How to know: The right check for industrial water

Industrial water used in industry must meet certain quality standards, which vary depending on the sector. Monitoring water quality in industrial processes is therefore crucial. It has been proven that regular monitoring of industrial water can mitigate risks and system complications before they become problematic. In addition, for many sectors there are concrete legal requirements for the type of control and the procedure for the investigations.

Pre-Treatment

Raw/Make-Up Water

Raw and Make-Up (MU) water should at least be tested for minimum values. Especially in the light of recent climate changes with droughts and floods, water quality can be significantly affected. Minimum controls are: 

• Conductivity  –    
  measures the dissolved solids level and can be used to quickly indicate if your site is             experiencing a change in MU water quality when compared to previous readings.
• pH –     
  confirms MU water quality and can indicate contamination, but the likelihood of mains water contamination is not very common
• Hardness –     
  confirms MU water quality and can be used to diagnose/set-up any pre-treatment plant that exists down-stream. If the hardness level has increased, an on-site water softener’s regeneration frequency will likely need resetting. Failure to address can result in softener overruns and scale control issues.
• Chloride –    
  needs to be checked periodically and whenever a change in 'normal' MU water quality is suspected.

Softened Water

Raw and Make-Up water must also be softened before it can be used in the industrial process. Here the performance of the water softening process must be checked. Conductivity/TDS is also provided, as it is a simple test that can quickly reveal problems in the regeneration of water softeners. A water softener must be regenerated with salt (sodium chloride) after it has been "exhausted" by removing hardness from the incoming make-up water.

• Hardness –     
  A properly functioning water softener should be capable of providing < 2 ppm hardness.
• Chloride (Conductivity/TDS)   –    This is to check if your water softener has regenerated properly and rinsed all the excess salt off the softener before going into service. To accurately assess this, you should test the softener immediately after it finishes the regeneration process, before it goes into    service, which is not always easy to catch naturally. Chloride levels should be no greater than the MU water level. If safe to do so, performing a regeneration while you are on-site can allow this check to be made.

Other forms of pre-treatment

There are many other forms of pretreatment available such as de-alkalisation, reverse osmosis, de-mineralisation, ultra-filtration etc. Overall recommendation is to assess what the pre-treatment is designed to do and then test if it was successful.
A quick example, a de-mineralisation plant can produce near distilled water quality with almost zero dissolved solids.
So, a simple conductivity check would be a good measurement to assess performance. Remember, these are the minimum recommended tests.

Closed Water Systems

Closed water systems are closed to the greater environment and use very little Make Up (MU) water in their normal operation. A properly 'tight' system will have no more than 5 % MU water annually, annually, as such closed water systems do not concentrate up the amount of dissolved solid present in the MU water. This fact usually means 'calcium' based scale formation is not as significant a risk as corrosion is for these systems.

Minimum recommended tests for a closed water system would include the following: 

• Conductivity –    
  the conductivity of a closed system can vary based on the initial MU water conductivity and the type of chemical treatments that are in use. Measuring the conductivity on start-up and on each service visit allows you to monitor in-spec trends and identify any large changes in readings. A significantly lower conductivity reading may suggest a system leak has occurred. A significantly higher reading may suggest that there has been an addition of a chemical(s) since the last visit, or some form of contamination has occurred. 
• pH-Value –     
  routine monitoring (trending) of the pH, likewise conductivity, will give a reasonable confirmation that the system is running well. High pH could mean excess chemical addition or some form of contamination. Low pH would usually mean some form of contamination has occurred or a     high level of sulphate reducing bacteria (SRB’s) may have taken hold in the system giving off low pH hydrogen sulphide as a byproduct. Systems with measurably high SRB’s will likely have a heavy biofilm formation as well.
• Hardness –     
  not normally required. As closed systems do not 'cycle-up' from evaporation, hardness levels should stay around MU water levels or just below as some hardness may have dropped out of solution. 
• Iron –    
  unlike hardness, corrosion is a common problem and is typically the main cause of failures with closed water systems. Corrosion problems can sometimes link right back to the installation of the system, and all too often are the result of an insufficient pre-commissioning program. 
• Inhibitor Levels  – 
  One of the more important tests that should be performed during each SV is a check on the inhibitor level. In general, most closed water systems tend to be treated with molybdate-based or nitrite-based corrosion inhibitors as the primary chemical component. Whichever inhibitor is in use, there should be clear guidance on the control levels needed and it should be tested for its active
  ingredient every SV. The test results are then used to assess whether any chemical additions need to be made to the operating system.

Note: other tests that can be done on closed systems include alkalinities (P & M), chloride, hardness etc. but the above water tests should be sufficient to allow an operator enough information to keep control of any closed water system.

