Urban Batch Anaerobic Biodigester

Open Biotecture

In recent days, the Open Biotecture team has been working to develop a portable bio-digester for urban area.

There is an increasing demand for organic waste management in Kathmandu, and some residents have asked us about cost-effective solutions. Currently, the residents of Kathmandu dispose their organic waste in the streets or pay for door-to-door waste collection services. For the latter, the monthly cost ranges anywhere between 200-500 NPR.

Is Portable Anaerobic Bio-digester a good idea?

Open Biotecture investigated existing solutions for treating organic waste in urban areas. The first consideration was given to aerobic composting, however, the process only produces fertiliser as an output. Also, prejudice regarding bad smells emanating from compost bins was considered a strong disincentive to make composting a valued solution.

Instead, we looked at the possibility of creating portable anaerobic biodigesters to process the waste while producing a certain amount of biogas. To simplify the process…

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Solid Waste Management: Vermicomposting and Its Upgrading

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Human beings live in nature and depend on the resources of nature. The utilization of soil, water, coal, forest, gas, etc, is very important for the development of nation. These resources have changed the level of living standard of man. Solid waste is regarded as the resources. Solid waste can be described as non-liquid refusals from household, industrial and commercial establishment, market and public places. These all refusals can be used as resources in one way or other, and can be recycled, reused or transform into other forms of resources.

Solid waste may be organic and inorganic. The best way to manage the organic waste is to make manure by composting. There are different methods of composting such as:

a)      Bin composting                                 d) pile composting

b)      Vermicomposting                            e) pit composting

c)       Box composting


Vermicomposting is one of the best ways of making compost. Vermicomposting is the process of…

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Plasma Gasification

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Clean Energy Diary

The World Bank development indicators 2008 shows that the wealthiest 20% of the world accounts for 76.6% of total private consumption. The poorest fifth just 1.5%.The report further states,

“Today’s consumption is undermining the environmental resource base. It is exacerbating inequalities. And the dynamics of the consumption-poverty-inequality-environment nexus are accelerating. If the trends continue without change — not redistributing from high-income to low-income consumers, not shifting from polluting to cleaner goods and production technologies, not promoting goods that empower poor producers, not shifting priority from consumption for conspicuous display to meeting basic needs — today’s problems of consumption and human development will worsen. The real issue is not consumption itself but its patterns and effects. Inequalities in consumption are stark. Globally, the 20% of the world’s people in the highest-income countries account for 86% of total private consumption expenditures — the poorest 20% a minuscule 1.3%. More specifically, the richest…

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Anaerobic Digestion of Agricultural Residues


The main problem with anaerobic digestion of crop residues is that most of the agricultural residues are lignocellulosic with low nitrogen content. To improve the digestibility of crop residues, pre-treatment methods like size reduction, electron irradiation, heat treatment, enzymatic action etc are necessary. For optimizing the C/N ratio of agricultural residues, co-digestion with sewage sludge, animal manure or poultry litter is recommended.

Several organic wastes from plants and animals have been exploited for biogas production as reported in the literature. Plant materials include agricultural crops such as sugar cane, cassava, corn etc, agricultural residues like rice straw, cassava rhizome, corn cobs etc, wood and wood residues (saw dust, pulp wastes, and paper mill. Others include molasses and bagasse from sugar refineries, waste streams such as rice husk from rice mills and residues from palm oil extraction and municipal solid wastes, etc. However, plant materials such as crop residues are more difficult to digest than animal wastes (manures) because of difficulty in achieving hydrolysis of cellulosic and lignocellulosic constituents.

Crop residues can be digested either alone or in co-digestion with other materials, employing either wet or dry processes. In the agricultural sector one possible solution to processing crop biomass is co-digestion together with animal manures, the largest agricultural waste stream. In addition to the production of renewable energy, controlled anaerobic digestion of animal manures reduces emissions of greenhouse gases, nitrogen and odour from manure management, and intensifies the recycling of nutrients within agriculture.

