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An activated sludge process removes carbonaceous pollution in waste water through two main stages. The first is the aeration tank where the raw water is mixed with air or oxygen. This aeration tank has a mixture of the raw water and some of the recycled sludge which acts as a seeding material. In the second part of the process, the aerated mixture is allowed to settle the biological flocculants, separating the sludge that is biological from the treated water. Nitrogenous matter or phosphate requires further treatment for its removal.
The removal of nitrogen and phosphorus from wastewater is nowadays considered essential to protect waterways. Excessive release of wastewater containing nitrogen and phosphorus can encourage the growth of algae and weeds in waterways. This can cause a rapid growth in the population which is not sustainable causing algae to die. These algae decompose due to bacterial action which further reduces the dissolved oxygen so necessary for health of the water. Phosphorous can also cause damage to reverse osmosis installations.
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The Bardenpho Process of Wastewater Treatment
One method of removal of the nutrients that nitrogen and phosphorus make up in treated wastewater is biological treatment through modifying the sludge system without any addition of chemicals. The organic matter of the sludge is used as the energy source and carbon required to remove the phosphorous and nitrogen. This method is also more cost-effective as no chemicals are needed.
The Bardenpho Process of Wastewater Treatment was developed by James Barnard of South Africa in the 1970’s. It is a biological nutrient removal process which goes through four stages. Anaerobic sludge is obtained from the anaerobic treatment tank of the plant and mixed with food waste, grass, or wastepaper. This organic waste material is subjected to anaerobic fermentation for a period of 2 to 4 days at a temperature of 30 to 40 ºC. Anaerobic sludge is used to obtain broth for fermentation. The fermentation broth is then sent through alternating anaerobic-aerobic-anoxic cycles in batch reactors.
When under anaerobic conditions, phosphorous is secreted from the microbes that accumulate phosphorous. When the fermentation broth is then subjected to aerobic conditions, the phosphorous is taken up by the accumulating microbes. Nitrifying bacteria oxidize the ammonia nitrogen in this stage. When the final anoxic tank is filled with this broth, the oxidized nitrogen is converted to nitrogen gas by the bacteria.
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Nitrogen in the original wastewater is mainly in the form of ammonia and this ammonia passes through the first two zones without any change. It is only in the third aerobic zone that the sludge has aged sufficiently for complete nitrification to take place and that the ammonia nitrogen gets converted to nitrates and nitrites. When this reaches the anoxic zone, because of the absence of dissolved oxygen, the nitrates are converted by the bacteria to nitrogen gas by using the organic carbon compounds as donors for hydrogen. This nitrogen escapes to the atmosphere. The effluent is then subjected to aeration in the final zone which raises the dissolved oxygen levels and prevents further denitrification.
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This is achieved through a step feed process in which wastewater influent is treated in at least one aerobic zone. This is again processed through at least one anoxic zone. A portion of the effluent from the anoxic zone is then sent to an anaerobic zone along with raw water. Influent from one anaerobic zone is sent to an anoxic zone and then sent to a downstream aerobic zone.
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Advantages of the Bardenpho Process
As no chemicals are used, operating costs are lower and there is also no problem with disposing of the sludge. Bardenpho Process plants are simple to operate and do not require any retraining of personnel. The sludge that is obtained in the final stages does not require any further treatment and can be easily disposed of.
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Disadvantages of the Bardenpho Process
One of the main disadvantages of the Bardenpho process is the number of tanks required, which greatly increases capital cost. Another disadvantage is detention times need to be very strictly monitored and evaluated.