Types of chemical dosing systems for odour control
- Calcium Nitrate dosing acts as a biological agent and reduces odours by 90%
- Magnesium Hydroxide dosing acts as a biological and chemical agent and reduces odours by >90%
- Ferrous Chloride chemical dosing acts as a chemical agent and reduces odours by 95%
What causes odour in waste streams?
Bacteria in waste streams, including sewage, consume oxygen to grow. At first they use up the free oxygen in the stream, but once this source is depleted they seek to release oxygen from compounds in the waste. Nitrates are the next target, which release nitrogen into the waste stream and oxygen for the bacteria to grow. The final target is sulphates, which also release oxygen for the bacteria, but it is the sulphides that convert into Hydrogen Sulphide gas, which give off the pungent odour.
How does a chemical dosing unit act as an odour neutraliser?
Chemical dosing units inject chemicals into the waste stream in order to minimise the production of sulphides or to keep them dissolved in the liquid stream. Different types of chemicals can be used to neutralise odour, each with their own characteristics, advantages and disadvantages.
Table of key differences
|Calcium Nitrate||Magnesium Hydroxide||Ferrous Chloride|
|Hazardous||No||No||Yes (Corrosive Class 8)|
|% Odour Reduction||90%||>90%||95%|
|Chemical / Biological||Biological||Biological and Chemical||Chemical|
|Dosing Point||Wet well / Pump Station||Wet well / Pump Station||Rising main|
|Special Features||Must be added upstream of sulphide formation||Must be operated and maintained to prevent blockages||Must conform to Australian Standard 3780|
In detail: chemical dosing unit types
The main principle in calcium nitrate dosing units is the addition of an alternative source of oxygen to the waste stream that prevents the bacteria from targeting the sulphates in the stream. If the sulphates are not converted by the bacteria, there will be no sulphides and H2S and hence no odour from the system. For this reason, it is important that calcium nitrate is injected into the system before the bacteria begin to target the sulphates.
Because calcium nitrate has a biological action on the waste stream, the dosing rate is affected by the changing conditions of the waste stream (e.g. temperature) as well as the flow rate. The ideal injection point for sewage systems is the wet well or pump station collecting manhole.
The main principle in magnesium hydroxide dosing units is to increase the pH of the sewage to at least 8.5, which reduces the production of H2S in two ways. It has a biological action on the waste stream in that it inhibits the bacteria from targeting sulphates as a source of oxygen. Biological conversion of sulphates occurs in the pH range 6.8 to 7.2. Magnesium Hydroxide also has a chemical action on the waste stream because it keeps more than 97% of the H2S present in the stream dissolved in the liquid. A lower pH increases the volatility of H2S and releases more H2S molecules into the atmosphere.
An advantage of Magnesium Hydroxide is that the pH is limited to a maximum of 9, no matter how much dosage is applied, which prevents downstream processing problems due to high pH. A disadvantage is the tendency of the dosing medium to solidify if not properly operated and maintained. Storage tanks require mixers, carrier water is often required to minimise dosing line blockages and regular maintenance is needed to keep the dosing system clear and functioning optimally.
Ferrous Chloride intervenes in the formation of odours at the final hurdle, after sulphides have already been formed in the waste stream. If dosed upstream of the presence of dissolved sulphides it remains available to react on contact as they are formed. It binds with dissolved sulphides forming a precipitate, thus locking in the sulphides and preventing the formation of hydrogen sulphide gas. Ferrous Chloride dosing is the most effective of all the chemical dosing systems for odour, acting as a chemical agent and removing up to 95% of odour from the waste stream.
The disadvantage of Ferrous Chloride is the hazardous nature of the product. It is classified as dangerous goods (corrosive class 8) and as such must be handled according to Australian Standard 3780. Overdosing can cause corrosion to metal components like pump impellers and spills can cause brown staining.
Contact OdourPro for more information about chemical dosing units
OdourPro has extensive experience in odour management. We treat the odour problem at its biological or chemical cause for a solution that is successful and sustainable. Our technical experts and proven solutions can help your business overcome the challenges of managing odours.
Call our expert team on 1800 510 704 today to discuss efficient solutions for odour control using our state of the art chemical dosing units.
OdourPro on 21 November 2016
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