the time of construction by use of engineered concrete mix, an internal liner, high build epoxy or polyurethane protective coating system.

Transcription

1 Durability Impacts to Treatment Plant Infrastructure due to conversion of Hydrogen Sulphide (H 2 S) Gas to Sulphuric Acid Presenting Author Jeremy Luke Smith 1 1 Hunter Water Australia, Mayfield West, NSW, Australia Keywords: Flow Split, Hydrogen Sulphide (H 2 S), Headworks, Inlet Works, Odour, Pretreatment, Preliminary Treatment Area (PTA), Wastewater Treatment Plant Abstract Asset owners should carefully consider long term durability issues of treatment infrastructure at planning stages or prior to any plant upgrade. Particular attention is required to minimise turbulent zones within structures that enable liberation of H 2 S gas which is shown to cause damage to unprotected concrete structures. Remedial solutions are costly and normally involve diversion of the inlet works which can result in untreated environmental discharges during bypass. Many systems are used at many Australian plants to minimise impacts such as engineered concrete mixes, protective coating systems or internal liners. Introduction Internal concrete surfaces of treatment plant structures such inlet works are susceptible to attack by sulphuric acid formed by conversion from H 2 S gas. the time of construction by use of engineered concrete mix, an internal liner, high build epoxy or polyurethane protective coating system. A decision to generally cover a treatment structure such as an inlet works is related to reduction of odour complaints for the community due to development in close proximity. Odour modeling is normally undertaken with impacts on potential sites known as receptors considered and odour contour plans prepared. Odour extraction systems that have been installed, often do not always adequately remove the H 2 S gas that is generated therefore degradation of concrete and structural support steelwork can occur. Remedial solutions are costly and normally involve diversion of the inlet works which can result in untreated environmental discharges during bypass. The conversion H 2 S gas is a well understood phenomenon which causes most treatment plant preliminary treatment areas (PTA) also referred to as inlet works to suffer significant concrete corrosion unless the gas is removed or the concrete protected from exposure at Page 1 of 5

2 Purpose Loss of asset serviceability due to concrete quality is frequently being observed at many of Australia s treatment plants in areas of high exposure to corrosive gases liberated by turbulence such as the inlet works area. Fig. 1. Structural degradation of inlet works flow splitter box (Regional Australian Plant) Fig. 2. Degraded flow splitter box Methods The cause of defects to treatment structures can be reduced by several options:- Removal of hydrogen sulphide gas in the network before entry to the headworks; Installation of an internal liner or suitable protective coating system at time of construction, as retrofits are complex; Efficient extraction of generated gas from within the covered inlet works such as use of biological filters; Removal of all or part removal of the inlet works covers to enable the gas to be released into the atmosphere, It is postulated that the cause, in principle, is the post 1980 design trend to seal the collection network and deal with the gases at specific vent sites. These have typically been at the head of the works. Degradation has also normally observed to be more severe at areas of turbulence, such as flow splitting zones. In hindsight, it is apparent that the ventilation systems are also not always sufficiently effective due to lower than optimum air change design and there has been a resultant accumulation of the corrosive gases in the headworks of the systems with ineffective ventilation in areas where gases can be trapped. Results The main objective for an asset owner should be to reduce the volume of hydrogen sulphide gas that is liberated or remains within the inlet works and associated structures. Options to achieve this are limited and may include: Review existing or install gas extraction system from a covered inlet works by use of biological filters or similar; Removal of all or part removal of the inlet works covers to enable the gas to be released into the atmosphere Elimination of elevated inlet pipework bellmouths which create Page 2 of 5

3 turbulence by adopting submerged inlets with non-return valves immediately outside the tank. Minimise use of splitter chambers as they normally incorporate integral weir arrangements which are used to ensure even flow distribution to downstream process units thus introducing further turbulence. Due to the difficulties in adopting any of the above strategies, a number of engineering solutions have been introduced by the water industry to address the problem with varying degrees of success. The following systems are used at many Australian plants: Engineered concrete mixes to address durability challenges; Coating systems such as epoxy, polyurethanes or rapid cure polyureas; Polyethylee (PE) Internal cast-in formwork liners Coating Systems Epoxy protective coating systems are no longer considered the ideal coating material for concrete protection in treatment plants as the system is considered brittle and can easily crack at construction joints and edges. More flexible surface coating materials such as polyurethanes / rapid cure polyureas and rubbers are now preferred. Fig. 3. Typical polyurethane coated inlet works structure (Regional Australian Plant) Polyethylene Internal Formwork Liner Other protection systems such as polyethylene liners cast integral with the formwork during construction have also been used with some success. However, as they are not fully adhered to the substrate there is always potential for attack behind the liner through a hole or liner failure which of course will go unseen. Liners are typically up to 3mm thick PE sheets with a large number of integral tags that enable adherence to the concrete surface. The major downside of using such techniques includes contractors who invariably nail the PE sheets to the internal formwork which results in a large number of small holes which are virtually unable to be detected. This results in failure of the liner option. PE cast-in liners have been found to have a lower capital cost. Page 3 of 5

