The principal results of many years of study of the removal of nutrients from municipal wastewater by a full-scale mainstream activated sludge process (capable of 95% removal of C, N and P without chemical dosing) indicate that anaerobic zones have two important functions, the second of which has not been reported previously in wastewater research publications:(1) Anaerobic zones provide ideal conditions for uptake of organic carbon and release of phosphate by Acinetobacter which accounted for about 50% of the heterotrophic organisms in the plant as determined by the API system.(2) Anaerobic zones also provide ideal conditions for reduction of influent ferric iron to ferrous iron and subsequent precipitation and adsorption of mixed metallic phosphates; such as a colloidal modification of ferrous calcium phosphate (1;0,4:1 molar) which forms in the pH range 6 to 7,5.
It was found that anaerobic retention times of short duration were sufficient to stimulate subsequent uptake of soluble phosphate by Acinetobacter in aerobic zones. However, much longer anaerobic retention times were necessary to lower the redox pote ntial sufficiently to reduce bound iron in the ferric state to ferrous ions which were identified in significant quantities in the sludge by using cold PCA fractionation procedures in conjunction with bathophenanthroline.
The partial transfer in anaerobic zones of phosphate, from intracellular polyphosphate in vivo to extracellularly adsorbed metallic phosphates in vitro, was found to be essential for truly enhanced removal of phosphate by this mainstream process which always removed more than 95% of the nitrogen from the municipal wastewater.
A further finding of significance was the limited phosphate storage capacity of Acinetobacter.
Metabolic phosphate, excess biological uptake and extracellular precipitation/ adsorption each accounted for about one third of the total phosphate removed.
About two thirds of the total influent nitrogen was removed by “simultaneous” nitrification and denitrification and most of the remainder was removed as organic nitrogen in the surplus sludge.
The continuous recycling of activated sludge containing extracellularly adsorbed colloidal phosphate precipitates eliminates the need for nucleation and induction and provides additional surface area with active sites for adsorption of excess phosphate. This could explain the observation that phosphate removal efficiencies improved when solids retention times and sludge concentrations were increased.
The plant and its performance have been described in detail in previous papers by the same Authors:1980 “The ability of the extended aeration activated sludge process to remove phosphorus consistently to less than 0,1 mg P/l in a simple surface-aerated rectangular reactor.” IWPC (South African Branch) Conference, Pretoria (2-6 June).1981 “Application of numerical models to design and operation of municipal wastewater treatment works incorporating multi-mini-step activated sludge reactors for enhanced P-removal.” Unpublished.1982 “Full scale phosphate removal experiences in the Umhlatuzana Works at different sludge ages.” IAWPR Post Conference Seminar on phosphate removal in biological treatment processes, Pretoria 5-6 ApriI 1982. Wat.Sci.Tech.Vol. 15, Cape Town, pp 261-281.1983 “Further developments in the understanding of phosphate removal at Umhlatuzana.” IWPC Conference, East London.
Operating at sludge temperatures in the range 17°C to 26°C for many years without chemical dosing, the plant exhibited, from time to time, the ability to remove consistently for many consecutive months 95% of the carbon, phosphate, and nitrogen from unsettled municipal wastewater (in a soft water area), with a TKN/COD ratio of 0,07; a P/COD ratio of 0,014; and a low alkalinity/COD ratio of 0,25 (as CaCO3/COD).
