1、 Definition of sewage load
1. Load: Generally speaking, the load includes sludge load and volume load, which respectively refer to the amount of COD removed by a certain amount of sludge (kg) in a certain time (day) and the amount of COD removed by a certain reaction volume (cubic meter) in a certain time (day) (kg).
2. Impact load: during the operation of sewage treatment, the sludge volume will generally be maintained at a certain level, and the volume of the reactor (aeration tank, anaerobic reactor, etc.) will not change. However, if the influent quality changes greatly (COD soars or drops sharply), the sludge load and volume load will change greatly, which will have an impact on sludge microorganisms, which is the so-called impact load.
3. In some treatment processes (especially those with large return flow or completely mixed type), due to some hydraulic or other design, the process has strong resistance to impact load. Even if the load increases, it will not collapse immediately and can recover relatively quickly. That is, it has strong impact load resistance.
2、 Understanding and calculation of sewage system load
1. Operating load rate = daily actual water inflow / daily design treatment capacity. Generally, the operating load rate is required to be no less than 60%. In 2010, although the average operating load rate of urban sewage treatment plants across the country has been close to 80%, and some even exceeded 100%, the operating load rate specified by the State shall not exceed 120% of the design treatment capacity.
2. Sludge volume: the volume of sludge with a concentration of 1% can be considered as the same as that of water. The volume of sludge with a concentration of 1 T / m3 and a concentration of 5% can be considered as the same as that of water. Sludge with a concentration of 1% means that there are 10 kilograms of solid matter in each ton of sludge. Sludge with a concentration of 5% means that there are 50 kilograms of solid matter in each ton of sludge. Therefore, the moisture content of sludge is 99% and reduced to 95%, that is, 5 tons of sludge becomes one ton of sludge In other words, the volume of sludge will be reduced by five times.
How much will the volume of activated sludge with 99% water content be reduced when it is concentrated to 97% water content?
Dry matter is conserved and the density is approximately 1v99 ×ρ× （1-99%）=V97 ×ρ× (1-97%) V97 / V99 = 1 / 3, so the volume is reduced from 3 to 1, about 66%.
3. BOD load = (inlet BOD) × Water inflow / (V tank capacity) × MLSS) this is MLSS load BOD load = (inlet BOD) × Water inflow / (V tank capacity) × MLVSS) this is the MLVSS load.
4. Sedimentation tank and outlet weir load: the surface load of sedimentation tank and outlet weir load belong to hydraulic load, which has little to do with biological treatment. All belong to some design parameters. Surface load of sedimentation tank: when a particle passes through a distance exactly equal to the depth of the tank within the theoretical residence time, its sedimentation rate is called overflow rate or surface load rate. The dimension is several cubic meters per square meter per unit time, that is, several meters per unit time. The efficiency of the sedimentation tank is usually based on the surface load rate and expressed in cubic meters of water flowing every day per square meter of water surface area. It is the water volume divided by the area of sedimentation tank and the outlet weir load: that is, the outlet flow of a certain length of weir, that is, the flow divided by the weir length.
5. The organic loading rate is the ratio of the amount of organic matter in the influent to the amount of sludge in the reactor.
6. Sludge age refers to the average residence time of microorganisms in the reaction system from their generation to discharge system, that is, the time required for all microorganisms in the reaction system to be renewed once. Under stable conditions, the difference between the total amount of activated sludge working in the aeration tank and the amount of residual sludge discharged every day is that the sludge age is longer than usual, the species diversity is more, and the organic load rate can be relatively increased. But it's not absolute.
From the perspective of dynamics, on the premise of keeping the biomass concentration MLVSS and influent flow in the tank unchanged (please pay attention to this prerequisite), the increase of load (increasing the influent COD concentration) will lead to the increase of effluent COD concentration and faster sludge growth. In order to maintain MLVSS, the sludge discharge is faster, that is, the sludge age becomes smaller. vice versa. But this kinetic reaction has a range.
Based on the following reactions: u = 1 / SRT = Umax * Se / (SE + KS) ---- monodns = Q * so / (V * x) ---- organic load for the increase of influent load in the actual project and the countermeasures, as well as the "organic load rate can be improved relatively. But it is not absolute." It can be analyzed as follows: for an existing system, it is impossible to adjust the residence time and change the structure. What can be changed is the control of sludge concentration and sludge discharge. In order to ensure the effluent quality (when se remains unchanged, the growth rate of unit microorganism and the absorption rate of pollutants remain unchanged), it is bound to need to improve MLVSS to realize the absorption of increased load. The actual operation is to reduce the sludge discharge, and then increase MLVSS. After the effluent reaches the standard, gradually increase the sludge discharge. The final balance is that MLVSS is larger than before the load increase, and the absolute sludge discharge also increases. Finally, under stable conditions, NS and SRT have not changed, but a new equilibrium point has been formed!
7. Sludge load the kilogram of biochemical oxygen demand per kilogram of activated sludge per unit time in the aeration tank. The unit of measurement is usually expressed in kg / (kg · d).
8. The flow per unit surface area of the sedimentation tank per unit time of surface load is called surface load or overflow rate, which is commonly expressed as Q, q = q / a (i.e. the ratio of flow to surface area)
Sludge load (NS) refers to the amount of pollutants removed by activated sludge per unit mass in unit time. The meaning of sludge load in microbial metabolism is the F / M ratio, in kgcod (BOD) / (kg sludge. D)
In different stages of sludge growth, the sludge load is different, and the purification effect is also different. Therefore, the sludge load is one of the main parameters for the design and operation of activated sludge process. Generally speaking, when the sludge load is in the range of 0.3 ~ 0.5kg / (kg. D), the BOD5 removal rate can reach more than 90%, and the SVI is 80-150. The adsorption performance and sedimentation performance of sludge are good.
Calculation method of sludge load:
NS - sludge load, kgcod (BOD) / (kg sludge. D);
Q -- daily water inflow, m3 / D;
S -- cod (BOD) concentration, mg / L;
V -- effective volume of aeration tank, m3;
X -- sludge concentration, mg / L.
9. The treatment load of full load sewage generally refers to the amount of sewage or the total amount of pollutants treated by the sewage treatment system on the premise that the incoming sewage can reach the standard stably. For example, the design of a sewage plant is 2000m3 / D and the influent cod1000mg / L. in fact, the influent is 1000m3 and the influent cod2000 is more than 1000. If the treated effluent is stable and up to standard, it can also be said that the system has reached full load. Of course, this full load is relative. The designer's design description will mention the organic load of microorganisms in the sewage treatment unit and the concentration of microorganisms in the tank. If you improve the biomass and treatment capacity in the tank through management during operation, you can also operate under overload. The general design indexes are parameters with relatively stable operation. It's not impossible to be higher or lower. In the process of increasing the load, gradually increase the biomass and unit removal capacity, and gradually increase the amount of sewage treatment. This process is the process of debugging. The target of this commissioning is that if the effluent quality is qualified and the effluent is stable, the sewage load can be slowly increased until it runs at full load.