• Microbiological testing  -
  There are several microbiological tests that can be done during a SV on a closed water system. These include dipslides for general bacteria levels (TVC’s), more specific dipslides for pseudomonas (aeruginosa or species) as well as tests for Nitrite Reducing Bacteria (NRB’s) and Sulphate Reducing Bacteria (SRB’s). For systems that contain glycol for freeze point depression, checks for yeast and moulds is a good recommendation. Any of these     microbiological     controls will require incubating your sample for prescribed times at set temperatures but will usually provide activity     levels well ahead of samples sent to a laboratory. 
• Turbidity/Suspended Solids –
  a visual check of the water clarity is the easiest of tests, yet always a good indicator of the condition of the operating system. Turbidity/suspended solids is usually left as the visual check and an 'appearance' result is entered. If a multi-parameter photometer is used for site testing it’s likely that one or both tests can be performed by the electronic meter.

Cooling Tower Systems

Cooling towers are used to expel heat from some process needing to be cooled (i.e., machine cooling/air conditioning/refrigeration) via a water re-circulation system that eventually flows over a cooling tower. When operating, a cooling tower by design is open to the environment and will incur evaporation losses to varying degrees. I tis this evaporation loss that results in the concentrating up of the MU water dissolved solids level in the system. Commonly known as 'cycling-up', this aspect of cooling towers tends to change the emphasis from corrosion to scale formation when looking at the major concerns with using water as a heat rejection medium, unless some form of water softening is involved. In very general terms 'all' of the tests that have been discussed with closed water systems are applicable when routinely testing cooling towers. However, there is significant interest in the alkalinities (M & P) and the hardness levels (particularly calcium hardness) as they relate to the likelihood of scale formation.

• Alkalinities (M, P & OH) –     
  There are generally 3 types of alkalinities that need to be discussed when talking about     general water chemistry. These are bicarbonate (HCO3-) alkalinity, carbonate (CO32-) alkalinity and hydroxide (OH) alkalinity. If we look in very general terms at the characteristics of these alkalinity types, we can see that they have varying levels of alkaline nature 
  •    HCO3-Alkalinity exists to a maximum pH of approx. 8.0
  •    CO32- Alkalinity exists to a maximum pH of approx. 10.5
  •    OH- Alkalinity exists to a maximum pH of approx. 14.0
  All raw waters around the world have varying levels of HCO3- alkalinity. As this form of alkalinity can only produce a pH of around 8.0 and most importantly has a high solubility level in the presence of water hardness, it would seem to be 'safe' to use water with HCO3- alkalinity for most cooling water applications. However, it’s well-documented that when water with HCO3- alkalinity is heated, it will convert through chemical process to form CO32- alkalinity which has a higher characteristic pH of around 10.5. 
  The more you use water for heat rejection which will naturally increase the water’s temperature, the more CO32- alkalinity we will generate with a corresponding increase in pH.  
• Hardness (Total & Calcium) –
  Testing the hardness levels during each SV is key to monitoring the system for potential scaling issues. Monitoring the calcium hardness levels in the MU water and comparing it to the level of calcium hardness in the cycled-up cooling tower is referred to as performing a 'calcium balance'. If MU water calcium hardness is 200 ppm, and you were controlling your cooling tower at 3.0 cycles of concentration (COC) then you would like to see a calcium hardness level of 600 ppm in the cooling tower. Anything less than a factor of 3 would suggest that calcium is dropping out as scale formation. We want to use water as a heat rejection medium: the more we heat the water the more CO32- alkalinity will form, and the more heat we pick up, the more CaCO3 wants to drop out of solution. 
• Conductivity/TDS –     
  Measuring conductivity/TDS has been mentioned under closed water systems but it is important to note that it is usually used as a controlling parameter for cooling tower systems. An automatic bleed control system based on an in-line conductivity probe will control a bleed valve to maintain the required CoC. 
• Biocides –         
  Control of the microbiological content in a cooling tower system is critical for several reasons including the need to control Legionella bacteria to reduce the risk of someone contracting Legionnaires’ Disease as well as the need to control other pathogenic microbes. Control usually involves a combination of physical control of the cooling tower to reduce the risk of exposure to an aerosol during operation (i.e., drift eliminators), with the use of chemical and/or non-chemical biocides. It should be noted that microbiological control will also help to minimize the formation of biofilms which will reduce the possibility of blockages, low flow areas, poor heat exchange and lessen the possibility of under-deposit corrosion. Oxidising biocides such as bromine, chlorine and chlorine dioxide are typically used on a continuous low level dosing program with evaporative cooling systems. These oxidising biocides are routinely backed up with the use of a non-oxidising 'shock dosed' biocide. It is important to not overdose the oxidising biocide as it can promote higher corrosion levels.

