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Switching to continuous water quality analyzers offers numerous opportunities

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water-quality-analyzer
There are numerous benefits to continually monitoring water quality throughout the treatment process.

By Redir Obaji

Today, the task of managing the quantity of water resources and the quality of drinking water is unimaginable without online instrumentation to assist water utilities in managing, treating, and delivering clean and safe drinking water to consumers.

With developments in technology, operators can increasingly make informed decisions based on real- or near real-time data, opening new possibilities not previously available through traditional water quality sampling methods. This is becoming increasingly desirable as the management of water supplies comes under mounting pressure from rapid population growth, rising urbanization, and steadily growing demand from industrial processes. These consume large amounts of water and generate effluent waste that needs to be treated and safely returned to the environment.

When it comes to potable water treatment, a variety of parameters need to be accurately and reliably measured, with each having their own potential impact on water quality if left unchecked.

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Water quality parameters

Aluminum in water can be attributed either to its natural presence in soil or as a result of its usage as a flocculant to remove impurities during treatment processes. Where used in water treatment processes, aluminum serves to reduce the turbidity and bacterial content of water prior to the final stages of treatment and disinfection.

There is dispute over its potential effects on health, with excessive levels thought to be linked to Alzheimer’s disease, although aluminum in drinking water represents only a very small percentage of the average person’s total daily intake. If left unchecked, excessive levels can lead to kidney dialysis problems. The Canadian guideline for aluminum in drinking water stipulates a maximum concentration of 0.2 mg/l.

Iron in potable water does not present a health hazard. However, if not closely controlled, its presence can cause problems. It can clog pipelines, pressure tanks, water heaters and water softeners. It also can cause staining of laundry and items such as crockery, cutlery, water fittings, sinks and bathtubs.

Sources of iron vary. In most cases, it will either be naturally present due to local geological conditions or will have been introduced as a result of water treatment or the corrosion of iron water mains. This last source is one of the key factors responsible for elevated iron levels in the majority of cases where water exceeds the permitted maximum level. Water that travels through distribution networks comprised of extensive runs of cast iron pipes is particularly prone to high levels of iron. Operational problems such as burst mains can disturb iron deposits in the pipes, discolouring the water and causing an unpleasant taste. To control levels and minimize the problems associated with excessive iron levels, the maximum permitted concentration for iron in potable water is ≤0.3 mg/l.

Manganese occurs naturally in many sources of water. Like iron, it has not been proven to pose a risk to human health but can have a negative impact on the appearance of drinking water if not properly treated. Failure to properly control manganese levels will result in black deposits collecting in pipe networks, which may turn potable water black if disturbed. Most complaints about manganese in potable water relate to staining of laundry or vegetables becoming discoloured during washing or cooking. The maximum permitted level for manganese in potable water is 0.1 mg/l.

Many water companies introduce phosphates to their water supplies to help prevent lead from old pipes dissolving into the water. The main concern relating to phosphate concentrations stems from the issue of eutrophication, where high levels of phosphate cause excessive growth of plants and algae that form on the surface of lakes, rivers and streams. By preventing light from penetrating through the water, these growths stop plants beneath the surface from photosynthesizing, causing them to die. Together, these dead plants, and the dead algal blooms that sink down, use up oxygen in the water as they decompose. This reduces the amount of oxygen available for other aquatic life, causing it to suffocate. Over time, this process can render a water body lifeless.

Sources of phosphate are numerous. As well as being added during the treatment process, phosphate levels in water can also be attributed to agricultural activities, animal wastes, human sewage, food wastes, urban run-off, vegetable matter, industry and detergents.

The amount of phosphate in water is not universally regulated. However, the World Health Organization sets a recommended maximum “safe” level of around 5mg/l and states that a person’s recommended daily allowance should not exceed 800 mg. Different countries may also have their own specific rules for permitted levels of phosphate.

Using colorimetric measurement to monitor potable water quality

Colorimetric measurement is used extensively throughout water, power and process industries. Simply described, the technique involves the colour-based measurement of a chemical in a solution. It is used to determine either the absorption or concentration of that chemical, based on the degree of colour and the ability of light to pass through it.

The colour comes from the absorption of certain wavelengths from the visible light spectrum within the range 400 to 700 nanometres.

Water quality analyzers
Colourless substances do not absorb light in the visible spectrum.

Where water is concerned, many of the substances that need to be measured are colourless and do not absorb light in the visible spectrum. To overcome this, and enable the substances to be measured, chemical reagents are used to create a reaction that forms a coloured compound. The reagents vary according to the parameter being measured.

Table 1. Reagents vary according to the parameter being measured.

ParameterChemical methodMax sample frequencyInstrument measurement range (including dilution)
AluminumPyrocatechol violet6/hr0.005 – 0.3mg/l Al 0.3 – 1.5mg/l Al
IronTPTZ6/hr0.005 – 1mg/l Fe 1 – 5mg/l Fe
ManganeseFormaldoxine6/hr0.020 – 2mg/l Mn 2 – 10mg/l Mn
PhosphateMolybdate4/hr0.050 – 8.5mg/l PO4 8.5 – 50mg/l PO4

Adding the reagent creates a dilute solution of molecules that absorb light. By measuring the absorption/passage of light through the coloured sample, the concentration of the parameter being measured can be ascertained.

