The first stage of the treatment process uses screens to remove the larger solid inorganic material such as paper and plastics. This is followed by the removal of particles such as grit and silt which are abrasive to plant equipment.
Following preliminary treatment, wastewater is passed through a primary sedimentation tank where solid particles of organic material are removed from the suspension by gravity settling.
The resultant settled primary sludge is raked to the centre of the tank where it is concentrated and pumped away for further treatment.
This next stage is a biological process which breaks down dissolved and suspended organic solids by using naturally occurring micro-organisms. It is called the activated sludge process.
The settled wastewater enters aeration tanks where air is blown into the liquid to provide oxygen for mixing and to promote the growth of micro-organisms. The “active biomass” uses the oxygen and consumes organic pollutants and nutrients in the wastewater to grow and reproduce.
From the aeration tanks, the mixture of wastewater and micro-organisms passes into a secondary sedimentation tank (also known as a clarifier) where the biomass settles under gravity to the bottom of the tank and is concentrated as sludge.
Some of this sludge is recycled to the inlet of the aeration tank to maintain the biomass, hence the name for the process – activated sludge. The remainder is pumped to anaerobic digesters for further treatment.
The clarified wastewater is discharged from the secondary clarifier and passes through for Tertiary Treatment.
All wastewater treatment plants use disinfection for tertiary treatment to reduce pathogens, which are micro-organisms which can pose a risk to human health.
Chlorine is usually dosed into the treated wastewater stream for disinfection. However, Bolivar uses large ponds in which sunlight and other micro-organisms reduce the pathogens.
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.
Tertiary treatment, such as that provided at Bolivar through the Dissolved Air Flotation Filtration (DAFF) plant, produces a much higher standard of treated wastewater suitable for these purposes.
The DAFF plant filters and disinfects the wastewater from the ponds allowing it to be used for direct irrigation of crops through the Virginia Pipeline Scheme.
Sludge collected during the treatment process contains a large amount of biodegradable material making it amenable to treatment by a different set of micro-organisms, called anaerobic bacteria, which do not need oxygen for growth.
This takes place in special fully enclosed digesters heated to 35 degrees Celsius, where these anaerobic micro-organisms thrive without any oxygen.
The gas produced during this anaerobic process contains a large amount of methane. At the Christies Beach plant it is used to heat the digesting sludge to maintain the efficiency of the process. Elsewhere the gas fuel is used to generate electricity, with the waste heat used to maintain the digestion process. This electricity is used in the plant, reducing our use of non-renewable energy sources.
Once the micro-organisms have done their work water is removed from the digested sludge through mechanical means such as centrifuging, or by natural solar evaporation in lagoons.
The liquid remaining at the end of the process is usually pumped back into the aeration tanks for further treatment.
The stable, solid material remaining, or biosolids, looks, feels and smells like damp earth and makes ideal conditioner for soil.