ferric chloride, pacl, microbiological removal performance

[Impdrew]

Can someone please advise comparison of efficiencies for removal of coliform and e-coli by ferric chloride and poly aluminium chloride for coagulation, flocculation, DAF and onto RGFs (and ph dependancies). We have a pre-treatment plant DAF plant with something like 75-80% turbidity removal at this early stage in its commissioning and optimisation. We dose ferric chloride as the coagulant but have found that we have majorly insufficient bacteria removal which are giving values above our clients trigger levels for water feeding disinfection systems.
The plant has a crypto risk and we have started to see some non-conforming cysts similar in size to oocysts coming through.
We have taken coliform and e-coli count samples and the removal is only around 80%. A similar plant we have built has a much higher removal of bactiria in the 99% etc percent range across the DAFs / RGFs. Although this plant is often assisted by ozone it operates well in bacteria removal with ozone off.
I have seen some preliminary notes that indicate PACL is much better at removing bacteria than ferric chloride and that at the lower pH towards 5-6 the removal is substantially higher for both(I am aware of the effieciencies and robustness of the two coagulants for NTU/solid loading, microbiological removal is where i am struggling). Concern is that ferric chloride is not a good bacteria removal coagulant at standard ph ranges.
Some current operating data:
pH = 7.8
Ferric Chloride Dose = 2.8mg/l
Raw Turbidity = 4.5 NTU
DAF outlet = about 1.5 NTU
Raw Water coliform counts = about 870/100ml
Raw water e-coli counts = about 100/100ml
DAF outlet coliform counts = about 130/100ml
DAF outlet e-coli counts = about 20/100ml
RGF outlet coliform = about 80/100ml
RGF outlet e-coli = about 10/100ml
Platn is Reservoir fed, worsening water quality due to weather changes. Raw water count could get much worse up into the 1000s and concerned plant can't perform.

 

[bimr]

I would not expect you to find the information that you request. There should not be major differences in treatment efficiences with different chemicals.

You probably should go back to jar testing to determine the optimum chemical feed dosage. Get some input from nearby water treatment operators if possible.

 

[Impdrew]

Bimr,
Thanks for the response
This source, although not definative, shows signs of a marked difference in microbiological removal outside of the normal solids loading removal.

http://books.google.co.uk/books?id=...X&oi=book_result&resnum=4&ct=result#PPA109,M1

 

[ch81pc]

I have in the past operated a plant on ferric in the summer (cheaper) and then PACl in the winter when turbidities rise and temperature drops. When dosing aluminium we found we could reduce chlorine set points on super chlor as there appeared to be a better bacterial kill/removal through the upstream processes.

I have also used PACl on waste treatment where we have found it has a dual purpose of coagulation and a biocidal action in preventing filamentous organisms growing in activated sludge. This activity wasn't shown when dosing iron based coags, and we have used it time and time again.
The aluminium ion is significantly smaller than the Fe3+ ion and hence has a higher charge desnity making it both a better coag. It may this fact that also seems to make it a better bug killer.