In co-digestion of plant material and manures, manures provide buffering capacity and a wide range of nutrients, while the addition of plant material with high carbon content balances the carbon to nitrogen (C/N) ratio of the feedstock, thereby decreasing the risk of ammonia inhibition. The gas production per digester volume can be increased by operating the digesters at a higher solids concentration. Batch high solids reactors, characterized by lower investment costs than those of continuously fed processes, but with comparable operational costs, are currently applied in the agricultural sector to a limited extent.

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Biomass Cogeneration

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Biomass conversion technologies transform a variety of wastes into heat, electricity and biofuels by employing a host of strategies. Conversion routes are generally thermochemical or biochemical, but may also include chemical and physical. Physical methods are frequently employed for size reduction of biomass wastes but may also be used to aggregate and densify small particles into pellets or briquettes.

A wide range of conversion technologies are under continuous development to produce biomass energy carriers for both small and large scale energy applications. Combustion is the most widely used technology that releases heat and can also generate power by using boilers and steam turbines. The simplest way is to burn the biomass in a furnace, exploiting the heat generated to produce steam in a boiler, which is then used to drive a steam turbine. At the smaller scale, biomass pellet and briquette combustion systems mainly used for domestic and industrial heat supply are experiencing growing demand in some countries due to their convenience.

Advanced technologies include biomass integrated gasification combined cycle (BIGCC) systems, co- firing (with coal or gas), pyrolysis and second generation Biofuels. Second generation Biofuels can make use of biochemical technologies to convert the cellulose to sugars which can be converted to bioethanol, biodiesel, dimethyl ester, hydrogen and chemical intermediates in large scale bio-refineries.

Biomass fuels are typically used most efficiently and beneficially when generating both power and heat through a Combined Heat and Power (or Cogeneration) system. A typical CHP system provides:

  • Distributed generation of electrical and/or mechanical power.
  • Waste-heat recovery for heating, cooling, or process applications.
  • Seamless system integration for a variety of technologies, thermal applications, and fuel types into existing building infrastructure.

CHP systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. The type of equipment that drives the overall system (i.e., the prime mover) typically identifies the CHP unit.

Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical energy.

A biomass-fueled Combined Heat and Power installation is an integrated power system comprised of three major components:

  1. Biomass receiving and feedstock preparation.
  2. Energy conversion – Conversion of the biomass into steam for direct combustion systems or into biogas for the gasification systems.
  3. Power and heat production – Conversion of the steam or syngas or biogas into electric power and process steam or hot water

The lowest cost forms of biomass for generating electricity are residues. Residues are the organic byproducts of food, fiber, and forest production, such as sawdust, rice husks, wheat straw, corn stalks, and sugarcane bagasse. Forest residues and wood wastes represent a large potential resource for energy production and include forest residues, forest thinnings, and primary mill residues.  Energy crops are perennial grasses and trees grown through traditional agricultural practices that are produced primarily to be used as feedstocks for energy generation, e.g. hybrid poplars, hybrid willows, and switchgrass. Animal manure can be digested anaerobically to produce biogas in large agricultural farms and dairies.

To turn a biomass resource into productive heat and/or electricity requires a number of steps and considerations, most notably evaluating the availability of suitable biomass resources; determining the economics of collection, storage, and transportation; and evaluating available technology options for converting biomass into useful heat or electricity.

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Biogas Upgradation

Enrichment of biogas is primarily achieved by carbon dioxide removal which then enhances the energy value of the gas to give longer, driving distances with a fixed gas storage volume. Removal of carbon dioxide also provides a consistent gas quality with respect to energy value. The latter is regarded to be of great importance from the vehicle manufacturers in order to reach low emissions of nitrogen oxide. At present four different methods are used commercially for removal of carbon dioxide from biogas either to reach vehicle fuel standard or to reach natural gas quality for injection to the natural gas grid. These methods are:

  • Water absorption
  • Polyethylene glycol absorption
  • Carbon molecular sieves
  • Membrane separation

 Water scrubbing

Water scrubbing is used to remove carbon dioxide but also hydrogen sulphide from biogas since these gases is more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurized and fed to the bottom of a packed column where water is fed on the top and so the absorption process is operated counter-currently.

