Chemical risk in wastewater treatment?
Due to our activities, the water we use every day at home or in our industries is loaded with various biological, physical and chemical substances (heavy metals, oils, pesticides, herbicides, food waste, organic waste, sand, parasites, toxic substances, biocides, etc.). This is what is commonly known as “wastewater”. In order to limit our impact on the environment as much as possible, it is essential to treat this wastewater before it can be discharged into waterways or reused.
This role of treating polluted water is carried out by wastewater treatment plants (WWTPs), whether they are internal (as may be the case in certain industries, where a site may have its own treatment plant) or external, covered or open-air.
This water treatment is broken down into several stages, where personnel may be exposed to risks at any time, whether physical (risks of falling, slipping), biological (asphyxiation, poisoning by contaminated materials) or chemical (projections, inhalation).
Description of the wastewater treatment process
The wastewater treatment process can be broken down into several main stages: inflow of effluent, pre-treatment (screening, grit removal, degreasing and oil removal), treatment (physical-chemical and/or biological, followed by clarification), disinfection, pH adjustment and sludge treatment. At the various stages of the treatment process, the water and sludge are monitored and analysed to ensure compliance with local environmental standards.
1 – The arrival of the effluent
The wastewater (also called effluent) arrives in the plant, after being processed in pumping stations or brought by tankers. At this stage, biological risks arise, such as asphyxiation and poisoning (in the case of sludge receiving) or the projection of contaminated materials (in the case of discharging).
2 – Pre-treatment
The wastewater first goes through a mechanical and physical pre-treatment.
A/ Screening and sieving
The wastewater passes through screens of different sizes (called bar screens), which have the role of retaining solid matter: coarse matter such as branches, leaves, various plastics, cans, as well as smaller particles (thanks to finer screens, via a sieving process).
B/ Sand and oil removal
In a first tank, the water is decanted in order to remove its content:
• fats and oils, which are recovered on the surface ;
• sands, shales and clays, which settle at the bottom of the tank.
During these pre-treatment stages, no chemicals are used in the process itself. Nevertheless, the risk of exposure to chemicals is high for maintenance and servicing operations, where bleach (sodium hypochlorite, formula NaOCl) or hydrochloric acid (HCl) is used to clean equipment and materials.
3 – Treatment
This treatment goes through several stages, in which various chemical products can be used.
A-1/ Physical-chemical treatment
Pre-treated water can then undergo a physical-chemical treatment (in addition to or instead of a biological treatment), which aims to eliminate dissolved and phosphorus pollution and suspended matter. Most of the time, iron salts such as ferric chloride (FeCl3) or aluminium chloride (AlCl3) are added to the water to react with the phosphorus, to which is added milk of lime (Ca(OH)2 in suspension), which has the role of helping to maintain an adequate pH for the coagulation of suspended solids and to eliminate heavy metals, sulphates and fluorides.
The insoluble phosphates resulting from this reaction are then agglomerated with a coagulant or flocculant (in powder or liquid form) and decanted.
Risk of falling: when in contact with water, powdered flocculants will dissolve and form a viscous gel, which will make installations and floors very slippery, thus increasing the risk of falls. If there is even a small amount of flocculant residue, the floor will become slippery again if water is added (e.g. during cleaning, rain or humidity in the air). It is therefore very important to get rid of all flocculant residues to avoid an accident..
A-2/ Biological treatment
There are several processes, which can be used alone or in combination: fixed cultures and biofilters, bacterial discs or biodisks, the fluidised support biological reactor and activated sludge (which is the most widely used process today).
This operation takes place in aeration tanks, where aerobic (nitrification) and anaerobic (denitrification) phases alternate.
Firstly, during the aerobic phase, the natural process of decomposition of organic matter by Nitrosomonas and Nitrobacter bacteria is encouraged and accelerated.
Thus, the ammonium (NH4+) present in the water is transformed into nitrate:
NH4+ + 2O2 -> NO3- + 2H+ + H2O (water)
Secondly, during the anaerobic phase, in the absence of oxygen in the water, the Pseudomonas bacteria use nitrate and nitrite as a source of oxygen. For this stage, it is important to have a sufficient amount of easily degradable carbon (mostly methanol). Ammoniacal nitrogen then changes to gaseous nitrogen (N2), which escapes to the atmosphere, while a deposit forms at the bottom of the tank (primary sludge) before being removed for treatment.
