CHAPTER 8: Hybrid nanofiltration process
8.3 RESULTS AND DISCUSSION
with stavudine in real urine. These aspects should be investigated further in future work though.
Alternatively, rejection may be influenced by differences in the osmotic pressure of real and synthetic urine caused by the rejection of organics (Pronk et al., 2006b). In general, both the NF90 and NF270 membrane are suitable for the removal of a broad spectrum of PhACs.
Urea purification using nanofiltration
To determine if a purer urea stream could be recovered from urine using NF, the urea, ion, and COD rejection were measured as a function of volume reduction factor (VRF), shown in Figure 8-2. Where VRF is the ratio of the starting volume to the brine volume remaining. Urea typically consists of 37 ± 4% of the TDS in urine (this will vary based on urine composition). For the loose NF270 membrane, urea rejection was low (less than 10%) and decreased further as more permeate was recovered. The rejection rates for real and synthetic urine were in a similar range to those achieved by Pronk et al.
(2006b) using the same membrane. For the synthetic urine experiments, 77.6% of the urea was recovered in the permeate, at an 80% water removal. Flux decline due to fouling was observed for real urine, as such, only 75% of the water could be recovered as permeate. However, the urea recovery in the permeate is comparable with synthetic urine (Figure 8-2B). Rejection of monovalent ions by the NF270 membrane was poor (< 55%) which can be observed by the high conductivity in the permeate (6 – 12 mS cm-1) as shown in Figure 8-2C. Rejection of ions was comparable for real and synthetic urine, the differences in permeate conductivity are due to the differences in initial conductivity of the two streams. The NF270 provided > 70% rejection of organic compounds measured as COD. At a VRF of 4, 46% of the ions, and 78% of the COD was removed. This increased the urea purity from 37 to 56%.
The urea rejection rates for the tight NF90 membrane were higher than for the NF270 membrane and decreased with increasing permeate recovery. These results agreed with those of Ray et al. (2020), who observed a urea rejection of approximately 55% for real fresh urine at a VRF of 1.1. Urea rejection by the NF90 membrane increased significantly (20%) between synthetic and real urine. Pronk et al.
(2006b) hypothesized that this phenomenon could be attributed to the formation of organic matrices that increased the size of urea and thus increased rejection. The NF90 provided high rejection of ions resulting in a permeate conductivity of < 0.6 mS cm-1 for real urine (Figure 8-2C). The rejection of COD was greater than 95%. At a water removal (permeate) of 75%(for real urine), 48% of the urea was recovered, 90% of the COD was removed, and 97% of the ions were removed, resulting in increased urea purity from 37% to 89%. Concentrating the NF permeate stream using SWRO membranes would further decrease the recovery of urea since approximately 14% of the total urea is lost using a SWRO membrane (Chapter 7).
Due to the poor rejection of salts by the NF270 membrane, only further concentration of the NF90 permeate using RO was investigated. However, the NF270 membrane is advantageous as it has a higher permeate flux and lower urea rejection compared to the NF90 membrane. This results in a smaller NF setup being required and increased urea recovery in the permeate. However, the low rejection of monovalent ions (< 55% compared to > 80% for divalent ions) does not significantly improve the urea purity. Replacement of monovalent ions with divalent ions using ion exchange resins would increase their rejection and thus result in a purer urea stream. Hilal et al. (2015a) exchanged up to 80% of the chloride ions with sulfate ions (depending on dose and contact time) in seawater.
Hilal et al. (2015b) further observed that permeate flux using an NF90 membrane increased from 4 to 11 L m-2
h-1 after seawater was treated with ion exchange. An ion exchange pre-treatment step should therefore be further investigated for this integrated urine treatment process.
Figure 8-2: Comparison of the NF270 and NF90 membranes, for both real and synthetic urine in terms of urea rejection (A), urea recovery in the permeate (B), total ion rejection (C), and COD rejection for real urine (D).
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Further concentration of NF permeate using RO
Whilst NF can be used to improve the purity of urine by removing organics, undesirable salts, and PhACs, these membranes do not sufficiently reduce the volume of the stream. Further concentration using RO is therefore still required for transportation of the final product to be economically feasible.
The NF90 permeate from real urine was further concentrated to a volume reduction factor of 2.5 and the urea rejection was monitored (Figure 8-3A). The VRF was limited due to the volume of permeate available and the minimum liquid volume required in the feed tank. The relationship between VRF and urea rejection was then used to predict urea recovery and purity if the permeate was concentrated beyond a VRF of 2.5 (Figure 8-3B). Rejection of urea for both real and synthetic permeate was consistent. This was expected as the majority of organics present in real urine are removed during the NF pre-treatment and can therefore no longer influence urea rejection as hypothesized by Pronk et al.
(2006b). Higher urea rejection by the NF90 membrane for real urine compared to synthetic urine results in lower urea recovery in the permeate and therefore lower overall recovery as the permeate is further concentrated using SWRO. The purity of synthetic urine was also slightly higher than real urine due to
the simplified concentration of organics used in the synthetic urine recipe. It is advised that overall water removal for the hybrid NF-RO system should be limited to 80% since above this, both urea purity and recovery decrease substantially.
Figure 8-3: Concentration of NF90 permeate via SWRO, urea rejection as a function of VRF (A), predicted urea recovery and purity as overall water removal increases.
Figure 8-4 gives a summary mass balance for the tight NF-RO process with an overall volume reduction of 80%. The final concentrated liquid fertilizer would have a urea purity of 89% with an overall urea recovery of 32.7%. In comparison to a different process to recover and purify urea from urine, Marepula et al. (2021) used an evaporation and ethanol purification process and they were able to recover a dry
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y = -0.0734x + 0.8407 R² = 0.9373
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considerable urea in the NF concentrate stream that could also be further treated to recover more urea.