Steam Boilers

The following types of water should be tested when used in steam boilers to ensure satisfactory operating conditions:

• Raw MU Water 
• Softened MU Water (requires ZERO hardness levels)
• Condensate returns
• Feedwater (combination of above waters in varying quantities)
• Boiler Water

The following factors should be tested:

• Alkalinity –     Due to the temperatures and pressures involved within an 'operating' steam boiler OH hydroxide alkalinity is formed in the boiler. Some might think this would be a problem due to the high expected pH but mild steel 'prefers'     pH to be in the region of pH = 11.0–12.5 which minimises the corrosion potential for the steel construction. Each boiler type (manufacturer) will have recommended levels for M & P alkalinity control which will provide suitable levels of OH alkalinity.
  It is this OH alkalinity, when combined with the proper levels of your sludge conditioner additive (typically phosphate), that will allow any calcium or magnesium hardness to be properly conditioned' as a fluid sludge that can be removed via bottom blowdowns.
  It is also important to note that allowing alkalinity levels to run too high in a boiler increases the surface tension of the water making it difficult for steam bubbles to break free at the water/steam interface and join the steam space in the boiler. This is referred to as 'wet' steam and results in carryover of boiler water into the steam which can cause problems related to steam use and steam trap operation.  
• Temperature –     
  Levels of dissolved gases (particularly O2 & CO2) in water are directly proportional to the temperature of the water. As we do not want either dissolved gas to enter the boiler, both can cause corrosion problems, we try to    maintain as high a feedwater temperature as possible. Condensate returns, live steam injection system and/or    deaerators can be beneficial in raising the feedwater temperature. 
• Oxygen Scavengers –
  As per above discussion on temperature, we do not want any oxygen in our boiler feedwater. As such it’s common to dose an oxygen scavenger into the feedwater tank (or hot well) or directly into the feedwater line ahead of the feedwater pump. It is important that the chemical addition is made with sufficient reaction time to scavenge all the oxygen before it reaches the boiler. Most sulphite-based oxygen scavengers are catalysed so the pick-up of oxygen is 10 to 100 faster than un-catalysed, sulphite. When cobalt is used as the catalyst it will become inactive at a pH of 9.3 or greater, as such it is important to use a separate mix/dosing tank just for the catalysed, sulphite solutions. The cobalt catalyst precipitates as a brown floc. If you see this material collecting in the dosing tank, your catalyst has dropped out. When testing your sampled boiler water it is important to test the sulphite level first as the level can change as your sample picks up atmospheric oxygen when cooling down.
  Other forms of oxygen scavenger include the below chemicals. Tannin is listed here but acts as both a filming agent (tannate film), as well as an oxygen scavenger.
  
  •    Sodium Sulphite
  •    Erythorbate
  •    Diethylhydroxylamine (DEHA)
  •    Hydroquinone
  •    Hydrazine
  •    Carbohydrazide
  •    Methyl Ethyl Ketoxime (MEKO)
  •    Tannin
  
  It is important to note that guidance for the control levels of each of these different types of oxygen scavengers is available from the boiler manufacturers or the chemical suppliers.
• Sludge Conditioners –
  As with oxygen scavengers, there are many different formats for sludge conditioner with likely hundreds of proprietary blends. Some will attempt to keep solids in solution, in solution so they can be removed with surface blowdown, e.g. chelant based conditioners. Others like phosphate-based conditioners will seek to form 'fluid' sludges for bottom blowdown removal. Unlike oxygen scavenger dosing, there is no real need to         increase the reserve level when the boiler is not in operation as without actual feedwater entering the boiler and therefore there is no increase in demand for the sludge conditioner. It is important to note that the boiler sample needs to be filtered prior to testing for a phosphate reserve in order to remove calcium/phosphate complexes that could be tested as reserve phosphate.
• pH (condensate) –
  During the breakdown of HCO3- to CO32- and finally to OH- alkalinity conversion reactions, CO2 is released, which as a gas flows off with the steam. When the steam has cooled down enough to condense so that CO2 is dissolved again in the form of HCO3-, a pH around 8.0 would be satisfactory. However, when the CO2 dissolves back into the condensate, it forms carbonic acid H2CO3, which can lower the pH of the condensate to 4.0–5.0. This low pH condensate can corrode the condensate return pipework especially at the bottom of the pipe exposed to the acidic liquid.

Understanding Interferences

When running analytical methods users need to remain diligent about the tests being carried out and pay particular attention to details such as pH of the sample, cleanliness of the sample container and the colour produced by the chemical reagents. 
In complex water systems such as those found in water treatment plants, there are many species of chemicals which
may possess cross reactivity to the chemistry involved in measurement. These cross reactivities may lead to the production of a different colour than expected.
Water treatment professionals should be aware of the make-up of their system but also, in addition, the chemistry behind the analysis techniques they are using. 
With knowledge of the chemistry, potential problems from interferences can be avoided or compensated for. If those interferences are unaccounted for, wrong decisions can be taken in the care of the water system treatment leading to problems including increased corrosion or biofilm build up. 
Below is a common list of interferences users should be aware of when using any analytical methods. Our instruments and reagents are designed to mitigate some of these interferences as much as possible but the user must also take responsibility for eliminating these common issues: