Toxic And Essential Plant Elements Biology Essay

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Kenya is a water-scarce country. Per capita available water is about 650 m3 per year. Future projections show that by the year 2025, per capita water availability will drop to 235 m3 as a result of population growth. With increasing competition for good-quality water among different sectors, availability of water for agriculture will likely decline especially in poor urban and arid areas. Water conservation measures therefore, should be practiced in order to sustain water demand. In Kenya, greywater is not adequately utilized probably due to inadequate information about its quality and safety. The present study investigated the levels of plant nutrients (K, Ca, Mg, Fe, Zn, NO3--N and P), toxic metals (Cd and Pb), and the pH values in kitchens, bathrooms, and laundries wash and laundries first rinse raw greywater from Kenyatta University staff quarters. The levels of the parameters were compared with the levels recommended by WHO for crop irrigation and Kenya Bureau of Standard (KEBS) for drinking water. The levels were found to be within the recommended limits. Therefore greywater can have dual usage as source of plant nutrient and for irrigation, thus preserving freshwater and increasing food production hence enhancing the achievement of Kenya's Vision 2030.

Key words: Greywater; plant nutrients; toxic metals; irrigation.

Abbreviations: BGW-Bathroom greywater, KGW-Kitchen greywater, LRGW-Laundry rinse greywater, LWGW-Laundry wash greywater, GW- Greywater

INTRODUCTION

Kenya is a water-scarce country. Surface waters cover only 2% of Kenya's total surface area (MWI, 2007). Future projections show that by the year 2025, per capita water availability will drop from 650 m3 to 235 m3 as a result of population growth (MWI, 2007). Therefore, Kenya faces serious challenges with regard to providing sustainable access to safe water, sewerage systems and basic sanitation particularly in its poor urban and semi-arid and arid areas due to its fast growing population. High water demand, lack of new water resources and the level of competition between different water uses like domestic, industrial and agricultural are expected to increase in the near future. Therefore, new methods of water conservation need to be exploited and especially in areas where large quantities of greywater are produced, such as higher institution of learning, boarding schools, big hotels and lodgings among others where available greywater treatment techniques are limited or none exists at all. Greywater is domestic wastewater that includes water from domestic cleaning operations such as bathing, cleaning dishes and washing clothes (Holtzhausen, 2005; Ahmed et al., 2001). However, it does not include soiled diapers laundry water (Ahmed et al., 2001; WHO, 2006; Wood, 2008). Greywater represents about 61% of the total wastewater streams according to WHO (2006), therefore greywater is the largest flow of wastewater. Greywater is characterized as high volume low strength stream that constitute about 50-80% of domestic wastewater (DHWA, 2002). Greywater contributes to over 70% of domestic wastewater in Kenya (MWI, 2007) and its potential has not been adequately investigated. Its magnitude and effect is quite enormous and could be devastating if not properly disposed off or re-used.

Greywater is no doubt a valuable resource that can be used to alleviate water shortage, and increase water conversation in individual households (Long et al., 2005). Greywater can be used untreated or it can be treated to varying degree to reduce nutrients and disease-causing micro-organism (EPA, 2007). Basically, the human mind tends to perceive water quality as being good if desirable water uses are possible, and bad if water quality involves water use problems (Hulzinger, 1991). Important to the approach of water recycling is the concept of the utility whereby water is used of a quality commensurate with its application that allows the use of large water resources that are not necessarily of the highest purity. Water of the highest quality is only needed for drinking and cooking. Greywater recycling is one aspect of this and relates principally to the reuse of lower polluted water generated within buildings for uses such as toilet flushing or irrigation (Jefferson et al., 1999). Kenyatta University (KU) main campus is located at Kahawa in between Nairobi city and Ruiru town. The main source of water for the university is from Nairobi Water Company and its boreholes. Due to its growing population and the national water shortage, KU has been experiencing water shortage to an extent of buying the same from available sources. High water demand due to its growing population, construction works, and irrigation among others water uses means extra measures are necessary in order to meet water needs. The university with a population of more than twenty thousand people produces an enormous amount of greywater from its premises such as hostels, hotels, residential areas, offices among other sources, which is not utilized in anyway but goes into waste as sewage. Therefore, this resource need be tapped and converted into utilizable resource. For example, it's a waste to irrigate with great quantities of drinking water when plants thrive on used water containing small bits of compost. Based on these problems and on the fact that greywater contains significant amounts of nutrients particularly nitrogen and phosphorus (WHO, 2006) which can be utilized as plant nutrients, the objective of this research study was to determine K, P, N, Mg, Ca, Fe, and Zn as selected plant nutrients in raw greywater, Cd and Pb as pollutants and pH in order to establish its safety for use in irrigation.

