Nutrient balance in waste treatment and disposal in Nairobi, Kenya : case of the Dandora dumpsite

Abstract

Nutrient loss to MSW streams is a major concern for food security and environmental protection. As such, the need to avoid waste and recover the useful fractions for future purposes is quickly becoming a necessity. Since more than 50% of organic materials are found in the waste streams of commingled garbage, this study focuses on the organic fraction of MSW mostly generated by households, markets and restaurants terminating at the landfill. The research reveals that in Nairobi a two-pronged waste management system of collection and disposal is operated. This is unsustainable as 2,000 t/d of MSW are generated, 420 t/d are illegally dumped after collection while 750 t/d are not collected and a total 200 t/d are recycled informally. Only 830 t/d MSW are landfilled and the impact on waste management from the population and its consumption yields that the HHs, restaurants and markets are the major contributors to the voluminous organic waste. The theoretical NB model relies on N, P and K as indicators to seek a sustainable solution through closing the nutrient cycle. These indicators show positive results of a net stock of nutrients in the landfill and in the dumped portions scattered around the city. This shows a correlation to nitrification and possibly high levels of heavy metals in the Nairobi River. As 51% of all waste flows are organic matter, the individual contribution to MSW volumes at 1.57 kg per day reveals that 0.8 kg per capita per day is exclusively OFMSW and thus the impact on nutrient flows to waste streams in Nairobi is quantified as between 1 kg to 3 kg/capita/yr NPK or 119 kg/capita/yr of generated OFMSW containing essential nutrients for animal and plant development. Of this input into the WMS, between 0.3 to 0.7 kg/capita/yr NPK is lost to leaching at the landfill as observed in this study. The NB presents a method to achieve sustainable nutrient balance through MFA and closing of the nutrient cycle. The study explores urban mining, composting and biogas/ cogeneration as ideal solutions to managing OFMSW in Nairobi. There was however, a gross lack of sufficient data to build the models thus extrapolations and estimations were made to remediate the theoretical modeling. Through modeling, a positive indication of 30% N, 70% P and an increment of 52% K was revealed to be present in the landfill in just a year while 70% N and 25% P is lost through leaching. The potential to recover these nutrients is proposed through the intermediate and optimized models of MFAs in the study.