Polyethylene glycol scrubbing

Polyethylene glycol scrubbing is a physical absorption process. Selexol is one of the trade names used for a solvent. In this solvent, like in water, both carbon dioxide and hydrogen sulphide are more soluble than methane. The big difference between water and Selexol is that carbon dioxide and hydrogen sulphide are more soluble in Selexol which results in a lower solvent demand and reduced pumping. In addition, water and halogenated hydrocarbons (contaminants in biogas from landfills) are removed when scrubbing biogas with Selexol.

Carbon molecular sieves

Molecular sieves are excellent products to separate specifically a number of different gaseous compounds in biogas. Thereby the molecules are usually loosely adsorbed in the cavities of the carbon sieve but not irreversibly bound. The selectivity of adsorption is achieved by different mesh sizes and/or application of different gas pressures. When the pressure is released the compounds extracted from the biogas are desorbed. The process is therefore often called “pressure swing adsorption” (PSA). To enrich methane from biogas the molecular sieve is applied which is produced from coke rich in pores in the micrometer range. The pores are then further reduced by cracking of the hydrocarbons. In order to reduce the energy consumption for gas compression, a series of vessels are linked together. The gas pressure released from one vessel is subsequently used by the others. Usually four vessels in a row are used filled with molecular sieve which removes at the same time CO2 and water vapour.


There are two basic systems of gas purification with membranes: a high pressure gas separation with gas phases on both sides of the membrane, and a low-pressure gas liquid absorption separation where a liquid absorbs the molecules diffusing through the membrane.

  • High pressure gas separation

Pressurized gas (36 bar) is first cleaned over for example an activated carbon bed to remove (halogenated) hydrocarbons and hydrogen sulphide from the raw gas as well as oil vapour from the compressors. The carbon bed is followed by a particle filter and a heater. The raw gas is upgraded in 3 stages to a clean gas with 96 % methane or more. The waste gas from the first two stages is recycled and the methane can be recovered. The waste gas from stage 3 (and in part of stage 2) is flared or used in a steam boiler as it still contains 10 to 20 % methane.

  • Gas-liquid absorption membranes

Gas-liquid absorption using membranes is a separation technique which was developed for biogas upgrading in the recent past. The essential element is a micro-porous hydrophobic membrane separating the gaseous from the liquid phase. The molecules from the gas stream, flowing in one direction, which are able to diffuse through the membrane will be absorbed on the other side by the liquid flowing in counter current. The absorption membranes work at approx. atmospheric pressure (1 bar) which allows low-cost construction. The removal of gaseous components is very efficient. At a temperature of 25 to 35°C the H2S concentration in the raw gas of 2 % is reduced to less than 250 ppm.

Different Strategies in Composting

Compost Pile

The methodology of composting can be categorized into three major segments—anaerobic composting, aerobic composting, and vermicomposting. In anaerobic composting, the organic matter is decomposed in the absence of air. Organic matter may be collected in pits and covered with a thick layer of soil and left undisturbed six to eight months. The compost so formed may not be completely converted and may include aggregated masses.

Aerobic compostingis the process by which organic wastes are converted into compost or manure in presence of air and can be of different types. The most common is the Heap Method, where organic matter needs to be divided into three different types and to be placed in a heap one over the other, covered by a thin layer of soil or dry leaves. This heap needs to be mixed every week, and it takes about three weeks for conversion to take place. The process is same in the Pit Method, but carried out specially constructed pits. Mixing has to be done every 15 days, and there is no fixed time in which the compost may be ready. Berkley Method uses a labor-intensive technique and has precise requirements of the material to be composted. Easily biodegradable materials, such as grass, vegetable matter, etc., are mixed with animal matter in the ratio of 2:1. Compost is usually ready in 15 days.

Vermicomposting involves use of earthworms as natural and versatile bioreactors for the process of conversion. It is carried out in specially designed pits where earthworm culture also needs to be done. Vermicomposting is a precision-based option and requires overseeing of work by an expert. It is also a more expensive option (O&M costs especially are high). However, unlike the above two options, it is a completely odorless process making it a preferred solution in residential areas. It also has an extremely high rate of conversion, so quality of the end product is very high with rich macro and micronutrients. The end product also has the advantage that it can be dried and stored safely for a longer period of time.

