Field Utilization of Dried Water Hyacinth for Phosphorous Recovery from Source-Separated Human Urine ()
1. Introduction
Dried petioles of water hyacinth (Eichhornia crassipes) were found useful for phosphorus removal from highly eutrophic water bodies [1,2]. In nature, phosphorus in soil is taken up by plants and then moves through the food chain. It is released back to the land when plant or animal matter decomposes. The cycle has been, however, disrupted as a result of man’s over-exploitation. Now many soils are fertilized using the elements from phosphate rocks. Phosphorus overloading has become a very high profile water quality issue. Meanwhile, the viable commercial reserve of phosphate is being depleted and estimated to run out within 50 - 100 years [3].
Hence there is a growing interest in phosphorus recovery from source-separated human urine for agriculture use [4]. It is estimated that in 2050 the available phosphorus from urine will increase to 2.16 million MT [5]. Various ecosan technologies have been invented to meet the objective [6,7], but most of them are either too expensive, impractical or both for developing countries.
There are other options available for phosphorous recovery from source-separated human urine, e.g., evaporation, precipitation, ammonia stripping and struvite crystallization [8,9]. Among the variety of techniques being studied, evaporation is currently the most feasible approach for agricultural zones, where solar radiation is an abundant energy source while convenient organic fertilizers are heavily demanded.
The air spaces in the petioles of water hyacinth are walled by single layered parenchyma cells, which give rise to a large surface area for fast evaporation as well as efficient moisture absorption and dissolved solids (DS) adsorption. This work focused on the practical aspects of using dried water hyacinth (DWH) as a natural adsorbent to retrieve urine dissolved solids (UDS), phosphorous in particular, through repeat absorption and solar thermal evaporation. Field application was performed at a guest farm to satisfy the goals. Some basic physical properties of the DWH material, such as water absorption capacity, porosity, relative density and the retention of dissolved solid (i.e., KH2PO4), were also investigated as a prelude to studying the subject.
2. Material and Methods
2.1. Material Preparation
The water hyacinth waste was collected from a wastewater treatment lagoon at the Torch Park, Xiamen, China. The petioles, with an average length of 70 cm, were sliced lengthwise using a cutting machine and sun-dried for storage.
2.2. Water Absorption and Evaporation
Water holding capacity (WHC) of the DWH was determined using a method modified from the ASTM D7367 (2007). 1.00 g of the specimen was submerged in 90 ml distilled water (dw) in a 100 ml screw-cap glass bottle and weighed after 2, 6, 12, 24 and 36 h, respectively. They were drained on a sieve for 3 min and weighed to the nearest 0.01 g using an electronic balance. The mean value of three replicates was used to calculate water absorption rate [Equation (1)] as well as the value of WHC [Equation (2)].
(1)
(2)
where Wd, Ww and Ws denote, respectively, the initial dry weight (i.e., 1.00 g), the wet weight and the saturated weight of the material.
The material porosity was estimated following a method of liquid immersion technique [10] with modification. The specimens were oven dried 24 h at 45˚C, weighed, and placed in a 30 cm × 30 cm plastic vacuum bag. The bag was vacuumized for 20 min and then injected with dw to cover the specimen. The immersion lasted for 60 min and the specimen was subjected to an immediate weighing. A dry towel was placed on the balance to collect drained water. The percentage porosity was calculated by Equation (3).
(3)
where Wd and Ww denote, respectively, the initial dry weight and the final wet weight of the specimen. The average volume of 1.00 g DWH was estimated by measuring the volume of water expelled when the specimen was submerged into 80.0 ml of dw contained in a 100 ml measuring cylinder (±1.0 ml). All data were expressed as the mean of three replicates.
The efficacy of DWH on water evaporation was tested in the following procedures. 1.00 g of DWH specimens was cut into about 1 cm long and laid on a piece of filter paper in a plastic sieve. 3 ml of dw was dropped onto the specimen through a 1.0 ml medical syringe to avoid excessive water dispensing. The specimens were then exposed to sunlight for evaporation. On day one (from 9:00 am to 6:00 pm), a total of 12.0 ml dw was consumed in four wet/dry cycles. On day two, the specimen absorbed a total of 10.0 ml dw. The average rate of absorption/ evaporation for the material was shown to be ~11 ml dw g−1·d−1.
2.3. Dissolved Solid Adsorption in DWH
This experiment was carried out to evaluate DS retention by DWH through absorption/evaporation cycling. 3.00 g of the DWH specimens were cut into approximately 4 cm long and placed on a plastic plate. A solution of 10% (w/v) KH2PO4 (AR) was transferred into the specimens from a 5.0 ml syringe. The wet/dry cycle was repeated until the salt precipitates covered the surface of the adsorbents, which were then weighed for the value of the observed final weight (OWf). The expected final weight (EWf) of the KH2PO4-bearing specimen was calculated using Equation (4).
