Effect of Greywater on Plant Growth
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Published: Thu, 07 Jun 2018
Water availability in South Africa is integral to the economy, but South Africa is a water scarce nation. An alternate solution for household waste water, excluding toilet waste commonly known as greywater is to use it for irrigation in rural community gardens. This is likely to decrease the stress on the current potable water supply and simultaneously improve food security. Indigenous African leafy vegetables are a staple diet throughout Africa. A viability trial highlighted three out of six African leafy vegetable species; Amaranthus terere, Corchorus olitorius and Cloeme gynandra. Two treatments were used as suitable for trials of germination and growth under irrigation of tap water and greywater. Greywater treatments throughout the species decreased germination and seedling height was diminished. A. terere was the most robust to both the treatments as well as weather variability. Continuous investigation is needed to address the water scarcity and subsequent food insecurity.
Keywords: greywater, irrigation trial, African leafy vegetables, germination and growth
Water scarcity in South Africa is an issue that requires robust discussion and debate. If not addressed, it is likely to have serious consequences for both economic growth and the country’s population (Momba et al., 2006). Agricultural industry constitutes 12% of South Africa’s GDP. Even though this sector is decreasing, it is still water intensive. Without aviable water source, economic input in this sector is likely to have ramifications on the country’s health (Morel and Diener, 2006). Statistics show 65% of the country receives less than 500mm of rainfall per annum (Schulze, 1997). The level of water insecurity places pressure on the existing water resource for irrigation. This shortage is felt disproportionally by small-scale subsistence farmers and community gardeners.
South Africa is one of twelve countries that have safe drinking in the world and is ranked third in this group. However, there are many rural communities with under-developed water supply systems or these communities lack access to potable water sources (Momba et al., 2006 and Mackintosh and Colvin, 2002). While the need is great these small communities, they account a small percentage of the customer base. This then fuels the vicious cycle of supply and demand. As a result alternative water sources need to be acquired to satisfy the demand.
Grey water is likely to be a viable prospect to efficiently mitigate this deficiency (Alcamo et al., 2000). It consists of domestic waste water excluding toilet waste. The use of potable water is not needed for all consumptive practices, example irrigation (Alfiya et al., 2011). The main objective of finding alternative and sustainable water usage is to attain water security. Water insecurity is highly interlinked with food insecurity (Al-Jayyousi, 2002 and Blaine, 2012). Thus the use of greywater for small scale agriculture has the potential to address both water insecurity and food insecurity (Rodda et al., 2011). They are most usually harvested from the wild. This practice is a threat to the continued survival of these plants Cultivating African leafy vegetables would also address their conservation need (Momba et al., 2006).
Indigenous African leafy vegetables are a part of the staple diet in South Africa (Momba et al., 2006). The challenge is to continue production of these vegetables without jeopardizing potable water supplies, but by utilizing alternate water source such as grey water as a means of irrigation.
The concept of grey water had both advantages and disadvantages (Rodda et al., 2011). Reducing stress on the potable supply is a main benefit but there are drawbacks to using waste material to grow plants, households have different proportions of additives, thus may effect plant growth (Roesner et al., 2006). Whereas the risks are divided into three main categories; possible detrimental effects on the environment which decreases the ability for soil to provide plant growth, subsequent effects on plant growth and yield, and risk to human health (Rodda et al., 2011).
The aim of this study was to determine whether irrigation with grey water had an effect on seed germination and seedling growth, and whether this effect differed with detergent formulation. The objective was to assess if grey water can replace potable water for irrigation of indigenous plants. It was predicted that seedlings under tap water-irrigated conditions would have a greater growth rate than under greywater conditions. It was further predicted that rate of germination would not be affected by the grey water.
Materials and methods
This investigation took place in 2 parts. This first was to assess the viability of the seeds and to select the species for further investigation. In the second, seed germination and seedling growth under grey water and tap water treatments irrigation were evaluated.
Germination trials were performed in the laboratory in the Biology Building at UKZN (Westville Campus).
An initial experiment was conducted with six species (Solanum nigrum, Amarathus terere, Corchorus olitorius, Solanum villosum, Amarathus dubois and Cloeme gynandra). Germination was tested. The most viable 3 were chosen to determine the germinability of the three selected species of African leafy vegetables. Only viability was tested for as this was pertinent to the success of the actual trial. The viability criteria were the speed at which germination took place. This indicated the viability of the seeds and validates the ability to germinate under controlled conditions.
The germination viability trial was conducted in the laboratory. Each of the three species (Cloeme gynandra; Amaranthus terere; Cochorus olitorius) had six replicates of ten seeds each. Seeds were placed randomly on filter paper in a Petri dish and a smaller piece of filter paper was placed over. They were watered with deionised water until moist. An equal number of seeds were placed under illuminated and dark conditions. These were then monitored every 24 hours and replenished with deionised water as necessary. Once germination had occurred and the radicle was greater than 1 cm, seedlings were moved to the left side of the Petri dish. This prevented recounting and recording. Percentage germination was recorded.
