Source and central level recovery of nutrients from urine and wastewater: A state-of-art on nutrients mapping and potential technological solutions
Graphical Abstract
Introduction
The demand for a constant supply of fertiliser has augmented in the past few decades because of the industrialisation of farming [1]. Globally, the demand for nitrogen (N), phosphate (P) and potassium (K) fertilisers increases every year by 1.5% [2]. It is forecasted that there will be a shortfall of at least one of the NPK fertilisers in most regions of the world by 2022 [3]. The limited availability of phosphate and the high production price of nitrogen has led scientists to consider alternative routes for providing these essential agriculture nutrients.
Notably, nitrogen is a renewable resource; however, the production of nitrogen fertiliser using the Haber-Bosch process is significantly energy-intensive. Moreover, industrial production of nitrogen fertiliser is dependent on the limited supply of natural gas and given its high and fluctuating price, and it is necessary to consider other production methods. Potassium, so far, has not created a substantial economic impact on farming compared to nitrogen and phosphorous as it has more than 330 years of reserves at its current consumption rate [4]. In contrast, phosphorus is a finite resource and is considered a strategic commodity as there is no substitute for this element in the agricultural production [5]. Australia and the rest of the world are currently dependent on phosphorus from finite phosphate rock ores. These ores are concentrated only in a few countries and becoming costlier day by day. Phosphorus scarcity is likely to threaten cost-effective global food production in the future. Australia has naturally phosphorus-deficient soils [6] and is substantially dependent on imported sources of phosphorus to maintain agricultural productivity. It means that the declining availability of phosphorus will inevitably threaten food production and its value as an export industry for Australia. Therefore, achieving a substantial improvement in the phosphorus balance of global agriculture, particularly in Australia, is essential for potential food production and environmental benefits. In this scenario, an integrated approach that recycles nutrients from multiple sources and finds innovative ways to recover them is highly merited.
Nutrients discharged in the form of human waste (such as urine, faeces and grey water) generated from household and industrial activities ends up in the wastewater which, if not treated to high standards, becomes a substantial source of nutrient release into natural water bodies such as rivers, lakes, and lagoons. The discharge of excessive nutrients due to their ineffective removal/recovery from the wastewater treatment plants (WWTPs) can promote algal growths and the proliferation of other aquatic plants, leading to eutrophication [7], [8]. Studies noted that the build-up of phosphorus in the water distribution system could lead to gradual precipitation of struvite, causing blockages and equipment scaling [9], [10]. To tackle the environmental pollution triggered by wastewater discharge, the European Union (EU) has issued discharge limits for total nitrogen and phosphorus from WWTPs (15 mg/L for nitrogen and 2 mg/L for phosphorus) [11], [12]. Also, in Australia, the South Australia Environmental Protection Authority (EPA) guideline suggests that the total nitrogen discharge from a septic system must not exceed 5 mg/L and this value is 0.5 mg/L for phosphorus [13]. To date, the majority WWTPs around the world do not recover nutrients but mainly remove them from wastewater and minimise the negative impact on the environment. However, the research and policy paradigms have shifted from removing nutrients to their recovery in recent times [14], [15]. This is because industrial and domestic wastewater and sludge are considered to be an untapped source of these nutrients and energy [16], [17], [18], [19]. It is notable that total phosphorus content in human waste (urine and faeces) can supply approximately 22% of global phosphorus demand [20].
Nutrient recovery from human waste such as urine, faeces and greywater can be approached in two ways. (i) tackle them at the source level (i.e., at house, apartments, estate or suburb levels) or (ii) handle them at the central level (i.e. at the WWTPs). Urine contains the majority of wastewater nutrient load, and therefore, there is a significant interest to recover nutrients from urine at the source level. Both centralised and source separation options have their own benefits, bottlenecks and disadvantages [21], [22]. Nutrient recovery from urine at the source level can significantly increase the life of WWTPs assets by minimising the need for nitrification and denitrification processes, resulting in lower energy demand for aerobic degradation of organic matters [23]. However, providing infrastructure at the source level for existing houses, apartments, the estate is a challenging task. For new houses, apartments, estate or suburb, this can be an attractive option; however, cost-effectiveness and suitable applications of nutrients are yet to be established. For central systems such as WWTPs, the nutrients are diluted to a greater extent, and there is a technological barrier to overcome for their recovery. Both areas require technology or process advancement/development, demonstrations and social research before they can be largely adopted into practice.
