Transformation of Organic Matter from Waste

 Waste Management Technology Vol II - Transformation of Organic Matter from Waste

Most of the content of municipal waste in the form of vegetable waste, fruit waste, meat fish, and leaves can be processed and utilized in various ways, such as through the process of degradation by microorganisms (composting and biogasification), as a source of food for living creatures whose products can be utilized (vermi compost from worms, fly larvae), or into livestock feed directly (see table).

                 Table 1: Products from processing waste organic matter

Waste component	Composting (Compost)	Biogasification (Methane)	Worm feed (Vermi Compost)	Black soldier fly (larva)
Rice and the like	V	V		V
Vegetables	V	V	V	V
Fruits	V	V	V	V
Fish and meat		V	V	V
Garden leaves	V

Biologically, organic matter will undergo decomposition by microorganisms in nature, which are generally bacteria, actinomycetes, fungi, and so on. In life, these microorganisms need food and nutrients for their cellular metabolism. The two main sources of substrate for cell wall formation are organic carbon and carbon dioxide (CO2). Most of the carbon content will be used by microorganisms as a source of energy, which will be burned in the process of respiration into CO2. The rest, together with nitrogen, will be used by microorganisms for cell synthesis in cell protoplasm, especially in cell wall formation.

In Indonesia, waste has a high content of organic matter and will naturally undergo biological decomposition, both under aerobic and anaerobic conditions. Based on the composition of waste in Indonesia, which is mostly food waste, especially kitchen waste, this type of waste will rot quickly or be decomposed easily by microorganisms that are abundant in nature. With relatively high humidity and air temperature conditions, the speed at which microorganisms destroy biological waste will be faster. Composting and biogasification are one of the processing techniques for biodegradable solid waste.

1. Aerobic process

Composting is the aerobic decomposition of solid organic matter, such as waste. residue is produced from the process as compost. Sometimes the residue resulting from anaerobic processes (biogasification) is also called anaerobic compost. Composting classifications can be grouped on the basis of:

• Oxygen availability: when the process uses oxygen (air), it is known as aerobic composting, and it is known as anaerobic composting when the process does not require oxygen. But the most commonly recognised composting process is aerobic.
• Temperature conditions: when it takes place at normal temperatures, known as mesophilic conditions, and known as thermophilic when it takes place above 40°C.The
• Technologies used are traditional composting (natural), for example, by windrow, and accelerated composting (high rate), which aims to speed up the process with engineering that will optimise the work of microorganisms, such as pH regulation, air supply, humidity, temperature, mixing, and so on.

In general, the aerobic transformation of organic matter can be explained as follows (Tchobanoglous et al., 1993):

• Input: organic matter + O₂+ nutrients
• Output: microorganism cells + organic matter + H₂0 +CO₂+ NH₃+ SO₄ = +∆E.

If the organic matter is C_aH_bO_c,N_d, and the bacterial biomass cells are ignored, and the reaction is not complete, leaving the organic matter not yet degraded C_wH_x O_y N_z, (as ½ mature compost) then the reaction that occurs is (Tchobanoglous et al.,1993):

C_aH_bO_cN_d+ 0,5 (ny + 2s + r -c) 0₂→nC_wH_xO_yN₂ + sC0₂+ rH₂0 + (d-nx) NH₃+∆E.(1) 

With: r = 0,5 * [(b - nx)- 3(d - nz)]

           s = a - nw 

NH₃ dioksidasi lagi dan akan menjadi NO₃

NH₃+ 20₂ →H₂0 + HN0₃

• C_aH_bO_cN_d: early organic matter.
• C_wH_xO_yN_z,: the final organic matter, in the form of compost, is free of pathogenic bacteria due to exothermal processes, is odourless, and relatively stable.
• ∆E is heat energy (exotherm), the magnitude of which varies depending on the initial organic matter being composted and the organic matter of the resulting product. If glucose, ∆E = 484 to 674 kcal/gram molecule.

If the reaction is complete (as in a good incinerator), meaning that all organic matter is demineralized (no compostable material remains), the reaction will be:

2. Anaerobic process

The aerobic process is more widely applied because it does not cause odor, has a faster process time, and is at a high temperature that can kill pathogenic bacteria and worm eggs, so the compost produced is more hygienic. Anaerobic processes usually occur in naturally occurring slurry fields or in anaerobic reactors, where odor occurs and composting time is longer. The difference between these two biological processes is shown in label 2.

