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Methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water.

Bacteria used: Methanosarcina, Methanobacterium, Methanococcus

CH₃COOH → CH₄ + CO₂
CO₂ + 4H₂ → CH₄ + 2H₂O

Byproducts: CH₄, CO₂, H₂O

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Simple molecules created from acidogenesis are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.

Bacteria used: Syntrophomonas, Syntrophobacter

C₆H₁₂O₆ + 2H₂O → 2CH₃COOH + 2CO₂ + 4H²
CH₃CH₂OH + 2H₂O → CH₃COO⁻ + 3H₂ + H⁺

Common byproducts: CH₃COOH, CH₃COO⁻

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Developing novel solutions to treat food waste

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Developing novel solutions to treat food waste

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Supported 24 premises, including hotels, malls and educational institutions, to install on-site food waste treatment systems under NEA’s 3R Fund

Conducted demonstration project to co-digest food waste and used water sludge. Results from a trial project have shown that the process can triple biogas yield, compared to the treatment of used water sludge alone.

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The Organic Loading Rate (OLR) is defined as the amount of organic matter per unit reactor volume

If the OLR is too high, too much acid might be produced too quickly. This would cause the pH to (increase/decrease). The digestor would then die.

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Retention time (also known as residence time) is defined as the amount of time that the feedstock stays in the digestor

Retention time can range between 5 and 55 days depending on whether it is a wet or dry anaerobic process.

The feedstock in a dry process has a dry mass percentage of 20%-40%.

The feedstock in a wet process has a dry mass percentage of less than 15%.

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Ongoing food waste reduction publicity and outreach programme to increase awareness of the food waste situation in Singapore, as well as to encourage consumers to adopt smart food purchase, storage and preparation habits that can help them minimise food wastage. More than 400 Food Waste Reduction Ambassadors have also been trained to help spread the word on food waste reduction with their communities, families and friends.

Developed Food Waste Minimisation Guidebooks for Food Retail Establishments/ Food Manufacturing Establishments/ Supermarkets to further reduce food waste across the supply chain. The guidebooks provide a step-by-step guide and case studies to help such establishments develop a food waste minimisation plan tailored to their needs.

Ongoing Love Your Food @ Schools project at 10 schools to encourage students and staff to reduce the amount of food waste they generate and to garner the participation of students, staff and canteen stall holders to segregate their food waste for treatment using on-site food waste digesters.

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Bacteria that thrive in different temperature ranges can be used in anaerobic digestion. Two examples are thermophilic and mesophilic bacteria.

Ideal temperature range for thermophilic bacteria is 50°C to 65°C.

Ideal temperature range for mesophilic bacteria is 25°C to 40°C.

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The range of acceptable pH values: 5.5 – 8.5.

Methanogens (bacteria involved in methanogenesis) function best at a pH value from 6.7 to 7.4.

If the conditions become too acidic, the methanogens cannot survive.

Since the methanogenesis stage is after the acidogenesis stage, the pH will decrease if the methanogens die – as the acids produced are not used up.

If the pH is too high, the methanogenesis stage is impeded.

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The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts.

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

Acid-phase bacteria/acidogens, such as Streptococcus and Enterobacterium

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This process breaks down large organic polymer chains and dissolves smaller molecules into a solution. During Hydrolysis, complex organic molecules are broken down into simple sugars, amino acids, and fatty acids

An example is represented by this equation
C₆H₁₀O₄ + 2H₂O → C₆H₁₂O₆ + H2

Organic waste C₆H₁₀O4 after adding water to it, forms glucose and hydrogen gas

Bacteria used:  Fermentative bacteria, such as Pseudomonas, Bacillus, Clostridium, Micrococcus, and Flavobacterium

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C/N ratio is the amount of carbon (g) compared to the amount of nitrogen (g) present

E.g. C/N = 30 means there is 30g of C to 1g of N

Used to measure the amount of waste-activated sludge needed for blending

A C/N ratio of 30 is ideal.

C/N < 30: Nitrogen will be supplied in excess and will be lost as ammonia gas, causing undesirable odors

C/N > 30: Insufficient nitrogen for optimal growth of the microbial populations, leading to slower decomposition of waste

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An aerobic method of decomposing solid waste. We can compost leaves, food waste, manure, fruits and vegetables. Composting produces gases such as carbon dioxide, water vapour, organic material

Optimum C/N ratio of 30:1

We can also use black soldier fly to compost. The larvae will feed on the compost and break down organic matter such as manure and household garbage. This breaking down makes it easier for common worms to digest while reducing the odour and improving hygiene. It also reduces the amount of waste present.

Dead larvae can be used as fertilizer or used to feed livestock because of their high protein and good fats.

While feeding, the larvae also leave behind rich, organic material called frass that can act as fertilizer for crops and plants. Frass is a better alternative to chemical fertilizers that degrade soil quality over time and harms ecosystems.