CHAPTER 10: Economic analysis
10.2 METHODS
System boundaries
Figure 10-2 shows the process flow diagram (PFD) for a potential urine source-separation,
chosen as Chipako and Randall (2020a) showed that the cost of transporting urine to a decentralized treatment facility was cheaper than on-site treatment (for an RO system). The decentralized system is based on the work of (Chipako and Randall, 2020a), which identified eight shopping centers in the Cape Town area from which 7 500 L of urine per week could be collected and then transported to a decentralized facility for treatment. Five different treatment configurations were identified, and the nutrient recoveries and water removals of each treatment configuration were experimentally determined. Regardless of treatment configuration, the urine would be stabilized with Ca(OH)2 and pre- treated with air bubbling before concentration. The operational cost of pre-treatment is discussed in Chapter 5 and Chapter 6.
The system boundary used for the analysis of the different treatment methods, therefore, considers only the treatment process and transport of the final fertilizer product as the inputs for all five different treatment methods would be the same. The treatment methods in this study were also compared to three additional urine treatment technologies: freeze concentration (FC) (Noe-Hays et al., 2021), alkaline dehydration (AD), and nitrification and distillation (ND) (Udert and Wächter, 2012). Five studies investigated AD of urine (Riechmann et al., 2021b; Simha et al., 2021; Simha et al., 2020a; Simha et al., 2020b; Vasiljev et al., 2022), each system operated at various temperatures and used different alkaline substrates. The ND system was designed to be used at the building scale and the AD system at the bathroom scale. This analysis considers urine collection and treatment at a decentralized facility (which was the most suitable for an RO process). Although this is at a different scale to what ND and AD systems were designed for, it was still deemed important to include these technologies in the analysis for comparison purposes.
The five AD studies all had comparable water removal, however, the nitrogen recovery and content in the final product varied between studies due to the different operating conditions. Riechmann et al.
(2021b) operated their AD system at ambient temperatures rather than at 50-60 °C (Simha et al., 2020a).
Nitrogen recovery achieved during short-term (3-4 days) laboratory experiments was high (97%), however, this was not the case during long-term(10-20 days) pilot-scale experiments where N-recovery was only 20%. This process still requires further optimization and for this reason, it was excluded from the fertilizer market analysis. An average N-content from the remaining four studies was used in the market analysis.
Phosphorus was excluded from this analysis as some treatment methods recover P separately as calcium phosphate (RO, RO-FC, RO-EFC, NF-RO) during the collection phase, other methods recover P as struvite (ND), and with other methods, the P is recovered in the liquid (FC) and solid (AD) products. It would therefore be difficult to make an equivalent comparison. However, the P recovered in the stabilization step could be added to the final product to meet the desired P concentrations. However, as
the P is recovered as a solid it would need to be digested first and then added as an aqueous solution at an additional expense. This aspect was not considered in the economic analysis.
Figure 10-2: Process diagram for a potential urine source-separation, transport, and treatment system, where the red line indicates the boundaries for the system investigated in this analysis, adapted from (Chipako and Randall, 2020a).
Mass and energy balance
Process flow diagrams for each treatment option were developed (Appendix G) and from this mass and energy balances were conducted. The energy balance considered the energy required for pre-treatment (air bubbling). Energy is reported as kWh (electrical energy) required to treat 1 m3 of fresh urine. A summary of the assumptions used for the mass balances is available in the appendices. The energy requirements of the lab-scale equipment are not an accurate reflection of a full-scale plant and therefore the energy requirements for RO, NF, and freezing were calculated using the energy requirements of full-scale systems obtained from the literature. The energy requirements for an FC process (with varying water recovery) were calculated in this study rather than using the energy requirements reported by Noe-Hays et al. (2021) as this is a theoretical value and not realistic of a full-scale FC treatment process.
The energy requirements for AD (Simha et al., 2020a) and ND (Udert and Wächter, 2012) were reported by the authors of those studies and are presented with the calculated energy requirements for the treatment configurations investigated in this study.
Fertilizer sales value
The two markets for fertilizer are typically commercial agricultural use and household fertilizer use.
The sales price for niche liquid and dry fertilizers was calculated using a least square regression analysis of the fertilizer sales price as a function of nutrient content (by weight) according to (Chipako and
Urine collection Urine
Ca(OH)2 Calcium
phosphate
Air bubbling
Calcium carbonate
Treatment process
Transport Liquid fertilizer
Wholesaler x
Transport
Product distribution
Resource recovery facility Shopping
centres
Legend Input Process Transport
Output Production stage
included those designed for use on ornamental vegetation, as well as vegetation intended for human consumption. All prices for niche liquid fertilizers were collected from the Stark Ayres Nursery in Rondebosch, Cape Town (18 August 2022). This ensured a similar pricing comparison was used.
A comparison of the UBFs to commercial fertilizers was based on an N-cost analysis as this is the main component of all the UBFs. The bulk fertilizer (25-50 kg bags) value was calculated based on a farmer using a 50/50 split of synthetic urea (R32 kg-1, (ChemLab Supplies, 2022)) and limestone ammonium nitrate (LAN) 28% N granular fertilizer (R23.57 kg-1, (Tack’nTogs, 2022)). The price that the urine- based fertilizer (UBF) can be sold for (in bulk) was based on the average Rand per kg-N that the synthetic fertilizer is sold for. For the AD fertilizer product, an average N-content (based on the compositions from four studies) was used to calculate the bulk price. A summary of the assumptions and values from the literature used for these calculations is available in Table G-2.
Niche fertilizer market
Gross fertilizer sales were calculated based on an input of 7 500 L per week as a case study for the Cape Town region. The volume and sales value of each fertilizer varied based on the treatment method and nitrogen content. A sensitivity analysis was conducted that compared gross fertilizer sales as a function of the niche fertilizer market size. Fertilizers not sold at niche prices were assumed to be sold at bulk prices. The system boundary was fixed to an input of 7 500 L of urine per week from which each treatment method produced a different amount of final product. The maximum revenue possible occurs if the total volume of fertilizer produced is sold at niche prices, after which, maximum revenue is fixed by the volume of fertilizer produced even if the niche fertilizer market would have allowed for more product to be sold. For the AD process, it was assumed that the mass of fertilizer produced from 1 L of urine was fixed regardless of operating conditions.
Bulk fertilizer market
Fertilizer is typically applied by calculating the kg-N required per hectare and then the appropriate amount of fertilizer is applied based on its N-content (Heyns, 2016). In this analysis, a basis of 1000 kg-N was used. As an example, a typical nitrogen application in a mature vineyard is 50-60 kg-N ha-1 (AWRI, 2010). An application of 1000 kg-N would therefore be sufficient for a 20 ha vineyard (which would be considered a small farm), however, maize requires approximately 100 kg-N ha-1 (FERTASA, 2016) and therefore 1000 kg-N would only be sufficient for a 10 ha farm. A sensitivity analysis was conducted to determine how the cost of transporting the fertilizer to the farm (as a percentage of the gross fertilizer value) varied by distance travelled. Liquid fertilizers are typically transported using tanker trucks and then stored in fertilizer tanks on the farm (Heyns, 2016). Granular fertilizers would
be transported and stored in bags. It was assumed, based on the volume of fertilizer required, that it would be transported using either a 4, 8, 14, or 18-ton truck. For the AD system, the transport costs were calculated based on the average N-content of the fertilizer produced in the four studies. A list of assumptions used is available in Table SX.