odour control in water treatmentReading time:
Before examining the treatment of gaseous effluent, consideration has to be given to action required in respect of structures, even upstream drains, in order to limit odour emissions (see section sources of emission of odorous compounds produced by a wastewater treatment plant).
When these adjustments have been made, there are three potential types of treatment that can be used to control airborne odour in wastewater treatment plants:
- physical-chemical treatments;
- biological treatments;
- odour control through adsorption over activated carbon.
physical-chemical treatment in scrubbing towers: the Azurair C
Gas is scrubbed in a packing tower, transferring pollution to the liquid phase; this is followed by a chemical reaction: acid-base reaction in the case of acid and base towers, oxidation-reduction reaction in the case of "bleach" (NaCℓO), thiosulphate and bisulphite towers.
Depending on the pollution mixture to be treated, one to four reagents will be needed for its removal. Accordingly, their use in the towers follows a sequence.
A sulphuric acid (H2SO4) scrubbing removes nitrogen compounds and especially ammonia and amines:
An oxidising scrubbing using sodium hypochlorite (NaCℓO) removed reduced sulphur compounds, especially hydrogen sulphide, organic sulphides, mercaptans and also ammonia and amines:
The alkaline scrubbing using sodium hydroxide (NaOH) fixes volatile fatty acids ( VFA ), reduced sulphur and residual chlorine. In particular, sodium hydroxide (NaOH) removes carboxylic acids, hydrogen sulphide and mercaptans as well as a proportion of the carbon (CO2) found in air and this has to be taken into consideration when calculating reagent consumption:
Reduction scrubbing using bisulphate (NaHSO3) or thiosulphate (Na2S2O3) is used as a polishing treatment on compounds such as VFA, residual chlorine, aldehydes and ketones.
Oxydising and alkaline scrubbers can be used together but to the detriment of the quantities of reagents used.
The above reactions will destroy compounds generating unpleasant odours (e.g. H2S via CℓO–) or, more frequently, fix them in the circulating solution which will then have to be drained from time to time and returned to the plant inlet.
schematic diagram of an odour control tower (figure 2)
air circulation (figure 2)
A fan (1) is used to send the foul air extracted from a confined building (or structure) into one or more towers in series. The air is injected into the tower beneath the floor (2) supporting the packing (3) through which the reagent percolates. The gaseous compounds are absorbed and the chemical reactions described earlier take place within this packing.
As the air leaves the towers, it goes through a demister (4) that limits the droplets carried, especially from one tower to the next.
circulation of the liquid solution (figure 2)
The solution is drawn up by a centrifugal pump (5) at the bottom of the tower, from a storage capacity, and delivered (QL) to a distribution system (ramp or sprayer nozzles) positioned above the packing.
The circulation flow is set between two limits according to the air flow to be treated: minimum ensuring that the entire packing surface area is wetted and maximum to avoid excess flow. Therefore, these values will depend on the packing selected.
The scrubbing solution’s reagent concentration is measured by:
- using a pH metre in the case of an acid, sodium hydroxide, bleach/sodium hydroxide and thiosulphates tower;
- measuring the available chlorine concentration in the case of a bleach and bleach/sodium hydroxide tower;
- measuring the redox potential in the case of a thiosulphate tower.
This concentration is kept to within two limit values by automatic in-line injection (6) of a reagent drawn from a storage capacity (7) (figure 2).
Make-up water (8) (QE) is required to at least compensate for evaporation and water carried away in the form of aerosols, despite the inclusion of a demister (4).
In standard towers, the make-up water is a continuous and calculated so that the storage capacity is renewed weekly. Once yearly, the storage capacity has to be emptied for cleaning and usual inspection procedures.
When the TH is greater than 5 °F, make-up water needs to be softened.
The treatment system is selected as detailed above, according to the nature and quantity of pollutants to be processed and to target discharge concentration levels. When designing the towers, the following also needs to be allowed for:
- type of packing selected (supplied in bulk or in an orderly sequence, specific surface area …);
- gas velocity through the towers (maximum 2.1 m3TPN·m–2·s–1 in the case of bulk packing), whence the tower’s diameter;
- the liquid solution flow rate (standard); 2.5 L·m–3TPN of air in the case of bulk packing).
