Tag Archives: digestate

Description of a Biogas Power Plant

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A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. The key components of a modern biogas power (or anaerobic digestion) plant include: manure collection, anaerobic digester, effluent treatment, biogas storage, and biogas use/electricity generating equipment.

anaerobic_digestion_plant

Working of a Biogas Plant

The fresh organic waste is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digester where stabilization of organic waste takes place.

In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.

Biogas Cleanup

Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.

Utilization of Biogas

Biogas is dried and vented into a CHP unit to a generator to produce electricity and heat. The size of the CHP system depends on the amount of biogas produced daily.

Treatment of Digestate

The digested substrate is passed through screw presses for dewatering and then subjected to solar drying and conditioning to give high-quality organic fertilizer.  The press water is treated in an effluent treatment plant based on activated sludge process which consists of an aeration tank and a secondary clarifier. The treated wastewater is recycled to meet in-house plant requirements.

Monitoring of Environmental Parameters

A chemical laboratory is necessary to continuously monitor important environmental parameters such as BOD, COD, VFA, pH, ammonia, C:N ratio at different locations for efficient and proper functioning of the process.

Control System

The continuous monitoring of the biogas plant is achieved by using a remote control system such as Supervisory Control and Data Acquisition (SCADA) system. This remote system facilitates immediate feedback and adjustment, which can result in energy savings.

Biogas from Kitchen Waste at Akshaya Patra Foundation

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The Akshaya Patra Foundation, a not-for-profit organization, is focused on addressing two of the most important challenges in India – hunger and education. Established in year 2000, the Foundation began its work by providing quality mid-day meals to 1500 children in 5 schools in Bangalore with the understanding that the meal would attract children to schools, after which it would be easier to retain them and focus on their holistic development. 14 years later, the Foundation has expanded its footprint to cover over 1.4 million children in 10 states and 24 locations across India.

Akshaya-Patra-Kitchen-BioGas

The Foundation has centralised, automated kitchens that can cook close to 6,000 kilos of rice, 4.5 to 5 tonnes of vegetables and 6,000 litres of sambar, in only 4 hours. In order to make sustainable use of organic waste generated in their kitchens, Akshaya Patra Foundation has set up anaerobic digestion plants to produce biogas which is then used as a cooking fuel. The primary equipment used in the biogas plant includes size reduction equipment, feed preparation tank for hydrolysis of waste stream, anaerobic digester, H2S scrubber and biogas holder.

Working Principle

Vegetable peels, rejects and cooked food waste are shredded and soaked with cooked rice water (also known as ganji) in a feed preparation tank for preparation of homogeneous slurry and fermentative intermediates. The hydrolyzed products are then utilized by the microbial culture, anaerobically in the next stage. This pre-digestion step enables faster and better digestion of organics, making our process highly efficient.

The hydrolyzed organic slurry is fed to the anaerobic digester, exclusively for the high rate biomethanation of organic substrates like food waste. The digester is equipped with slurry distribution mechanism for uniform distribution of slurry over the bacterial culture.

Optimum solids are retained in the digester to maintain the required food-to-microorganism ratio in the digester with the help of a unique baffle arrangement. Mechanical slurry mixing and gas mixing provisions are also included in the AD design to felicitate maximum degradation of organic material for efficient biogas production.

After trapping moisture and scrubbing off hydrogen sulphide from the biogas, it is collected in a gas-holder and a pressurized gas tank. This biogas is piped to the kitchen to be used as a cooking fuel, replacing LPG.

