DOC (Dissolved Organic Carbon) is an important parameter in industrial water and wastewater treatment and indicates the concentration of organic carbon compounds present in water in dissolved form. This parameter plays a decisive role in the assessment of water quality, as organic compounds influence various treatment processes and can have a negative impact on the efficiency of the systems.
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Technical background and significance of DOC
DOC is a sum parameter that includes all dissolved organic substances in water. This includes both naturally occurring compounds, such as humic substances and fulvic acids, as well as synthetic and anthropogenic contaminants, such as industrial chemicals, pesticides and pollutants from wastewater treatment. DOC is particularly relevant in drinking water treatment, as organic impurities affect taste and odor and can lead to the formation of harmful by-products such as trihalomethanes (THM) during disinfection with chlorine.
DOC as an indicator in water treatment
The measurement of DOC is crucial for monitoring the efficiency of biological and chemical-physical treatment processes. A high DOC content in water can promote the formation of biofilms on membrane surfaces, which leads to biofouling. In biological treatment processes, the DOC serves as an indicator for the degradability of organic substances and is decisive for the control of activated sludge or membrane bioreactors.
Determination methods of the DOC
The DOC is determined by oxidizing the organic carbon compounds and then measuring the resulting carbon dioxide (CO₂). Common methods include:
- High-temperature combustion: In this process, the water is oxidized at high temperatures (650-900 °C). The carbon released is measured as CO₂ and reflects the DOC content. This method is used particularly in process control and drinking water analysis.
- UV persulphate oxidation: The organic compounds are oxidized by irradiation with UV light and the addition of persulphate ions. This method is particularly suitable for low concentrations and sensitive measurements.
The choice of method depends on the type of water sample, the required precision and the specific requirements of the application.
Applications and influence of DOC on industrial water and wastewater treatment processes
Drinking water treatment:
- A high DOC content in raw water can promote the formation of disinfection by-products that are potentially harmful to human health. In drinking water treatment, DOC is often removed by activated carbon or biological filters to minimize these risks.
Reverse osmosis and membrane filtration:
- Dissolved organic matter contributes to the formation of biofilms on membranes, which is known as biofouling. In reverse osmosis systems, high DOC can significantly reduce efficiency and shorten membrane life. Pretreatments such as biofiltration or the use of antifouling agents are used to reduce the risk of biofouling.
Biological wastewater treatment:
- In biological processes, such as activated sludge or membrane bioreactors, the DOC serves as a food source for microorganisms. An adequate DOC content promotes microbial degradation and improves the performance of the biological reactors. In systems with insufficient DOC, it may be necessary to add organic matter to stabilize the bacterial population and optimize the degradation of organic compounds.
Industrial wastewater treatment:
- Many industrial wastewaters, particularly from food processing, the pharmaceutical industry and chemical production, contain high DOC values. These compounds can be toxic or increase oxygen depletion in downstream waters. Here, the DOC is removed by processes such as precipitation and flocculation, biofiltration or chemical oxidation processes such as Fenton process specifically reduced.
- An increased DOC content in the cooling water can promote the growth of biofilms and microorganisms, which leads to fouling in the cooling circuits. In open cooling circuits, the DOC is monitored in order to optimize the risk of fouling and the efficiency of cooling. Biocides or suitable oxidation processes are used for this purpose.
Photo: Biofouling on a heat transfer surface of a cooling system
Measures to reduce DOC
The choice of methods for reducing DOC depends heavily on the type of compounds and the requirements for the final water:
- Activated carbon adsorbs dissolved organic compounds and is often used in drinking water treatment. This is a cost-effective method for DOC reduction and is particularly suitable for removing odors and flavors.
- Biological activated carbon or sand filters use microorganisms to break down organic compounds. These filters are used in biological pre-treatment and are particularly effective for wastewater with a high biodegradability of DOC.
Fenton process and ozonation:
- The oxidation using Fenton reagents (H₂O₂ and Fe²⁺) and the ozonation are proven methods for reducing DOC in industrial wastewater. They break down organic substances efficiently and are used in particular for compounds that are difficult to break down.
- Membrane processes such as reverse osmosis and nanofiltration can effectively retain DOC and can be used to produce high-purity water. However, these processes are susceptible to fouling and require pre-treatment to maximize efficiency.
Photo: ALMA OSMO VE reverse osmosis system for the production of demineralized water and DOC reduction
Benefits and challenges of DOC monitoring and treatment
Advantages:
- Improved water quality: The reduction of DOC leads to improved water quality and minimizes the formation of undesirable by-products.
- Protection against biofouling: By controlling the DOC, biofouling and deposits in membrane systems and cooling circuits can be reduced.
- Compliance with environmental standards: DOC control is crucial to meet legal requirements for water quality and reduce pollution.
Challenges:
- Complexity of organic compounds: DOC comprises a large number of organic substances that can vary greatly in their structure and chemical reactivity. It is therefore difficult to apply a universal treatment method.
- Cost-intensive measurement methods: DOC determination and monitoring often requires specialized equipment and is costly, especially in industrial applications with continuous monitoring.
- Fouling problems in membrane filtration: A high DOC content increases the risk of fouling, especially in reverse osmosis. This leads to higher maintenance costs and requires effective pre-treatment methods.
Conclusion
DOC is a key parameter in industrial water and wastewater treatment that has a direct impact on treatment processes and water quality. Its measurement and control are crucial to maximize the efficiency of biological and physico-chemical processes and to avoid biofouling. ALMAWATECH offers specialized solutions for the reduction of DOC, from biological pretreatments to chemical oxidation processes, which make it possible to ensure high water quality and reduce operating costs through fouling and biofouling prevention.
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