Abstract
Water treatment efficiencies of different five treatment plants providing drinking (potable) water to the greater population size of Dakahleia governorate in Egypt were investigated and compared. The selected plants vary widely in hydrology of surface raw water resource, engineering design and certain performance characteristics. All the selected water treatment facilities adopt the conventional water treatment processes that involve the successive treatment stages of coagulation, flocculation and sedimentation (clarification) followed by granular bed filtration. Raw, clarified and filtered water samples were seasonally collected between mid-spring 2008 and mid-winter 2009. The treatment efficiency tools include comparative evaluation of certain physical and chemical characteristics (e.g. turbidity, ammonia-N, BOD and COD) of raw and treated water, removal of suspended algae and water toxicity testing with standard algal biotest. The results indicated substantial reduction in water turbidity of raw water with a mean annual value of 0.6 NTU of the potable water. Significant (P ≤ 0.05) increase in dissolved oxygen coupled with sharp decline of BOD and COD of drinking water were recorded. Potable water was entirely nitrite-N and ammonia-N free. The phytoplankton-based biological indices indicated water quality consequences agreed well with that revealed by the physical and chemical characteristics of raw water. After the clarification stage, the major treatment plants DWTPs 1, 2, and 3 maintained superior removal efficiencies (77.3% - 90.3%) of suspended micro-algae compared with the compact units DWTPs 4 and 5 (63.4% - 82.3%). Except the obvious failure of compact unit DWTP4 that removed only 76.4% of algal biomass of filtered water in spring, all the selected plants removed about 99.9% of algal biomass after filtration stage. In conclusion, the results indicated that, the selected water treatment plants provide drinking water with acceptable quality coping with both WHO and Egyptian guidelines.

Key words: Drinking water, suspended micro-algae, compact units, turbidity