The benefits of continuous online measurement

Traditionally, many companies have operated measurement routines based on spot testing, with samples being sent to a laboratory for testing to produce a water quality measurement. This technique has inherent drawbacks, not least of which is the fact that a sample will only ever be indicative of a particular set of conditions for a particular moment in time. Added to this is the risk of uncertainties being introduced into the measurement as the sample is transported between locations, potentially compromising the accuracy of the resulting data.

Continuous measurement using online analyzers overcomes these problems. By increasing the frequency of sampling, online analyzers provide a true indication of water quality under a variety of conditions. Furthermore, as samples are measured in situ within the analyser itself, any uncertainties that could affect the accuracy of the measurement are removed.

Aztec 600 Iron Analyzer
Aztec 600 Iron Analyzer.

ABB’s Aztec 600 series of large case analyzers use the principle of colorimetry to measure concentrations of aluminum, iron, manganese and phosphates. Able to measure up to six samples an hour, the analyzers use an LED and detector to measure the passage of light through a sample. A single precisely controlled piston pump provides all the sample and reagent fluid handling for measurement, mixing and disposal. Measurements are taken before and after the addition of reagents to compensate for background colour and turbidity. These measurements are compared against the calibrated values to calculate the value of the sample being measured.

The inherent benefits of these analyzers enable them to be used in a variety of applications.

Removing iron, manganese and phosphate

As naturally occurring minerals, manganese and iron can easily find their way into raw water supplies, with levels tending to fluctuate according to conditions such as climate, ground erosion, thermal changes and disturbances of water sources.

Removal plants are installed to reduce the levels of these substances and to help control taste, odour and fouling caused by their presence. Close control of the treatment processes is required to help reduce energy consumption, optimize chemical usage and maximize treatment efficiency.

The control of iron and manganese levels varies according to whether they are in insoluble/particulate or soluble form. Insoluble/particulate iron or manganese is a characteristic of well oxygenated water sources and can be easily removed through filtration.

The soluble forms tend to be encountered at the deeper sedimentary levels in wells, rivers and other water sources, most often during periods of hotter weather when water is less abundant. Removing soluble iron and manganese is more difficult and requires using different processes, first to convert them to particulates and then to remove them through filtration. Phosphate removal is done through either a chemical or biological process.

In all these processes, manganese, iron and phosphate analyzers should be used to monitor water quality at several key stages, namely:

Pre-treatment – This stage sees the quality of the incoming water being measured to assess initial levels of iron, manganese or phosphate present before the application of treatment processes, including aeration and chemical dosing.

Treatment stages – After any pre-treatment process, the water will again be monitored to check iron, manganese and phosphate concentrations. Where a clarification process is used, water may also be monitored to assess whether coagulants used in the treatment process are being under- or overdosed, which can involve using other analyzers to detect colour or turbidity levels.

Post filtration – A final measurement will be carried out after the filtration process to ensure that any residual levels of iron, manganese and phosphate meet the required treatment standards. Where iron-based/ferric chloride coagulants are used, this measurement will also be used to assess the efficiency of the coagulation process, by identifying any potential overdosing. Overdosing of water with ferric chloride can lead to sludge formation and discoloration, which can cause problems for customers, as outlined earlier in this article.

Waste discharge – In addition to the above, one or more online analyzers such as the Aztec 600 can also be used to monitor the effluent discharge from the sludge holding tanks, again to assess the efficiency of the treatment process. This helps to identify whether the correct levels of treatment chemicals are being used, which can help operators to find potential areas for cost savings through greater efficiency.

Residual coagulant monitoring

Coagulation is a safe and effective method of treating surface water. It is used to improve water quality by reducing levels of organic compounds such as manganese, dissolved phosphorus, colour, iron and suspended particles.

Coagulation techniques have been developed to bind together very small particles in water that will not settle or float and which cannot be removed by filtering. A chemical salt is added to electrically charge small waterborne particles (known as “colloidal matter”) so that they attract and bind to form larger particles, or “floc”, which can then float or settle. These salts are either ferric (iron) or aluminum based.

Using an online analyzer can help to regulate the dosing of these salts to ensure the treated water meets the required standards.

Monitoring coagulation efficiency also provides an added safeguard against the risk of waterborne diseases such as cryptosporidiosis. Failure to correctly coagulate and filter water can increase the risk of a cryptosporidium outbreak, which can have serious repercussions on water companies if they are found not to have exercised every care in treating their water supplies.

Cost saving benefits

As a means of continually monitoring water quality throughout the water treatment process, continuous online analyzers offer a range of cost saving benefits, including:

  • Reduced failures and maintenance issues caused by underdosing, such as plant shutdowns or increased cleaning of sand filters.
  • Reduced delays and failures caused by overdosing, such as discoloration, plus the associated costs of chemical dosing for pH correction.
  • Reduced operator intervention through the ability to carry out automatic monitoring.
  • Decreased levels of sludge, leading to a reduction in the cost and resources connected with sludge disposal.

Improvements in any of these areas can offer potential savings of thousands of dollars, either directly through operational savings, or indirectly through avoidance of fines and penalties arising from regulatory non-compliance or incorrect measurement.

Redir Obaji is with ABB Measurement and Analytics. This article appears in ES&E Magazine’s August 2019 issue.

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