1 Introduction

Biomass is the most promising among the renewable energy sources. Biomass refers to different forms of organic matter including crop residues, product-based agro-industrial, urban and municipal waste, animal dung [1]. A detailed study of world energy demands shows that firewood, charcoal, and crop residues make up more than 75% of the total energy consumed. It means the loss of the forest cover at a faster rate than attempts to raise trees. All of this study implies the loss of the forest cover at a faster rate than attempts to raise trees. Not only the cost involved, but also the very unpredictable weather patterns largely attributed to global warming, electrical power etc. [2, 3]. Anaerobic digestion was theoretically seen as a desirable tool for stabilizing waste before landfills as a pretreatment to minimize substantial contamination in the atmosphere [4, 5]. And to power a generator that generates electricity, it can be used to fuel internal combustion engines [6]. Biogas typically contains around 55–65% of methane, 30–35% of carbon dioxide and hydrogen, nitrogen and other impurities [7]. The first step includes a group of anaerobic bacteria that produces organic acids as a by-product of the initial organic degradation, known as the acid formers. The second step includes a group of bacteria that breaks down the organic acids and produces methane as a by-product of organic acid degradation [8]. Anaerobic digestion was theoretically seen as a desirable tool for stabilizing waste before landfills as a pre-treatment to minimize significant environmental pollution [9, 10]. Anaerobic digestion technology, as described above, is ideal for use in the production of biogas. Biogas generated by anaerobic digestion can be used to power internal combustion engines to operate an electric generator [6]. The requirement to collect landfill gas has the effect of reducing the cost of landfill gas conversion to electricity [11]. The processing of biogas by anaerobic digestion (AD) is an environmentally friendly method that utilizes the growing quantities of organic waste generated worldwide [12]. With a strong agro-sector in these countries, reducing fertilizer emissions and generating renewable energy are equally strong driving forces that promote the growth of biogas in other countries, such as Portugal, Greece, Ireland, and many of the modern East [13]. The rate of biogas production is up to organic content of waste as well as masses of parameters that defines the conditions of decomposition (such as age and constitution of waste, temperature, humidity, pH-varies with depth of filling, population of microbes, quality and quantity of alimentary substances) [14,15,16]. Biogas is combusted as it is in the generator unit and produces electricity [15, 17]. The biomass is converted into biogas (methane, carbon dioxide and traces of other contaminant gases) after the anaerobic digestion process is completed. Like liquid digester (a fertilizer rich in nutrients) [18].

2 Research methodology

This report practically collects various samples of waste (cow dung, rice waste, papers waste, tissue waste, vegetable waste) and measuring the pH value. Load calculation has been done for the IUBAT University to measure the total amount of loads utilized in this organization (Fig. 1).

Fig. 1
figure 1

Flow chart of methodology

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3 Waste condition in Bangladesh

This paper focuses on the status of Bangladesh’s solid waste generation system, waste management system and waste management problems. Every day, the country generates around 8000 tons of solid waste from the six major cities (Dhaka, Chittagong, Khulna, Rajshahi, Barisal and Sylhet), of which Dhaka alone contributes about 70%. Efforts are underway to develop the waste collection, storage, recycling, incineration and land-filling system [19] (Fig. 2).

Fig. 2
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Waste condition in Bangladesh

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3.1 Biogas generation

Figure 3 shows the basic operation of biogas generation process. An inlet is used for feeding the waste for digestion process and an out is used for taking out the digested waste after a fixed time interval. The produced gas is bypasses through a block where moisture is absorbed which may increase the potentiality of the biogas. After that this gas can be processed for power generation.

Fig. 3
figure 3

Proposed block diagram of biogas generation

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4 pH Measurement and calculation

Low pH prevents growth and gas production of methaneorganic bacteria and is often the result of overloading. Efficient digestion takes place at a pH similar to equilibrium, between 6.0 and 8.0. A slightly alkaline state is an indication of not being too drastic in pH fluctuations. Upon dilution or by adding acid, low pH can be relieved [18]. A low pH in the digester decreases the activity of especially bacteria that are involved in the digestion process [16] (Fig. 4).

Fig. 4
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pH measurement experiment

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Practical experiment has been done for the measurement of pH value of cowdung, vegetable waste, tissue paper, News papers and rice waste for one week.

Figure 5 shows the measurement of pH value of rice waste. In that case, the pH value gives lower value. That means in rice waste acidity is high.

Fig. 5
figure 5

Experiment set up for rice waste

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The Table 1 is about the rice waste experimental and cumulative pH value with respect to retention time. This table is showing the pH value of rice waste for a week. If the amount of waste is increased the electricity of production also will be increased.

Table 1 pH measurement for rice waste
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Figure 6 shows the measurement of pH value of tissue waste. In tissue waste firstly pH value has around the 7. After some days its pH value decreased. After 7 or 10 days it reached to around pH value 6.

Fig. 6
figure 6

Experiment set up for Tissue waste

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Table 2 is about the rice waste experimental and cumulative pH value with respect to time. This table expressed the result of tissue paper waste with measured the pH value.

Table 2 pH measurement of tissue waste
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Figure 7 shows the measurement of pH value of paper waste. In paper waste firstly pH value has around the 7. After some days its pH value decreased. After 7 or 10 days it has reached to around pH value 5.

Fig. 7
figure 7

Experiment set up for paper waste

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The Table 3 is about the paper waste. The table is showing the cumulative pH value. In paper waste pH is not varied more. If the pH value is more (within 5 to 9), that means its batter for production of biogas.

