030929
Biological Wastewater Treatment in a fluidized-bed reactor
(MBBR-process)
To remove organic matter, phosphorus and nitrate from wastewater biological processes are used. A high process stability, even in case of hydraulic jumps, underload, and overload of the system, a nearly completely elimination of nitrogen and very ambitious target values for the required discharge parameters are to achieve with a sewage treatment process by combining activated-sludge process and fluidized-bed biofilm process. This biotechnology for the effective elimination of nutrients during the traditional wastewater treatment was developed by us in the years 1984 to 1989.
Since 1989 this fluidized biofilm technology was put into operation at more than 170 WWTP`s with a capacity 50 to 50.000 PE.The largest part of the micro-organisms, which are necessary for the treatment of wastewater for removal of organic matter, nitrogen compounds (nitrification of ammonium and denitrification of oxidized nitrogen compounds) as well as the phosphorus elimination, grows up with this process combination on a cylindrical carrier material (biofilm), which is mechanically and biologically resistant. Diameter of carriers is general 3 to 8 mm, the length 4 to 8 mm with a specific surface of 800 to 950 m2/m3 bulk volume of the carrier material.
The microorganisms that are concentrated in this biofilm, have depending on the environmental conditions of the respective successive stages of treatment highly efficient specialized metabolic effects. In the unaerated, anaerobic reactor sections are an acidification and the hydrolysis of organic substances, their results are needed of the phosphorus eliminating micro-organisms and the denitrificants. In the anoxic reactor sections, in which there are indeed no dissolved oxygen, but still as nitrate chemically bonded oxygen is present, there is a reduction of the oxidized nitrogen compounds (nitrate) to nitrogen gas, which escapes into the atmosphere. In the aerated turbulent reactor sections is determined by the metabolism of the microorganisms in the biofilm the oxidation of organic substances and the nitrification of the Ammonium.
The coarse-dispersed and dissolved organic substances (putrescible material) and nutrients (nitrogen and phosphorous compounds) are stored in the biomass, mainly in the biofilm on the carrier material and in the activated sludge flakes, which in this procedure are very well settleable. The biomass on the carrier material has a high degree of specialization of the microorganisms specifically for nutrient elimination, nitrification, and denitrification as well as the acidification and hydrolysis of organic substances. The concentration of microorganisms in the aeration tank is approximately five times higher than for the classic activated sludge process, although the concentration in the liquid phase in order to achieve good outflow conditions in the following secondary settlement can be set rather low.
The higher quantity of biomass concentration results in an increase in efficiency and increase of process stability without detrimental effect on the secondary sedimentation tank, that cannot prevent with the classical activated sludge process.For the dimensioning of the secondary sedimentation tanks the dry solid concentration in the aeration tanks for the suspended substances will be calculated as 2 to 5 kg DS/m3 , although the real concentration of dry solids including biomass on the carrier material general is 12 to 16 kg DS/m3. The biomass on the carrier material do not adversely affect the dimensioning of the secondary sedimentation tanks although its share is very much higher than the share of the activated-sludge flocs. The sludge volume index of this good settleable activated sludge is in the range of 70 to 100 ml/g.
Carrier-fixed micro-organisms remain in the corresponding process stage in case of hydraulic jumps or excess sludge removal by a special hydraulic retention system, which cannot block. The biomass can therefore not flush out as in the traditional activated sludge process, if it is necessary for the treatment in case of load shocks . This hydraulic retention system ensures, in contrast to mechanical equipment, such as sieves or grids that the system is absolutely clog-proof.
The potential of the suspended activated sludge flakes (usually used only for the internal recirculation for biological phosphorus removal) and the micro-organisms in the form of biofilm on the carrier in this process will be effectively combined with each other. In the anaerobic zones with floating carrier material layers with a floating grain filter effect is an acidification and hydrolysis of organic substances in the wastewater. In the anoxic zones without aeration takes place in the fluidized carrier material layers the denitrification and in the aerated aerobic zones in dependence on the intensity of the aeration in an ideal homogeneously mixed turbulent or fluidized bed the nitrification and oxidation of organic substances. This ensures that in this process no external carbon compounds (ethanol, methanol, etc.) are required, however, the effectiveness of the primary treatment steps should be reduced accordingly.
