dosage of coagulant (and Auxipó, when required) and a pH, alkalinizing or acid dosage, was given, thus resulting in values for a wide final pH range. Hence, it is possible to observe that for the same dosage of coagulant up to six values of remaining turbidity were obtained, at different pH values.
All data was tabulated, so after the final preparation of the diagram, it was possible to know the dosage for each point.
The best point observed in Figure 7 was with the pairs of coagulant 35 mg/L and 10 ml of NaOH. This result was possible due to the dosage of 10 ml of Auxipó.
In general, the aluminum sulfate coagulant acidifies the sample, so the higher the dosage, the lower the resulting pH, requiring higher dosages of the base and lower dosages of the acid. Therefore, as the dosages of higher aluminum sulfate are increased, the sodium hydroxide dosage should be used to balance the pH.
It is noteworthy the improvement in the data of Figure 7, leading to a lower dosage of chemicals, corroborating a lower expenditure in wastewater treatment plant.
With the present work, it was possible to achieve the following conclusions:
✓ The data collection on the wastewater treatment plant demonstrated its regularity in wastewater characteristics regarding Temperature, BOD, COD and pH. The characteristics of TSS and Color showed small variations.
✓ Continuous monitoring of the analyzed parameters was observed;
✓ The removal efficiency for the parameters in the analyzed period was:
- Temperature: 33%;
- TSS: 84%;
- BOD5: 92%;
- COD: 78%;
- Color: 46%.
✓ The removal efficiency showed good response to the proposed treatment; however, the removal efficiency of Turbidity could not be analyzed due to lack of samples in the analyzed period. The analyses performed with the two characterization samples indicate a removal efficiency of approximately 60%.
✓ With the addition of the coagulation and flocculation step, the turbidity removal efficiency can reach 95%, since values of remaining turbidity only up to 17 NTU were observed.
✓ With the flocculation diagrams, it was possible to corroborate its effectiveness in the removal of Turbidity;
✓ The flocculation diagrams show a better efficiency when coagulant aid was applied with the coagulant. It is worth mentioning that with the combined use of coagulation aid and coagulant there is a reduction in expenses.
In order to obtain a better precision of the design parameters for the addition of a coagulation and flocculation unit in the wastewater treatment plant, a research with variation of agitation coefficient and time of fast mixing and slow mixing is suggested.
The author is grateful to the company that gave the samples, to the Center for Paper and Cellulose Technology (SENAI) and to UTFPR.