Elimination of organic micropolluants in wastewater treatment plants
In recent years, organic micropollutants such as pharmaceutical residues, hormones and other endocrine disrupters, biocides, or other additives have been detected in various surface and groundwaters around the world. They often enter surface waters via domestic wastewater and wastewater treatment plant (WWTP) effluents have been identified as one of the main sources of these substances. This is because existing treatment plants were not designed to eliminate substances of this kind, but to reduce inputs of solids, organic loads and nutrients.
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Micropollutants in existing wastewater treatment plants
Nevertheless, modern sewage treatment plants remove a large proportion of micropollutants. The removal mechanisms are either sorption to the activated sludge, biological degradation/transformation and - of minor importance - stripping. However, residual contamination with pharmaceuticals, hormones or other micropollutants can still cause problems in aquatic ecosystems.
A lot of research has been conducted in order to find out, whether existing wastewater treatment plants (activated sludge plants) can be modified in order to enhance the removal of such contaminants, e.g. by increasing the sludge age. The reasoning behind is, that biodegradation may be enhanced by slowly growing organisms and a better adapted microbial community. The results of various are somewhat contradictory, there is a slight tendency towards better biodegradation with increasing sludge age, however, this only holds true for specific substances. Fully nitrifying treatment plants (sludge ages between 5 and 15 days) have been shown to achieve higher elimination rates than non-nitrifying plants (sludge age around 1-3 days), increasing the sludge to 50 days or more only has a minor effect on overall removal of organic micropollutants.
Advanced treatment methods for municipal wastewater
In order to remove organic micropollutants in municipal wastewater treatment plants, additional processes are necessary. Such an additional process needs to meet various requirements:
- Broad spectrum of action: it must be possible for a wide range of problematic substances to be largely eliminated.
- No problematic by-products: the formation of toxic or otherwise problematic products in the additional step must be avoided.
- Ease of use: it must be straightforward to operate and should not call for specialist staff.
- Cost/benefit ratio: the use of resources (materials, energy, staff, costs) must be reasonable and provide appropriate benefits.
In fact, a number of existing methods make it possible to eliminate micropollutants effectively. Some of these are already used in the treatment of drinking water, although the requirements differ markedly in the case of wastewater treatment:
- Background contamination: the concentration of organic substances in treated wastewater is usually around 5–50 times higher than in drinking water. Micropollutants account for less than 1% of this total – i.e. more than 99% consists of “harmless”, natural substances. At the same time, these natural substances influence the effectiveness of the methods under consideration, thus often leading to reduced efficiency and increased costs.
- Inflow variation: both the volumes of wastewater to be treated and the composition can vary significantly (by a factor of 10). The process in question has to be able to respond appropriately to such fluctuations.
When all these aspects are taken into account, two-three methods emerge as suitable candidates for advanced wastewater treatment – ozonation, powdered activated carbon adsorption and “dense membrane” technologies (i.e. nanofiltration or reverse osmosis).
Ozonation
Ozone has a strong oxidizing action – i.e. many substances are attacked and transformed by this agent. Since ozone is highly unstable, it has to be generated at the site of application – in an energy–intensive process – from dry air or from liquid oxygen. It is added in gaseous form to the wastewater stream, and sufficient time must then be available for it to react with the wastewater constituents (Fig. 1A). The amount of ozone required depends on various parameters, such as the level of background dissolved organic matter (DOC), wastewater pH and alkalinity, as well as the desired elimination performance. At the wastewater treatment plant in Regensdorf, Switzerland, a full-scale ozonation was operated during 1.5 years. During this time, an in-depth investigation of wastewater ozonation was conducted. The topics covered were elimination efficiency, formation of by-products, operation, control strategies, energy demand and costs. It was shown, that ozonation is an efficient process to remove a broad range of organic micropollutants from wastewater. Most substances were degraded to over 90% even with fairly low doses of ozone around 0.6 - 0.8 gO3/gDOC. Antibiotics, hormones, analgesics etc, were almost completely removed, while others were hardly affected, like iodinated X-ray contrast media.
One problem with ozonation is that in general the target compounds are not fully mineralized, but merely transformed, and so even more harmful substances may be produced as a result. Accordingly, experience at Regensdorf indicated that after ozonation an additional step is required – e.g. sand filtration – to break down reactive oxidation products.
However, as well as removing micropollutants, ozone reduces not only the microbial count but also odour, colour and foam. Because ozone is a potent irritant, safety measures are also required to protect staff in the event of malfunctions. Again, according to the study in Regensdorf, ozonation is associated with increases of around 10–20% in both energy consumption and costs, although these figures depend on various local factors, as well as the size of the plant, wastewater composition, existing treatment etc.