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Biomass Energy Resources in Indonesia

With Indonesia’s recovery from the Asian financial crisis of 1998, energy consumption has grown rapidly in past decade. The priority of the Indonesian energy policy is to reduce oil consumption and to use renewable energy. For power generation, it is important to increase electricity power in order to meet national demand and to change fossil fuel consumption by utilization of biomass wastes. The development of renewable energy is one of priority targets in Indonesia.

It is estimated that Indonesia produces 146.7 million tons of biomass per year, equivalent to about 470 GJ/y. The source of biomass energy is scattered all over the country, but the big potential in concentrated scale can be found in the Island of Kalimantan, Sumatera, Irian Jaya and Sulawesi. Studies estimate the electricity generation potential from the roughly 150 Mt of biomass residues produced per year to be about 50 GW or equivalent to roughly 470 GJ/year. These studies assume that the main source of biomass energy in Indonesia will be rice residues with a technical energy potential of 150 GJ/year. Other potential biomass sources are rubber wood residues (120 GJ/year), sugar mill residues (78 GJ/year), palm oil residues (67 GJ/year), and less than 20 GJ/year in total from plywood and veneer residues, logging residues, sawn timber residues, coconut residues, and other agricultural wastes.

Sustainable and renewable natural resources such as biomass can supply potential raw materials for energy conversion. In Indonesia, they comprise variable-sized wood from forests (i.e. natural forests, plantations and community forests that commonly produce small-diameter logs used as firewood by local people), woody residues from logging and wood industries, oil-palm shell waste from crude palm oil factories, coconut shell wastes from coconut plantations, as well as skimmed coconut oil and straw from rice cultivation.

The major crop residues to be considered for power generation in Indonesia are palm oil sugar processing and rice processing residues. Currently, 67 sugar mills are in operation in Indonesia and eight more are under construction or planned. The mills range in size of milling capacity from less than 1,000 tons of cane per day to 12,000 tons of cane per day. Current sugar processing in Indonesia produces 8 millions MT bagasse and 11.5 millions MT canes top and leaves. There are 39 palm oil plantations and mills currently operating in Indonesia, and at least eight new plantations are under construction. Most palm oil mills generate combined heat and power from fibres and shells, making the operations energy self –efficient. However, the use of palm oil residues can still be optimized in more energy efficient systems.

Other potential source of biomass energy can also come from municipal wastes. The quantity of city or municipal wastes in Indonesia is comparable with other big cities of the world. Most of these wastes are originated from household in the form of organic wastes from the kitchen. At present the wastes are either burned at each household or collected by the municipalities and later to be dumped into a designated dumping ground or landfill. Although the government is providing facilities to collect and clean all these wastes, however, due to the increasing number of populations coupled with inadequate number of waste treatment facilities in addition to inadequate amount of allocated budget for waste management, most of big cities in Indonesia had been suffering from the increasing problem of waste disposals.

The current pressure for cost savings and competitiveness in Indonesia’s most important biomass-based industries, along with the continually growing power demands of the country signal opportunities for increased exploitation of biomass wastes for power generation.

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Composting Guidelines

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It seems everyone is concerned about the environment and trying to reduce their “carbon footprint”.  I hope this trend will continue and grow as a nationwide way to live and not turn into a fad.  Composting has been around for MANY years.  Composting is a great way to keep biodegradables out of the landfill and to reap the reward of some fabulous “black gold”.  That’s what master gardeners call compost and it’s great for improving your soil.  Plants love it.  Check out 10 Rules to Remember About Composting.

