Helena Joyce
Pollution is the introduction of harmful substances into the environment at a rate faster than it can be dispersed, diluted, decomposed, recycled, or stored harmlessly. It is usually classified by environment as air, water, and land pollution. However, modern society is also concerned about specific types of pollutants, such as noise, light, and plastic pollution. One such pollutant is dissolved organic carbon (DOC), which is defined as organic matter that can pass through a filter that removes material between 0.22 and 0.7 mm in size. DOC can originate from within or outside any given body of water and is an important factor in the functioning of marine ecosystems.
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What You'll Learn
Dissolved organic carbon (DOC) in wastewater treatment
Dissolved organic carbon (DOC) is a parameter of organic pollution for waters and wastewaters. It is defined as organic matter that can pass through a filter with a pore size between 0.22 and 0.7 micrometres. The fraction remaining on the filter is called particulate organic carbon (POC). DOC is a component of dissolved organic matter (DOM), which refers to the total mass of dissolved organic matter, including elements such as nitrogen, oxygen, and hydrogen.
DOC plays an important role in the global carbon cycle and supports the growth of microorganisms. It is also an indicator of organic loadings in streams and terrestrial processing of organic matter. DOC concentrations can vary across ecosystems, with higher concentrations found in environments like the Everglades and lower concentrations in the middle of oceans.
In wastewater treatment, the occurrence and removal of DOC during the treatment process are important considerations. The type of wastewater influences the quality of DOC, which can contain aquatic humic substances, hydrophobic and hydrophilic bases and neutrals, and acids. As a significant fraction of DOC is non-biodegradable, it may be released into aquatic environments along with treated effluent. Studies have investigated the removal rates of DOC during primary, secondary, and overall wastewater treatment, with calculated removals of 0.8% in primary treatment, 63% in secondary treatment, and 69% in overall treatment.
The presence of DOC in wastewaters can significantly affect the partitioning of hazardous pollutants. DOC has been found to correlate with the partition coefficients of pollutants, influencing the efficiency of wastewater treatment plants (WWTPs) in removing organic pollutants. A negative linear relationship between DOC concentrations and the distribution coefficients (Kd) of various persistent organic pollutants (POPs) and heavy metals (HMs) has been observed, suggesting that DOC facilitates the retention of hydrophobic pollutants in the dissolved phase, leading to lower removal percentages.
Additionally, DOC plays a role in soil processes such as denitrification, acid-base reactions, nutrient retention, and immobilization of heavy metals and xenobiotics. DOC can reach the water table through well-drained soils, releasing nutrients and pollutants that can contaminate groundwater.
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DOC's impact on the efficiency of wastewater treatment plants (WWTPs)
Dissolved Organic Carbon (DOC) is defined as organic matter that can pass through a filter that removes material between 0.70 and 0.22 mm in size. The type of wastewater impacts the quality of DOC, which in turn affects its fate in a wastewater treatment plant (WWTP). DOC may contain aquatic humic substances, hydrophobic bases, hydrophobic neutrals, hydrophilic acids, hydrophilic bases, and hydrophilic neutrals.
The presence of DOC in wastewaters can significantly affect the partition of hazardous pollutants between the dissolved and sorbed phases. This can decrease the efficiency of WWTPs to remove persistent pollutants quantitatively. DOC has been correlated with the partition coefficients of pollutants in wastewater to examine the effect of 'solubility enhancement' in WWTPs and evaluate the results on the efficiency of a WWTP to remove organic pollutants.
Studies have shown that primary treatment has a minor effect on the DOC content of wastewater, with similar mean concentrations of DOC in raw wastewater and the effluent of the primary sedimentation tank. However, the secondary treatment showed significantly lower DOC concentrations due to the degradation of organic compounds in the biological reactor. Overall, the removal of DOC was found to be independent of its concentration in raw wastewater.
To improve the efficiency of WWTPs in removing DOC, it is important to control this parameter in wastewaters. Additionally, the adoption of a performance assessment system can help identify critical aspects of WWTPs that negatively affect their effectiveness, efficiency, and reliability.
Furthermore, WWTPs can increase their self-sufficiency by exploiting the energy potential of the entering organic matter, which is recovered as sludge and can be converted into electrical energy and heat. Implementing new technologies, such as chemical feed pumps, online analyzers, and flow switches, can also help improve efficiency by allowing operators to do more with fewer resources.
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The sources of DOC in bodies of water
Dissolved Organic Carbon (DOC) is an important component of the water chemistry of freshwaters. It is a potential source of carbon and energy for heterotrophic organisms and plays a central role in many limnological processes. DOC is often largely derived from terrestrial vegetation (allochthonous sources) and deposited from lake catchments either by streams or by overland flow. In some environments, in situ algal and macrophyte production can give rise to large quantities of DOC (autochthonous sources).
