Investigating the feasibility and logistics of decentralized urine treatment for resource recovery

Chipako, Tinashe

Investigating the feasibility and logistics of decentralized urine treatment for resource recovery

Master Thesis

2019

Abstract

Background Phosphorous levels around the world are decreasing rapidly, as urbanization increases. This is significant as phosphorus is a resource that is required by all living organisms and is a key ingredient in many fertilizers. Similar to the peak oil phenomenon, phosphorus will soon experience a peak in its production, and it is predicted that within the next century, naturally occurring phosphorous will be completely depleted. Moreover, methods of nitrogen production such as the Haber‐Bosch process contribute largely towards global energy expenditure and greenhouse gas emissions. Considering this, researchers have aimed to investigate methods of recovering phosphorus and nitrogen to help cope with the increasing global demand and promote environmental sustainability. Urine has been identified as a potential source of recoverable phosphorus and nitrogen. Urine accounts for approximately 1% of the total volume of domestic wastewater. Conversely, urine accounts for 80%, 60% and 63% of the nitrogen, phosphorus and potassium in domestic wastewater, respectively. Moreover, wastewater treatment plants specifically target the removal of nitrogen and phosphorus. This is because these substances can cause a toxic environment in surface water, which can have a negative effect on aquatic organisms. This research aimed to evaluate a novel mode of resource recovery, through the assessment of a decentralized approach to urine treatment. Methodology Two methodological approaches were adopted to evaluate this system. In the first, a thorough review of literature was conducted to assess current innovations pertaining to urine treatment technologies. Public perceptions regarding the collection and recycling of urine were also researched. This culminated in the creation of design charts depicting treatment sequences for fresh and hydrolyzed urine, aimed at maximum resource recovery. These charts were entirely based on values from published literature and basic calculations. Secondly, geographic information systems (GIS) were used to assess the transportation and logistics of a decentralized urine treatment system, using the City of Cape Town as an illustrative case study. In this model, existing urinals at frequently visited shopping centres are theoretically replaced with waterless nutrient recovery urinals. Within these urinals, urea hydrolysis is prevented from occurring in an attached urine collection container. This minimizes nitrogen losses and allows for a solid, phosphorous based fertilizer to form. The collected urine is retrieved from individual buildings and transported to a resource recovery facility (RRF) by truck. The collected urine is filtered to remove the solid fertilizer, while the remaining liquid is concentrated to produce a liquid fertilizer. Finally, the recovered material is then sold as fertilizers to wholesalers. The implication of transportation and logistics was also assessed through four scenarios of decentralization. In scenario one, one RRF was used. In scenarios two, three and four; two, four and eight RRFs were used, respectively. The economic and environmental implications of each scenario were then evaluated through standard engineering economics and potential greenhouse gas (GHG) emissions. Ideal treatment sequence It was deduced that the most promising treatment sequence for maximum resource recovery, based on nutrient recovery rates and operating conditions, incorporated a combination of alkaline stabilization and volume reduction. Calcium hydroxide and reverse osmosis (RO) were the chosen mediums for stabilization and volume reduction. If this sequence is used, almost all urine constituents can be recovered. Moreover, a liquid fertilizer with a 3.3 - 0 ‐ 0.8 NPK rating, and 11 grams of calcium phosphate, per litre of urine treated, can theoretically be produced. This was the chosen treatment sequence for the decentralized urine transportation system. Decentralized urine transportation treatment It was found that the main contributor to GHG emissions in the decentralized system was the truck. Driving distance decreased as the number of RRFs increased, which led to a decrease in the GHG emissions because of fuel consumption. However, warehouse rental costs were a large contributor to operating expenditure (OPEX) and increased proportionately as the degree of decentralization increased. Therefore, a globally optimal solution incorporating the minimum cost, minimum GHG emissions and shortest travel distance was not possible. From a financial perspective, increased decentralization was not appealing, meaning the use of one RRF was the most favourable scenario. Weight limitations of the truck were found to influence the travel route designations within the model road network. However, transportation had a small effect on the systems monetary cost, as it only accounted for 2% to 6% of the total OPEX across all design scenarios. This is likely due to the geographical configuration of Cape Town. Similar studies in larger areas with more dispersed collection locations may yield different results. It was found that a positive net present value was achieved if the recovered fertilizer was capable of being sold at prices in line with commercially available liquid fertilizers, with similar nitrogen content. However, it is likely overly optimistic to believe the recovered liquid fertilizer could break into the South African fertilizer market and immediately compete with established products. Although, it was shown that the liquid fertilizer produced would only need to be sold at R22.75/L to equate the total system expenditure to the total income, over a five‐year period. Conclusion and Outlook It was determined that the decentralized approach to urine treatment, investigated in this research, exhibited several advantages over biological nutrient removal at conventional wastewater treatment plants. These advantages included lower GHG emissions and energy expenditure for a similar operating cost. This study ultimately shows that the collection of source‐separated urine for the purposes of resource recovery holds significant potential from a monetary and an environmental perspective. Furthermore, the combination of transportation planning and waste management could play an important role in future studies aiming to improve decentralized sanitation systems.

Keywords

Civil Engineering

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