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To explain the nature of hybrid biological treatment it is first necessary to explain what to water treatment experts is known as biological treatment.

Biological treatment is a process to remove dissolved organic matter from wastewater. To do this. sewage microorganisms are cultivated and added to the wastewater. This type of treatment is provided by completely mixing aeration tanks, and the biologically treated waste is directed to a means of clarification that separates the biological solids from the treated liquid. The clarified liquid is then removed and becomes the treated effluent which is discharged.

Our video below shows an example of a hybrid biological treatment process which in this case is being used to treat landfill leachate (which is the polluted water from a landfill):-

Biological treatment is a natural process and is the process whereby contaminants in soil, sediments, sludge or groundwater are transformed or degraded into innocuous substances such as carbon dioxide, water, fatty acids and biomass, through the action of microbial metabolism.

Biological processes are typically a low cost solution when compared with alternatives and for that reason will always be high in the options considered by water process designers. For what they do for us biological treatment is a highly successful method.

Pressures to Improve Upon Simple Biological Aeration Treatment

However, increasingly, higher water discharge quality is being required and environmental regulators worldwide are listing additional substances which must now be removed in addition to the simple biologically treatable contaminants. Simple biological treatment by aeration in the presence of a biological sludge alone, will no longer be adequate as population pressure on watercourses increases.

Population growth is reducing the ability of water regulators to allow the use of the old standby of relying on the ample dilution available when a small sewage works discharges its treated effluent into a large river. In other words, it used to be acceptable to use the natural processes in our rivers to complete the job we started but only 80% (say) completed, in our sewage works – but is less so now.

Furthermore, neither solids removal nor disinfection alone can be guaranteed to effectively remove viruses, or parasites such as cryptosporidium and giardia, and it has limited effectiveness in removing soil bacteria which will be present although not nearly in such high concentrations as in the untreated water.

So it should now be clear what simple biological treatment is and that something more than just biological treatment is now needed.

Hybrid Biological Treatment

So, to meet this challenge hybrid biological treatment processes have been devised which are designed to utilize the same breakdown mechanisms employed by nature in biological aeration treatment to degrade nutrients, but also include other biological and physical processes as well.

Also, toxic chemicals that persist in nature, such as heavy metals, may be present in the sewage and they can survive biological treatment systems and therefore, hybrid biological treatment systems are necessary to remove them when the environmental regulator requires this.

The best known type of hybrid biological treatment system that combines secondary treatment and settlement is the sequencing batch reactor (SBR). In this process the incoming raw sewage is mixed with activated sludge and then further mixed and aerated. The resultant mixture (mixed liquor) is then allowed to settle, producing a high quality effluent – plus a waste activated sludge.

Waste activated sludge produced during biological treatment processes is usually stabilized by aerobic digestion in which the degradable solids are oxidized by prolonged aeration, and self digest. Nevertheless, other options such as incineration and water based anaerobic processes are also used.

The following processes have been included in hybrid biological treatment plants, and no doubt there are others that could be added:-

• BAF units (Biological Aerated Filters)
• DAF units (Dissolved Air Flotation)
• Reverse Osmosis
• Ultrafiltration
• Activated Carbon Absorption
• Irrigation
• Reed Beds
• Anoxic biological treatment