Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-08-30
2004-07-06
Prince, Fred G. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S630000, C210S259000, C210S260000
Reexamination Certificate
active
06758972
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to methods and devices for treatment of wastewater. More particularly, the methods of the present invention are designed for the biological removal from wastewater of contamination in the from of insoluble suspended solids and soluble and insoluble organic and inorganic material including nitrogen and phosphorus nutrients.
BACKGROUND OF THE INVENTION
Wastewater which is produced every day by domestic activities (wastewater from bath, kitchen, shower, toilet, washing-machine, etc.) and in industry, must be purified for both organic materials and inorganic materials such as nitrogen compounds and phosphorus nutrients, before being discharged in surface waters or being treated for reuse (irrigation, cleaning or process water).
Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or “open” systems) rather than as “batch” or a series of periodic treatment processes in which treatment is carried out on parcels or “batches” of wastewaters. While most wastewater treatment processes are continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are routinely handled as periodic batch operations.
Wastewater treatment, however, can also be categorised by the nature of the treatment process operation being used; for example, physical, chemical or biological. A complete treatment system may consist of the application of a number of physical, chemical and biological processes to the wastewater. The present invention relates to biological wastewater treatment. Biological treatment methods use micro-organisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products. More micro-organisms, or sludges, are formed and a portion of the waste is converted to carbon dioxide, water and other end products. Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on presence of dissolved oxygen.
Basics of biological wastewater treatment are given hereinafter.
Aerobic biological wastewater treatment may be carried out in an aerobic activated sludge reactor, where activated sludge (flocculated aggregates of micro-organisms) is aerated and fed with wastewater. The activated sludge is a microbial mass which has increased biological activity through aeration in a vessel.
The suspended and dissolved organic matter of the wastewater is in a first enzymatically induced step, adsorbed and absorbed by the micro-organisms in the sludge flocks (the accumulation process).
The accumulated or stored substrates are then oxidised in carbon dioxide and water, while producing energy (the dissimilation process).
The generated energy is used to take up the substrates and generate new micro-organisms (the assimilation and regeneration processes).
In a clarifier, the mixture of the sludge and treated wastewater is separated by gravitational sedimentation in a bottom sludge blanket and supernatant effluent. Depending on the technology, the bottom sludge is recycled to the aeration reactor or re-activated later in the same compartment.
Next to organic matter, wastewater usually also contains nitrogen and phosphorus compounds. These compounds exist under different forms, like particulate or dissolved organic bound nitrogen and phosphorus, and dissolved ammonia, nitrates, nitrites and ortho-phosphates.
European and world-wide legislative bodies have issued strict discharge limits for nitrogen (<10 mg N/I) and phosphorus (<1-2 mg P/I).
For this reason enhanced biological nitrogen and phosphorus removal is often incorporated in biological wastewater treatment processes.
To activate biological nutrient removal in wastewater treatment plants, specific conditions should be applied to the activated sludge micro-organisms. A distinction can be made between assimilative and dissimilative nutrient removal.
Hydrolysis of organic bound nitrogen or phosphorus to dissolved ammonia and ortho-phosphates is a process that is important to make the nitrogen and phosphorus available to the micro-organisms.
During the assimilation process the micro-organisms incorporate nitrogen and phosphorus in new cell material according to a specific ratio based on an ideal cell material composition. This ratio is estimated to be approximately 100:5:1 as BOD:N:P. This is a rather conservative value and depending on the F/M ratio, a measure of food provided to micro-organisms in an aeration tank, and thus depending on sludge loading. BOD is the Biological Oxygen Demand, the quantity of oxygen necessary for breakdown of readily decomposable organic matter added to water (or a liquid mix such as liquid manure).
If the nutrient content of the wastewater is low, then nutrients should be added to obtain a good working activated sludge. Addition of nutrients is often necessary when treating high organically loaded wastewaters from the food industry.
In general the assimilative nutrient removal is depending on the sludge production and thus on the organic loading of the plant. Since sludge production is normally kept as low as possible, the assimilative removal capacity is rather limited.
The specific conditions for dissimilative enhanced biological nitrogen and phosphorus removal processes are different. Therefore these processes are described separately.
The dissimilative nitrogen removal is a process that is carried out by a specific group of bacteria in the activated sludge during different environmental conditions or states.
A first step in the dissimilative nitrogen removal is called nitrification or ammonia oxidation, carried out by autotrophic nitrifying organisms during aerobic conditions. The nitrifying micro-organisms in the activated sludge oxidise ammonia to nitrite and nitrate. This process uses oxygen, according to the following reaction (nitrification reaction):
ammonium-N+oxygen→nitrate-N+water+protons+energy
A second step in the dissimilative nitrogen removal is called denitrification, carried out by facultative anaerobic heterotrophic bacteria. During denitrification the nitrate is transformed via nitrite and several intermediates to atmospheric nitrogen. The denitrification reactions are as follows:
nitrate-N+electrons+protons→nitrogen(gas)+water+energy
carbon+water→carbon dioxide+electrons+protons
In this process, nitrates are used for the oxidation of carbon, and this process provides the energy for cell growth.
Denitrification is achieved under low or zero dissolved oxygen concentrations (anoxic phase). As carbon source for denitrification, raw wastewater should be used to the maximum.
Oxygen and alkalinity are partly recovered during this process.
Important control parameters for dissimilative nitrogen removal are the aerobic and anoxic retention time of the bacteria, the availability of rapidly biodegradable organic substrates (volatile fatty acids), pH and temperature and especially the overall sludge retention time in the system (typically >10 days at 15° C.).
Biological phosphorus (BioP) removal is achieved in a sequence of anaerobiosis (the presence of life in the absence of air or free oxygen) and aerobiosis (the presence of life in presence of air or free oxygen).
Strictly aerobic organisms can survive in anaerobic conditions on short chain fatty acids. These acids are taken up using energy from hydrolysis of polyP (polyphosphate). Finally polyP is hydrolysed, ortho-phosphates are released in the liquid and an internal C-source (carbon source) is formed.
Oxidation of the internal C-source in the subse
Gerards Ron
Vriens Luc
Barnes & Thornburg LLP
Prince Fred G.
LandOfFree
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