Penrith Wastewater Treatment Plant

Transcription

1 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. 1

2 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. 2

3 2. Intermittently Decanted Aerated Lagoon The two Intermittently Decanted Aerated Lagoons (IDALs) were built in 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. 3

4 Removing nitrogen and phosphorous Nitrogen 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 (NO 2 ). The Nitrobacter convert nitrate to nitrite (NO 3 ). 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: 2NH 4 + 3O 2 2NO H 2 O + 4H + new cells For Nitrobacter: 2NO O 2 2NO 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 (N 2 ). The basic path for reducing nitrate to nitrogen gas is: Phosphorus 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. NO 3 - NO 2 - NO N 2 O N 2 nitrite nitrate nitric oxide nitrous oxide nitrogen gas 4

5 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 m 2. With five filter bays online, and the sixth offline for backwash and maintenance, the plant can filter 35.6 ML/d on average and 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. 5

6 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 6