Basic Design Data and Performance Projections

Waste handling capacity 1 ton per day cooked and uncooked food waste with 1 ton per day ganji water

Input Parameters                      

Amount of solid organic waste 1000 Kg/day
Amount of organic wastewater ~ 1000 liters/day ganji (cooked rice water)

Biogas Production

Biogas production ~ 120 – 135 m3/day

Output Parameters

Equivalent LPG to replace 50 – 55 Kg/day (> 2.5 commercial LPG cylinders)
Fertilizer (digested leachate) ~ 1500 – 2000 liters/day

Major Benefits

Modern biogas installations are providing Akshaya Patra, an ideal platform for managing organic waste on a daily basis. The major benefits are:

  • Solid waste disposal at kitchen site avoiding waste management costs
  • Immediate waste processing overcomes problems of flies, mosquitos etc.
  • Avoiding instances when the municipality does not pick up waste, creating nuisance, smell, spillage etc.
  • Anaerobic digestion of Ganji water instead of directly treating it in ETP, therefore reducing organic load on the ETPs and also contributing to additional biogas production.

The decentralized model of biogas based waste-to-energy plants at Akshaya Patra kitchens ensure waste destruction at source and also reduce the cost incurred by municipalities on waste collection and disposal.

akshayapatra-kitchen

An on-site system, converting food and vegetable waste into green energy is improving our operations and profits by delivering the heat needed to replace cooking LPG while supplying a rich liquid fertilizer as a by-product.  Replacement of fossil fuel with LPG highlights our organization’s commitment towards sustainable development and environment protection.

The typical ROI of a plug and play system (without considering waste disposal costs, subsidies and tax benifts) is around three years.

Future Plans

Our future strategy for kitchen-based biogas plant revolves around two major points:

  • Utilization of surplus biogas – After consumption of biogas for cooking purposes, Akshaya Patra will consider utilizing surplus biogas for other thermal applications. Additional biogas may be used to heat water before boiler operations, thereby reducing our briquette consumption.
  • Digested slurry to be used as a fertilizer – the digested slurry from biogas plant is a good soil amendment for landscaping purposes and we plan to use it in order to reduce the consumption of water for irrigation as well as consumption of chemical fertilizers.

How Can You Produce Your Own Biogas?

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The idea of biogas is anything but new. People have been experimenting with making biogas for many generations. Biogas is made by converting organic waste into energy. It’s a huge win for the environment because it utilizes what is otherwise considered waste, but it’s a big win for pocketbooks too.

Organic waste includes the byproducts of human food production (think potato peels, carrot peels, the tops of turnips, etc) but it also includes manure. Any manure is fair game, think about cows, pigs, chickens, rabbits, goats — virtually any farm animal produces mounds of this each day.

This manure produces very high levels of methane gas which is horrible for the environment. By using this manure to create biogas, we remove the danger of creating heat-trapping gases in our atmosphere that raises the temperature of the entire planet. Using it for biogas production can also help to reduce global warming.

How Do We Produce Biogas?

Biogas is produced from the breakdown of organic waste in an environment that is void of oxygen. We call this environment anaerobic and the process is process is called anaerobic digestion. Two products are created from this process. One is digestate. Digestate can be used for fertilizer and even as livestock bedding.  The other product is biogas. Biogas can be used for heating, electricity production and as a clean vehicle fuel.

It’s essentially like composting all of the materials, but in an environment without oxygen and in the temperature range of around 35 to 40 degrees Celsius and pH of around 7. This is optimal to produce biogas. Biogas can be converted into an upgraded form of gas by removal of carbon dioxide that can be used like natural gas. It can be used as-is as an engine fuel. It can be used as fuel in a vehicle, sometimes without modification.

How Can You Produce Your Own Biogas?

Just imagine being on your own off-grid property, running a hundred head of cattle, growing your own food and canning it. You’ve got meat covered, your food is stocked and you are prepared for just about anything. But what about fuel? Imagine what a game-changer it could be if you were able to produce your own fuel from the waste from your cattle and your garden scraps or food residuals! You can!

The Biogas Digester makes it possible, and fairly easy, for you to start producing your own biogas. Buy a ready-made biodigester for around $700-$1000 dollars and start producing your own biogas to meet your fuel requirements. They are containers designed to do the work for you and help you collect the fruits of your composted and digested waste.

