/sub>solution absorbed in DWH evaporated faster than when it was contained in a beaker. The dried DWH became hard as the precipitates accumulated. Under SEM, the precipitated salt appeared to fuse into either a stiff layer covering the pore wall surface or amorphous solids lying on the outside surface of the material (Figures 3(a) and (b)). Hardly any independent crystals were shown held in the pore cavity or the channels.
KH2PO4 has a specific density of 2.338 g·cm−3. Its maximum storage in DWH was expected to be 15.15 g·g–1 at 64% RH, 31˚C, regarding the adsorbent porosity. In the experiment, each of the specimens received 335 ml of 10% KH2PO4 solution during 12 sunny days, undertaking 3 cycles·d–1 on average. Upon saturation, the mean storage of KH2PO4 was about 11.27 g·g–1 (Table 1), which realized approximately 74.4% of the assumed capacity. No significant difference was determined between EWf and OWf when analyzed using chi-square tests (p > 0.95).
The present results demonstrated the feasibility and effectiveness of using DWH for DS recovery through absorption/evaporation. This innovation can be expected to be useful in the recycling of some dissolved hazardous materials. Meanwhile, the results helps explain why floating water hyacinth straws absorb more phosphorus than the fully hydrated counterparts . Physical adsorption and surface precipitation are regarded as possible mechanisms with respect to the observed phenomena and
Figure 3. SEM photos of dried water hyacinth petiole tissues. (a) An oblique section covered with precipitated KH2PO4 salt. Arrow indicates an area without the salt; (b) A cross section showing a stiff layer of KH2PO4 coating the pore walls; (c) A longitude section showing pore walls deposited with urine paste (lower arrow) and salt crystals (upper arrow); (d) A cross section showing one-cell thick pore walls.
Table 1. Weight change in DWH adsorbed with salt from a 10% KH2PO4 solution.
by reference to metal biosorption in wheat straw and grass .
3.3. UDS and Urine P retention in DWH
Figure 4 illustrates result of the pilot experiment on UDS retention. An inverse relationship occurred between the values of UAF and the amount of UDS accumulated in the adsorbent (reflected by Wd). The end product weighed 1.65 g (30˚C, 64% RH) on average, gained ~65% compared with the initial. The total reduction in UAF was ~48%, while the difference in net urine absorption (Wu) was ~17%.
The experiment was halted on day 7 due to humidity accumulation in the adsorbent. The presence of both hydrate salts and complex compounds from the urine had affected . Apparently, the pore structure played a functional role in liquid absorption, while the physical properties of the precipitates determined the efficiency
Figure 4. Urine absorption and urine TDS adsorption behaviours in DWH. Wu: Urine absorbed (g); Wd: Dried weight of the specimen (g).
and effectiveness of adsorption as well as the other well known factors. Figure 3(c) shows an SEM image of a UDS-DWH specimen. The pore walls are densely covered with diffused urine paste. The cubic particles represent, most possibly, the crystals of sodium chloride. No denaturation of the plant tissue was observed.
In normal adult humans, the value of urine specific gravity ranges from 1.001 to 1.030 . Accordingly, the herein total mean urine absorption by DWH was 18.78 ml·g−1 on average, with ~3.46% UDS retrieved.
Table 2 presents NPK levels measured for the UDSDWH specimens. P and K increased simultaneously with the amount of urine absorbed. Although the net amount of TN might have increased in the DWH regarding weight enhancement of the adsorbent over the time, it was still much smaller than the amount expected theoretically. The DWH’s capacity of urine absorption declined from an initial 2.73 L·kg−1·d−1 to 0.68 L·kg−1·d−1with a requirement of material change in about 25 effective days. The final P(P2O5) level in the adsorbent was shown to be 3.14%, suggesting a satisfactory P recovery during the process.
Amino groups on the surface of a cellulose-based adsorbent played an important role in phosphate removal from solutions . The carboxyl group of the cellulose in dried water hyacinth was also considered to be relevant . In addition, the dry matter of water hyacinth petioles contained about 2.13% Ca and 1.69% Mg , which would also accelerate P adsorption by forming insoluble hydroxyapatite and struvite, i.e., Ca5(PO4)3(OH) and NH4MgPO4.
