The increasing role of UV disinfection and filtration in server farms

Written by Jon C McClean, President of ETS-UV by Neptune Benson

An increasing number of technology industries are turning to cooling towers to remove excess heat from buildings or processes. Server farms or server clusters are typically located between the system switches and the routers, and the removal of heat from these facilities is critical to their optimal function. The advances in cluster computing, scientific simulation (such as Computational Fluid Dynamics), the rendering of detailed 3D images for health care and the complex transactions required by web enterprises are all processed at server farms. Performance of the servers is limited by cooling, rather than processing speed, and in many cases for every 100 watts used to power the server, 50 watts is required to cool it. The critical design parameter for these large and complex, continuous systems is performance per watt. Maintaining effective and continuous cooling is critical.

Facebook has established a server cluster in Lulea, Northern Sweden within 62 miles of the Arctic Circle to benefit from the availability of cold air. High speed fiber optic cables link the USA to cooler climates such as Iceland. Google operates 12 data centers globally, with 6 in the USA, and uses 260 million watts of power, or 0.01% of global power consumption. Amazon operates 450,000 servers across 9 locations globally, with a 10th under construction in Ningxia, China. These complex, large scale operations require a great deal of cooling, and for some time now the trend has been to move away from the use of chemicals and towards nonchemical, more water efficient and critically robust disinfection processes. UV disinfection of the cooling water plays a central role in these process critical applications, preventing harmful microbial growth that can pose a danger to people, or can affect the performance of the cooling system.   

How does the cooling take place?

Evaporative cooling occurs when water evaporates, changing state from liquid to vapor and requires an input of heat energy - the latent heat of evaporation. The input of heat is drawn as a waste product directly from the server facility.

Modern heat rejection requirements employ Cooling Towers or Evaporative Condensers as the most efficient and cost effective method by maximizing the contact between air and the water to be cooled.

Cooling tower circulation cycle - Illustration courtesy of Neptune Benson

Cooling towers used in evaporative cooling water systems and domestic hot and cold water systems are a common source of legionella. The disease is transmitted via the inhalation of mist droplets containing the bacteria. The use of UV water treatment ensures that microbial contaminants are effectively inactivated, and also slime formers that impair cooling performance are eliminated.  Unlike chemical disinfection systems, organisms do not demonstrate a tolerance or resistance to UV light. Typically, cooling towers require nearly 66% less power to reject a given amount of heat than alternative “dry methods”. In addition, they occupy a smaller footprint and are significantly quieter. Some server farms use reclaimed water for cooling, although all need optimal performance from their cooling loops.  

ETS UV disinfection system - Image courtesy of Neptune Benson

Cooling towers evaporate pure water, leaving any suspended or dissolved solids, such as minerals etc, behind in the retained water. This resultant build-up of solids, or concentration factor would leave the water unusable, reduce operating efficiency, and potentially damage the recirculating system.

In an effort to reduce build up, it is necessary to blowdown or bleed a proportion of the system water. In the US, the total dissolved solids (TDS) of the supply water requires that the concentration factor within an evaporative cooling system is maintained at 3 to 3.5 times, requiring an amount of water equivalent to up to 50% of the evaporation losses is bled to waste. For a typical MW (1000Kw) of heat rejected, this equates to 150 to 200 gallons per hour that is drained to waste. Several novel approaches are being utilized for cooling water systems, included using reclaimed or wastewater for cooling in an attempt to reduce the use of potable water. The selection of filtration products that minimize the backwash water loss is critical, and high efficiency filters such as the Defender® Regenerative Media Filter (RMF) from Neptune Benson.

Cooling towers are effective air scrubbers. As a consequence of the cooling method, they flush airborne contaminates into the system where they deposit on and foul the heat exchange surfaces. Suspended matter in the cooling water also supplies waterborne microorganisms with a supply of nutrients. Modern UV systems use automatic wipers to keep the optical path free from contamination. Many of these airborne contaminants, as well as iron in solution in the water will foul the quartz sleeves and prevent optimal disinfection of the cooling water.

Fouled UV systems do not perform well.  Effective wipers are critical.  Picture courtesy of Neptune Benson

A permanent full flow filtration and UV combination can remove these contaminants before they increase the cost of operation, cause infection, or shut the system down. The early application of UV for cooling water loops was to disinfect a side-stream flow. Modern, high capacity UV systems, when used with the correct separation processes, can deliver a high dose of UV to the cooling loop and turn over the entire reservoir frequently.  A key benefit of UV disinfection is that the water cannot be overdosed.

Typically a cooling system will turn over the entire volume of water every several times each hour.  A typical reservoir might contain 7000-15,000 gallons, with a filtration rate of 500 to 1000 gallons per minute. A 100-ton cooling tower for would recirculate the cooling water at 300-500 gallons per minute. Side-stream technologies are a lower cost, but less effective method to disinfect the cooling system. Process critical applications such as server farms need to have full flow, automated disinfection operating 24 hours each day, 365 days per year.

The Defender® Filter from Neptune Benson is a replacement to the older sand filters. It uses perlite, a regenerative media that provides 90% more filtration surface area than traditional sand filters. This allows both footprint reduction and notably, it uses 90% less backwash water than a sand filter. The Defender® filters the water to a 1-micron particle size, and provides excellent pre-treatment for a UV system.

Defender® Filter - Courtesy of Neptune Benson

Microbial concerns in cooling systems

The dynamics of flora and fauna in cooling water systems are beginning to be better understood. In systems where a single microbial group or species dominates, fouling problems can often occur. In a balanced population mix, often little or no fouling is evident. It is probable that when mixed populations co-exist, they compete for the available oxygen and nutrients, and so control each other’s growth. When one group successfully displaces the others, its growth can proceed without competition and biofilm and slime formation rapidly follow. 

ETS UV provides effective full flow disinfection - Picture courtesy of Neptune Benson

A wide variety of bacteria, including Klebsiella Pneumoniae, Bacillus Emegaterium can colonize cooling systems. Spherical, rod-shaped, spiral, and filamentous forms are common. Some are spore producing to survive adverse environmental conditions such as dry periods or high temperatures. Both aerobic bacteria, needing oxygen to survive and anaerobic bacteria such as Desulfovibrio Desulfurcans (SRB), which can survive in the absence of oxygen are found in cooling systems. The SRB species are directly linked to Microbial Induced Corrosion (MIC), as they metabolize Sulfur and form Hydrogen Sulfide as a waste product. This then leads to hydrochloric acid formation, and causes corrosion of pipe and structures.

Several forms of fungi are encountered in cooling systems, including Candida Krusei and Trichoderma Viride. Filamentous molds will lead to rot of any exposed wood and as with yeasts; they are prolific slime formers that will impair cooling performance.  

Algae, including Chlorella Pyrenoidosa and Scenedesmus Obliquus, are commonly found in cooling systems. Green and blue-green algae are very common in cooling systems. Several species of algae will produce the growths that foul screens and block distribution decks. Without disinfection, algae fouling will lead to unbalanced water flow and dramatically reduced cooling tower efficiency.

Summary

Modern UV disinfection systems are capable of disinfecting the full flow of modern cooling loops, and disinfecting the entire water system many times each hour. These UV systems use automated wipers and sophisticated UV dose control algorithms to ensure that microbial growth within cooling systems is effectively eliminated. System feedback is provided thru MODBUS or Ethernet connection, with SCADA integration a standard feature. These tools give the operators of the server farms optimized loop cooling that minimizes the use of chemicals and ensures a highly robust and simple disinfection process.