Description of Species
The initial viability trial revealed that the following three Kenyan species were the most viable. C. gynandra is commonly known as spider plant. It is used as a component of a high fibre diet and, from indigenous knowledge, has medicinal properties (Mauyo et al., 2008). A. terere is another widely grown consumable in East Africa (Nabulo et al., 2011). The final species used was Corchorus olitorius, Jew’s mallow, a dark green leafy vegetable high in protein which is consumed in most African communities.
Trails of irrigation with greywater and tap water were then performed in the Biology greenhouse at UKZN (Westville campus).
Synthetic greywater (10 l) was made up freshly weekly (Table 1). It was stored in the cold storage to impede bacterial and algal growth.
Detergent products used to generate the greywater were representative of solid or powder detergent products typically used in lower income households, which are those most likely to benefit from the use of greywater for irrigation of subsistence crops. The flour, nutrient broth and cooking oil were used to represent carbohydrates, salts and proteins, and greases respectively in the synthetic greywater.
Seedling trays (6) were filled with Berea red soil. For three days prior to planting, the seedling trays were watered with tap water and greywater respectively until they were saturated to field capacity. The seeds were then planted into seedling trays. Species were randomized per tray. Sixty seeds of each species were watered with tap water and the other sixty seedlings were watered with the synthetic grey water. For the first 14 days, trays were watered every 24 hours. Each seed was hydrated with 0.25 ml of either synthetic grey water or tap water. Thereafter, trays were watered every second day for the remainder of the trial. The experiment was repeated three time under three treatment groups; the first treatment group was tap water for germination and subsequent growth, tap water for germination and then greywater for growth and the final treatment of grey water throughout the lifespan of the plants. Height was measured weekly. Productivity was measured by destructive harvesting (dry mass production) at end of experiment. However plant height was gauged growth during the experiment.
On two occasions there was death of seedlings due to severe weather conditions and this restricted the growth period. Since this investigation was over a short time span. The weather impacted the progress of experiment. Weather variability such as intense heat, humidity and berg winds, and strong rains affected the seedlings. Even though they were protected in the shade house, the extreme elements could have inhibited their germination and growth.
The data were analyzed using SSPS version 19. Two sets of statistical analyses were performed. The first test was to show the difference between greywater and tap water in terms of growth (height). A Kolmogorov-Smirnov test was performed to test for normality. Levenes test for Equality of Variances was performed, the assumption homoscedasticity was violated but all other assumptions were satisfied. Since the data was not normally distributed a more robust Mann-Whitney U test was done, to evaluate the differences in germination for each species under the two conditions (greywater-irrigated and tap water-irrigated). An excel graph was then used to show the rate at which the all three species comparatively germinated in terms of the two treatments (greywater-irrigated and tap water-irrigated).
Seed germination and seedling growth are gauged by the germination totality and seedling height measured weekly. Initial germination was 70% in tap water-irrigated seeds whereas as 45% in greywater-irrigated seeds.
Figure 1 shows the totality of germinated seeds present over time for each of the three species. A. terere and C. olitorius had the highest totality under controlled laboratory conditions; C. gynandra seeds had the lowest survival percentage >40%. A. terere has the highest standard deviation, indicating the data is wide spread.
Figure 2 the initial and final number of seedlings present per species and the treatment. Co. olitorius under the grey water treatment had the least number of seedlings that survived. This species also had the greatest difference between the treatments. A. terere had the greatest number of individuals that survived in both the treatments.
The results of the Mann-Whitney U test rejected the H0 that there will be no difference between the two treatments, there is a significant difference between height of the three species per treatment. Therefore the distributions of height for each species across the treatments are different. Plant height differed significantly among the treatment (p < 0.05). Greywater -irrigated seedlings consistently attained a lesser height than tap water-irrigated seedlings across all species (Figure 3).
Figures 3 indicate the difference in height between tap water irrigated and greywater-irrigated plants for each species. The standard deviation is shown as an error bar. Seedling height 18days after germination was lower in grey-water irrigated plants than in tap water-irrigated plants for all 3 species. A. terere had the largest standard deviation of tap water-irrigation with 10.197 whereas the greywater-irrigation treatment was 9.1197. C. olitorius which had a visibly lower standard deviation than A. terere tap water-irrigated treatment was 5.753186 and the greywater-irrigated treatment was 1.558646. Finally with the lowest standard deviation, C. gynandra tap water-irrigated treatment was 0.588196 and the greywater-irrigated treatment was 0.316563. C. olitorius had the greatest discrepancy for tap water-irrigated and greywater-irrigated.
Discussion and Conclusion
Africa, according to Morel and Diener (2006) is known as a water insecure continent. As adjustments are discussed on the efficient use of potable water, reusing waste water is seen as a possible solution. Alternative irrigation methods are needed for progress.
Greywater is a possible alternative water source, however contrasting evidence in Morel and Diener (2006) indicates that the potential drawbacks, even though greywater is less contaminated than other waste water. Untreated greywater contains solid particles, pathogens, grease and oils, salts, and chemicals. According to Rodda et al. (2011) these impurities could have negative effects on soil quality, ground water supply and human health.
