EXTENDED ABSTRACT

Introduction

Sewage sludge (SS), as observed, is a by-product of wastewater treatment process. Its treatment and management incur about 50% of the total operating costs in wastewater treatment plants. In terms of generation capacity, approximately 10 million tons of sewage sludge are produced in Europe, 8million in the United States, and 4 million tons in China annually. The generated sewage sludge poses a threat to the environment and humans alike due to the presence of heavy metal pollutants, a high portion of organic, and toxin contents. Despite it being laden with pollutants, the sustainable usage of this growing quantity of produced sewage sludge can make it a valuable resource. Upon processing, sewage sludge can serve as a feedstock or substrate for energy generation.

Materials and methods
In this study, instead of a full case-specific design study, an initial potential calculation was performed. Equation No (1) was used to calculate the amount of biogas produced from industrial sewage sludge.
V_CH4=(0.4)[(S_0-S)(Q)/(1000gr/kg)-1.42 P_x] (1)
The value of P_x is obtained from equation (2):
Px=(YQ(S_0-S)(1000gr/kg))/(1+((K_d )*SRT)) ‌ (2)
In relation (2), (Y=0.08) and (0.02Vbiogas=VCH_4/0.65 ‌ (3)
Internal combustion engines, fuel cells, micro gas turbines are the main devices for biogas electrical conversion. The internal combustion engine (gas engine) is selected as the conversion device for evaluation.

Results and discussion
Having the value of Q, the total volatile solid production per day can be calculated.
Px=(YQ(S_0-S)(1000gr/kg))/(1+((K_d )*SRT))=(0.08×2400(1/666-0/055))/(1+(0/03×13))=223.3
Having all the required values, the volume of methane produced is calculated from the following equation.
V_CH4=(0/4)[(S_0-S)(Q)/(1000gr/kg)-1.42P_x ]=(0/4)[(1/666-0/055)×2400-1.42×223.3]= 1229.474m^3/day
The average thermal energy of biogas is 5.96 kilowatts per cubic meter. Therefore, the amount of biogas energy produced per day will be equal to 11273 kilowatts. According to the value of each cubic meter of natural gas based on the tariff of 1401 in the amount of 1350 Rials, the value of biogas in Rials per year is calculated as follows:
biogass_(economic value=) 1891.498×1350×365=932035639/5 Rial/year
The emission factor of greenhouse gases per cubic meter of biogas is equivalent to 0.005 kg of carbon dioxide and the amount of biogas produced is 1891.498 cubic meters per day. Based on the greenhouse gas emission factor for biogas and the amount of daily biogas production, the amount of carbon dioxide produced by burning it is equal to 9.457 kg/day. Each liter of diesel weighs 0.85 kg and releases 2.6 kg of carbon dioxide. In the cold seasons of the year, due to the pressure drop of domestic gas, the consumption of industrial gas will be limited and they will be forced to use fuel oil, and Jovein sugar factory is no exception. As mentioned, 2.6 kg of carbon dioxide will be released for each liter of diesel fuel. So, for 1 cubic meter which is equivalent to 1000 liters; we will have:

The amount of carbon dioxide emission per cubic meter of fuel oil= 1000 liter × 2.6 = 2600
The reduction of carbon dioxide emissions

Therefore, 2600 kg of carbon dioxide is released into the atmosphere for every cubic meter of fuel oil. As mentioned earlier, the amount of nitrogen dioxide emission per cubic meter of biogas is 0.005, and if biogas is used instead of fuel oil, we will see a significant decrease in the emission of carbon dioxide into the atmosphere. In 2015, Iglinski et al calculated the potential of biogas production from different substrates as follows: 82 million cubic meters from municipal waste, 20 million cubic meters from sewage sludge, 1603 million cubic meters from animal waste, 551 million cubic meters from corn and 254 million cubic meters from grass (Igliński et al., 2015). In 2013, Maghanaki et al estimated that the usual amount of biogas production from agricultural waste, sewage and industrial and urban waste, and taking into account the appropriate safety factor, biogas in Iran will produce about 16146.35 million cubic meters, which is approximately 323 petajoules of energy. Maghanaki et al., 2013). In 2013, Lohani investigated the potential of biogas production in Nepal and found that the potential of biogas production from urban sewage is about 22 million cubic meters per year, equivalent to 14.5 MWh, at the same time, the potential for reducing carbon dioxide emissions is about 94,500 tons per year (Lohani., 2013).


Conclusion
The main goal of this research is to evaluate the feasibility of producing biogas from raw sewage and reducing greenhouse emissions (case study: sugar factory). According to the findings, the following can be concluded:
Considering that in the cold seasons of the year due to the increase in domestic gas consumption, most industries are facing a shortage of gas, biogas can be introduced as a suitable alternative.
Due to the country's high potential in biogas production, if all refineries in the country use biogas technology, the added value of the energy sector will increase significantly due to the production of biogas and electricity.
3- Due to the significant amount of biogas energy produced, in the hot seasons of the year due to the increase in per capita electricity consumption in the country and the severe lack of electricity, the produced electric energy can compensate a part of the unit's electricity shortage in the hot seasons.
It is suggested that other researchers investigate the potential of biogas and energy produced from dry household waste in future studies.