Constructed in three stages from 1966-69, Bolivar is the largest wastewater treatment plant in Adelaide with
a drainage area stretching from Gawler to Mitcham. On average each day, 135 million litres of household and industrial
wastewater enters the Bolivar plant. The plant has the capacity to handle the wastewater treatment needs of 1.3
million people.
The wastewater reaches the Bolivar treatment plant via two gravity sewers, one from the North and one from
the South. To reduce the nuisance of odour from gases released from raw sewage, oxygen is injected into the Trunk
Sewer before the treatment plant. At the plant inlet, higher concentrations of odorous hydrogen sulphide gas are
reduced by collecting and treating the foul air from the covered screens and inlet channel in an odour control tower
with sodium hydroxide and sodium hypochlorite dosing.
Initial screening of incoming wastewater removes large suspended matter which is achieved by four rotating 3mm
wedgewire drum screens. The screenings are then washed and conveyed to a large bin for daily collection and disposal
by a contractor at an off-site landfill.
Screened wastewater is then split 50:50 in the inlet channel into two parallel process units, each unit consisting
of grit removal in the first bay followed by three bays of pre-aeration tanks in series and then two primary settling
tanks in parallel. In the grit tank, the combined effect of flow and aeration produces a helical motion which deposits
heavier grit to one side from where air lift pumps lift the grit into a channel. The grit then flows by gravity into a sump
from where it is pumped to grit settling lagoons on the western side of the treatment works. Supernatant from the
grit lagoons is pumped back to the treatment works.
Pre-aeration tanks follow grit removal which 'freshens' the sewage improving the settling ability of the organic
suspended solids. The freshened sewage then flows slowly through primary sedimentation tanks allowing suspended
solids to settle by gravity to the bottom. There are four parallel primary sedimentation tanks, each 68.4 m long, 23.3 m
wide and 4.2 m deep. Skimmings (which include fats and greases) are transferred to one end of the tanks by surface
water sprays (using plant effluent) for pumping to the digesters for stabilisation. Flight and chain scrapers automatically
convey the settled sludge to the inlet end of the tank from where it is pumped to 2 gravity thickening tanks and then
to the digesters for stabilisation. Sludge transfer is controlled automatically by flowmeters or density meters.
The grit removal and pre-aeration tanks plus the effluent launders of the primary sedimentation tanks are also
covered. The foul air from these areas is collected and delivered to the inlet of the aeration blowers for the activated
sludge process. This foul air is mixed with clean air and scrubbed by passing through the aeration tanks.
Primary effluent is pumped to the new Activated Sludge process for secondary treatment.
The activated sludge process is known as a suspended growth biological treatment process because active
micro-organisms are maintained in suspension in the wastewater.
Wastewater flows to 4 reactors or aeration tanks where aerobic bacteria are maintained in suspension and utilise
organic waste and nutrients for cell growth and synthesis. Compressed air is diffused into the reactor tank to provide
the required dissolved oxygen levels. Rising air bubbles also provide mixing which ensures contact with the microbes,
maintains a uniform environment and prevents accumulation of decomposed by-products.
As wastewater flows through the 4 reactor tanks, organic matter is stabilised by micro-organisms. After the reactors,
the wastewater flows to eight 40 m diameter circular settling tanks or clarifiers. Nitrogen in the wastewater is removed in
the reactors by bio-chemical processes in 2 stages. In the aeration tank, Ammonia (NH4) is converted to Nitrite and Nitrate
(NO2). The reactor also has zones of no aeration where micro-organisms convert the Nitrate to Nitrogen gas (N2) which
escapes to the atmosphere.
The clarifiers have a side wall depth of 3 m with shallow conical bottoms to a maximum depth of 6.6 m. In the clarifier
the increased microbe population forms flocs and settle out. A portion of biological floc or ‘sludge’ is then pumped from
the settling tank floor back to the reactor tank to maintain the active bacteria population. The remaining portion of new
cells are wasted (waste activated sludge) to the digesters after being thickened by a dissolved air flotation (DAF) process
in which fine bubbles float the floc to the tank surface which is then removed as a surface scum. The supernatant from
the gravity and DAF thickening processes is pumped to the Activated Sludge reactors.
Treated wastewater or effluent is decanted from the surface of the settling clarifier tank and then flows by gravity
to stabilisation lagoons. The six stabilisation lagoons cover a total area of 347 hectares, providing a nominal detention time
of about 30 days, for further treatment by micro-organisms, including algae, which decompose organic matter in a symbiotic
relationship. The lagoons are approximately 1.2 m deep.
The Bolivar DAFF Plant
A significant proportion of the final effluent from the lagoons is pumped to the Dissolved Air Flotation and Filtration (DAFF) plant
where the water is filtered and disinfected with chlorine. This water travels through the Virginia pipeline and is used for
irrigating market garden crops in the Virginia area. Some water is used at the plant for cleaning and general use or for
irrigation on site.
The DAFF plant produces two wastewater streams. Filter backwash water, which relatively low in suspended solids,
is returned to the Activated Sludge process while the sludge, which is high in suspended solids is pumped directly to the
digesters.
