Wastewater pumping stations sometimes become sources of emission of odours and polluting compounds that can influence on the staff health, the equipment and facilities state and the perception of general public about this activity.
F. Andrés Tomás*, J. A. Mut Noguera, M. Carrero Planes, J. Solís Sánchez, V. Fajardo Montañana.
Competing interests: The author has declared that no competing interests exist.
Academic editor: Carlos N Díaz.
Content quality: This paper has been peer reviewed by at least two reviewers. See scientific committee here
Citation: Fernando Andrés Tomás, José Alberto Mut Noguera, Marina Carrero Planes, Javier Solís Sánchez, Vicente Fajardo Montañana, 2015, Sewage odor removal. Case studies, III International Conference of Odours in the Environment, Bilbao, Spain, www.olores.org
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ISBN: 978-84-608-2262-2.
Abstract
Wastewater pumping stations sometimes become sources of emission of odours and polluting compounds that can influence on the staff health, the equipment and facilities state and the perception of general public about this activity. Anaerobic conditions that occur at sanitation systems favour the formation of harmful gases such as hydrogen sulfide, whose effects can be mitigated with the dosage of calcium nitrate that prevent their formation or by use of systems of volatile compounds adsorption.
1. Introduction
Pumping stations and pre-treatment wastewater treatment plants are essential to protect the environment. However, their presence may cause the rejection of the population from the surrounding area due to the visual perception of installations and to the generation of bad smells. So, it is of great importance to take into account any action aimed to minimise this effect.
Odours generated at these installations have their origin mainly in the bacterial metabolic activity. Among the generated volatile compounds, it should be stand out the hydrogen sulfide (H2S) which is classified as very toxic (T+) according to the Real Decreto 363/1995 for its harmful effects on health. Furthermore, it causes damages and corrosion of materials on site and generates bad odours, which are easily detected due to the sensitivity of human to this compound (2ppb).
As reported in the Real Decreto 486/97, the exposure to environmental conditions at the workplace should not present a risk to the health and safety of employees. The reference values for Diary Exposure (VLA-ED) and the Short term exposure (VLA-EC) defined on the Professional Exposure limits for chemical agents from the “Instituto Nacional de Seguridad e Higiene en el Trabajo” for 2015 are shown below:
Table 1. H2S concentration without dosing
|
Adopted limits |
VLA-ED (ppm) |
VLA-EC (ppm) |
|
H2S |
5 |
10 |
The objective of this project is to present the action measures performed at three collection and wastewater treatment installations in order to reduce inconveniences and risks associated with the generation of hydrogen sulfide.
2. Material and methods
Although three studies were carried out in different plants and they share a common purpose, some specific terms were established for each of them based on technical and economical criteria:
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Actions in Peñíscola. A dosage of calcium nitrate as oxidative agent to prevent H2S production is proposed in the collector network between the pre-treatment station and the different pumping stations.
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Actions in Canet d’En Berenguer. The replacement of the activated charcoal bed impregnated with NaOH/KOH by a catalytic carbon bed as adsorbent agent in the deodorisation system in pumping stations is studied considering its adsorption capacity and its regeneration properties.
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Actions in Calp. Odour removal is studied by dosing calcium nitrate and replacement of the adsorbent agent from the deodorisation system; this will confirm the results obtained from previous actions.
3. Results and discussions
3.1. Actions in Peñíscola
The sanitation and sewage treatment systems in Peñíscola have five pumping stations and one wastewater pretreatment plant with an obsolete activated charcoal deodorization system and an odour neutraliser compact equipment. The odour neutraliser equipment dispenses a masking product that reduces odour without eliminating the toxic compound, so this would difficult the identification of the toxic gas due to the absence of odour which would risk the health of the employees.
First, the odour generation points were identified in the sewerage system, concluding that they were placed in the inner walls of the pipes forming the collectors and in the pumping wells where suitable anaerobic conditions occur for the growth of sulfate-reducing bacteria, which reduce sulfates to H2S.
The use of calcium nitrate to avoid the production of H2S has significant advantages in terms of cost, effectiveness, danger and need of equipment compared with other methods as injection of air or oxygen, addition of chlorine gas, sodium hypochlorite or potassium permanganate to oxidise H2S, or the dosage of zinc or iron salts to generate the precipitation of sulfides. The biggest drawback of using this reagent is the difficulty of establishing the appropriate dose, so a rigorous study was necessary to determine this dosage
Given the inefficiency of the existing deodorization system and the risk associated with the use of the odour neutraliser, it was installed a continuous controller to the inlet manifold on the pre-treatment plant to determinate the levels and the hourly profiles of the detection of H2S, while other test were performed for the dosage of calcium nitrate in the external pumping systems. This dosage reduces the production of hydrogen sulfide, because its oxidative properties inhibit the formation of dissolved sulfides and H2S as it provide the correct amount of NO3. So, denitrifying bacteria perform their function and inhibit the development of anaerobic and sulfate-reducing bacteria, preventing arrival of high concentrations of H2S to the pre-treatment plant.
The study of the correct dose of reagent was made by adjusting the results obtained from the measures of H2S, achieving a minimum effective dosage for the most critical periods in terms of generation of the toxic gas. Figure 1 shows wide intervals of time in which VLA-ED and VLA-EC are exceeded by 5 and 10 ppm of H2S respectively. Although, the maximum concentration is registered between 0:00 and 5:00 am, it should be noted that employees have guards of 24h, and they could also have alarms to attend during this time.
Figure 1. H2S concentration without dosing
While adjustments in reagent dosing are performed, it can be observed that between 9.00 and 22.30 the concentration of H2S is close to 0 ppm. That means that the H2S emission in the pre-treatment plant was reduced during the working hours (Figure 2).
