Abstract
1. Introduction
Pulp and paper industry is a big consumer of energy and water. Its industrial process produces significant amounts of organic effluent with a high chemical oxygen concentration (COD). According to the raw materials used in the process, there are two types of paper production industry: pure pulp and recycled paper. In this work, we focus on the effluent of the recycled paper industry. A large amount of organic waste produced from the recycled paper industry may be converted into biomethane and converted into renewable energy. Different methods for pulp and paper wastewater treatment, physicochemical treatment methods such as (sedimentation and flotation, coagulation and sedimentation, filtration, reverse osmosis, absorption, wet oxidation, ozonation and other advanced oxidation processes) and biological treatment methods such as Anaerobic digestion, aerobic treatment and fungal treatment). Among these techniques, we can mention anaerobic digestion, which is a sustainable process for wastewater treatment. In fact, it significantly reduces the input mass and COD concentration compared to the activated sludge process. This technique has more advantages than aerobic and physicochemical processes, including simplicity of design and less energy requirement. Note that in the case of aerobic treatment, the thick sludge produced in large quantities by wastewater treatment facilities of paper recycling plants is a serious disposal problem that requires an immediate solution, the degradation process is effective compared to conventional aerobic processes, and only 5 It produces up to 10% sludge. This results in significant savings in costs and measures associated with sludge disposal. In addition, UASB digester can be used in small and large scale. Many different types of anaerobic reactors such as high flow anaerobic filter, high flow coated anaerobic sludge (UASB) + partial recirculation, submerged anaerobic membrane bioreactor (SAnMBR) and modified anaerobic baffled reactor (MABR). The most commonly used for industrial and municipal wastewater treatment is the UASB digester, which is operationally stable and energy efficient. UASBreacto is an encouraging system where it is located at the bottom of granular sludge because it can provide relatively high COD removal efficiency and energy production in the form of methane. Due to the low yield of biogas, anaerobic processing of bio sludge has not yet been carried out in pulp mills.
2- Materials and methods
2-1- Setting up and test method











Figure 1. shows the experimental setup consisting of an inlet storage buffer tank installed before the UASB digester; The role of the buffer tank is to control and adjust the input physicochemical parameters before feeding to the reactor. A peristaltic pump maintains digester feeding at a flow rate of 0.5 to 4.5 liters per hour. The results of the digester design were 70 liters with a height of 1 meter and a diameter of 0.30 meters. The upper part of the UASB digester includes a gas-liquid-solid separator with a height of 0.30 m. This device allows the recovery of biogas. In order to maintain the mesophilic conditions of the process, a spiral exchanger is fixed inside the digester.
Fig. 1. Schematic of a UASB digester, designed to treat wastewater from recycled pulp and paper industry. Fig. 1 Illustrates the designed UASB digester used in this work, influent tank (1), influent feed pump (2), UASB cylinder tank digester (3) with a total volume of 70 L, "solid-liquid-gas" separator (4), including thermostat heating water (5) for mesophilic conditions (37°C) C, effluent tank (6), Gazometer (7), graduated tube (8) for measuring the water volume displaced from the gasometer.
The volume of biogas produced during the process is measured using a 16-liter water tank connected to the reactor (Boyle-Marriott tank). The volume of biogas displaces an equal measurable volume of water from the tank. The volume of biogas is expressed in normal conditions (0 °C and 760 mmHg) taking into account the effect of pressure and temperature on a standardized laboratory scale.
2-3-1-granular sludge bed
The reactor is inoculated with 25 liters of granular sludge. The granular sludge bed used in this experiment is recovered from a full-scale UASB digester. Table 1 shows the characteristics of the granular sludge bed used in this study.

Table 1 Physicochemical characteristics of the granular sludge bed
Average value Unit Parameter
7.4 - PH
5330.5 mg/L COD (chemical oxygen demand)
3250 mgCaCO3/L ALK (alkalinity)
32.99 g/L TS (total solid)
27.28 g/L VS (volatile solid)
5.71 g/L MS (mineral solid)
3.21 % %TS
82.70 % VS (%TS)
96.79 % Moisture (%)
1-1.5 Mm Diameter
Black - Color

The initial granular sludge had respectively 27.28 and 32.99 g/L as VS and TS value, and the COD equals to 5330.5 mg/L. In General, the alkalinity varies from 1000 to 5000 mg/L as CaCO3 in anaerobic.
3- Results and discussion
The purpose of monitoring stability parameters is to control and ensure process performance. The main control parameters are: evolution of pH, alkalinity (Alk), volatile fatty acid (VFA), VFA/Alk ratio and chemical oxygen demand (COD).
3-1- Stability parameters
3-1-1- Evolution of pH
pH is an essential parameter used to control the stability of the anaerobic digestion process. The average value of input and output pH is calculated during the process. The pH fluctuation was in the neutral range, which confirms the correct operation of the process and the stability of the UASB digester. Indeed, several studies reported that most anaerobic bacteria, especially methanogens, increase biogas production in the pH range of 6.5 to 7.5. A previous study on RPMW treatment using a modified anaerobic baffled reactor (MABR), reported that the pH decreased slightly from 7.3 to 6.2 during the process.
3-1-2- Evolution of volatile fatty acids
Volatile fatty acid content is an essential parameter for monitoring the anaerobic digestion process. Accumulation of VFA causes displacement of bicarbonate to dissolved CO2, and CO2 partially passes into the gas phase. Their accumulation in the digester leads to organic overload and acidification of the digester content, which leads to process failure. Several studies show that organic overload in digesters is due to VFA accumulation, and often acetate accumulates first, the total VFA concentration increases slightly or steadily.
4 - Conclusion
It is predicted that UASB digester granules are effective in converting organic matter from wastewater to biogas. The reactor seems to support high organic loads; Also, this technique is an excellent opportunity to exploit the economic benefits of the energy produced from this process, which can contribute to the development of more efficient and sustainable systems.