Other microorganisms can also be used to capture dissolved phosphorus from the water.
Contact with these chemicals, which are necessary for the biological treatment to work, is risky. Operators are exposed to the risk of splashing, which can have serious consequences (for more information on the risks, see our table in the appendix to this article).
The mixture is then directed from the aeration tank to the clarifier, where the biological sludge (or activated sludge) is separated from the treated water by settling. Some of this sludge will be returned to the aeration tank to increase the bacterial concentration, help propagation and accelerate the degradation of organic matter. The surplus, called “secondary sludge”, is sent to the sludge treatment.
4 – Disinfection
1/ Water disinfection
Disinfection is the final stage of water treatment. The aim is to eliminate the pathogenic micro-organisms remaining in the water after the aeration stage, before it is returned to the distribution network and consumed again.
Chemicals that can be used for disinfection include chlorine (dose 2-10mg/L), chlorine dioxide (ClO2), ozone (O3) and sodium hypochlorite (although the latter is used less and less). Ozone also removes certain pollutants that may still be present in the water (nitrites, iron, manganese, cyanide, pesticides, nitrogen oxides, chlorinated hydrocarbons, PCBs, etc.).
Although chlorination is the most common type of disinfection, it is good to know that there is also a possibility of disinfection by UV.
2/ Air disinfection
In order to keep the air free of any unpleasant odours for the inhabitants near the water treatment plant, the air is also washed with chemicals. These include hydrogen peroxide (H2O2), sulphuric acid (H2SO4), bleach (NaOC) and caustic soda (NaOH).
Contact with these chemicals for disinfection is risky. Indeed, operators are exposed to the risk of splashes which can have serious consequences (for more information on the risks, see our table in the appendix to this article).
5 – Adjusting the pH in the water
This step must be carried out before each discharge into the natural environment. In Europe, the pH level of discharged water must be below 8.5.
To lower the pH, operators use strong acids, such as hydrochloric acid (HCl), sulphuric acid (H2SO4) or nitric acid (HNO3). As an alternative to these strong acids, it is also possible to use carbon dioxide (CO2), which can neutralise the alkalis present in the water, as well as having an anti-scaling effect on the cooling circuits. Conversely, to increase the alkalinity, they use caustic soda (NaOH), calcium carbonate (CaCO3) or lime suspension (Ca(OH)2).
Contact with these chemicals, which are necessary to neutralise the pH, poses a risk to the operator (for more information on risks, see our table in the annex).
6 – Sludge treatment
The waste sludge collected during the various stages of the treatment of polluted water arrives in the sludge treatment room.
Depending on its final destination, it can then undergo one or more of the following treatments: thickening, dewatering, drying, incineration.
Thickening is the first stage of treatment applied to sludge from water treatment. This first treatment increases the dryness rate of the sludge (between 6 and 8%), in order to obtain “quality” sludge for the following treatments.
This thickening is possible through two processes, namely static thickening (decantation under the action of gravity alone) and dynamic thickening (concentration using mechanical energies, such as flotation, dewatering/filtration or centrifugation). These two methods can be combined in the same plant.
In most cases, static thickening does not use any polymer on the organic sludge. However, lime may be used to maintain a pH of 7 to 8 when the sludge is likely to ferment, especially if the sludge is left in the tank for a long time (e.g. weekends without dewatering) or in hot regions. For hydroxide sludge, on the other hand, the use of flocculants makes it possible to significantly increase the admissible flows.
Excess sludge is dewatered to a dryness of between 15% and 40%. The sludge then takes on a more pasty or solid appearance, before being dried.
There are 2 dewatering techniques: mechanical dewatering, which consists of filtration or centrifugation (mainly concerns large plants), and dewatering by geomembranes (a more recent technique and more suitable for small plants).
When the sludge is to be used as fertiliser, it can be mixed with quicklime powder to stop fermentation and reduce odours. For chemical risks related to lime, you can refer to our table in appendix.
This process makes it possible to make the sludge solid by drying it, in order to increase its calorific value (if it is subsequently incinerated) or to facilitate its storage and transport (if it is reused).