MATERIALS AND METHODS

Sampling and sample pretreatment

The study was conducted in Kenyatta university (KU) main campus, Kenya. Kenyatta University is the second largest university in Kenya after University of Nairobi and is located along Nairobi-Thika superhighway about 20 km from Nairobi city centre. In addition to the students, it is inhabited by university workers both teaching and non-teaching staffs. The university population is estimated to be more than twenty thousand people. This study targeted the analysis of greywater from staff quarters. Four types of greywater (kitchen, bathroom, and laundry wash and laundry first rinse greywater) were sampled from selected household except greywater from soiled diapers. A total of 360 samples were analysed over a period of six months. At each sampling site, bathroom and kitchen greywater samples were taken directly from an outlet into clean plastic bottles while laundries greywater were taken from wash basins/buckets in duplicate, and then labeled accordingly. Samples were then taken to Kenyatta University chemistry Research laboratory which is few metres from the collection sites. The pH of each sample was taken and samples for metal and phosphorus analysis were acidified to pH 2.0 with concentrated nitric acid. All samples were then kept under refrigeration.

Analysis of greywater

The analysis was mainly through the use of appropriate laboratory standard methods APHA (1998, 2005) and PYE UNICAM (1970). The greywater samples for metal analysis were first digested using concentrated nitric acid then analyzed separately according to the type using Atomic Absorption Spectrophotometer (Varian spectra AA10, Australia). The greywater samples for phosphorus were digested using sulphuric acid-nitric acid method while nitrate-nitrogen samples were prepared using spectrophotometric method and then analyzed separately according to the type using T80+ UV/Vis spectrometer PG instruments Ltd. The pH was measured using a pH meter (model 290A, USA).

Results and Discussion

The data collected was subjected to SPSS statistical Program. Mean values were determined for each type of greywater and one way ANOVA done to test any significance difference between them. The results are presented in table1.

From the results, the mean pH values of greywater were significantly higher than in tap water (α=0.05, p<0.05). This could be attributed to the presence of dissolved ions from cleaning products like soaps, detergents as well as from body oils, toothpaste among others. However, the mean pH values of laundry wash and bathroom greywater, differed significantly from other types of greywater (α=0.05, p<0.05). The KGW and LRGW did not differ significantly (α=0.05, p>0.05). The pH range for the various types of greywater was 6.36 to 8.78 with an overall mean of 7.82±0.04). Laundry wash greywater was on average more basic than other types of greywater, probably due to ionization of carbonates usually added as builders or water softeners. Laundry wash GW could be appropriate for use in acidic soils because it would help in soil neutralization. The pH values obtained in this study concurs with those reported by WHO (2006), Peter et al. (2002), Raude et al. (2009) and Bhausaheb et al. (2010) for various types of raw greywater.

The main effect of water pH on plant growth is through control of nutrient availability. For example, for most crops, soil pH values of 5.5 to 7.5 are suitable for availability of most nutrients (Muchukuri et al., 2004). Therefore, from the results of the present study raw bathroom GW would be the most suitable for use in growing of most crops since its pH values are within those of the soil reported by Muchukuri et al. (2004). However, according to Tchobanoglous (2003) the concentration range suitable for the existence of most biological life is quite narrow and critical and is typically 6 to 9. Also, the pH values recommended by KEBS (2007) for drinking water are in the range of 6.5-8.5. Therefore, this implies that the four types of greywater analysed in this study can be recommended for irrigation because their pH values are within the required range.

There was no significance difference between the mean levels of lead in tap water and bathroom GW (α=0.05, p>0.05). However KGW, LWGW and LRGW differed significantly from tap water (α=0.05, p<0.05). The concentration of lead in the four types of RGW was in the range of 0.01 to 0.19 mg/L with an overall mean of 0.11±0.00 mg/L. The potential sources of lead in domestic wastewater are household products such as toothpaste, mouth wash, facial cream, powder and liquid detergents, dishwashing products (dishwashing tablets and powders), hair conditioners among others (Tjandraatmadja et al., 2008). Also lead may originate from the plumbing systems. The lead levels from this study are much lower than the maximum concentration threshold of 5.0 mg/L for lead recommended by WHO (2006) for safe use of wastewater, excreta and greywater for crop production. Therefore, it can be concluded that all the four types of raw greywater analysed in this study are recommendable for use in irrigation. The problems associated with Pb to plants for example (In most cases lead blocks the entry of cations (K+, Ca2+, Mg2+, Mn2+, Zn2+, Cu2+, Fe3+) and anions (NO3-) in the root system (Kabata-Pendias and Pendias, 1992). Lead levels should be minimized to the lowest level possible by treating greywater because KEBS (2007) limit Pb to 0.01 mg/L for primary drinking water.

