Treatment of water closet flush water for recycle and reuse

Results from the operation of a 37.8 m³/d extended aeration and sand filtration system in the closed‐loop treatment of water closet flush water are presented.

The system has operated for four and one‐half years at 95 percent recycle. During this period over 30,000 m³ of flush water was treated and reused. Water inputs into the recycle system resulted from liquid human wastes plus wastage form potable water uses. Wasted potable water inputs were from wash basins, water fountains, and custodial services.

Operation of both the biological treatment unit and the pressure sand filter followed acceptable conventional practice. Chemicals were not required to adjust pH, aid solids settleability or enhance treatment efficiency. Equilibrium concentrations followed predicted values except for pH, alkalinity, ammonia, nitrite and nitrate. The concentration of these constituents was governed by urine content after flushing, agitation before biological treatment, temperature and biological oxidation. Variations in nitrogen (ammonia, nitrite and nitrate), pH and alkalinity that were observed could be accounted for through fundamental biological, chemical and physical relationships. The pH throughout the entire recycle system varied between 5.5 and 8.4. Recycled water pH rose from a preflush pH of approximately 7.0 to a pH of 8.4 immediately after flushing. The biological unit lowered the pH and functioned between pH values of 5.5 and 7.0. A slight rise in pH between the biological unit (through storage and filtration) and water closets was observed.

The predominate biomass in the biological unit was fungi. Biological solids were threadlike; however, they readily separated by gravity settling. Wastage of biological solids from the biological unit in the recycle‐reuse system was the same experienced for a comparable biological unit used to treat water closet wastewater that was not recycled.

Results from this study have conclusively demonstrated on a full‐scale basis the acceptability of using biological oxidation and sand filtration as a treatment train in the reuse of water closet wastewater with a recycle ratio of 20.