Does the sludge contain appreciable concenreations of Al or Fe?

Paul Milham: Al or Fe concentrations are minimal.

Hej Shiv,

not sure which was the pH before adding HCl?! I guess it was around or even higher than 8?! As Paul was asking if iron and/or aluminum hydroxides are present lowering the pH to 5 actually favor the binding of phosphorus (P) since the surface of hydroxides becomes positive and thus attractive for P binding. For example, the zero point charge of amorphous FeOOH happens at pH 7.9–8.1 (Buffle, 1988. Accordingly a positive charged surface can be assumed at pH 5. However, the role of iron for P binding is more important at aerated conditions (Zak et al. 2004). May be the role of metals is negligible in your substrate and with some likelihood changes of the organic matrix and bacteria dominating the P cycle in your system. Thus, it is very well known that under anaerobic conditions, P- removing bacteria convert volatile fatty acids synthesized in the zone by fermenters to polyhydroxybutyrate (PHB) which is stored intracellularly while under aerobic conditions, stored PHB is used to generate cell growth, poly-P synthesis and glycogen formation and maintenance, resulting in the uptake of phosphate (Bdrjanovic et al., 1997).

Coming back to the pH effect. As for metals binding of P at the organic matrix is also pH dependent. I am not sure right now but as for metal hydroxides lower pH should favor P binding?! For example carboxyl groups become more protonated so that the negative surface charge is reduced and some organic acids precipitate and might co-precipitate P which is either incorporated in the organic matrix or bound at the surface., e.g. over metal bridges (Zak et al. 2004). Did you measured phosphate or total dissolved P? Anyway the common phosphate analysis by the molybdenum method might also detect P which is associated with dissolved organic compounds. So why the P release was increasing in your case at lowering pH to 5? Actually, I would suspect that acid hydrolysis of organic matter containing P or cell lysis of bacteria has only a minor impact on the P release in your case?! These processes should be first relevant if pH values lower than 4 become established?! I would not excludes this idea completely since some organic matter and also Bacteria might be affected already depending on the original pH.

Best wishes,

Dominik

References

BDRJANOVIC D, VAN LOOSDRECHT MCM, HOOIJMANS CM, MINO T, ALAERTS GJ and HEIJNEN JJ (1997) Bioassay for Glyco- gen determination in biological phosphorus removal systems. In: Proc. 2nd Int. Conf. on Microorganisms in Activated Sludge and Biofilm Processes, July 21-23, Berkeley, California, USA. 335-342.

Buffle J. 1988. Complexation properties of homologous comlexants and choice of measuring methods.

In: Complexation Reactions in Aquatic Systems: an Analytical Approach. Ellis Horwood, Chichester,

UK, pp. 304–383.

Zak, D., Gelbrecht, J., and Steinberg, C.E.W. (2004) Phosphorus retention at the redox interface of fens in Northeast Germany. Biogeochemistry. doi.org/10.1007/s10533-003-0895-7

Dominik Zak: thanks

I'd be nterested in the washup