Cadmium was not detected in any of the samples analysed in this study. Therefore, all the four types of greywater could be recommended for irrigation. The average levels of potassium in greywater were significantly higher than in tap water (α=0.05, p<0.05). However, there was no significance difference between the mean levels of potassium in kitchen, bathroom and laundry first rinse greywater (α=0.05, p>0.05). Also the mean levels of potassium in LWGW were significantly higher than the rest of the GW analysed (α=0.05, p<0.05). The mean levels of potassium in the four types of GW were in the range of 2.59 to 28.40 mg/L with an overall mean of 10.83±0.50 mg/L. The possible sources of potassium in greywater include cleaning products such as soaps, detergents among others. These results are in accordance with those documented by Bhausaheb et al. (2010) in raw bathroom and basin greywater, Murphy (2000) for dishwater and bathwater. Since potassium is one of the fertilizer element (others are nitrogen and phosphorus), greywater can be used to supplement this nutrient.

The concentration of calcium in greywater was significantly higher than in tap water (α=0.05, p<0.05). Only laundry wash grey water differed significantly from the other types of GW analysed (α=0.05, p<0.05). The concentration range of calcium ions in the four types of greywater was 8.94 to 50.00 mg/L with an overall mean of 25.53±0.86 mg/L. The calcium levels were lower than the maximum level recommended for drinking water by KEBS (2007) and Will et al. (1999) for irrigation water. Therefore, from the results of this study all the types of greywater could be recommended for use in irrigation. The calcium mean values obtained in this study are within the ranges documented by Murphy (2000) for dishwater and bathwater.

Magnesium ions in greywater differed significantly from tap water (α=0.05, p<0.05). Likewise LWGW differed significantly from the other types of greywater (α=0.05, p<0.05). However, KGW, LRGW and BGW did not differ significantly (α=0.05, p>0.05). The mean levels of magnesium for the four types of greywater were in the range of 1.57 to 13.24 mg/L with an overall mean of 6.31±0.29 mg/L. According to Will et al. (1999), desirable levels of magnesium for irrigation water is 30 to 50 mg/l and KEBS (2007) maximum threshold for drinking water is 100mg/L, therefore greywater analysed could be recommended for irrigation. Therefore, greywater could be used as an effective nutritional supplement for Mg to crops. These results are in agreement with those reported by Bhausaheb et al. (2010) in raw bathroom and basin greywater, Murphy (2000) in dishwater and bathwater.

For zinc, there was no significance difference among all the types of greywater as well as with the tap water (α=0.05, p>0.05). The concentration range of zinc in the four types of greywater was 0.07 to 0.70 mg/L with an overall mean of 0.38±0.02 mg/L. The possible sources of zinc in greywater could be bar soaps, liquid and powder detergents, shampoo, sunscreens, and dishwashing products among other household products according to Tjandraatmadja et al. (2008). The threshold levels of zinc recommended by WHO (2006) as maximum concentration for crop production is 2.0 mg/L for safe use of wastewater, excreta and greywater, while KEBS (2007) is 5 mg/L for drinking water. Therefore, greywater produced in KU could be recommended for use in irrigation since its levels are lower than the recommended/allowed threshold. However, care should be taken since zinc is toxic to many plants at widely varying concentrations, although the toxicity reduces at pH > 6.0 and in fine textured organic soils (WHO, 2006).

There was no significance difference in mean levels of iron in KGW, BGW and LRGW (α=0.05, p>0.05). The LWGW differed significantly from the other types of grey water (α=0.05, p<0.05). All the four types of GW differed significantly from tap water (α=0.05, p<0.05). The concentration of iron in the four types of greywater ranged from 0.26 to 4.04 mg/L and an overall mean of 1.71±0.08 mg/L. Use of iron wool could be the cause of high levels of iron in kitchen greywater, in addition to cleansing agents used. Also according to Tjandraatmadja et al. (2008), iron is a common element in majority of cleaning and personal products. The levels of iron are below those recommended by WHO (2006) for safe use of greywater for crop production. However, it's necessary to note that, iron is not toxic in aerated soils but can contribute to soil acidification and loss of availability of essential phosphorus and molybdenum WHO (2006). Therefore GW studied should be recommended for irrigation uses since iron is only required in trace amounts by plants.