Table 3 PH measurement for paper waste
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Figure 8 shows taking the pH value of cow dung slurry. This slurry is made by the combination of water and cow dung. Its slurry ratio is 1:1. It is mixed until all the liquids are mixed in a bottle. In that case, the pH value decreased. That means in cow dung acidity is high. After a certain period it will be constant or increase the pH value (Fig. 9).

Fig. 8
figure 8

Experiment set up for cow dung

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Fig. 9
figure 9

Experiment set up for vegetable waste

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Table 4 is showing the cumulative pH value of cow dung. In cow dung, the pH value is found between 5.55 and 5.99. That means in cow dung acidity is increased. After a certain period it will be constant or increase the pH value.

Table 4 pH measurement of cow dung
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Table 5 is showing the cumulative pH value of vegetable waste. In vegetable, we find the pH value is around 5.24 to 6.01. That means in vegetable waste acidity is increased. After a certain period it will be constant or increase the pH value.

Table 5 PH measurement for Kitchen waste
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Table 6 is showing the load calculation for a day. Regarding this table daily total energy needs 6913.548 kW h for IUBAT. In university 988 ceiling fan rated 75 W each, 546 wall fan rated 50 W each, 2616 lights rated as 35 W each, 704 computers rated 65 W each, 68 projectors rated as 150 W each, 54 AC rated as 1988 Watt each. IUBAT total energy demand is 6,913,548 W h.

Table 6 Data table of IUBAT loads
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Cow dung calculation:

Daily time duration t = 9 h

Total Power p = 768,172 W

Total Energy = p*t = 768172*9 = 6913.548 kW h

1 kW h = 3.6*106 j = 6913.54*3.6*106 j

So daily energy consumption = 2.488*1010 j per day

Using cow dung production of biogas per kg of fresh dung = 0.09 m3

Calorific Value of the biogas produce = 20 \( \frac{\text{Mj}}{{{\text{m}}^{3} }} \) = 20*106 \( \frac{\text{j}}{{{\text{m}}^{3} }} \)

Total amount of biogas required = \( \frac{{2.488*10^{10} }}{{20*10^{6} }} \) = 1244 m3

Total amount of cow dung required = \( \frac{{1244{\text{m}}^{3} }}{{0.09\frac{{{\text{m}}^{3} }}{\text{kg}}}} = 13822.22\,{\text{kg}} \)

Slurry

Cow dung slurry ratio = 1:1

Mass of slurry per day = (mass of cow dung + mass of water) = 13,822.22 kg +13,822.22 kg = 27,644.44 kg/day

Density of the slurry, D = 1000 \( \frac{\text{kg}}{{{\text{m}}^{3} }} \)

Volume of the slurry v = \( \frac{m}{D} \) = \( \frac{{27644.44\,{\text{kg}}}}{{1090\frac{\text{kg}}{{{\text{m}}^{3} }}}} \) = 25.36 m3/day

The retention time 30 days digester Volume = Volume of the slurry *Retention time = (25.36 m3/day)*30 day = 760.8 m3

Actual volume should be 10% more to probity, some space for proper digestion.

Actual digestion value = 760.8*10% + 760.8 = 836.8 m3

Vegetable wastecalculation

Vegetable Waste Products:

Daily time duration t = 9 h

Total Power P = 768,172 W

Total Energy = p*t = 768172*9 = 6913.54 kW h

1 kW h = 3.6*106 j = 6913.54*3.6*106 j

So daily energy consumption = 2.488*1010 j per day

Using Vegetable waste production of biogas per day kg = 0.05 m3

Calorific Value of the biogas produce = 20 \( \frac{\text{Mj}}{{{\text{m}}^{3} }} \) = 20*106 \( \frac{\text{j}}{{{\text{m}}^{3} }} \)

Total amount of biogas required = \( \frac{{2.488*10^{10} }}{{20*10^{6} }} \) = 1244 m3

Total amount of vegetable waste bio gas required = \( \frac{{1033\,{\text{m}}^{3} }}{{0.05\,\frac{{{\text{m}}^{3} }}{\text{kg}}}} \) = 24,880 kg/per day

Slurry

Vegetable waste slurry ratio = 1:1

Mass of slurry per day = (mass of vegetable wastes + mass of water) = 24,880 + 24880 = 49,760 kg

Density of the slurry D = 1000 \( \frac{\text{kg}}{{{\text{m}}^{3} }} \)

Volume of the slurry v = \( \frac{m}{D} \) = \( \frac{{49760\,{\text{kg}}}}{{1090\frac{\text{kg}}{{{\text{m}}^{3} }}}} \) = 35.65 m3

The retention time 30 days digester Volume = Volume of the slurry*Retention time = 35.65*30 = 1069.5 m3

Actual volume should be 10% more to probity, some space for proper digestion (Table 7).

Table 7 Digester volume table
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Actual digestion value = 1137*10% + 1137 = 1176.45 m3

5 Conclusion

pH is a very important parameter in the anaerobic digestion process. It has been observed that the variation of pH value is directly related with the anaerobic digestion process. The experiment has been done for one week. pH measurement has been done for cow dung, kitchen waste, rice waste, tissue waste and paper waste. For Kitchen waste pH is maximum 5 which is less than cow dung. The pH value is highest for cow dung than the value of different samples like vegetable waste, rice waste, tissue waste, paper waste. From the load calculation, it has been perceived that how much biogas is needed to produce the electricity for IUBAT. Daily total energy consumption is 6913.54 kW h of IUBAT.