OXYGEN SYSTEMS

Description

An oxygen system injects oxygen or oxygen-enriched air into liquid effluent, providing a much higher dissolved oxygen concentration in the effluent than could be supplied by aeration. As a result, a larger or more active population of aerobic bacteria can be supported in a smaller, more intensive reactor, and larger amounts of BOD, COD and ammoniacal nitrogen can be removed.

An oxygen system has the advantage of producing only small amounts of surplus sludge. In some cases virtually no sludge is produced, but where the inorganic content of the waste is high some sludge disposal will be necessary. The oxygen is usually delivered to site and stored in bulk, but may be produced on site by pressure-swing absorption.

Application

Oxygen systems are used in the chemical and pharmaceutical industries to treat effluents containing intractable organic compounds such as chlorinated phenolics, and in the food and drink industry to reduce the amount of sludge for disposal or to avoid odour problems. Oxygen may be used instead of air to treat effluents in open tanks in completely mixed activated sludge systems, or at the front end of plug flow systems where the demand for oxygen is highest. Alternatively, oxygen may be introduced into covered vessels within which transfer is effected by surface aerators. In all these cases a high sludge return rate is used to maintain large populations of microorganisms. Mixed liquor suspended solids concentrations may be as high as 20 kg/m3 and up to 40 kg/m3 in fluidised bed reactors operating on oxygen.

The use of oxygen can produce so much carbon dioxide that pH depression occurs. To avoid this, oxygen is not used in the final stages of plug flow systems and aerators are employed to introduce air to strip out excess carbon dioxide.

Sizing

The capital cost of an oxygen system may be less than the capital cost of an aeration system for the same duty. The operating costs (for oxygen and electricity) may be greater than those for aeration (for electricity), but the costs of sludge disposal should be much less.

An oxygen system providing 1 tonne of oxygen per day might require a 15 kW oxygen transfer system, a 2 kW carbon dioxide removal system and 2 kW of pumping and have a transfer efficiency of about 2-25 kg O2/kWh used. Such a system might be able to treat a liquid effluent flow of about 30 m3/hour with a COD concentration of about 1000 mg/l. Where sludge is being digested an amount of oxygen must be allowed for in addition to that required to oxidise and remove the BOD.