Wastewater Treatment Plant

Effects of managing WWTP odour through the addition of ferrous chloride
J Barrnett
Publication Date (Web): 5 April 2017
DOI: https://doi.org/10.21139/wej.2017.011

TasWater operates the Ti-Tree Bend (TTB) wastewater treatment plant (WWTP), a continuous mixed activated sludge (CMAS) plant, in Launceston, Tasmania. Launceston was one of the first cities in the southern hemisphere to have a sewer system dating back to the 1860’s. The WWTP was commissioned in 1974 and receives combined wastewater including stormwater, domestic and industrial waste.

Sulphide in wastewater can cause significant odour issues through the generation of hydrogen sulphide gas (H2S) in sewers and wastewater treatment plants. Ferrous iron salts (FeCl2) are added in excess to enable the precipitation of iron sulphide, effectively removing H2S. Flow paced iron salts dosing started on 31 August 2015. 

The pH is also an important factor controlling the formation of H2S. The optimum pH to reduce the formation of H2S is approximately 8. The purpose of this report was to determine the optimal iron salts dosing levels and the effect of the dosing on the operability of Ti Tree Bend (TTB) sewage treatment plant (STP).

Effects of Iron Salts Dosing on WWTP H2S Levels
Iron salts dosing showed a statistically significant decrease in the mean monthly H2S levels detected at the wet well before the plant; between 20 – 70%. The optimal dose rate was estimated relative to the residual H2S detected on site, however, with inline monitoring the dose rate could be optimised further. Although H2S was significantly reduced, olfactometry testing showed that there was minimal difference in the total odorous gas on site.

There were significant H2S diurnal patterns identified at TTB, with spikes identified throughout the system. Tankered waste could be contributing to the H2S levels at the inlet and there is considerable H2S generated on site within the treatment train.

Iron Salts and the Effects on the WWTP 
Iron salts precipitate phosphate and a  30% reduction in effluent total phosphorus  (TP) cuts the total TP load discharged by approximately 11,000 kg/y;  a significant benefit for the receiving environment. Alkalinity in the AST can be lower than the recommended levels for effective biological treatment, yet, the data indicated that iron salts dosing had no effect on the levels in the STP. However, there were significant reductions in the pH in both the AST and the effluent. The mean effluent pH after the start of dosing was 6.51, this will result in pH non-compliances for the effluent.  

Community Engagement
Following commissioning of the iron dosing unit and a community engagement program, total odour complaints dropped from 18 to 10 in 2014-15 and 2015-16 respectively. Many complaints were associated with the dosing system being offline and/or maintenance work being carried out. 


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