From Table 1 the mean levels of phosphorus in the different types of GW were significantly higher than in tap water (α=0.05, p<0.05). Equally laundry wash GW differed significantly from other types greywater, but the mean levels of phosphorus in KGW, LRGW and BGW did not differ significantly (α=0.05, p>0.05). The concentrations of phosphorus in the four types of greywater were in the range of 0.71 to 15.58 mg/L with an overall mean of 5.56±0.30 mg/L. The high levels of phosphorus in laundry wash GW could be due to the use of more detergents, while in the kitchen GW could be attributed to presence of food particles. The phosphorus levels obtained in this study concurs to those documented by Raude et al. (2007) in the range of 1.2 to 13.1 mg/L; Jerpperson et al. (1994) in the range of 0.6 to 27.3 mg/L with a mean of 8 mg/L while Murphy (2000) in the range of 0.87 to 131 mg/L with a mean of 14 mg/L for dishwater, and < 0.1 to 14 mg/L with a mean of 2 mg/L for bathwater. All the four types of greywater analysed in this study could be recommended for use for crop production because their mean levels were within the recommended range by WHO (2006). Also they would provide this vital macro-nutrient needed by plants for various functions such as photosynthesis, energy storage and transfer among others.

There was a significance difference between the concentration of nitrate-nitrogen for tap water and all the types of greywater (α=0.05, p<0.05). Likewise on comparing the four types of greywater, a significance difference existed between kitchen GW and the other types of GW (α=0.05, p<0.05). However, there was no significance difference between bathroom GW and laundry first rinse GW (α=0.05, p>0.05). Kitchen greywater is usually heavily polluted with food particles which could be contributing to high concentrations of nitrate-nitrogen, while in laundry wash greywater it may be due to presence of soil and faecal particle on the clothes. These results concur to those reported by Peter et al. (2002), Al-Jlil (2009), and Skudi (2011). The levels of nitrate-nitrogen obtained in this study are lower than the maximum concentration for crop production recommended by WHO (2006) and KEBS (2007) for primary drinking water. Therefore, all the four types of greywater analysed in this study should be taken into account in the fertility program in irrigation.

Conclusion and Recommendations

From this study it can be concluded that raw greywater generated in Kenyatta University can be used for irrigation since the levels of all the parameters were within the ranges recommended/allowed by WHO for crop irrigation and Kenya Bureau of Standard (KEBS) for drinking water. Therefore greywater can have dual usage as source of plant nutrient and for irrigation, thus preserving freshwater and increasing food production hence enhancing the achievement of Kenya's Vision 2030.

Table 1: Mean Levels of Parameters in Raw Greywater

Parameters/

Units

Tap water (control)

Mean±SE n=30

Kitchen

(KGW)

Mean ±SE

n=30

Bathroom

(BGW)

Mean ±SE n=30

Laundry wash

(LWGW)

Mean±SE n=30

Laundry rinse

(LRGW)

Mean±SE n=30

pH(pH scale)

7.18±0.10a

7.84±0.08c

7.47±0.09b

8.23±0.06d

7.75±0.06c

Pb(mg/L)

0.06±0.02a

0.11±0.01b

0.08±0.01a

0.13±0.01b

0.11±0.01b

Cd(mg/L)

ND

ND

ND

ND

ND

K(mg/L)

1.76±0.07a

9.74±0.64b

8.60±0.76b

15.51±1.24c

9.45±0.78b

Ca(mg/L)

5.80±1.16a

20.94±1.60b

23.88±1.23b

33.10±1.72c

24.20±1.50b

Mg(mg/L)

1.64±0.12a

6.21±0.53b

5.52±0.54b

7.53±0.63c

5.98±0.56b

Zn(mg/L)

0.25±0.06

0.36±0.03

0.39±0.03

0.42±0.03

0.37±0.04

Fe(mg/L)

0.76±0.11a

1.73±0.16b

1.28±0.12b

2.36±0.16c

1.45±0.12b

TP(mg/L)

0.59±0.13a

4.93±0.41b

3.39±0.51b

9.34±0.51c

4.56±0.26b

NO3--(mg/L)

0.53±0.15a

4.99±1.04c

2.55±0.58b

3.62±0.54bc

2.87±0.44b

NB: Same superscript means no significant difference and different superscript indicates significant difference at α=0.05

ND-Not Detected

Acknowledgement

I expess my gratitude to J. Munyao, D. Osoro, E. Maina and J. Wambua for their substantial guidance and advice during the study.

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