The general transformation of solid matter under anaerobic conditions is:

• Input: organic matter + H₂0 + nutrients.
• Output: set of microorganisms + stabii organic matter + CO₂ + CH₄ + NH3 + H2S + ∆E.

If the conversion is partial (incomplete), then:

C_aHbO_cNd →nC_wH_xO_yN_z+ mCH₄ + sC0₂ rH20 + (d-nz)NH₃

Where

• C_aH_bO_cN_d: early organic matter.''
• C_wH_x,O_yN_z,: the final organic matter, as slury or digestate, will be compost which becomes a breeding ground for pathogenic bacteria due to its 'cold' process, sour smell, but can be used as organic fertilizer. If disposed of in the environment, it will increase the organic load in the environment.

                 Table 2: Comparison of aerobic and anaerobic composting

Characteristics	Aerob	Anaerob
Reaction formation	Exothermic requires external energy outside for oxygen supply and heat generation.	Endothermic, no need for external energy, bio-gas produced energy source energy source.
Final product	Humus (compost), CO2 , H20	Digestate sludge, CO2, CH4
Mass reduction	Will not exceed the C-organic content.	Will not exceed C-organic content.
Process time	(20-30) days (1/2 cooked)	(20-40) days
The main purpose for waste	Volume reduction (mass and water), resulting in compost	Gasbio's mass is reduced, but its volume is increased because it is mixed with water in a digestate form.
Aesthetics	No odor	Causes odor
Hygienic	Free of pathogenic bacteria	Potential pathogenic bacteria

Example 1:

It is known that 2000 kg of wet  waste with 50% moisture content has an initial composition of [C₆H₇0₂(0H₃)]₅ undergoing aerobic processes. After 3 weeks, it becomes ½ mature compost [C₆H₇0₂(0H₃)₂]₂ with a weight of 40 kg.

Calculate: air requirement in biologically complete oxidation.

Answer:

Weight of dry solids = 50% x 2000 kg= 1000 kg.

The atomic weight is C = 12; H = 1; O = 16; N = 14.

From the reaction:

Initial molar: final molar: n = 1.23/1.23 = 1.00

Values a, b, c, d, w, x, y and z:

Senyawa awal: [C₆H₇0₂(0H₃]₅=C₃₀H₅₀O₂₅ atau a= 30, b =50, c =25, d = 0

Senyawa akhir: [C₆H₇O₂ (0H₃)₂. C₁₂H₂₀O₁₀ atau w = 12, x =20, y =10, z = 0

Thus: 

r = 0,5*[(b - nx)- 3(d- nz)]= 0,5*[(50-1*20)- 3(0 -1*)) =15

s =a - nw = 30-1*12 =18

1 mol 0₂ = 32 gram 0₂

Oxygen demand (in air):

0,5 (ny + 2s + r-c) 0₂=0,51(1*10 + 2*18 + 15- 25)) * 1,23 *32 = 708 kg-0₂

1. Biogas = CH₄ + CO₂; ∆E is the heat energy produced, but it is very small because the main energy conversion is not in the form of heat but in the form of methane gas (CH₄).
2. Specific gravity (SG) CH₄ = 0.557, SG CO₂ = 1.519, and SG air = 1.00. Density: CH₄ density = 0.656 kg/m³ and CO₂ density = 1.977 kg/m³.

If it is perfect (which in reality is unlikely), then:

Anaerobic biodegradation of complex organic matter involves various levels of parallel processes and reactions, and can be grouped into 4 stages: hydrolysis, acetogenesis, acidogenesis, and methanogenesis. Each stage will involve different groups of microorganisms.

1. Hydrolysis process:

The process of dissolving insoluble organics and breaking down long-chain (complex) organic compounds such as proteins, carbohydrates, fats, cellulose, and hemicelluloses into smaller-molecule materials or into soluble and simpler-chain compounds, such as glucose, fatty acids, alcohols, and amino acids, Extracellular enzymes that the bacteria released into the media are responsible for catalyzing this reaction. The bacteria responsible for this stage are those that will hydrolyze carbohydrates, proteins, fats, and other minor components of biomass into fatty acids, H2, and CO₂.

2. Proses asidogenesa: 

Compounds resulting from the hydrolysis process will be fermented by acid-producing microorganisms into organic acids, especially short-chain volatile acids (acetate, propionate, and butyrate), H₂, CO₂, and other lower molecular weight compounds. (acetic, propionic, and butyric), H₂, CO₂, and other lower molecular weight compounds.