(4)
where WC1 and WC2 indicate respectively the amount of solid KH2PO4 supplied in total and that collected from the plate at the end of the experiment. Wi represents the initial weight of the DWH specimen (i.e., 3.00 g). Data were expressed as the mean ± SD and presented as an average of three replicate.
There were two controls. Control 1 duplicated the above procedures except that the solution of KH2PO4 was replaced by dw. In Control 2, 50 ml of 10% KH2PO4 solution was contained in an open 100 ml beaker for evaporation so that weight change between the solid and the precipitate could be measured.
2.4. Urine Dissolved Solids Adsorption in DWH
The DWH adsorption of urine dissolved solids (UDS) was investigated in laboratory. The specimens were cut into 1 cm long and weighed to 1.00 g (30˚C, 64% RH) in three replicates. Each absorption/evaporation cycle began with the material’s absorption of fresh human urine transferred from a 1 ml syringe. Weighing was performed before and after the wetting. Two layers of filter paper were put under the specimen to absorb excess moisture during the procedure and weighed separately after the specimen’s removal for sun drying. The urine absorption fraction (UAF) was calculated using Equation (5).
(5)
(6)
where Wd indicates the weight of specimen before respective absorptions; Wwp and Wwh indicate, respectively, the wet weight of filter paper and the wet weight of the filter paper and the specimen as a whole; Wu represents the weight of urine absorbed.
2.5. Urine Phosphorus Recovery in DWH
A pilot experiment was performed in the summer of 2011 at an apartment with a large eastern balcony. Five plastic sieves, each had an inside dimension of 6 cm × 338 cm2 and contained ~80 g of DWH, were stacked on top of one another for urine absorption by the DWH. A plastic tray, which contained ~40 g of the material, was put at the bottom of the stacked sieves to absorb excess moisture. The device was placed on balcony to receive adequate sun exposure and air circulation while avoiding being wet by the rain. At the beginning of the experiment, the DWH materials were absorbed with 1.0 L fresh urine in a tray to overcome the effect of surface tension. To ensure an adequate absorption throughout, the order of the sieves was changed by moving the topmost to the bottom each time when a new cycle started. Fresh urine was poured slowly and evenly onto the absorbents from the top of the stack usually upon sunrise in the early morning and the midday. The UDS adsorbed DWH (UDS-DWH) was sampled randomly for total phosphorus (TP) determination. The contents of total nitrogen (TN) and potassium (K) were also measured for relevant discussions.
Field application was realized in March 2012 at #3 Dongshan Guest Farm, Xiamen. The urine was discharged, not in wet weather, onto the DWH via a tube connected to a waterless urinal (Figure 1). 2% dilute
Figure 1. Field utilization of dried water hyacinth for phosphorous recovery from source-separated human urine. A: Male urinal; B: Urine outlet; C: Stacked sieves containing dried water hyacinth petioles; D: Tray for excess urine.
acetic acid was used to remove urine odor and spots from the urinal when necessary. Each of the three stacked baskets contained ~6 kg of the DWH materials. The adsorbents were also laid inside the tray at the bottom.
2.6. Chemical Analyses
NPK contents in DWH specimens were measured using the following methods: Determination of total nitrogen content for Compound fertilizers (GB/T8572-2001, PRC), Determination of organic-inorganic compound fertilizers—Part 2: Total phosphorus content (GB/T17767.2- 2010, PRC) and Determination of potassium content for compound fertilizers—Potassium tetraphenylborate gravimetric method (GB/T8574-2010, PRC).
By convention, TP was expressed as P2O5 concentration in fertilizer and as 
ion concentration in the urine. To convert 
concentration to P2O5, the data were multiplied by a factor of 0.746.
2.7. Scanning Electron Microscopy
DWH specimens adsorbed with either UDS or KH2PO4 precipitates were mounted onto respective metal stubs and sputter-coated with gold. They were subsequently imaged in a scanning mode using conventional SEM techniques (JEOL/EO JSM-6380, Japan).
3. Results and Discussion
3.1. Water Absorption Capacity
Figure 2 illustrates result of the present water absorption experiment. On average, it took about 24 h to saturate a DWH specimen. In the summer (July), the immersion bottle gave foul smell after 24 h, indicating an active microbial growth. Temperature could affect the rate of absorption significantly, e.g., the mean value of WHC was 475% at 15˚C ± 1˚C, but being 648% at 30˚C ± 1˚C. Compared with many other plant materials, the present