The treatment of wastewater in Denmark and Europe becomes centralised and simultaneous are several low-density residential zones getting sewerage. This can imply further hydrogen sulphide problems from pressure mains with smaller dimensions, which give a requirement of a method that, prevent hydrogen sulfide, along with low-cost and an advantage when used on pressure mains with these dimensions.
The project examines FNA-dosing (Free Nitrous Acid; HNO2), which is a newly developed method for hydrogen sulfide prevention. The project is divided in four parts; literature review, batch experiment, trial on pressure main, and economy. The four parts leads to an evaluation about if FNA is rentable in proportion to the conventional methods under Danish conditions.
Hydrogen sulfide is formed by sulfate reduction to sulfide in anaerobe pressure mains and becomes volatile, when the wastewater meets the atmosphere. Volatile hydrogen sulfide can then give malodours and corrosion problems. Sulfide is mostly produced in the biofilm of the pressure main, and especially in small dimensions, the production from the biofilm will be significant. For that reason, is FNA based on inhibition of the biofilm and particularly the sulfate reducing bacteria, since these are slow growing.
FNA was dosed as sodium nitrite and hydrochloric acid to anaerobe wastewater in batch reactors at different concentrations, and the sulfide concentration was measured at different time steps. The experiment showed that FNA had a negative effect on the sulfide production. On the basis of the experiment it was decided to test a composition of FNA at pH 6 and 40 mg-N/L nitrite on a pressure main with a exposure of 2,2 hours.
Trials were executed on a pressure main in Frejlev, Denmark. Where the first three dosages showed an inhibition of the sulfate reduction at 80 % for 3 days, hereafter the effect continually began to decrease, for that reason it was concluded that a selection of microorganisms in the biofilm had taken place. The last dosage performed on the same level as the first experiments, where the same composition of FNA was used with an exposure of 37 hours. A dosing strategy that could retain the sulfide production at a decent level through the main was not found, since the pressure main had an abnormal high retention time and area/volume ratio.
A qualified guess on a dosing strategy for the pressure main was determined and showed that FNA-dosage was competitive in terms of chemical expenses on the pressure main. Further, it was concluded that usage of FNA compared to the conventional would be advantageous at short pressure mains with high area/volume ratio.