As the NF brine still contains the majority of the organics, further treatment with a membrane process (forward osmosis, membrane distillation, or RO) would likely result in fouling. Further concentration with a process such as evaporation or freeze concentration would be suitable as these processes are not affected by fouling. A process such as the one developed by Marepula et al. (2021) could also be used as there is a much smaller volume of water to remove by evaporation. This would increase the overall urea recovery to 67%. A freeze concentration process, such as the one developed by (Noe-Hays et al., 2021) could also be used. Their process treated hydrolyzed urine and achieved a nitrogen recovery of 92% with a VRF of 5.8. However, ss this process treated hydrolyzed urine, it is unclear what the urea recovery would be if stabilized urine was treated with a freeze concentration process.
Figure 8-4: Summary mass balance for the hybrid NF90-SW30 treatment process assuming an overall water removal of 80%.
Whilst almost 68% of the urea would be lost using an NF-RO treatment process, this process cannot be directly compared to studies that investigated nutrient recovery from hydrolyzed or fresh (non- stabilized) urine as many of these previous studies did not account for N lost through ammonia volatilization during the urine collection or storage phase. An analysis of at least 10 studies using hydrolyzed urine estimated that the N losses due to volatilization could range anywhere from 0 – 44%
(see Appendix E). Nitrogen loss from non-stabilized urine will vary significantly depending on collection methods (urinals/toilets with in-situ collection tanks vs. piped systems) (Udert et al., 2003a), whether collection tanks are sterilized (presence of urease-producing bacteria), storage duration, whether or not storage tanks are sealed (potential for ammonia to escape), and if the pH is adjusted (NH3 is volatile, whilst NH4+ ions are not).
10 L NF90 (tight) 7.5 L VRF = 4
2.5 L
NF permeate
45.8 g urea (89% purity) 48.2% urea recovery 0.65 mS cm-1 232 mg O2L-1 Brine
49.2 g urea 38.3 mS cm-1 23 050 mg O2L-1
Feed 95 g urea 37% urea purity 11.9 mS cm-1 7 280 mg O2L-1
SW30
VRF = 3.75 5.5 L Permeate 14.8 g urea 0.03 mS cm-1 2.3 mgO2L1 Concentrated fertilizer 31 g urea, (85% purity) 32.7% urea recovery 1.5 mS cm-1
870 mg O2L1 2.0 L
Treatment process decision tree
The optimal treatment process of urine to recover nutrients will vary depending on the desired end product and its use. A decision tree is shown in Figure 8-5 that can be used to decide which membrane treatment option to use. Whilst a treatment process with only high-pressure SWRO offers the highest urea recovery, the value and use of the product may be limited as it can only be used as a fertilizer on non-edible plants (to reduce pharmaceutical uptake concerns) with some level of salt tolerance. The final product value can be increased by recovering urea in a purer form; however, it ultimately comes at the cost of urea recovery. Although the loose NF-RO configuration provided better urea recovery (and reasonable pharmaceutical removal) compared to the tight NF-RO configuration the purity was significantly lower (56% compared to 85%). As the aim is to produce a purer urea product the additional treatment cost may not be worth the small improvement in purity. The loose NF membrane is advantageous for its higher permeate flux (Figure E-1) and it is advised that ion exchange resins be investigated to replace monovalent ions in urine with divalent ones to improve salt rejection. A detailed economic analysis would be required to determine feasibility based on the product use and value.
Whilst these scenarios would still need to be tested at a pilot scale level and beyond, this study provides a proof of concept for producing different urea streams directly from stabilized urine.
Figure 8-5: Decision tree to determine the preferred treatment process based on the desired final product.
Recoveries and purities were calculated assuming all streams were treated such that for the overall process 80% of the water was removed. Results were calculated by extrapolating data from this study (NF90 and NF270) and from the RO experiments in Chapter 7. Purity was determined based on urine composition U5. Importantly, purity will vary slightly for different urine compositions. Pharmaceutical rejection by SWRO was assumed to be comparable to (if not greater than) the NF90 membrane based on similar observations by Radjenović et al. (2008).
Will the final product yes be used as a fertilizer?
Will edible foods be fertilized (do PhACs need to be removed)?
Is the plant being fertilized salt tolerant?
SWRO only 73% urea recovery
± 39% urea purity
> 98% PhAC rejection (99% in product) no
yes
yes
Loose NF - RO 52% urea recovery
56% urea purity
>73% PhAC rejection (< 30% in product)
Tight NF - RO 33% urea recovery
85% urea purity
>98% PhAC rejection (< 1.1% in product) no
Does the final product require urea in a purer
form?
no
yes
no
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The advantage of a hybrid NF-RO process is that it recovers urea in a purer form. Other membrane processes such as forwards osmosis (FO) and membrane distillation (MD) would also result in salts and pharmaceuticals being concentrated along with the nutrients such as with the RO process. It is also important to note that as MD operates at elevated temperatures fresh or stabilized urine would undergo chemical urea hydrolysis and therefore can’t be used If the aim is to recover urea. However, NF could be used as a pre-treatment step before FO or MD to also remove salts and pharmaceuticals. It is likely that it would also optimize both processes as the fouling compounds would be removed. And in the case of FO, as the NF permeate has a lower osmotic pressure the concentration of draw solution required to achieve a defined flux would not have to be as high. Ray et al. (2020) compared the cost of ammonia recovery for RO, NF, and FO and found that FO cost 2 to 13 times more than RO and NF (depending on the treatment scenario). Whilst this comparison was for hydrolyzed urine it is likely that the comparative costs would be similar for stabilized urine