Increasing urbanization and development and the corresponding increase in industrialization and climate change means wastewater treatment plays a major role in tackling the challenge of freshwater scarcity. Converting wastewater for reuse, particularly for agricultural and industrial purposes, which collectively consume 89% of water globally, means more freshwater can be made available for domestic use. Moreover, wastewater, if not processed and treated properly, may also have a negative impact on the environment, polluting rivers and seas and affecting animals, fisheries, biodiversity and public health.
One of the areas in wastewater treatment that has always posed a challenge is the removal of natural organic matter (NOM). NOM exists in all sources of fresh water, but left untreated, it can affect the efficiency of potable water-treatment processing and impair the quality of the finished water. At a time when every drop of water counts, removing NOM is essential for optimizing process efficiency and public health.
Why Is NOM a Problem in Drinking Water?
NOM occurs when decayed vegetable matter mixes with water. It can also be found in wastewater or industrial effluent. It is a multicomponent organic mix of several compounds, humic acids, fulvic acids, carbohydrates, proteins and carboxylic acids, most of which derive from soil and the decomposition of plants and animals that enter streams, rivers, lakes and aquifers. The chief indicator of humid and fulvic contamination is discoloration in the yellow-brown spectrum.
While not considered toxic in drinking water, NOM can detrimentally affect color, taste and odor. More importantly, it has been found to act as a precursor for toxic and mutagenic compounds like trichloromethane and chlorophenols, which originate from disinfection of the drinking water by chlorine, and which increase the risk of pathogen-induced diseases.
At certain pH levels and temperatures, a reaction with a disinfectant like chlorine that is intended to make water safer can have the opposite effect. Trihalomethanes (THMs) in drinking water have been linked to heart, lung, kidney, liver and central nervous system damage, as well as bladder or colorectal cancer. High levels of THMs have also been associated with above-average risk of miscarriages. THM levels in public water supplies must therefore be carefully monitored. Reducing the risk from THMs requires highly accurate detection methods. The challenge for water utilities is to identify and remove these contaminants so that they can treat the water effectively.
Methods of NOM-Removal
Coagulation, the chemical water-treatment process that manipulates the electrostatic charge of particles in suspension to remove solids, is one of the main methods of NOM-removal and is considered essential for the control of pathogens. In terms of cost efficiencies, getting coagulation right is important. Suboptimal coagulation causes high residuals, post-treatment problems and microorganisms getting into the water supply, as well as additional costs in waste disposal. Optimal coagulation minimizes turbidity and removes microorganisms/pathogens including cryptosporidium parvum, a chlorine-resistant protozoan, which can cause diarrhea.
All coagulants have an optimum pH value at which they work most efficiently. Continuous monitoring of the pH values of raw water is necessary in order to optimize the dosage of coagulant.
To minimize the risk of undesirable disinfectant by-products (DBPs) creeping into the water system, utilities use modified conventional processes, such as enhanced coagulation and softening, to ensure organic matter is dissolved to safe levels. Many processes also use an activated carbon stage as an additional means of capturing dissolved organic matter that may have escaped the coagulation stage. Granular activated carbon (GAC) filters remove dissolved organic compounds and are effective for drinking water purification, as well as decontamination of polluted water.
Instrumentation in NOM-Removal: Color Monitoring & UV Detection
Accurate, reliable and representative information on the performance of the coagulation, clarification and filtration stages is paramount to successful NOM-removal. Near-instantaneous online analyzers that measure levels of dissolved organics are playing an important role in mitigating health and efficiency issues that arise from contaminants. Technological advances have enabled water companies to invest in safety measures at a lower cost than ever before.
When detecting dissolved organics in water during the potable water treatment processes, two methods apply: color monitoring and ultraviolet (UV) detection. Coloration of water, specifically the degree of color intensity and changes in color, indicates the levels of either the absorption or concentration of certain chemicals and is regarded as a test of treatment plant efficiency.
Of course, many dissolved substances and chemicals do not absorb light in the visible spectrum and are therefore colorless. Chemical reagents are therefore introduced to create a reaction that forms a legible color complex that can be measured.
Color is the main parameter for coagulant-dose demand. By monitoring coloration of raw uncoagulated water, color analyzers can provide predictive coagulant-control, thereby ensuring optimal levels of coagulants are administered throughout the process. Installing color analyzers at the final treated water storage phase helps verify the treatment process is functioning correctly and water quality standards are met.
Another important factor when measuring color is turbidity. Turbidity—cloudiness or opacity—is a key test of water quality. However, it causes problems when one is differentiating between true and apparent color. For analytical purposes, true color is attributed to dissolved matter and apparent color to suspended matter. Turbidity can result in the apparent color showing a higher value than the true color, which can affect color-determination and therefore impair results. That is why, when turbidity is a factor, it is imperative to properly filter a water sample before introducing it to the analyzer to obtain a true color-intensity and an accurate result.
The most advanced instrumentation measures the three main parameters in the water treatment process (color, pH and turbidity) and feeds the numbers into an algorithm, which then calculates the required dose in milligrams per liter (mg/L). Taking the incoming flow of water, it then equates that mg/L reading to the volumetric dose of milliliters per minute (mls/min), which produces an accurate figure for the required dose. Assuming all instrumentation is functioning correctly, one should end up with good, clarified water ready for filtration.
The latest instrumentation in the detection of NOM has been designed specifically to measure coloration for coagulation optimization, ensuring water is sufficiently dosed and treated to reduce dissolved concentrations of NOM to approved safe levels.
UV monitors measure the capacity of water to absorb ultraviolet light at 254 nanometers (nm), commonly known as UV 254; the UV-light absorbent qualities of water are directly related to levels of organic matter. These levels are calculated by measuring a pulse of light at two wavelengths. One wavelength provides a reading from a turbidity photo detector; the other, from a UV photo detector. Put simply, in addition to the absorption measurement at 254 nm, a second measurement at 400 nm enables the monitor to compensate automatically for fluctuations in turbidity.
Many manufacturers offer UV monitors that can be configured to measure UV transmission, known as %UVT. UV transmission measures the light that passes through a sample, enabling it to produce a %UVT value for key parameters: color, turbidity, particulates and organic and non-organic matter. In potable treatment processes, measuring %UVT can be used for checking the quality of treated water and assessing the efficiency of the coagulation, flocculation, sedimentation, filtration and carbon adsorption phases. Measuring UV transmission can also help detect residual dissolved organics that could generate DBPs during chlorination. Online analyzers can detect dissolved organics faster and more effectively than traditional methods, in most cases eliminating the need for laboratory testing, which leads to cost savings.
These advances in detection devices are helping to reduce the likelihood of contamination by dissolved organics, leading to safer supplies of drinking water.
Avoiding DBPs
There are some simple ways of minimizing risk of DBPs. Instrumentation should be installed as close to sampling points as possible and in places where it can be easily accessed, operated and maintained. Sample lines should be straight and smooth to avoid sedimentation traps. When supplying the coagulant dose, ensure dosing pumps are correctly sized to cover the entire dosage requirement. The coagulant flow meter should be installed on the side of the dose pump. Where automated control of dosing pumps is employed, the control software needs to be capable of responding to real-time dose-demand changes.
Controlling dissolved organics has historically been a problem for the water industry. The presence of NOM-based THMs in municipal water supplies remains a daily challenge for water treatment operators, especially in countries which commonly treat drinking water with chlorine which can lead to DBPs. Optimization of coagulation is central to the drinking water industry’s NOM-removal goals. By providing insights into dissolved organics levels in real- or near-real-time, color and UV analyzer technologies can make a big impact in ensuring water quality meets required safety standards.