As an essentially advantage of this technology can the quantity of carrier material adapted to standards on purification efficiency without consequence to required reactor volume. Seasonally pollution load can therefor exactly reliable treated.
Effluent concentrations for phosphorus 0,1…0,5 mg/l, for NH4–N 0,1…0,6 mg/l, CSB 23…60 mg/l can be achieved dependent on quantity of carrier material also in case of difficult BOD/TKN relation.
In case the requirements for the quality of the effluent are not so high, can be saved up to 60% reactor volume, and increasing loads or higher requirements without structural measures can only be taken into account by adding additional carrier.
The comparison showed that this process is therefore the most economical (low investment and operating costs) and technically most advantageous process technology for the wastewater purification with the highest operational safety as a result of process according to the state of the art. The special process technology makes it possible to eliminate phosphorus and nitrogen even in traditional wastewater treatment plants with a significant smaller volume of the biologically reactors. This is very important for an effective recovery.
The first tank is used as a phosphate release basin. The biomass on the carrier material in this tank acidifies and hydrolyses the organic substances in the wastewater. The supplied return sludge accepts this easily assimilable low molecular substances, stores them as hydroxybutyric acid and press out polyphosphates (Valutingranula) in the liquid phase. A prerequisite for this is that neither dissolved oxygen or as nitrate chemically bonded oxygen in this reactor part are present. The carrier material is designed to act as a floating particle filter and is equipped with an agitator. In this layer are all rough and coarse disperse organic substances by microorganisms on the carrier material hydrolysed. In the following denitrification step, the hydrolysis products are used as hydrogen donors for further denitrification of nitrates.
The separation of the metabolic processes of the biofilm (acidification and hydrolysis) from the metabolic processes of the return sludge (phosphate storage and denitrification) results in a high degree of specialisation of the micro-organisms which happens because such short retention periods are sufficient. In case of residual phosphate content can be separated as flocculent with dosing of Me3+ salts.
The next section of the activation tank is used as the subsequent control step in order to compensate weekly or seasonal load fluctuations. This reactor section can be used both for denitrification and nitrification. It will be equipped with aerators and agitators.
The nitrification and denitrification steps are filled at the beginning with carrier material at 20 to 30 %. This corresponds to a biofilm load of ≤ 5 g BOD per m² biofilm. Thus, a total nitrification is possible at 450 % recirculation. The max. carrier material filling more than 60%, so that there is a sufficient margin for load peaks or extensions. The existing shearing forces in the turbulent bed guarantee, thereby, a highly active thin biofilm.
The micro-organisms are supplied with oxygen by means of fine-bubble aeration. The required oxygen will be introduced to the system via membrane aerators. These entrain oxygen from the atmosphere, keep the tank mixed and the carrier material is brought to turbulence. The oxygen input to the wastewater is determined by the submergence of the membrane aerators. It is usually calculated with a water depth of about 5 to 6 m. In the reference plant WWTP Köthen were reachedoxygen utilisation values of 5.2 kg O2/kWh in clear water. The alpha value is in this system 0.8.
A degassing zone should be provided at the end of the aeration tank and before the outlet. There, the formed CO2, the residual air, and other gaseous substances will escape. The internal recirculation pump for the denitrification will be installed in this degassing zone. Each pump is variable speed controlled. These pumps extract simultaneous a defined amount of carrier material to the denitrification zone.
This moved-bed biofilm-technology is particularly suitable for the upgrading of existing sewage treatment plants, which works as conventional activated sludge process. As a rule, in the existing tank volume can be reached by use of the carrier for the biofilm a nitrogen and phosphorus elimination. Due to the very compact design, can be reached a small land consumption and good integration of plants in the landscape, especially in sensitive areas. The biofilm carrier on the anaerobic and anoxic basin sections at the same time do the deodorierung of odorous substances through the wastewater biology, which is why no odour nuisance occurs. Noise does not occur because of the lack of any mechanical noise sources to the reactors are present. Especially for seasonal load fluctuations and for increasing demands on the cleaning performance is this technology because of its extremely high stability and reliability even in small plants very well-suited.
The carrier biology generates less excess sludge as a classical activated sludge plant, it is in this respect comparable with a trickling filter biology.
Through the turbulence of the air with the pearl-shaped sources is guaranteed a rational energy utilization for oxygenation.
