It is generally accepted that domestic sewage does not attack concrete when it is in direct contact. Untreated urban or industrial effluents on the other hand may attack concrete if they contain aggressive chemicals (for example sulfates) and/or have a low pH-value (below 7). However, in the upper part of closed wastewater and sewerage systems, above the water line, material attack is often reported on the underside of concrete covers or roofs.
If the void above the sewage in a closed tank, sewer or manhole is not well ventilated, anaerobic (oxygen deficient) conditions can prevail. In this case, hydrogen sulfide gas is produced in the sewage by bacteria. The hydrogen sulfide gas collects in the void and mixes with any moisture, for example condensation, on the surfaces above the sewerage to produce sulfuric acid. Sulfuric acid has a low pH-value and thus attacks the concrete.
It is thus not unusual to find significant damage on the underside of the crown of concrete pipes, storage tanks and/or the inner faces of upper portions of precast concrete manholes in poorly ventilated and enclosed sewerage systems.
Prior to the 1980s, levels of hydrogen sulfide in urban wastewater were typically in the range of 10 parts per million. The reason for this was the quantity of water used for flushing toilets and the presence of heavy metals, which reduced the presence of sulfide in the water.
Since then, regulations as well as certain programs aimed at reducing household water consumption and increasing the reuse of rainwater and greywater appear to have reduced the presence of heavy metals in the wastewater. These regulations and practices have caused gaseous H2S levels to multiply tenfold (from 10 ppm to 100 ppm, in some cases even 300 ppm). As a result of this development, the composition of wastewater has also become more aggressive. The consequences of this are
Coal tar epoxy has a very long history of use in wastewater applications, offering good performance and effective protection for several years
Coal tar epoxies are relatively easy to apply, cost-effective coatings that show resistance to low concentrations of sulfuric acid over a certain period of time. They are adhesive on dry concrete, but their capacity to bridge active cracks is very limited.
During application, coal tar epoxy coatings require concrete surfaces with a residual humidity below 4%. It normally takes a minimum of 7 days at temperatures above 10ºC to complete curing before the coating can be immersed. The total thickness of the coating does usually not exceed 0.5 mm as it is difficult to achieve a thicker application, which may be necessary on irregular surfaces or to increase protection.
Inspections of existing facilities show that the move towards higher aggressive exposure conditions limits the durability of coatings based on this technology. This is because coal tar epoxies show good chemical resistance to low concentrations of acid but fail if the concentration increases. Additionally, the strict application requirements, limited crack bridging capacities and low film thicknesses reduce the safety factor of these products. This puts at risk the continuity of the applied film, which is meant to protect the concrete and steel surfaces from the action of the acids.
When developing a new product for the protection of wastewater facilities, one of the reasons for the difference between lab results and the real-life performance of different technologies used in wastewater treatment facilities is that typical laboratory testing is limited to sulfuric acid resistance and does not include resistance to bacterial action.
Bacterial action is difficult to simulate in the laboratory. This is because the test chamber needs not only to replicate the conditions for the bacteria in terms of H2S concentration, nutrient content, moisture and temperature. It also has to optimize them to provide accelerated weathering. Extensive experimentation is necessary to match the optimized conditions and the weathering situation on a real-time scale
Today’s more aggressive conditions suggest that the specifications for coatings or membranes used in sewage pipes and wastewater treatment facilities should contain the following features:
The quality of the membrane is also essential to ensure its barrier capacity and long-term performance. For this reason, in addition to the performance criteria listed above, some application criteria need to be included in the specifications of the coatings to avoid initial membranes defects:
Changing conditions in the composition of wastewater mean that traditional systems that were used in the past do not perform adequately in the long term.
Coal tar epoxies are cheap and easy to apply and require only very low volumes. However, adhesion on humid concrete is poor, curing at low temperatures is very slow and increasingly aggressive conditions mean that their durability is very limited.
Traditional testing of a coating’s resistance against sulfuric acid does not reflect the complexity of the chemical and biological processes that take place in wastewater environments. To evaluate durability, resistance to bacterial action must also be considered. Finally, the selected coatings must show not only chemical and biological resistance but also good application properties, including on wet substrates and in high thicknesses. More importantly, they need excellent crack bridging capacities to prevent cracks in the substrate from damaging the film.
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