Dried water hyacinth displayed significant K loss when placed in biogas fluid in contrast to the constant P accumulation [1,2]. In the present experiment, however, the material was shown to be attractive to both elements, implying the involvement of different adsorption mechanisms. Although nitrogen makes up ~20% of UDS , it becomes decomposed into ammonia gas (NH2) on exposure to air. This explains why both TN level and UDS accumulation in the present DWH samples were lower than mathematical expectations.
The experiment was suspended on day 6 as it began raining. Things came back normal about one week later. It took, however, 20 days for the absorption of other 8 L of the urine (Table 3). An average daily treatment was realized to be ~1.1 L·urine·kg−1 DWH during the effective days, and the maximum adsorption was ~25 L·urine·kg−1 DWH.
It is worthy mentioned that a continuous direct sunlight exposure could have helped killing most forms of microorganism in the UDS-DWH. In addition, the ultraviolet radiation and infrared heat are known to be effective in the degradation of micropollutants . The high sodium
Table 2. NP(P2O5)K levels in the present UDS-DWH randomly sampled during experiment.
Table 3. The amounts of urine continuously absorbed by DWH during experiment.
level raised by the urine deposit would also inhibit microbial growth in UDS-DWH.
3.4. Field Application
The male toilet involved in the field application of this research is currently used by two or three people during the weekdays and 8 - 10 in the weekend. Since average urine production in adult humans is about 1 - 2 L·d−1 , the device is expected to serve well in about 60 effective days according to the herein revealed experimental results. Basically, it is economic and easy to handle in practice.
Human UDS contains high concentrations of plant nutrients. Urine P provides a soluble, plant-available form . There are several ways to use UDS-DWH as a P(K)-rich fertilizer. For example, the straws can be submerged in rice water in a weight ratio of about 1:100 - 1000 for direct spraying on the foliage of house plants. On the farm, they can be put in rain water for vegetable spraying to encourage prolific flowering and fruiting. Another research project is in plan to collect and provide information on fertilizer use of the UDS-DWH product.
This work demonstrated the feasibility of utilizing water hyacinth waste to convert source-separated human urine into a P(K)-rich fertilizer. The present end product of UDS-DWH contained up to 3.14% P2O5, which is commonly 0.075% - 0.164% in fresh human urine, ~0.4% in dried water hyacinth and ~3% - 8% in phosphate rock regarding the fraction available to plant . The urine absorption in DWH was shown to be in a ratio of 25 (L) to 1 (kg) during 25 effective days through a continuous absorption/evaporation cycling. Compared with other intensive techniques concerning urine nutrient recovery, the present device is economic and convenient to operate in small farms.
In spite of its tremendous application in waste water treatment, water hyacinth proliferation is considered to be a significant economic and ecological burden to many sub-tropical and tropical regions of the world. In China, 10 s millions RMB a year are spent on removing the plants from infested water bodies, the waste transportation and landfill. Meanwhile, direct discharge of urine into water bodies in some rural areas has made eutrophication worse and encouraged the spread of water hyacinth. In developed countries and areas, human urine typically contributes around 80% of the nitrogen, 50% of the phosphorus, and 90% of the potassium in the total nutrient load arriving at a treatment plant . In conclusion, this proposed simultaneous utilization of both water hyacinth waste and source-separated human urine will benefit the environment from a wide range of perspectives.
This project was part of the Science and Technology Innovations Programme, executed and financially supported by the Fujian Academy of Agricultural Science. The authors are grateful to the Central Laboratory, Fujian Academy of Agricultural Sciences, for assistance on SEM and chemical analyses; to Xiamen Greenwave Biotechnology Co. Ltd. for assistance in preparing the DWH materials and to Mr. X. Gao for providing the site at #3 Dongshan Guest Farm, Xiamen, to promote the present methodology innovation and its field application.