Download Penrith Wastewater Treatment Plant

Penrith Wastewater
Treatment Plant
Treating wastewater at Penrith Wastewater Treatment Plant
Wastewater that comes into a wastewater treatment plant is about 99% water and about one percent
solids and other pollutants. Unless these solids and pollutants are effectively removed, the treated
wastewater can’t be safely discharged to the environment or recycled.
The pollutants in wastewater include organic matter, plastic, rags, oil, grease, nutrients and other contaminants
in typically very low quantities. Because the physical properties and nature of these contaminants are quite
varied, we need a number of different treatment processes to efficiently remove them.
Preliminary treatment
Preliminary treatment involves removing grit (sand and similar material), plastic and rags. Even though the
quantities are relatively small, if these are not removed at this stage, they may damage the pumps and other
mechanical equipment, interfering with the rest of the treatment. Penrith Wastewater Treatment Plant has
5 mm fine screens to remove plastic and rags and a 6.1 metre diameter vortex tank to remove grit particles.
Primary treatment
During primary treatment, heavy wastewater solids (primary sludge) sink to the bottom of the circular
sedimentation tanks. Oil and grease float to the surface where they are skimmed off. Mechanically driven
scrapers continually drive the sludge on the bottom towards a hopper from where it is removed.
Primary treatment:
●● removes a large proportion (around 60%)
of organic solids in wastewater with minimal
use of energy
●● removes larger wastewater particles that help
biological treatment processes to remove organic
material and nutrients
●● reduces fouling of filters, membranes and other
mechanical equipment.
Primary treatment does not remove:
●● contaminants that don’t float or sink (neutral
buoyancy contaminants)
●● very small dissolved or colloidal pollutants
●● other contaminants such as nutrients
and pathogens
These contaminants are removed in secondary
or more advanced treatment processes.
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Secondary treatment
In the secondary or biological treatment stage, billions of microorganisms (including bacteria) are
suspended in the wastewater and most of the remaining organic matter and nutrients (nitrogen and
phosphorus) are removed through a combination of aerobic (plenty of oxygen), anoxic (low oxygen)
and anaerobic (no oxygen) processes (see page 4).
Types of biological treatment at Penrith Wastewater Treatment Plant
Penrith Wastewater Treatment Plant has two types of secondary treatment processes:
1.
Biological reactor
The four stage biological reactor consists of a fermenter, and anaerobic, anoxic and aeration zones.
The purpose of each zone is to create conditions that allow the nutrient removing bacteria to
effectively compete with other bacteria and remove nitrogen and phosphorus.
Fermenter
Primary sludge is pumped into the fermenter, where it’s broken down under anaerobic conditions,
and produces volatile fatty acids (VFAs). These VFAs are then introduced to the anaerobic zone
of the biological reactor.
Anaerobic zone
Wastewater from the primary sedimentation tanks and biomass (mainly VFAs) are fed into the
anaerobic zone forming mixed liquor.
With a lot of VFAs for food and low levels of dissolved oxygen and nitrite, phosphorus accumulating
organisms (POAs) release phosphate from their cells so they can take up more food.
Anoxic zone
In this zone, nitrogen is removed from the wastewater through denitrification (see page 4).
Aeration zone
During this step, an oxygen rich environment is created by pumping air through the mixed liquor
from fine bubble diffusers. Ammonia in the wastewater is converted to nitrates (nitrification) and
POAs absorb phosphorus in the wastewater.
Flow is recycled back from this tank to the anoxic zone where denitrification occurs.
Secondary clarifiers
The mixed liquor drains into four secondary clarifiers. These can treat 22 Ml/d of average dry weather
flows. The basic purpose is to separate the sludge solids from liquids through gravity sedimentation.
As the flow enters the large tank, it slows down. Biological sludge settles to the bottom. It is collected
by hydraulic suction and returned to the anaerobic zone for further treatment. The clarified (clear)
liquid flows over a weir at the outside edge of the tank and on to tertiary treatment.
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2. Intermittently Decanted Aerated Lagoon
The two Intermittently Decanted Aerated Lagoons (IDALs) were built in 2003–04 to increase the
plant’s capacity.
IDAL’s have some significant advantages over biological reactors. They can provide effective treatment
to a wider range of flows, including significant wet weather flows. They are also much simpler to
construct than the various stages of the four stage biological reactor.
The IDAL cycle usually takes about three hours. The difference between the IDAL and the biological
reactor is that aeration, settling and decanting all take place in the one tank.
Anaerobic zone
While the IDAL units remove nitrogen effectively and provide treatment over a wide range of flows,
they are not as efficient at removing phosphorus biologically.
