Source of Wastewater

Water, sewage and other liquid wastes can come from any sector of the community and can contain anything.  Fortunately there are legal restrictions on some substances. (Please refer to tradewaste information sheet).   Wastewater comes from residences, local commercial and light industrial sites such as clubs, caravan parks, restaurants, commercial laundries, oil company retail outlets and hospitals but it does not include roof water or surface runoff.

Everyday each Tweed resident uses about 450 litres of water.  About 65 percent of this is returned to the sewage system within minutes.  The wastewater drains to around 190 pumping stations which transfer the sewage to the local wastewater treatment plant for treatment. These treatment plants are located at Banora Point, Hastings Point, Kingscliff, Murwillumbah, Uki, Tumbulgum and Tyalgum.   A treatment plant and sewer reticulation system is also planned for Burringbar in the near future.

When the raw sewage or influent arrives at the plant it contains a lot of material which has to be removed before effluent can be returned to the river or land.  The sorts of material that must be extracted includes decaying organic matter from faeces, sediments and grits, grease, heavy metals, disease causing viruses and bacteria, nutrients such as phosphates and nitrates, plus insecticides and many other household and industrial chemicals.  Added to this are objects ranging from clothing, plastic nappies, sanitary items, plastic toys, metal and wood.  The aim of wastewater treatment is to remove as many of these materials as possible.

Stages in Wastewater Treatment

Primary Treatment

This involves the removal of the inorganic solids and rags from the incoming wastewater by passing the water through screens.  These solids are removed and buried on site.  By reducing the water velocity grit and organic settleable solids are removed.  During this primary sedimentation stage, floating grease and scums are removed by a surface skimmer.

During this stage the flow is also measured.

Secondary Treatment

This is a biological process where naturally occurring micro-organisms break down organic matter and nutrients.  A high level of organic matter in water requires a high level of dissolved oxygen to decompose and break it down quickly and effectively.  If such water was released into waterways untreated then aquatic animals that also need dissolved oxygen to respirate cannot survive.  For this reason the biological oxygen demand (BOD) in wastewater is reduced to an acceptable level before being returned to the environment.

The secondary treatment units include biological filters, activated sludge tanks and oxidation ponds followed by settling tanks or clarifiers to separate micro-organisms from the purified liquid.  These can be combined i.e. activated sludge and clarifier in IDEA tank.  Secondary treatment at most plants include nutrient removal of Nitrogen and Phosphorus.  Sometimes this is chemical, sometimes biological, depending upon the size of the plant.  Usually with larger plants it is more cost effective to use a biological process, with chemical backup to get very low levels depending on EPA licence conditions.

Tertiary Treatment

The standard of effluent, produced in the secondary wastewater treatment plants is not high enough to be discharged into natural waters so this additional stage is called Tertiary treatment.  This treatment is intended to reduce pathogens, which are micro-organisms that can pose a risk to human health that have carried over.

The most common methods of tertiary treatment for the effluent are disinfection and filtration.

This may be done by:

• Chlorination followed by dechlorination,

• holding the effluent for 10 or more days in tertiary effluent ponds in which sunlight and other micro-organisms reduce the pathogens,

• filtering using microfiltration or cloth filters,

• Ultra Violet and/or Gamma radiation to disinfect the effluent.

Additional treatment may be required if the treated wastewater is reused for purposes such as irrigation of food crops or where close human contact may result

Water quality (BOD) readings for the Tweed.

• Normal River quality < 5 mg/L BOD₅

• Flood River quality > 1000 mg/L BOD₅

•  Sewage in to the treatment plants 250 mg/L BOD₅ and NFR

• Sewage after extended aeration 20 mg/L BOD₅ and NFR

• Final effluent discharged < 10 mg/L BOD₅ and NFR

• NFR is non filterable residues.

• BOD₅ is the biochemical oxygen demand of the micro-organisms in a 5 day period.

Environmental Protection Authority (EPA) licence range

The Environmental Protection Authority (EPA) sets the standards for water quality for water being discharged.

• Generally for BOD₅ < 15 mg/L 90% of the time

• NFR < 20 mg/L 90% of the time

• Oil and Grease <10 mg/L

The licence standards vary between plants depending on the sensitivity of the receiving waters, how well it is flushed and what it is used for. Some examples of 100 percentile limits are

• pH between 6.5 and 8.5

• Faecal Coliforms - < 600 cfu/100mL

• Total Nitrogen < 5 mg /L

• Total Phosphorus < 2 mg/L