|Physical description: ||The Bondi plant is a primary sewage treatment plant, i.e. the treatment process is a physical one, employing screening and settlement to treat the flow of liquid sewage before discharge to the ocean. These works are particularly noteworthy in that the greater part of the works are situated nearly 40m underground in large caverns excavated out of solid rock. The main underground installation covers an area approximately 120m x 55m with some of the chambers being up to 14m high.
The plant provides what is known as full primary treatment with sludge digestion, the components of the latter process being located on the surface. The main treatment processes, in order, are:
(i) Grit Removal
Removes sludge and screenings as well as sand sized particles (mainly inorganic 'heavy' particles), collectively called ‘grit,’ from the screened sewage, in specially designed channels. At Bondi, this process will be upgraded after 2010.
(ii) Fine Screening
In the Bondi Sewage Treatment Plant, fine drum screens (with 5mm apertures) have been inserted between the grit channels and the sedimentation tanks to remove additional solid material finer than that removed by the coarse bar screens. This process is also to be upgraded after 2010.
(iii) Primary Sedimentation
This takes place in large, long tanks, where the velocity of flow is greatly reduced allowing the heavier organic materials to settle out to become raw sludge and the lighter organic material, oil and grease to float to the surface to become scum. These tanks are larger and longer in Bondi Sewerage Treatment Works than normal. The sludge and scum are then transferred to the surface to the Sludge Digestion Tanks. The ongoing flow of treated effluent is then pumped 2.2km out to sea through the submarine ocean outfall tunnel, commissioned in 1991.
(iv) Sludge Digestion
The raw sludge and scum from the sedimentation process are pumped up to four large steel tanks, lined externally in brick, located on the surface (just to the north of the Bondi Golf Course) where they undergo anaerobic digestion (i.e. in the absence of air) by various micro-organisms. This process produces two main end products:
1. Digested Sludge which is converted to sludge cake in centrifuges for biosolid production. This material is relatively inert and is utilised for a range of commercial and industrial uses such as a soil conditioner; and
2. Methane Gas, which can be used for stirring the tanks by recirculating it through them, or be reticulated throughout the works and used for heating. This gas can also be used, with the addition of diesel fuel, as fuel for powering ‘dual fuel’ internal combustion engines.
After the digestion is complete, the digested sludge is sent for dewatering. The dewatering plant comprises two centrifuges and is currently located in the former engine room of the decommissioned electricity generating plant. The dewatered sludge is referred to as sludge cake.
At Bondi STP, most of the present equipment is not original but consists of improved modern replacement equipment undertaking the same treatment processes.
The size of the Bondi Sewer where it enters the treatment plant is about 2.1m x 1.8m. The flow is then divided into four main streams to pass through four identical sets of screens, grit channels and sedimentation tanks before being re-united in a common channel for discharge to the submarine ocean outfall pumping station (previously, it discharged directly to the submerged cliff face outfall). This system is licensed to give full primary treatment for up to about 450 megalitres per day of raw sewage, with the average dry weather flow of about 130 megalitres per day. However, wet weather flows can reach 700 megalitres per day and, to provide for this, there are two extra flow lines which can accept the surplus of 180 megalitres per day but provide only coarse screening and grit removal.
In the 1960s, the methane gas from the sludge digestion process was used as fuel for three dual fuel engines (previously mentioned), each of 450 HP and coupled to a 312 KW alternator, located in a building on the western side of the Digesters. These provided sufficient electric energy to operate the auxiliary machinery of the whole treatment works. This arrangement attempted to make the plant self-sufficient but, unfortunately, the operating and depreciation costs of the engines were greater than the cost of electricity purchased from the State grid, so the dual fuel engines were phased out in the late 1970s.
Above ground, the Bondi STP has two areas of structures, one on the east side of Military Road and one on the west, between Blair and O'Donnell Street. The eastern area contains four circular Digesters and their ancillary equipment, with a two storey concrete post and beam building between these and Military Road. Below this building is the inclined tunnel entrance to the underground works, which is a vehicular road from the surface which spirals down to the underground areas. South of this, connected to the underground works but located above ground in the adjacent golf course, is the Ben Buckler Ventilation Stack (separately listed). On the western side of Military Road are the administration offices for the treatment plant, housed in a two-storey brick building dating from the 1960s. It is a plain, functional building. Two electric passenger lifts and stairs connect vertically from this building (and its gatehouse) to the underground works. The treatment plant, particularly the eastern section, is one of the most easterly structures on the coastline and the setting for the treatment plant is spectacularly scenic, with views down the coast across Bondi Beach. The old ventilation stack is a local landmark on the coast.
The physical curtilage is defined by the property boundary of the site.