DECLARATION 2 PUBLICATIONS
3. RESULTS
3.3 Microbiological quality of below-ground crops
3.3.1 Beetroot
As beetroot is a root crop, it was exposed directly to greywater during sub-surface irrigation. It is however generally eaten after having been cooked thoroughly and is therefore regarded as a low risk crop. Observation of the crops did not show evidence of differences in crop size between crops irrigated with greywater and hydroponic solution, but the tap water-irrigated
Gext n= 9 Hext n= 9 Gint n= 9 Hint n= 9
crop was stunted, as can be seen in Figure 3.11. Median values per gram dry weight for each of the microorganisms are presented in Table 3.10.
Figure 3.11: Examples of tap water irrigated beetroot plants showing stunting of growth
Table 3.10: Median number of organisms per gram dry weight on external surfaces (ext) or the interior (int) of beetroot irrigated with greywater (G), hydroponic solution (H), tap water (T) or purchased commercially (C). Results are based on all samples taken over the complete trial period. No Ascaris ova, Pseudomonas, somatic coliphages, E. coli was detected internally or externally for any of the water treatments. E. coli was detected externally for all three water treatments and internally for greywater irrigated crop.
Enterococcus was detected internally and externally for greywater and hydroponic solution irrigated crop and externally for the commercial crop. Staphylococcus was detected internally and externally on crops from all three water treatments but not on the commercially bought crop. Total coliforms were detected internally and externally for all treatments.
Gext Hext Text Cext Gint Hint Tint Cint
E. coli 0 0 0 0 0 0 0 0
Enterococcus 0 0 0 60 0 0 0 0
Staphylococcus 0 0 0 0 0 0 0 0
Total coliforms 1100 900 80 17550 10 200 0 30
E. coli was detected externally for all three water treatments and internally for greywater irrigated crop. Enterococcus was detected internally and externally for greywater and hydroponic solution irrigated crop and externally for the commercial crop. Staphylococcus was detected internally and externally on crops from all three water treatments but not on the commercially bought crop. Total coliforms were detected internally and externally for all treatments. The statistical data are presented in Figure 3.12 to Figure 3.15.
Tap water
Median 5%-95%
Min-Max G ext H ext T ext C ext G int H int T ext C int
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
log (cfu/g)
Figure 3.12: Box and whisker plot of E. coli counts on external surfaces (ext) or the interior (int) of beetroot irrigated with greywater (G), hydroponic solution (H), tap water (T) or purchased commercially (C). Results are based on all samples taken over the complete trial period. No statistical difference was found between the treatments for either the internal or external sets when tested using the Kruskal-Wallis test (p=0.05)
Median 5%-95%
Min-Max G ext H ext T ext C ext G int H int T ext C int
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
log (cfu/g)
Figure 3.13: Box and whisker plot of Enterococcus counts on external surfaces (ext) or the interior (int) of beetroot irrigated with greywater (G), hydroponic solution (H), tap water (T) or purchased commercially (C). Results are based on all samples taken over the complete trial period. No statistical difference was found between the treatments for either the internal or external sets when tested using the Kruskal-Wallis test (p=0.05)
Gext n= 34 Hext n= 34 Text n= 27 Cext n= 2 Gint n= 34 Hint n= 39 Tint n= 27 Cint n= 2
Gext n= 34 Hext n= 37 Text n= 28 Cext n= 2 Gint n= 34 Hint n= 39 Tint n= 28 Cint n= 2
Median 5%-95%
Min-Max G ext H ext T ext C ext G int H int T int C int
-1 0 1 2 3 4 5 6 7
log (cfu/g)
Figure 3.14: Box and whisker plot of Staphylococcus counts on external surfaces (ext) or the interior (int) of beetroot irrigated with greywater (G), hydroponic solution (H), tap water (T) or purchased commercially (C). Results are based on all samples taken over the complete trial period. No statistical difference was found between the treatments for either the internal or external sets when tested using the Kruskal-Wallis test (p=0.05)
Median 5%-95%
Min-Max G ext H ext T ext C ext G int H int T int C int
-1 0 1 2 3 4 5 6
log (cfu/g)
Figure 3.15: Box and whisker plot of total coliform counts on external surfaces (ext) or the interior (int) of beetroot irrigated with greywater (G), hydroponic solution (H), tap water (T) or purchased commercially (C). Results are based on all samples taken over the complete trial period. No statistical difference was found between the treatments for either the internal or external sets when tested using the Kruskal-Wallis test (p=0.05)
No significant difference was apparent for any of the treatments for any of the indicators when tested with the Kruskal-Wallis test at the p=0.05 level. These results are given in Table 3.11.
There was little penetration of microorganisms into the tissue of the beetroot as is shown by the Gext n= 34
Hext n= 37 Text n= 27 Cext n= 2 Gint n= 34 Hint n= 39 Tint n= 27 Cint n= 2
Gext n= 34 Hext n= 37 Text n= 27 Cext n= 2 Gint n= 34 Hint n= 39 Tint n= 27 Cint n= 2
low interior counts of bacteria. No Staphylococcus was detected on or in the purchased produce. Except for total coliforms, the median values for all treatments for each of the detected microorganisms were generally zero. No significant increase in contamination was found over the growing cycles so all data were used in the statistical analysis.
The programme @RISK was used to fit distributions to the data for later use in determining the risk of infection. In each case the best fit was determined using the chi-squared test.
Table 3.11: Statistical comparison, using non-parametric (Kruskal-Wallis) methods, of means of bacterial load for the interior (int) and exterior (ext) of beetroot irrigated with greywater (G), hydroponic solution (H) or tap water (T). No significant differences were found at the p=0.05 level
E. coli Enterococcus Staphylococcus Total coliforms
Ext Int Ext Int Ext Int Ext Int
Kruskal- Wallis
NS NS NS NS NS NS NS NS
NS – no significant difference at p=0.05 level - No bacteria detected