With such strong findings there are studies that have shown greywater reuse as a viable alternative to 100% potable water. Greywater has been implemented a cost-effective means to reduce domestic water levels. According to Morel and Diener (2006) greywater reuse produced average yield, with decrease in water usage and fertilizer requirements. In both Cyprus and Israel domestic water used was reduced by effective greywater management schemes (Moral and Diener, 2006).
In this study, greywater-irrigated seeds and plants consistently yielded poorer germination (Figure 1 and 2) and growth (Figure 3) of three species of African leafy vegetables. The germination trial (Figure1) yielded a higher percentage of germinated seeds than the outdoor irrigation trial. This is possibly due to the controlled, pathogen-free environment in the laboratory. Cited by Pinto (2010) experiment alternate watering regimes of potable water and greywater resulted in the growth of the plants very similar to 100% potable water. This is a means to mitigate the soil health risks related with greywater reuse. Even though Pinto (2010) had no significant change of plant biomass in the control and treatment, it differed in this investigation.
Figure 2 indicates that A. terere were unable to acquire a high germination percentage in grey-water irrigated treatments but acquired the highest tap water-irrigated germination percentage. Hence the treatment of greywater-irrigated seeds affected their ability to germinate, with initial germination at 70% in tapwater-irrigated seedlings whereas as 45% in greywater-irrigated seedlings. The best germination in greywater-irrigation was observed by A. terere, possibly reflecting its resilience under a wide range of conditions as mentioned by Nabulo et al. (2011). Cl. gynandra had an average of ±7% greater tapwater-irrigated seedling germination than greywater- irrigated seedling germination. Conversely Co.olitorius had the greatest variability between tapwater-irrigated seedling germination than greywater- irrigated seedling germination. Since a significant difference was calculated, greywater does effect the germination of seeds and subsequently the amount of germinated seeds able to grow.
A possible factor in poor survival of both tap water- and greywater-irrigated seedlings, in addition to weather conditions, is nutrient depletion. Berea red sand had a composition of 62.68% SiO2 which is generally used and is nutrient poor (Okonta and Manciya, 2010)
Since a watering regime observed, nutrients to the plant was not considered. Other nutrients found in soil are needed for healthy growth. Seedling trays were used to separate species and keep difference treatment uncontaminated but after the 2 week germination period, nutrients are need for plant growth. Each seed had ± 18cm2 of Berea red soil, this soil consists of 12-64% and 15-57% of fine and medium sand respectively (Hamel, 2006). Water holding capacity of the soil is thus diminished due to porosity. This could have exacerbated the depletion of nutrients in the volume of sand thus leading to their inability to withstand weather variability.
Soapy residue may have contributed to poor performance of the greywater-irrigated seeds and seedlings. Mataix-Solera et al. (2011) point out that the detergents in greywater cause soil water repellency of soil. It can be argued that greywater might be an interim solution, but posed long-term effects that might not be easily remedied. Soapy soil could cause hydrophobic soil properties which have poor water hold capacity. This could have hampered the germinated seed’s shoot from emerging through the soil due to the coagulated surface. An alternate solution can be found according to Pinto (2010), where altering water regimes between grey water and potable resembled the results observes in 100% potable water. The pH levels remains similar between water regimes. In household greywater system the proposal ceramic pot filter is used this eradicates the large particles.
Another caveat of this investigation is changing the watering regime. Initially seeds are watered every day until germination which is ± 10days and then changed to every alternate day. Since plants are sensitive to change, the watering regime should be carefully monitored in conjunction with weather patterns. This ensures a smooth transition for the seedlings.
According to Roesner et al. (2006) household waste contains 2500-5000 chemicals which if used as greywater could cause coagulation at the soil surface. More organic products could be used to reduce the amount of chemicals in the greywater (Al-Jayyousi, 2002). Pre-treatment of greywater and limiting its used only to salt-tolerant crops could allow wide use of greywater for irrigation (Al-Jayyousi, 2002). In this investigation germination of all three species was diminished under greywater-irrigated conditions, this being said with calculated changes to the experiment, greywater could possibly be a viable option in the future.
An observation was made during the experiment, refer to appendix image 1 and 2 of A. terere, the leaf colour in greywater-irrigated treatment was lighter than the tap water-irrigated treatment. Image 3 and 4 also exhibit the same phenomenon in C. gynandra. Cultivation in Jordan of different crops yielded a similar observation, this was attributed to the solids and increased salinity of the greywater (Al-Jayyousi, 2002).
Although the results obtained conclusively show that greywater does effect the both the germinability of the seeds and subsequent growth. It is recommended that seeds should not be irrigated with grey water, possibly increasing the percentage of seed germination. Organizations such as the Water research council are investigating innovative ideas to alleviate the pressure on South Africa’s stressed water system.
Primary greywater systems in community gardens should be not be implement immediately rather as in Pinto et al. (2010) a combination of greywater and tap water should be used. This will relieve the possibility of failing crops. Social and environmental sustainability are interlinked which fuels the economy. Water is an integral part life and therefore should be continuously well-managed. Further research is necessary as water scarcity and availability still threatens food security around Africa.
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