Nutrient recovery from source separated human urine and the treatment of the residual urine with anaerobic processing and ion exchange/adsorption

Abstract

Global population is continuously increasing with a predicted increase to 9.7 billion by 2050. On the other hand, resources are facing with extinction. Sustainable food production greatly depends upon fertilizer production, which has nitrogen, phosphorus and potassium as key elements. Nitrogen, which is abundant in the air, is fixated by Haber-Bosch process, which consumes enormous amounts of energy, to be used in the fertilizer production. Phosphorus, which is a vital element, has limited resources, which is distributed unevenly around the world. The current domestic wastewater management, which adopts mixed collection of wastewater, is based on "treat" and "discard". Valuable materials in wastewater cannot be recovered with the current management practices. Therefore, an alternative way is needed to be generated. Segregation of domestic wastewater into different streams at the source of generation is suggested to get benefit from each stream. ECOSAN is an alternative sanitation concept that claims wastewater is not waste to be discarded but a source to be revaluated. Within this context, each stream is separately collected at the source and is processed for the recovery/reuse of valuable materials. ECOSAN is based on the separation of different domestic wastewater streams into three streams as yellow water, grey water and brown water. Yellow water, which is mainly source separated human urine, is a valuable waste stream in terms of macro plant nutrients (N, P, K). Yellow water constitutes only 1 % of conventional domestic wastewater by volume; however, it contains over 80% of nitrogen, over 50% of phosphorus and over 50% of potassium. This rich nutrient content makes urine a potential source of fertilizers. There are two routes to use source separated human urine as fertilizer in agriculture; (i) direct application and (ii) indirect application. Direct application of human urine as fertilizer is based on the collection of human urine, followed by transport (if required), storage of human urine to destruct pathogens and direct application of stored urine onto soil as fertilizer. Indirect use of human urine as fertilizer necessitates processing urine before intended use. Urine is frequently processed to produce urine-based fertilizers through struvite precipitation, ammonia stripping/absorption and ion exchange/adsorption. Nutrient removal/recovery from source separated human urine was widely investigated in literature. After nutrient removal, the residual liquid phase needs to be handled in an appropriate way as it still contains appreciable amounts of organic matter and nutrients. However, studies on the handling of residual urine are scarce. This study aims the investigation of nutrient recovery from source separated human urine by ion exchange/adsorption on one hand, while investigating organic matter and nitrogen removal from the residual urine by anaerobic processing and ion exchange/adsorption. The behavior of organic matter was closely monitored during different phases of the investigation. Within the scope, urine was collected from two urine diverting toilets and a urinal, and then it was stored for the conversion of urea to ammonium, which is the desired form of nitrogen for ion exchange. Ion exchange/adsorption was employed for nutrient removal from stored urine. The residual urine was processed with anerobic processing and second stage ion exchange/adsorption. Anaerobic processing was suggested to reduce organic matter content of the residual urine and to investigate possible production of biogas. Second stage ion exchange/adsorption was employed to reduce organic matter content of the residual urine and to maximize nutrient removal from the residue. The results of this study revealed that storage was a crucial step not only for urea hydrolysis but also for organic matter removal as between 25% to 39% of COD in urine was removed during storage. Through ion exchange/adsorption for nutrient removal/recovery with the initial loading of 15 mg NH4+/g clinoptilolite, 82% of ammonium and 28% of COD were removed from stored urine. The residual urine still contained appreciable amounts of COD and ammonium, and a high level of salinity for which a special care should be taken. During nutrient recovery, 99% of ammonium and 94% of phosphorus were recovered from the surface of nutrient enriched clinoptilolite upon contact with tap water with the contact time of 5 min in 16 days. 63% of ammonium and 100% of phosphorus were recovered with the contact time of 300 min in 35 days. The COD release from the surface was not considerable for both contact times, indicating that organic matter is not released appreciably from the surface of the clinoptilolite upon contact with water. This is beneficial from the standpoint of pollution prevention when nutrient enriched clinoptilolite is applied as fertilizer in agriculture. Anaerobic granular sludge from a confectionery industry was adapted to highly saline human urine using synthetic urine as feed in attempt to control adaptation conditions. During adaptation the effect of salinity and COD concentration on the removal of organic matter were investigated. During adaptation to high salinity levels at constant COD, organic matter removal efficiency was decreased from 90% to 85% when electrical conductivity was gradually increased from 14000 to 32000 µs/cm, indicating that organic matter removal was not considerably affected by salinity. For different COD concentrations at constant salinity, organic matter removal efficiency was decreased from 83% to 53% when COD was reduced from 2000 mg/L to 750 mg/L, indicating that organic matter removal efficiency was greatly affected by COD concentration. The results showed that selection of the treatment process for residual urine was case specific. Anaerobic processing seems to be a better option for organic matter removal in case of higher COD in residual urine. However, second stage ion exchange/adsorption seems to be a better choice for the treatment of residual urine in case of lower COD concentrations. Anaerobic processing has a potential of a calculated biogas production between 0.2 to 0.46 L CH4/L urine. However, not all of this could be collected under the conditions of the experiments in this work. Second stage ion exchange/adsorption, on the other hand, was advantageous in terms of complete removal of ammonium from residual urine. This study showed that the suggested processing layout was applicable for simultaneous recovery of nutrients and treatment of residual urine. For residual urine, process selection should be evaluated based on the conditions of specific cases to be handled.