3. Proses asetogenesa:

At this point, acetogenic microorganisms will utilize short-chain fatty acids, butyrate, and propionate to produce acetic acid, CO2, and H2.

4. Methanogenesis process:

All the products of the previous stage are utilized by the methane bacteria to produce CH₄ and CO₂ gas. At this stage, the conditions must be strictly anaerobic. About 70% of the methane produced is formed from acetate, with the following reaction:

CH₃COO + H₂O →CH₄ +HCO₃+ energi.

Tahapan proses degradasi materi organik secara anaerob

According to the following reaction, bacteria that oxidize H₂ and reduce bicarbonate degrade the remaining material:

 4H₂+HC0₃ + H⁺→ CH₄+3H₂0 + energi

Most methanogenic bacteria use H₂ and CO2 for their growth. Reactions that occur:

 4H₂ + CO₂ → CH₄ + 2H₂0

Of all the types of bacteria involved, methane bacteria have the highest sensitivity to environmental factors and the slowest growth. Therefore, it is important to maintain optimum environmental conditions to avoid unstable conditions. Methanogenic bacteria can be divided into two  major groups, namely:'

Ocetoclastic methanogenes that use acetic acid as their substrate for methane formation; and hydrogen-utilizing bacteria that use hydrogen for methane formation; and some can oxidize alcohols such as ethanol or isopropanol to acetate and acetone. The resulting acetate is then used to form methane.

Example 2:

The amount of waste is 100 kg (wet), with an organic matter formula C₆₀H₉₄0₃₈N Moisture content of waste = 40%; the amount of volatile matter is 75% of dry weight. Density: CO² = 1.977 kg/m³ and CH₄ = 0.656 kg/m³

Count:

Determine the maximum amount of biogas formed from the organic waste fraction that is considered to be undergoing complete anaerobic decomposition.

Answer:

From the reaction equation, it is obtained:

C₆₀H₉₄0₃₈N + 18,25 H₂O → 31,88 CH₄ + 28,13 CO₂ + NH₃

Dry weight of waste = 60% x 100 kg= 60 kg-dry

Total volatile matter= 75% x 60 kg= 45 kg-dry

1 mol C₆₀H₉₄0₃₈N = (60*12) + (94*1) + (38*16) + (1*14)= 1.436 kg

From the reaction equation obtained:

CH₄= (31,88 x 16 x 45)/1.436 = 15,98 kg 

       = 15,98/0,656 m³= 24,35 m³

CO₂= (28,13 x 44 x 45)/1.436 = 38,79 kg

       = 38,79/1,977 = 19,62 m3

% gas CH₄ = 24,35/(24,35 + 19,62) x 100%= 55,35%

% gas CO₂=19,62/(24,35 + 19,62) x 100% = 44,62%

Total gas formed = (24.35 +19.62)/100

                            = 0.439 m3 / kg-wet waste Or = 0.439/45 = 0.98 m3/kg-volatile

Degradation rate of waste organic matter

The degradation rate of municipal solid waste organic matter can be explained based on the rate of degeneration of volatile matter using a first-order equation, where the rate of degradation (decay) is proportional to the residual organic matter left behind, i.e., the rate of degradation of volatile matter is proportional to the rate of degradation of volatile matter.

dS/dt = - Kd S

with:

S = biodegradable matter concentration (weight of volatile matter per unit time)

Kd = degradation constant (1/day or day^-1)

t = time (days)

Obtained:

S = So e^kd

^{So = organic matter content at the beginning}

^{In steady state, the change that occurs is = O so:}

QSo/V- QS/V- Kd S= 0

t =(So-5)/Kd S

With batch experiments, the value of Kd will be generated by plotting the log S/So value against time and calculating the slope as Kd/2,303.

Previously, estimating the biodegradation of waste organic matter with an exponential equation was:

Pt = Po ^-kl

^with:

Pt = organic matter in waste (kg/ton) at time t.

Po = organic matter at time t = O, and k = degradation rate (1/year).

Based on the waste samples in the Netherlands, the value of k is obtained as follows:

• 22.6% degraded rapidly (k = 0.693/year);
• 22.6% degraded moderately (k = 0.139/year);
• 30% degraded very slowly (k = 0.046/year);
• the rest (25%) did not degrade.

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