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. 2014 Mar 10;11(3):2876–2898. doi: 10.3390/ijerph110302876

Figure 1.

Comparison of the characteristics of Unit-3 and Unit-5 at the Northern Wastewater Treatment Works, Johannesburg.

Figure 1

Notes: Legend: Unit-3: A: Primary settling tanks (2×); B: Elutriation tank (2 open tanks). Sludge from PST is pumped into one tank; retention time of 4-6 days. Waste out with settled sewage from PST. Other tank is then filled up; C: Balancing tank; D: Division box, outflow from balancing tank; E: Pre-anoxic tank; receives the RAS; F: Anaerobic zone; receives outflow from balancing tank; G: Anoxic zone; receives recycle from aerobic zone; H: Aerobic zone; I: Secondary settling tank; 3 suction lift tanks (difficult to control); J: Return activated sludge (RAS) from the bottom of the SSTs; K: Recycle stream from aerobic to anoxic zone; L: Water effluent from SST goes to disinfection and river; M: Waste activated sludge; wasted directly from aeration tank; N: Anaerobic digestion; Unit-5 (design is similar to that of Unit-3 with slight modifications): B: Elutriation tank (1×) is covered; receives sludge from PST; retention time of 4 days. The VFA is washed out with settled sewage from the PST; D: Division box; outflow from balancing tank. Some turbulence may ingress oxygen; F: Anaerobic zone; receives outflow from balancing tank. Excessive “bubbling” where water enters the zone; I: Secondary settling tank; mechanically scraped tanks (3×) remove sludge better.