Production of Biogas and Biofertilizer by the Co-digestion of Effluent From Cellulose Industry With Addition of Glycerin

The present study aimed to assess the anaerobic digestion of sewage from cellulose industry mixed with doses of glycerin, in order to produce biofertilizer and biogas, for agriculture and energy purposes. Five anaerobic reactors were developed, which were fed with effluent and doses of glycerin (0, 5, 10, 15 and 20% v v) and evaluated during the hydraulic retention time of 42 days. The physical and chemical parameters were evaluated every seven days: temperature; pH; turbidity; series of solids; total phosphorus (Ptotal); Kjeldahl nitrogen (Ntotal) and COD; as well as the levels of K, Ca, Mg, Cu, Zn, Fe, Mn and Cd, Pb and Cr; biogas production was evaluated daily. The anaerobic treatment produced biofertilizer with excellent nutritional conditions, with adequate levels of P, K, Ca, Mg, Mn, Fe and Zn. The addition of glycerin increased biogas production, with 15% and 5% of glycerin, respectively producing 0.54 and 0.40 mL L d. The treatments of 15% and 5% of v v glycerin also exhibited greater removal of COD. However, the low levels of N have caused problems in the anaerobic process. Anaerobic digestion of the cellulose industry effluent with 5 and 15% of glycerin (v v) provided increase in biogas production of 0.54 and 0.40 mL L d, respectively. K, Ca, Mg, Zn, Fe and Mn at the final biodigestion remained with excellent levels for use in crops as biofertilizer.


Introduction
Brazil is a major world producer of paper, besides supplying the domestic market, it exports products mainly to countries of Latin America, the European Union and North America (Bracelpa, 2013).
This industrial segment is one of the most competitive in the country, the quality standard is one of the best in world, and the market is globalized and strongly active. The majority of paper production comes from the pulp of short eucalyptus fibers, which the country is the largest producer in the world. These fibers are mainly used in the manufacture of papers with high benefit (Montebello & Bacha, 2013).
In contrast to the considerable economic benefits, the pulp and paper industry is responsible for the emission of pollutants in the solid, liquid and gaseous states (CETESB, 2008). The sector generates a large amount of effluent, reaching a value of 1 m 3 per ton of produced paper (Priadi et al., 2014), and therefore has been seeking development in environmental issues, especially liquid effluents generated in production processes, which, due to highly polluting characteristics and high volume, when released without adequate treatment, aggressively impair the quality of the receiving water body (CETESB, 2008).
Among the new technologies that have emerged in recent years, which can be applied in the treatment of effluents from the pulp and paper industries, effluent co-digestion has gained prominence because, besides biostabilizing the effluent, it enables the production of biogas during the process. The addition of glycerin to the effluent may potentiate the biogas production, making it an activity of greater attractiveness and profitability (Priadi et al., 2014).
Some studies have demonstrated that anaerobic digestion of crude glycerin, mixed with effluents from different jas.ccsenet.

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Gasometer
Five gasometers were made in plastic bottles, which were filled with water and sealed with a lid to which two hoses were connected, one for biogas inlet and another for the water outlet. As the gas was produced, it was directed through the hoses for the bottles, the volume of biogas produced ended up generating pressure and displacing the water to a second bottle, thus providing quantification of the volume of the produced gas ( Figure  1B).
For the construction of the gasometers, 10 plastic bottles of 2 liters were used; 4.5 meters of hose Rodoar soft blue PU 8 mm; 5 metal fittings; 5 plastic valves and silicone for sealing.

Sampling of Waste and Evaluated Parameters
The cellulose effluent sampling was carried out in a pulp paper production unit located in the state of Paraná, southern Brazil.
In addition, the volume of the biogas produced was determined daily, by the gasometers linked to the biodigesters, the measurement was carried out with graduated cylinder.

Results
The reactor containing 0% of glycerin exhibited low variation of pH, with maximum and minimum values of 6.16 to 6.81. The presence of glycerin acidified the medium, reaching the lowest pH (4.77) in the dose of 15%.
The higher the dose of glycerin added to the system, the higher is the concentration of TS, because with such increase, the concentration of VS is increased, since the glycerin [C 3 H 5 (OH) 3 ] is basically composed of carbon, hydrogen and oxygen. In addition to increasing TS levels, the addition of glycerin favors the production of biogas components, mainly CH 4 , CO 2 , NH 3 and H 2 S.
The turbidity is higher in the glycerin treatments than in the 0% dose treatment, due to the multiplication of microorganisms.
The treatments with glycerin addition exhibited higher values for COD, this is an expected fact, since a liquid with higher turbidity and total and volatile solids tends to have higher COD values.
In this research, the treatments with the highest COD removal at the end of HRT are the treatments with the highest biogas production (Dose 0%, 5% and 15%).
Both, the industrial effluent and the glycerin, did not have N contents in their initial composition, a condition that was maintained for almost all treatments until the end of the process.
It was observed a reduction of Ptotal for the treatments with glycerin addition. The stability of P becomes attractive for biofertilization purposes, since the final effluent contains considerable P contents, being this one of the most important macronutrients for the development of plants. jas.ccsenet.

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In the trea observed a occurrence The presen methanoge required a trends that and remna org Figure 5. B ect is discussed he co-digestion ds the result of for these auth with 0%.     Note. Total contents of K, Ca, Mg, Cu, Zn, Fe and Mn of cellulose industry effluent and glycerin, and comparison with other organic wastes.
In addition to being used in electron transport, Fe is a constituent of the metalloenzymes involved in methanogenesis, in the form of Fe 2 S 2 , Fe 3 S 2 or Fe 4 S 2 (Rao et al., 2011). Another important element is Ca, this macronutrient when in the substrate contributes to pH equilibrium by neutralizing H + , thus avoiding intensive acidification of the medium (Matheri et al., 2016). On the other hand, Suárez et al. (2014) states that excess of Ca can inhibit biogas production by favoring the precipitation of carbonate and calcium phosphate, which can result, for example, in the loss of buffer capacity and essential nutrients for anaerobic digestion.
It is also necessary to be cautious about Zn, because in high concentrations it can cause toxicity to the microorganisms, inhibiting important biochemical reactions. However, adequate levels of Zn have been shown to potentiate the formation of CH 4 (Matheri et al., 2016).
The Mn is another element that can positively interfere in the production of biogas, since besides accelerating the methanogenesis, can increase the final produced volume. In addition, the element can also serve as an oxidizing agent to donate electrons to form methane from carbon dioxide (Qiao et al., 2015). Similarly, K + ions also are not directly involved in methanogenesis, however, a large part of the methanogenic enzymes exert their function effectively when K + is present in the medium at high concentrations (Glasemacher et al., 1996).
Since several soils have micronutrient deficiency (Silva et al., 2014), the final concentrations of these in the biofertilizer, although not expressive, are a good alternative for supplying Zn, Fe, Mn, K, Ca and Mg to the soil, without causing toxicity to the plants.
Anaerobic digestion of the cellulose industry effluent with 5 and 15% of glycerin (v v -1 ) provided a satisfactory increase in biogas production (0.54 and 0.40 mL L -1 d -1 , respectively).
Anaerobic digestion provided considerable reductions for TS and VS, confirming the feasibility of using biodigesters as treatment of these residues.
The concentrations of P were reduced during HRT, and the nutrients (K, Ca, Mg, Zn, Fe and Mn), although with high levels during the process (which is considered normal by several authors) are in adequate concentrations for use in crops as biofertilizer.