Figure 2. H2S concentration with dosage
Successive studies for the adjustment of reagent dosing shown very satisfactory results in which were achieved a reduction of the concentration of the H2S in the range of 0:00 to 9:00 am
3.2. Actions in Canet d’En Berenguer
The sanitation and wastewater treatment system in Canet d’En Berenguer has a pumping station in the urban area, which has a deodorisation tower with an activated charcoal bed impregnated with NaOH/KOH as adsorbent agent. This deodorisation system has the disadvantage of having to be regenerated after a period of use when the saturation point is reached.
This recovery can be performed thermally, which means the transport of the saturated product to suitable installations where it can be subjected to a pyrolysis at 800ºC in a controlled atmosphere, a fact that would make this treatment very expensive also being necessary the handling of this product which is very uncomfortable and annoying. Another way for the regeneration is the re-impregnation with alkali compounds (usually NaOH). This process is characterised by being very tedious and difficult to perform because successive washes has to be done with solutions of different concentrations of NaOH and then the residue must be neutralised, and finally the adsorbent agent has to be washed with water. All this process can bring to a loss of 10% of the mass that should be added and in addition, it generates a reduction on its adsorption capacity, reducing its effectiveness gradually until it becomes more convenient substrate turnover.
Due to the location of the pumping station where the deodorization equipment is placed, the manipulation actions of the adsorbent agent are critical because the possible negative effects on the urban area.
The alternatives to the activated charcoal that have been studied were: catalytic carbon and mix of activated carbons and alumina. The use of a mix of activated carbon-alumina was an interesting option because it would increase the adsorption capacity and the COV retention spectrum. However, finally the use of catalytic carbon was chosen because it can be regenerated a higher number of times than the impregnated carbon simply by successive washes with water until elimination of the retained adsorbate. This facilitates the recovery work and allows to work more safely and in less time without being necessary the neutralization of the residue.
Before the industrial implantation phase, some simulation essays were performed in laboratory to confirm the increased capacity of the catalytic carbon until saturation, and optimisation of the regeneration methodology by determination on the number of operations that support this adsorbent agent. Once this phase was finished, it was implemented in the plant, showing a higher adsorption capacity with consequent reduction of H2S emissions.
3.3. Actions in Calp
The sanitation and wastewater treatment system in Calp, has fourteen pumping stations throughout the municipality and they bring the wastewater to a main pumping station called “Emissary”, located in a urban area, from where they are driven to the WWTP. Due to the characteristics of this sanitation system, there are many areas where low circulation rates of the wastewater are recorded which promote sedimentation of residues and increase the retention time causing the generation of gases and bad smells. This problem is aggravated by groundwater intrusion, with high conductivity and high concentration of SO 42-, in some sections of the sewerage that are below the sea level.
As in the pumping station located in the urban area of Canet d’En Berenguer, the pumping station “Emissary” from Calp has a conventional activated carbon deodorization system that has been frequently saturated in the past. So, it has been necessary to regenerate it with NaOH solutions and to change the adsorbent agent multiple times and these operations are characterized by their difficulty (manipulation of alkaline solutions and extremely fine particle size of the adsorbent difficult to handle). Due to the sensitive location of this station and to the generation of H2S in the sewage system, there are fixed equipments for continuous measurements of this gas in the input and in the output of the deodorisation tower. Figure 3 shows the evolution in time of the peaks of H2S when connecting the pump (depending on the time).
Figure 3. Graphic log of H2S concentration in the deodorization tower. Input = Buiding (red); output = chimney (purple)
Because the constant need for regeneration/replacement of the adsorbent agent, and the high concentration of H2S in the entrance of the pumping station, exceeding sometimes values of 60 ppm, it was decided to execute the two actions described previously, substituting the active charcoal for catalytic carbon and reducing the H2S production in the network with the addition of calcium nitrate.
As in the action taken in Peñíscola, once the H2S production points were identified, coinciding with the areas where higher amounts of odour complains were recorded, the calcium nitrate dosing points were established and were installed two systems for continuous measurement of H2S.
4. Conclusions
- The catalytic carbon provides higher adsorption of H2S compared to conventional activated carbon, and a longer life as it can be regenerated by washing with water becoming an economically favourable alternative and reducing occupational risks for workers avoiding the manipulation of alkaline solutions.
- The calcium nitrate dosage systems are easy and cheap and they can be adjusted to the real needs of the moment to remove H2S.
- The optimisation in the tested systems for the elimination of H2S favours the conservation of material resources of the installations, minimise the risk of exposition of the employees to toxic compounds and improve the image to the local environments.
- In view of the obtained results, some criteria are defined to decide when it is preferable to use one methodology or the other or a combination of both, being possible the application of the obtained results to other areas.
5. References
- CEDEX (2013). XXX Curso sobre tratamiento de aguas residuales y explotación de estaciones depuradoras. Tomo nº2. Ministerio de Fomento. Madrid.
- EPA (1985). Odor and corrosion control in Sanitary Sewerage Systems and Treatment Plants. Office of Research and Development. Cincinnati (USA). EPA/625/1-85/018.
- Frechen, F.B. (2001). Prediction of odorous emissions in odours in wastewater treatment. Ed. by R. Stuetz and F-B. Frechen. IWA Publishing London, págs. 201-213.
- WEF (2004). Control of Odors and Emissions from Wastewater Treatment Plants. Manual of Practice nº 25. Water Environment Federation, Alexandria (USA).
See the article in other languages:
Odor control in waste water treatment plants and pumping stations is already a priority in the integration with the neighborhood. Lately the energetic and economic optimization of deodorization has become very important.