Build your own! China has approximately 30 million Biodigesters in use in its rural areas. Rural Chinese areas are far removed from cities that have gas stations. It simply isn’t accessible as it is in the US. Many rural people have learned to make their own biodigesters to fill their fuel needs.

offgrid-biodigester

You need a tank that is sealed with an access hole on one side for adding organic waste. You have another access to an outlet. That is where you collect the liquid run-off that can be used for fuel.

The bottom of the main unit is the digestion chamber. From that is an outlet where the digestate can be collected and used as fertilizer. The main chamber typically has a domed top to allow for the room that will be necessary for the expansion of the gases formed inside. By being sealed, the unit creates that all-important anaerobic environment.

Useful Links

A tank that demonstrates the size and simplicity of a tank that can be purchased and used in the backyard.

https://www.etsy.com/listing/705458580/portable-home-biogas?gpla=1&gao=1&

This is a very in-depth article with directions for creating your own biodigester from Science Direct – https://www.sciencedirect.com/topics/engineering/biogas-digester

Miscanthus: Reducing the Establishment Costs

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Miscanthus has been lauded as a dynamic high potential biomass crop for some time now due to its high yields, low input requirements and perennial nature. Miscanthus is commonly used as a biomass fuel to produce heat and electricity through combustion, but studies have found that miscanthus can produce similar biogas yields to maize when harvested at certain times of the year.  Miscanthus is a C4 grass closely related to maize and sugarcane, it can grow to heights of three metres in a single growing season.

Miscanthus-Elephant-Grass

High Establishment Costs

However, high establishment costs have impeded the popularity of the crop. High establishment costs of miscanthus are as a result of the sterile nature of the crop, which means that miscanthus cannot be propagated from seed and instead must be propagated from vegetative material. The vegetative material commonly used is taken from the root structure known as rhizomes; rhizome harvesting is a laborious process and when combined with low multiplication rates, results in a high cost for miscanthus rhizomes. The current figure based on Irish figures is €1,900 ha for rhizomes.

Promising Breakthrough

Research conducted in Teagasc Oak Park Carlow Ireland, suggests that there may be a cost effective of method of propagating miscanthus by using the stem as the vegetative material rather than having to dig up expensive rhizomes. The system has been proven in a field setting over two growing seasons and plants have been shown to be perennial.

A prototype planter suitable for commercial up scaling has been developed to sow stem segments of miscanthus. Initial costs are predicted at €130 ha for plant material. The image below shows the initial stem that was planted in a field setting and the shoots, roots, and rhizome developed by the stem at the end of the first growing season.

miscanthus-stem

Feedstock for AD Plants

Switching from maize to miscanthus as a feedstock for anaerobic digestion plants would increase profitability and boost the GHG abatement credentials of the systems. Miscanthus is a perennial crop which would provide a harvest every year once established for 20 years in a row without having to be replanted compared to maize which is replanted every year. This would provide an obvious economic saving as well as allowing carbon sequestration in the undisturbed soil.

There would be further GHG savings from the reduced diesel consumption required for the single planting as opposed to carrying out heavy seedbed cultivation each year for maize. Miscanthus harvested as an AD feedstock would also alleviate soil compaction problems associated with maize production through an earlier harvest in more favourable conditions.

Future Perspectives

Miscanthus is a nutrient efficient crop due to nutrient cycling. With the onset of senescence nutrients in the stem are transferred back to the rhizome and over-wintered for the following year’s growth. However the optimum date to harvest biomass to produce biogas is before senescence. This would mean that a significant proportion of the plants nutrient stores would be removed which would need to be replaced. Fertiliser in the form of digestate generated from a biogas plant could be land spread to bridge nutrient deficiencies. However additional more readily available chemical N fertiliser may have to be applied.

Some work at Oak Park on September harvested miscanthus crops has seen significant responses from a range of N application rates. With dwindling subsidies to support anaerobic digestion finding a low cost perennial high yielding feedstock could be key to ensuring economic viability.