PETRO concept: fortunate process similarities for both biological excess phosphorus (BEPR) and micro-algae removal in pond-based BNR system
Abstract:The PETRO system is a mature cost-effective technology widely used for treatment of municipal and industrial wastewater in the developed and developing communities in South Africa. Integration of ponds with a downstream polishing facility, trickling filter (TF) or activated sludge process (ASP), allows for substantial savings while upgrading existing pond- or TF-based plants which fail to meet stringent discharge requirements. Up to 60% of the COD load is reduced in the low-tech ponding section. Effective removal of pond micro-algae and nitrogen occurs in the ASP or TF (final effluent: BOD-10, SS—10, N-NO3 -5, N-NH3 -1 mg/L). The current effort is to incorporate BEPR into the PETRO concept and achieve a higher degree of treatment (P<1mg/L). Production of readily biodegradable organics (RBCOD) plays a crucial role in both the PETRO and BEPR concepts. Availability of RBCOD produced anaerobically in the primary pond is a key factor in the heterotrophic autoflocculation/removal of micro-algae in the PETRO ASP or TF while RBCOD fraction is the driving force behind the activity of phosphate accumulating organisms in a conventional BNR reactor. A novel concept of the RBCOD generating pit (GP) was formulated and full-scale trials performed. Introduction of GP in place of the primary pond pit does not involve major structural alterations and is based on the existing pond layout. Certain changes in the regime of the pit operation are suggested. RBCOD generation in the pit avoids high tech options (e.g. acidogesters, etc) and malodours. The concept combines bulk COD removal in the ponds with a concomitant solubilization of sludge solids under oxygen-stressed anaerobic conditions. Conditions for the enhanced RBCOD production in the GP are: (i) high organic load on the primary pond (0.21–0.82 kg COD/m3/d); (ii) an extensive pool of sludge in the pit; (iii) short hydraulic retention time (<15 hours); (iv) high rate recirculation, providing algae-rich oxygenated water from the secondary ponds to the surface of the primary pond. It is a crucial factor in the enhancement of the RBCOD production. Acidogenesis predominates due to oxygen inhibition of extremely sensitive methanogens offsetting digestion. Sludge production rate is intermediate between that of acidogenic and methanogenic reactors (up to 0.18 kg SS/ kg treated COD). RBCOD generating potential correlated with the sludge age and elutriation rates compare well with those of conventional acidogesters.
Document Type: Research Article
Publication date: January 1, 2000
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