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A new paradigm for clean city and nutrient recycling for agriculture

Publication Date - 2015-06-13 00:00:00

Publication Title : A new paradigm for clean city and nutrient recycling for agriculture

Publicationed By : Dr. Shamim Mia

Publication Publication Date : 2015-06-13 00:00:00

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Publication Description :

The city corporation elections were held recently. If we analyse manifestos of all candidates, probably we will get one point common – promise to keep the city clean. Unfortunately, there is no outline about how the manifesto will be implemented. Here, the author would like to propose a sustainable clean city programme with utilisation of organic wastes for agricultural uses.

Waste generation scenario of Bangladesh

Every day we are generating wastes from our daily leftover uses. In Bangladesh, the per capita waste generation is 0.5 kg a day. However, in a recent study, it is claimed to be 0.54 kg per day for the year 2014. The waste generation of municipal areas of Bangladesh is estimated to be between 23,294 to 27,352 tons per day for the same year. The urban population is increasing at a rate of about 3.0 %. On the other hand, the per capita GDP and GNI are increasing and therefore, the consumption of people is also increasing. The simultaneous pressure of population and their escalated consumption are generating continuously increasing amount of wastes which are crossing the managing capacity of municipalities. In a recent study, the total waste generation of Dhaka, Chittagong, Khulna, Barisal, Rajshahi, and Sylhet city are estimated to be respectively 7909, 2407, 604, 85, 346, 126 tons per day while the total wastes for municipal areas of Bangladesh is estimated to be 23,294 tons per day. Most of the wastes (>75%) is organic in nature consisting of food, vegetables and fruits. Paper, plastic, textile and wood, leather and rubber constitute respectively about 6.5%, 4.3% 3.4% and 0.9% of the total wastes. Although, a part of the wastes such as plastic, glass and iron are collected by garbage collector and recycled, the organic waste generated in our municipalities, is primarily dumped in open fields without any sanitary managements. Moreover, the municipality can only collect and dump half of the wastes while the rest remains unmanaged. Therefore, the role of municipalities is often criticised. As a consequence of open field dumping and unmanaged waste scattering, the city dwellers are facing serious problems including bad smell, health risks with diarrhoea and other water borne diseases. Moreover, wastes are also polluting air and water. In addition, unmanaged wastes, which are dumped to sewerage channels, cause congestion in sewerage lines resulting water logging of cities.  The wastes generated in the cities can be turned into resources if they are managed in sustainable ways. There are several ways to manage the organic solid wastes such as open pit composting, anaerobic composting and biogas generation, incineration and sanitary dumping. Composting is facing some problems as some of the organic wastes such as coconut shell and lignified trashes are not suitable for composting and some of them take long time for decomposition. Waste concern, Bangladesh, in fact, is trying to promote waste composting. Secondly, open incarnation is causing serious air pollution and aggravating climate change as it emits carbon dioxide and other greenhouse gases. Moreover, burning is only possible when the waste biomass is dry. Anaerobic decomposition and biogas production could be a good technology, but there are problems relating to disposal of slurries and it often involve high installation cost. In recent years, there is a growing interest about waste biomass pyrolysis, heating of biomass in absence or limited oxygen. This technology provides bio energy in the form of heat which can be used to generate electricity simultaneously producing biochar as by-product. The solid organic waste including the non-compostable organic wastes, such as lignified trashes, coconut shells, paper, wood etc., can be used as pyrolysis feedstock. There are considerable amount of paper and apparel industry by-products that might be used as fuel for pyrolysis. After critical analysis, the author proposed a new approach of waste management and nutrient recycling which is presented in the next section.Simultaneous waste composting and pyrolysingIn this new approach, a business-driven waste collection and recycling are envisaged. The highest possible waste collection is proposed in this system. The collected wastes would be separated from inorganic wastes that would be sold for profit. For example, the iron, plastic and polyethylene could be sold for recycling. The organic wastes would be separated into two parts namely compostable and non-compostable depending on the decomposability and state of collection. The amount of organic wastes production in Bangladesh is about more than 17,759 tons per day. About 65% of the solid waste is considered to be compostable while 35% could be used for pyrolysis. Paper, textile or wood waste could be used as fuel energy for pyrolysis of 35% wastes biomass. When the feedstock pyrolyses, the biomass also generates heat energy which can be used for drying the biomass that would be used as fuel for pyrolysis. During compositing, it needs comparatively high temperature to accelerate the decomposition processes. Heat is also needed to get the compost dried. The heat energy generated from pyrolysis process could be used for better management of composting process. Therefore, if pyrolysis and composting are conducted simultaneously both of the systems can get synergistic benefits. Moreover, in some studies, it has been found that if biochar is added as composting materials, the nitrogen loss during composting is reduced significantly. On top of these, the extra heat generated during pyrolysis might be used for power generation. The compost and biochar, thus produced, can be marketed and used for agricultural purposes. A further value addition is also possible by making compound fertiliser incorporating major nutrients such as nitrogen or phosphorous.Figure- Schematic diagram for new paradigm of waste collection and recycling. Nutrient recycling and agricultural benefitsThe author calculated carbon, nitrogen (N), phosphorous (P) and potassium (K) recovery from the wastes for the proposed models using average recovery rate of these nutrients presented in science journals. The carbon recovery is estimated to be over 50%. Among the recycled carbon, a considerable part, 39% is biochar carbon, which is very stable and retains in soil for many years. If the total organic wastes of Bangladesh can be utilised using the proposed approach, 1.0 to 1.5 million tons of organic matter can be recycled in a year. As one of the biggest challenges of our agriculture is to replenish and maintain soil organic matter, biochar and compost will definitely contribute to propel our efforts.  A considerable amount of N, P and K could be harvested and recycled with this new paradigm. The percentage of recovery could be as much as 30, 68 and 69% of the biomass N, P and K. The amount of N, P and K recovery potentials from the total wastes of the country would be respectively 9968, 8893, and 24588 tons per year. The monetary value of the produced biochar and compost from total biomass of the country would be Tk. 10,000 million if they are sold at a rate of Tk. 10 per kilogram. Although the figure seems highly sceptic, in fact, it could be possible if we could process 6.4 million tons of organic wastes. Therefore, there is good scope of investment. Moreover, the power generation from this process could also add additional returns.Implication for environmental pollution reduction. The new paradigm, business oriented waste management and recycling, might be sustainable as it will generate money and therefore, we would be able to collected highest amount of wastes. Accordingly, it would reduce environmental pollution considerably including control odour and stop diseases from spreading. Emission of other harmful gases will also be reduced. The nutrient loading in the waterways and sea would be reduced. One of the import reasons for absence of fish and other animals in the river Buriganga is the lack of sufficient oxygen in the river water. Decomposition of organic wastes radically reduces oxygen levels in water. If we could manage our organic wastes in a sustainable manner, there is a hope to revive life in our polluted rivers. Furthermore, the biochar produced is this process can be used to trap organic and inorganic pollutant from our river water as in many scientific studies it has been confirmed that it is a good pollutant adsorbing agent.Research needs. The new approach is proposed based on scientific literature review and personal experience. However, scientific investigation and experimentation are obligatory before starting this kind of project. Nevertheless, this technology may open an avenue in waste recycling research and business.

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