To help remove phosphorus in the IDAL, ‘spent pickle liquor’ is added to the wastewater in an
anaerobic zone after preliminary treatment. Spent pickle liquor is an acidic mixture left over from
metal treatment. It contains iron sulphate, which reacts with phosphorus to form iron phosphate.
This new compound separates from the wastewater in the settling phase and is disposed of as
sludge or returned to the anaerobic zone to provide ‘seed sludge’.
Aeration
During aeration, air is fed into the IDAL through diffusers. These aerating devices are made up
of a membrane with fine pores, through which fine bubbles are blown. In this phase organic matter
is broken down and ammonia converted to nitrates and water.
Settling
During settling the aerators are turned off. Sludge settles to the bottom, leaving clear treated
water near the surface. Anoxic conditions develop which enable denitrification to occur. Sludge
that settles to the bottom is removed. As the sludge contains the microorganisms needed for
wastewater treatment, most is returned to the start of the IDAL.
However, unless some sludge is ‘wasted’ from the system (and sent for digestion) the amount
of microorganisms and the chemical sludges will increase. It is by wasting excess sludge,
that we remove phosphorus and other organic matter from the wastewater.
Decanting
The final treatment stage in the IDAL is decanting. Once the sludge has fully settled, clear treated
wastewater remains near the surface of the tank. A weir is lowered to decant the clear effluent
into an equalisation basin. This basin controls the flow of treated water entering tertiary treatment.
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Removing nitrogen and phosphorous
Nitrogen
Phosphorus
Nitrogen removal is done through aerobic
and anoxic processes.
Aerobic zone
Nitrification is an aerobic process, which involves
converting ammonium in wastewater into
nitrates. Two types of bacteria are responsible
for nitrification, Nitrosomonas and Nitrobacter.
The Nitrosomonas oxidise ammonia (largely
from urine) to the intermediate product nitrate
(NO2). The Nitrobacter convert nitrate to nitrite
(NO3). The conversion of ammonia to nitrite
involves a complex series of reactions. Significant
oxygen is required for the conversion. Aeration
supplies the oxygen needed by the bacteria to
drive the reaction.
Approximate equations for the reactions can
be written as:
For Nitrosomonas:
2NH4 + 3O2  2NO2- + 2H2O + 4H + new cells
For Nitrobacter:
2NO2- + O2  2NO3- + new cells
Anoxic zone
The next step in the process is removing nitrate
and forming nitrogen gas (or denitrification).
An anoxic environment is critical to enable
denitrification.
Under anoxic conditions, there is no ‘free’ oxygen
in the water. Facultative bacteria use either
‘free’ oxygen, or the oxygen in nitrate for their
metabolic processes. When there is no ‘free’
oxygen, they use the oxygen from nitrate for
their metabolic processes, resulting in the release
of nitrogen gas.
Several types of bacteria are responsible for
converting nitrate to nitrogen gas (N2). The basic
path for reducing nitrate to nitrogen gas is:
NO3-
 NO2- 
NO
 N2O 
N2
nitrite
nitrate
nitric
oxide
nitrous
oxide
nitrogen
gas
Removal of phosphate by phosphate
accumulating organisms (POAs) is a two-step
process. POAs initially release phosphorus to the
mixed liquor under anaerobic conditions, and
later on in an aerobic phase of treatment, take
up much larger quantities of phosphorus. We
can then remove this phosphorus as part of
excess sludge.
Anerobic zone
In the anaerobic zone, volatile fatty acids
(VFAs) are introduced as additional food for the
POAs. When there is no oxygen available, the
POAs take up the VFAs and release phosphate
to the mixed liquor.
The POAs now have a large supply of energy
in the form of stored volatile fatty acids for
metabolism and growth.
Aerobic zone
In the aerobic zone, there is a large increase
in the mass of POAs, which are now capable
of absorbing much more phosphorus from the
mixed liquor than they released during the
anaerobic stage. The phosphate Is now part
of the bacterial cell mass and is removed as
sludge in the clarifier.
Removing phosphorous is important as
it contributes to eutrophication, where
nutrient-rich water leads to an overgrowth
of weeds, algae and cyanobacteria (blue-green
algae), causing algal blooms, depleting oxygen
and killing animal life.
Removing phosphorous is also important to
improving efficiency at water recycling plants,
as high phosphorous levels can effect water
recycling equipment such as reverse osmosis
membranes.
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Tertiary treatment
Neither the IDAL (which is far more effective at reducing nitrogen than phosphorus) or the biological
reactor, can reduce phosphorus to discharge limits. Tertiary treatment further improves the quality
of wastewater before it is re-used, recycled or discharged to the environment.
Tertiary treatment helps remove any remaining inorganic compounds and nutrients, such as nitrogen and
phosphorous. Bacteria, viruses and parasites harmful to public health are removed in the disinfection stage.