DOC originating from within the body of water is known as autochthonous DOC and typically comes from aquatic plants or algae. DOC originating outside the body of water is known as allochthonous DOC and typically comes from soils or terrestrial plants. When water originates from land areas with a high proportion of organic soils, these components can drain into rivers and lakes as DOC.
DOC in marine systems originates from either autochthonous or allochthonous sources. Autochthonous DOC is produced within the system, primarily by plankton organisms and in coastal waters additionally by benthic microalgae, benthic fluxes, and macrophytes. Allochthonous DOC is mainly of terrestrial origin supplemented by groundwater and atmospheric inputs. In addition to soil-derived humic substances, terrestrial DOC also includes material leached from plants exported during rain events, emissions of plant materials to the atmosphere, and deposition in aquatic environments.
DOC concentrations in sediments are often an order of magnitude higher than in the overlying water column. This concentration difference results in a continued diffusive flux and suggests that sediments are a major DOC source. Marine sediments represent the main sites of OM degradation and burial in the ocean, hosting microbes in densities up to 1000 times higher than found in the water column. Globally, groundwater accounts for an unknown part of the freshwater DOC flux to the oceans.
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DOC's role in the carbon cycle
Dissolved organic carbon (DOC) is a key component of the Earth's carbon cycle. DOC is a pool of organic matter that contains a similar amount of carbon to the Earth's atmosphere. It is produced in the near-surface layers of oceans during primary production and zooplankton grazing processes. Other sources include dissolution from particles, terrestrial and hydrothermal vent input, and microbial production.
DOC plays a crucial role in the functioning of marine ecosystems. It is at the interface between the chemical and biological worlds, fuelling marine food webs. DOC is a basic nutrient that supports the growth of microorganisms and is an important part of the global carbon cycle through the microbial loop. It affects various processes in soil, including denitrification, acid-base reactions, nutrient retention and translocation, and the immobilization of heavy metals and xenobiotics.
In the carbon cycle, DOC interacts with climate on both short and long timescales. It is a reactive carbon reservoir that mediates the oceanic microbial food web. DOC cycling in the deep ocean is intricate and potentially variable on shorter timescales than previously thought. It has two components of differing radiocarbon ages: a modern DOC with a fast turnover time and an older, more depleted fraction.
DOC also contributes to the production of volatile trace gases in the water column, such as methanol, acetone, and carbon monoxide. These gases play a significant role in the chemistry of the atmosphere and, consequently, the climate.
Furthermore, DOC is an essential factor in carbon sequestration and greenhouse gas emissions. It influences soil acidification and climate warming, and its behaviour is affected by soil properties, environmental factors, and human activities. Understanding the dynamics of DOC is crucial for managing and mitigating the impacts of global change on the Earth's carbon cycle and climate system.
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DOC sensors and monitoring
Dissolved Organic Carbon (DOC) is defined as organic matter that can pass through a filter removing material between 0.22 and 0.70 mm in size. DOC is an important parameter in water quality measurement, especially for drinking water, as elevated levels can interfere with disinfection processes. DOC also influences water colour, the transport and degradation of pollutants, and can cause the formation of disinfection by-products.
DOC sensors are used to measure the concentration of DOC in aqueous solutions. DOC sensor technology has advanced to include innovative, reagent-free sensors that use UV LEDs to measure organics in a multi-dimensional way. These sensors can detect changes in the composition of organics independently of their concentration. This is important because DOC concentration is not always correlated with the presence of pollutants.
DOC sensors are also used for real-time monitoring of water and wastewater. The Proteus sensor, for example, is a multiparameter sensor that provides a reliable, robust, and low-maintenance platform for measuring DOC in real-time. It incorporates a CDOM fluorometer, turbidity sensor, and thermistor to provide real-time measurement of the bulk dissolved organic load.
CSIRO has also developed an inexpensive hand-held device for monitoring DOC in aqueous solutions. This device uses a UV255 LED as the light source, a reflecting mirror, and a UV-sensitive photodiode as a detector. Laboratory trials revealed that the device exhibited good linear absorbance with DOC from 5 to ~100 ppm.
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Frequently asked questions
Dissolved Organic Carbon (DOC) pollution refers to the presence of organic carbon compounds in water. DOC can originate from within or outside any given body of water. It is an important parameter to control in wastewaters as it can affect the ability of wastewater treatment plants (WWTPs) to remove toxic pollutants.
Sources of DOC pollution can be autochthonous or allochthonous. Autochthonous DOC originates from within the body of water, typically from aquatic plants or algae. Allochthonous DOC comes from outside the body of water, usually from soils or terrestrial plants.
DOC pollution can have negative impacts on the environment by facilitating the discharge of toxic pollutants into aquatic ecosystems. It can also affect the functioning of marine ecosystems as DOC is a basic nutrient that fuels marine food webs and plays a role in the global carbon cycle.
