Anaerobic processing of and nutrient recovery from source separated human urine

Abstract

Due to the increase in world population which is around 7.7 billion in early 2019 and it was estimated to increase to 9.0 billion by 2050, the stress on the available resources of water, food, energy, etc. increases as well. This enormous increase in world population will put the mankind under a critical challenge related to resource security. Part of the Millennium Development Goals as well as Sustainable Development Goals are to reduce the degradation and provide a sustainable environment that the current and next generation can live in with an adequate and healthy resources to insure the prosperity of mankind. To meet the increasing needs for resources, the needs for solutions to overcome resource depletion had been increased too, and alternative resources must be found beside those that exists to maintain permanence and sustainability of these resources. It was assessed that the need for food, water and energy will show an increase of 35, 40 and 50% respectively, owning to that increasing demand of the increased number in world population by 2030 which will be 8.3 billion. Ecological Sanitation or shortly named ECOSAN is a new management concept for domestic wastewater that based on separation at source of generation. According to ECOSAN approach, domestic wastewater can be divided into three streams as grey water (all wastewater generated in household except that one originating from toilets), yellow water (human urine) and brown water (mainly feces and flush water). Human urine is known as a nutrient rich solution, and highly saline with quite considerable amount of organic matter. Human urine consist of 80% nitrogen, over 50% of phosphorus and potassium. Separation of human urine from the rest of the domestic wastewater will enable closing the nutrient loops in domestic wastewater. Several pieces in the literature studied the possibility of recovering nutrient from source separated human urine using different processes. Struvite precipitation, stripping absorption and ion exchange/adsorption are among the available processes studied in the literature. Ion exchange is one of these processes that showed a remarkable recovery of nitrogen, phosphorus and potassium from source separated urine. The studies about the organic matter fate in source separated human urine after employing ion exchange for removal/recovery of nutrients was not reported yet in the literature. This work is aiming to investigate the removal of organic matter from the liquid residue of ion exchange process using fixed bed clinoptilolite columns by suggesting the use of anaerobic process. Different configurations based on the combination between ion exchange and anaerobic processes was investigated to achieve the best results of concurrent nutrient and energy recovery. Part of the investigation is the adaptation of anaerobic granular sludge that was brought form confectionery wastewater treatment plant to a highly saline solution like urine. Natural fresh and mainly stored urine were used in this work. This work investigated the possibilities of ammonium release from anaerobic processes and its effect on the selection of the experiment configurations. Fixed bed clinoptilolite column was used for ion exchange process and Expanded Granular Sludge Bed (EGSB) reactor was used for anaerobic process. Human urine was collected from separation system the separates urine from men's toilet at the Department of Environmental Engineering in Istanbul Technical University. The results revealed that the adaptation of anaerobic granular sludge was successful under very diluted fresh urine solution in the feeding with COD removal efficiency of 75%. While under higher fresh urine concentration in the feeding the COD removal reduced to reach 40% with 65% fresh urine. The release of ammonium was monitored at the adaptation with fresh urine and it was observed that the percent of release was not appreciable with maximum of 6% release only. The poor performance of anaerobic sludge adaptation using fresh urine as its feeding solution was attributed to the increased level of ammonium and salinity. Urine was stored to increase the amount of ammonium as urea in human urine will hydrolyze during storage. The results from urine storage were in line with previous studies specifically in terms of nutrient concentration, pH and electrical conductivity. During this work a considerable reduction of COD concentration was observed through long storage period of about 4 months that counted for almost 65% of COD reduction from its initial state. This observation was not reported by any of the previous studies used natural stored human urine. COD reduction through storage has an important impact on anaerobic processes as the amount of organic matter in the feed is expected to be lower. On the other hand, the reduced COD concentration will be beneficial for protection of the environment. After hydrolysis was completed, clinoptilolite was used to concentrate nutrients from the stored urine through ion exchange process. The results from this stage was in line with previous studies used ion exchange to remove and recover nutrients from human urine. 80% removal of ammonium from liquid phase was obtained with 99% and 70% of removal for phosphorus and potassium, respectively. It was observed that COD was removed during ion exchange process with a removal efficiency of 25 – 35%. This observation has an influence on the use of anaerobic processing for removing organic matter from the liquid residue of ion exchange process, in which lesser amount of organic matter will be present in the feeding solution. Stored urine in which nutrients had been removed then was used as a feeding for the EGSB reactor. COD removal efficiency was ranged between 60 – 85%. Under 50% stored urine in the feeding solution COD removal was observed to be the best with 85%. Regarding the use of 100% stored urine in the feeding the removal efficiency was reduced to 60%. Through these stages the salinity level had a major impact on COD removal efficiency. The quality of the EGSB reactors with stored urine as a feeding solution was evaluated for the sack of environmental protection in case the effluent was discharged without further treatment. The results revealed that the effluent of EGSB was still has a considerable amount of nutrients and COD, thus ion exchange employed with stage wise manner and variable initial loadings. The results of the stage wise operation aid to reduce ammonium, phosphorus and COD considerably that the discharge of the effluent to sewer may be possible. About biogas production up on COD removal from human urine, the results were theoretically appreciable and observable with gas counter. Methane was evolved with a range of 0.3 – 0.8 l CH4/day that corresponds to 0.19 – 0.5 l CH4/ l of urine. The effect of salinity on COD removal using anaerobic process was investigated also in this work. Synthetic solution was used to simulate stored urine that was subjected to single stage ion exchange. Synthetic urine was used to create a controlled condition regarding salinity. The results of this experiment indicated that salinity had a considerable negative impact on anaerobic process at high level like 32000 µS/cm. COD removal efficiencies were ranged between 40 – 90% with salinity level between 32000 – 10000 µS/cm. This work suggest that more effort should focus on adjusting the recommended salinity inhibition threshold in the literature. This work shows that the combination between ion exchange and anaerobic processes is possible and nutrients recovery with organic matter removal is achievable, but factors like adaptation, dilution, storage period, best operational conditions, inhibition from salinity and ammonium must be taken in consideration. This study recommends the combination of ion exchange and anaerobic process in the manner that nutrients will be removed at the first place with single stage ion exchange followed by anaerobic processes, then stage wise operation of ion exchange to improve the effluent quality form environmental protection. Recovery experiment were conducted under two different contact times, 5 and 300 min in an attempt to mimic two different irrigation type. The results revealed that most of the nitrogen and phosphorus could be recovered from the clinoptilolite surface. The results were in line with previous studies. COD was not recovered from the clinoptilolite surface. Potassium was recovered with a very limited percentage which is in contrast to previous research that reported no potassium recovery at all. Plant experiments conducted to show the effectiveness of nutrient enriched clinoptilolite as a fertilizer using pepper and tomato. The results showed that clinoptilolite had a considerable performance as a n alternative fertilizer compared to synthetic fertilizer that was tested in the same experiment. Plant height, texture and no of fruits that indicate possible fruits in clinoptilolite pots were higher than that one of synthetic fertilizer.