STABILIZATION PONDS WITH RECIRCULATION

The first facultative pond in a series of stabilization ponds, or else the first part of a large pond, is sensitive to organic overloading c reating anoxic or anaerobic conditions at the pond's surface, resultin g in malodors and nuisances. Such adverse characteristics are usually manifested seasonally when climatic conditions change to lower tempera tures and/or reduced solar irradiance. The design organic loadings on such ponds are therefore determined by the critical season and they ar e lowered accordingly. Introducing recirculation of effluent from a la ter pond in the series (usually from the second or third pond) back to the inlet of the first one, at a ratio of 1.0 - 2.5 (recirculated eff luent) to 1 (raw sewage influent), can be most advantageous, as follow s: (1) organic loadings on the first facultative pond in the series ca n reach 400-600 kg BOD5 per hectare per day (khd) during summer time a nd 300-400 khd as a yearly average, compared with a yearly average of 60-140 khd on ordinary facultative ponds, while still maintaining odor -free facultative conditions; (2) reseeding the first pond with active adapted algal biomass; (3) mixing the influent (which is often septic ) with oxygen-rich recirculated effluent, thus enhancing the biologica l process in the first pond and suppressing septic odors, and (4) the increased inlet flow (by combining influent flow with the recirculated effluent) increases the area of solids (sludge) settling in the first pond. Altogether, the recirculation is manifested by reduced land req uirements, better stability in pond operation, improved pond's perform ance and reduction or elimination of malodors and nuisances. Step feed ing of the ponds further accentuates the effect of recirculation. Obvi ously, recirculation requires pumping (low head),energy, piping and co nnection to a power supply. The cost of operation and maintenance amou nt to US $0.01-0.02 per cubic metre of treated wastes. The advantages of recirculation nevertheless significantly outweigh the added costs. The paper describes the experience and data which have been gathered d uring the operation of 120 hectares of ponds with recirculation in the Dan Region (Greater Tel-Aviv) over a period of almost 20 years.