When it comes to solid waste generation, Kathmandu valley alone produces about 500 metric tons of solid waste per day. Valley’s daily solid waste constitutes of 70% organic wastes, 12% plastics, 8% paper products, 1% hazardous wastes and remaining 5% miscellaneous wastes. Besides organic wastes most of the other wastes are collected, reused or recycled by various industries. Organic wastes in other hand is the least managed and most troublesome waste that contributes to street pollution, hazard in landfills and environmental toxicity. One of the most common method promoted by NGOs and government bodies to treat organic waste in urban area is composting. But since many people in cities are not into urban farming so they do not have motivation to segregate organic wastes and would not invest time and labor to do composting. Due to this reason, unless and until the central government comes up with a proper plan and policy to divert organic waste from landfill to recycling facility. It is in the hands of general people to keep their city clean, which would only happen if they have an alternative method to convert their organic waste into value.
Before understanding how organic wastes can be turned into value we should first know little more about biodegradable wastes coming out of our kitchens. For easy understanding, biodegradable kitchen wastes can be segregated into two groups such as dry wastes and wet waste. Dry wastes includes tea bags, fibrous vegetable peels, papers, vegetables stems, etc. Dry wastes are basically those low caloric waste that do not go anaerobic, that is, will not rot or smell even if kept openly for few days. These dry wastes are ideally managed with composting. Wet wastes are those high caloric wastes such as uneaten foods and fruits, dairy wastes, meats and fat wastes, rancid oils, etc. that if left open will rot and smell. These wet waste are very hard to compost so, ideally they are either buried into ground or can be recycled into biogas and fertilizer with biodigester. Biodigester can manage both dry as well as wet wastes. Biodigester is an organic waste recycling system that works on the principle of anaerobic digestion (AD). AD is a natural process of biodegradation of organic materials that takes place in an environment with no oxygen such as in the intestine of living organisms, underneath the lake and river, etc. End products of AD is biogas and digestate slurry or bioslurry. Unlike composting, AD gives two products and is least labor intensive. Biogas is about 60% methane by weight and can be used to cook foods while reducing LPG use. If a house hold produces about 2 kg of organic wastes daily then with biodigester, up to 9 LPG cylinders can be saved per year, simultaneously giving equivalent of 300kg of compost fertilizer in the form of liquid each year. Quantity of biogas produced from high caloric wastes such as food wastes is way higher than those produced from animal or human manure. For an example, since cow dung is already digested byproduct so in order to produce 1000 liters of biogas about 40kg of cow dung is needed while for the same on average 4kg of food wastes is enough.
Even with such tempting figures, so why is biodigester still not popular in our cities, you may ask. Problem lies with most people, even many experts, not understanding biodigester technology and operating it the way gober gas plant is operated. For easier understanding, we can take cow as an analogy. A cow eats certain portions of grass and grains per day and gives certain amount of milk and manure per day. If a cow is not fed enough then it cannot give optimum amount of milk and if the cow is fed too much it will get indigestion and may even get sick. Similarly, food waste based biodigester can be compared to cow husbandry. If it is fed too little it will give low biogas but if it is fed consistently beyond its capacity it will get indigestion (acidic) and stops working. Unlike cow, once biodigester gets overloaded (acidic) it should not be fed and takes several weeks to get back in balance. But gober gas plant, also known as cow dung based biodigester, is different. Gober gas plant hardly ever gets overloaded even when consistently fed beyond its capacity. This is because cow dung which is digested byproduct coming out of cow’s intestine already contains both caloric value as well as methane producing live microbial population. Due to this reason even if the gober gas plant is operated as dustbin for cow dung it can rarely go acidic and constantly produces biogas once established. To optimally operate biodigester we should keep two important points in our mind as a user. First, never to feed biodigester beyond its daily capacity for more than 3 consecutive days. Second, well mixed food wastes should be fed. As an example for latter, feeding only one type of wastes such as only egg shells, or only rice, or only fats, etc. is strictly avoided.
By properly understanding and operating biodigester, one can rip tremendous benefits from it for more than 20 years. For those involved in urban farming adding biodigester becomes excellent complement to their farm. Biodigester is one time investment that has virtually no recurring or maintenance cost. For bigger institutions like schools and hotels, customized food waste based biodigester can be a solid green investment that would not only save cost but also help them in portraying themselves as socially and environmentally responsible institutions. Institution can easily self assess their biogas and fertilizer production capacity by following simple formula. Roughly 100kg of wet waste gives biogas equivalent to 1 LPG cylinder. As an example, let us say school ‘X’ produces about 150kg of wet waste daily so per year if all wet waste is utilized with biodigester, about 550 cylinders can be saved. Calculation is pretty simple, 150kg times 365days gives 54,750kg wet wastes per year. Dividing this by 100 gives equivalent LPG cylinders i.e. 547.5. Calculating fertilizer production is also simple. Suppose 54,750kg of food waste when properly composted would generated 35% by weight of compost that would be about 19,000kg of compost per year. So, with this simple calculation one can see that if school ‘X’ which produces daily 150kg of food waste would get about 550 LPG cylinders worth of biogas and 19,000kg worth of compost every year if biodigester customized to its requirement is installed.
JUAS biodigester is a re-engineered open source design based on original ARTI floating drum design. With this open source technology anyone with creativity and motivation can build and run their own biodigester designed and built according to their need. Our re-desinged model through our R&D got so much efficient that we decided and built up to 10 cubic meters of biodigesters for ourselves in our commercial kitchen to make ghee and liquid stevia utilizing biogas. So, next time when we complain about not getting enough LPGs due to shortage or see organic wastes littering our streets. We should blame no one else but ourselves. Viable technology to totally manage and convert organic waste into value is already available but the choice to implement remains ours.