Comparative study was conducled to evaluate lhe quality of secondary
treated sewage effluent (TSE) and groundwater of Riyadh region. Water samples from
each type of water were collected from three sites at the four different seasons. EC of
groundwater was significantly higher than TSE. Co~', Mg2', Na", Cl', HeO)' of
groundwater was significantly higher than TSE. Both irrigation waters had low amount
of NH; - N. N03' - N. Fe, Mn, Zn, and Cu which below the levels recommended for
water quality. Bacterial populations of Daryiah and Waseel sites did not show any
significant differences. however, their counts were significantly higher in Ammaryah
site. There were no significant differences between groundwater and TSE in their
content of bacteria, although TSE contained higher numbers of bacteria than
groundwater has. Fungal counts were Significantly higher at Daryiah site as
compared with the other sues. In contrast, Ihe highest density of streptomycetes was
recorded at Waseel site. Fungal and streptomycetes counts were significantly higher
in groundwater than TSE. Results also revealed that different microbial groups
differed inconsistently between sites, seasons and irrigation type. The highest most
probable numbers of different groups were found in TSE as cornea red to groundwater
(1.3 to 22 fold higher than groundwater). Microbial communities appeared to be
differed between sites where nitrifiers and sulfate reducers were found in high density
at different sites than the other groups of microorganisms. Conversely, the density of
denitrifiers and N2-fixers were significantly higher at Daryiah site lhan the other sites.
Sutfate-reducers showed their highest density at Waseel site. Treated sewage effluent
of Riyadh region has a good quality for irrigation and did not show any deleterious
effects as compared to groundwater. Due to the presence of fecal coliform in this
effluent which is used as indicators of sewage pollution, it can be safely used for
irrigation the wood trees, forests and landscape.