The "stringent" guarantee conditions often demanded have led us to regularly put forward a system combining four towers in series: «acid, bleach, bleach/sodium hydroxide, sodium thiosulphate».
activated carbon treatment: the Azurair A
This process is recommended for low gas flow rate systems with low pollution loading. Consideration can also be given to its use as a polishing stage after biological treatment or, if applicable, after a gas scrubbing.
The treatment principle consists in bringing the air to be purified into contact with a mass of granular activated carbon ( GAC ) over which the pollution compounds are adsorbed. Specially impregnated activated carbon is used for this application. This preparation makes GAC less sensitive to humidity.
An activated carbon filter works satisfactorily within an air temperature range of 5 to 60 °C.
The main parameter is the contact time: 2 to 3 seconds. Gas velocity is approximately of 0.5 m·s–1 in order to avoid creating too much of a head loss, the carbon depth being nearly of 1 m.
The adsorption capacity depends on the substance(s) adsorbed and supposes that the carbon is not saturated. The air needs to be heated when the air contains water particles (mist) or when there is a danger of condensation.
Provided the activated carbon is not saturated, activated carbon efficiencies are excellent:
- approximately 99% efficient H2S elimination (for concentrations above 5 mg·m–3);
- possibility of treating punctual H2S peaks up to approximately 100 mg·m–3;
- wide spectrum of adsorbed compounds, especially VOC (volatile organic compounds) of which some are difficult to reduce using other means.
Consideration can be given to a "biflow" design (figure 3) that restricts the system’s footprint.
biological treatments: the Azurair B
Fixed bacteria are used to purify the air; therefore, this is a biological filter and in the Azurair B, the support material is a biolite which, however, is not submerged as is the case for a Biofor.
The following schematic diagram applies (figure 4):
In this instance, the air to be treated circulates down flow at the rate of 400 to 1,200 m3 air·m–2·h–1 depending on the incoming concentration and on the target residual concentration; water is sprayed on the support (600 litres per m3 of biolite and per day). It is used to keep the bacteria film wet and to provide it with the necessary nutrients.
The bacteria oxidise the biodegradable pollutants and the hydrogen sulphide converts into sulphuric acid and the ammonia into nitrate. The pH of dripping can drop down to 1.5 depending on the hydrogen sulphide content and, therefore, block other biological reactions. Drips are returned to the plant inlet.
As in the case of all biological treatment, this process is temperature-sensitive and starts up progressively. The following minimum temperatures apply:
- minimum air temperature of 10 °C;
- minimum spray water temperature of 12 °C.
A wash may be required depending on the loading eliminated and, therefore, on bacterial growth. The wash will be actuated by the head loss.
However, in the case of a properly managed installation, if the pollution load is not too high and if it does not contain any source of carbonated pollution, the wash frequency will be low (1 wash every 2 to 5 years) and, in such cases, a manual wash system will be sufficient.
Less compact and with limited efficiency compared with chemical scrubbing systems, this type of odour control is normally used on small to medium-size plants or on heavily loaded air as part of an odour "pre-treatment" (upstream from an activated carbon or chemical scrub treatment). It can then be used either to comply with highly stringent discharge standards or more often to reduce the consumption of reagents (acid, bleach, sodium hydroxide) and activated carbon.
It should be noted that there are many filters using coarser supports such as peat, compost, bark. However, these need to be calculated for use with gas velocity that are 3 to 5 times lower than for the Azurair B. In addition, these beds require regular monitoring to prevent their clogging and that is why we prefer the Azurair B.
a few typical results
Classic wastewater treatment plant with four-tower odour control, acid, bleach, sodium hydroxide, sodium thiosulphate (table 3):
Classic wastewater treatment plant with Azurair B (biological odour control using granular materials) and activated carbon for polishing (table 4) :