Wastewater treatment: how activated carbon compares with other technologies
Wastewater is a problem for numerous sectors, particularly as more stringent pollution limits are introduced. Several technologies are used, often in combination, to make wastewater safe for recycling or discharge.
In many situations and sectors, activated carbon is an effective option, reducing costs and providing a reliable, environmentally sound alternative or back-up to technologies such as bioreactors and membranes.
Typical challenges in wastewater purification
Wastewater typically comes from three main sources: industrial wastewater and process water, municipal wastewater or landfill leachate water.
Contaminants depend on the source, with the main ones as follows.
- Organic components, as measured by Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD). Excessive amounts can cause oxygen depletion in water sources and result in insufficient levels to sustain life.
- Nitrates and phosphates, often from fertilisers. These can cause eutrophication, where excessive amounts of algae form and starve aquatic organisms of light and oxygen.
- Adsorbable organic halides (AOX). Organic components containing e.g. chlorine or bromine, many carry environmental risks.
- Trace components, such as residues from pesticides, hormones, antibiotics, and industrial chemicals. Many are classified as Substances of Very High Concern (SVHC), believed to be harmful even in low concentrations. Others are Persistent Organic Pollutants (POPs) that can accumulate in water, soil and potentially the food chain.
- Metals, which can damage damage the environment or aquatic life if levels are too high.
Not all contaminants can be removed via adsorption by activated carbon. Nitrates, phosphates and metals are inorganic components and have no affinity with activated carbon and thus require an alternative purification technology. However, for the removal of COD, BOD, AOX and trace components, activated carbon is certainly worth investigation.
Legislation protects the natural watercourses and biodiversity; and reduces risks to human health. Pollution limits vary according to the component, region, and proximity to natural resources or drinking water wells.
Sectors and scenarios
Activated carbon can be used in a wide variety of industries and situations, especially in the treatment of organic contaminants in industrial wastewater.
Petrochemical, chemical and pharmaceutical industries produce wastewater in their production processes, rinsing water and rainwater contaminated with organics such as COD and BOD and trace components. Colour in process water or wastewater can also be removed.
Scrap firms, landfills and waste treatment sites may produce contaminated run-offs when it rains. The composition of the pollution depends on the type of waste on site. Metals from e.g. old car parts and shredded material are often present but cannot be removed via adsorption. Activated carbon typically tackles organic components resulting from leaching of solid waste or spills from gasoline and cooling fluids.
Disasters such as train accidents may result in contaminated water, for example from liquids being transported or from water used to extinguish fires, which typically contains fluorinated components such as PFAS.
Treatment companies can also benefit from activated carbon filtration, as can festivals producing wastewater from kitchens and showers.
Wastewater treatment technologies
Wastewater is generally treated in stages.
- Primary treatments
These remove the biggest non-soluble particles such as leaves and sand. A standard solution to remove this category is via sedimentation, where water is left in a tank to allow these particles to settle.
In physico-chemical treatment, chemicals are added to increase coagulation and flocculation, making it easier to remove impurities. Metals and some pharmaceuticals can be reduced in this way. Powder activated carbon can be added at this stage to remove toxicity to protect bacteria at the next stage.
- Secondary treatments
These remove organic impurities as gauged by COD and BOD, such as those produced by the food industry and the chemical and pharmaceutical sectors.
Bioreactors, which use bacteria to break down organic components, are used in either aerobic or anaerobic conditions depending on the components that have to be removed.
Aerobic bioreactors consume a lot of energy, otherwise the bacteria may die off. Toxicity in wastewater can also kill the bacteria, and it is hard to restart the colony. Bioreactors require on-site expertise and close monitoring to ensure survival of the bacteria.
Powder activated carbon can be added as a kind of vitamin, improving flocculation and protecting bacteria. This is also known as the PACT process.
Sometimes this secondary stage is sufficient to purify water, but usually a tertiary step is required. Furthermore, biofiltration leaves sludge, which itself requires additional treatment.
- Tertiary treatments
This is the ‘polishing’ stage, ensuring water is safe for recycling or discharge. Several technologies are used, alone or in combination.
Membranes separate the wastewater into a purified flow for recycling or discharge, leaving behind a concentrate which still requires treatment. Membrane technology is effective, but the energy consumption required to pump water over the membranes is high.
They are expensive and liable to become fouled by bacteria, particles or insoluble salts, forcing operations to cease while they are cleaned. They require careful monitoring to ensure there is no breakthrough of contaminants.
Activated carbon filtration adsorbs COD, AOX and many trace components. It is especially useful for organic components which do not biodegrade.
As well as working alone or in series, filters can be placed before membranes to protect them from fouling. This makes membranes last longer, and avoids shutdowns. Placed after membranes, they can act as a final polishing step in case of any breakthrough.
Sand filtration removes suspended solids, sometimes in conjunction with activated carbon.
Advanced oxidation processes (AOP) use chemicals such as ozone to remove the last traces of COD, often in conjunction with membranes.
Disinfection and distillation are unusual techniques, rarely found outside laboratory environments.
DESOTEC activated carbon filtration solutions
DESOTEC provides a 24/7 service across Europe that takes care of clients’ wastewater concerns so they can focus on their core business.
Our solutions are particularly useful where pollution is discontinuous, for example to treat contaminated rainwater at a scrapyard. Whereas biofilters need a steady supply of contaminants to feed bacteria, as well as close monitoring, activated carbon filters can simply be left in place until required.
DESOTEC supplies all mobile filters on a rental basis, making them ideal for short-term situations such as emergencies or maintenance. Companies can test them out at full scale and tweak them or remove them as necessary, rather than make an upfront investment.
DESOTEC’s system is modular, giving it great flexibility. This benefits municipal WWTPs, for example, which need to increase treatment as communities expand. It also helps companies bolster existing installations to meet newer, more stringent pollution limits.
Clients do not handle the saturated carbon themselves. Instead, DESOTEC removes spent filters, allowing the plant to resume its work quickly. We then transport them to our furnaces, where the contaminants are destroyed and the carbon reactivated, making DESOTEC filtration an environmentally conscious option.
How can DESOTEC help you?
Across Europe, we are seeing clients switching to activated carbon for wastewater treatment, or including our filters in their biofiltration or membrane systems.
To discuss how our solutions could work for your company, contact our engineers today.
At DESOTEC’s facilities, all used carbon is analysed so the right measures can be taken for handling and removing the saturated carbon out of the mobile filters. All molecules that were adsorbed on the activated carbon at the customers’ site, are desorbed inside DESOTEC’s reactivation furnaces. These contaminants are then fully destroyed, in accordance with National and European legislation, by an incineration and neutralisation setup. The entire installation and it's emissions are under continuous on-line monitoring, which guarantees that only harmless water vapour is seen exiting the chimney.