Ca. 60 % of the Dutch activated sludge plants consist of completely mixed systems, experiments have been carried out in completely mixed pilot plants to study the biological P-removal.

The research was carried out in two pilot plants.

The pilot plants consisted of: anaerobic reactor, anoxic reactor, aerobic reactor and a clarifier. All the reactors were completely mixed.

Both plants were fed with settled domestic waste water at a sludge loading of 400 and 250 g COD/kg sludge.day respectively. The results are given below:

sludge loading (g COD/kg sludge.day) 400 400 250 
ratio Anaerobic : Anoxic : Aerobic 1: 1:2,7 1:1:4,1 1:1:2,7 
P-removal (%) 80 28 75 
N-removal (%) 50 50 65 
COD-removal (%) 85 85 85 
sludge loading (g COD/kg sludge.day) 400 400 250 
ratio Anaerobic : Anoxic : Aerobic 1: 1:2,7 1:1:4,1 1:1:2,7 
P-removal (%) 80 28 75 
N-removal (%) 50 50 65 
COD-removal (%) 85 85 85 

It has been shown that there is no significant difference between the results at the two different sludge loadings.

Remarkable is the difference between the ratio 1:1:2,7 in combination with the internal recirculation flow anoxic-anaerobic of 160 % and the ratio 1:1:4,1 with a recirculation flow of 30 %.

During the start-up at a sludge loading of 250 g COD/kg sludge.day and an internal recirculation flow of 30 %, bulking sludge developed almost immediately. The Premoval was completely disturbed.

Increasing the internal recirculation flow to 160% had a positive effect on settling properties and P-removal.

This investigation has pointed out that a completely mixed system is suitable for biological P-removal, without negatively affecting the nitrification. Important factors in the process are the ratio anaerobic:anoxic:aerobic and the recirculation flows.

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