Iron and hydrogen sulfide (H2S) are common well problems that frequently occur together and often lend themselves to similar treatment. The standard methods used to treat both are variations on the same three-step principle of oxidation, precipitation, and filtration. An oxidiser is added to the water, which induces precipitation of the iron and hydrogen sulphide, and the precipitated contaminant is then filtered out of the water.
Aeration as a water treatment practice is used for the following operations:
carbon dioxide reduction (decarbonation)
oxidation of iron and manganese found in many well waters (oxidation tower)
ammonia and hydrogen sulphide reduction (stripping)
Aeration is also an effective method of bacteria control.
Methods for aeration:
The most common in industrial use is the water-fall aerator. Through the use of spray nozzles, the water is broken up into small droplets or a thin film to enhance countercurrent air contact.
- In the air diffusion method of aeration, air is diffused into a receiving vessel containing counter-current flowing water, creating very small air bubbles. This ensures good air-water contact for “scrubbing” of undesirable gases from the water.
From the standpoint of minimising capital, operating and maintenance costs, an economical solutionfor commercial applications is an induced draft, cross flow stripper with structured packing. They are simple to build and operate and have several auxiliary benefits including biofiltration and re-oxygenation of the water. For potable water supply in domestic situations or rural water supply, air can be introduced via a venturi or pressurised via compressor, followed by a static mixer to enable intimate contact between water and air bubbles. A pressure vessel or vessel at atmospheric pressure downstream of the aeration system serves for removing entrained air bubbles and degassing of the water.For a venturi based air injection a few design rules need to be considered:
- The venturi and static mixer should be sized to suit flow conditions of the water.
- The air inlet into the venturi should be controlled and monitored as excess air allowed to enter into the water flow can severely restrict flow.
- The solubility of air( oxygen and nitrogen) in water is low, and apart from some oxygen demand for oxidation and removal of CO2 out of solution, the relative amount of air required is often overestimated.
- Sizing of pressure or holding vessel downstream of the venturi injector should be sized to allow for some contact time for oxidation reactions to take place and/or degassing of the liquid. The oxidation reactions can be accelerated by replacing oxygen with ozone.