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Side Stream Filtration Rate For Cooling Towers

Side Stream Filtration Rate For Cooling Towers Dear All,

Suspended solids (particulates or turbidity) are frequently found in cooling towers. At best these suspended solids will tend to settle out in low velocity areas of the water system (such as the cooling water basin) where they can become a breeding ground for bacteria, requiring frequent cleaning and flushing of the basin. At worst, they can degrade system heat transfer capacity and lead to steep increase in dosing rate and consequently the cost of water treatment chemicals (e.g bactericides).

Modern day cooling tower systems have thus adopted side stream filtration of the water from the cooling tower. Various schemes can be implemented. The basin water inventory may only be filtered and / or side stream filters may be employed at the discharge of the CW recirculation pumps. Refer the simplified flow scheme of side stream filtration for a cooling tower:

Attached Image

When the side stream filter is employed in the discharge of the cooling water recirculation pump discharge, the flow rate required for filtration through side stream filter can be calculated by the following formula:

Side Stream Flow Rate, L/s = ((ppm suspended solids in recirculating cooling water / 200) - 1)*Blowdown Rate, L/s - Metric

Side Stream Flow Rate, gpm = ((ppm suspended solids in recirculating cooling water / 200) - 1)*Blowdown Rate, gpm - USC

From the above formula, it becomes obvious that if the ppm level of suspended solids is ≤200 ppm in the recirculation cooling water, side stream filtration may not be required. However, installation of side stream filter is recommended since suspended solid concentration builds up over time and periodic high build-up would require side stream filtration for the aforementioned reasons.

Pressure sand filters with backwash facilities are commonly employed as side stream filters for cooling towers.
Backwash water for the filters can either be from another source, or if cooling water is used, this water should be considered as part of the cooling tower blowdown. In either case, backwash effluent water must be sent to the waste water treatment plant.

That is all for today's blog entry. Comments are most welcome.


I would like to propose the following idea to calculate the side stream circulation rate.


The main idea is the following water and suspended solid(SS) balance in the whole cooling water(CW) system;  no loss in CW, thus no SS loss in circulating loop is assumed:


Overall water mass balance(flow rate):    M = E + B + W................(1)

SS mass balance:   aM = cB + cS ......(2)  <--all SS in M is to be removed by B and S.

Side filter SS balance:  cS = dW  .......(3)  <--all SS in S is to be removed completely by the side filter.



M=make-up CW,  E=evaporation loss, B=blow-down, W=waste sludge purge (from side filter), S=side stream 

a=SS conc. in make-up, c=("ALLOWABLE") SS conc. in circulating CW system, d=SS conc. in sludge (to be purged from side filter); the unit of a,c,d is in wppm                 


Compared with M, E, and B the amount W in eq. (1) could be neglected, and flow rate of side filter can be calculted from eq.(1) and (2):   


S = (aM-cB)/c  = (a/c)M - B  = M/α  - B ................(4)

where     α = c/a  = "RATIO" Allowable Maximum SS to make-up SS ....(5)


If the net SS make-up, aM, is to be totally purged as blow-down, cB, then S becomes 0; so "NO" side filter would be required.   If there is "NO" blow-down, then all the sludge shall be purged as d*W (=c*S) by circulating with side-stream rate of S ( =(a/c)*M).  


S=0  if a*M=c*B .................................................(4a)

S=(a/c)M  if  B=0................................................(4b)  


The maximum "Allowable" SS concentration quoted by Ankur seems to be 200 wppm.   


Please comment on my idea above.



Oct 18 2016 12:36 AM


 how to calculate inlet dia.,throat dia, length of conv., throat & divergent of venturi scrubber.

Nice Post. Thanks!

Nice idea dear Sir Ankur (y) .

kamal Nashar
May 09 2017 12:50 PM

Really Nice Post, Thanks  

Can we consider pump minimum circulation flow rate as flow rate for side stream filter. Altimately that is the minimum flow a pump can deliver 

Aug 07 2018 12:41 PM



I am working as an intern for a company and i have been given a project for water conservation.

My prime focus is on reducing the water consumption in the cooling tower.


The company has a zero blowdown: induced draft, counter flow cooling tower and it operates 3-4 times a week.


I  calculated the Cycle of concentration(COC) for the present situation and found it to be around 1.23


With the help of makeup water parameters, I was able to calculate the optimum COC level of the tower to be 3.67


I am really confused if it is giving any relevant inference, as the water is not blown down but rather it circulates.


They have a side filtration tank in order to keep the water soft, they also feed the chemicals in timely manner to control the water quality. 


Does this mean that the cooling water system is overly designed to keep the COC much less than the optimum level?


How can we make this cooling tower run close to 3.67 ?


I am not an expert in this field, so please let me know if i am missing something here?


Thank You 

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