Treated water not pumped through the Bolivar DAFF plant flows north for 7km through an open channel before
discharge through mangrove trees into the Gulf St Vincent.
Solids pumped from the primary tanks are stabilised anaerobically for around 20 days in 6 x 7.8 ML digesters that are
mixed and heated to 35°C to maintain optimum conditions for anaerobic digestion. Each 30.5 m diameter digester has a
floating cover for storage of generated gas, which is used as fuel for power generation. Mixing of the digesters is
achieved by recirculating digester gas through draft tubes using gas compressors.
The digester gas is used to fuel a 3.5MW gas turbine that provides a substantial portion of the power requirements
for the whole site. Heat generated in the turbine is used to raise the temperature of the sludge digestion tanks to 35C
for optimum efficiency. The remaining gas is flared via the waste gas burners.
Reduction in the volatile organic matter produces a relatively stable digested sludge that is pumped to eight
evaporation lagoons with a total area of 119 hectares for air drying. Digested sludge from Port Adelaide and Glenelg is
also discharged to the evaporation lagoons. After drying for up to three years sludge is dug out of the lagoons and
stockpiled on site. The stockpiled sludge can then be used by farmers to recondition soil.
Bolivar Plant tours can be arranged by contacting United Water on (08) 8259 0234.
The Bolivar Dissolved Air Flotation Filtration (DAFF) Plant
The Bolivar DAFF Plant
Commissioned in 1999, the $30 million Dissolved Air Flotation Filtration (DAFF) plant can treat up to 150 million litres per day of high quality effluent suitable for spray irrigation of horticulture, including crops eaten raw.
The plant was the first in Australia to apply a traditional water treatment process to wastewater on a large scale. Designed by an Alliance of United Water, Thames Water Asia-Pacific and Kinhill Engineers, it was awarded an Engineering Excellence Award by the Institution of Engineers (SA).
Owned by the State Government and SA Water, the plant is managed, maintained and operated by United Water in accordance with international standards ISO 9001 and environmental standards 14001.
Treated effluent is sent via the privately operated Virginia Pipeline Scheme (VPS) for use by market gardeners at Virginia, 30 km north of Adelaide. Previously, the growth of this industry was limited by the availability of irrigation water from already depleted underground aquifers. The effluent must meet stringent guidelines set by the Department of Human Services, Environmental Protection Agency (EPA) and SA Water.
During summer, it is anticipated that most of the daily flow from the Bolivar Wastewater Treatment Plant will receive tertiary treatment and be used for irrigation. Besides economic benefits for SA through increased production, there will also be a significant environmental benefit with a dramatic reduction in the amount of wastewater being released into the marine environment.
Currently in winter or when demand is low, the treated water is returned to the sea via an outfall channel. In association with its research partners, United Water is investigating options for storing and retrieving this treated effluent.
The Bolivar DAFF Treatment Process
Water is pumped into the plant from the Bolivar Wastewater Treatment Plant stabilisation lagoons, where it has been settling for approximately 28 days. Aluminium Sulphate (Alum) is now mixed with the raw water to bond all the tiny suspended particles together. A polymer is also added to help the particles bond together and increase in size. The water and chemicals are stirred together to ensure proper mixing and bonding. The 'floc' size in now large enough to be separated from the water in the next treatment stage. Chlorine can be added at this stage if necessary.
The water is mixed with air under pressure (500kPa) until it becomes saturated. The release of pressure causes billions of microscopic air bubbles to form and rise to the surface. These rising air bubbles lift the 'floc' particles to the surface, forming a thick scum. This is scraped off, collected and pumped back to anaerobic digesters at the Bolivar WWTP to produce methane gas (used to generate some of the site power) and a sludge which can be dried and used for soil improvement programs. Most of the algae and suspended particles are captured and removed in this process.
The water under the surface scum but above the filter is called floated water. This water still contains some suspended particles and microscopic pathogens and requires further treatment through multimedia rapid sand filters. The floated water enters the filters and the suspended particles are caught as they pass through several different layers of media such as anthracite (filter coal), sand and gravel. The rapid filters are regularly cleaned to remove the suspended particles by forcing water and air in from below the filter bed. Water left over from this process is called washwater and is collected and pumped to the Bolivar WWTP for re-treatment.
The filtered water is now disinfected by chlorination before it flows via a covered channel to a storage lagoon. It is from here that the water goes to Virginia for irrigation. If the water is to be returned to the sea, it is not chlorinated and travels via an open channel to Gulf St Vincent.
The Bolivar High Salinity Plant Treatment Process
Construction of the Bolivar High Salinity Plant will be completed in mid 2004 and will be followed by several months of commissioning. The new plant will treat highly saline wastewater from the Port Adelaide region, which will arrive at the Bolivar site via a new 17.5 km pipeline.
The plant will have a treatment capacity of 32 ML/ day and will utilise Sequencing Batch Reactor technology, a process used around the world but not previously used in Adelaide.
Full details of the treatment process will be available from late 2004.
