Abstract
Most urban cities are facing severe problems relating to the delivery of urban services, including waste management, which requires the immediate attention of policymakers. Moreover, as cities continue to expand at rapid rates regional land and water resources are encroached on by urban housing and industrial establishments for dumping urban solid and liquid wastes. Urban yard, green belts and roof farming practices have also developed to improve local food and agricultural productivity and reduce urban impacts. However, they have a large water demand in high climatic temperature conditions 35±5 °C and nearly half of N and P fertilizer inputs are not used by plants and are lost into the environment via drainage. It is projected that the N pollution level by 2050 will be 150% higher than in 2010 out of which the agricultural sector is accounting for 60% of this increase. Nitrogen losses are a major economic concern in the urban landscape as excess N runoff will convert to nitrous oxide (a potent greenhouse gas) if not managed properly. During the past two decades, under the frameworks of sustainability and circular economy, there has been a high legislative focus on the management of organic waste to prevent it from being landfilled and to promote its valorization. Biochar is a suitable route to achieve this goal that has the potential to enhance agricultural productivity in a climate-friendly manner and provide a valuable soil amendment agent. If carbon offset markets develop, biochar can also provide income for farmers and landowners who use it to sequester carbon in the soil. The carbon market prices in the European trading system have ranged between $5 to 30.ton-1 of CO2e during the year of 2008 to 2013, whereas in long term practice it projected a value of $100.ton-1 CO2e. Biochar those contains approximately 75% of C stable over 100 years and one ton of biochar could sequestered 2.06 tons of CO2e.The applications of biochar from food wastes have been shown to reduce N and P leaching, improve plant growth particularly, in organic farming and carbon sequestration. According to FAO, Recycling of food waste supports the circular economy, given 1/3 billion tonnes of food waste are produced with a cost of $1 trillion. Production of biochar from food wastes by pyrolysis requires minimum energy inputs as the pyrolysis process is typically self-sustaining on flue gases once ignited. However, drying of food wastes requires around 0.55 kWh.kg-1 using solar drying to prepare the feedstock. The production of biochar is directly related to the cost of the feedstock, collection transportation cost and processing. Production of biochar from green wastes were found to cost between $150 to $260.ton-1, while application costs for 25 tons.ha-1 in agricultural fields is approximately $63.ha-1 to $70.ha-1 using the trench and fill method. This provides an important context for the economic feasibility of biochar applications. A cost of $70.2 ton-1 biochar application could achieve 10 to 40% increase in yield coupled with a 25% savings in fertilizer. Biochar applications implies less conventional fertilizers needed to achieve crop yield. The vast majority of nitrogen fertilizer is derived from natural gas (CH4) via the Haber-Bosch process. Biochar could be able to reduce greenhouse gases and have estimated a 10 to 30% reduction of nitrogen fertilizer use. It was estimate that for approximately every ton of N fertilizer manufactured and utilized, 13.5 tons CO2e is emitted. This study demonstrated biochar application in urban agriculture is a sustainable approach to mitigate water stress, improve plant growth and subsequently benefit urban policymakers.