  1. Layer your compost bin with dry and fresh ingredients: The best way to start a compost pile is to make yourself a bin either with wood or chicken wire.  Layering fresh grass clippings and dried leaves is a great start.
  2. Remember to turn your compost pile: As the ingredients in your compost pile start to biodegrade they will start to get hot.  To avoid your compost pile rotting and stinking you need to turn the pile to aerate it.  This addition of air into the pile will speed up the decomposition.
  3. Add water to your compost pile: Adding water will also speed up the process of scraps turning into compost.  Don’t add too much water, but if you haven’t gotten any rain in a while it’s a good idea to add some water to the pile just to encourage it along.
  4. Don’t add meat scraps to your pile: Vegetable scraps are okay to add to your compost pile, but don’t add meat scraps.  Not only do they stink as they rot, but they will attract unwanted guests like raccoons that will get into your compost bin and make a mess of it.
  5. If possible have more than one pile going: Since it takes time for raw materials to turn into compost you may want to have multiple piles going at the same time.  Once you fill up the first bin start a second one and so on.  That way you can allow the ingredient in the first pile to completely transform into compost and still have a place to keep putting your new scraps and clippings.  This also allows you to always keep a supply of compost coming for different planting seasons.
  6. Never put trash in your compost pile: Just because something says that it is recyclable it doesn’t mean that it should necessarily go into the compost bin.  For example, newspapers will compost and can be put into a compost pile, but you will want to shred the newspapers and not just toss them in the bin in a stack.  Things like plastic and tin should not be put into a compost pile, but can be recycled in other ways.
  7. Allow your compost to complete the composting process before using: It might be tempting to use your new compost in your beds as soon as it starts looking like black soil, but you need to make sure that it’s completely done composting otherwise you could be adding weed seeds into your beds and you will not be happy with the extra weeds that will pop up.
  8. Straw can be added if dried leaves are not available: Dried materials as well as green materials need to be added to a compost bin.  In the Fall you will have a huge supply of dried leaves, but what do you do if you don’t have any dried leaves?  Add straw or hay to the compost bin, but again these will often contain weed seeds so be careful to make sure they are completely composted before using them.
  9. Egg Shells and Coffee grounds are a great addition: Not only potato skins are considered kitchen scraps.  Eggshells and coffee grounds are great additions to compost piles because they add nutrients that will enhance the quality of the end product.
  10. Never put pet droppings in your compost pile: I’m sure you’ve heard that manure is great for your garden, but cow manure is cured for quite a while before used in a garden.  Pet droppings are far to hot and acidic for a home compost pile and will just make it stink.

Contributed by Roxanne Porter whose original blogpost can be viewed at http://www.nannypro.com/blog/10-rules-to-remember-about-composting/

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Image via Wikipedia
VACAVILLE, CA - APRIL 20:  Birds fly over a co...
Image by Getty Images via @daylife

The composting process is a complex interaction between the waste and the microorganisms within the waste. The microorganisms that carry out this process fall into three groups: bacteria, fungi, and actinomycetes.Actinomycetes are a form of fungi-like bacteria that break down organic matter. The first stage of the biological activity is the consumption of easily available sugars by bacteria, which causes a fast rise in temperature. The second stage involves bacteria and actinomycetes that cause cellulose breakdown. The last stage is concerned with the breakdown of the tougher lignins by fungi.

Central solutions are exemplified by low-cost composting without forced aeration, and technologically more advanced systems with forced aeration and temperature feedback. Central composting plants are capable of handling more than 100,000 tons of biodegradable waste per year, but typically the plant size is about 10,000 to 30,000 tons per year. Biodegradable wastes must be separated prior to composting: Only pure foodwaste, garden waste, wood chips, and to some extent paper are suitable for producing good-quality compost.

 The composting plants consist of some or all of the following technical units: bag openers, magnetic and/or ballistic separators, screeners (sieves), shredders, mixing and homogenization equipment, turning equipment, irrigation systems, aeration systems, draining systems, bio-filters, scrubbers, control systems, and steering systems. The composting process occurs when biodegradable waste is piled together with a structure allowing for oxygen diffusion and with a dry matter content suiting microbial growth. The temperature of the biomass increases due to the microbial activity and the insulation properties of the piled material. The temperature often reaches 65 degrees C to 75 degrees C within a few days and then declines slowly. This high temperature hastens the elimination of pathogens and weed seeds.

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