Treatment Processes

What are the options?

Pre-treatment

This process involves screens removing the solids from the sewage. This is usually to remove larger matter put into the sewer that may damage the process equipment.

Primary Waste Treatment.   

This is the first step in wastewater treatment where screens and sedimentation tanks are used to remove solids. The sewage is passed through a settling tank and the solids that have settled to the bottom are removed (raw sludge). About one third of organic oxygen-demanding substances (Biochemical Oxygen Demand or BOD) is removed from domestic sewage and two third of suspended solids (SS). Oxygen-demanding substances must be removed to reduce adverse effects in the water course that the effluent goes to. Effectively, if effluent with a high oxygen demand goes into a water course, an amount of dissolved oxygen is consumed by the organisms and causes a further disruption to the ecosystem. Disinfection of effluent is included if this is the only level of treatment provided.

 Secondary Treatment

The second step involves bringing together waste, bacteria, and oxygen in the activated sludge process. The objective of secondary treatment is to remove BOD and suspended solids The water leaving the primary clarifier has most of the solid organic matter removed but still contains a high demand for oxygen.

Most secondary treatment processes use microbial action to reduce the BOD in the waste, through an aeration device. This involves an aeration tank filled with clarified liquid from the primary treatment process and a mass of microorganisms. Air is bubbled into the aeration tank waste to provide oxygen for the aerobic organisms (oxygen consuming organisms) to break down the effluent. The microorganisms adsorb (process by which one material is attached to another) the organics on their surface. The waste material is broken down to carbon dioxide, water, compounds and other microorganisms. Microorganisms are separated from the liquid in a settling tank, or a secondary clarifier. The liquid is removed and the microorganisms exist without additional food and become "hungry" or activated, hence the name of the process being activated sludge. At this stage this sludge is return-activated sludge and more clarified liquid from the primary treatment phase is brought into the aeration tank, in a continuous process. At times, some sludge has to be removed as more microorganisms are produced and the system will become clogged. This waste activated sludge must be processed and disposed of which is a difficult aspect of the process as the contaminants removed from the water are now concentrated in the sludge.

This treatment removes 85 to 90 % of floating and suspended solids (SS), and organic oxygen-demanding substances (BOD). Disinfection is the final stage of secondary treatment. Secondary treatment - achieves stabilisation of biodegradable material through biochemical degradation. This treatment typically removes 85-90 % of BOD and SS. If secondary treatment is the final treatment process, effluent is disinfected with chlorine prior being discharged to environment.

Tertiary Treatment

Effluent from a secondary treatment plant still contains significant amounts of pollutants, such nitrogen, bacteria, viruses, phosphorus, further of BOD and suspended solids. In the marine environment at Boags Rocks, Gunnamatta, the sediment is observed several inches thick over the rocks that can be seen at low tide. Apart from the chemicals in the water, the sediment has a physical effect of clogging up marine plants and stops light getting through the water that is essential for plants to grow.

To achieve a potable standard of water (treated to a level safe for human consumption) requires a high level of disinfection. The use of a certain technology does not necessarily mean potable standard is achieved, the technology must perform to achieve EPA water quality regulations.

Besides disinfection, reduction of phosphorus and nitrogen (ammonia) must be incorporated in this step to make the effluent potable for reuse.

Nitrogen is removed commonly through a process known as nitrification/denitrification.

The first step is nitrification which is the conversion of ammonia (NH4+) to nitrate (NO3-) in covered reactors in combination with the final sedimentation tanks. The second step is an aerobic biological process called denitrification, employed to convert the nitrate-nitrogen, in the effluent, from the activated biosolids-nitrification process into nitrogen gas. Denitrification usually takes place in denitrification filters.

During the normal filter cycle, the nitrified effluent with the supplemental carbon source enters the filters and passes through the filter media where it comes in contact with the anaerobic denitrifying bacteria. It is here that the bacteria biologically dissimulate the nitrogen to nitrogen gas. Greater than 90 percent of the nitrogen in the influent to this process will be converted to nitrogen gas, and thereby removed from the sewage.

Reduction of phosphorous can be achieved with the use of a 'stripper system', This stripper system consists of phosphorous accumulating bacteria which accumulate phosphorous compounds. This 'stripper' can be installed into the sedimentation tanks. Phosphorous reductions of 90 % can be achieved.

Ultra-Violet Disinfection

Ultra violet light is shone through a secondary treated effluent. The UV rays will penetrate the liquid and breakdown the DNA of the bacteria and viruses that are found in the effluent, reducing reproduction incapability.

Light can be prohibited from reaching the bacteria by suspended solids trapping the bacteria within the particle. The light may also be prohibited from reaching the bacteria if there is a high content of protein and urea which absorbs the light. Passing the effluent through reed beds or rapid sand filtration will improve the conditions for the UV to perform the disinfection.

Microfiltration or membrane microfiltration uses extremely fine screens/filters through which the effluent is passed to remove bacteria and viruses. The effluent passes through a hollow cylinder with multi-layered walls containing microscopic pores. The membranes are 2 microns in diameter and bacteria and viruses larger than this are removed. The build up on the outside of the filter is regularly washed off therefore the system remains continuous.

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