sources of emission of odorous compounds produced by a wastewater treatment plantReading time:
raw effluent inlet and pre-treatment (grit removal, oil removal and primary settling)
With these structures, the main danger lies in the potential transfer into the surrounding air of the unpleasant smelling compounds contained in the water to be treated. Their properties and their origins are, therefore, determining factors that have to be taken into account when assessing their odour potential (septicity, redox potential, temperature, network length, BOD concentration, intermediate lifting stations, presence of industrial discharge …). Action is possible with a view to restricting this input by fighting against the network’s tendency to anaerobiosis: injecting an oxidizing (e.g. H2O2), injecting NO3– (anoxia and not anaerobia), or by precipitating S2–via ferrous or ferric salts.
Any mixing (lifting pump, agitation at the foot of the screens, bubbling in the grease removers) will tend to "strip" volatile and extremely malodorous sulphur and nitrogen compounds released by anaerobic fermentation taking place in the sewers (degradation of proteins, production of mercaptans ...).
An aggravating factor: recycling hight loaded and septic liquor generated by sludge treatment to "the inlet".
At primary settling, odour emissions remain limited (drop at the weirs) and the concentrations measured are markedly lower than those found at the plant inlet. Nevertheless, fresh sludge pits still continue to generate quite strong odours.
biological treatment and associated clarifier
These units make a relatively minor contribution.
The sulphur compound concentration measured at the surface of aeration tanks will always be low and, if we seek to classify structures according to the odour intensity produced by a treatment, we will have :
- for aeration: fine bubbles < surface aeration < large bubbles (see increasing stripping);
- for the load: low < medium < high (see water quality).
The clarifier has virtually no involvement in odour emission when clarification is carried out correctly. Similarly, very few problems are created by tertiary treatments.
This is undoubtedly the source of the main olfactory compounds emitted by a water treatment plant.
The type of treatment applied will have an impact on the intensity of the nuisance:
- aerobically or anaerobically stabilised sludge will restrict odours.
- lime treatment will alter the spread of odorous compounds: by raising pH, the sulphur products are locked into the water in a (soluble) ionised form while the nitrogen compounds are released.
There are anaerobic areas at the bottom of thickening tanks and the acidogenesis phase of fermentation will produce volatile fatty acids.
For dewatering, table 1 summarises the contribution to emissions depending on the type of dewatering and sludge quality.
Now, either the entire plant or at least the main units generating odours will be covered and ventilated and all the gas output from all these units will be fed into the odour control unit (see section odour control in water treatment).