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The average values of physicochemical, mineral, heavy metal and microbial properties of the two water sources met irrigation water quality standards according to FAO guidelines (Ayers & Westcot 1985), as shown in Table 1. The pH of both the water sources was acidic and was lower than that of the quality guidelines. However, it is suggested that a variation of 10% to 20% above or below the guideline value has little to no significant effects on the yield or quality if considered in proper perspective with other factors (WHO 2012). The EC of both water sources were well within the threshold, with the lake water having a significantly higher EC compared to the treated waste water. Both COD and BOD5 levels were significantly higher in the lake water compared to treated waste water. However, COD and BOD5 levels in both water sources were lower than that of the quality guidelines. There were no significant differences between the water sources with respect to DO. Both water sources tested contained levels of DO that were higher than the minimum limit. DO is an important attribute when it comes to irrigation water, as an increased level of DO is perceived to increase root mass and fine root hairs, which eventually leads to increased growth, better fertilization and overall plant health (Becker 2014). The nutrient content of the different water sources was observed to impact the nutrient content of the guava fruits. As for the treated waste water used for irrigation, the content of calcium, magnesium and potassium was well within the range of the irrigation water quality guidelines. Total nitrogen in treated waste water, however, was higher than the guideline level. Lake water used for irrigation on the other hand contained calcium and magnesium levels that were within the acceptable range, while total nitrogen and phosphorus levels were higher than the level of the irrigation water quality guidelines. An excess of nitrogen may prove to be detrimental to highly sensitive crops. However, most crops are relatively unaffected until nitrogen levels exceed 30 mg/l (Hassan 1990). The total nitrogen content of the treated waste water did not exceed the 30 mg/l threshold, while the content of total nitrogen in the lake water was above the level that is perceived to have adverse effects on crops. The nutrients present in irrigation water are vital for plant growth: calcium and magnesium are essential plant nutrients and cations responsible for good soil structure; potassium influences rooting, drought, heat, cold and disease tolerance; nitrogen is one of the main chemical elements required for plant growth and reproduction; phosphorus is known to play vital roles in photosynthesis and plant growth (Uchida 2000). Results from the microbial analysis revealed that both water sources had faecal coliform, E. coli and helminth egg values that were below the recommended values set by WHO (1989). Treated waste water, which may be a carrier of a variety of bacteria and viruses when used for irrigation purposes, could cause diseases through contamination of crops. However, in our study, relatively low microbial values were obtained, which could be attributed to the use of chlorine as a disinfectant. Heavy metals (Ni, Cd, Cr, Cu, Pb, Zn) levels in both water sources were observed to be well within the respective recommended range. This reduces the potential accumulation of toxic heavy metals in the soil that will lead to adverse effects on plant growth (Rattan et al. 2005). Through comparison of the different water sources’ properties against irrigation water quality guidelines, it is evident that the treated waste water was of better quality for irrigation purposes.

Table 1

Physicochemical and mineral analysis of the two different water sources used for the irrigation of the Lohan guava crops

 TWWLWIrrigation water quality guidelines
Physicochemical parameters 
 pH 5.83 ± 0.18a 5.52 ± 0.23b 6.50–8.40 
 EC (ppm) 53.15 ± 3.51a 138.61 ± 1.98b <448 
 COD (mg/L) 205.4 ± 3.14a 232.7 ± 2.88b <300 
 BOD5 (mg/L) 95.3 ± 1.92a 102.5 ± 1.32b <150 
 DO (mg/mL) 5.4 ± 0.2a 5.6 ± 0.3a >5 
 Turbidity (NTU) 1.54 ± 0.33a 5.55 ± 0.22b 0–5 
Macroelements 
 Calcium (mg/l) 4.58 ± 0.42a 16.3 ± 0.53b 0–20 
 Magnesium (mg/l) 1.18 ± 0.19a 4.81 ± 0.23b 0–5 
 Potassium (mg/l) 1.81 ± 0.09a 13.5 ± 0.11b 0–2 
 Total nitrogen (mg/l) 26.5 ± 1.07a 39.3 ± 1.19b 0–10 
 Phosphorus (mg/l) ND (<0.01) ND (<0.01) 0–2 
Heavy metals 
 Nickel, Ni (ppm) 0.16 ± 0.03a 0.20 ± 0.04a 0–2 
 Cadmium, Cd (ppm) 0.03 ± 0.02a 0.05 ± 0.03a 0–0.05 
 Chromium, Cr (ppm) 0.07 ± 0.03a 0.09 ± 0.02a 0–1 
 Copper, Cu (ppm) 0.04 ± 0.01a 0.03 ± 0.01a 0–3 
 Lead, Pb (ppm) 0.02 ± 0.02a 0.03 ± 0.01a 0–1 
 Zinc, Zn (ppm) 0.07 ± 0.01a 0.08 ± 0.02a 0–1 
Microbiological parameters 
 Faecal coliforms (CFU/100 ml) <10 <10 0–200 
E. coli (CFU/100 ml) <10 <10 0–100 
 Helminth (eggs/10 l) <10 <10 <1 
 TWWLWIrrigation water quality guidelines
Physicochemical parameters 
 pH 5.83 ± 0.18a 5.52 ± 0.23b 6.50–8.40 
 EC (ppm) 53.15 ± 3.51a 138.61 ± 1.98b <448 
 COD (mg/L) 205.4 ± 3.14a 232.7 ± 2.88b <300 
 BOD5 (mg/L) 95.3 ± 1.92a 102.5 ± 1.32b <150 
 DO (mg/mL) 5.4 ± 0.2a 5.6 ± 0.3a >5 
 Turbidity (NTU) 1.54 ± 0.33a 5.55 ± 0.22b 0–5 
Macroelements 
 Calcium (mg/l) 4.58 ± 0.42a 16.3 ± 0.53b 0–20 
 Magnesium (mg/l) 1.18 ± 0.19a 4.81 ± 0.23b 0–5 
 Potassium (mg/l) 1.81 ± 0.09a 13.5 ± 0.11b 0–2 
 Total nitrogen (mg/l) 26.5 ± 1.07a 39.3 ± 1.19b 0–10 
 Phosphorus (mg/l) ND (<0.01) ND (<0.01) 0–2 
Heavy metals 
 Nickel, Ni (ppm) 0.16 ± 0.03a 0.20 ± 0.04a 0–2 
 Cadmium, Cd (ppm) 0.03 ± 0.02a 0.05 ± 0.03a 0–0.05 
 Chromium, Cr (ppm) 0.07 ± 0.03a 0.09 ± 0.02a 0–1 
 Copper, Cu (ppm) 0.04 ± 0.01a 0.03 ± 0.01a 0–3 
 Lead, Pb (ppm) 0.02 ± 0.02a 0.03 ± 0.01a 0–1 
 Zinc, Zn (ppm) 0.07 ± 0.01a 0.08 ± 0.02a 0–1 
Microbiological parameters 
 Faecal coliforms (CFU/100 ml) <10 <10 0–200 
E. coli (CFU/100 ml) <10 <10 0–100 
 Helminth (eggs/10 l) <10 <10 <1 

Values followed by different letters within the same column are significantly different for each fruit (p < 0.05) (n = 9). TWW: treated waste water; LW: lake water.

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