The trick is to dry the sludge sufficiently so that it is stabilised and hygienised, while always ensuring that its dryness is not too high, at the risk of generating dust and certain risks, such as self-heating or explosion reactions.
They are incinerated when they are considered either as waste or as fuel.
Chemical risk prevention and decontamination methods
1 – How to protect yourself?
There are different ways to protect yourself, both with collective protection equipment (CPE), such as gas detectors (fixed or portable), showers or a general ventilation system, and with personal protection equipment (PPE) adapted to the risks encountered (e.g: a disposable half-mask FFP2 in the presence of bioaerosols, with anti-gas filtration if necessary; goggles, to protect yourself from splashes when taking water samples in pools or cleaning with a water jet; waterproof and washable gloves to avoid biological contamination…).
To protect oneself from chemical products, there are the classic goggles, gloves, smock, full-body suits, but also special equipment adapted to a specific handling. For example, when handling sulphuric acid, the material of the gloves worn will depend on its concentration: for concentrated sulphuric acid (>70%), only butyl rubber or polyethylene gloves will be compatible. On the other hand, if the acid is less concentrated (<30%), the gloves can be made of rubber (natural, butyl or nitrile), neoprene (polychloroprene), polyethylene or polyvinyl chloride…) .
So remember to check the adequacy of your personal protective equipment, to wear it and adjust it correctly. If PPE fails (e.g. a hole in the glove, chemical getting under the goggles or running onto the face) you need to react quickly to clean up and prevent the chemical from having time to penetrate and react.
2 – Contact with chemicals, what to do?
A chemical spill can have serious consequences for the victim if it is not dealt with quickly and effectively.
The general chain of action we propose below involves responding in accordance with the recommended protocol for the chosen washing solution.
3 – Decontaminate, yes, but with what?
Option 1: washing with water
Water safety showers are a common option in industry generally. This water wash must be undertaken within an optimum response time of 10 seconds. This means that the facility must be easily accessible so that the operator suffering from chemical contact can begin washing within this time.
Fortunately, accidents do not happen every day. However, this also means that safety showers are activated only very rarely, usually at the time of the accident. This lack of activity causes water to stagnate in the shower and in the specific part of the supply. The stagnant water deposits scale; causes the shower and pipework to rust and become bacteriologically contaminated, sometimes rendering it unusable in an emergency. This is of particular concern if the safety shower has a special connection. To avoid this, the standards recommend that the shower be operated weekly.
In the case of teams moving around the installations (such as maintenance and service workers or workers at the sludge receiving station) or the presence of an internal treatment plant within an industrial complex, quick access to safety showers can be difficult. It is therefore interesting to be able to equip such employees with mobile solutions, so that a first aid solution is always at hand.
Option 2: DIPHOTERINE® solution
The DIPHOTERINE® solution is an active washing solution, which washes and removes the hazardous chemical faster with less volume.
This allows it to quickly remove the chemical that is present on the tissue and has not yet reacted with the tissue, in order to avoid or limit its action. This reduces pain, decreases the severity of injury and the need for care, greatly reduces the number of showers and the costs of maintaining them, and can potentially be portable to begin decontamination quickly.
Washing should be initiated within the first minute after spraying for maximum effectiveness. This gives the operator more time to get to the wash point. Transportable containers also allow any operator to be equipped for isolated operations.
To learn more about the different benefits of DIPHOTERINE®, please visit our page!
MAIN SOURCES :
INRS, Station d’épuration des eaux usées – Prévention des risques biologiques (2013)
INRS, Assainissement et traitement des eaux usées (2017)
INRS, Fiches toxicologiques (FT30, FT238, FT154, FT157, FT43, FT5, FT32, FT16, FT51, FT258, FT123, FT13,…)
Officiel Prévention – Santé et sécurité au travail, La prévention des risques professionnels des agents d’assainissement et de traitement des eaux usées (2009)
Officiel Prévention – Santé et sécurité au travail, La prévention de la pollution des eaux
SUEZ International SAS, Memento Degremont® – Eau et généralités
SUEZ International SAS, Memento Degremont® – Procédés et technologies
Société Publique de Gestion de l’Eau (SPGE), Fonctionnement d’une station d’épuration
JCFrance Industrie, Comment fonctionne une station d’épuration ? (2022)