Adsorption to activated carbon
By bringing the wastewater in contact with activated carbon - either powdered activated carbon (PAC) or granulated activated carbon (GAC) - contaminants may adsorb to the surfaces of the carbon. While the PAC process has been investigated in detail, only little data for the GAC process is available.
In the PAC process, PAC (particle diameter 10–50 μm) is added to the wastewater. Thanks to the huge surface area (1000 m2/g) and other specific chemical properties (e.g. charge, arrangement of molecules), many substances adsorb onto the particles. Activated carbon adsorption is a highly promising method for the removal of numerous micropollutants: elimination rates of more than 80% are achieved for many (but not all) substances in treated wastewater with a dose of 10–20 mg PAC per litre. In contrast to ozonation, activated carbon adsorption is a slow process. For many substances, equilibrium concentrations are only attained after several hours. One way of accelerating and optimizing the adsorption process is to circulate the carbon so that – as with activated sludge – it remains in the system longer than the water (Fig. 1B). The general difficulty with PAC treatment lies in separating the carbon from the water. Various options are available: it can be done either via sedimentation, which necessitates the use of precipitants, or via (membrane) filtration, which requires additional energy. With sedimentation, a downstream sand filter is needed to retain carbon that has not been removed. The used carbon is then incinerated, and the sorbed organic substances are thus completely mineralized. The sludge increase is around 5-10%.
Another way of improving the effectiveness of activated carbon adsorption would be to recycle the carbon to the biological step of the treatment plant. As activated carbon is normally only used after the biological step – i.e. when concentrations of contaminants are already very low – its surface is only partly loaded and its full purification potential is not effectively exploited. When carbon is recycled to the biological step, where contaminant concentrations are higher, additional loading occurs. With this set-up, the carbon would be continuously removed from the system with the activated sludge.
The additional energy required for activated carbon adsorption at the treatment plant is low. However, as the production of activated carbon is highly energy-intensive, primary energy consumption is higher than with ozonation. The costs are also estimated to be slightly higher than for ozonation and are largely dependent on the costs of powdered activated carbon.
If GAC is to be used, the wastewater flows through a GAC filter. While passing this filter, the contaminants adsorb to the surface of GAC. This process is assumed to be less efficient than the PAC process because the contact time between wastewater and activated carbon is lower. However, the advantages are that the activated carbon may be regenerated and therefore the production of waste products (activated carbon slurry) is strongly reduced.
Dense membranes
Dense membranes (as used in nanofiltration and reverse osmosis) are made of a material that is much more permeable to water than to dissolved substances, while particles are fully retained. At an operating pressure of 5–40 bar, relatively pure water can thus be obtained from feed water rich in dissolved substances and particles (Fig. 1C). After biological treatment, the wastewater generally has to be prefiltered (microfiltration) and the pressure boosted before it passes through the filter modules. The water is circulated several times so as to increase the flow rate across the membrane, wash away deposits and thus slow down the formation of a cake layer. Depending on the composition of the wastewater and the type of membrane, conditioning – i.e. the addition of chemicals – will also be needed to prevent precipitation and membrane fouling. Even so, membranes will require regular chemical cleaning.
The concentrate held back by the membrane is known as the retentate, while the treated water is known as the permeate. The yield, i.e. the permeate/wastewater ratio, typically lies between 75% and 90%. Consequently, between 10% and 25% of the wastewater – in the form of contaminated retentate – has to be further treated and disposed of. In addition, both the energy requirements (due to the high operating pressure) and the costs are substantially higher than with ozonation or PAC adsorption. The energy required is estimated at 1–2 kWh/m3. Given the energy and cost considerations and the lack of disposal options for the retentate, dense membrane technologies do not appear to be suitable for municipal wastewater treatment. However, in areas of water scarcity, where drinking water is to be prepared directly or indirectly from treated wastewater, these technologies – especially nanofiltration – are certainly an option to be considered.
Advanced oxidation processes (AOP)
Advanced oxidation processes cover a broad range of different technologies, e.g. UV/H2O2, Fenton processes, semiconductor photolysis, photoelectrocatalyisis, ... The idea behind is to produce OH-radicals, known to be a very strong oxidizing agent, able to oxidize hardly all organic substances. This strong oxidizing capacity is in the meantime the disadvantage of theses processes, as also “harmless” natural organic matter is oxidized, decreasing the efficiency of the process and increasing energy demand and costs. Therefore, AOPs seem currently not suited for the treatment of municipal wastewater, but may be very useful for the treatment of specific wastewaters with high concentrations of very persistent contaminants.