The three main processes involved in tertiary treatment are:
1.
Flash mixing
Wastewater flows from the biological reactor and IDAL to a pumping station which pumps it to a
flash mixer. In the flash mixer, alum is used as a coagulant to help remove additional phosphorous.
Alum is a salt consisting of an alkali metal such as sodium, potassium or ammonium and a trivalent
metal such as aluminium, iron or chromium.
Solid particles in wastewater coagulate into tiny particle clusters or ‘floc’, making them bigger, heavier
and easier to remove in the deep bed sand filters. This process is also called flocculation.
2.
Filtration
Wastewater from the flash mixers is gravity fed through six deep bed sand filters, which trap the
floc. These have a total surface area of 450 m2. With five filter bays online, and the sixth offline for
backwash and maintenance, the plant can filter 35.6 ML/d on average and 106.9 ML/d during peak
periods. The clear water that passes through the filters is gravity fed into chlorine contact tanks for
disinfection.
Trapped floc from the filter beds is removed by backwashing the filters every 24 hours in dry weather.
The backwash is returned to primary treatment.
3.
Disinfection
Chlorine in the chlorine contact tank kills microorganisms, including bacteria, viruses and parasites
like Giardia and Cryptospiridium. Chlorine is a powerful oxidant and reagent and the most common
disinfectant used in industry. Two dosing pumps deliver gaseous sodium hypochlorite into the
chlorine contact tank, which holds the wastewater until the chlorine has time to react.
As chlorine can be harmful to aquatic life in high concentrations, the remaining chlorine is neutralised
(dechlorination) by adding liquid sodium bisulphate to the wastewater before it is discharged.
Dechlorination dosing facilities were added to Penrith Wastewater Treatment Plant in 2000,
and expanded later.
Further treatment
Tertiary treated water from Penrith Wastewater Treatment Plant (as well as from Quakers Hill and St Marys
Wastewater Treatment Plants) is piped to the St Marys Water Recycling Plant. This recycled water forms part
of the Replacement Flows Project.
The St Marys Water Recycling Plant produces up to 50 million litres of recycled water a day, using
microfiltration and reverse osmosis. The very fine filters in microfiltration trap nutrients, chemicals, bacteria
and viruses. Treatment is further enhanced by reverse osmosis, a process where pressure is applied to a
solution when it is on one side of a selective membrane. The solute is returned on the pressurised side of
the membrane and the solute passes to the other side. Large molecules and ions are removed.
This water helps maintain healthy water flows in the Hawkesbury-Nepean River.
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Biosolids treatment
At Penrith Wastewater Treatment Plant sludge is collected from the IDAL process, or from ‘wasting’ excess
biological matter from the biological reactor. In biological treatment the population of microorganisms will
increase as they consume the available ‘food’ from the wastewater. This excess or (waste activated) sludge
needs to be removed to maintain a stable population of microorganisms.
At Penrith, there are several steps to treat this sludge. These are thickening, aerobic digestion, and dewatering.
Thickening
Sludge from the treatment tanks contains a lot of water. It is fed into a dissolved air floatation (DAF) tank
where very fine bubbles of pressurised air are injected below the waste activated sludge (WAS) inlet.
As the fine bubbles rise to the surface they attach themselves to the solids in the WAS. The WAS is lifted
to the surface where it is scraped from the top of the tank and sent to aerobic digesters. The remaining
effluent flows underneath the layer of sludge and returns to the treatment tanks.
As a result of thickening the sludge to solids, the volume of sludge that has to be treated in the aerobic
digesters is dramatically reduced, meaning we can have smaller digestion tanks and aeration equipment.
However, the sludge can’t be too thick when it goes to the digesters or it won’t mix properly and proper
oxygen transfer cannot be maintained.
Aerobic digestion
An aerobic digester is an oxygen-rich tank where bacterial processes partially breakdown organic matter
in the sludge (carbon dioxide is one of the end products). This ‘sludge stabilisation’ reduces the organic
material, odours and pathogens in the sludge. When the sludge has been stabilised it is suitable for
beneficial uses, such as land application and composting. Stabilised sludge is known as biosolids.
Dewatering
The biosolids from the digester still have only a small percentage of solids (for every three kilograms
(kg) of solids, there are still 97 kgs of water). The liquid sludge is fed through high-speed centrifuges
to remove excess moisture.
Penrith Wastewater Treatment Plant produces about 2,000 tonnes of biosolids a year, which are
sold as part of Sydney Water’s biosolids land application program under the registered business
name of Biosoil®.
Sydney Water uses 100% of captured biosolids. About 190,000 tonnes of Biosoil® a year is produced and
used as fertilisers to improve soil quality in agriculture and horticulture after testing at laboratories
to make sure it is safe for the environment and human health.
SW7 07/10
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