A large number of review articles on the nutrient’s recovery from different sources of wastewater have been published in the current literature [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. However, the published review papers mostly focussed either on a specific recovery technology [25], [26] or a specific nutrient [24], [26], highlighting fundamentals of the technologies [30], influential factors for nutrient recovery and associated challenges [27], [28], [29], [31] with a very limited focus on benchmarking of nutrients (i.e., N, P and K) recovery technologies, their techno-economic and suitability analysis for WWTPs. To the best of the authors’ knowledge, there is no review article which provides a systematic understanding of mapping of nutrients and their fate in WWTPs together with their recovery using different processes and understanding the source and centralised level feasibility of each technology. Therefore, a state-of-the-art review focusing on the detailed mapping of nutrients and benchmarking of the recovery technologies in-line with their applicability at source and centralised level is essential to understand the scope for developing feasible nutrient recovery technology for WWTPs, which is the novelty of the current review. The current paper aims to critically review the literature on nutrient recovery from urine and wastewater, which include (i) mapping the nutrients from source to centralised levels, (ii) reviewing process/technologies critically for nutrients recovery including their advantages, disadvantages and their suitability, (iii) social acceptance studies, (iv) case studies on land application of urine and urine based fertilisers, (v) techno-economic analysis on currently available technologies/processes, and (vi) benchmarking and qualitative comparison of the reviewed technologies that can be applied at both source and central levels. The paper also discusses the opportunities and challenges for various nutrient recovery technologies and has identified key knowledge gaps and developed a set of future recommendations.
Section snippets
Source of nutrients in human waste
Although human urine accounts only 1% of wastewater entering the treatment plants, human urine is by far the largest contributor of nutrients to wastewater [36], [37], [38], as shown in Table 1. Several studies reported that approximately 80% of nitrogen, 50% of phosphorus, and 60% of potassium in wastewater generally come from urine [39], [40], [41]. In addition to nutrients, Rocha et al. identified a total of 294 metabolites present in human urine [42]. However, the concentrations of these
Nutrient mapping from household to wastewater
There are mainly four streams that form domestic wastewater: (i) urine stream, (ii) faces, (iii) greywater, and (iv) toilet flushing water [59]. Greywater comes from household sinks, showers, tubs and washing machines [60]. In addition to these four domestic wastewater streams, trade wastewater enters to the wastewater system. Toilet flushing water usually does not contain any nitrogen, phosphorus and potassium nutrients. However, all other streams contain these nutrients and create their load
Ammonia stripping
Ammonia stripping is a physicochemical process where ammonia is stripped from wastewater by air/steam. This technology is mainly applicable at centralised WWTPs. A pilot scale ammonia stripping plant was installed in a dormitory at the University of CanTho in South Vietnam and tested for the nitrogen recovery as NH3 from source separated urine, collected from no-mix toilet [80]. The stripping feed pump was run at 10 and 80 L/h flow rate. Another pilot-scale ammonia stripping plant of capacity
Community acceptance
Reshaping human behaviour for a source-separated toilet and applying human excreta in agricultural purpose are sensitive matters. It raises many questions including: (i) how users would respond to source-separated/no-mix toilets, (ii) how farmers feel in using recovered urine for fertiliser purpose, (iii) what concerns farmers would have on using urine as a fertiliser, and (iv) what type of crops farmers would consider to grow using urine as a fertiliser. There have been social studies on this
Case study 1: urine diverting toilets
A urine-diverting toilet is a specially designed toilet, which discharges urine and faces separately. Urine is often considered storing at 20 ºC for 6 months to remove pathogens and recommended to use for almost all types of crops [268]. In Australia, Commonwealth Scientific and Industrial Research Organisation (CSIRO) and RMIT University with the financial assistance of Yarra Valley Water (a retail water utility in Melbourne) carried out a study on the urine-diverting toilet [269]. It is known
Techno-economic analysis
The techno-economic analysis is considered to be one of the important tools for assessing the commercial-scale feasibility of technologies for nutrient recovery for WWTP. The following discussion focuses on the findings of the techno-economic analysis of a few nutrient recovery technologies at pilot-scale investigation.
Lin et al. [283] developed a comprehensive economic and environmental model for assessing the feasibility of ion exchange technologies (Section 4.5) to recover nitrogen in a WWTP
Benchmarking and qualitative comparison among the reviewed technologies
Centralised and source-separated technologies have been benchmarked for exploring their potential applications and performances. Current challenges and opportunities are highlighted in Table 5, while a detailed qualitative assessment is presented later in this section. Both centralised and source-separated systems have several challenges in terms of nutrients recovery, the cost for infrastructure modification and effective managements. The possible presence of pharmaceutically active
Conclusions, current challenges, research gaps and future perspectives
Nutrients present in human urine, if recovered can make significant contributions to continuous and sustainable supply of fertiliser, reduce our reliance on synthetic fertiliser and demonstrate a circular economy approach. This is highly important in the current scenario as food security in many countries is threatened by limited supply of cost-effective nutrients against the growing global population and increased food demand. Nutrients recovery from human urine cannot only reduce our reliance
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financial support provided by South East Water Ltd., Address: 101 Wells St, Frankston VIC 3199101 Wells St, Frankston VIC 3199.Project ID: RE-04437 for the present study.
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