Side Stream Filtration

Cooling towers, by design, are the world's finest air scrubbers. As part of the cooling method, they flush airborne contaminates into the system, where they deposit on heat exchange surfaces. Suspended matter in the cooling water also supplies living micro-organisms with a constant food source for sustaining life and multiplying.

A permanent medium filtration system can remove these contaminants before they increase the cost of operating the water-cooled equipment or shut the system down. And, combined with UV sterilization, constant side-stream filtration will result in a much cleaner system, with the UV providing a constant suppression of micro-organism growth far below those achievable with chemical biocides. However, it should be noted that a side stream filtration system will only complement, not replace a chemical water treatment program and vice versa. Chemicals cannot remove sediment and filters cannot control the chemical makeup of the water.

Cooling water system treatment programs must include the following four specific parameters of control for successful equipment protection: Scale, Corrosion, Biological fouling, and Airborne contaminants. Airborne dirt affects scale control, corrosion and biofouling, and is the only one that cannot be measured. No one knows which way the wind will blow and what it will pick up in its travels and deposit into the system.

The most effective means to control all of these problems is cleanliness. Filtration of sump water helps keep the tower clean. Removal of as many contaminants, both airborne and byproducts of the system makes the treatment program easier. Filtration and UV provide a daily cleaning of the water.

With side-stream filtration, a portion of the water filters continuously. The whole system is filtered every hour. Side-stream filtration works on the principle that continuous particle removal will keep the system clean. The filtration goal is not to make drinking water but to remove the fine dirt, dust, smoke and organic particles that collect in water.

The most effective filter system for cooling towers and process water is a permanent medium unit, one that requires very little attention or maintenance. The medium must be finer than 50 microns to trap the average airborne contaminant. Things to look for in filter systems are: self cleaning, simplicity, compactness and ease of operation. Also important are screens with pore sized down to 15 microns, providing consistent filtration and a short automatic backflush cycle

Typically, a side-stream representing about 5-10% of the total recirculated flow is adequate for sizing purposes. For example, a 100-ton cooling tower will have a total recirulating flow rate of 19 litres per second. Sizing the filter and UV sterilizer to handle flows of 2 litres per second would be sufficient for cleaning the entire capacity of the tower once every hour.

In summary, when you add up all the benefits, permanent side stream filtration with UV sterilization pays for itself in less than two years. No water-cooled system, especially cooling towers, should operate without a filter/UV combination

If suspended materials are degrading the quality of cooling tower water, consider installing a side stream filtration system to clean the water. These are rapid sand filters or high efficiency cartridge filters that draw water from the basin, filter out sediment and return the filtered water to the tower, enabling the system to operate more efficiently with less water. This system is particularly effective where the water is cloudy, airborne contaminants are common or cooling water pipes are small and susceptible to clogging. Removing particles or suspended solids from the recirculating water enables the system to operate more efficiently with less maintenance. Some systems, however, use a lot of water to backwash the filter. In this case, consider capturing bleed-off in a backwash holding tank and use it to backwash the side-stream filters, but firstly evaluate the potential contaminants/dissolved solids in the bleed water. Using a bag or cartridge filter saves even more water, as it does not entail a backwash cycle. Before installing a side stream filter, assess all costs, maintenance, and shutdown requirements and the disposal of spent cartridges to landfill.

Calculating Total Water Volume
Step 1. Calculate Litres of Water in Tower Basin (Sump)
Length x Width x Depth of water = m3 of water
m3 in sump x 7.5
Litres in Sump
Step 2. Calculate Litres in System Piping
Start with the size of your system piping to find litres per metre of pipe size (see pipe volume chart) and multiply by length of pipe in metres.
Litres per Metre x Length of pipe
+ Ls in System Piping
Nominal Pipe SizeLitres per Metre of LengthNominal Pipe SizeLitres per Metre of Length
50mm1.96250mm49.08
75mm4.41300mm70.68
100mm7.85350mm96.21
125mm12.27400mm125.66
150mm17.67450mm159.04
200mm31.41500mm196.34
Step 3. Litres of Water in Tower
Add one litre per ton or rated capacity of cooling tower
Ton cooling capacity x 1 litre
+ Litres in tower
Step 4. Calculate Total System Water Volume
Add step 1 + step 2 + step 3
= Total Litres in System
Step 5. Decide Turnover rate
Assuming a 1 hour (60 min) turnover rate,
Divide the result in step 4 by 60
= Minutes
Step 6. Filter Flow Rate for Sizing
Side-Stream Filter system
= Litres/Minute

Side stream filtration draws a percentage of the cooling water from the basin or, alternatively, the balance line. Sediment is then filtered out and the water pumped back into the system via eductor nozzles to the main condenser line or the basin.

Please refer to the calculation table to assess the required size for your tower.

Eductor nozzles agitate the water in the cooling tower which serves two purposes; to blend in the condenser water and assist the sediment to pass through a filtration system rather than settle and cause a decrease in efficiency.