4 cement systems and curing is potentially 30mm over the design life. This is less than the minimum reinforcement cover of exposed concrete and hence corrosion due to carbonation is not considered to lead to loss of serviceability over the Design Life. Figure 4: PE Liner cast-in integral to formwork Concrete Durability Considerations The basic requirements for durable concrete are: Concrete design mix i.e. blended cement, low water cement ratios, limit shrinkage and slump; Adequate concrete strength; Correct batching and mixing; Adequate cover; Adequate compaction; Limit concrete drop during placement to avoiding segregation; Curing methodology; Workmanship and supervision; Chloride Induced Corrosion The diffusion of chlorides can normally be controlled by limiting the adsorption and diffusion of chlorides through the cured concrete and by minimising the initial chloride content in the concrete. The latter is normally achieve by use of reinforcement without chloride surface contaminants and blended cements and coatings to provide a durable concrete which will meet the specified design life. Carbonation The carbonation impact on 40MPa compressive strength concrete using blended Concrete Curing Effective concrete curing is considered critical in achieving proven long term durability of concrete structures and the required design life for the structures. Curing is undertaken to satisfy short term requirements to prevent plastic shrinkage cracking and long term requirements of ensuring continued hydration. Water curing has proven traditionally to be the most effective curing technique that achieves maximum durability. Durability Risks The following durability risks are apparent at inlet works: Hydrogen sulphide attack on the concrete resulting in corrosion of reinforcement. Lower than specified cover over reinforcement and poor placement of concrete. Exposure to ground water Surface abrasion Carbonation of surfaces exposed to atmospheric carbon dioxide resulting in corrosion of reinforcement. Cracking due to flexural problems Corrosion damage at locations which are difficult to detect and repair. There are a number of issues to consider in designing water retaining concrete structures to maximise durability. Aside from cement quality, the most significant issues from a durability aspect concern the design, the Page 4 of 5

5 selection of a suitable concrete mix and the placement and curing of the concrete. Asset Life Considerations The asset life can be compromised if deterioration of concrete and subsequent corrosion of reinforcement occurs which will result in maintenance and repairs being required to be carried out. Based on the current version of the Australian Standard for liquid retaining structures, the structure designer is given some freedom in selecting parameters which can significantly impact a utility in the future. A prime example is where the incoming sewage is assessed as stale due to low flow and extended detention times in a rising main a concrete cover of 75mm would be required. Whereas if interpreted as fresh sewage, under wet / dry conditions would be considered a different exposure classification where a 50mm cover is designated. Note the standard does not allow for relaxing of these requirements when using a protective coating system. attack are typically flow balancing tanks and flow splitter boxes at preliminary treatment areas or inlet structures which enable liberation of H 2 S gas. Consideration should be given to elimination of turbulent zones within the structures such as at bellmouth inlets and weirs within splitter structures. A strategy for managing H 2 S gas in the system should be reviewed by the asset owner at planning stages or prior to upgrade to ensure long term durability of the infrastructure. Use of polyurethane or rapid cure polyureas can prevent impact to the community in terms of potential untreated inlet works bypass discharges during remedial works, odour impacts and cost. Acknowledgments Alan Thornton and Barry Horvath. Disclosures Nil to disclose. The Australian Standards relevant to liquid retaining structures are AS 3600, AS 1379 and AS These standards are based on a "normal" design life of 40 to 60 years. Where the required design life is specified by the water utility as being higher, say 100years, then either cover requirements need to be increased to prevent ingress of contaminants and or use of protective coating or linings is considered essential. Discussion and Conclusions The key infrastructure assets at treatment plants that are identified as susceptible to Page 5 of 5