An activated sludge contains three phases: the liquid, the suspended solid and the colloidal phases. However, the common analyses include only liquid and suspended solid quantifications, the colloidal phase is usually counted with the liquid or the suspended solids or with either. So, is their a way to quantify this phase separately?
Hi Dr Semrany,
Determination of the colloidal particular fraction of wastewater or activated sludge system is difficult. However some researchers developed various methods and we can see them in research sites. The methods are generally depend on particle size. One of them is "COD fractionation of tannery wastewaters—Particle size distribution, biodegradability and modeling"
O. Karahan, S. Dogruel, E. Dulekgurgen, D. Orhon, WATER RESEARCH 42 (2008) 1083 – 1092" and may be it will be help you.
Olcayto Keskinkan
Hi Dr Semrany,
This would depend on how much effort you want to go to and what equipment you have. Also if you are wanting absolute measurements or comparative ones to track over time?. In a domestic system these colloids will largely be biological (EPS) products. However with some wastewater (for example the pulp and paper wastewater I study) there can be a high fraction of colloids originating in the influent.
For low tech approach
For an operationally relevant measurement the colloidal fraction can be measured as the difference in COD or between a GFC filtered (~1.2um) and 0.45 or filtered activated sludge sample (reccomend centrifuging first). It would proabably be more correct to use a 0.22um but these are much more expensive. The method is not concrete as the GFC filtration is only around 1.2um not exactly.
Also low tech, but works if you want comparitive measurements over time.
Centrifuge 50ml at set speed (we used 2400rpm) then measure the turbidity of the supernatant. This can be done using a stand alone turbidity meter or absorbance at 560nm on any spectrophotometer.
Medium Technology
We have come to favour measurement of dTOC/ TNb of a centrifuged and filtered (0.45um) activated sludge sample. Although this may miss some of the larger 'colloidal' matter, we have found this is very good for tracking the retention of colloids in the MBR. In this case these colloids are caclulated as the dTOC of the above sample minus the TOC of the UF filtrate (nominally 0.08um). This works well for us and correlates with CST. Its just that the colloids measured are in the 0.08-0.45um range.
High Technology
You will find a lot of papers surround the sorts of analytics, depending on what you want to know about the colloids. I have used size exclusion chromatography on 0.45um filtered supernatant, this will give you PSD and can give some good information Lots of people do chemical/ ultrasound/CER extractions to try to separate the cellular and non cellular materials. Then they measure the carbohydrate and protein fractions of the supernatant. Then of course are the people with excessive curiosity who do proteomics studidies.
Happy to answer any questions you have about any of this.
Regards,
Emily
In waste-water treatment, the solids can be classified according to (a) size and state, (b) chemical characteristics and (c) settleability. Solids with diameter less than 10 exp-3 µm are considered dissolved solids, colloidal solids are in the range 10 exp-3 to 10 exp 0 µm and suspended solids diameters are greater than 10 exp 0 µm. Filter paper will separate the three solid sizes. Conventionally 1.0 µm GF will trap suspended solids and 0.45 µm filter will trap colloidal solids while dissolved solids will pass through.
If you want to be more accurate The flocculation/filtration method can be used as detailed below:-
1. 1mL of 100g/L zinc sulphate solution is added to 100 mL of wastewater
2. The sample is mixed vigorously for approximately 1 minute
3. The sample pH is adjusted to approximately 10.5 using 2M sodium hydroxide solution with slow mixing for 5 minutes
4. The sample is then allowed to settle for a few minutes and a volume of clear supernatant is withdrawn for filtration through a 0.45µm membrane filter.
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