Method diagram
Method and installation description
The aim of neutralisation is to modify an acid or base water flow to a neutral pH (approximately 7). The most important objectives are to make the wastewater treatable using biological purification and/or make it comply with discharge criteria. In addition to neutralisations, various pH corrections are also performed for a variety of reasons:
- Conditioning, stabilising raw wastewater
- Emulsion breaking
- Precipitation reactions / resolving precipitation
- Checking chemical reaction speeds
Neutralisation generally takes place by adding chemicals directly to the wastewater flow. Thus equipment consists of storage and reaction tanks and a dosage and mixing installation.
To increase pH one uses a leachant (NaOH) or lime milk (Ca(OH)2); to decrease pH sulphuric acid (H2SO4), hydrochloric acid (HCl) or carbon dioxide (CO2) is used. Other acids and bases are also possible (organic acids/bases, for example), but are less commonly used for treating wastewater.
Specific advantages and disadvantages
Biological treatment of wastewater is improved because this is often less effective at extreme pH levels.
When treating wastewater with particular salts, toxic gases may be created when the neutralisation product is added. This could present risks to health and the environment.
An additional disadvantage of acidification via sulphuric acid or hydrochloric acid is an increase of sulphate or chloride in the wastewater, whereby discharge norms may be exceeded.
Application
Neutralisation is implemented in a lot of waste waters in almost all industrial sectors.
Boundary conditions
Neutralisation can be implemented in just about all types of wastewater but, in particular cases, can drastically alter the composition of the wastewater. In general, neutralisation is realised using inorganic acids and/or bases. The neutralisation reaction results in the forming of potentially insoluble salts. The choice of reactants (base/acid) must also be compatible with the desired result, taking into account the discharge permit, for example.
Effectiveness
In principle, all types of wastewater can be neutralised, but the quantity of reactants needed to realise this may vary greatly. The reason for this is the buffer capacity in wastewaters. This reflects the resistance of the wastewater against a pH change and is caused by the presence of carbonates, phosphates, free hydroxide ions, sulphides and silicates. This buffer capacity (also referred to as alkalinity) can be determined by titrating the wastewater with acid and/or base.
Support products
Inorganic and organic acids and bases.
Environmental issues
No residual products are released.
Costs
Investment costs for such an installation are very low in comparison with the total cost price of water purification. The operational costs could rise if the wastewater has a very high or low pH or if the wastewater has a high buffer capacity.
Comments
None
Complexity
The installation is very simple.
Level of automation
The installation can be fully automated.
References
- AEA Technology, Manual of Effluent Process Technology, Environmental & Process Engineering Department, Harwell (GB), 1991
- VITO-SCT, revision of technical files WASS, 2009
Version February 2010